Magnetic flares and the optical variability of the X-ray transient XTE J1118+480 |
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| Authors: | A Merloni T Di Matteo † A C Fabian |
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| Institution: | Institute of Astronomy, Madingley Road, Cambridge CB3 0HA;Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA |
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| Abstract: | The simultaneous presence of a strong quasi-periodic oscillation, of period ∼10 s, in the optical and X-ray light curves of the X-ray transient XTE J1118+480 suggests that a significant fraction of the optical flux originates from the inner part of the accretion flow, where most of the X-rays are produced. We present a model of magnetic flares in an accretion disc corona where thermal cyclo-synchrotron emission contributes significantly to the optical emission, while the X-rays are produced by inverse Compton scattering of the soft photons produced by dissipation in the underlying disc and by the synchrotron process itself. Given the observational constraints, we estimate the values for the coronal temperature, optical depth and magnetic field intensity, as well as the accretion rate for the source. Within our model we predict a correlation between optical and hard X-ray variability and an anticorrelation between optical and soft X-rays. We also expect optical variability on flaring time-scales (∼tens of ms), with a power-density spectrum similar to that observed in the X-ray band. Finally, we use both the available optical/extreme-ultraviolet/X-ray spectral energy distribution and the low-frequency time variability to discuss limits on the inner radius of the optically thick disc. |
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| Keywords: | accretion accretion discs magnetic fields radiation mechanisms: thermal binaries: general stars: individual: XTE J1118+480 |
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