Swift-UVOT captures the earliest ultraviolet spectrum of a Gamma Ray Burst

TL;DR

This paper reports the earliest ultraviolet spectrum of a gamma-ray burst (GRB 081203A) captured by Swift-UVOT, revealing key absorption features and demonstrating UVOT's capability to rapidly determine redshifts and study host galaxy properties for bright GRBs.

Contribution

First ultraviolet spectrum of a GRB obtained within minutes of trigger, showing damped Ly-alpha and Ly-beta lines, and establishing UVOT's rapid redshift measurement potential.

Findings

Detected damped Ly-alpha and Ly-beta lines at z=2.05

Measured neutral hydrogen column density typical for GRB hosts

UVOT can provide redshifts within 4-6 minutes for bright GRBs

Abstract

We present the earliest ever ultraviolet spectrum of a gamma-ray burst (GRB) as observed with the Swift-UVOT. The GRB 081203A spectrum was observed for 50 seconds with the UV grism starting 251 seconds after the Swift-BAT trigger when the GRB was of u ~13.4 mag and still rising to its peak optical brightness. The UV grism spectrum shows a damped Ly-alpha line, Ly-beta, and the Lyman continuum break at a redshift z = 2.05 +/- 0.01. A model fit to the Lyman absorption implies log N(HI) = 22.0 +/- 0.2 cm-2, which is typical for GRB host galaxies with damped Ly-alpha absorbers. This observation of GRB 081203A demonstrates that for GRBs brighter than v ~14 mag and with 0.5 < z < 3.5 the UVOT will be able to provide redshifts, and probe for damped Ly-alpha absorbers within 4-6 minutes from the time of the Swift-BAT trigger.