Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2012-02-20
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
7 pages, 8 figures
Scientific paper
The X-ray transient source Sw J1644+57 recently discovered by Swift is believed to be triggered by tidal disruption of a star by a rapidly spinning supermassive black hole (SMBH). For such events, the outer disk is very likely misaligned with respect to the equatorial plane of the spinning SMBH, since the incoming star before disruption most likely has an inclined orbital plane. The tilted disk is subject to the Lense-Thirring torque, which tends to twist and warp the disk due to the Bardeen-Petterson effect. The inner disk tends to align with the SMBH spin, while the outer region tends to remain in the stellar orbital plane, with a transition zone around the Bardeen-Petterson radius. The relativistic jet launched via the Blandford-Znajek mechanism from the spinning SMBH would undergo precession. The X-ray lightcurve of Sw J1644+57 shows a quasi-periodic (2.7-day) variation with noticeable narrow dips. We numerically solve a warping disk solution and propose a jet-processing model by invoking a Blandford-Znajek jet collimated by a wind launched near the Bardeen-Petterson radius. Through simulations, we show that the narrow dips in the X-ray lightcurve can be reproduced for a range of geometric configurations. From data we infer that the inclination angle of the initial stellar orbit is in the range of $10^{\circ}-20^{\circ}$ from the SMBH equatorial plane, that the jet should have a moderately high Lorentz factor, and that the inclination angle, jet opening angle, and observer's viewing angle are such that the duty cycle of the line-of-sight sweeping the jet cone is somewhat less than 0.5.
Gao He
Lei Wei-Hua
Zhang Bing
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