The Afterglows of Swift-era Gamma-Ray Bursts II.: Type I GRB versus Type II GRB Optical Afterglows

Abstract

Gamma-Ray Bursts (GRBs) have been separated into two classes, originally along the lines of duration and spectral properties, called "short/hard" and "long/soft". The latter have been conclusively linked to the explosive deaths of massive stars, while the former are thought to result from the merger or collapse of compact objects. In recent years indications have been accumulating that the short/hard vs. long/soft division does not map directly onto what would be expected from the two classes of progenitors, leading to a new classification scheme called Type I and Type II which is based on multiple observational criteria. We use a large sample of GRB afterglow and prompt-emission data (adding further GRB afterglow observations in this work) to compare the optical afterglows (or the lack thereof) of Type I GRBs with those of Type II GRBs. In comparison to the afterglows of Type II GRBs, we find that those of Type I GRBs have a lower average luminosity and show an intrinsic spread of luminosities at least as wide. From late and deep upper limits on the optical transients, we establish limits on the maximum optical luminosity of any associated supernova, confirming older works and adding new results. We use deep upper limits on Type I GRB optical afterglows to constrain the parameter space of possible mini-SN emission associated with a compact-object merger. Using the prompt emission data, we search for correlations between the parameters of the prompt emission and the late optical afterglow luminosities. We find tentative correlations between the bolometric isotropic energy release and the optical afterglow luminosity at a fixed time after trigger (positive), and between the host offset and the luminosity (negative), but no significant correlation between the isotropic energy release and the duration of the GRBs. We also discuss three anomalous GRBs, GRB 060505, GRB 060614, and GRB 060121, in the light of their optical afterglow luminosities.

VLT/X-shooter spectroscopy of the GRB 090926A afterglow

“…For most GRB afterglows, when accurate multi-band photometry is available (e.g., Covino et al 2008, Schady et al 2010, Kann et al 2010, the derived extinction curve is in fair agreement with what observed locally in the Small Magellanic Cloud (SMC, Pei 1992) although often, due to the limited wavelength resolution, this simply means that the observed extinction curve has to be chromatic (i.e. wavelength dependent) and featureless.…”

GRB060614: a “fake” short GRB from a merging binary system

“…However, its morphology is different from typical long GRBs, being similar to the one of GRB050724, traditionally classified as a short GRB (Zhang et al, 2007;Piro, 2005). Its optical afterglow luminosity is intermediate between the traditional long and short ones (Kann et al, 2008). Its host galaxy has a moderate specific star formation rate (R Host ≈ 2M s y −1 (L * ) −1 , M vHost ≈ −15.5; Fynbo et al, 2006;.…”

A SHORT GAMMA-RAY BURST “NO-HOST” PROBLEM? INVESTIGATING LARGE PROGENITOR OFFSETS FOR SHORT GRBs WITH OPTICAL AFTERGLOWS

“…The detection of short GRB afterglows starting in mid-2005 provided an opportunity to investigate the various progenitor models through a range of observational tests: the redshift distribution Gal-Yam et al 2008), the host galaxy demographics , the afterglow properties Gehrels et al 2008;Kann et al 2008;Nysewander et al 2009), and perhaps most importantly, their locations relative to the host galaxies (Fong et al 2010). As of mid-2010, X-ray and optical afterglows have been detected from 40 and 20 short GRBs, respectively, with the latter sample providing accurate sub-arcsecond positions.…”