E. Berger, W. Fong, R. Chornock
We present Hubble Space Telescope optical and near-IR observations of the short-hard GRB 130603B (z=0.356) obtained 9.4 days post-burst. At the position of the burst we detect a red point source with m(F160W)=25.8+/-0.2 AB mag and m(F606W)>27.5 AB mag (3-sigma), corresponding to rest-frame absolute magnitudes of M_J -15.2 mag and M_B>-13.5 mag. A comparison to the early optical afterglow emission requires a decline rate of alpha_opt<-1.6 (F_nu t^alpha), consistent with the observed X-ray decline at about 1 hr to about 1 day. The observed red color of V-H>1.7 mag is also potentially consistent with the red optical colors of the afterglow at early time (F_nu nu^-1.6 in gri). Thus, an afterglow interpretation is feasible. Alternatively, the red color and faint absolute magnitude are due to emission from an r-process powered transient ("kilonova") produced by ejecta from the merger of an NS-NS or NS-BH binary, the most likely progenitors of short GRBs. In this scenario, the observed brightness implies an outflow with M_ej 0.01 Msun and v_ej 0.1c, in good agreement with the results of numerical merger simulations for roughly equal mass binary constituents (i.e., NS-NS). If true, the kilonova interpretation provides the strongest evidence to date that short GRBs are produced by compact object mergers, and places initial constraints on the ejected mass. Equally important, it demonstrates that gravitational wave sources detected by Advanced LIGO/Virgo will be accompanied by optical/near-IR counterparts with unusually red colors, detectable by existing and upcoming large wide-field facilities (e.g., Pan-STARRS, DECam, Subaru, LSST).
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http://arxiv.org/abs/1306.3960
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