Gamma Ray Bursts (GRBs) are bright, brief flashes of high energy photons that have fascinated scientists for 30 years. They come in two classes 1 : long (>2 s), softspectrum bursts and short, hard events. The major progress to date on understanding GRBs has been for long bursts which are typically at high redshift (z ~ 1) and are in sub-luminous star-forming host galaxies. They are likely produced in core-collapse explosions of massive stars 2 . Until the present observation, no short GRB had been accurately (<10") and rapidly (minutes) located. Here we report the detection of X-ray afterglow from and the localization

Abstract. The luminous infrared-loud quasar IRAS 13349+2438 was observed with the XMM-Newton Observatory as part of the Performance Verification program. The spectrum obtained by the Reflection Grating Spectrometer (RGS) exhibits broad (v ∼ 1400 km s −1 FWHM) absorption lines from highly ionized elements including hydrogen-and helium-like carbon, nitrogen, oxygen, and neon, and several iron L-shell ions (Fe xvii-xx). Also shown in the spectrum is the first astrophysical detection of a broad absorption feature around λ = 16−17Å identified as an unresolved transition array (UTA) of 2p-3d inner-shell absorption by iron M-shell ions in a much cooler medium; a feature that might be misidentified as an O vii edge when observed with moderate resolution spectrometers. No absorption edges are clearly detected in the spectrum. We demonstrate that the RGS spectrum of IRAS 13349+2438 exhibits absorption lines from at least two distinct regions, one of which is tentatively associated with the medium that produces the optical/UV reddening.

-We have obtained deep optical images with the Very Large Telescope at ESO of the first well-localized short-duration gamma-ray burst, GRB 050509B. From V and R imaging, initiated ∼ 2 days after the GRB trigger and lasting up to three weeks, we detect no variable object inside the small Swift/XRT X-ray error circle down to 2σ limits of V = 26.5 and R = 25.1. The X-ray error circle includes a giant elliptical galaxy at z = 0.225, which has been proposed as the likely host of this GRB. Our limits indicate that if the GRB originated at z = 0.225, any supernova-like event accompanying the GRB would have to be over 100 times fainter than normal Type Ia SNe or Type Ic hypernovae, 5 times fainter than the faintest known Ia or Ic SNe, and fainter than the faintest known Type II SNe. Moreover, we use the optical limits to constrain the energetics of the GRB outflow. Simple models indicate that, unless the intrinsic energy in the outflow from GRB 050509B was ≪ 10 51 erg, there was very little radioactive material with efficient decay timescales for generating a large luminosity. These limits strongly constrain progenitor models for this short GRB.