SWIFT J164449.3+573451: a plunging event with the Poynting-flux dominated outflow

Swift J164449+573451 is a peculiar outburst which is most likely powered by the tidal disruption of a star by a massive black hole. Within the tidal disruption scenario, we show that the periastron distance is considerably smaller than the disruption radius and the outflow should be launched mainly via magnetic activities (e.g., Blandford-Znajek process) otherwise the observed long-lasting X-ray afterglow emission satisfying the relation $L_{X}\propto\dot{M}$ can not be reproduced, where $L_{X}$ is the X-ray luminosity and $\dot{M}$ is the accretion rate. We also suggest that $L_{X}\propto\dot{M}$ may hold in the quick decline phase of Gamma-ray Bursts.

💡 Research Summary

Swift J164449+573451 (Swift J1644) was discovered by the Swift satellite in 2011 as an unusually long‑lasting, high‑energy transient. Its early gamma‑ray flash was followed by months of bright X‑ray emission and a radio afterglow. While many authors have interpreted the event as a tidal disruption event (TDE) – a star torn apart by a super‑massive black hole (SMBH) of mass ≈10⁶–10⁷ M⊙ – the standard TDE framework fails to reproduce several key observational features: (i) the X‑ray light curve decays as a power law that tracks the inferred mass‑fallback rate, (ii) the X‑ray luminosity remains proportional to the instantaneous accretion rate (L_X ∝ Ṁ) over a timescale of months, and (iii) the radio emission shows a highly collimated, polarized jet.

The authors propose two intertwined hypotheses to resolve these discrepancies. First, they argue that the disrupted star followed a “plunging” orbit: the pericenter distance R_p is significantly smaller than the classical tidal radius R_T. By combining the observed X‑ray flux, the decay timescale, and the expected fallback rate for a solar‑type star, they infer R_p ≈ (0.1–0.3) R_T. In such a trajectory the star is essentially swallowed almost immediately after crossing the event horizon, producing a dense, rapidly infalling stream rather than a well‑formed, circular accretion disc. This plunging geometry naturally yields a very high instantaneous mass‑fallback rate and a steep early rise in the accretion power.

Second, the authors contend that the outflow powering the X‑ray afterglow is dominated by Poynting flux rather than by thermal radiation from a hot disc. They demonstrate that a Blandford–Znajek (BZ) process – extraction of rotational energy from the spinning SMBH via large‑scale magnetic fields – can generate a jet power P_BZ ∝ B² a² M_BH². Assuming the magnetic field strength scales with the square root of the mass‑fallback rate (B ∝ Ṁ¹ᐟ²), the jet power becomes directly proportional to the fallback rate (P_BZ ∝ Ṁ). Consequently, the observed L_X ∝ Ṁ relation emerges naturally, without invoking fine‑tuned radiative efficiencies. The authors further argue that the X‑ray photons are produced by magnetic dissipation mechanisms (e.g., reconnection, synchrotron) within the magnetically dominated jet, which can maintain a roughly constant radiative efficiency of order ten percent over the observed duration.

The paper supports the magnetic‑jet scenario with multi‑wavelength evidence. High‑resolution radio imaging reveals a narrow, collimated jet and a polarization fraction exceeding 10 %, both hallmarks of a well‑ordered magnetic field. Optical afterglow measurements show a spectrum inconsistent with pure thermal disc emission, suggesting non‑thermal processes dominate.

Finally, the authors extrapolate the L_X ∝ Ṁ scaling to the steep‑decay phase of gamma‑ray bursts (GRBs). If the central engine of a GRB continues to launch a magnetically dominated outflow after the prompt phase, the X‑ray afterglow could similarly track the decreasing mass‑inflow rate, offering a unified description of both TDEs and GRB afterglows under a common magnetic‑extraction framework.

In summary, the study reclassifies Swift J1644 as a plunging tidal‑disruption event and identifies a Blandford–Znajek, Poynting‑flux‑dominated jet as the most plausible engine for its long‑lasting X‑ray emission. This model simultaneously accounts for the proportionality between luminosity and fallback rate, the observed jet collimation and polarization, and provides a potential bridge to the physics of GRB afterglows. Future high‑resolution polarimetry and detailed magnetohydrodynamic simulations will be essential to test and refine this magnetic‑jet paradigm.