Evidence for energy injection and a fine-tuned central engine at optical wavelengths in GRB 070419A |
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| Authors: | A Melandri C Guidorzi S Kobayashi D Bersier C G Mundell P Milne A Pozanenko W Li A V Filippenko Y Urata M Ibrahimov I A Steele A Gomboc R J Smith N R Tanvir E Rol K Huang |
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| Institution: | Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD;Dipartimento di Fisica, Universitàdi Ferrara, via Saragat 1, I-44100 Ferrara, Italy;INAF-Osservatorio Astronomico di Brera, via Bianchi 46, I-23807 Merate (LC), Italy;Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA;Space Research Insitute (IKI), 84/32 Profoyuznaya Str, Moscow 117997, Russia;Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA;Department of Physics, Saitama University, Saitama 338-8570, Japan;Ulugh Beg Astronomical Institute, Tashkent 700052, Uzbekistan;Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia;Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH;Academia Sinica Institute of Astronomy and Astrophysics, Taipei 106, Taiwan |
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| Abstract: | We present a comprehensive multiwavelength temporal and spectral analysis of the 'fast rise exponential decay' GRB 070419A. The early-time emission in the γ-ray and X-ray bands can be explained by a central engine active for at least 250 s, while at late times the X-ray light curve displays a simple power-law decay. In contrast, the observed behaviour in the optical band is complex (from 102 up to 106 s). We investigate the light-curve behaviour in the context of the standard forward/reverse shock model; associating the peak in the optical light curve at ～450 s with the fireball deceleration time results in a Lorenz factor Γ≈ 350 at this time. In contrast, the shallow optical decay between 450 and 1500 s remains problematic, requiring a reverse shock component whose typical frequency is above the optical band at the optical peak time for it to be explained within the standard model. This predicts an increasing flux density for the forward shock component until t ～ 4 × 106 s , inconsistent with the observed decay of the optical emission from t ～ 104 s . A highly magnetized fireball is also ruled out due to unrealistic microphysic parameters and predicted light-curve behaviour that is not observed. We conclude that a long-lived central engine with a finely tuned energy injection rate and a sudden cessation of the injection is required to create the observed light curves, consistent with the same conditions that are invoked to explain the plateau phase of canonical X-ray light curves of γ-ray bursts. |
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| Keywords: | gamma-rays: bursts |
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