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Fast orbital decays of black hole X-ray binaries: XTE J1118+480 and A0620–00.

Author/s: Jonay I. González Hernández, Rafael Rebolo and Jorge Casares

Reference: 2013, Jonay I. González Hernández et al. MNRAS letters | Link

Fig. 1: <em>Top panel</em>: 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 BHXB XTE J1118+480 versus the orbital cycle number, n, folded on the best-fit parabolic fit. The error bars give the observational uncertainties. Green filled circles are spectroscopic determinations, the blue filled triangle is a photometric measurement, red filled diamonds, and the new violet square are GTC/OSIRIS spectroscopic determinations. The small panels show two MonteCarlo (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 center of the MC distributions (left small panel), and (ii) using the points on the parabolic fit (rigth small panel). <em>Bottom panel</em>: residuals of the fit of the <em>T<sub>n</sub></em> values versus the cycle number <em>n</em>.
Fig. 1: Top panel: orbital phase shift at the time of the inferior conjunction (orbital phase 0), Tn, of the secondary star in the BHXB XTE J1118+480 versus the orbital cycle number, n, folded on the best-fit parabolic fit. The error bars give the observational uncertainties. Green filled circles are spectroscopic determinations, the blue filled triangle is a photometric measurement, red filled diamonds, and the new violet square are GTC/OSIRIS spectroscopic determinations. The small panels show two MonteCarlo (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 center of the MC distributions (left small panel), and (ii) using the points on the parabolic fit (rigth small panel). Bottom panel: residuals of the fit of the Tn values versus the cycle number n.

We present new 10.4 m-GTC/OSIRIS spectroscopic observations of the black hole X-ray binary XTE J1118+480 that confirm the orbital period decay at (dP/dt) = −1.90 ± 0.57 ms yr−1. This corresponds to a period change of −0.88 ± 0.27 μs per orbital cycle. We have also collected observations of the black hole X-ray binary A0620–00 to derive an orbital period derivative of (dP/dt)= −0.60 ± 0.08 ms yr−1 (−0.53 ± 0.07 μs/cycle). Angular momentum losses due to gravitational radiation are unable to explain these large orbital decays in these two short- period black hole binaries. The orbital period decay measured in A0620–00 is very marginally consistent with the predictions of conventional models including magnetic braking, although significant mass-loss ((dMBH/dt)/(dM2/dt) ≤ 20 per cent) from the system is required. The fast spiral-in of the star in XTE J1118+480, however, does not fit any standard model and may be driven by magnetic braking under extremely high magnetic fields and/or may require an unknown process or non-standard theories of gravity. This result may suggest an evolutionary sequence in which the orbital period decay begins to speed up as the orbital period decreases. This scenario may have an impact on the evolution and lifetime of black hole X-ray binaries.

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