Wei-Hua Lei, Bing Zhang, He Gao
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.
View original:
http://arxiv.org/abs/1202.4231
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