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Extremely fast orbital decay of the black hole X-ray binary Nova Muscae 1991

Author/s: J. I. González Hernández, L. Suárez-Andrés, R. Rebolo, J. Casares

Reference: 2016 MNRAS 465 L15 | Link

Top panel: orbital phase shift at the time of the inferior conjunction (orbital phase 0), <em>T<sub>n</sub></em>, of the secondary star in the black hole X-ray binary Nova Muscae 1991 versus the orbital cycle number, <em>n</em>, folded on the best-fitting parabolic fit. The error bars give the uncertainties <em>δT<sub>n</sub></em>. Blue filled circles are previous literature spectroscopic determinations, and the red filled triangle is the new VLT/X-Shooter spectroscopic P-fix MCMC determination. The small panels show two Monte Carlo (MC) simulations of 10 000 realizations taking into account the uncertainties of each <em>T<sub>n</sub></em> point: (i) with the observed data set, i.e. using <em>T<sub>n</sub></em> values as a centre of the MC distributions (left small panel), and (ii) using the points on the parabolic fit (right small panel). Bottom panel: residuals of the fit of the <em>T<sub>n</sub></em> values versus the cycle number <em>n</em>.
Top panel: orbital phase shift at the time of the inferior conjunction (orbital phase 0), Tn, of the secondary star in the black hole X-ray binary Nova Muscae 1991 versus the orbital cycle number, n, folded on the best-fitting parabolic fit. The error bars give the uncertainties δTn. Blue filled circles are previous literature spectroscopic determinations, and the red filled triangle is the new VLT/X-Shooter spectroscopic P-fix MCMC determination. The small panels show two Monte Carlo (MC) simulations of 10 000 realizations taking into account the uncertainties of each Tn point: (i) with the observed data set, i.e. using Tn values as a centre of the MC distributions (left small panel), and (ii) using the points on the parabolic fit (right small panel). Bottom panel: residuals of the fit of the Tn values versus the cycle number n.

We present new medium-resolution spectroscopic observations of the black hole X-ray binary Nova Muscae 1991 taken with X-Shooter spectrograph installed at the 8.2-m VLT telescope. These observations allow us to measure the time of inferior conjunction of the secondary star with the black hole in this system that, together with previous measurements, yield an orbital period decay of (dP/dt)= −20.7±12.7 ms yr−1 (−24.5 ± 15.1 μs per orbital cycle). This is significantly faster than those previously measured in the other black hole X-ray binaries A0620-00 and XTE J1118+480. No standard black hole X-ray binary evolutionary model is able to explain this extremely fast orbital decay. At this rate, the secondary star would reach the event horizon (as given by the Schwarzschild radius of about 32 km) in roughly 2.7 Myr. This result has dramatic implications on the evolution and lifetime of black hole X-ray binaries.

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