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arXiv · 2207.01628

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

J. Casares·T. Muñoz-Darias·M. A. P. Torres·D. Mata Sanchez·C. T. Britt·M. Armas Padilla·A. Alvarez-Hernandez·V. A. Cuneo·J. I. Gonzalez Hernandez·F. Jimenez-Ibarra·P. G. Jonker·G. Panizo-Espinar·J. Sanchez-Sierras·I. V. Yanes-Rizo
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Abstract

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 Ha emission profiles arising in accretion discs. We find that the inclination angle (i) is linearly correlated with T in phase-averaged spectra with sufficient orbital coverage (>~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 de-projected velocity of the outer disc. Interestingly, such scaling allows to estimate the fundamental ratio M1/Porb 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) implies a BH mass of 2.7+0.7-0.5 Msun 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+1.7-1.6 Msun BH seen nearly edge on (i=87.4+2.6-5.6 deg). Such extreme inclination, however, should be treated with caution since it relies on extrapolating the T-i correlation beyond i>~75 deg, where it has not yet been tested.

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J. Casares, T. Muñoz-Darias, M. A. P. Torres, D. Mata Sanchez, C. T. Britt, M. Armas Padilla, A. Alvarez-Hernandez, V. A. Cuneo, J. I. Gonzalez Hernandez, F. Jimenez-Ibarra, P. G. Jonker, G. Panizo-Espinar, J. Sanchez-Sierras, I. V. Yanes-Rizo. 2022-07-19. A correlation between Ha trough depth and inclination in quiescent X-ray transients: evidence for a low-mass black hole in GRO J0422+32. https://doi.org/10.1093/mnras%2Fstac1881

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