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The Fast Spiral-in of the companion star to the black hole XTE J1118+480.

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

Reference: 2012 ApJ, 744, L25 | 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 low-mass black hole X-ray binary XTE J1118+480 versus the orbital cycle number, <em>n</em>, folded on the best-fit parabolic fit. Green filled circles are spectroscopic determinations, the blue filled triangle is a photometric measurement and red diamonds are the new 10.4m-GTC/OSIRIS spectroscopic determinations. Solid error bars show the uncertainties associated to the <em>T<sub>n</sub></em> determinations, whereas the dashed error bars also include the uncertainties associated to the <em>P<sub>0</sub></em> and <em>T<sub>0</sub></em> determinations. <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 low-mass black hole X-ray binary XTE J1118+480 versus the orbital cycle number, n, folded on the best-fit parabolic fit. Green filled circles are spectroscopic determinations, the blue filled triangle is a photometric measurement and red diamonds are the new 10.4m-GTC/OSIRIS spectroscopic determinations. Solid error bars show the uncertainties associated to the Tn determinations, whereas the dashed error bars also include the uncertainties associated to the P0 and T0 determinations. Bottom panel: residuals of the fit of the Tn  values versus the cycle number n.

We report the detection of an orbital period decay of (dP/dt)= -1.83+-0.66 ms yr–1  in the black hole X-ray binary XTE J1118+480. This corresponds to a period change of –0.85 ± 0.30 μs per orbital cycle, which is ~150 times larger than expected from the emission of gravitational waves. These observations cannot be reproduced by conventional models of magnetic braking even when including significant mass loss from the system. The spiral-in of the star is either driven by magnetic braking under extremely high magnetic fields in the secondary star or by a currently unknown process, which will have an impact on the evolution and lifetime of black hole X-ray binaries.

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