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

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