Happy birthday Swift: Ultra-long GRB 141121A and its broadband afterglow

Cucchiara, A. and Veres, Péter and Corsi, A. and Cenko, S. B. and Perley, D. A. and Lien, A. and Marshall, F. E. and Pagani, C. and Toy, V. L. and Capone, J. I. and Frail, D. A. and Horesh, A. and Modjaz, M. and Butler, N. R. and Littlejohns, O. M. and Watson, A. M. and Kutyrev, A. S. and Lee, W. H. and Richer, M. G. and Klein, C. R. and Fox, O. D. and Prochaska, J. X. and Bloom, J. S. and Troja, E. and Ramirez-Ruiz, E. and De Diego, J. A. and Georgiev, L. and González, J. and Román-Zñiga, C. G. and Gehrels, N. (2015) Happy birthday Swift: Ultra-long GRB 141121A and its broadband afterglow. ASTROPHYSICAL JOURNAL, 812 (2). ISSN 1538-4357

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Abstract

We present our extensive observational campaign on the Swift-discovered GRB 141121A, al- most ten years after its launch. Our observations covers radio through X-rays, and extends for more than 30 days after discovery. The prompt phase of GRB 141121A lasted 1410 s and, at the derived redshift of z = 1.469, the isotropic energy is Eγ,iso = 8.0 × 1052 erg. Due to the long prompt duration, GRB 141121A falls into the recently discovered class of UL-GRBs. Peculiar features of this burst are a flat early-time optical light curve and a radio-to-X-ray rebrightening around 3 days after the burst. The latter is followed by a steep optical-to-X-ray decay and a much shallower radio fading. We analyze GRB 141121A in the context of the standard forward-reverse shock (FS,RS) scenario and we disentangle the FS and RS contributions. Finally, we comment on the puzzling early-time (t . 3 d) behavior of GRB 141121A, and suggest that its interpretation may require a two-component jet model. Overall, our analysis confirms that the class of UL-GRBs represents our best opportunity to firmly establish the prominent emission mechanisms in action during powerful GRB explosions, and future missions (like SVOM, XTiDE, or ISS-Lobster) will provide many more of such objects.

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