Closing in on a Short‐Hard Burst Progenitor: Constraints from Early‐Time Optical Imaging and Spectroscopy of a Possible Host Galaxy of GRB 050509b

Abstract

The localization of the short-duration, hard-spectrum gamma-ray burst GRB 050509b by the Swift satellite was a watershed event. Never before had a member of this mysterious subclass of classic GRBs been rapidly and precisely positioned in a sky accessible to the bevy of ground-based follow-up facilities. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. Though the Swift X-ray Telescope (XRT) discovered an X-ray afterglow of GRB 050509b, the first ever of a short-hard burst, no convincing optical/infrared candidate afterglow or supernova was found for the object. We present a re-analysis of the XRT afterglow and find an absolute position of R.A. = 12 h 36 m 13. s 59, Decl. = +28 • 59 ′ 04. ′′ 9 (J2000), with a 1σ uncertainty of 3. ′′ 68 in R.A., 3. ′′ 52 in Decl.; this is about 4 ′′ to the west of the XRT position reported previously. Close to this position is a bright elliptical galaxy with redshift z = 0.2248 ± 0.0002, about 1 ′ from the center of a rich cluster of galaxies. This cluster has detectable diffuse emission, with a temperature of kT = 5.25 +3.36 −1.68 keV. We also find several (∼11) much fainter galaxies consistent with the XRT position from deep Keck imaging and have obtained Gemini spectra of several of these sources. Nevertheless we argue, based on positional coincidences, that the GRB and the bright elliptical are likely to be physically related. We thus have discovered evidence that supports the notion that at least some short-duration, hard-spectra GRBs are at cosmological distances.We also explore the connection of the properties of the burst and the afterglow, finding that GRB 050509b was underluminous in both of these relative to long-duration GRBs. However, we also demonstrate that the ratio of the blast-wave energy to the γ-ray energy is consistent with that of long-duration GRBs. This suggests a comparably high efficiency of γ-ray conversion as in long GRBs as might be expected if the same emission mechanism is at work in short and long GRBs. Based on this analysis, on the location of the GRB (40 ± 13 kpc from a bright galaxy), on the galaxy type (elliptical), and the lack of a coincident supernova, we suggest that there is now observational consistency with the hypothesis that short-hard bursts arise during the merger of a compact binary (two neutron stars, or a neutron star and a black hole). In this context, we limit the properties of a Li-Paczyński "mini-supernova" that is predicted to arise on ∼day timescales. Other progenitor models are still viable, and additional rapidly localized bursts from the Swift mission will undoubtedly help to further clarify the progenitor picture.

Optical and near-infrared follow-up observations of fourFermi/LAT GRBs: redshifts, afterglows, energetics, and host galaxies

“…One possible exception is the case of GRB 090426 at z = 2.609 (Levesque et al 2010;Antonelli et al 2009;Zhang et al 2009) for which absorption line spectroscopy of the afterglow was successfully obtained, but may be considered on balance by the authors to be associated with the population of long gamma-ray bursts. The host galaxies of the short bursts were found initially to be mostly elliptical galaxies, e.g., GRB 050509B (Gehrels et al 2005;Bloom et al 2006) and GRB 050724 (Berger et al 2005;Gorosabel et al 2006) and that an old progenitor population was required. However, as the sample has since increased, all types of galaxies, from elliptical to star-forming, have been found to be associated with short bursts and the majority of short bursts are now found to reside in star forming galaxies (e.g., Berger et al 2007;Berger 2009;Prochaska et al 2006).…”

The optical afterglows and host galaxies of three short/hard gamma-ray bursts

“…1, the study of the offset distribution of the afterglows with respect to their host galaxies is a key issue 8 With the significant exception of GRB 050724 and possibly GRB 050509B, the only short GRBs firmly associated with early-type galaxies and specific SFR lower than 0.01 and 0.03 M yr −1 L * −1 Bloom et al 2006). in the study of short GRBs progenitors.…”

“…Although our knowledge of GRBs has significantly improved in recent years, optical and near-infrared (NIR) studies of the afterglows and host galaxies of short bursts are still limited to a small sample (e.g. Fox et al 2005;Hjorth et al 2005b;Castro-Tirado et al 2005;Bloom et al 2006;Covino et al 2006; Levan et al 2006;see Nakar 2007, for a review). While it has been firmly established that long GRBs (or at least a significant fraction of them) originate in core-collapse supernova (SN) explosions, the nature of short GRB progenitors is still under debate.…”

The BATSE-Swift luminosity and redshift distributions of short-duration GRBs

“…In some cases, optical (Covino et al 2005;Fox et al 2005;Bloom et al 2006;Price et al 2005;Jensen et al 2005;Hjorth et al 2005;Castro-Tiraldo et al 2005;Gal-Yam et al 2005b;Cobb et al 2005;Wiersema et al 2005) and radio (Cameron & Frail 2005;Berger 2005) afterglow was detected as well. This has lead to the identification of host galaxies and to redshift measurements.…”