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The nature of the late achromatic bump in GRB 120326A

By A. Melandri, F.J. Virgili, C. Guidorzi, M.G. Bernardini, S. Kobayashi, C.G. Mundell, A. Gomboc, B. Dintinjana, V.-P. Hentunen, J. Japelj, D. Kopač, D. Kuroda, A.N. Morgan, I.A. Steele, U. Quadri, G. Arici, D. Arnold, R. Girelli, H. Hanayama, N. Kawai, H. Mikuž, M. Nissinen, T. Salmi, R.J. Smith, L. Strabla, M. Tonincelli and A. Quadri

Abstract

The long Swift gamma-ray burst GRB120326A at redshift z = 1.798 exhibited a multi-band light-curve with a striking feature: a latetime, long-lasting achromatic rebrightening that is rarely seen in such events. Peaking in optical and X-ray bands ~35 ks (~12.5 ks in the GRB rest frame) after the 70 s GRB prompt burst, the feature brightened nearly two orders of magnitude above the underlying optical power-law decay. By modelling the multi-wavelength light-curves, we investigated possible causes of the rebrightening in the context of the standard fireball model. We excluded a range of scenarios for the origin of this feature: reverse-shock flash, late-time forward-shock peak caused by the passage of the brightest synchrotron frequency through the optical band, late central engine optical or X-ray flares, interaction between the expanding blast wave and a density enhancement in the circumburst medium, and gravitational microlensing. Instead we conclude that the achromatic rebrightening may be caused by a refreshed forward shock or a geometrical effect. In addition, we identify an additional component after the end of the prompt emission, which shapes the observed X-ray and optical light-curves differently, and which rules out a single overall emission component to explain the observed early-time emission

Year: 2014
DOI identifier: 10.1051/0004-6361
OAI identifier: oai:iris.unife.it:11392/2257814
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