Swift J1357.2-0933: the faintest black hole?

Swift J1357.2−0933 is a confirmed very faint black hole X-ray transient and has a short estimated orbital period of 2.8 h. We observed Swift J1357.2−0933 for ∼50 ks with XMM-Newton in 2013 July during its quiescent state. The source is clearly detected at a 0.5-10 keV unabsorbed flux of ∼3 × 10−15 erg cm−2 s−1. If the source is located at a distance of 1.5 kpc (as suggested in the literature), this would imply a luminosity of ∼8 × 1029 erg s−1, making it the faintest detected quiescent black hole low-mass X-ray binary. This would also imply that there is no indication of a reversal in the quiescence X-ray luminosity versus orbital period diagram down to 2.8 h, as has been predicted theoretically and recently supported by the detection of the 2.4 h orbital period black hole MAXI J1659−152 at a 0.5-10 keV X-ray luminosity of ∼1.2 × 1031 erg s−1. However, there is considerable uncertainty in the distance of Swift J1357.2−0933 and it may be as distant as 6.3 kpc. In this case, its quiescent luminosity would be LX ∼ 1.3 × 1031 erg s−1, i.e. similar to MAXI J1659−152 and hence, it would support the existence of such a bifurcation period. We also detected the source in optical at r′ ∼ 22.3 mag with the Liverpool telescope, simultaneously to our X-ray observation. The X-ray/optical luminosity ratio of Swift J1357.2−0933 agrees with the expected value for a black hole at this range of quiescent X-ray luminosities.