Dynamical Confirmation of a Black Hole in MAXI J1820+070
Abstract
We present time-resolved 10.4 m Gran Telescopio Canarias and 4.2 m William Herschel Telescope intermediate resolution spectroscopy of the X-ray transient MAXI J1820+070 (=ASASSN-18ey) obtained during its decline to the quiescent state. Cross-correlation of the 21 individual spectra against late-type templates reveals a sinusoidal velocity modulation with a period of 0.68549 ± 0.00001 day and semi-amplitude of 417.7 ± 3.9 km s-1. We derive a mass function f(M) = 5.18 ± 0.15 M ⊙, dynamically confirming the black hole (BH) nature of the compact object. Our analysis of the stellar absorption features supports a K3-5 spectral classification for the donor star, which contributes ≈20% of the total flux at 5200-6800 Å. The photometric 0.703 ± 0.003 day periodicity observed during outburst is 2.6% longer than the orbital period supporting the presence of a superhump modulation in the outburst light curves. In line with this interpretation, we constrain the binary mass ratio to be q ≃ 0.12. In addition, we observe a sharp increase in the Hα emission line equivalent width during inferior conjunction of the donor star that we interpret as a grazing eclipse of the accretion disk and allows us to constrain the binary inclination to i ≳ 69°. On the other hand, the absence of X-ray eclipses during outburst imply i ≲ 77°. These inclination limits, together with our dynamical solution, lead to a BH mass in the range 7-8 M ⊙. We also measure a systemic velocity γ = -21.6 ± 2.3 km s-1, which, combined with the Gaia second data release proper motion and parallax, implies a large peculiar velocity ∼100 km s-1.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 2019
- DOI:
- 10.3847/2041-8213/ab39df
- arXiv:
- arXiv:1907.00938
- Bibcode:
- 2019ApJ...882L..21T
- Keywords:
-
- Astrophysical black holes;
- Low-mass X-ray binary stars;
- Stellar accretion disks;
- X-ray binary stars;
- 98;
- 939;
- 1579;
- 1811;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 7 pages, 3 figures, accepted for publcation in ApJ Letters