Merger delay time distribution of extended emission short GRBs

Abstract

The most popular progenitor model for short duration Gamma-Ray bursts (sGRBs) is the merger of two compact objects. However, the short GRB population exhibit a certain diversity: some bursts display an extended emission (EE), continuing in soft γ-rays for a few hundreds of seconds post the initial short pulse. It is currently unclear whether the origin of such bursts is linked to compact object mergers.Within the merger hypothesis, the redshift (z) distribution of short GRBs is influenced by the merger delay time, i.e., time elapsed between the merger and the formation of the binary star system, which is dominated by the time-scale for gravitational wave losses during the compact binary phase. We examine redshift distributions of short GRBs with extended emission to see whether their formation channel requires considerable delay post the star formation episode. Our results show that the z distribution of EE bursts is consistent with the merger model. We attempted to compare the delay time distribution of the EE and the non-EE short bursts. However, no statistically significant difference could be seen within the limited sample size.

Evidence of Extended Emission in GRB 181123B and Other High-redshift Short GRBs

“…Possible explanations for their phenomenology include the formation of a long-lived highly magnetized NS (magnetar; Gompertz et al 2013), an NS-black hole (BH) encounter (Troja et al 2008), a core-collapse fallback SN (Valenti et al 2009), or more simply viewing angle effects (Barkov & Pozanenko 2011;Oganesyan et al 2020). The latter scenario is consistent with the redshift distribution found by Anand et al (2018), who observe no significant difference between the two classes and support an old progenitor system. Alternatively, sGRBEEs may not fit at all into the collapsar/ merger dichotomy and herald a novel and rare channel of GRB production (e.g., Fryer et al 1999;King et al 2007;Lyutikov & Toonen 2017).…”

“…Anand et al (2018) found no difference in the redshift distribution of sGRBs with and without EE; however, they studied an older sample of sGRBs with EE, not including the z > 1 EE discussed in this work.…”

“…While the GW signal from these mergers can be detected out to a few hundred Mpc (Abbott et al 2020), sGRBs span a wider range of redshifts (from z ∼ 0.1 to z  2; Selsing et al 2019) and therefore are a unique tool to pinpoint NS mergers across all cosmic times. The redshift distribution of sGRBs is a key observational input to infer the age of their stellar progenitors (Zheng & Ramirez-Ruiz 2007;Behroozi et al 2014;Anand et al 2018;McCarthy et al 2020) and thus estimate their contribution to the cosmic chemical evolution. For instance, sGRBs can be used to infer the observational delay time 12 distribution (DTD) of NS mergers, which can then be compared to the theoretically predicted DTDs for different formation channels.…”

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

“…This serves as an initial basis for comparison to predicted redshift distributions with varying underlying DTDs, star formation histories, and luminosity functions. Much work has been done in the literature to perform the convolution between these functions and predict the observed redshift distributions (Guetta & Piran 2005;Nakar et al 2006;Hao & Yuan 2013;Wanderman & Piran 2015;Anand et al 2018). From these works, we collect eight representative predicted distributions that cover the entire observed SGRB redshift range (z∼0.1-2.5) and are not already ruled out by current observations.…”

“…In the absence of other mechanisms, the merger timescale is determined by the loss of energy and angular momentum due to GWs (Peters 1964), which can be tied to the parameters of the binary (e.g., initial separation, ellipticity; Postnov & Yungelson 2014;Selsing et al 2018). Many studies have constrained the SGRB DTD by fitting the SGRB redshift distribution, predominantly focused on the z<1 population (Nakar et al 2006;Berger et al 2007;Jeong & Lee 2010;Hao & Yuan 2013;Wanderman & Piran 2015;Anand et al 2018). Other constraints on the DTD have come from studies of the Galactic population of NS binaries (Champion et al 2004;Beniamini et al 2016a;Vigna-Gómez et al 2018;Beniamini & Piran 2019) and SGRB host galaxy demography, as longer delay times will result in an increase in host galaxies with old stellar populations (Zheng & Ramirez-Ruiz 2007;Behroozi et al 2014).…”