A correlation between H α trough depth and inclination in quiescent X-ray transients: evidence for a low-mass black hole in GRO J0422+32. Issue 2 (7th September 2022)

• Record Type: Journal Article
• Authors: Casares, J; Muñoz-Darias, T; Torres, M A P; Mata Sánchez, D; Britt, C T; Armas Padilla, M; Álvarez-Hernández, A; Cúneo, V A; González Hernández, J I; Jiménez-Ibarra, F; Jonker, P G; Panizo-Espinar, G; Sánchez-Sierras, J; Yanes-Rizo, I V
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 516:Issue 2(2022)
• Page Start: 2023
• Page End: 2037
• Publication Date: 2022-09-07
• Subjects: accretion, accretion discs -- stars: individual: GRO J0422+32, Swift J1357−0933 -- X-rays: binaries -- stars: black holes, dwarf novae, neutron; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac1881
• Languages: English
• ISSNs: 0035-8711

Abstract

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 H α 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 × 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 ○ ) implies a BH mass of $2.7^{+0.7}_{-0.5}$ M⊙ 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 $10.9^{+1.7}_{-1.6}$ M⊙ 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 ○, where it has not yet been tested.