Swift/UVOT: 18 Years of Long GRB Discoveries and Advances

TL;DR

This paper reviews 18 years of discoveries of long gamma-ray bursts using the Swift/UVOT instrument, highlighting advances in understanding their early optical/UV afterglow emissions.

Contribution

It provides a comprehensive overview of Swift/UVOT's long GRB observations and summarizes key scientific advances over 18 years.

Findings

Enhanced understanding of early optical/UV afterglow emission.

Identification of new features in GRB light curves.

Improved models of GRB emission mechanisms.

Abstract

The Neil Gehrels Swift Observatory (Swift) has been in operation for 18 years. The Ultra-Violet/Optical Telescope (UVOT) onboard Swift was designed to capture the earliest optical/UV emission from gamma-ray bursts (GRBs), spanning the first few minutes to days after the prompt gamma-ray emission. In this article, I provide an overview of the long GRBs (whose prompt gamma-ray duration is >2 s) observed by the Swift/UVOT and review the major discoveries that have been achieved by Swift/UVOT over the last 18 years. I discuss where improvements have been made to our knowledge and understanding of the optical/UV emission, particularly the early optical/UV afterglow.

Full text

\pubvolume

1 \issuenum1 \articlenumber0

\externaleditorAcademic Editor: Peter Roming, Michael Siegel

\datereceived4 January 2023 \daterevised15 February 2023 \dateaccepted16 February 2023 \datepublished \hreflinkhttps://doi.org/

\TitleSwift/UVOT: 18 Years of Long GRB Discoveries and Advances \TitleCitationSwift/UVOT: 18 Years of Long GRB Discoveries and Advances

\AuthorSam Oates \orcidA \AuthorNamesSam Oates \AuthorCitationOates, S.

1 Introduction

Gamma-ray bursts (GRBs) are brief, intense flashes of gamma-rays, often accompanied by a longer-lasting emission in the X-ray to radio wavelengths. The duration of the gamma-ray emission may be as short as a few milliseconds or may last for as long as several hundreds of seconds, during which the GRB ‘outshines’ all objects in the known universe. The initial gamma-ray emission is followed by an afterglow, longer-lived emission that is usually observed from X-ray through to radio wavelengths (Sari, 1997; Sari et al., 1998).

GRBs can be divided approximately into two classes, short and long, by the duration of their gamma-ray emission, with the division at $\sim$2 s (Kouveliotou et al., 1993). The short GRBs are thought to be a result of the merger of two compact objects, either two neutron stars or a neutron star and a black hole (Paczynski, 1986; Eichler et al., 1989). This was recently confirmed (in one case) with the detection of a GRB associated with the GW signal of binary neutron star merger (GW 170817/AT2017gfo; e.g., (Abbott et al., 2017a, b; Coulter et al., 2017; Evans et al., 2017; Goldstein et al., 2017)). Long GRBs are thought to be the collapse of massive stars (Woosley, 1993; MacFadyen and Woosley, 1999); evidence for this is the association of supernovae a few days after the gamma-ray emission (e.g., (Galama et al., 1998; Kulkarni et al., 1998; Hjorth et al., 2003; Woosley and Bloom, 2006)).

Following the launch of the Neil Gehrels Swift Observatory (henceforth, Swift; (Gehrels et al., 2004)), it was shown that the empirical categorisation of short and long GRBs, by their gamma-ray duration, quantified by the ${\rm T_{90}}$ parameter (${\rm T_{90}}$ is the time interval over which 90% of the gamma-ray emission is measured) with a division at 2 s, is too simplistic. There is likely some natural overlap in duration between the classes (Bromberg et al., 2013). Some GRBs that are categorised as long or short have been shown to have other properties that suggest that they belong to the other class. The prototypical event was the Swift discovered GRB 060604 (Della Valle et al., 2006; Fynbo et al., 2006; Gehrels et al., 2006; Gal-Yam et al., 2006). More recent examples of each type are GRB 200826A (Ahumada et al., 2021; Zhang et al., 2021; Rossi et al., 2022) and GRB 211211A (Rastinejad et al., 2022; Troja et al., 2022; Yang et al., 2022). In addition, different detectors with different sensitivities and different methods for determining $\rm T_{90}$ may result in slightly different values for $\rm T_{90}$, adding to the complication of classifying GRBs solely by their duration (e.g., (Sakamoto et al., 2011; Savchenko et al., 2012; Bromberg et al., 2013; Qin et al., 2013)). Because of such complications, there is some preference to categorise GRBs by their progenitors, into merger and collapsar categories, which avoids the pitfalls of classifying GRBs by their gamma-ray durations. However, classifying GRBs into merger (Type I) and collapsar (Type II) classes (Zhang, 2006) does often require a more involved analysis and may not be possible for all observed GRBs, for instance, due to insufficient observations. For simplicity, in this review, I will use the empirical short and long GRB dichotomy unless otherwise stated.

In this review, I will discuss the advances in long GRBs and their afterglows as a result of optical/UV observations performed by the Ultra-violet/optical telescope (UVOT) onboard Swift. The review is divided into Section 2, a discussion of the discovery and status of long GRBs before the launch of Swift; Section 3, an overview of long GRBs observed with Swift/UVOT; Section 4, advances in our understanding of the optical/UV emission as a result of Swift/UVOT; and summarise in Section 5. A review of short GRBs observed by Swift/UVOT is provided in an accompanying article in this special issue.

2 Long GRBs Pre-Swift

In this section, to put Swift/UVOT and its discoveries into context and where it fits into the timeline of long GRB discoveries, I provide a short overview of our understanding of long GRBs up until the launch of Swift.

2.1 The Discovery of GRBs

Gamma-ray bursts were initially detected during the late 1960s by the 4 Vela spacecraft (Klebesadel et al., 1973). These unknown sources of gamma-rays were found not to be associated with any object within our solar system, and therefore were assumed to be of Galactic or extra-galactic origin. Further progress had to wait until the launch of the Compton Gamma Ray Observatory (CGRO) in 1992, where, using the Burst Alert Transient Source Experiment (BATSE) onboard the CGRO, it was found that the distribution of GRBs was isotropic, suggesting a cosmological origin Meegan et al. (1992). Also discovered using BATSE was the bimodal distribution of the duration of the gamma-ray emission: GRBs with hard spectra and of $<$2 s duration were classified as short-hard GRBs, and those with soft spectra and of $>$2 s duration were classified as long-soft GRBs Kouveliotou et al. (1993), providing the first evidence for two different types of progenitors.

Emission at longer wavelengths, X-ray to radio, was predicted. This emission, the ‘afterglow’, was expected to be longer lived and to be observed shortly after the prompt gamma-ray emission (Liang, 1985; Hartmann et al., 1988; Rappaport and Joss, 1985). A detection was only achieved at longer wavelengths, with the launch of BeppoSax in 1996, which had instruments to observe 0.1–300 keV X-rays (Boella et al., 1997; Parmar et al., 1997; Boella et al., 1997; Frontera et al., 1997). The first GRB to be detected in X-rays, and subsequently in the optical, was GRB 970228 (Costa et al., 1997; van Paradijs et al., 1997). Observations of GRB 970508 provided the first redshift, $0.835\leq z\leq 2.3$ (Metzger et al., 1997), confirming the extra-galactic origin of these sources, and provided the first indications of how energetic these sources are, with an isotropic energy of $7\times 10^{51}$ erg calculated for this GRB (Metzger et al., 1997). One last further major development was the detection of supernovae coincident with long GRBs; the first was SN1998bw, associated with GRB 980425 (Pian et al., 1999; Galama et al., 1998); and the second, SN2003dhm associated with GRB 030329 (Hjorth et al., 2003). With GRB 980425, there was some concern that the supernova was a chance coincidence, but with a second GRB associated with a supernova, the connection between the collapse of massive stars and long GRBs was solidified (Woosley and Bloom, 2006).

2.2 Expectations and Observations of Long GRBs, Pre-Swift

Prior to the launch of Swift, the theoretical understanding of long GRBs and their emission in the optical/IR was well developed. The collapse of the massive star to a black hole was predicted to result in the production of relativistic bipolar jets (Katz and Piran, 1997). The relativistic jets can be regarded as a series of relativistic shells with varying Lorentz factors (Kobayashi et al., 1997, 2002), which result in internal shocks when the shells with high Lorentz factors catch up with shells of lower Lorentz factor that were emitted earlier. The internal shocks produce the prompt gamma-ray emission. Once the relativistic jet reaches the external medium and ploughs into it, it begins to slow down and produces the external shocks, which comprise of a forward and a reverse shock (Sari, 1997). The forward shock propagates into the external medium, while the reverse shock travels back through the relativistic ejecta. The emission produced in both shocks is released by synchrotron radiation. This emission results in a long-lived afterglow, which for the most part is emission from the forward shock. The emission produced by the external shocks can be described by power-laws for both the observed spectra and the light curves. The temporal and spectral indices, $\alpha$ and $\beta$, are given using the standard convention of $F(t)\propto t^{\alpha}\nu^{\beta}$, where $t$ is time and $\nu$ is frequency. The expected value of the spectral and temporal indices can be described analytically by a set of closure relations (Sari et al., 1998). For a given observing band, the choice of closure relation depends on its location relative to the synchrotron frequencies (the synchrotron cooling frequency $\nu_{c}$, the synchrotron peak frequency $\nu_{m}$, and the synchrotron self-absorption frequency $\nu_{a}$), which in turn depends on the values of the microphysical parameters (the kinetic energy of the outflow $E_{k}$, the fraction of energy given to the electrons $\epsilon_{e}$, the fraction of energy given to the magnetic field $\epsilon_{B}$, the structure and density of the external medium, and the electron energy index $p$).

The behaviour of the afterglow light curve is also dependent on whether or not the observer is seeing all of the emission from the jet, which has an opening angle $\theta$. Initially, the radiation contained within the jet will move at a bulk Lorentz factor $\Gamma$ and will be beamed into an opening angle $\Gamma^{-1}$, which is smaller than the jet opening angle, $\theta$. As $\Gamma$ decreases, the observer will begin to see more and more of the jet until they see the emission from the entire jet at $\Gamma\sim\theta^{-1}$ (Rhoads, 1997; Piran, 1999). At this point, an achromatic break in the light curve will occur, known as the ‘jet break’, after which the light curves will decay at a much steeper rate (Rhoads, 1997, 1999; Sari and Piran, 1999).

Prior to the launch of Swift, the number of GRBs with optical/IR detections was small, around 60 GRBs had optical/IR detections, and only a handful of these had well-sampled optical/IR light curves (Zeh et al., 2006). The optical/UV behaviour was consistent with power-law behaviour, often with a break in the light curve. The average power-law indices of optical/IR afterglows with observed breaks in their light curves were $\alpha_{1}=1.05\pm 0.1$ and $\alpha_{2}=2.12\pm 0.14$ (Zeh et al., 2006). The first decay index is consistent with the predictions of the closure relations for an isotropic outflow, while the latter decay index is consistent with the decay expected after a jet break. Observations of the optical emission typically began a significant amount of time after the gamma-ray detection, commencing typically more than 0.1 days after the trigger (Zeh et al., 2006). This left a significant gap in our knowledge of the early optical/IR emission, between the onset of the GRB through to the first few thousand seconds after the trigger. There were also questions as to why not all GRBs had bright optical or radio afterglows, while X-ray afterglows were expected for all GRBs (Gehrels et al., 2004). It was thought that these GRBs were ‘dark’, potentially due either to dust extinction (Lamb, 2003), being (intrinsically) faint and thus not detected when observations occurred several hours after the trigger (Crew et al., 2003; Pandey et al., 2003), that the optical decayed more rapidly than the X-ray (Groot et al., 1998), or that they were at high redshift (Gehrels et al., 2004). Prompt high-quality X-ray, UV, and optical observations over the first minutes to hours of the afterglow were thus crucial to obtain a better understanding of GRBs.

3 The Era of Swift/UVOT

Swift, launched in November 2004, has been in successful operation for 18 years. Swift was designed specifically to detect GRBs, and observe rapidly with its narrow field instruments. Swift houses three instruments that are designed to capture gamma-ray, X-ray, and optical/UV emissions. The Burst Alert Telescope (BAT (Barthelmy et al., 2005)) detects the prompt gamma-ray emission, while the narrow field instruments, the X-ray Telescope (XRT (Burrows et al., 2005)), and the Ultra-violet/Optical Telescope (UVOT (Roming et al., 2005)), observe the X-ray and optical/UV afterglow, respectively. The co-alignment of the XRT and Swift/UVOT instruments is ideal for observing GRB afterglows because it allows the X-ray and optical/UV emission to be observed simultaneously. Swift’s capacity to slew rapidly to point the narrow field of view telescopes at the GRB location, enables regular and early observations of GRBs at longer wavelengths, starting as early as a few tens of seconds after a BAT trigger.

3.1 An Overview of Long GRBs Observed with Swift/UVOT

At the time of writing Swift/UVOT  has observed $\sim$1339 long GRBs (https://swift.gsfc.nasa.gov/archive/grb_table/; accessed on 20th October 2022) with $\rm T_{90}>2$ s. The majority of GRBs observed by Swift/UVOT were initially detected by Swift/BAT, resulting in Swift automatically slewing to point the Swift/XRT and Swift/UVOT instruments at the GRB location to rapidly commence observations in the X-ray and optical/UV. However, for a small number of GRBs, Swift/UVOT began observations much later, since the gamma-ray emission did not trigger Swift/BAT, but was detected either through BAT ground analysis, or by other X-ray/gamma-ray observatories, including the High Energy Transient Explorer-2 (HETE2), the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), Konus-Wind, the Interplanetary Network (IPN), Fermi, Astro-rivelatore Gamma a Immagini Leggero (AGILE), and MAXI and CALET.

After an automatic slew, Swift/UVOT takes its first exposure. This short, typically 10 s, $v$ band exposure is taken while the reaction wheels are stabilising Swift, such that the pointing direction is in transition from a motion of many arcseconds per second to a stable pointing. During the slew, Swift/UVOT is protected from damage by bright stars passing through the field of view by maintaining the photocathode at a low-voltage state. As such, the settling exposure is frequently ignored or discarded (Oates et al., 2012), since the photometry may be uncertain due to the Swift/UVOT photocathode voltage changing at the beginning of the exposure, or concerns regarding the rapidly changing spacecraft attitude (Roming et al., 2017). Ref. Page et al. (2019) examined the settling exposure for 26 GRBs detected by Swift/UVOT. They found an issue in only 2 out of the 26 GRBs, and only in the first second of the settling exposures. Fortunately, the settling exposure is taken in event mode (in event mode, the arrival times and positions of the individual photons are recorded) and therefore, when issues arise, it does not have to be discarded completely, because it can be cut to exclude the affected parts of the exposure. The settling exposure usually begins 10–15 s before the first settled exposure. The fastest time to settled observations commencing for a long GRB is 52 s after the BAT trigger for GRB 050509A, and the average time to the start of settled observations is 120 s (calculated using GRBs observed by Swift/UVOT within the first 500 s, observed as a result of an automatic slew).

Of the $\sim$1339 long GRBs observed by Swift/UVOT, 514 have a reported Swift/UVOT detection in at least one Swift/UVOT filter (the Swift/UVOT filters are $white$, $v$, $b$, $u$, $uvw1$, $uvm2$, and $uvw2$ (Roming et al., 2005; Poole et al., 2008; Breeveld et al., 2011)); 415 are within the first 500 s after the BAT trigger. This equates to a detection rate of 42% if observed within the first 500 s. This value is consistent with that determined from the second Swift/UVOT catalogue, which, when using a sample of 538 GRBs, reported a detection rate of 43% (with a detection threshold of at least $3\sigma$) for long GRBs observed within the first 500 s (Roming et al., 2017). If considering a detection threshold of $2\sigma$, rather than $3\sigma$, the detection rate increases to 64% (Roming et al., 2017). This is in contrast to 80–90% of GRBs having optical/IR detections observed by ground-based telescopes (Cenko et al., 2009; Greiner et al., 2011). Examining the number of GRBs with a photometric or spectroscopic redshift, 477 GRBs (36%) observed by Swift/UVOT have redshifts (these GRBs may or may not have been detected by Swift/UVOT); of these, 289 (22%) have at least one detection in Swift/UVOT. The redshift distribution for Swift/UVOT GRBs is given in Figure 1. The average redshift for the Swift/UVOT observed GRBs is 2.0, while for the Swift/UVOT detected GRBs, the average redshift is 1.7. Figure 2 displays the total number of GRBs observed by Swift/UVOT per year, together with the number of GRBs observed by Swift/UVOT with a measured redshift per year, and the number detected by Swift/UVOT with a measured redshift per year. The second panel also provides the fraction of GRBs observed by Swift/UVOT with redshift and the fraction of GRBs detected by Swift/UVOT with redshift. It is concerning that the number of GRBs observed with Swift/UVOT with a reported redshift has decreased over the last 5–10 years, the fraction of GRBs observed with redshift in the first 5 full years of operation (2005–2009) is 47%, while in the last 5 years of operation (2017–2021), this has reduced to 23%. However, while the number of GRBs detected by Swift/UVOT with redshift shows a similar decreasing trend, the reduction is not as large, changing from 25% in the first 5 years to 18% in the last 5 years.

In addition to the six lenticular optical/UV filters, Swift/UVOT’s filter wheel also contains two grisms. These provide low-resolution field spectroscopy in the ultraviolet and optical bands. The UV grism covers the range $\lambda$1700–$5000$ Å  with a spectral resolution ($\lambda/\Delta\lambda$) of 75 at $\lambda 2600$ Å  for the source magnitudes of $u$ = 10–16 mag, while the visible grism covers the range $\lambda$ 2850–6600 Å  with a spectral resolution of 100 at $\lambda 4000$ Å  for source magnitudes of $b$ = 12–17 mag (Kuin et al., 2015). Since November 2008, the automated response sequence of the Swift/UVOT, which governs the early exposures after a BAT trigger (Roming et al., 2005), includes a 50 s UV grism exposure, provided that the burst is bright enough in the gamma-rays. So far, this has resulted in two well-exposed UV spectra for GRB afterglows: GRB 081203A (Kuin et al., 2009) and GRB 130427A (Maselli et al., 2014). The light curve and grism spectrum for GRB 081203A is given in Figure 3. Swift/UVOT has also been able to increase the number of GRBs with known redshift by obtaining photometric redshifts through the analysis of spectral energy distributions built using XRT and Swift/UVOT observations of the afterglow, supplemented by ground-based photometry (where available) (De Pasquale et al., 2007; Krühler et al., 2011; Oates et al., 2012; Gupta et al., 2021).

4 Swift/UVOT Long GRB Discoveries

GRB 050318 was the very first GRB to be detected by Swift/UVOT (Still et al., 2005). Observations began 3200 s after the BAT trigger, with an initial detection in the $v$ band of 17.8 mag. The optical emission was observed to decay, and the derived spectral and temporal indices were consistent with the expectations of the standard afterglow model, with the fireball expanding into a constant density medium, and the synchrotron cooling frequency lying in between the optical and X-ray bands.

Since GRB 050318, we now have over 1300 GRBs observed with Swift/UVOT, and over 500 with a detection by Swift/UVOT. Importantly, Swift/UVOT has provided a view of the first minutes to hours after the detection of the gamma-ray emission, which was lacking prior to Swift, and which provided large numbers of GRBs with simultaneous X-ray and optical/UV observations. While the standard afterglow model, as described in Section 2.2, has been shown to be a good basic representation of the afterglow emission of Swift GRBs, post-Swift observations of GRBs often require a more complex model, for instance, with additional emission components or complex jet geometry, which I discuss further in this section. Many advances in GRB science have been through the analysis of multi-wavelength observations, and so, simultaneous observations with Swift/UVOT and Swift/XRT have been key in unravelling the temporal and spectral behaviour of GRBs. In the rest of this review, I will focus on the most significant advances in GRB science as a result of Swift/UVOT’s 18 years of operation. I will discuss the most momentous GRBs observed, which due to their brightness, are the best-sampled and best-studied events. I will also discuss rare and usual GRBs observed by Swift, and improvements to our understanding of dark GRBs, the morphology of optical/UV light curves and correlations discovered with optical/UV observations.

4.1 Record-Breaking GRBs

The detection of GRB 050318 was just the start of many major discoveries by Swift and Swift/UVOT. Over the years, Swift has detected several GRBs that have pushed the boundaries of our expectations and knowledge. Some of the most striking and informative are the brightest GRBs observed via UVOT. These GRBs have the best signal-to-noise Swift/UVOT light curves, and they are events that tend to be intensely observed by a whole range of ground- and space-based facilities. Every few years, Swift delights us with an even more remarkable example. These events tend to be bright, due to a combination of being intrinsically bright (isotropic gamma-ray energy, $E_{iso}$, $\sim$10${}^{54}\,{\rm erg}$) and relatively nearby ($z\lesssim 1$), compared to the typical redshift of Swift/UVOT-detected GRBs (see Figure 1).

The first markedly bright GRB was GRB 061007. At the time, it was the brightest observed by Swift/UVOT, and one of the brightest to be detected by Swift  in BAT and XRT (Schady et al., 2007; Mundell et al., 2007). This GRB was at a redshift of $1.26$ (Jakobsson et al., 2006) and had an $E_{iso}\sim 1\times 10^{54}\,{\rm erg}$ (Golenetskii et al., 2006). The afterglow had a $v$-band magnitude <11.1 at 80 s, after the prompt emission (Schady et al., 2007). This GRB was surpassed in brightness, a couple of years later, with the detection of GRB 080319B. This GRB was at a redshift of 0.937 (Vreeswijk et al., 2008) and had a V-band magnitude of 5.3 during the prompt emission. It was dubbed the ‘naked eye GRB’ as it would have been visible to the naked eye for a few tens of seconds for an observer in a dark location looking at the right time and place. GRB 080319B saturated the Swift/UVOT $white$ filter for the first $\sim$970 s after the BAT trigger. From $\sim 280$s, a measurement could, however, be achieved in the $v$ filter. For this GRB afterglow, a two-component afterglow model was necessary to explain the complex behaviour of the optical-X-ray emission (Racusin et al., 2008). The previous record holder for optical brightness was a pre-Swift GRB, GRB 990123, with a ninth magnitude V-band observation.

GRB 1304027A was a $v$=12th mag, luminous ($E_{iso}\sim 8\times 10^{53}\,{\rm erg}$; (Maselli et al., 2014)) and unusually close by ($z=0.34$; (Levan et al., 2013)) GRB. Such a nearby and luminous event was predicted to occur once every 60 years (Maselli et al., 2014). Long GRBs at $z<0.4$ tend to be sub-luminous ($E_{iso}<10^{52}\,{\rm erg}$, see also Section 4.2), and so GRB 130427A was unusual in that its behaviour is more consistent with a GRB occurring at $z\sim$ 1–2. It was thus given the moniker ’a nearby ordinary monster’ (Maselli et al., 2014). GRB 160625B was another bright GRB. It triggered Fermi (Dirirsa et al., 2016; Burns, 2016), but unfortunately not Swift/BAT, and so the early optical behaviour was missed by Swift/UVOT. Observations only began with Swift/UVOT 9ks after the Fermi trigger (Troja et al., 2017). GRB 190114C had similar redshift and isotropic energy as GRB 130427A (Ajello et al., 2020); it had a bright, 13th mag optical/UV afterglow, and was the first GRB to have TeV photons reported and detected in the range of 0.2–1 TeV (MAGIC Collaboration et al., 2019).

The latest addition to this collection of bright events observed by Swift/UVOT is the very recently detected GRB 221009A (Dichiara et al., 2022; Kennea et al., 2022). This GRB was initially thought to be an X-ray transient due to its unusual gamma-ray behaviour observed by Swift/BAT, and its apparent location within the Galactic plane. However, its status as a GRB was confirmed with the X-ray light curve decaying as expected for a GRB afterglow (Kennea et al., 2022), and subsequently, a redshift determination of 0.151 (de Ugarte Postigo et al., 2022; Castro-Tirado et al., 2022; Malesani et al., 2023). Most unusually, many facilities, including Fermi, AGILE, Konus-Wind, and INTEGRAL, reported detecting X-ray and gamma-ray emission from this GRB an hour prior to the Swift detection (Veres et al., 2022; Bissaldi et al., 2022; Ursi et al., 2022; Gotz et al., 2022; Xiao et al., 2022; Frederiks et al., 2022; Ripa et al., 2022; Liu et al., 2022). It is also the second GRB to be detected at TeV energies, and the first with photons of above 10 TeV (Huang et al., 2022), with one photon being reported at 251 TeV (Dzhappuev et al., 2022). The isotropic energy from this event is still to be confirmed, but initial estimates suggest it is $E_{iso}\sim$ 1–6$\,\times\,10^{54}$ (Gotz et al., 2022; Frederiks et al., 2022; Kann and Agui Fernandez, 2022; Kann et al., 2023), close to that of GRB 160625B (Kann and Agui Fernandez, 2022). Since Swift/BAT triggered on the afterglow emission (the first time in 18 years) rather than the prompt gamma-ray emission, Swift/UVOT observations only began an hour after the Fermi trigger (Williams et al., 2023). In addition, due to the GRB’s closeness to the Galactic plane, the optical/UV emission is affected by significant galactic extinction ($A_{v}\simeq 4$ mag increasing in the UV to an $A_{uvw2}\simeq 10$ mag). Given these restrictions, GRB 221009A was still detected in the initial Swift/UVOT $v$ image at 16.6 mag, and marginally detected in $uvw1$. Such a nearby and bright event is only expected once in 1000 years (Williams et al., 2023). The results of the detailed analysis of this GRB are starting to be published; see (Ai and Gao, 2022; Alves Batista, 2022; Carenza and Marsh, 2022; Cheung, 2022; Galanti et al., 2022; González et al., 2022; Li and Ma, 2022; Murase et al., 2022; Nakagawa et al., 2022; Ren et al., 2022; Romanov, 2022; Rudolph et al., 2022a, b; Sato et al., 2022; Smirnov and Trautner, 2022; Troitsky, 2022; Vardanyan et al., 2022; Xia et al., 2022; Zhang et al., 2022; Zheng et al., 2022; Zhu and Ma, 2022; Abbasi et al., 2023; Camisasca et al., 2023; Das and Razzaque, 2023; Finke and Razzaque, 2023; Fulton et al., 2023; Guarini et al., 2023; Kann et al., 2023; Laskar et al., 2023; Levan et al., 2023; Liu et al., 2023; Malesani et al., 2023; Negro et al., 2023; O’Connor et al., 2023; Ripa et al., 2023; Sahu et al., 2023; Shrestha et al., 2023; Vasilopoulos et al., 2023; Williams et al., 2023; Zhao et al., 2023; Zhang and Ma, 2023). The Swift/UVOT light curves of all these bright GRBs are given in Figure 4.

4.2 Rare and Unusual GRBs

As well as spectacular GRBs in terms of optical brightness, Swift/UVOT has observed GRBs with rare and unexpected behaviour. Prior to Swift, the nearest GRBs to us were GRB 980425 (Tinney et al., 1998; Foley et al., 2006) at $z=0.0085$, followed by GRB 031203A at 0.105 Prochaska et al. (2004); Margutti et al. (2007), and GRB 030329 at z = 0.168 (Greiner et al., 2003; Thöne et al., 2007). GRB 980425 and GRB 031203 were of low luminosity, atypical of the bulk of cosmological GRBs. Swift has increased the number of detected rare and nearby long GRBs, with GRB 060218 at z=0.033 (Mirabal and Halpern, 2006), GRB 100316D at z = 0.0591 (Starling et al., 2011), and GRB 111005A at z = 0.01326 (MichałowskI et al., 2018; Tanga et al., 2018). These GRBs were also of low luminosity, and along with GRB 980425 and GRB 031203, they are categorised as low-luminosity GRBs (LLGRBs). All of these events, except GRB 111005A (MichałowskI et al., 2018; Tanga et al., 2018), are associated with a type Ib/c supernova, supporting the connection between GRBs and the collapse of massive stars. Unfortunately, GRB 111005A was constrained by the Sun and thus was not observable with Swift/UVOT, and no variable optical or UV source was detected for GRB 100316D, which was hampered by the bright underlying host galaxy (Starling et al., 2011). However, Swift/UVOT did detect optical/UV emission from GRB 060218, with observations starting $\sim$100 s after the gamma-ray trigger. In this case, Swift/UVOT observations were important to constrain a thermal component, found initially in the X-ray spectrum, as it cooled and shifted with time into the optical/UV (Campana et al., 2006; Pian et al., 2006; Emery et al., 2019). This was suggested to be the ‘shock break out’ of the supernova, the moment where the shock wave breaks out of the star (Campana et al., 2006; Pian et al., 2006). This was the first time such an observation was observed with a GRB.

Over the years, a small number of GRBs have been found by Swift and other gamma-ray detectors to have ${\rm T_{90}}$ $>1000$ s (Gendre et al., 2013; Levan et al., 2014; Kann et al., 2018), including GRB 060218 and GRB 100316D. An even smaller number are ultra-long GRBs (ULGRBs), with ${\rm T_{90}}$ $>2000$ s (Gendre et al., 2013; Levan et al., 2014). Of these, Swift detected: GRB 101225A at $z=0.847$ (Thöne et al., 2011; Campana et al., 2011; Levan et al., 2014), GRB 111209A at $z=0.677$ (Vreeswijk et al., 2011; Gendre et al., 2013; Stratta et al., 2013; Kann et al., 2018), GRB 121027A at $z=1.773$ (Levan et al., 2014), and GRB 130925A at $z=0.347$ (Vreeswijk et al., 2013; Evans et al., 2014; Greiner et al., 2014) (other candidate ULGRBs are discussed in Kann et al. (2018)). While all four were observed by Swift/UVOT, only GRB 101225A and GRB 111209A have well-sampled Swift/UVOT light curves. GRB 130925A was not detected by Swift/UVOT, and GRB 121027A was only weakly detected in $white$, which may be due to its larger distance compared to GRB 101225A and GRB 111209A (Marshall and Evans, 2012; Levan et al., 2014). The main difficulty in determining the nature of ULGRBs is in explaining their long gamma-ray duration. It has been suggested that they are caused by the collapse of massive stars with radii that are larger than that considered for typical GRB progenitors (Gendre et al., 2013; Levan et al., 2014). An alternative suggestion is that these are the tidal disruption of white dwarfs as they pass close to a supermassive black hole (SMBH), for which the black hole is at the lower mass end of the SMBH distribution (e.g., <$10^{5}M_{\odot}$) (Levan et al., 2014). An analysis of the Swift/UVOT and GROND data of GRB 111209A by Kann et al. (2018) suggests that the optical/UV afterglow is consistent with the main population of long GRBs in terms of its luminosity distribution. They also state that while GRB 111209A has an isotropic energy and peak energy at the high end of the GRB distribution in terms of prompt emission parameters, these parameters are consistent with the Amati and Ghirlanda relations (Amati et al., 2002; Ghirlanda et al., 2004), suggesting that this and potentially other ULGRBs may not be a distinct class. Ref. Kann et al. (2018) also notes that GRB 101225A is more unusual in its optical/UV behaviour compared to the other ULGRBs, and may have a distinctive progenitor. GRB 101225A has been explained by Thöne et al. (2011) as an inspiral of a neutron star into a helium star, creating a central engine similar to a GRB.

4.3 Dark GRBs

Before moving on to discuss what we have learned about the optical/UV behaviour of GRBs, it is important to discuss the large fraction of long GRBs that were observed by Swift/UVOT but not detected, $\sim$40–60%, depending on the detection threshold (see Section 3.1). The optical/UV afterglows of some GRBs may not be detected due to observational constraints such as the rapidity of telescope pointing; and the GRB location relative to the Sun, Moon, and other bright sources. Restricting the sample to those observed by Swift/UVOT within the first 500 s after the gamma-ray detection and without observing constraints, there is still a significant fraction that are ‘dark’. Formally, GRBs are considered to be “dark” if their X-ray to optical spectral index is $\beta_{OX}<\beta_{X}-0.5$ (van der Horst et al., 2009), and so, some GRBs may be considered to be ‘dark’, even with the detection of the optical/IR emission. The percentage of ‘dark’ GRBs typically found for different instruments is 20–50% (Melandri et al., 2008; Cenko et al., 2009; Zheng et al., 2009; Greiner et al., 2011). With the rapid slewing capabilities of Swift, observations with Swift/UVOT have enabled us to rule out these dark GRBs as being due to factors such as a lack of sensitivity, late observation times, and rapid temporal decays (Melandri et al., 2012). The low detection rate or “darkness”, could however, be due to one or more of the following: a high background due to a small Sun-to-field angle (Fynbo et al., 2009), a large galactic extinction (e.g., (Fynbo et al., 2001, 2009)), a high circumburst extinction (e.g., (Roming et al., 2006; D’Elia and Stratta, 2011; Jeong et al., 2014)), intrinsic faintness (D’Elia and Stratta, 2011), and Ly$\alpha$ damping due to high redshift (e.g., (Roming et al., 2006; D’Elia and Stratta, 2011; Chrimes et al., 2019)). Several studies, including those using Swift/UVOT observations (e.g., (Krühler et al., 2011)), have suggested that high circumburst extinction and high-redshift are the two main causes for why GRBs are ‘dark’ (Perley et al., 2009; Zheng et al., 2009; Greiner et al., 2011; Melandri et al., 2012).

4.4 The Optical/UV Behaviour of Swift/UVOT Observed GRBs

Swift/UVOT has provided the GRB community with the largest sample of optical/UV observations of GRBs to date, with observations beginning typically 120 s after the detection of the gamma-ray emission. We now have a good idea of how GRBs behave in the optical/UV range from the first few minutes after detection, through to the first few days, beyond which they typically fade below the detection sensitivity of Swift/UVOT.

Since X-ray and optical/UV observations are taken simultaneously, it is useful to compare the temporal behaviour in both bands. Not long after the launch of Swift, the X-ray light curves were shown to have two unexpected features: an initial steep decay, thought to be due to the tail of the prompt emission, and a shallow decline phase whose nature is not yet resolved, but may be due to a long lived central engine (Zhang and Mészáros, 2001) or a short lived central engine that emitted shells over a wide range of velocities, with the slower shells catching up at a later time to continuously eject energy into the ejecta (Rees and Meszaros, 1998). These two power-law segments, in addition to the two power-law segments observed pre-Swift, the normal decay and the post-jet break decay, lead to the creation of a ‘canonical’ X-ray light curve, whereby most X-ray light curves can be fitted with one or more of these power-law decays, and X-ray flares may also be additionally superimposed (Nousek et al., 2006; Zhang et al., 2006).

In the optical/UV, it was shown that the optical/UV light curves behave more simply (Oates et al., 2009), with an initial study of optical/UV light curves observed by Swift/UVOT, showing that the light curves decay as a simple or broken power-law, and with a small number displaying an initial rise to a peak before they decay. Later, with a larger sample of GRBs, the optical/UV light curves could be grouped into different morphologies (Roming et al., 2017) in a similar fashion, as was achieved for the X-ray light curves (Evans et al., 2009). The morphologies are shown in the eight panels of Figure 5, (a) a ‘canonical’ light curve, (b) a break to a shallower decline, (c) a break to a steeper decline, (d) no break, (e) a gentle rise, transitioning to a steep decay, followed by a shallow decay, then another steep decay, and finally a more gentle decay; (f) starts with a rapid, steep decay, then a rise to peak (in some instances, with a break within the rise), a steep decay, and a final less-steep decay. Panels (a–d) are the same as those used for the X-ray light curves, while (e) and (f) are specific to the optical/UV emission. The final shallow decay in (f) is likely a result of poor background subtraction due to the background host signal dominating over the GRB signal. Using this scheme, approximately half of the optical/UV afterglows observed by Swift/UVOT are consistent with panel (d), a simple power-law, 21% are consistent in behaviour with the scheme in panel (e), with a few percent being consistent with each of the other panels. In comparison for the X-ray light curves, 42% are consistent with panel (a) canonical, 30% are consistent with one break, 15% with panel (b) and 15% with panel (c), 4% are consistent with panel (d) a simple power-law, and a further 24% are considered to be oddballs, displaying a range of behaviour that is not consistent with these specific types. This morphological categorisation provides some indication of the number and frequency of emission components producing the X-ray and optical light curves. For the optical/UV, at least $\sim$50% are consistent with panel d), which suggests that typically, a single emission component is sufficient to produce all of the optical/UV emission. For the X-ray light curves, the largest fraction of GRBs are consistent with panel (a), suggesting a more complex scenario with multiple emission components producing the X-ray emission, as described in the previous paragraph. However, ref. Roming et al. (2017) states that poorly sampled light curves tend to be consistent with a power-law, and so light curves with poor sampling may have more complex behaviour than that observed.

The range in optical/UV behaviour can also be displayed as a canonical optical light curve (Li et al., 2012) in a similar way as was done for the X-ray canonical light curve (Nousek et al., 2006; Zhang et al., 2006), with the optical/UV afterglow comprised of or more components: prompt optical flares, an early optical flare from the reverse shock, shallow-decay segment, the standard afterglow component (an onset hump followed by a normal decay segment), the post-jet-break phase, optical flares, rebrightening humps, and late supernova bumps (Li et al., 2012). I will now focus on the particular features observed in the optical/UV light curves.

4.4.1 Early Optical Emission

Some of the most informative and important observations Swift/UVOT has performed have been in relation to the earliest phases of the optical/UV GRB emission, within the first few hundred seconds. Optical/UV emission, when detected, is observed from the earliest moment that Swift/UVOT observations begin (Page et al., 2019). In the majority of cases, the optical/UV emission is already declining by the time Swift/UVOT observations begin, and only around 20% are observed to have an initial rise to a peak in their light curves (Oates et al., 2009; Roming et al., 2017; Page et al., 2019). A correlation has been observed by Liang et al. (2010); Panaitescu and Vestrand (2011); Liang et al. (2013) between the peak time and the peak brightness of optical afterglows, but for the majority of GRBs, we do not have a measure for either parameter. Since all of the optical/UV light curves must rise initially, this suggests the need for an even faster response to catch the start of the optical/UV emission.

There are two possible origins for the early optical/UV emission. It could either be produced by the external shocks that are generated as the ejecta are slowed by the external medium, or the optical emission could be related to prompt gamma-ray emission, which in turn is thought to be produced internally to the outflow. The observed emission may also be a combination of both possibilities, with different components dominating at different times for different GRBs. For the majority of GRBs, the prompt emission is not thought to be the dominant emission mechanism producing the early optical/UV emission of Swift/UVOT observed GRBs, but it is observed in some (Kopač et al., 2013; Page et al., 2019). Examples where the prompt emission contributes to the early optical/UV emission observed by Swift/UVOT are GRB 061121 (Page et al., 2007; Oganesyan et al., 2019), GRB 080928 (Rossi et al., 2011; Oganesyan et al., 2019), GRB 110205A (Oganesyan et al., 2019), and the ULGRB GRB 111209A (Stratta et al., 2013; Kann et al., 2018). In addition, prompt emission produced some of the very early optical emission of GRB 080319B (Racusin et al., 2008), before Swift/UVOT began observations. Simultaneous observations of the prompt emission across multiple wavelengths are rare but important for understanding the origin of the prompt emission and the transition to the afterglow phase (Oganesyan et al., 2019).

For the small fraction of GRBs with early observed rises in their optical/UV light curves, there are a number of mechanisms that may produce it. In some cases, an initial rise may be due to the prompt emission (Kopač et al., 2013). Observed rises may also be produced by the external shock. Both the forward and reverse shocks could produce a rise in the optical/UV afterglow, and may even be a combination of both (Zhang et al., 2003; Zhang and Kobayashi, 2005; Gomboc et al., 2009; Gao et al., 2015); see Figure 6. In the forward shock, a rise may be produced as a result of the jet slowing down as it ploughs into the external medium (the onset of the afterglow) or the passage of $\nu_{m}$ through the optical bandpass (Sari and Piran, 1999; Zhang et al., 2003). If the rise is due to the start of the forward shock, it should also be observed in the X-rays, but it is generally masked by the tail of the prompt emission (Oates et al., 2011; Willingale and Mészáros, 2017). In a sample of Swift/UVOT optical/UV light curves with observed initial rises, the passage of $\nu_{m}$ could be excluded as the cause of the initial peak, and there was no evidence for reverse shock emission (Oates et al., 2009). The rise in these optical/UV light curves and other early optical light curves samples suggested that the rise could be attributed to the start of the forward shock (Molinari et al., 2007; Oates et al., 2009; Rykoff et al., 2009). More recent work (Gao et al., 2015), with a larger sample of GRBs suggests that optical rises are produced by all possible combinations of forward and reverse shocks, as shown in Figure 6, with $\sim 50\%$ of rises being consistent with the start of the forward shock (bottom left of Figure 6). The least common type of rise was initially dominated by the reverse shock that quickly fades, and then a second peak occurs due to the passage of $\nu_{m}$ through the optical band of the forward shock (Gao et al., 2015). It is possible, however, that some of these optical rises occur while the prompt emission is still active, and so they could also be explained as internal origin (Kopač et al., 2013).

For optical rises produced by the onset of the afterglow, the peak time of the optical light curve depends on whether the shell that collides with the surrounding interstellar medium is thick or thin. For thick shells, the peak time, $t_{peak}$, of the optical emission will be comparable to ${\rm T_{90}}$ (Sari et al., 1999; Kobayashi, 2000). In the thin shell case, the optical peak is expected after ${\rm T_{90}}$ (Molinari et al., 2007; Rykoff et al., 2009). Providing that the peak occurs when t_{peak}>$${\rm T_{90}} (thin shell regime), then the Lorentz factor of the shell at the moment of the peak, $\Gamma_{peak}$, can be derived (Molinari et al., 2007; Sari, 1997; Sari and Piran, 1999); $\Gamma_{peak}$ is expected to be half of the initial value $\Gamma_{0}$ (Panaitescu and Kumar, 2000; Meszaros, 2006). A lower limit can be obtained for those GRBs where the peak occurs before the onset of observations ($>$100; (Oates et al., 2009)). For GRBs with observed rises, $\Gamma_{0}$ ranges from 100 to 1000 (Molinari et al., 2007; Oates et al., 2009; Rykoff et al., 2009; Liang et al., 2010), consistent with expectations that $\Gamma_{0}$ must be at least 100 in order to produce gamma-ray emission (Fenimore et al., 1993; Piran, 2004). Using $\Gamma_{peak}$, it is possible to deduce two more quantities: the isotropic-equivalent mass of the baryonic load, $M_{fb}$, and the deceleration radius, $R_{dec}$ (Molinari et al., 2007). The deceleration radius defines the radius, in a thin shell regime, at which the accumulated external medium is $1/\Gamma_{0}$ of the ejecta mass. The mean mass of the baryonic load and the mean deceleration radius for GRBs with an observed rise is $\langle{M_{fb}}\rangle\sim 4\times 10^{-3}\,{\rm M}_{\odot}$ and $\langle{R_{dec}}\rangle\sim 2\times 10^{17}$ cm. For GRBs without observed rises, the quantities are $\langle{M_{fb}}\rangle\lesssim 1\times 10^{-3}\,{\rm M}_{\odot}$ and $\langle{R_{dec}}\rangle\lesssim 1\times 10^{17}$ cm (Oates et al., 2009; Liang et al., 2010). These deceleration radii are consistent with the value expected from the forward shock model, $R_{dec}\sim 10^{16}$ cm (Rees and Meszaros, 1992). The internal shocks, that are believed to power the prompt emission, are expected to occur at \sim$$10^{15}\,{\rm cm} (Meszaros and Rees, 1997; Rees and Meszaros, 1994).

4.4.2 Optical Flares

Flares in X-ray light curves had been seen prior to Swift, but only a handful of times (e.g., (Piro et al., 1998, 2005; Galli and Piro, 2006)). With the launch of Swift, it was quickly shown that they are quite common (Burrows et al., 2005; Roming et al., 2006), appearing in approximately 50% of XRT afterglows (O’Brien et al., 2006), occurring generally after the end of ${\rm T_{90}}$, and were superimposed on the X-ray light curve. These X-ray flares are thought to be due to the same processes that produce the prompt gamma-ray emission (Falcone et al., 2007; Chincarini et al., 2007). Flares are also observed in the optical/UV light curve (see example in Figure 7). However, they are generally not as prominent as those in the X-rays, not as frequent (Li et al., 2012), and were likely to be overlooked or dismissed as noise (Swenson et al., 2013).

Using a blind, systematic search for flares, ref. Swenson et al. (2013) analysed 201 Swift/UVOT GRB light curves, and found episodes of flaring in 68 ($\sim$34%); a lower fraction than that found in X-ray light curve samples ($50\%$), but higher than previous optical studies (12% (Li et al., 2012)). On average, two flares were found in each of the 68 GRBs with flares. Ref. Swenson et al. (2013) found that most of the flares occur within the first 1000 s of the afterglow, but could be observed and detected beyond $10^{5}$ s. More than 80% of the flares detected are short in duration, with $\Delta t/t$ of $<$0.5. Ref. Yi et al. (2017) investigated further the Swift/UVOT optical flares found by Swenson et al. (2013). Ref. Yi et al. (2017) found a correlation between the rise and decay times of the optical flares, and a correlation between their duration and peak time. These correlations are consistent with the results of X-ray flares, suggesting they share the same physical origin, and both being possibly produced via internal emission as a result of central engine activity (Yi et al., 2017).

4.4.3 Optical Rebrightenings

Another feature found in the Swift/UVOT observations are late optical/UV rebrightening bumps (e.g., GRB 100815A; (Nardini et al., 2014; De Pasquale et al., 2015)). These have been detected in around 18% of optical light curves (Liang et al., 2013). These bumps occur after the initial onset and detection of the optical emission, and they are distinctive from bumps produced by an accompanying supernova. Supernovae occur later, peaking around one to two weeks after a GRB trigger, and they are generally red in colour. While some rebrightening bumps can be explained through a combination of reverse and forward shocks (e.g., the top left panel of Figure 6; (De Pasquale et al., 2015)), many require a structured jet Liang et al. (2013). In this case, the jet is not a simple uniform jet, but the energy and Lorentz factor of different parts of the jet varies and is dependent on the angle away from the jet axis. The simplest structured jet is a two-component outflow, which has an inner, narrow jet and an outer, wider jet encompassing the narrow jet. The narrow jet produces the prompt emission together with the initial optical emission, and the wider outflow dominates the optical emission at later times (Granot et al., 2002). This model interprets the rebrightening bump as the deceleration of the second slow jet. A two-component outflow model has a great deal of flexibility. Variations on this model have been used to explain the behaviours of GRBs with breaks in the X-ray light curves, which are geometric in nature (e.g., no spectral evolution is observed at the time of the X-ray break; spectral evolution would have indicated that the break was due to the passage of a synchrotron frequency through the observing band), but they have no corresponding break in the optical/UV light curves, which would otherwise be expected in the uniform jet model (e.g., (Oates et al., 2007; De Pasquale et al., 2009; Racusin et al., 2008)).

4.4.4 Jet Breaks

Jet breaks were thought to be observed in pre-Swift observations of GRBs (e.g., (Frail et al., 2001; Bloom et al., 2001; Zeh et al., 2006)), however, only a few were confirmed as being achromatic (Kulkarni et al., 1999; Harrison et al., 2001; Klose et al., 2004). While there have been some cases of confirmed achromatic jet breaks in post-Swift GRBs (e.g., GRB 050525A and GRB 140629A; (Blustin et al., 2006; Hu et al., 2019)), they are small in number (Burrows and Racusin, 2006; Liang et al., 2008; Evans et al., 2009; Racusin et al., 2009), and for some GRBs, no jet break has been detected for months (Grupe et al., 2007) or even years after the initial gamma-ray detection (De Pasquale et al., 2015). These extreme cases can pose problems for the external shock models, requiring extreme values of the physical parameters of the explosion, the emission mechanism, and the environment to explain them (De Pasquale et al., 2015). In general, however, the lack of detected jet breaks for many GRBs may likely be, at least in part, due to the lack of good coverage, particularly in the optical/UV. Of 900 GRBs studied by Wang et al. (2015), only 85 had well-sampled optical and X-ray afterglows. Within these 85, ref. Wang et al. (2018) found that around half had achromatic breaks, consistent with being a jet break (Wang et al., 2018), with a wide range of jet break time, from a few hundred seconds, up to $250\,{\rm ks}$. When a jet break can be determined, it can be used to constrain the opening angle of the jet, with values found to range from \sim$$1^{\circ} to \sim$$50^{\circ} (e.g., (Tanga et al., 2018; Zhao et al., 2020), see also references therein) and a typical value $\theta_{j}=(2.5\pm 1.0)^{\circ}$ (Wang et al., 2018). The jet opening angle can be used to compute the geometrically corrected energy of a GRB as the energy of a GRB is concentrated with a jet and not emitted isotropically. The authors of Wang et al. (2018) compute a beaming-corrected gamma-ray energy $\log\,E_{\gamma}=(49.54\pm 1.29)\,{\rm erg}$ and a geometrically corrected kinetic energy ($E_{K}$), the blastwave kinetic energy is computed from the optical afterglow, $\log\,E_{K}=(51.33\pm 0.58)\,{\rm erg}$. With $E_{\gamma}$ and $E_{K}$, the radiative efficiency of the jet can be computed (Zhang et al., 2007; Racusin et al., 2011). Most GRBs in Wang et al. (2018) have a small radiative efficiency of $<$10% at the time of the jet break. Determining accurate values for the $\theta_{j}$ and $E_{\gamma}$, as well as the time of the jet break, $t_{jet}$, are important for GRB correlations (e.g., (Frail et al., 2001; Ghirlanda et al., 2004; Liang and Zhang, 2005)). The opening angle of the GRB jets, $\theta_{j}$, is also important in constraining the total GRB event rate density, with ref. (Wang et al., 2018) finding twice that of pre-Swift estimates (Frail et al., 2001; Guetta et al., 2005).

4.4.5 Afterglow Luminosity and Correlations

Soon after the launch of Swift, several works found a bimodal optical luminosity distribution (Nardini et al., 2006, 2008; Liang and Zhang, 2006), implying two populations of optically bright GRBs. However, other studies (Melandri et al., 2008; Cenko et al., 2009; Kann et al., 2010, 2011; Zaninoni et al., 2013), including one study consisting only of Swift/UVOT light curves (Oates et al., 2009), only required a single population to describe the optical luminosity distribution. A single population seems to best represent the distribution even as the sample continues to increase in size (e.g., (Kann et al., 2020)).

Studies of single GRBs provide exceptional detail on the behaviour and physical properties of individual events. However, statistical investigations of large samples of GRBs aim to find common characteristics and correlations that link individual events and that therefore provide insight into the mechanisms common to GRBs. Statistical investigations have benefited greatly post-Swift launch from the observation of large numbers of GRBs with well-sampled X-ray and optical/UV observations. This has led to a number of correlations being discovered in the Swift era within the afterglow emission and linking the prompt gamma-ray emission (Liang and Zhang, 2005; Kann et al., 2010; Bernardini et al., 2012; Margutti et al., 2013; Zaninoni et al., 2016). This is most notable between the intrinsic afterglow brightness and the isotropic energy of the prompt emission (Kouveliotou et al., 2004; De Pasquale et al., 2006; Nysewander et al., 2009; D’Avanzo et al., 2012; Margutti et al., 2013; Oates et al., 2015), which suggest the most energetic GRBs in the prompt emission have also the brightest afterglows. Since the isotropic energy of the prompt emission is correlated with the intrinsic prompt emission peak energy (Amati et al., 2002; Amati, 2006), this also implies a three-parameter correlation between the isotropic energy of the prompt emission, the intrinsic prompt emission peak energy, and the X-ray afterglow brightness (Bernardini et al., 2012; Margutti et al., 2013; Zaninoni et al., 2016). The intrinsic brightness of the afterglow in the X-ray and optical light curves are also correlated, such that GRBs with bright X-ray afterglows have also bright optical afterglows (Oates et al., 2015), see also (Jakobsson et al., 2004; Gehrels et al., 2008; Berger, 2014). The Liang–Zhang correlation is another three-parameter correlation between the isotropic energy of the prompt emission, the intrinsic prompt emission peak energy, and the jet break time (Liang and Zhang, 2005). For recent reviews describing all GRB correlations in the prompt and afterglow emission, see Wang et al. (2015); Dainotti and Del Vecchio (2017); Dainotti et al. (2018) and references therein. For a recent, comprehensive, and systematic study of all GRB parameters and correlations, see (Wang et al., 2020). Three correlations have been discovered using only optical/UV afterglow observations, which I will expand upon below.

Refs. Panaitescu and Vestrand (2008); Liang et al. (2010); Panaitescu and Vestrand (2011); Panaitescu et al. (2013) find a significant correlation between the peak time and peak afterglow brightness in both the X-ray and optical light curves of those GRBs with observed rises. Ref. Liang et al. (2010) used the peak time to determine the initial Lorentz factor of the outflow, finding it to be correlated with the isotropic gamma-ray energy.

Within samples of optical light curves Li et al. (2012); Dainotti et al. (2020, 2022) a correlation is found between the restframe time at the end of the plateau phase, with the luminosity at the same time (see also (Panaitescu and Vestrand, 2011)). This correlation is also present in the X-ray light curves (Dainotti et al., 2008; Ghisellini et al., 2009; Dainotti et al., 2010, 2013), and in GeV light curves (Dainotti et al., 2021). A theoretical interpretation of this correlation is that it may be explained within the context of the standard fireball model through the evolution of the microphysical parameters (van Eerten, 2014a, b). Alternatively, it may be explained by the spindown of a newly born magnetar (Dai and Lu, 1998; Zhang and Mészáros, 2001; Metzger et al., 2011; Bernardini et al., 2012; Wang et al., 2022), or by structured jets viewed over a range of viewing angles (Beniamini et al., 2020). An extension of this optical luminosity-plateau end time correlation (also known as the Dainotti relation) has been obtained by adding the peak prompt luminosity, leading to a three-parameter ’Fundamental Plane’ correlation (Dainotti et al., 2022), which was also shown to be present in the X-ray light curve sample (Dainotti et al., 2016, 2017).

Using the Swift/UVOT optical light curves, Oates et al. (2012) discovered a correlation between the early luminosity (at restframe 200 s) and the average rate of decay (measured with a single power-law from 200 s until the end of observations), see Figure 8. This correlation was also shown to be present at X-ray wavelengths (Oates et al., 2015; Racusin et al., 2016), and most recently, in a small sample of GeV light curves (Hinds et al., 2023). The early X-ray and optical/UV luminosity also correlates with the isotropic energy of the prompt emission (D’Avanzo et al., 2012; Margutti et al., 2013; Oates et al., 2015), implying that the most energetic GRBs have the brightest and fastest decaying afterglows (Oates et al., 2015). This correlation may be explained via a parameter or mechanism that controls the energy release in GRBs, such that the most energetic GRBs lose their energy quicker, or may be a viewing angle effect, such that GRBs viewed off-axis have fainter and slower decaying light curves (Granot et al., 2002; Panaitescu and Vestrand, 2008; Oates et al., 2012). As this correlation is present in samples of X-ray light curves with and without plateau phases (Racusin et al., 2016), it will be important to determine how the luminosity-plateau end time relation relates to the luminosity-decay rate correlation.

Since the luminosity-decay rate and luminosity-plateau end-time correlations relate intrinsic parameters to observed ones, it is hoped that these and other GRB correlations may be used to standardise GRBs, to determine redshifts for GRBs without a spectroscopic or photometrically measured redshift, and for use as standard candles, as was achieved using the Philips relation (Phillips, 1993) for SNe Ia (e.g., (Riess et al., 2022)). This has so far been implemented for several correlations involving prompt emission parameters (e.g., (Schaefer, 2003; Amati et al., 2008; Capozziello and Izzo, 2008; Liang et al., 2008; Wang and Dai, 2011; Izzo et al., 2015; Wang et al., 2015; Muccino et al., 2021)), and the X-ray versions of the luminosity-plateau end-time correlation and the three-parameter ’Fundamental Plane’ correlation Cardone et al. (2009, 2010); Dainotti et al. (2013); Postnikov et al. (2014); Dainotti et al. (2022); Wang et al. (2022); Dainotti et al. (2023).

5 Conclusions/Looking to the Future

Swift has been a highly successful GRB mission for the last 18 years and continues to be the workhorse of the GRB and transient community. The flexibility and responsiveness of Swift, in part due to its excellent and dedicated team, ensures that Swift continues to provide key observations to the astronomical community, and continues to be a successful top-ranked NASA mission.

Swift/UVOT observations have revolutionised our understanding of the optical/UV emission of GRBs, providing information on the behaviour of the optical/UV emission in the first minutes to hours after the gamma-ray emission, which was unknown prior to Swift. Over the past 18 years, Swift/UVOT has provided large numbers of optical/UV observations of GRBs with simultaneous X-ray observations. Swift/UVOT and Swift/XRT have highlighted the importance of multi-wavelength observations and analyses, which have been key in unravelling the temporal and spectral behaviour of GRBs. The optical/UV light curves can be more complex than expected pre-Swift, with optical flares and optical rebrightenings sometimes being superimposed on the expected standard afterglow emission. This behaviour does not always trace the X-ray light curves, and in some cases, it suggests multiple emission components or that a complex jet structure is needed to explain the observations. In particular, 20% of the early optical light curves are observed initially to rise, which is not observed in the X-ray.

The number of well-sampled optical/UV light curves observed by Swift/UVOT has enabled statistical analyses to be performed, resulting in the discovery of correlations between parameters measured from the optical/UV afterglows, and also from the gamma-ray emission, thus connecting to the processes resulting in the prompt and afterglow emission. The optical/UV capabilities of Swift/UVOT also enable the dust content along the line of sight to be studied, and photometric redshifts to be obtained for GRBs where a spectroscopic redshift has not been possible.

Swift/UVOT will continue to build the samples of the optical/UV light curves required to explain the collective behaviours of GRBs, and to continue to observe and to detect unique GRBs and transients that push the boundaries of our understanding. There are still many questions to be addressed regarding long GRBs, such as when is the onset of the afterglow for $\sim$80% of GRBs, what is the structure of the jet, and whether there is any variation in observer viewing angle; Swift will continue to provide exquisite data to address these issues.

\funding

This research received no external funding.

\dataavailability

The data presented in Section 3.1 of this review are openly available at https://swift.gsfc.nasa.gov/archive/grb_table/, accessed 20th October 2022. For all other sections, no new data were created nor analysed, and data sharing is not applicable.

Acknowledgements.

The author would like to acknowledge and thank all the engineers and scientists that built and calibrated Swift/UVOT, analysed Swift/UVOT GRB data, and supported and planned Swift/UVOT observations. This review paper would not be possible without their time and dedication. This research has made use of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC) and the Leicester Database and Archive Service (LEDAS), provided by NASA’s Goddard Space Flight Center and the School of Physics and Astronomy, University of Leicester, UK, respectively. \conflictsofinterestThe authors declare no conflicts of interest. \abbreviationsAbbreviations The following abbreviations are used in this manuscript:

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