Long γ-ray bursts (GRBs) are the most dramatic examples of massive stellar deaths, often associated with supernovae1. They release ultra-relativistic jets, which produce non-thermal emission through synchrotron radiation as they interact with the surrounding medium2. Here we report observations of the unusual GRB 101225A. Its γ-ray emission was exceptionally long-lived and was followed by a bright X-ray transient with a hot thermal component and an unusual optical counterpart. During the first 10 days, the optical emission evolved as an expanding, cooling black body, after which an additional component, consistent with a faint supernova, emerged. We estimate its redshift to be z = 0.33 by fitting the spectral-energy distribution and light curve of the optical emission with a GRB-supernova template. Deep optical observations may have revealed a faint, unresolved host galaxy. Our proposed progenitor is a merger of a helium star with a neutron star that underwent a common envelope phase, expelling its hydrogen envelope. The resulting explosion created a GRB-like jet which became thermalized by interacting with the dense, previously ejected material, thus creating the observed black body, until finally the emission from the supernova dominated. An alternative explanation is a minor body falling onto a neutron star in the Galaxy3.
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This Letter is based on observations collected at CAHA/Calar Alto, GTC/La Palma, the Liverpool Telescope at ORM/La Palma, the McDonald Observatory at the University of Texas at Austin, and Gemini-North and Keck on Hawaii. We thank J. S. Bloom for helping with the Keck observations. The Dark Cosmology Centre is funded by the DNRF. K.L.P., S.R.O. and M.D.P. acknowledge the support of the UK Space Agency. J.G., S.G. and P.K. are partially supported by MICINN. M.A.A. and P.M. are supported by an ERC starting grant. H.T.J. acknowledges support by a DFG grant. M.I., W.-K.P., C.C., J.L. and S.P. acknowledge support from CRI/NRF/MEST of Korea. A.M. acknowledges support from the Russian government. We thank A. J. Castro-Tirado for help in obtaining the data from the BTA 6-m telescope and comments on an early draft.
This file contains Supplementary Text and Data 1 - 8, Supplementary Figures 1-19 with legends, Supplementary Tables 1- 7 and additional references.
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Frontiers of Physics (2013)