The nature of the late achromatic bump in GRB 120326A

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. KawaiH. Mikuž, M. Nissinen, T. Salmi, R. J. Smith, L. Strabla, M. Tonincelli, A. Quadri

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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.
Original languageEnglish
Article numberA55
Number of pages7
JournalAstronomy & Astrophysics
Publication statusPublished - 23 Sept 2014


  • astro-ph.HE


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