GRB 090313: X-shooter's first shot at a GRB

TL;DR

X-shooter, a new VLT spectrograph, successfully observed a high-redshift GRB afterglow, capturing broad spectral features and intervening systems, demonstrating its potential for rapid, detailed GRB studies and early universe exploration.

Contribution

First demonstration of X-shooter's capabilities in GRB afterglow spectroscopy, covering a broad wavelength range simultaneously and revealing new high-redshift absorption features.

Findings

Detected multiple absorption features at redshift 3.3736

Observed intervening systems at redshifts 1.8005 and 1.9597

Proved X-shooter's ability to observe faint GRB afterglows quickly

Abstract

Context. X-shooter is the first second-generation instrument to become operative at the ESO Very Large Telescope (VLT). It is a broad-band medium-resolution spectrograph designed with gamma-ray burst (GRB) afterglow spectroscopy as one of its main science drivers. Aims. During the first commissioning night on sky with the instrument fully assembled, X-shooter observed the afterglow of GRB 090313 as a demonstration of the instrument's capabilities. Methods. GRB 090313 was observed almost two days after the burst onset, when the object had already faded to R~21.6. Furthermore, the 90% illuminated Moon was just 30 degrees away from the field. In spite of the adverse conditions, we obtained a spectrum that, for the first time in GRB research, covers simultaneously the range from 5700 to 23000 Angstroms. Results. The spectrum shows multiple absorption features at a redshift of 3.3736, the redshift of the GRB. These features are composed of 3 components with different ionisation levels and velocities. Some of the features have never been observed before in a GRB at such a high redshift. Furthermore, we detect two intervening systems at redshifts of 1.8005 and 1.9597. Conclusions. These results demonstrate the potential of X-shooter in the GRB field, as it was capable of observing a GRB down to a magnitude limit that would include 72% of long GRB afterglows 2 hours after the burst onset. Coupled with the rapid response mode available at VLT, allowing reaction times of just a few minutes, X-shooter constitutes an important leap forward on medium resolution spectroscopic studies of GRBs, their host galaxies and intervening systems, probing the early history of the Universe.