Constraints on the circumburst environments of short gamma-ray bursts

Abstract

Observational follow up of well localized short gamma-ray bursts (SGRBs) has left $20\!-\!30{{\ \rm per\ cent}}$ of the population without a coincident host galaxy association to deep optical and NIR limits (≳26 mag). These SGRBs have been classified as observationally hostless due to their lack of strong host associations. It has been argued that these hostless SGRBs could be an indication of the large distances traversed by the binary neutron star system (due to natal kicks) between its formation and its merger (leading to an SGRB). The distances of GRBs from their host galaxies can be indirectly probed by the surrounding circumburst densities. We show that a lower limit on those densities can be obtained from early afterglow light curves. We find that ${\lesssim}16{{\ \rm per\ cent}}$ of short GRBs in our sample took place at densities ≲10−4 cm−3. These densities represent the expected range of values at distances greater than the host galaxy’s virial radii. We find that out of the five SGRBs in our sample that have been found to be observationally hostless, none are consistent with having occurred beyond the virial radius of their birth galaxies. This implies one of two scenarios. Either these observationally hostless SGRBs occurred outside of the half-light radius of their host galaxy, but well within the galactic halo, or in host galaxies at moderate to high redshifts (z ≳ 2) that were missed by follow-up observations.

Kilonova and Optical Afterglow from Binary Neutron Star Mergers. I. Luminosity Function and Color Evolution

“…The simulated afterglow lightcurves can basically reproduce the observed cosmological sGRB afterglows in Xray, optical, and radio bands. The distributions of these afterglow parameters, which are consistent with the constraints by Fong et al (2015); Wang et al (2015); Wu & MacFadyen (2019); O'Connor et al (2020), are listed in Table 1.…”

Discovery of the Optical Afterglow and Host Galaxy of Short GRB 181123B at z = 1.754: Implications for Delay Time Distributions

“…In the case of an afterglow detection, the lack of redshift can be attributed to the lack of a coincident host galaxy due to significant kicks and merger timescales, leaving large displacements between the burst and host galaxy (Berger 2010;Tunnicliffe et al 2014); offsets of 10 kpc are predicted to comprise as much as 40%-50% of the total population (Wiggins et al 2018). 28 A faint host galaxy may also escape detection due to a low-luminosity or high-z origin (O'Connor et al 2020). In this case, an apparently faint galaxy is more likely to originate at lower redshifts due to the increase in the faint-end slope of the galaxy luminosity function (Blanton et al 2005;Parsa et al 2016), although Figure 8 shows that SGRB hosts overall are drawn from the brighter end of the galaxy luminosity function.…”

The Broadband Counterpart of the Short GRB 200522A at z = 0.5536: A Luminous Kilonova or a Collimated Outflow with a Reverse Shock?

“…Rapid-response, radio observations at 1 day have enabled the detection of early excess emission compared to expectations from the forward shock model, interpreted as reverse shock emission in two events, GRBs 051221A and 160821B (Soderberg et al 2006;Lloyd-Ronning 2018;Lamb et al 2019). As a population, the lack of optical and radio afterglow emission for a majority of short GRBs is a direct reflection of their low beamingcorrected kinetic energy scales (≈ 10 49 erg, two orders of magnitude lower than long-duration GRBs; Panaitescu 2006;Gehrels et al 2008), and their low circumburst densities of ≈ 10 −3 −10 −2 cm −3 (Panaitescu et al 2001;Soderberg et al 2006;Fong et al 2015;O'Connor et al 2020).…”

“…Their hosts have a range of stellar population ages of ≈ 0.5 − 8 Gyr (Leibler & Berger 2010;Nugent et al 2020), which can naturally be explained by the wide expected range of delay times for their BNS merger progenitors (Belczynski et al 2006;Paterson et al 2020). The low densities, weak correlation with host stellar mass or star formation, and origin from a diverse range of host galaxies are all hallmarks of the short GRB population (Zheng & Ramirez-Ruiz 2007;Tunnicliffe et al 2014;Wiggins et al 2018;O'Connor et al 2020).…”