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We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 (Sw 1644+57) extending to t ≈ 600 days. The data were obtained with the JVLA and AMI Large Array as part of our on-going study of the jet energetics and the density structure of the parsec-scale environment around the disrupting supermassive black hole. 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 days, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of σt/t ≲0.2 (and by a factor of 15 in σt/t ≈ 0.05). The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at ≲500 days, 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 days 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 ~2-10Rs 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 ~ MEdd ≈ 0.006M☉ yr-1 (for a 3 × 106 M☉black hole and order unity efficiency) indicating that the peak accretion rate was about 330~MEdd, and the total accreted mass by t ≈ 500 days is about 0.15M☉. From the radio data we further find significant flattening in the integrated energy of the forward shock at t ≳ 250 days with Ej,iso ≈ 2 × 1054 erg (Ej ≈ 1052 erg for a jet opening angle, θj = 0.1) following a rise by about a factor of 15 at ≈30-250 days. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates. [ABSTRACT FROM AUTHOR]

We present continued radio observations of the tidal disruption event Swift J164449.3+573451 extending to δt ≈ 216 days after discovery. The data were obtained with the EVLA, AMI Large Array, CARMA, the SMA, and the VLBA+Effelsberg as 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 δt ≈ 1 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 ρ∝r–3/2 (0.1-1.2 pc) with a significant flattening at r ≈ 0.4-0.6 pc. The increase in energy cannot be explained with continuous injection from an L∝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(> Γj)∝Γ–2.5j. The similar ratios of duration to dynamical timescale for Sw 1644+57 and gamma-ray bursts (GRBs) suggest that this result may be applicable to GRB jets as well. The radial density profile may be indicative of Bondi accretion, with the inferred flattening at r ∼ 0.5 pc in good agreement with the Bondi radius for a ∼few × 106M☼ black hole. The density at ∼0.5 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 (δt ≈ 216 days) we find that the jet energy is Ej, iso ≈ 5 × 1053 erg (Ej ≈ 2.4 × 1051 erg for θj = 0.1), the radius is r ≈ 1.2 pc, the Lorentz factor is Γj ≈ 2.2, the ambient density is n ≈ 0.2 cm–3, and the projected angular size is rproj ≈ 25 μas, below the resolution of the VLBA+Effelsberg. Assuming no future changes in the observed evolution and a final integrated total energy of Ej ≈ 1052 erg, we predict that the radio emission from Sw 1644+57 should be detectable with the EVLA for several decades and will be resolvable with very long baseline interferometry in a few years. [ABSTRACT FROM AUTHOR]

The standard model of gamma-ray burst afterglows is based on synchrotron radiation from a blast wave produced when the relativistic ejecta encounters the surrounding medium. We reanalyze the refreshed shock scenario, in which slower material catches up with the decelerating ejecta and reenergizes it. This energization can be done either continuously or in discrete episodes. We show that such a scenario has two important implications. First, there is an additional component coming from the reverse shock that goes into the energizing ejecta. This persists for as long as the reenergization itself, which could extend for up to days or longer. We find that during this time the overall spectral peak is found at the characteristic frequency of the reverse shock. Second, if the injection is continuous, the dynamics will be different from that in constant energy evolution and will cause a slower decline of the observed fluxes. A simple test of the continuously refreshed scenario is that it predicts a spectral maximum in the far-infrared or millimeter range after a few days.

We discuss the spectrum arising from synchrotron emission by fast cooling (FC) electrons, when fresh electrons are continually accelerated by a strong blast wave, into a power-law distribution of energies. The FC spectrum has so far been described by four power-law segments divided by three break frequencies nusa

We report the discovery of a radio transient VLA 232937.2-235553, coincident with the proposed X-ray afterglow for the gamma-ray burst GRB 981226. This gamma-ray burst (GRB) has the highest ratio of X-ray to gamma-ray fluence of all the GRBs detected by BeppoSAX so far, and yet no corresponding optical transient was detected. The radio light curve of VLA 232937.2-235553 is qualitatively similar to that of several other radio afterglows. At the subarcsecond position provided by the radio detection, optical imaging reveals an extended R=24.9 mag object, which we identify as the host galaxy of GRB 981226. Afterglow models that invoke a jetlike geometry for the outflow or that require an ambient medium with a radial density dependence, such as that produced by a wind from a massive star, are both consistent with the radio data. Furthermore, we show that the observed properties of the radio afterglow can explain the absence of an optical transient without the need for large extinction local to the GRB.