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Register a new userRadio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. IV. Continued Fading and Non-Relativistic Expansion
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We present continued radio and X-ray observations of the previously relativistic tidal disruption event (TDE) Swift J164449.3+573451 (sw) extending to about 9.4 years post disruption, as part of ongoing campaigns with the Jansky Very Large Array (VLA) and the textit{Chandra} X-ray observatory. We find that the X-ray emission has faded below detectable levels, with an upper limit of $lesssim 3.5times 10^{-15}$ erg cm$^{-2}$ s$^{-1}$ in a 100 ks observation, while the radio emission continues to be detected and steadily fade. Both are consistent with forward shock emission from a non-relativistic outflow, although we find that the radio spectral energy distribution is better fit at these late times with an electron power law index of $papprox 3$ (as opposed to $papprox 2.5$ at earlier times). With the revised spectral index we find $epsilon_Bapprox 0.01$ using the radio and X-ray data, and a density of $approx 0.04$ cm$^{3}$ at a radius of $Rapprox 0.65$ pc ($R_{rm sch}approx 2times 10^6$ R$_odot$) from the black hole. The energy scale of the blastwave is $approx 10^{52}$ erg. We also report detections of sw at 3 GHz from the first two epochs of the VLA Sky Survey (VLASS), and find that $sim 10^2$ off-axis sw-like events to $zsim 0.5$ may be present in the VLASS data. Finally, we find that sw itself will remain detectable for decades at radio frequencies, although observations at sub-GHz frequencies will become increasingly important to characterize its dynamical evolution.
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We present continued radio and X-ray observations of the relativistic tidal disruption event Swift J164449.3+573451 extending to $delta t approx 2000$ d after discovery. The radio data were obtained with the VLA as part of a long-term program to monitor the energy and dynamical evolution of the relativistic jet and to characterize the parsec-scale environment around a previously dormant supermassive black hole. We combine these data with $textit{Chandra}$ X-ray observations and demonstrate that the X-ray emission following the sharp decline at $delta t approx 500$ d is due to the forward shock. Using the X-ray data, in conjunction with optical/NIR data, we constrain the synchrotron cooling frequency and the microphysical properties of the outflow for the first time. We find that the cooling frequency evolves through the optical/NIR band at $delta t approx 10 - 200$ d, corresponding to a magnetic field energy density fraction of $epsilon_B approx 10^{-3}$, well below equipartition; the X-ray data demonstrate that this deviation from equipartition holds to at least $delta t approx 2000$ d. We thus recalculate the physical properties of the jet over the lifetime of the event, no longer assuming equipartition. We find a total kinetic energy of $E_K approx 4 times 10^{51}$ erg and a transition to non-relativistic expansion on the timescale of our latest observations ($delta t approx 700$ d). The density profile is approximately $R^{-3/2}$ at $lesssim 0.3$ pc and $gtrsim 0.7$ pc, with a plateau at intermediate scales, characteristic of Bondi accretion. Based on its evolution thus far, we predict that Sw 1644+57 will be detectable at centimeter wavelengths for decades to centuries with existing and upcoming radio facilities. Similar off-axis events should be detectable to $z sim 2$, but with a slow evolution that may inhibit their recognition as transient events.
We present continued multi-frequency radio observations of the relativistic tidal disruption event Sw1644+57 extending to dt~600 d. The data were obtained with the JVLA and AMI Large Array. We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at ~500 d, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of dt/t<0.2. The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at <500 d, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at ~610 d is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of 30-60 eV and inner radius of 2-10 R_s cannot accommodate the observed flux level or the detected emission at >1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below Mdot_Edd~0.006 Msun/yr (for a 3x10^6 Msun black hole and order unity efficiency) indicating that the peak accretion rate was about 330 Mdot_Edd, and the total accreted mass by 500 d is about 0.15 Msun. From the radio data we further find significant flattening in the integrated energy of the forward shock at >250 d with E_j,iso~2x10^54 erg (E_j~10^52$ erg for a jet opening angle, theta_j=0.1) following a rise by about a factor of 15 at 30-250 d. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.
We present continued radio observations of the tidal disruption event SwiftJ164449.3+573451 extending to sim216 days after discovery. The data are part of a long-term program to monitor the expansion and energy scale of the relativistic outflow, and to trace the parsec-scale environment around a previously-dormant supermassive black hole (SMBH). The new observations reveal a significant change in the radio evolution starting at sim1 month, with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude, and an overall density profile around the SMBH of rho propto r^{-3/2} (0.1-1.2 pc) with a significant flattening at rsim0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an L propto t^{-5/3} tail, which is observed in the X-rays. Instead, we conclude that the relativistic jet was launched with a wide range of Lorentz factors, obeying E(>Gamma) propto Gamma^{-2.5}. The similar ratio of duration to dynamical timescale for Sw1644+57 and GRBs suggests that this result may be applicable to GRBs as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at rsim0.5 pc in good agreement with the Bondi radius for a sim10^6 M_sun black hole. The density at sim0.5 pc is about a factor of 30 times lower than inferred for the Milky Way galactic center, potentially due to a smaller number of mass-shedding massive stars. From our latest observations (sim216 d) we find that the jet energy is E_{iso}sim5x10^{53} erg (E_jsim2.4x10^{51} erg for theta_j=0.1), the radius is rsim1.2 pc, the Lorentz factor is Gammasim2.2, the ambient density is nsim0.2 cm^{-3}, and the projected size is r_{proj}sim25 microarcsec. Assuming no future changes in the observed evolution we predict that the radio emission from Sw1644+57 should be detectable with the EVLA for several decades, and will be resolvable with VLBI in a few years.
A tidal disruption event (TDE) is an astronomical phenomenon in which a previously dormant black hole (BH) destroys a star passing too close to its central part. We analyzed the flaring episode detected from the TDE sources, Swift~J1644+57 and Swift J2058+05 using RXTE, Swift and Suzaku data. The spectra are well fitted by the so called Bulk Motion Comptonization model for which the best-fit photon index Gamma varies from 1.1 to 1.8. We have firmly established the saturation of Gamma versus mass accretion rate at Gamma_{sat} about 1.7 -- 1.8. The saturation of Gamma is usually identified as a signature of a BH now established in Swift~J1644+57 and Swift J2058+05. In Swift~J1644+57 we found the relatively low Gamma_{sat} values which indicate a high electron (plasma) temperature, kT_e ~ 30 -- 40 keV. This is also consistent with high cutoff energies, E_{cut} ~ 60 -- 80 keV found using best fits of the RXTE spectra. Swift~J2058+05 shows a lower electron temperature, kT_e ~ 4-10 keV than that for Swift~J1644+57. For the BH mass estimate we used the scaling technique taking the Galactic BHs, GRO J1655--40, GX~339--4, Cyg~X--1 and 4U~1543--47 as reference sources and found that the BH mass in Swift~J1644+57 is M_{BH}> 7x10^6 solar masses assuming the distance to this of 1.5 Gpc. For Swift J2058+05 we obtain M_{BH}> 2x 10^7 solar masses assuming the distance to this source of 3.7 Gpc.
A small fraction of candidate tidal disruption events (TDEs) show evidence of powerful relativistic jets, which are particularly pronounced at radio wavelengths, and likely contribute non-thermal emission at a wide range of wavelengths. A non-thermal emission component can be diagnosed using linear polarimetry, even when the total received light is dominated by emission from an accretion disk or disk outflow. In this paper we present Very Large Telescope (VLT) measurements of the linear polarisation of the optical light of jetted TDE Swift J2058+0516. This is the second jetted TDE studied in this manner, after Swift J1644+57. We find evidence of non-zero optical linear polarisation, P_V ~ 8%, a level very similar to the near-infrared polarimetry of Swift J1644+57. These detections provide an independent test of the emission mechanisms of the multiwavelength emission of jetted tidal disruption events.
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