T. Muñoz-Darias, A. de Ugarte Postigo, D. M. Russell, S. Guziy, J. Gorosabel, J. Casares, M. Armas Padilla, P. A. Charles, R. P. Fender, T. M. Belloni, F. Lewis, S. Motta, A. Castro-Tirado, C. G. Mundell, R. Sánchez-Ramírez, C. C. Thöne
We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745-26, starting only 19 minutes after the discovery of the source. We observe the system peaking at i' ~17.6 on day 6 (MJD 56192) to then decay at a rate of ~0.04 mag/day. We show that the optical peak occurs at least 3 days later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 magnitudes, which favours an orbital period < 21 h and a companion star with a spectral type later than ~ A0. Spectroscopic observations taken with the GTC-10.4 m telescope reveal a broad (FWHM ~ 1100 km/s), double-peaked H_alpha emission line from which we constrain the radial velocity semi-amplitude of the donor to be K_2 > 250 km/s. The breadth of the line and the observed optical and X-ray fluxes suggest that Swift J1745-26 is a new black hole candidate located closer than ~7 kpc.
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http://arxiv.org/abs/1303.6317
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