Spectroscopy of the short-hard GRB 130603B: The host galaxy and environment of a compact object merger

Short duration gamma-ray bursts (SGRBs) are thought to be related to the violent merger of compact objects, such as neutron stars or black holes, which makes them promising sources of gravitational waves. The detection of a kilonova-like signature associated to the Swift-detected GRB 130603B has suggested that this event is the result of a compact object merger. Our knowledge on SGRB has been, until now, mostly based on the absence of supernova signatures and the analysis of the host galaxies to which they cannot always be securely associated. Further progress has been significantly hampered by the faintness and rapid fading of their optical counterparts (afterglows), which has so far precluded spectroscopy of such events. Afterglow spectroscopy is the key tool to firmly determine the distance at which the burst was produced, crucial to understand its physics, and study its local environment. Here we present the first spectra of a prototypical SGRB afterglow in which both absorption and emission features are clearly detected. Together with multiwavelength photometry we study the host and environment of GRB 130603B. From these spectra we determine the redshift of the burst to be z = 0.3565+/-0.0002, measure rich dynamics both in absorption and emission, and a substantial line of sight extinction of A_V = 0.86+/-0.15 mag. The GRB was located at the edge of a disrupted arm of a moderately star forming galaxy with near-solar metallicity. Unlike for most long GRBs (LGRBs), N_HX / A_V is consistent with the Galactic ratio, indicating that the explosion site differs from those found in LGRBs. The merger is not associated with the most star-forming region of the galaxy; however, it did occur in a dense region, implying a rapid merger or a low natal kick velocity for the compact object binary.

Discovery of the afterglow and host galaxy of the low redshift short GRB 080905A

We present the discovery of short GRB 080905A, its optical afterglow and host galaxy. Initially discovered by Swift, our deep optical observations enabled the identification of a faint optical afterglow, and subsequently a face-on spiral host galaxy underlying the GRB position, with a chance alignment probability of <1%. There is no supernova component present in the afterglow to deep limits. Spectroscopy of the galaxy provides a redshift of z=0.1218, the lowest redshift yet observed for a short GRB. The GRB lies offset from the host galaxy centre by ~18.5 kpc, in the northern spiral arm which exhibits an older stellar population than the southern arm. No emission lines are visible directly under the burst position, implying little ongoing star formation at the burst location. These properties would naturally be explained were the progenitor of GRB 080905A a compact binary merger.

A new analysis of the short-duration, hard-spectrum GRB 051103, a possible extragalactic SGR giant flare

GRB 051103 is considered to be a candidate soft gamma repeater (SGR) extragalactic giant magnetar flare by virtue of its proximity on the sky to M81/M82, as well as its time history, localization, and energy spectrum. We have derived a refined interplanetary network localization for this burst which reduces the size of the error box by over a factor of two. We examine its time history for evidence of a periodic component, which would be one signature of an SGR giant flare, and conclude that this component is neither detected nor detectable under reasonable assumptions. We analyze the time-resolved energy spectra of this event with improved time- and energy resolution, and conclude that although the spectrum is very hard, its temporal evolution at late times cannot be determined, which further complicates the giant flare association. We also present new optical observations reaching limiting magnitudes of R > 24.5, about 4 magnitudes deeper than previously reported. In tandem with serendipitous observations of M81 taken immediately before and one month after the burst, these place strong constraints on any rapidly variable sources in the region of the refined error ellipse proximate to M81. We do not find any convincing afterglow candidates from either background galaxies or sources in M81, although within the refined error region we do locate two UV bright star forming regions which may host SGRs. A supernova remnant (SNR) within the error ellipse could provide further support for an SGR giant flare association, but we were unable to identify any SNR within the error ellipse. These data still do not allow strong constraints on the nature of the GRB 051103 progenitor, and suggest that candidate extragalactic SGR giant flares will be difficult, although not impossible, to confirm.