Long [gamma]-ray bursts (GRBs) are the most dramatic examples of massive stellar deaths, often associated with supernovae (1). They release ultra-relativistic jets, which produce non-thermal emission through synchrotron radiation as they interact with the surrounding medium (2). Here we report observations of the unusual GRB 101225A. Its [gamma]-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 Galaxy (3).
On 25 December 2010, at 18:37:45 UT, the Burst Alert Telescope (BAT, 15-350 keV) on board the Swift satellite detected GRB 101225A, one of the longest GRBs ever observed by Swift (4) (see Supplementary Information); this GRB had [T.sub.90] > 2,000 s ([T.sub.90] is the time in which 90% of the [gamma]-ray energy is released (5)). A bright X-ray afterglow was detected for two days, and a counterpart in the ultraviolet, optical and infrared bands could be observed from 0.38 hours to two months after the event (see Supplementary Information). No counterpart was detected at radio frequencies (6,7).
The most surprising feature of GRB 101225A is the spectral energy distribution (SED) of its afterglow. The X-ray SED is best modelled with a combination of an absorbed power-law and a black body. The ultraviolet/optical/near-infrared (UVOIR) SED (see Fig. 1) can be fitted with a cooling and expanding black-body model until 10 days after the burst (see Supplementary Information), after which we observe an additional spectral component accompanied by a flattening of the light curve (Fig. 2). This behaviour differs from a normal GRB where the SED follows a power law owing to synchrotron emission created in shocks when the jet hits the interstellar medium (see, for example, ref. 2).
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
An optical spectrum taken two nights after the burst does not show any spectral lines (see Supplementary Information). We fitted the SED and light curve with the template of SN 1998bw, a type Ic supernova associated with GRB 980425 (8), and obtained a redshift of z = 0.33 (see Supplementary Information). At this distance, the supernova has an absolute peak magnitude of only [M.sub.V,abs] = -16.7 mag, which makes it the faintest supernova associated with a long GRB (9,10). In contrast, the [gamma]-ray isotropic-equivalent energy release at z = 0.33 is >1.4 x [10.sup.51] erg, typical of other long GRBs but more luminous than most other low-redshift GRBs associated with supernovae (11). We detect a possible host galaxy in g' and r' bands with the OSIRIS instrument on GTC (Gran Telescopio Canarias) at 6 months after the burst with an absolute magnitude of only [M.sub.g,abs] = -13.7 mag ~2 mag fainter than any other GRB host (12). Although its blue colour matches that of a star-forming galaxy, our observations do not allow us to resolve it as an extended source.
At z = 0.33, the X-ray black body has a radius of ~2 x [10.sup.11] cm (~3 solar radii) and a temperature of ~1 keV ([10.sup.7] K) at 0.07 d with little temporal evolution. Such a thermal component, attributed to the shockbreakout from the star, has also been observed for XRF 060218 (13) , XRF 100316D/SN 2010dh (14) and GRB 09061815, all nearby GRBs associated with type Ic supernovae (14,16,17,18), with similar temperatures but larger radii. The UVOIR black body starts with a radius of 2 x [10.sup.14] cm (~ 13 AU) and a temperature of 8.5 x [10.sup.4] K at similar times and evolves considerably over the next 10 days, reaching a radius of 7 X [10.sup.14] cm and temperature of 5,000 K. The evolution of the two black-body components suggests that they must stem from different processes and regions (see Supplementary Information).
An appealing model is a helium star-neutron star merger with a common envelope phase, a model that has been proposed earlier as a possible progenitor for GRBs (19-21). In this scenario, a binary system consisting of two massive stars survives the collapse of the more massive component to a neutron star. When the second star leaves the main sequence and expands, it engulfs the neutron star, leading to a common-envelope phase and the ejection of the hydrogen envelope and part of the helium core as the remnant spirals into the centre of the second star. When the neutron star reaches the centre, angular momentum forms a disk around the remnant of the merger, allowing for the formation of a GRB-like jet. This remnant might be a magnetar whose prolonged activity can explain the very long duration of the
The interaction of this ultra-relativistic, well-collimated jet with the previously ejected common-envelope material can explain both the X-ray and UVOIR emission components. Estimating that the in-spiral takes 5 orbits or 1.5 yr and material is ejected at escape velocity, the outer ejecta are at a distance of a few times [10.sub.14] cm at the time of the merger, consistent with the radius of the UVOIR black body. We assume that the ejecta form a broad torus with a narrow, low-density funnel along the rotation axis of the system that permits the passage of the [gamma]-radiation generated in the jet. Most of the jet hits the inner boundary of the common-envelope ejecta and only a small fraction of it propagates through the funnel. The X-ray emission is produced by shocks created by the interaction of the jet with the inner boundary of the common-envelope shell. As the jet passes through the funnel, it decelerates owing to the increased baryon load and shear with the funnel walls so that a regular afterglow signature is suppressed. When the now mildly relativistic, mass-loaded jet breaks out of the common-envelope ejecta, it produces the UVOIR emission in the first 10 d. As the supernova shock expands beyond the common-envelope shell, we observe a small bump in the light curve at ~30 d. This helium star/neutron star merger scenario naturally assumes the production of a relatively small amount of radioactive nickel, leading to a weak supernova (for a detailed description of the different processes, see Supplementary Information).
A similar scenario might explain another, previously detected, event, XRF 060218 (13), which showed a thermal component both in X-rays and at optical wavelengths (see Supplementary Information), albeit with a different progenitor system producing a brighter supernova and a fainter GRB. On the other hand, a class of GRBs exist that show a thermal component in X-rays, but have a classical afterglow with a power-law SED, such as GRB 090618 (15). Finally, SN 2008D, a type lb supernova in NGC 2770 (22) showing X-ray emission, had an early thermal component in the optical emission (see Supplementary Information), attributed to the shock breakout and independent of the supernova emission itself. GRB 101225A might hence be, together with XRF 060218, a member of a newly defined class of 'black-body-dominated', supernova-associated, long-duration GRBs, which arise in very dense environments that are created by the progenitor systems themselves; this dense environment thermalizes the high-energy output from the collapsing star. The non-relativistic, uncollimated emission in this scenario makes it difficult to detect such events at higher redshifts. This makes GRB 101225A a fortunate case that allows us to derive conclusions about the progenitor system and its environment from a new variety of massive stellar death, which had so far been only proposed to exist theoretically.
Acknowledgements 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 thesupportof the UK Space Agency. J.G., S.G. and P.K. are partially supported by MICINN. M.A.A. and P.M. are supported byan 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.
Received 10 April; accepted 3 October 2011.
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Supplementary Information is linked to the online version of the paper at www.nature.com/nature.
Author Contributions C.C.T. did the overall management of the observations and modelling, the analysis of the spectra and wrote most of the manuscript. A.d.U.P. did the UVOIR black-body modelling, supernova template fitting, most of the optical/infrared photometry and lead the GTC observations. C.L.F. suggested and investigated the progenitor system. K.L.P. did the X-ray analysis, J.G. worked on the supernova templates and the photometric calibrations for the optical/infrared data. M.A.A. did the modelling of the UVOIR black body and X-ray emission from numerical simulations. D.A.P. contributed to the observation and analysis of the late Gemini and Keck data. C.K. investigated possible progenitor models. H.TJ., P.M. and A.L. contributed to the theoretical modelling. J.L.R., H.K., J.C., S.R.O., S.T.H., M.H.S., M.D.P. and E.S. did the analysis of the Swift data. M.I., W.-K.P., C.C., H.J., J.L. and S.P. contributed the McDonald 2.1-m data, A.M. the late BTA 6-m data, K.B. and I.P. the late Keck spectrum. D.A.K. did the comparison of supernova stretching factors and luminosities. S.G. and L.H.G. helped with the optical photometry. H.K. and T.M.-D. investigated alternative interpretations of the event, and P.K. assisted with the manuscript.
Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of this article at www.nature.com/nature. Correspondence and requests for materials should be addressed to C.C.T. (firstname.lastname@example.org).
C. C. Thone [1,2], A. de Ugarte Postigo , C. L. Fryer , K. L. Page , J. Gorosabel , M. A. Aloy , D. A. Perley , C. Kouveliotou , H. T. Janka , P. Mimica , J. L. Racusin , H. Krimm [10,11,12], J. Cummings , S. R. Oates , S. T. Holland [10,11,12], M. H. Siegel ,M. De Pasquale , E. Sonbas [10,11,15],M. Im , W.-K. Park , D. A. Kann , S. Guziy [1,18], L. Hernandez Garcia , A. Llorente , K. Bundy , C. Choi , H. Jeong , H. Korhonen [21,22], P. Kubanek [1,23], J. Lim , A. Moskvitin , T. Munoz-Darias , S. Pak  & I. Parrish 
 IAA-CSIC, Glorieta de la Astronomia s/n, 18008 Granada, Spain.  Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark.  Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark.  Los Alamos National Laboratory, MS D409, CCS-2, Los Alamos, New Mexico 87545, USA.  Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK.  Departamento de Astronomia y Astrofisica, Universidad de Valencia, 46100 Burjassot, Spain.  Astronomy Department, UC Berkeley, 601 Campbell Hall, Berkeley, California 94720, USA.  Science and Technology Office, ZP12, NASA/Marshall Space Flight Center, Huntsville, Alabama 35812, USA.  Max-PlanckInstitut fur Astrophysik, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany.  NASA, Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.  Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, Maryland 21044-3432, USA.  Center for Research and Exploration in Space Scienceand Technology (CRESST), 10211 Wincopin Circle, Suite 500, Columbia, Maryland 21044-3432, USA.  Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK.  Department of Astronomy and Astrophysics, Pennsylvania State University, 104 Davey Laboratory, University Park, Pennsylvania 16802, USA.  University of Adiyaman, Department of Physics, 02040 Adiyaman, Turkey.  Center for the Exploration of the Origin of the Universe, Department of Physics and Astronomy, Seoul National University, 56-1 San, Shillim-dong, Kwanak-gu, Seoul, South Korea.  Thuringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany.  Nikolaev National University, Nikolska 24, Nikolaev, 54030, Ukraine.  Herschel Science Operations Centre, INSA, ESAC, Villafranca del Castillo, PO Box 50727, I-28080 Madrid, Spain.  School of Space Research, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea.  Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Vaisalantie 20,21500 Piikkio, Finland.  Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30,2100 Copenhagen, Denmark.  Institute of Physics, Na Slovance 2, 180 00, Prague 8, Czech Republic.  Department of Astronomy and Space Science, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea.  Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz 369167, Russia.  INAF- Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy.