Cenko 2007 GRB 070125 long burst not very in a galaxy

主要内容:
The difference of this burst is its host galaxy: none, or maybe have both. More accurate may be collision of the two galaxies.

精彩摘抄:
Two circles point to the same region, in different time. From the right (cross), it may be in the collision of two galaxies B1 and B2.

文章信息:

arXiv:0712.2828 [ps, pdf, other]

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

Authors: S. B. Cenko, D. B. Fox, B. E. Penprase, A. Cucchiara, P. A. Price, E. Berger, S. R. Kulkarni, F. A. Harrison, A. Gal-Yam, E. O. Ofek, A. Rau, P. Chandra, D. A. Frail, M. K. Kasliwal, B. P. Schmidt, A. M. Soderberg, P. B. Cameron, K. C. Roth

Comments: 8 pages, accepted in ApJ

Subjects: Astrophysics (astro-ph)

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 <> 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.