Very large telescope/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 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 (∼2000kms^-1), double-peaked emission profiles of H, He I, and He II, 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 A_V ≃ 0.4mag. Using the neutral hydrogen column density obtained from the X-ray spectrum we estimate A_V ≃ 1mag. The radial velocity structure of the interstellar Na I D and Ca II H&K lines results in a lower limit to the distance of ∼4± 1kpc, consistent with previous estimates. With this distance and A_V, the dereddened spectral energy distribution represents a flat disk spectrum. The two 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.