XRF 060428B: Observational evidence for a strongly lensed burst

Abstract

Long-duration GRBs and X-ray flashes (XRFs softer-spectrum brethren of long GRBs) are thought to occur following the core-collapse of massive stars. We report here on observations of a recent X-ray flash (XRF 060428B) that occurred 2.6$^″$ in projection from the center of a massive red galaxy at redshift z = 0.348, well within its detectable light. While initial probabilistic arguments suggested a physical connection, deep Keck imaging reveals a compact blue source at the burst position, likely a higher-redshift host galaxy. Since the observed offset is approximately equal to the Einstein radius of the foreground elliptical, we suggest that XRF 060428B may have been strongly gravitationally lensed, allowing us to detect an underluminous burst at high-z. This would naturally explain the otherwise coincidental proximity to a nearby galaxy.

Publication
Supernova 1987A: 20 Years After: Supernovae and Gamma-Ray Bursters