The nature of the late achromatic bump in GRB 120326A

• Published in: arXiv.org 2014-09
• Main Authors: Melandri, A, Virgili, F J, Guidorzi, C, Bernardini, M G, Kobayashi, S, Mundell, C G, Gomboc, A, Dintinjana, B, V -P Hentunen, Japelj, J, Kopač, D, Kuroda, D, Morgan, A N, Steele, I A, Quadri, U, Arici, G, Arnold, D, Girelli, R, Hanayama, H, Kawai, N, Mikuž, H, Nissinen, M, Salmi, T, Smith, R J, Strabla, L, Tonincelli, M, Quadri, A
• Format: Article
• Language: English
• Subjects: Emission; Gamma ray bursts; Gamma rays; Light curve; Microlenses; Red shift
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1409.6467

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.