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The nature of the late achromatic bump in GRB 120326A

Melandri, A, Virgili, FJ, Guidorzi, C, Bernardini, MG, Kobayashi, S, Mundell, CG, Gomboc, A, Dintinjana, B, Hentunen, V-P, Japelj, J, Kopač, D, Kuroda, D, Morgan, AN, Steele, IA, Quadri, U, Arici, G, Arnold, D, Girelli, R, Hanayama, H, Kawai, N , Mikuz, H, Nissinen, M, Salmi, T, Smith, RJ, Strabla, L, Tonincelli, M and Quadri, A (2014) The nature of the late achromatic bump in GRB 120326A. ASTRONOMY & ASTROPHYSICS, 572 (A55). ISSN 0004-6361

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The long ${\it Swift}$ gamma-ray burst GRB 120326A at redshift $z=1.798$ exhibited a multi-band light curve with a striking feature: a late-time, long-lasting achromatic rebrightening, rarely seen in such events. Peaking in optical and X-ray bands $\sim 35$ ks ($\sim 12.5$ ks in the GRB rest frame) after the 70-s GRB prompt burst, the feature brightens nearly two orders of magnitude above the underlying optical power-law decay. Modelling the multiwavelength light curves, we investigate possible causes of the rebrightening in the context of the standard fireball model. We exclude a range of scenarios for the origin of this feature: reverse-shock flash, late-time forward shock peak due to the passage of the maximal synchrotron frequency through the optical band, late central engine optical/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, that shapes the observed X-ray and optical light curves differently, ruling out a single overall emission component to explain the observed early time emission.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences, 0305 Organic Chemistry, 0306 Physical Chemistry (Incl. Structural)
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
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Date Deposited: 19 Nov 2015 09:35
Last Modified: 03 Sep 2021 20:46
DOI or ID number: 10.1051/0004-6361/201424338
URI: https://researchonline.ljmu.ac.uk/id/eprint/1493

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