Evidence that Gamma-ray Burst 130702A Exploded in a Dwarf Satellite of a Massive Galaxy

TL;DR

This paper presents evidence that Gamma-ray Burst 130702A occurred in a dwarf satellite galaxy of a larger, metal-rich galaxy, highlighting the importance of considering superpositions in LGRB host galaxy studies.

Contribution

It provides observational evidence that some long gamma-ray bursts occur in dwarf satellite galaxies rather than the main host galaxy, challenging assumptions about their typical environments.

Findings

The GRB exploded in a dwarf galaxy offset from a massive galaxy.

The dwarf galaxy has a star formation rate of about 0.05 solar masses per year.

The explosion site is approximately 61 kpc from the center of the massive galaxy.

Abstract

GRB 130702A is a nearby long-duration gamma-ray burst (LGRB) discovered by the Fermi satellite whose associated afterglow was detected by the Palomar Transient Factory. Subsequent photometric and spectroscopic monitoring has identified a coincident broad-lined Type Ic supernova (SN), and nebular emission detected near the explosion site is consistent with a redshift of z=0.145. The SN-GRB exploded at an offset of ~7.6" from the center of an inclined r=18.1 mag red disk-dominated galaxy, and ~0.6" from the center of a much fainter r=23 mag object. We obtained Keck-II DEIMOS spectra of the two objects and find a 2{\sigma} upper limit on their line-of-sight velocity offset of ~<60 km/s. If we project the SN-GRB coordinates onto the plane of the inclined massive disk galaxy, the explosion would have a ~61+-10 kpc offset, or ~6 times the galaxy's half-light radius. This large estimated nuclear offset suggests that the faint source is not a star-forming region of the massive red galaxy but is instead a dwarf galaxy. The star-formation rate of the dwarf galaxy is ~0.05 solar masses per year, and we place an upper limit on its oxygen abundance of 12 + log(O/H) < 8.16 dex. The identification of an LGRB in a dwarf satellite of a massive, metal-rich primary galaxy suggests that recent detections of LGRBs spatially coincident with metal-rich galaxies may be, in some cases, superpositions.