Joshua S. Bloom

The host to gamma-ray burst 970508: a distant dwarf galaxy?

Abstract If the optical transient to gamma-ray burst (GRB) 970508 is indeed associated with the 8 May burst and is connected with the z = 0.835 absorption system reported by Metzger et al. (1997) [Natur, 387, 879], then either the GRB originated from an intrinsically very faint galaxy ( L ≲ 0.01L ∗ ) or it occurred at a large distance from a host galaxy (≳25 h70−1kpc); a large offset of GRBs from galaxies would tend to favour the merging neutron star-neutron star (NS-NS) model (Meszaros & Rees, 1993 [ApJ, 405, 278]; Narayan et al., 1997 [ApJL, 395, 83]). Here we show that the properties of a suspected host galaxy, particularly its intrinsic brightness and comoving distance from the transient, can be constrained indirectly using the Mg II absorption features detected in the spectrum of the optical transient (Metzger et al., 1997 [IAUC, 6676]) by examining the galaxies in the vicinity of the optical transient from ground-based and Hubble Space Telescope (HST) images. This is an independent test of the brightness of GRB host galaxies irrespective of distances implied from logN-logP distributions. The spectral lines most likely arise from absorption by very underluminous galaxies; in which case the optical transient has revealed the presence of a population of intermediate-redshift dwarf galaxies.

Photometry and Spectroscopy of GRB 060526 : A detailed study of the afterglow and host of a high-redshift gamma-ray burst

Aims. With this paper we want to investigate the highly variable afterglow light curve and environment of gamma-ray burst (GRB) 060526 at z = 3: 221. Methods. We present one of the largest photometric datasets ever obtained for a GRB afterglow, consisting of multi-color photometric data from the ultraviolet to the near infrared. The data set contains 4 12 data points in total to which we add additional data from the literature. Furthermore, we present low-resolution high signal-to-noise spectra of the afterglow. The afterglow light curve is modeled with both an analytical model using broken power law fits and with a broad-band numerical model wh ich includes energy injections. The absorption lines detec ted in the spectra are used to derive column densities using a multi-ion single-component curve-of-growth analysis from which we derive the metallicity of the host of GRB 060526. Results. The temporal behaviour of the afterglow follows a double broken power law with breaks at t = 0: 090�0: 005 and t = 2: 401�0: 061 days. It shows deviations from the smooth set of power laws that can be modeled by additional energy injections from the central engine, although some significant microvariability remains. The broadband spect ral-energy distribution of the afterglow shows no significa nt extinction along the line of sight. The metallicity derived from S II and Fe II of [S/H] = ‐0.57� 0.25 and [Fe/H] = ‐1.09� 0.24 is relatively high for a galaxy at that redshift but comparable to the metallicity of other GRB hosts at similar redshifts. At the position of the afterglow, no host is detect ed to F775W(AB)= 28.5 mag with the HST, implying an absolute magnitude of the host M(1500 A)>‐18.3 mag which is fainter than most long-duration hosts, although the GRB may be associated with a faint galaxy at a distance of 11 kpc.

Gamma-ray astronomy: Bursting out all over

The origin of γ-ray bursts, events which release massive amounts of energy, has been an enduring puzzle in astronomy. As discussed at a meeting last month and papers in this weeksNature, however, evidence from X-ray, optical and radio afterglows continues to stream in and harden two conclusions that most, perhaps all, of these bursts come from the merger of two neutron stars, or a neutron star and a black hole; and that the bursts are occurring at cosmological distances near the edge of the observable Universe.

Using STIS to find gamma-ray burst redshifts

A recent spectrum of the optical afterglow of GRB 970508 suggests that gamma-ray bursts (GRBs) are cosmological in origin, and it is of crucial importance to derive an accurate distance to each burst. If GRBs occur near their host galaxies (much less than 40 kpc) then Lyman limit absorption should be observable in roughly half the GRB afterglow spectra. Here we outline the methodology to obtain a redshift from the GRB afterglow spectrum using the recently installed Space Telescope Imaging Spectrograph (STIS) instrument on board HST. A low-resolution spectrum with the Multi-Anode Microchannel Array (MAMA) detector gives complete spectral coverage over the wavelength range 1570-3180 A and 1150-1740 A. Assuming that a Target of Opportunity observation is conducted soon (about 3 wk) after a bright burst, a relatively small integration time (about three orbits) would be sufficient to detect the Lyman limit over a wide redshift range (0.3-2.2). Detection (or nondetection) of the Lyman limit, in concert with ground-based observations of nearby galaxies and Mg II and C IV absorption lines, should provide meaningful constraints on the relationship of GRBs to galaxies.