GRB 090902B: afterglow observations and implications

TL;DR

This paper reports detailed optical-infrared afterglow observations of GRB 090902B, confirming synchrotron fireball model predictions, and discusses implications for the burst's energetics and jet structure.

Contribution

It provides comprehensive multi-wavelength afterglow data for GRB 090902B, analyzing its decay, spectral behavior, and energy, advancing understanding of high-energy GRB mechanisms.

Findings

Optical afterglow detected 80 minutes post-burst

Cooling break between optical and X-ray frequencies at 1.9 days

Jet break not observed within 6 days, indicating high collimation energy

Abstract

The optical-infrared afterglow of the LAT-detected long duration burst, GRB 090902B, has been observed by several instruments. The earliest detection by ROTSE-IIIa occurred 80 minutes after detection by the GBM instrument onboard the Fermi Gamma-Ray Space Telescope, revealing a bright afterglow and a decay slope suggestive of a reverse shock origin. Subsequent optical-IR observations followed the light curve for 6.5 days. The temporal and spectral behavior at optical-infrared frequencies is consistent with synchrotron fireball model predictions; the cooling break lies between optical and XRT frequencies ~ 1.9 days after the burst. The inferred electron energy index is $p = 1.8 \pm 0.2$, which would however imply an X-ray decay slope flatter than observed. The XRT and LAT data have similar spectral indices and the observed steeper value of the LAT temporal index is marginally consistent with the predicted temporal decay in the radiative regime of the forward shock model. Absence of a jet break during the first 6 days implies a collimation-corrected $\gamma$-ray energy $E_{\gamma} > 2.2\times10^{52}\rm$ ergs, one of the highest ever seen in a long-duration GRBs. More events combining GeV photon emission with multi-wavelength observations will be required to constrain the nature of the central engine powering these energetic explosions and to explore the correlations between energetic quanta and afterglow emission.