DISCERNING THE PHYSICAL ORIGINS OF COSMOLOGICAL GAMMA-RAY BURSTS BASED ON MULTIPLE OBSERVATIONAL CRITERIA: THE CASES OFz= 6.7 GRB 080913,z= 8.2 GRB 090423, AND SOME SHORT/HARD GRBs

Abstract

The two high-redshift gamma-ray bursts, GRB 080913 at z = 6.7 and GRB 090423 at z = 8.3, recently detected by Swift appear as intrinsically short, hard GRBs. They could have been recognized by BATSE as short/hard GRBs should they have occurred at z ≤ 1. In order to address their physical origin, we perform a more thorough investigation on two physically distinct types (Type I/II) of cosmological GRBs and their observational characteristics. We reiterate the definitions of Type I/II GRBs and then review the following observational criteria and their physical motivations: supernova association, specific star forming rate of the host galaxy, location offset, duration, hardness, spectral lag, statistical correlations, energetics and collimation, afterglow properties, redshift distribution, luminosity function, and gravitational wave signature. Contrary to the traditional approach of assigning the physical category based on the gamma-ray properties (duration, hardness, and spectral lag), we take an alternative approach to define the Type I and Type II Gold Samples using several criteria that are more directly related to the GRB progenitors (supernova association, host galaxy type, and specific star forming rate). We then study the properties of the two Gold Samples and compare them with the traditional long/soft and short/hard samples. We find that the Type II Gold Sample reasonably tracks the long/soft population, although it includes several intrinsically short (shorter than 1s in the rest frame) GRBs. The Type I Gold Sample only has 5 GRBs, 4 of which are not strictly short but have extended emission. Other short/hard GRBs detected in the Swift era represent the BATSE short/hard sample well, but it is unclear whether all of them belong to Type I. We suggest that some (probably even most) high-luminosity short/hard GRBs instead belong to Type II. Based on multiple observational criteria, we suggest that GRB 080913 and GRB 090423 are more likely Type II events. In general, we acknowledge that it is not always straightforward to discern the physical categories of GRBs, and re-emphasize the importance of invoking multiple observational criteria. We cautiously propose an operational procedure to infer the physical origin of a given GRB with available multiple observational criteria, with various caveats laid out.

Short versus long gamma-ray bursts: a comprehensive study of energetics and prompt gamma-ray correlations

“…Early hints to the existence of distinct populations of gamma-ray transients (e.g., Mazets et al 1981;Norris et al 1984), and at least two classes of short-hard (Type-I) & long-soft (Type-II) GRBs (Dezalay et al 1992) have now been extensively corroborated and confirmed by the prompt-emission data E-mail: amir@physics.utexas.edu (AS); nemiroff@mtu.edu (RJN) from independent gamma-ray detector missions (e.g., Kouveliotou et al 1993a;Gehrels, Ramirez-Ruiz & Fox 2009; or follow-up observations of the afterglows or host galaxies (e.g., Zhang et al 2009;Berger 2011Berger , 2014. Although the possibility of more than two classes of GRBs with distinct progenitors has been extensively discussed and considered (e.g., Horvath 1998;Mukherjee et al 1998;Hakkila et al 2001;Balastegui, Ruiz-Lapuente & Canal 2001;Hakkila et al 2004b;Horvath et al 2006;Gehrels et al 2006;Chattopadhyay et al 2007;Horvath et al 2008;Virgili, Liang & Zhang 2009;Gao, Lu & Zhang 2010;Horvath et al 2012;Levan et al 2014;Kbori et al 2014), it has remained a matter of debate and speculation to this date (e.g., Hakkila et al 2000cHakkila et al ,a,b, 2003Hakkila et al , 2004aShahmoradi 2013b;Zhang et al 2014;Levan et al 2014).…”

“…cal classification of GRBs based on empirical relations that are believed to hold for only a specific class of GRBs or based on the prompt or afterglow emission data (e.g., Zhang et al 2009;L et al 2010;Qin & Chen 2013). These methods however, either suffer from sample incompleteness or require information, such as redshift, that might not be available for the majority of GRBs (e.g., Coward et al 2012Coward et al , 2013.…”

“…Nevertheless, the detector & energy dependence of the observed GRB duration (e.g., Fenimore et al 1995;Nemiroff 2000; has prompted many studies in search of alternative less-biased methods of GRB classification, typically based on a combination of the prompt gamma-ray emission and/or afterglow/host properties (e.g., Gehrels, Ramirez-Ruiz & Fox 2009;Zhang et al 2009;Goldstein, Preece & Briggs 2010;L et al 2010;Shahmoradi & Nemiroff 2011;Zhang, Chen & Huang 2012;Shahmoradi 2013b;L et al 2014) or using the prompt-emission spectral correlations in conjunction with the traditional method of classification (e.g., Qin & Chen 2013).…”

Gravitational-Wave Observations May Constrain Gamma-Ray Burst Models: The Case of Gw150914–gbm

“…The central engine timescale (the accretion timescale in our example; Zhang et al 2009) can be constrained to be at most the observed duration or t acc 1 s (Aloy et al 2005). This constraint will carve out a region in Figure 2 to the left of the t acc =1 s or R out =2.3×10 8 cm line.…”

Classification of gamma-ray burst durations using robust model-comparison techniques

“…The progenitors of long GRBs consist of supernovae related to the collapse of massive stars (Woosley & Bloom 2006) and those of short GRBs are thought to be binary compact object mergers (Nakar 2007). There are however exceptions to this general picture (Zhang et al 2009;Bromberg et al 2013).…”