Multiwavelength spectroscopy of the black hole candidate MAXI J1813-095 during its discovery outburst

MAXI J1813-095 is an X-ray transient discovered during an outburst in 2018. We report on X-ray and optical observations obtained during this event, which indicate that the source is a new low-mass X-ray binary. The outburst lasted ∼70 d and peaked at L_X(0.5-10 keV) ∼ 7.6 × 10³⁶ erg s^-1, assuming a distance of 8 kpc. Swift/XRT follow-up covering the whole activity period shows that the X-ray emission was always dominated by a hard power-law component with a photon index in the range of 1.4-1.7. These values are consistent with MAXI J1813-095 being in the hard state, in agreement with the ∼30 per cent fractional root-mean-square amplitude of the fast variability (0.1-50 Hz) inferred from the only XMM-Newton observation available. The X-ray spectra are well described by a Comptonization emission component plus a soft, thermal component (kT ∼ 0.2 keV), which barely contributes to the total flux (≲8 per cent). The Comptonization y-parameter (∼1.5), together with the low temperature and small contribution of the soft component supports a black hole accretor. We also performed optical spectroscopy using the Very Large Telescope and Gran Telescopio Canarias telescopes during outburst and quiescence, respectively. In both cases, the spectrum lacks emission lines typical of X-ray binaries in outburst. Instead, we detect the Ca II triplet and H α in absorption. The absence of velocity shifts between the two epochs, as well as the evolution of the H α equivalent width, strongly suggest that the optical emission is dominated by an interloper, likely a G-K star. This favours a distance ≳3 kpc for the X-ray transient.