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

Melandri, A, Virgili, F, Guidorzi, C, Bernardini, MG, Kobayashi, S, Mundell, CG, Gomboc, A, Dintinjana, B, Hentunen, V, Japelj, J, Kopac, D, Kuroda, D, Morgan, AN, Steele, IA, Quadri, U, Arici, G, Arnold, D, Girelli, R, Hanayama, H, Kwai, N , Mikuz, H, Nissinen, M, Salmi, T, Smith, R, Strabla, L, 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.