Poonam Chandra's Academic Page

 

Reverse shock in radio afterglows of Gamma Ray Bursts

The relativistic fireball that gives rise to a gamma-ray burst drives both a forward shock into the surrounding circumburst medium, and a reverse shock into the material that was ejected from the GRB central engine. While the fireball dissipates comparable amount of internal energy in the ejecta and the circumburst gas, the deceleration time of the reverse shock in the ejecta is much smaller, resulting in a bright, short-lived flash of emission. The nearly self-similar behavior of a forward shock means that little information is preserved about the central engine properties that gave rise to the GRB. In contrast, the brightness of the short-lived reverse shock depends in part on the initial Lorentz factor and the magnetization of the ejecta. Thus, the study of reverse shocks has much to tell us about the acceleration, the composition and the strength and orientation of any magnetic fields in the relativistic outflows from GRBs.
The launch of the Swift satellite, working in tandem with ground-based robotic telescope, greatly increased the capability to detect optical flares contemporaneously with the gamma-ray emission. Despite this, only eight additional candidates have been identified from hundreds of events.

We have searched the known sample of radio afterglows looking for the characteristic signature from a reverse shock. We see possible reverse shock emission from 24 radio afterglows. This strengthens the view that the reverse shock phenomenon is quite common in radio GRBs, despite the well-known paucity of optical reverse shocks. If our sample is representative of the GRB population as a whole then 12% of GRBs have radio reverse shocks, compared to 4% of optical afterglows. These data lend support for the hypothesis that the peak frequency of the reserve shock for some GRBs is shifted below the optical band. This is a work in progress.

References: P. Chandra et al. 2015, work in progress

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