iPTF16fnl: a faint and fast tidal disruption event in an E+A galaxy

TL;DR

iPTF16fnl is a notably faint and fast tidal disruption event in an E+A galaxy, expanding the diversity of known TDEs and indicating a potentially higher occurrence rate of such low-luminosity events.

Contribution

This paper reports the discovery and detailed multi-wavelength analysis of a faint, fast TDE, highlighting its unique properties and implications for TDE rates.

Findings

Faint optical luminosity peak at M_g=-17.2 mag

Exponential decay with a timescale of ~15 days

No radio detection, marginal X-ray detection

Abstract

We present ground-based and \textit{Swift} observations of iPTF16fnl, a likely tidal disruption event (TDE) discovered by the intermediate Palomar Transient Factory (iPTF) survey at 66.6 Mpc. The lightcurve of the object peaked at absolute $M_g=-17.2$ mag. The maximum bolometric luminosity (from optical and UV) was $L_p~\simeq~(1.0\,\pm\,0.15) \times 10^{43}$ erg/s, an order of magnitude fainter than any other optical TDE discovered so far. The luminosity in the first 60 days is consistent with an exponential decay, with $L \propto e^{-(t-t_0)/\tau}$, where $t_0$=~57631.0 (MJD) and $\tau\simeq 15$ days. The X-ray shows a marginal detection at $L_X=2.4^{1.9}_{-1.1}\times 10^{39}$ erg/s (\textit{Swift} X-ray Telescope). No radio counterpart was detected down to 3$\sigma$, providing upper limits for monochromatic radio luminosity of $\nu L_{\nu} < 2.3\times10^{36}$ erg/s and $\nu L_{\nu}<1.7\times 10^{37}$ erg/s (VLA, 6.1 and 22 GHz). The blackbody temperature, obtained from combined \textit{Swift} UV and optical photometry, shows a constant value of 19,000 K. The transient spectrum at peak is characterized by broad He II and H$\alpha$ emission lines, with an FWHM of about 14,000 km/s and 10,000 km/s respectively. He I lines are also detected at $\lambda\lambda$ 5875 and 6678. The spectrum of the host is dominated by strong Balmer absorption lines, which are consistent with a post-starburst (E+A) galaxy with an age of $\sim$650 Myr and solar metallicity. The characteristics of iPTF16fnl make it an outlier on both luminosity and decay timescales, as compared to other optically selected TDEs. The discovery of such a faint optical event suggests a higher rate of tidal disruptions, as low luminosity events may have gone unnoticed in previous searches.

Tables

Table 1. Table 1: Archival photometry of Mrk 950.

Survey	Band	Magnitude	Reference
		(mag)
GALEX	FUVAB	21.22$\pm$0.39 a	[1]
GALEX	NUVAB	20.191$\pm$ 0.13 a	[1]
SDSSDR12	$u$	17.005$\pm$0.012 b	[2]
SDSSDR12	$g$	15.491$\pm$0.003 b	[2]
SDSSDR12	$r$	14.913$\pm$0.003 b	[2]
SDSSDR12	$i$	14.585$\pm$0.003 b	[2]
2MASS	$J$	13.212 $\pm$0.040	[3]
2MASS	$H$	12.545$\pm$0.052	[3]
2MASS	$K$	12.360$\pm$0.069	[3]
WISE	W1	13.075$\pm$0.029	[4]
WISE	W2	13.086$\pm$0.034	[4]
WISE	W3	12.331$\pm$0.285	[4]
WISE	W4	$>$9.136	[4]
NVSS	1.4 GHz	1.7 mJy	[5]
ROSAT	0.1-2.4 keV	1.04$\times {10}^{-12}$ erg s-1	[6]

Table 2. Table 2: 3 σ 𝜎 \sigma upper limits on radio emission for iPTF16fnl.

Date	Telescope	Frequency	Flux
(UT)		(GHz)	(Jy)
2016 Aug 31	VLA	6.1, 22
2016 Sep 01	AMI	15
2016 Sep 05	AMI	15
2016 Sep 09	JCMT/SCUBA2	352
2016 Sep 10	JCMT/SCUBA2	352
2016 Sep 17	AMI	15
2016 Oct 22	AMI	15

Table 3. Table 3: Flux values for H α 𝛼 \alpha and He II 4868Åfor the lines shown in Figure 8 . The values were derived from the best model fit line profile. From the fit uncertainties, we estimate errors of 40% and 30% of the total flux for H α 𝛼 \alpha and He II lines respectively.

MJD	Phase	H$\alpha$	He II
(d)	(d)	(erg  s-1)	(erg  s-1)
57630.4	$-$1.7	2.1$\times {10}^{40}$	8.9$\times {10}^{40}$
57631.0	$-$0.8	3.8$\times {10}^{39}$	1.5$\times {10}^{40}$
57631.3	0.0	6.6$\times {10}^{39}$	8.3$\times {10}^{39}$
57661.4	29.3	3.0$\times {10}^{39}$	3.0$\times {10}^{39}$
57661.4	29.3	2.1$\times {10}^{39}$	2.5$\times {10}^{39}$
57687.3	55.2	–	1.1$\times {10}^{40}$
57694.4	62.3	–	2.7$\times {10}^{39}$
57720.3	88.2	–	1.1$\times {10}^{39}$

Table 4. Table 4: Log of spectroscopic observations of iPTF16fnl.

MJD	Slita	Telescope+Instrument	Grism$/$Grating	Dispersion	Resolutionb	Exposure
(d)	(arcsec)			(Å/pix)	Å km/s)	(s)
57630.2	3.0	P60+SEDM	none	25.5	540 (2900 km s-1)	900
57630.4	2.0	LCO 2-m+FLOYDS	–	1.7	15.3 (820 km s-1)	2700
57631.0	1.0	GTC+Osiris	1000B+2500I	2.12+1.36	7.1 (380 km s-1)	300
57631.0	1.0	GTC+Osiris	2500R	1.04	7.1(380 km s-1)	300
57631.2	2.2	P60+SEDM	none	25.5	504 (2700 km s-1)	900
57631.3	1.5	P200+DBSP	600/4000	1.5	10.5 (560 km s-1)	600
57633.5	3.6	P60+SEDM	lenslet arr.	25.5	672 (3600 km s-1)	1800
57634.2	3.0	P60+SEDM	lenslet arr.	25.5	613 (3300 km s-1)	1200
57636.2	3.0	P60+SEDM	lenslet arr.	25.5	659 (3500 km s-1)	1200
57637.2	3.6	P60+SEDM	lenslet arr.	25.5	589 (3200 km s-1)	1200
57638.2	3.0	P60+SEDM	lenslet arr.	25.5	556 (3000 km s-1)	1200
57643.22	1.0	VLT Kueyen+UVES	437+860	0.030+0.050	0.15 (8 km s-1)	2x1500
57643.25	1.0	VLT Kueyen+UVES	346+580	0.024+0.034	0.15 (8 km s-1)	2x1500
57643.4	1.5	DCT+Deveny	300	2.17	6.9 (370 km s-1)	600
57661.4	1.0	Keck I+LRIS	400/3400+400/8500	2.0	6.5 (350 km s-1)	600
57661.4	1.0	Keck I+LRIS	400/3400+400/8500	2.0	6.5 (350 km s-1)	600
57676.3	1.0	GeminiN+GMOS	–	1.35	8.1 (435 km s-1)	600
57683.4	1.0	GeminiN+GMOS	–	1.35	8.1 (435 km s-1)	1200
57687.3	1.5	P200+DBSP	600/4000	1.5	10.5 (560 km s-1)	600
57694.4	1.0	Keck I+LRIS	400/3400+400/8500	2.0	7.2 (390 km s-1)	1350
57720.3	1.0	Keck I+LRIS	400/3400+400/8500	2.0	6.5 (350 km s-1)	1800
57751.3	1.0	Keck I+LRIS	400/3400+400/8500	2.0	6.5 (350 km s-1)	1800

Table 5. Table 5: Optical and UV photometry of iPTF16fnl in AB magnitude system. These are the originally measured magnitudes with Swift and difference imaging piplines. For P48+CFH12k, the r 𝑟 r -band column contains measurements in Mould- R 𝑅 R filter system. These measurements are not corrected for Galactic extinction. Table 5 is published in its entirety in the machine-readable format. A portion is shown here for guidance regarding its form and content.

MJD	Telescope	$UVW1$	$UVM2$	$UVW2$	$U$	$B$	$V$	$u$	$g$	$r$	$i$
(days)	+ Instrument	(mag)	(mag)	(mag)	(mag)	(mag)	(mag)	(mag)	(mag)	(mag)	(mag)
57626.4	P48+CFH12k	–	–	–	–	–	–	–	17.50$\pm$0.07	–	–
57629.4	P48+CFH12k	–	–	–	–	–	–	–	17.23$\pm$0.09	17.54$\pm$0.08	–
57630.8	Swift+UVOT	16.83$\pm$0.04	16.92$\pm$0.04	16.61$\pm$0.04	16.35$\pm$0.04	15.68$\pm$0.03	15.19$\pm$0.04	–	–	–	–
57631.7	P60+SEDM	–	–	–	–	–	–	17.13$\pm$0.09	17.17$\pm$0.07	17.42$\pm$0.12	–
…	…	…	…	…	…	…	…	…	…	…	…

Full text

accretion, accretion discs – black hole physics – stars: individual (iPTF16fnl), galaxies: nuclei

††software: AstroPy, EMCEE (Foreman-Mackey et al., 2013), IRAF, LMFIT, pPXF (Cappellari, 2016), PTFIDE (Masci et al., 2017), PYRAF, UVOTSOURCE

1 Introduction

A tidal disruption event (TDE) is the phenomenon observed when a star is torn apart by the tidal forces of a supermassive black hole (SMBH), usually lurking in the core of its galaxy. As a consequence, a bright flare is expected when some of the bound material accretes onto the SMBH. Although such events were theoretically predicted a few decades ago (Hills, 1975; Lacy et al., 1982; Rees, 1988; Evans & Kochanek, 1989; Phinney, 1989), observational signatures are more recent. The first detection of TDEs were made in the soft X-ray data. The flares, consistent with the proposed stellar disruption scenario, were identified in ROentgen SATellite (ROSAT) all-sky survey (Bade et al., 1996; Komossa & Bade, 1999; Saxton et al., 2012). Detection in gamma-ray data of Swift events Swift J1644+75 (Bloom et al., 2011; Levan et al., 2011; Burrows et al., 2011; Zauderer et al., 2011; Cenko et al., 2012) and Swift J1112.2-8238 (Brown et al., 2015) were attributed to relativistic outbursts caused by jetted emission. We refer the reader to Komossa (2015); Auchettl et al. (2016) for a broader review of the status of observations in different wavelengths.

Ultraviolet detections of nuclear flares were reported from the GALEX survey (Gezari et al., 2006, 2008, 2009). TDEs are now being discovered by optical surveys such as the Sloan Digital Sky Survey (SDSS; van Velzen et al., 2011), PanSTARRS-1 (PS1; Gezari et al., 2012; Chornock et al., 2014), Palomar Transient Factory (PTF; Arcavi et al., 2014), All Sky Automated Survey for Supernovae (ASAS-SN; Holoien et al., 2014, 2016b, 2016a), Robotic Optical Transient Search Experiment (ROTSE Vinkó et al., 2015), Optical Gravitational Lensing Experiment (OGLE; Wyrzykowski et al., 2017) and the intermediate Palomar Transient Factory (iPTF; Hung et al. (2017), Duggan G. et. al. in prep and the work presented here). The optical sample has revealed that an important fraction of the TDEs appear to be found in E+A (“quiescent Balmer-strong”) galaxies (Arcavi et al., 2014; French et al., 2016), which can be interpreted as middle-aged ($<1$ Gyr) post-starburst galaxies (Zabludoff et al., 1996; Dressler et al., 1999).

Here we present iPTF16fnl, an optical TDE candidate discovered by iPTF. The event is localized in the center of an E+A galaxy. With a distance of 66.6 Mpc this is the closest well-studied event in optical/UV wavelengths.

2 Discovery and Host Galaxy

2.1 Discovery and classification

iPTF16fnl was discovered on UT 2016 August 29.4 in an image obtained during the $g$+R experiment (Miller et al., 2017): during the night, one image each was obtained in $g$ and R band; the images are separated by at least an hour in order to filter out asteroids. The event was identified by two real-time difference imaging pipelines (Cao et al., 2016; Masci et al., 2017), shown in Figure 1. The discovery magnitudes were $g=17.11\pm 0.09$ and $R=17.39\pm 0.09$ (see Ofek et al. (2012) for photometric calibration of PTF). Given an RMS of 0.5*′′*, the coordinates of the source, $\alpha_{J2000}=00^{\rm{h}}29^{\rm{m}}57^{\rm{s}}.04\ \delta_{J2000}=+32^{\rm{h}}53^{\rm{m}}37^{\rm{s}}.5$ (ICRS) are consistent with central position of the galaxy, as provided by the Sloan Digital Sky Survey (SDSS) catalogue (Alam et al., 2015).

The brightness, blue colour ($g-{\rm R}=-0.28$ mag) and central location of the transient in its host galaxy, made it a prime candidate for prompt follow-up observation. On the night after discovery, we observed the source with the FLOYDS spectrograph (Sand et al., 2011) on the Las Cumbres Observatory (LCO; Brown et al., 2013) 2-m telescope and the Spectral Energy Distribution Machine (SEDM) on the Palomar 60-inch (P60) telescope. FLOYDS are a pair of robotic low-resolution (R $\sim$ 450) slit spectrographs optimized for supernova classification and follow-up. The SEDM is a ultra-low resolution (R$\sim 100$) integral-field-unit (IFU) spectrograph, dedicated to fast turnaround classification (Blagorodnova N. et. al. in prep). The IFU’s wide field-of-view of 28*′′*, allows for robotic spectroscopy. Their data reduction pipeline allow rapid reduction with minimal intervention from the user (aperture placing). Figure 2 shows the classification spectra, displaying a blue continuum and broad He II and H$\alpha$ emission lines, characteristic of previously observed optical TDE spectra (Arcavi et al., 2014). The fast spectral identification of iPTF16fnl as a TDE candidate allowed us to rapidly inform the astronomical community (ATel #9433; Gezari et al., 2016), which in turn enabled numerous multi-wavelength follow-up campaigns.

2.2 Host galaxy

The host galaxy of iPTF16fnl is Markarian 950 (Mrk 950) located at $z$=0.016328 (Cabanela & Aldering, 1998; Petrosian et al., 2007). Given the edge-on host inclination and the existence of a peculiar bar and nucleus, the galaxy is classified as Sp (spindle). The luminosity distance is DL=66.6 Mpc (distance modulus $\mu=$ 34.12 mag) using H0 = 69.6 km s*-1*  Mpc*-1*, $\Omega_{M}$ = 0.29, $\Omega_{\Lambda}$ = 0.71 in the reference frame of the 3 K cosmic microwave background (CMB; Fixsen et al., 1996).

The estimated Galactic colour excess at the position of the transient is $E({\rm B}-{\rm V})~{}=~{}0.062~{}\pm~{}0.001$ mag (from NED111The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.), after adopting the extinction law of Fitzpatrick (1999) with corrections from Schlafly & Finkbeiner (2011). Assuming R${}_{\rm V}~{}=~{}3.1$, the Galactic visual extinction is AV = 0.192 mag.

The archival host magnitudes are shown in Table 1. The derived $K$-band luminosity of the galaxy is $L_{K}=1.15\times 10^{10}$L*⊙. Using the galaxy color $u-g=1.5$, we compute a mass to light ratio for the $K$-band, log${}_{10}(M/L)=-0.0755$ (Bell et al., 2003), corresponding to a stellar mass $M_{*}\simeq 9.7\times 10^{9}$M⊙. Provided that size of the PSF in SDSS DR13 is an upper limit for the angular size of an unresolved bulge, we assume that an upper limit for the bulge-to-total ratio (B/T) can be estimated with an average (across all bands) value of psfFlux/cModelFlux$\sim 0.19\pm 0.05$. We use this ratio to scale the total galaxy mass and derive $M_{BH}\leq 10^{6.6\pm(0.1+0.34)}$ M⊙*, according to the $M_{BH}-M_{\rm{bulge}}$ relation (McConnell & Ma, 2013), including the 1$\sigma$ scatter of 0.34 dex.

The age and metallicity of the host galaxy were determined by fitting a grid of galaxy models from Bruzual & Charlot (2003), using stellar evolutionary models from (Chabrier, 2003). The fit was done using the pPXF code (Cappellari, 2016). The best model was in agreement with a single burst of star formation with an age of 650$\pm$300 Myr and a metallicity of $Z$=0.18 (here, $Z=0.2$ corresponds to that of the Sun).

We use the high resolution spectrum taken with VLT/UVES taken two weeks after discovery (see log in Table 4, PI: P. Vreeswijk) to measure the host velocity dispersion. We obtain $v_{\rm{host}}=89\pm$1 km s*-1*, from the Ca II $\lambda\lambda$8544,8664 absorption lines. According to the M$-\sigma$ relation (McConnell & Ma, 2013), this corresponds to a $M_{BH}=10^{6.33\pm 0.38}$ M*⊙*, consistent with our previous estimate.

The field of iPTF16fnl was extensively observed for the last six years by PTF/iPTF. No prior activity in the host is detected with upper limits of $\sim$20-21 mag. The most recent non-detection is from MJD 57432.6, around 194 days before discovery. The host galaxy was also monitored by The Catalina Real-Time Transient Survey (CRTS; Drake et al., 2009) during 2006-2016 period. The magnitude of the galaxy was stable within the errors, with an unfiltered average magnitude of 14.88$\,\pm\,$0.05 mag.

3 Follow-up observations

3.1 Photometric observations

Following spectroscopic identification of iPTF16fnl as a TDE candidate, the source was monitored at Palomar and the Ultraviolet and Optical Telescope (UVOT; Roming et al., 2005) on board the Swift observatory (Gehrels et al., 2004). The UVOT observations were taken in UVW2, UVM2, UVW1, U, B and V; see Table 5. The data were reduced using the software UVOTSOURCE using the calibrations described in Poole et al. (2008) and updated calibrations from Breeveld et al. (2010). We use a 7.5*′′* aperture centered on the position of the transient.

At Palomar, photometry in the $g$ and Mould-$R$ bands were obtained with the iPTF mosaic wide-field camera on the Palomar 48-inch telescope (P48; Rahmer et al., 2008). Difference-image photometric measurements were provided by the IPAC Image Subtraction and Discovery Pipeline developed for the iPTF survey (PTFIDE; Masci et al., 2017). Difference-imaging photometry in the u’g’r’i’ bands, obtained with the SEDM, were computed using the FPipe software (Fremling et al., 2016). The zeropoints were calibrated using stars in the SDSS footprint. Table 5 reports the measured Swift aperture photometry magnitudes and the difference imaging photometry for the Palomar data.

The multi-band Swift and optical lightcurve, corrected for Galactic extinction, is shown in Figure 3. To correct the UV bands for host light contamination, we used the average of the last four epochs of Swift data, from MJD 57712.7 to 57724.7 ($>$ 80 days), to subtract from the early part of the lightcurve. The measurements were constant with an RMS of $\sim$0.05 mag, comparable to the measurement error. Optical Swift data were excluded from the analysis, as they were dominated by the host, not by the central point source.

In order to estimate the extinction in the host galaxy, we use all available Swift UV data and difference imaging photometry in $ugri$ bands to fit blackbody emission curves, as detailed in Section 4. For each epoch, the photometry is corrected for both Galactic (fixed) and additional host extinction E($B-V$) from 0 to 0.25 mag in 0.05 steps. The likelihood between the de-reddened photometry and a blackbody model is computed for each epoch. In a final step, we marginalize over all epochs to derive the final value. We find that the best fit corresponds to E($B-V$)=0, with an upper limit of E($B-V$)=0.05. The selection of different extinction laws for host extinction (Calzetti et al., 2000; Fitzpatrick, 1999) does not change our conclusions. Given the assumption that the emission from iPTF16fnl in fact follows a distribution, from now on we will assume the reddening in the host to be negligible.

3.2 Radio observations

Radio follow-up observations of iPTF16fnl were taken with the Jansky Very Large Array (VLA; PI A. Horesh), the Arcminute Microkelvin Imager (AMI; PI K. Mooley) and the James Clerk Maxwell Telescope and the Submillimetre Common-User Bolometer Array 2 (JCMT/SCUBA-2; PI A. K. H. Kong). The upper limits corresponding to the observations are shown in Table 2. The limits for the monochromatic radio luminosities corresponding to the first VLA epoch are $\nu L_{\nu}<2.3\times 10^{36}$  erg s*-1* and $\nu L_{\nu}<1.7\times 10^{37}$  erg s*-1* at 6.1 and 22 GHz. These limits are respectively two and one order of magnitude deeper than the VLA detection of ASASSN-14li at peak for a similar frequency range (van Velzen et al., 2016; Alexander et al., 2016).

We can use the limits reported here to argue against the presence of an on-axis relativistic outflow, or at least constrain the energy of the jet, $E_{j}$. We compare our limits in 6 and 15 GHz with the analytical lightcurves for on-axis TDE radio emission from Generozov et al. (2017). Figure 4 shows that our early time observations suggest jet energies with $E_{j}<10^{49}$ erg s*-1*.

Additionally, we contrast our measurements, scaled to the redshift of PS1-11af, with the GRB afterglow models of van Eerten et al. (2012) presented in Chornock et al. (2014) (their figure 13). Based on our non-detections, we can rule out the existence of a relativistic jet, as viewed 30*∘* off-axis. For larger angles, the radio emission is expected to arise at later times ($>$ 1 year after disruption). Therefore, continuous monitoring of the event at radio wavelengths is encouraged.

3.3 X-ray observations

We observed the location of iPTF16fnl with the X-Ray Telescope (XRT; Burrows et al. 2005) on-board the Swift satellite beginning at 19:32 UT on 30 August 2016. Regular monitoring of the field in photon counting (PC) mode continued over the course of the next four months (PIs T. Holoien and B. Cenko).

No significant emission is detected in individual epochs (typical exposure times of $\approx$2 ks). Using standard XRT analysis procedures (e.g., Evans et al. 2009), we place 90% confidence upper limits ranging from $(3.8$–$12.1)\times 10^{-3}$ counts s*-1* in the 0.3–10.0 keV bandpass over this time period.

Stacking all the XRT data obtained over this period together (58 ks of total exposure time), we find evidence for a weak ($\approx 4\sigma$ significance) X-ray source at this location with a 0.3–10.0 keV count rate of $(2.6\pm 1.2)\times 10^{-4}$ counts s*-1*.

With only 15 source counts we have limited ability to discriminate between spectral models; however, with several photon energies detected above 1 keV. We derive response matrices for the stacked XRT observations using standard Swift tools. Adopting a power-law model for the spectrum with a photon index of 2 and accounting for line-of-sight absorption in the Milky Way (Willingale et al., 2013), we find the measured count rate corresponds to an unabsorbed flux of 0.3–10.0 keV flux of $4.6_{-2.0}^{+3.7}\times 10^{-15}$ erg cm*-2* s*-1*.

At the distance of Mrk 950, this corresponds to an X-ray luminosity of $L_{X}=2.4_{-1.1}^{+1.9}\times 10^{39}$ erg s*-1*. Without additional information (e.g., variability and/or spectra), we cannot determine conclusively if this X-ray emission is associated with the transient iPTF16fnl, or if this is unrelated X-ray emission from the host nucleus (e.g., an underlying active galactic nucleus) or even a population of X-ray binaries or ultra-luminous X-ray sources. However, the lack of evidence for ongoing star formation or AGN-like emission lines in the late-time optical spectra of Mrk 950 (§5) suggest an association with the tidal disruption event.

If this is indeed the case, the implied X-ray emission would be extremely faint, both in an absolute and a relative sense. We can contrast, for example, with the X-ray emission observed from ASASSN-14li (Holoien et al., 2016b; van Velzen et al., 2016), with a peak luminosity approximately four orders of magnitude above that seen for iPTF16fnl. Even sources with much fainter X-ray emission, such as ASASSN-15oi (Holoien et al., 2016a), still outshine iPTF16fnl by more than a factor of 100.

3.4 Spectroscopic Observations

Spectroscopic follow-up observations of iPTF16fnl have been carried out with numerous telescopes and instruments, summarized in the spectroscopic log in Table 4. Figure 5 shows the spectral sequence for iPTF16fnl, spanning three months. The spectroscopic data are made public via WISeREP (Yaron & Gal-Yam, 2012).

Given the brightness of the galactic bulge relative to the TDE, the interpretation of the TDE spectrum poses some challenges. A noticeable feature is the strong host component in all the available spectra. Different slit widths, the variable seeing, different orientations of the slit during the acquisition of the data (generally taken at the parallactic angle) and the highly elongated geometry of the host galaxy, contribute to create a strong variation in the contribution from the host component, which appears to vary from one instrument to another.

Early epoch spectra ($<50$ days), the most prominent emission lines correspond to broad He II and H$\alpha$, shown in Figure 8. These lines have an average FWHM of $\sim 14,000$ km s*-1* and $\sim 10,000$ km s*-1* respectively. The analysis and evolution of their profiles is further explored in Section 4.2.

Several narrow absorption lines, associated with the host galaxy, were identified. The region around H$\alpha$ contains an emission line that can be associated with [N II] at $\lambda$ 6583. We also observed narrow absorption lines corresponding to Ba II at $\lambda\lambda$ 6496, the Na I D doublet at $\lambda\lambda$ 5889, 5896, Mg I $\lambda\lambda$ 5167, 5173, 5184, Fe I $\lambda\lambda$ 5266, 5324 Ca II is detected at $\lambda\lambda$ 3934, 3968 and as strong NIR triplet absorption at $\lambda\lambda$ 8498, 8542, 8662 Å.

4 Analysis

4.1 SED and bolometric lightcurve

Swift host subtracted $UVW1$, $UVM2$, $UVW2$ photometry and Palomar data were used to fit the object blackbody temperature and radius. We fit the fluxes derived from each band with spherical blackbody emission models using Markov Chain Monte Carlo (MCMC) simulations, based on the Python package emcee (Foreman-Mackey et al., 2013). The blackbody bolometric luminosity, along with the best fit for the temperature and the radius are shown in Figure 6. Because only $g$-band measurements were available for our first detection epoch, we assumed that the luminosity follows a blackbody emission with a temperature of 21,000 K (average for the first 2 weeks) and scaled the flux to match the $g$-band magnitude.

The bolometric luminosity at peak is $L_{p}~{}\simeq~{}(1.0\,\pm\,0.15)\times 10^{43}$ erg s*-1*. This is one order of magnitude lower than most of the optical TDEs (PS1-10jh, ASASSN-14ae, ASASSN-14li and ASASSN-15oi; see Figure 6). If compared to ASASSN-15lh, a TDE candidate from a rotating high-mass SMBH (Leloudas et al., 2016) 222This object is somehow controversial and has been initially interpreted as a superluminous supernova (Dong et al., 2016), the peak is two order of magnitude fainter. Based on our $M_{BH}$ estimate, we derive its Eddington luminosity $L_{\rm{Edd}}=2.7^{+3.7}_{-1.6}\times 10^{44}$ erg s*-1*, implying that at peak, iPTF16fnl shines only 2$-$10% of $L_{\rm{Edd}}$. Assuming a radiative efficiency of $\eta=0.1$, this translates into a peak accretion rate of $\dot{M}_{\rm{peak}}\sim 1.8\times 10^{-3}$ M*⊙*  yr*-1* ($L=\eta\dot{M}c^{2}$).

Integrating the available bolometric luminosity (see Figure 6), we find the radiated energy to be $E_{R}=(2.0\pm 0.5)\times 10^{49}$ erg. The accreted mass for this interval is $M_{acc}\sim 7.3\times 10^{-5}$ M*⊙*.

The rest-frame blackbody bolometric luminosity (Figure 6) is fit with the characteristic power law $L\propto((t-t_{0})/\tau)^{-5/3}$ and an empirically motivated exponential profile, $L\propto e^{-(t-t_{0})/\tau}$, where $t_{0}$ and $\tau$ are free parameters. For the decaying part of the lightcurve, the exponential model fits the data better ($\chi^{2}$= 17 vs. 110), and best fit parameters are $\tau_{0}\simeq 15$ and $t_{0}\simeq-6$. For comparison, the decay for other optical TDEs is slower: ASASSN-14ae had $\tau$ = 30 days, whereas ASASSN-15oi and ASASSN-14li faded on timescales of 46.5 and 60 days respectively. Continued, high quality photometric monitoring would be required to draw conclusive results on long-term evolution, beyond the initial fading stage.

We estimate a lower limit for the time from disruption to peak light $t_{\rm{peak}}\geq 11$  days from the bolometric lightcurve. We select the measurements at $\pm$10 day from the peak in $g$-band and fit the luminosity with a 2 degree polynomial. For the raising part of the lightcurve, we use our only available $g$-band measurement. While this approach is widely used in estimating the explosion time for supernovae, the emission mechanism for TDEs is different and therefore it only yields to lower limits, as seen when applied to PS1-10jh.

Assuming that our bolometric lightcurve traces the rate of mass falling into the black hole, $\dot{M}(t)$, we use the lightcurve models from Guillochon & Ramirez-Ruiz (2013) (hereafter GR13), scaling them to the peak accretion mass rate and time to peak. The lightcurves are defined for a range of impact parameters $0.5\leq\beta\leq 2.5$, where $\beta$ is defined as the depth of the encounter $\beta=R_{T}/R_{p}$, $R_{T}$ is the tidal disruption radius and $R_{p}$ the pericentre radius. We impose a $M_{BH}=2\times 10^{6}$  M*⊙, but we leave the mass and radius of the disrupted star as free parameters. Our best fit corresponds to a star with polytropic index $\gamma=4/3$ (fully radiative) with $M_{*}\sim 0.03$ M⊙, $R_{*}\sim 0.3$ R⊙* and a depth of the encounter comparable to the disruption radius ($\beta\simeq 1$). The lightcurve for the best fit model is shown in Figure 6. We obtain $t_{peak}\sim 11$  days, comparable with our previous naive estimate. If we impose that low mass stars are fully convective and fit for an object with $\gamma=5/3$, we find a relatively good fit for a partial disruption ($\beta\sim 0.6$) and a similar value of $M_{*}\sim 0.06$ M*⊙*, although in this case $t_{peak}$ is shorter than our observations suggest. Although these values illustrate that the disrupted object was likely a low-mass star, detailed modeling of the event would be required to draw quantitative results.

The blackbody model has an average temperature of T${}_{BB}=19,000\pm$2000 K, which does not vary significantly over time, as shown Figure 6. At later epochs, the increased uncertainties in the host subtraction lead to increased scatter in TBB. Given the uncertainty of the extinction in the host, these values can be assumed as lower limits. The model blackbody radius, RBB starts at $\sim$2.5$\times 10^{14}$ cm, linearly declines for the first twenty days and then flattens to $5\times 10^{13}$ cm. These radii are much larger than the Schwarzschild radius $r_{Sch}\sim 6\times 10^{11}$ cm of the nuclear black hole. In comparison, we note that the tidal disruption radius of such SMBH for a main sequence Solar-like star is $R_{T}\simeq 1\times 10^{13}$ cm. Photospheric emission at radii larger than $R_{T}$ is commonly observed for the optical sample of TDEs. This has been attributed to the existence of a reprocessing layer at larger radii, which re-emits the X-ray and UV in optical bands (Loeb & Ulmer, 1997; Guillochon et al., 2014). Alternatively, the emission mechanism may originate from the energy liberated by shocks between streams in the apocenter, during the formation of the accretion disk (Piran et al., 2015). Such an optically thick layer, mainly formed of stellar debris, is associated with the origin of the emission line signature for optical TDEs (Roth et al., 2016; Metzger & Stone, 2016).

4.2 Spectroscopic analysis

The early time spectrum of iPTF16fnl is dominated by blue continuum radiation and the characteristic broad He II $\lambda$4686 line. The emission around 6500 Å can be attributed to H$\alpha$, although the He I $\lambda$6678 line is also detected. In the region around He II, we also detect H$\beta$ in emission. However the strong host contribution makes its identification challenging at late times. In our analysis, we use the late time host spectrum (+119.2 days) as our template. We select the highest signal-to-noise (S/N) spectra, and fit them with a combination of host and a blackbody continuum, as show in Figure 7. On the residual spectrum, we fit a line model using the python package lmfit (Non-Linear Least-Squares Minimization and Curve-Fitting for Python). After masking the regions affected by telluric absorption, He II + H$\beta$ and H$\alpha$ + He I lines are fit using two component Lorentzian model, in order to derive the width (FWHM) and central location of the emission. The results, plotted as insets, are shown in Figure 8. The fluxes for each line are derived from the best fit model and shown in Table 3. As discussed in Brown et al. (2017), if all the flux in the He II line would be attributed to recombination produced by black body photoionizing radiation, the observed flux of $>10^{40}$ erg s*-1* would require black body radiation with temperature $\sim 4\times 10^{4}$ K, which is higher than our fit, requiring an additional energy source to power this line.

Around peak, the He II lines appear to have higher velocity, showing an average value of FWHM${}_{\rm{HeII}}\simeq 14,000\,\pm 3,000$ km s*-1*, in contrast to the H$\alpha$ line, with FWHM${}_{H\alpha}\simeq 10,000\pm 500$ km s*-1*. At +30 and +45 days after peak, the FWHM narrows down to $8,500\pm 1500$ km s*-1* for He II and $6,000\pm 600$ km s*-1* for H$\alpha$. The center of the lines appears constant within the scatter for the first 90 days: for He II, the lines appear marginally blueshifted with velocity of $-700\pm 700$ km s*-1*, while the H$\alpha$ lines appear to be consistent with the reference wavelength, with a shift in velocity of $-800\pm 1200$ km s*-1*.

5 Discussion

iPTF16fnl is the faintest and fastest event in the current sample of optically discovered TDEs. Assuming our extinction estimation method is accurate, its luminosity at peak is one order of magnitude lower than any other optical/UV TDE discovered so far. Its timescale, as shown in Figure 10, also makes it as an outlier among the existing sample.

The host of iPTF16fnl is another example of a TDE in a post-starburst galaxy, further linking the propensity of TDEs to such galaxies (Arcavi et al., 2014; French et al., 2016). Moreover, E+A galaxy hosts seem to be exclusive for the lowest redshift TDEs ($z<0.05$) (see Figure 10). The origin of the burst could be associated with a merger episode, as discussed in the case of ASASSN-14li (Prieto et al., 2016). The violent relaxation in the stellar orbits could enhance the rate of captures, as stars can undergo encounters that will scatter them towards the SMBH.

Lower SMBH masses ($<10^{7}$M*⊙) can increase the number of deeper encounters (Kochanek, 2016), allowing for disruptions with smaller pericenter radius, $R_{p}$. However, theoretical works (Guillochon & Ramirez-Ruiz, 2013; Stone et al., 2013) only show a weak correlation between the impact parameter $\beta$, and the peak of the flare. Therefore, the low luminosity and fast timescales shall be attributed to a lower mass black hole and/or lower mass for the disrupted star. Using our fit to TDE lightcurves from Guillochon & Ramirez-Ruiz (2013) with our estimated $M_{BH}\sim 2\times 10^{6}$M⊙*, we obtain the mass of the disrupted star to be $M_{*}\sim 0.03$ for $t_{peak}$ value of 11 days.

iPTF16fnl has clearly faster decay timescales than other TDEs, but also lower $M_{BH}$. Figure 11 shows a comparison of the e-folding timescale for iPTF16fnl and other optical TDEs, computed from exponential decay models, and the galaxy $M_{BH}$. There seems to be a trend between these two values for the optical/UV TDE population, in general agreement with the theoretical scaling between fallback timescale and black hole mass, $t\propto M_{B}H^{1/2}$. As a cautionary note, while literature generally reports $M_{BH}$ based on bulge mass/luminosity, our best measurement is based on the $M-\sigma$ relation, although the bulge luminosity method yielded to similar results. Figure 10 shows that the most luminous flares ($L_{\rm{bol}}>10^{44}$  erg s*-1*) tend to fade on intermediate timescales, $\sim$50 days. However, there does not seem to be an evident correlation between the peak luminosity and the black hole mass, as discussed in Hung et al. (2017) (see their figure 15).

The tension between theoretical prediction of TDE rates and the ones inferred from observations is an active field of research. While it is difficult to explain the differences in terms of host galaxy properties (Stone & Metzger, 2016), an observational bias towards the brighter events seems to offer a more plausible explanation. The discovery of iPTF16fnl has consequences for previous optical searches for nuclear tidal disruptions. In fact, its peak absolute magnitude $M_{g}=-17.2$ mag, and fast decay timescales, may mimic the behavior of a SN exploding close to the galaxy nucleus. Therefore, such faint events may have gone unnoticed in searches for bright ($M_{g}\sim-20$ mag) nuclear flares (Arcavi et al., 2014).

Systematic searches using the colour (including UV) and location of the transient, rather than its absolute magnitude, will increase our sensitivity to fainter flares. Consistent candidate selection using future surveys such as ZTF or LSST will allow us to explore the full luminosity function of tidal disruption flares. Spectroscopic confirmation of the candidates will be essential to identify this faint population. Dedicated instruments for transient classification such as SEDM will become the big players in this new era.

6 Conclusions

We have presented the discovery and follow-up data for iPTF16fnl, a TDE candidate discovered by the iPTF survey on 2016 August 29th. The real-time image-subtraction pipeline and rapid spectroscopic classification allowed us to initiate a timely follow-up campaign. The photometric and spectroscopic signatures of iPTF16fnl are consistent with the sample of previous optically selected TDEs. As observed in other TDEs, the object shows very strong emission in UV wavelengths, with a $T_{BB}\simeq 19,000$ K. The temperature does not show strong evolution and the decrease in luminosity is best explained as a decrease in the size of the radiating region. In agreement with previous work, the size of this region, defined by its photospheric radius, is also about an order of magnitude larger than $R_{T}$. The early times, the spectroscopic signature of iPTF16fnl is dominated by He II and hydrogen lines, although we also detect emission from He I. After two months after peak light, most of the lines have faded. The exception is He II, which can be identified with a relatively constant FWHM of $\sim$7,000 km s*-1*.

iPTF16fnl is remarkable in three ways: it is the nearest well studied optical/UV TDE (66.6 Mpc), and it has one of the shortest exponential decay timescales (about 15 days) and one of the lowest peak luminosities, $L_{p}~{}\simeq~{}(1.0\,\pm\,0.15)\times 10^{43}$ erg s*-1*. Also, its host galaxy has the lowest $M_{BH}$ among the optical sample of TDEs. Although this could justify the fast decay, its low luminosity may be related to the disruption of a lower mass star, or even a partial disruption.

Our work demonstrates that TDEs cover a wide range of luminosities and timescales. Current and future surveys, such as ATLAS, PS-1, ZTF and LSST, will provide large numbers of events with exquisite temporal coverage. Multifrequency follow-up of the candidates will lead to a better understanding of the underlying emission mechanism, hopefully leading to a unified multifrequency emission model. Large samples obtained with well understood selection criteria will be key to the study of TDE demographics, providing a unique link between theoretical studies of SMBH astrophysics and observations.

Acknowledgments

We thank M. Urry for the use of her programme time to obtain target of opportunity observations of iPTF16fnl with DBSP on 200-inch telescope on Palomar. We also thank Nathaniel Roth, Sjoert Van Velzen and Nicholas Stone for useful comments and discussions, and James Guillochon for his support with TDEFit.

This work was supported by the GROWTH project funded by the National Science Foundation under Grant No 1545949. We thank the Gemini Fast Turnaround program (PI: T. Hung). S.G. is supported in party by NSF CAREER grant 1454816 and NASA Swift Cycle 12 grant NNX16AN85G. A.Y.Q.H. was supported by a National Science Foundation Graduate Research Fellowship under Grant No. DGE‐1144469. A.J.C.T. acknowledges support from the Spanish Ministry Project AYA 2015-71718-R. Support for I.A. was provided by NASA through the Einstein Fellowship Program, grant PF6-170148. G.H. is supported by the National Science Foundation (NSF) under Grant No. 1313484. This work makes use of observations from the LCO network. G.H. is supported by the National Science Foundation (NSF) under Grant No. 1313484. This work makes use of observations from the LCO network.

We acknowledge the use of public data from the Swift data archive. The CSS survey is funded by the National Aeronautics and Space Administration under Grant No. NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. The CRTS survey is supported by the U.S. National Science Foundation under grants AST-0909182. LANL participation in iPTF was funded by the US Department of Energy as part of the Laboratory Directed Research and Development program. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Part of this work is based on observations obtained at the Gemini observatory under the Fast Turnaround program. The Gran Telescopio Canarias (GTC) operated on the island of La Palma at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. The James Clerk Maxwell Telescope is operated by the East Asian Observatory on behalf of The National Astronomical Observatory of Japan, Academia Sinica Institute of Astronomy and Astrophysics, the Korea Astronomy and Space Science Institute, the National Astronomical Observatories of China and the Chinese Academy of Sciences (Grant No. XDB09000000), with additional funding support from the Science and Technology Facilities Council of the United Kingdom and participating universities in the United Kingdom and Canada. VAF is supported by the Russian Science Foundation under grant 14-50-00043.