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An unusually brilliant transient in the galaxy M85


Historically, variable and transient sources have both surprised astronomers and provided new views of the heavens. Here we report the discovery of an optical transient in the outskirts of the lenticular galaxy Messier 85 in the Virgo cluster. With a peak absolute R magnitude of -12, this event is distinctly brighter than novae, but fainter than type Ia supernovae (which are expected in a population of old stars in lenticular galaxies). Archival images of the field do not show a luminous star at that position with an upper limit in the g filter of about -4.1 mag, so it is unlikely to be a giant eruption from a luminous blue variable star. Over a two-month period, the transient source emitted radiation energy of almost 1047 erg and subsequently faded in the optical sky. It is similar to, but six times more luminous at peak than, an enigmatic transient in the galaxy M31 (ref. 1). A possible origin of M85 OT2006-1 is a stellar merger. If so, searches for similar events in nearby galaxies will not only allow study of the physics of hyper-Eddington sources, but also probe an important phase in the evolution of stellar binary systems.

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Figure 1: Optical images of the field around M85 OT2006-1 obtained at two epochs.
Figure 2: Temporal evolution of M85 OT2006-1.
Figure 3: Optical spectra of M85 OT2006-1.
Figure 4: Phase space of cosmic explosive (supernovae) and eruptive (novae and LBVs) transients.

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We thank D. Frail for discussion and constructive criticism. We also thank the astronomers who maintain the NED database at IPAC and the data archives of the Hubble Space Telescope, the Spitzer Space Telescope and the Chandra X-ray Telescope. Our work has been funded in part by NASA, NSF, the Sylvia and Jim Katz Foundation and the TABASGO Foundation.

Author Contributions S.R.K. recognized the importance of a sub-luminous ‘supernova candidate’ in M85 (E.O.O. and A.M.S. both separately alerted him to it). The discovery was reported by W.L. and A.V.F. The paper was written mainly by S.R.K. and A.R. E.O.O. and A.R. contributed to the analysis of spectra and the summary figure. Much of the photometry came from the Palomar 60-inch automated telescope. The Palomar 60-inch project is a result of the effort of S.B.C., D.B.F., A.G.-Y., D. S.M. and A.M.S. P.L.C., E.E., J.K. and D.B.S. contributed near-infrared measurements.

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Correspondence to S. R. Kulkarni.

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Supplementary information

Supplementary Table

This file contains Supplementary Table 1. This table provides the optical and near-infrared photometry for the unusually brilliant transient M85OT2006-1 from discovery to approximately 110 days post-eruption. It includes the facility/telescope, photometric band, time (in Julian Date – 2453000), brightness (in magnitudes) and uncertainty. (PDF 223 kb)

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Kulkarni, S., Ofek, E., Rau, A. et al. An unusually brilliant transient in the galaxy M85. Nature 447, 458–460 (2007).

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