Fast Extragalactic X-ray Transients From Gamma Ray Bursts Viewed Far Off Axis

TL;DR

This paper classifies fast extragalactic X-ray transients into two populations: short-duration XRF pulses from off-axis long GRBs and longer-duration afterglows from off-axis short GRBs, based on archival Chandra data.

Contribution

It identifies and characterizes two distinct populations of X-ray transients, providing insights into their origins and relation to gamma ray bursts viewed off-axis.

Findings

Two populations of XRTs with distinct durations identified

Short XRTs linked to off-axis long GRBs (XRFs)

Long XRTs associated with off-axis short GRB afterglows

Abstract

The observed lightcurves and estimated sky rate of fast extragalactic x-ray transients (XRTs) discovered in archival Chandra data indicate that they belong to two distinct XRT populations. The first population of relatively short duration pulses, which typically last less than few minutes seems to be pulses of x-ray flashes (XRFs), which are nearby long duration gamma ray bursts viewed from far off axis. The second population of much longer pulses, which typically last hours, seems to be the early time afterglows of short gamma ray bursts (SGRBs) which are beamed away from Earth, as was shown in a previous paper.

Tables

Table 1. Table 1: Best fit parametess of XRT pulse shape

Pulse	start t [s]	$\Delta$ [s]	$\tau$ [s]	$E_p\left(0\right)$ [keV]	${\chi }^2/dof$
000519 P1	9570.10	9.65	17.50	1.37	0.39
000519 P2	9613.28	35.54	28.58	2.71	1.28
110103 P1	6564.90	45.03	23.16	1.22	1.20
CDF-S XT1	23.48	167.06	$>>\Delta$		1.13

Equations

$$(1+z)E_p\propto E_{iso}^{1/3},$$

$$E_p\propto 1/FWHM,$$

$$V_{\perp }=\frac{\beta csin\theta }{1-\beta cos\theta }\approx \frac{2c}{\theta }\gg c,$$

$$P\approx \frac{2{\gamma }^2{\theta }^2}{1+{\gamma }^4{\theta }^4}\approx 2/{\gamma }^2{\theta }^2\ll 1,$$

$$\frac{d{N}_{\gamma }(E>E_m)}{dt}\propto \frac{t^{2}exp[-E_m/E_p(0)(1-t/\sqrt{t^2+{\tau }^2})]}{(t^2+{\Delta }^2{)}^2}$$

Full text

gamma-ray bursts, x-ray transients, neutron stars

1 Introduction

Two new types of populations of fast extragalactic X-ray transients (XRTs) were discovered in the past few years in archival images taken before by the Chandra X-ray observatory (CXO). Junker et al. (2013) and Glennie et al. (2015) reported the discoveries of relatively nearby fast XRT 000519 close to M86 in the Virgo cluster at a distance of $\sim 16.5$ Mpc (Mei et al. 2007), and XRT 110103 in the galaxy cluster ACO 3581 at a distance $\sim 94.9$ Mpc (Jhohnstone et al. 2005), respectively, with pulses that lasted less than few tens of seconds. XRTs at large cosmological distances that lasted more than $10^{4}$ s were discovered in the Chandra Deep Field-South (CDF-S) observations; CDF-S XT1 associated with a faint distant galaxy at unknown redshift, that lasted more than a day was discovered by Bauer et al (2017), and CDF-S XT2 associated with a galaxy at redshift $z=0.738$ that lasted more than $2\times 10^{4}$ s was discovered by Xue et al. (2019).

In a recent publication (Dado and Dar 2019a) we have shown that the observed light curve of CDF-S XT2 (Xue et al. 2019) and the estimated full sky rate of such XRTs (Yang et al. 2019) indicate that they are early time x-ray afterglows of short gamma ray bursts (SGRBs), which point away from Earth. Their light curves are indistinguishable from those of the early time X-ray afterglows of SGRBs which do point to Earth. Their full sky rate is consistent with the estimated rate of newly born millisecond pulsars in binary neutron stars mergers (Dado and Dar 2019a) whose spin down seems to power the early time quasi isotropic afterglow of SGRBs (Dado and Dar 2019b).

In this letter we show supportive evidence that the population of XRTs which was discovered by Jonker et al. (2013) and by Glennie et al. (2015) consists of x-ray flashes (XRFs: Heise et al. 2003; Barraud et al. 2003; Sakamoto et al. 2005), which, in the cannonball (CB) model of GRBs, are highly beamed ordinary long GRBs (Shaviv and Dar 1995) that are viewed from far-off axis (Dar and De Rujula 2000, Dado, Dar and De Rujula 2004). In particular, such ordinary GRBs viewed far off-axis were predicted by Dar and De Rújula (2000, Eqs. 35-40) to have much lower luminosity, peak energy and linear polarization, and a much longer duration, compared to those of ordinary GRBs viewed from near axis. As such, they are visible only from relatively nearby distances (Dar and De Rújula 2000; Dado, Dar and De Rújula 2004). Moreover, their observed isotopic equivalent energy, $E_{iso}$, and peak energy, $E_{p}$, were predicted (Dar and De Rujula 2000, Eq.35) to satisfy the correlation $(1+z)E_{p}\propto[E_{iso}]^{1/3}$ rather than the $(1+z)E_{p}\propto E_{iso}^{1/2}$ correlation, which was later discovered empirically by Amati et al. (2002) for ordinary, near axis, long duration GRBs (LGRBs).

2 XRFs properties in the CB model

In the CB model, a core collapse supernova of Type Ia result in a compact remnant and a fast rotating torus of non ejected fallen back ejecta. Matter acreting onto the compact central object produces a jet of highly relativistic plasmoids (CBs) of ordinary matter. Glory photons - light emitted/scattered by the supernova and presupernova ejecta- which undergo inverse Compton scattering by CB electrons (Shaviv and Dar 1995) produce a gamma ray pulse in a narrowly beamed GRB. Although the detailed properties of these CBs and their emission times are not predictable, several correlations and various typical properties of XRF pulses, which result from a large viewing angle which satisfies, $1\gg\theta^{2}\gg 1/\gamma^{2}$ are predictable. They include (Dar and De Rújula 2000, Dado, Dar and De Rújula 2004) the $[E_{p},E_{iso}]$ correlation,

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the $[E_{p},FWHM]$ ”anti” correlation between $E_{p}$ and the full width at half maximum,

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the early time large sky projected super luminal velocity,

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the small linear polarization,

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and a typical pulse shape and afterglow light curve. The pulse shape above a minimal energy $E_{m}$ has the behavior,

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For XRFs with $\tau\!\gg\!\Delta$, the exponential factor on the right hand side of eq.(5) can be neglected, which yields a full width at half maximum $FWHM\!\approx 2\,\Delta$, a rise time $RT\!\approx\!0.59\,\Delta$ from half to peak value at $t=\Delta$, and a decay time $DT\!\approx\!1.41\,\Delta$ from peak to half peak.

3 Comparison with observations

To test whether the x-ray transients XRT 000519 (Jonker et al. 2013) and XRT 110103 (Glennie et al. 2015) and CDF-S XT1 (Bauer et al. 2017) in the Chandra archival data could have been XRFs, i.e., LGRBs viewed far off-axis, we have fitted the 0.3-7 keV light curves of their pulses with Eq.(5). These fits with quite a satisfactory $\chi^{2}/dof$ are shown in Figures 1,2.

To test further whether the x-ray transients XRT 000519 (Jonker et al. 2013) and XRT 110103 (Glennie et al. 2015) discovered in the Chandra archival data could have been XRFs we have plotted the best fit CB model correlation $(1+z)\,E_{p}\propto E_{iso}^{1/3}$ (solid line) obtained for low luminosity long GRBs, where we included XRT 000519. As shown in Figure 3 the values $E_{p}\approx 1.5\pm 0.5$ keV and $E_{iso}\approx(4\pm 2)\times 10^{44}$ erg, reported by Bauer et al. (2017) satisfy well the CB model $[E_{p},E_{iso}]$ correlation obeyed by XRFs.

4 Conclusions

The observed lightcurves, sky rate and distances of the population of nearby extragalactic fast x-ray transients with a duration less than few minutes discovered in archival Chandra data (Jonker et al. 2013, Glennie et al. 2015) are consistent with being ordinary GRB pulses of ordinary long duration GRBs viewed from far off axis. This population is different from the population of much longer (hours) duration of XRTs at large cosmological distances discovered also in archival Chandra data (Bauer et al. 2017, Xue et al. 2019) which seems to be the early time isotropic x-ray afterglow of SGRBs which beamed away from Earth (Xue et al 2019, Dado and Dar 2019a).