A correlation between Hα trough depth and inclination in quiescent X-ray transients: evidence for a low-mass black hole in GRO J0422+32

We present a new method to derive binary inclinations in quiescent black hole (BH) X-ray transients (XRTs), based on the depth of the trough (T) from double-peaked H-alpha emission profiles arising in accretion discs. We find that the inclination angle (i) is linearly correlated with Tin phase-averaged spectra with sufficient orbital coverage (greater than or similar to 50 per cent) and spectral resolution, following i(deg) = 93.5 x T+ 23.7. The correlation is caused by a combination of line opacity and local broadening, where a leading (excess broadening) component scales with the deprojected velocity of the outer disc. Interestingly, such scaling allows to estimate the fundamental ratio M-1/P-orb by simply resolving the intrinsic width of the double-peak profile. We apply the T-i correlation to derive binary inclinations for GRO J0422+32 and Swift J1357-0933, two BH XRTs where strong flickering activity has hindered determining their values through ellipsoidal fits to photometric light curves. Remarkably, the inclination derived for GRO J0422+32 (i = 55.6 +/- 4.1 degrees) implies a BH mass of 2.7(-0.5)(+0.7) M-circle dot thus placing it within the gap that separates BHs from neutron stars. This result proves that low-mass BHs exist in nature and strongly suggests that the so-called 'mass gap' is mainly produced by low number statistics and possibly observational biases. On the other hand, we find that Swift J1357-0933 contains a 10.9(-1.6)(+1.7) M-circle dot BH, seen nearly edge on (i = 87.4(-5.6)(+2.6) deg). Such extreme inclination, however, should be treated with caution since it relies on extrapolating the T-i correlation beyond i greater than or similar to 75 degrees, where it has not yet been tested.

Automated summary

Researchers present a new method to derive binary inclinations in quiescent black hole X-ray transients based on the depth of the trough from double-peaked H-alpha emission profiles. They find that the inclination angle is linearly correlated with the trough depth and apply this correlation to derive inclinations for two black hole X-ray transients.