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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, whichindicate that the source is a new low-mass X-ray binary. The outburst lasted~70 d and peaked at Lx(0.5-10keV)~7.6 x 10^36 erg s-1, assuming a distance of 8kpc. Swift/XRT follow-up covering the whole activity period shows that theX-ray emission was always dominated by a hard power-law component with a photonindex in the range of 1.4-1.7. These values are consistent with MAXI J1813-095being in the hard state, in agreement with the ~30 per cent fractionalroot-mean-square amplitude of the fast variability (0.1-50 Hz) inferred fromthe only XMM-Newton observation available. The X-ray spectra are well describedby a Comptonization emission component plus a soft, thermal component (kT ~0.2keV), which barely contributes to the total flux (<8 per cent). TheComptonization y-parameter (~1.5), together with the low temperature and smallcontribution of the soft component supports a black hole accretor. We alsoperformed optical spectroscopy using the VLT and GTC telescopes during outburstand quiescence, respectively. In both cases the spectrum lack emission linestypical of X-ray binaries in outburst. Instead, we detect the Ca II triplet andH_alpha in absorption. The absence of velocity shifts between the two epochs,as well as the evolution of the H_alpha equivalent width, strongly suggest thatthe optical emission is dominated by an interloper, likely a G-K star. Thisfavours a distance >3 kpc for the X-ray transient.

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