Optical and Infrared Observations of Gamma-Ray Burst Afterglows

Columbia University, New York NY

Investigators

Abstract

ABSTRACT AST-0071108 Halpern This project has as its objective the rapid follow-up of gamma-ray burst (GRB) afterglows using ground-based, optical/IR photometry and spectroscopy on the Michigan-Dartmouth-MIT (MDM) 2.4m telescope on Kitt Peak. The PI has been one of the important players in this type of research since 1998 and has the good, prompt access to the telescope which is necessary for this type of follow-up work. Using the approximately 6 to 8 bursts per year in the northern hemisphere that are located within hours by the BeppoSAX and/or RXTE satellites, this project will continue a successful program of discovering and characterizing the optical afterglows of gamma-ray bursts with the two telescopes of the MDM Observatory. Starting with the year 2000 launch of the HETE-II satellite, it will become possible to make optical identifications within minutes of a GRB event, which affords the opportunity to obtain multicolor optical light curves on small telescopes such as the MDM 1.3m, and absorption-line redshifts on medium-sized telescopes such as the MDM 2.4m. These data can be used to study the temporal decay and spectral evolution of the afterglow in order to diagnose the energetics of the fireball, and to model the circumstellar environments in which these explosions occur, e.g., uniform interstellar medium vs. stellar wind. HETE-II will also be sensitive to the class of short-duration gamma-ray bursts, which have heretofore not been identified, and are distinct in their gamma-ray spectral properties and flux distribution from the long-duration bursts which have triggered all previous after- glow detections. This work has the goal of revealing a distinct population of GRB progenitor, for example, old ones that have traveled outside their galaxy's interstellar medium, versus those that end their short lives while still embedded in dense, star-forming regions. Infrared photometry is of utmost importance for this distinction. Prompt IR photometry, particularly in the K band, offers the opportunity to identify the highest redshift and/or most obscured GRBs. The MDM 2.4m and ONIS IR camera, can easily detect a moderately obscured GRB to z = 10 within an hour of the event. On longer time scales, the light curves can be followed for several days in order to discover the predicted (and occasionally detected) temporal steepening which is the signature of jet-like afterglows. Measuring such breaks is crucial to quantifying the effects of beaming on the energetics of the fireball and on the global statistics of GRBs and their afterglows. Finally, the afterglows of the brighter and closer events can be monitored for several weeks in order to search for the signatures of underlying supernovae in their late-time optical light curves. The possible association of at least some GRBs with supernovae of massive stars promises to be the long-sought handle on the physics of the GRB itself. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC). ***