The extreme, red afterglow of GRB 060923A: Distance or dust?

Gamma-ray bursts (GRBs) are powerful probes of the early Universe, but locating and identifying very distant GRBs remain challenging. We report here the discovery of the K-band afterglow of Swift GRB 060923A, imaged within the first hour post-burst, and the faintest so far found. It was not detected in any bluer bands to deep limits, making it a candidate very high-z burst (z≳ 11). However, our later-time optical imaging and spectroscopy reveal a faint galaxy coincident with the GRB position which, if it is the host, implies a more moderate redshift (most likely z≲ 2.8) and therefore that dust is the likely cause of the very red-afterglow colour. This being the case, it is one of the few instances so far found of a GRB afterglow with high-dust extinction

The nature of the X-ray flash of August 24 2005 Photometric evidence for an on-axis z = 0.83 burst with continuous energy injection and an associated supernova?

Aims.Our aim is to investigate the nature of the X-Ray Flash (XRF) of August 24, 2005. Methods.We present comprehensive photometric R-band observations of the fading optical afterglow of XRF 050824, from 11 min to 104 days after the burst. In addition we present observations taken during the first day in the $\it BRIK$ bands and two epochs of spectroscopy. We also analyse available X-ray data. Results.The R-band lightcurve of the afterglow resembles the lightcurves of long duration Gamma-Ray Bursts (GRBs), i.e., a power-law, albeit with a rather shallow slope of $\alpha=0.6$ ( $F_{\nu} \propto t^{-\alpha}$). Our late R-band images reveal the host galaxy. The rest-frame B-band luminosity is ~0.5 L*. The star-formation rate as determined from the [O II] emission line is ~ $1.8~M_{\odot}$ yr-1. When accounting for the host contribution, the slope is $\alpha=0.65$ $\pm$ 0.01 and a break in the lightcurve is suggested. A potential lightcurve bump at 2 weeks can be interpreted as a supernova only if this is a supernova with a fast rise and a fast decay. However, the overall fit still shows excess scatter in the lightcurve in the form of wiggles and bumps. The flat lightcurves in the optical and X-rays could be explained by a continuous energy injection scenario, with an on-axis viewing angle and a wide jet opening angle ( \theta_j \ga {10}^\circ). If the energy injections are episodic this could potentially help explain the bumps and wiggles. Spectroscopy of the afterglow gives a redshift of z=0.828 $\pm$ 0.005 from both absorption and emission lines. The spectral energy distribution (SED) of the afterglow has a power-law ( $F_{\nu} \propto \nu ^{-\beta}$) shape with slope ${\beta}=0.56$ $\pm$ 0.04. This can be compared to the X-ray spectral index which is ${\beta_{\rm X}}=1.0$ $\pm$ 0.1. The curvature of the SED constrains the dust reddening towards the burst to $A_{\rm v}<0.5$ mag

The blue host galaxy of the red GRB 000418

We report on multi-band ( $UBVRIZJ_{\rm s}K_{\rm s}$) observations of the host galaxy of the April 18, 2000 gamma-ray burst. The Spectral Energy Distribution (SED) is analysed by fitting empirical and synthetic spectral templates. We find that: (i) the best SED fit is obtained with a starburst template, (ii) the photometric redshift is consistent with the spectroscopic redshift, (iii) the colours of the host are inconsistent with an old stellar population, and (iv) the global extinction is constrained to be in the range AV=0.12- 0.61 mag. The derived global extinction agrees with the one reported for the afterglow ( AV = 0.4- 0.9 mag), suggesting a homogeneous distribution of the interstellar medium (ISM) in the host galaxy. These findings are supplemented by morphological information from Hubble Space Telescope (HST) imaging: the surface brightness profile is smooth, symmetric and compact with no underlying structures (like dust lanes, spiral arms or disks). A natural scenario which accounts of all the above results is a nuclear starburst that harbours a young population of stars from which the GRB originated