Late-Time Optical Afterglow Observations with LBT and MDM

TL;DR

This study used large telescopes to observe nine gamma-ray burst afterglows at late times, detecting jet breaks, flares, and supernova bumps, thereby extending understanding of their long-term behaviors and confirming the collimated nature of GRBs.

Contribution

First systematic late-time optical observations of GRB afterglows with deep flux limits, revealing jet breaks and late-time phenomena, and clarifying the nature of missing Swift jet breaks.

Findings

Detected four jet breaks and one candidate, extending temporal baseline.

Identified late-time flares and supernova bumps in afterglows.

Supported the collimated jet model for GRBs.

Abstract

Using the 2.4m MDM and 8.4m Large Binocular Telescope, we observed nine GRB afterglows to systematically probe the late time behaviors of afterglows including jet breaks, flares, and supernova bumps. In particular, the LBT observations have typical flux limits of 25-26 mag in the Sloan r' band, which allows us to extend the temporal baseline for measuring jet breaks by another decade in time scale. We detected four jet breaks (including a "textbook" jet break in GRB070125) and a fifth candidate, all of which are not detectable without deep, late time optical observations. In the other four cases, we do not detect the jet breaks either because of contamination from the host galaxy light, the presence of a supernova bump, or the intrinsic faintness of the optical afterglow. This suggests that the basic picture that GRBs are collimated is still valid and that the apparent lack of Swift jet breaks is due to poorly sampled afterglow light curves, particularly at late times. Besides the jet breaks, we also detected late time flares, which could attribute to late central engine activities, and two supernova bumps.