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Published October 10, 2015 | Published + Submitted
Journal Article Open

The Unusual Radio Afterglow of the Ultra-Long Gamma-Ray Burst GRB 130925A


GRB 130925A is one of the recent additions to the growing family of ultra-long gamma-ray bursts (GRBs; T90≳1000 s). While the X-ray emission of ultra-long GRBs have been studied extensively in the past, no comprehensive radio data set has been obtained so far. We report here the early discovery of an unusual radio afterglow associated with the ultra-long GRB 130925A. The radio emission peaks at low-frequencies (~7 GHz) at early times, only 2.2 days after the burst occurred. More notably, the radio spectrum at frequencies above 10 GHz exhibits a rather steep cut-off, compared to other long GRB radio afterglows. This cut-off can be explained if the emitting electrons are either mono-energetic or originate from a rather steep, dN/dE ∝ E^(−4), power-law energy distribution. An alternative electron acceleration mechanism may be required to produce such an electron energy distribution. Furthermore, the radio spectrum exhibits a secondary underlying and slowly varying component. This may hint that the radio emission we observed is comprised of emission from both a reverse and a forward shock. We discuss our results in comparison with previous works that studied the unusual X-ray spectrum of this event and discuss the implications of our findings on progenitor scenarios.

Additional Information

© 2015. The American Astronomical Society. Received 2015 May 26; accepted 2015 September 10; published 2015 October 9. We thank R. Barniol Duran, T. Piran, E. Nakar, R. Sari, and K. Mooley for useful discussions. We thank the VLA staff for promptly scheduling the observation of this target of opportunity. We also acknowledge the use of the Astronomical Matlab Packages by Ofek (2014). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Research leading to these results has received funding from the EU/FP7 via ERC grant 307260; ISF, Minerva, and Weizmann-UK grants; as well as the Quantum Universe I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation. S.B.C. acknowledges funding from NASA grant NNH13ZDA001N. Support for D.A.P was provided by NASA through an award issued by JPL/Caltech, and through Hubble Fellowship grant HST-HF-51296.01-A awarded by the Space Telescope Science Institute.

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Published - Horesh_2015p86.pdf

Submitted - 1506.00636v1.pdf


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