GRB070125: The First Long-Duration Gamma-Ray Burst in a Halo Environment

TL;DR

This paper reports the discovery of a long-duration gamma-ray burst, GRB070125, occurring in a halo environment with minimal host galaxy absorption features, suggesting it originated far from typical star-forming regions.

Contribution

It provides the first spectroscopic evidence of a long-duration GRB in a galactic halo environment, indicating a possible origin in distant stellar clusters rather than within the host galaxy disk.

Findings

Absence of strong absorption features in the afterglow spectrum.

Detection of weak Mg II absorption at z=1.5477.

GRB likely occurred far from the host galaxy's main star-forming regions.

Abstract

We present the discovery and high signal-to-noise spectroscopic observations of the optical afterglow of the long-duration gamma-ray burst GRB070125. Unlike all previously observed long-duration afterglows in the redshift range 0.5 < z < 2.0, we find no strong (rest-frame equivalent width W > 1.0 A) absorption features in the wavelength range 4000 - 10000 A. The sole significant feature is a weak doublet we identify as Mg II 2796 (W = 0.18 +/- 0.02 A), 2803 (W = 0.08 +/- 0.01) at z = 1.5477 +/- 0.0001. The low observed Mg II and inferred H I column densities are typically observed in galactic halos, far away from the bulk of massive star formation. Deep ground-based imaging reveals no host directly underneath the afterglow to a limit of R > 25.4 mag. Either of the two nearest blue galaxies could host GRB070125; the large offset (d >= 27 kpc) would naturally explain the low column density. To remain consistent with the large local (i.e. parsec scale) circum-burst density inferred from broadband afterglow observations, we speculate GRB070125 may have occurred far away from the disk of its host in a compact star-forming cluster. Such distant stellar clusters, typically formed by dynamical galaxy interactions, have been observed in the nearby universe, and should be more prevalent at z>1 where galaxy mergers occur more frequently.