A 2-hr binary period for the black hole transient MAXI J0637-430

We revisit various sets of published results from X-ray and optical studies of the Galactic black hole (BH) candidate MAXI J0637-430, which went into outburst in 2019. Combining the previously reported values of peak outburst luminosity, best-fitting radii of inner and outer accretion disc, viewing angle, exponential decay time-scale, and peak-to-peak separation of the He II lambda 4686 disc emission line, we improve the constraints on the system parameters. We estimate a heliocentric distance d (8.7 +/- 2.3) kpc, a projected Galactocentric distance R approximate to (13.2 +/- 1.8) kpc and a height vertical bar z vertical bar (3.1 +/- 0.8) kpc from the Galactic plane. It is the currently known Milky Way BH candidate located farthest from the Galactic Centre. We infer a BH mass M-1 approximate to (5.1 +/- 1.6)M-circle dot, a spin parameter a(*) less than or similar to 0.25, a donor star mass M2 approximate to (0.25 +/- 0.07)M-circle dot, a peak Eddington ratio lambda approximate to 0.17 +/- 0.11 and a binary period P-orb approximate to 2.2(-0.6)(+0.8) hr. This is the shortest period measured or estimated so far for any Galactic BH X-ray binary. If the donor star is a main-sequence dwarf, such a period corresponds to the evolutionary stage where orbital shrinking is driven by gravitational radiation and the star has regained contact with its Roche lobe (low end of the period gap). The three Galactic BHs with the shortest period (less than or similar to 3 hr) are also those with the highest vertical distance from the Galactic plane (greater than or similar to 2 kpc). This is probably because binaries with higher binding energies can survive faster natal kicks.

Automated summary

By revisiting the published results of X-ray and optical studies on the black hole candidate MAXI J0637-430, we have improved the understanding of its system parameters. This black hole is the farthest known from the Galactic Center and has the shortest period among all Galactic black hole X-ray binaries.