Afterglow model for the radio emission from the jetted tidal disruption candidate Swift J1644+57

Abstract

The recent transient event Swift J1644+57 has been interpreted as emission from a collimated relativistic jet, powered by the sudden onset of accretion on to a supermassive black hole following the tidal disruption of a star. Here we model the radio-microwave emission as synchrotron radiation produced by the shock interaction between the jet and the gaseous circumnuclear medium (CNM). At early times after the onset of the jet (t 5-10 d) a reverse shock propagates through and decelerates the ejecta, while at later times the outflow approaches the Blandford-McKee self-similar evolution (possibly modified by additional late energy injection). The achromatic break in the radio light curve of Swift J1644+57 is naturally explained as the transition between these phases. We show that the temporal indices of the pre-and post-break light curve are consistent with those predicted if the CNM has a wind-type radial density profile n ∝ r −2 . The observed synchrotron frequencies and self-absorbed flux constrain the fraction of the post-shock thermal energy in relativistic electrons e ≈ 0.03-0.1, the CNM density at 10 18 cm n 18 ≈ 1-10 cm −3 and the initial Lorentz factor j ≈ 10-20 and opening angle θ j ∼ (0.3-1) −1 j ∼ 0.01-0.1 of the jet. Radio modelling thus provides robust independent evidence for a narrowly collimated outflow. Extending our model to the future evolution of Swift J1644+57, we predict that the radio flux at low frequencies (ν few GHz) will begin to brighten more rapidly once the characteristic frequency ν m crosses below the radio band after it decreases below the self-absorption frequency on a time-scale of months (indeed, such a transition may already have begun). Our results demonstrate that relativistic outflows from tidal disruption events provide a unique probe of the conditions in distant, previously inactive galactic nuclei, complementing studies of normal active galactic nuclei.

Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. Ii. The Relativistic Jet Shuts Off and a Transition to Forward Shock X-Ray/Radio Emission

“…−5/3 at t 15 days, motivated by the X-ray light curve (Burrows et al 2011;De Colle et al 2012;Metzger et al 2012), we findṀ p ≈ 350Ṁ Edd , in good agreement with the predictions of De Colle et al (2012) for a 3 × 10 6 M black hole. Integrating the mass accretion rate to t ≈ 370 days in the rest-frame, we find a total accreted mass of ≈0.15 M .…”

“…In particular, we conclude that the mechanism powering the X-ray emission at δt 500 days has ceased to operate. The absence of a similar rapid decline in the radio band supports earlier conclusions that the radio and X-ray emission arise from distinct physical components (Bloom et al 2011;Zauderer et al 2011;Metzger et al 2012;Liu et al 2012).…”

“…We model the radio emission from Sw 1644+57 following the approach detailed in Paper I, which is based on the afterglow formulation of Metzger et al (2012) and Granot & Sari (2002).…”

Late-time observations of the relativistic tidal disruption flare candidate Swift J1112.2−8238

“…The spectral evolution of all three candidates (where contemporaneous observations in two or more bands allow) also shows a striking similarity (see Figure 6). Both Swift J1644+57 and Swift J2058+05 begin with a negative spectral index that evolves as expected towards synchrotron selfabsorption at late times (α∼1, Metzger et al 2012). The uncertainties on the spectral indices of the observations of Swift J1112-8238 are large, but are nonetheless consistent with the same spectral evolution.…”

Swift J2058.4+0516: Discovery of a Possible Second Relativistic Tidal Disruption Flare?

“…Late-time radio observations should be able to distinguish between these two possibilities (Zauderer et al 2011;Metzger et al 2012;Berger et al 2012). Much as has been done with longduration GRBs (Berger et al 2004;van der Horst et al 2008), accurate calorimetry can be performed in the nonrelativistic regime when the outflow is spherical, bypassing any concerns about geometric corrections.…”