# The nature of the late achromatic bump in GRB 120326A

Melandri, A and Virgili, F and Guidorzi, C and Bernardini, MG and Kobayashi, S and Mundell, CG and Gomboc, A and Dintinjana, B and Hentunen, V and Japelj, J and Kopac, D and Kuroda, D and Morgan, AN and Steele, IA and Quadri, U and Arici, G and Arnold, D and Girelli, R and Hanayama, H and Kwai, N and Mikuz, H and Nissinen, M and Salmi, T and Smith, R and Strabla, L and Tonincelli, M and Quadri, A (2014) The nature of the late achromatic bump in GRB 120326A. Astronomy and Astrophysics, 572 (A55). ISSN 0365-0138

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.