GRB 070125: The First Long-Duration Gamma-ray Burst in a Halo Environment

Abstract

We present the discovery and high signal-to-noise spectroscopic observations of the optical afterglow of the long-duration gamma-ray burst GRB 070125. 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_r ≳ 1.0 Å) absorption features in the wavelength range 4000–10000 Å. The sole significant feature is a weak doublet that we identify as Mg II λλ 2796 (W_r = 0.18±0.02 Å), 2803 (Wr = 0.08±0.01 Å) 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 GRB 070125; the large offset (d ≳ 27 kpc) would naturally explain the low column densities. To remain consistent with the large local (i.e. parsec scale) circum-burst density inferred from broadband afterglow observations, we speculate GRB 070125 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.