The face-on disk of MAXI J1836-194

Abstract: We present Very Large Telescope optical spectra of the black hole candidate X-ray binary MAXI J1836-194 at the onset of its 2011 outburst. Although the spectrum was taken at the beginning of the outburst and contains a significant contribution from the optically-thin synchrotron emission that originates in the radio jet, we find that the accretion disk was already large and bright. Single-peaked, narrow H$\alpha$ and He II $\lambda$4686 lines imply the most face-on accretion disk observed in a black hole low-mass X-ray binary to date, with an inclination angle between 4$^{\circ}$ and 15$^{\circ}$, assuming a black hole mass of between 5 M$\odot$ and 12 M$\odot$, for distances of between 4 and 10 kpc. We use New Technology Telescope observations of the system in quiescence to place strong upper limits on the mass and radius of the donor star and the orbital period. The donor is a main sequence star with a mass < 0.65 M${\odot}$ and a radius < 0.59 R${\odot}$ with an orbital period of < 4.9 hours. From those values and Roche lobe geometry constraints we find that the compact object must be >1.9 M${\odot}$ if the system is located 4 kpc away and >7.0 M${\odot}$ at 10 kpc.