VERY LARGE TELESCOPE/X-SHOOTER SPECTROSCOPY OF THE CANDIDATE BLACK HOLE X-RAY BINARY MAXI J1659-152 IN OUTBURST

Abstract

We present the optical to near-infrared spectrum of MAXI J1659-152, during\nthe onset of its 2010 X-ray outburst. The spectrum was obtained with X-shooter\non the ESO - Very Large Telescope (VLT) early in the outburst simultaneous with\nhigh quality observations at both shorter and longer wavelengths. At the time\nof the observations, the source was in the low-hard state. The X-shooter\nspectrum includes many broad (~2000 km/s), double-peaked emission profiles of\nH, HeI, HeII, characteristic signatures of a low-mass X-ray binary during\noutburst. We detect no spectral signatures of the low-mass companion star. The\nstrength of the diffuse interstellar bands results in a lower limit to the\ntotal interstellar extinction of Av ~ 0.4 mag. Using the neutral hydrogen\ncolumn density obtained from the X-ray spectrum we estimate Av ~1 mag. The\nradial-velocity structure of the interstellar NaI D and CaII H & K lines\nresults in a lower limit to the distance of ~ 4 +/- 1 kpc, consistent with\nprevious estimates. With this distance and Av, the dereddened spectral energy\ndistribution represents a flat disk spectrum. The two subsequent 10 minute\nX-shooter spectra show significant variability in the red wing of the\nemission-line profiles, indicating a global change in the density structure of\nthe disk, though on a timescale much shorter than the typical viscous timescale\nof the disk.\n