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Post Info TOPIC: Sw J1644+57 and Sw J2058+05


L

Posts: 131433
Date:
Sw J1644+57
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Title: On the origin of the radio emission of Sw 1644+57
Authors: Rodolfo Barniol Duran, Tsvi Piran

We apply relativistic equipartition synchrotron arguments to the radio emission of the tidal disruption event candidate Sw 1644+57. We find that, regardless of the details of the equipartition scenario considered, the energy required to produce the observed radio (i.e., energy in the magnetic field and radio emitting electrons) must increase by a factor of ~20 during the first 200 days. It then saturates. This energy increase cannot be alleviated by a varying geometry of the system. The radio data can be explained by: (i) An afterglow like emission of the X-ray emitting narrow relativistic jet. The additional energy can arise here from a slower moving material ejected in the first few days that gradually catches up with the slowing down blast wave (Berger et al. 2012). However, this requires at least ~4x10^{53} erg in the slower moving outflow. This is much more than the energy of the fast moving outflow that produced the early X-rays and it severely constrains the overall energy budget. (ii) Alternatively, the radio may arise from a mildly relativistic outflow. Here, the energy for the radio emission increases with time to at least ~10^{51} erg after 200 days. This scenario requires, however, a second X-ray emitting narrow relativistic component. Given these results, it is worthwhile to consider models in which the energy of the magnetic field and/or of the radio emitting electrons increases with time without a continuous energy supply to the blast wave. This can happen, for example, if the energy is injected initially mostly in one form (Poynting flux or baryonic) and it is gradually converted to the other form, leading to a time-varying deviation from equipartition. Another intriguing possibility is that a gradually decreasing Inverse Compton cooling modifies the synchrotron emission and leads to an increase of the available energy in the radio emitting electrons (Kumar et al. 2013).

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L

Posts: 131433
Date:
Swift J1644+57
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Title: Swift J1644+57 gone MAD: the case for dynamically-important magnetic flux threading the black hole in a jetted tidal disruption event
Authors: Alexander Tchekhovskoy (1), Brian D. Metzger (2), Dimitrios Giannios (3), Luke Zoltan Kelley (4) ((1) Princeton, (2) Columbia, (3) Purdue, (4) Harvard-CfA)

The unusual transient Swift J1644+57 likely resulted from a collimated relativistic jet powered by accretion onto a massive black hole (BH) following the tidal disruption (TD) of a star. Several mysteries cloud the interpretation of this event: (1) extreme flaring and 'plateau' shape of the X-ray/gamma-ray light curve during the first 10 days after the gamma-ray trigger; (2) unexpected rebrightening of the forward shock radio emission months after trigger; (3) no obvious evidence for jet precession, despite misalignment typically expected between the angular momentum of the accretion disk and BH; (4) recent abrupt shut-off in jet X-ray emission after 1.5 years. Here we show that all of these seemingly disparate mysteries are naturally resolved by one assumption: the presence of strong magnetic flux Phi threading the BH. Initially, Phi is weak relative to high fall-back mass accretion rate, Mdot, and the disk and jets precess about the BH axis = our line of sight. As Mdot drops, Phi becomes dynamically important and leads to a magnetically-arrested disk (MAD). MAD naturally aligns disk and jet axis along the BH spin axis, but only after a violent rearrangement phase (jet wobbling). This explains the erratic light curve at early times and the lack of precession at later times. We use our model for Swift J1644+57 to constrain BH and disrupted star properties, finding that a solar-mass main sequence star disrupted by a relatively low mass, M~10^5-10^6 solar masses, BH is consistent with the data, while a WD disruption (though still possible) is disfavoured. The magnetic flux required to power Swift J1644+57 is too large to be supplied by the star itself, but it could be collected from a quiescent 'fossil' accretion disk present in the galactic nucleus prior to the TD. The presence (lack of) of such a fossil disk could be a deciding factor in what TD events are accompanied by powerful jets.

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Posts: 131433
Date:
Sw J1644+57
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Title: Frame-dragging, disk warping, jet precessing, and dipped X-ray lightcurve of Sw J1644+57
Authors: 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 10-20 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.

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L

Posts: 131433
Date:
Sw J1644+57 and Sw J2058+05
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Title: Black hole Spin in Sw J1644+57 and Sw J2058+05
Authors: Wei-Hua Lei, Bing Zhang

Recently a hard X-ray transient event, Sw J1644+57, was discovered by the Swift satellite, which marks the onset of a relativistic jet from a supermassive black hole, likely triggered by a tidal disruption event (TDE). Another candidate in the same category, Sw J2058+05, was also reported. The low event rate suggests that only a small fraction of TDEs launch relativistic jets. A common speculation is that these rare events are related to rapidly spinning black holes. We attribute jet launching to the Blandford-Znajek mechanism, and use the available data to constrain the black hole spin parameter for the two events. It is found that the two black holes indeed carry a moderate to high spin, suggesting that black hole spin is likely the crucial factor behind the Sw J1644+57 - like events.

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