The nature of the late achromatic bump in GRB 120326A

¹ INAF – Osservatorio Astronomico Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
e-mail: andrea.melandri@brera.inaf.it
² ARI – Liverpool John Moores University, IC2 Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK
³ Dipartimento di Fisica e Scienza della Terra, via Saragat 1, 44122 Ferrara, Italy
⁴ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
⁵ Črni Vrh Observatory, Predgriže 29A, 5274 Črni Vrh nad Idrijo, Slovenia
⁶ Taurus Hill Observatory, Härkämäentie 88, 79480 Kangaslampi, Finland
⁷ Okayama Astrophysical Observatory – NAOJ, 3037-5 Honjo, Kamogata, Asakuchi, 719-0232 Okayama, Japan
⁸ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁹ Osservatorio Astronomico Bassano Bresciano, via San Michele 4, 25020 Bassano Bresciano ( BS), Italy
¹⁰ Osservatorio Astronomico di Cima Rest, via Rest, 25080 Magasa ( BS), Italy
¹¹ Ishigakijima Astronomical Observatory – NAOJ, 1024-1, Arakawa, Ishigaki, 907-0024 Okinawa, Japan
¹² Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551 Tokyo, Japan

Received: 4 June 2014
Accepted: 22 September 2014

Abstract

The long 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 that is rarely seen in such events. Peaking in optical and X-ray bands ~35 ks (~12.5 ks in the GRB rest frame) after the 70 s GRB prompt burst, the feature brightened nearly two orders of magnitude above the underlying optical power-law decay. By modelling the multi-wavelength light-curves, we investigated possible causes of the rebrightening in the context of the standard fireball model. We excluded a range of scenarios for the origin of this feature: reverse-shock flash, late-time forward-shock peak caused by the passage of the brightest synchrotron frequency through the optical band, late central engine optical or 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, which shapes the observed X-ray and optical light-curves differently, and which rules out a single overall emission component to explain the observed early-time emission.