GRB 090426: the farthest short gamma-ray burst?

Abstract

Aims. With an observed and rest-frame duration of <2 s and <0.5 s, respectively, GRB 090426 could be classified as a short GRB. The prompt detection, both from space and ground-based telescopes, of a bright optical counterpart to this GRB offered a unique opportunity to complete a detailed study. Methods. Based on an extensive ground-based observational campaign, we obtained the spectrum of the optical afterglow of GRB 090426, measuring its redshift and obtaining information about the medium in which the event took place. We completed follow-up observations of the afterglow optical light curve down to the brightness level of the host galaxy that we firmly identified and studied. We also retrieved and analyzed all the available high-energy data of this event, and compared the results with our findings in the optical. This represents one of the most detailed studies of a short-duration event presented so far. Results. The time properties qualify GRB 090426 as a short burst. In this case, its redshift of z = 2.61 would be the highest yet found for a GRB of this class. On the other hand, the spectral and energy properties are more similar to those of long bursts. LBT late-time deep imaging identifies a star-forming galaxy at a redshift consistent with that of the GRB. The afterglow lies within the light of its host and shows evidence of local absorption.

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

“…See however Levan et al (2007). 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.…”

The afterglow and host galaxy of GRB 090205: evidence of a Ly-αemitter at z = 4.65

“…These values make GRB 090205 consistent with the observed E p,i −E γ,iso correlation (Amati et al 2008), which is known to be followed only by long GRBs (see also Piranomonte et al 2008) and proposed as an indicator of GRBs with a massive stellar collapse origin (type II GRBs, Zhang et al 2009). The E p,i −E γ,iso correlation has indeed been used to support the long classification of a few rest-frame short duration bursts such as GRB 090423 (Salvaterra et al 2009) and GRB 090426 (Antonelli et al 2009). A type II classification for GRB 090205 is also supported by applying the classification method reported in Lv et al (2010), which implies that = E γ,iso,52 /E p,z,2 5/3 ∼ 0.26, where E γ,iso,52 = E γ,iso /10 52 erg and E p,z,2 = E p (1 + z)/10 2 keV.…”

GRB 071227: an additional case of adisguisedshort burst

“…The existence of the Amati relation, discovered when analyzing the BeppoSAX long duration bursts, has been confirmed by studying a sample of GRBs observed by Swift, intense or soft, with available measurements of redshift and spectral parameters, and also by HETE-2 and Konus/WIND. When the "afterglow revolution" allowed the redshift estimation of also some short GRBs, it was found that these bursts are inconsistent with the Amati relation, holding instead for long GRBs (Amati 2006(Amati , 2010Antonelli et al 2009). The most recent updating of the correlation (95 GRBs with the data available up to April 2009) also includes two high-energetic events detected by Fermi (GRB 090323 and GRB 080916C), which are fully consistent with the E p,i -E iso relation (Amati et al 2009).…”