GRB 070306: A Highly Extinguished Afterglow

TL;DR

This paper reports on the highly extinguished afterglow of GRB 070306, highlighting the importance of early near-infrared observations for detecting dark bursts and characterizing their host galaxies.

Contribution

It presents the first detailed analysis of a highly extinguished GRB afterglow using near-infrared data and models the extinction properties of its host galaxy.

Findings

The afterglow was detected in near-infrared but not optical.

The host galaxy is at redshift z = 1.49594.

The line-of-sight extinction A_V is approximately 5.5 mag.

Abstract

We report on the highly extinguished afterglow of GRB 070306 and the properties of the host galaxy. An optical afterglow was not detected at the location of the burst, but in near-infrared a doubling in brightness during the first night and later power-law decay in the K band provided a clear detection of the afterglow. The host galaxy is relatively bright, R ~ 22.8. An optical low resolution spectrum revealed a largely featureless host galaxy continuum with a single emission line. Higher resolution follow-up spectroscopy shows this emission to be resolved and consisting of two peaks separated by 7 AA, suggesting it to be [O II] at a redshift of z = 1.49594 +- 0.00006. The infrared color H-K = 2 directly reveals significant reddening. By modeling the optical/X-ray spectral energy distribution at t = 1.38 days with an extinguished synchrotron spectrum, we derive A_V = 5.5 +- 0.6 mag. This is among the largest values ever measured for a GRB afterglow and visual extinctions exceeding unity are rare. The importance of early NIR observations is obvious and may soon provide a clearer view into the once elusive 'dark bursts'.