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VLT/X-shooter spectroscopy of the candidate black-hole X-ray binary MAXI J1659-152 in outburst

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