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Register a new userYears Delayed Gamma-ray and Radio Afterglows Originated from TDE Wind-Torus Interactions
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Tidal disruption events (TDEs) that occur in active galactic nuclei (AGN) with dusty tori are a special class of sources. TDEs can generate ultrafast and large opening-angle wind, which will almost inevitably collide with the preexisting AGN dusty tori a few years later after the TDE outburst. The wind-torus interactions drive two kinds of shocks: the bow shocks at the windward side of the torus clouds, and the cloud shocks inside the torus clouds. In a previous work, we proved that the shocked clouds will give rise to considerable X-ray emissions which can reach $10^{41-42}$ erg s$^{-1}$ (so called emph{years delayed X-ray afterglows}). In this work, we focus on the radiations of high energy particles accelerated at both shocks. Benefitting from the strong radiation field at the inner edge of the torus, the inverse Compton scatterings of AGN photons by relativistic electrons at bow shocks dominate the overall gamma-ray radiation. The gamma-ray luminosity can reach $10^{41}~{rm erg s^{-1}} (L_{rm kin}/10^{45}{rm erg s^{-1}})$, where $L_{rm kin}$ is the kinetic luminosity of TDE wind. Synchrotron radiation at bow shocks contributes to the radio afterglow with a luminosity of 10$^{38-39} ~{rm erg s^{-1}} (L_{rm kin}/10^{45}{rm erg s^{-1}})$ at 1-10 GHz if the magnetic field is 100 mGauss, and extends to infrared with a luminosity of $sim 10^{39-40}~{rm erg s^{-1}} (L_{rm kin}/10^{45}{rm erg s^{-1}})$. Our scenario provides a prediction of the years delayed afterglows in multiple wavebands for TDEs and reveals their connections.
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Tidal disruption events (TDEs) occurred in active galactic nuclei (AGNs) are a special class of sources with outstanding scientific significance. TDEs can generate ultrafast winds, which should almost inevitably collide with the preexisting AGN dusty tori. We perform analytical calculations and simulations on the wind-torus interactions and find such a process can generate considerable X-ray afterglow radiation several years or decades later after the TDE outburst. This provides a new origin for the years delayed X-rays in TDEs. The X-ray luminosity can reach 10^{41-42} erg/s, and the light curve characteristics depend on the parameters of winds and tori. We apply the model to two TDE candidates, and provide lower limits on the masses of the disrupted stars, as well as rigorous constraints on the gas densities of tori. Our results suggest that the observations of the time delay, spectral shape, luminosity and the light curve of the X-ray afterglow can be used to constrain the physical parameters of both TDE winds and tori, including the wind velocity, wind density and torus density.
We systematically analyze three GRB samples named as radio-loud, radio-quiet and radio-none afterglows, respectively. It is shown that dichotomy of the radio-loud afterglows is not necessary. Interestingly, we find that the intrinsic durations ($T_{int}$), isotropic energies of prompt gamma-rays ($E_{gamma, iso}$) and redshifts ($z$) of their host galaxies are log-normally distributed for both the radio-loud and radio-quiet samples except those GRBs without any radio detections. Based on the distinct distributions of $T_{int}$, $E_{gamma, iso}$, the circum-burst medium density ($n$) and the isotropic equivalent energy of radio afterglows ($L_{ u,p}$), we confirm that the GRB radio afterglows are really better to be divided into the dim and the bright types. However, it is noticeable that the distributions of flux densities ($F_{host}$) from host galaxies of both classes of radio afterglows are intrinsically quite similar. Meanwhile, we point out that the radio-none sample is also obviously different from the above two samples with radio afterglows observed, according to the cumulative frequency distributions of the $T_{int}$ and the $E_{gamma, iso}$, together with correlations between $T_{int}$ and $z$. In addition, a positive correlation between $E_{gamma, iso}$ and $L_{ u,p}$ is found in the radio-loud samples especially for the supernova-associated GRBs. Besides, we also find this positive correlation in the radio-quiet sample. A negative correlation between $T_{int}$ and $z$ is confirmed to hold for the radio-quiet sample too. The dividing line between short and long GRBs in the rest frame is at $T_{int}simeq$1 s. Consequently, we propose that the radio-loud, the radio-quiet and the radio-none GRBs could be originated from different progenitors.
Tidal disruption event (TDE) can launch an ultrafast outflow. If the black hole is surrounded by large amounts of clouds, outflow-cloud interaction will generate bow shocks, accelerate electrons and produce radio emission. Here we investigate the interaction between a non-relativistic outflow and clouds in active galaxies, which is manifested as outflow-BLR (broad line region) interaction, and can be extended to outflow-torus interaction. This process can generate considerable radio emission, which may account for the radio flares appearing a few months later after TDE outbursts. Radio observations can be used to directly constrain the physics of outflow, instead of indirectly providing a lower limit of the outflow energy by estimating the electron and magnetic field energy as in the outflow-CNM (circumnuclear medium) model. Benefitting from efficient energy conversion from outflow to shocks and the strong magnetic field, outflow-cloud interaction may play a non-negligible, or even dominating role in generating radio flares in a cloudy circumnuclear environment if the CNM density is no more than 100 times the Sgr A*-like one.
Afterglows of gamma-ray bursts often show flares, plateaus, and sudden intensity drops: these temporal features are difficult to explain as coming from the forward shock. We calculate radiative properties of early GRB afterglows with the dominant contribution from the reverse shock (RS) propagating in an ultra-relativistic (pulsar-like) wind produced by the long-lasting central engine. RS emission occurs in the fast cooling regime -- this ensures high radiative efficiency and allows fast intensity variations. We demonstrate that: (i) mild wind power, of the order of $sim 10^{46}$ erg s$^{-1}$, can reproduce the afterglows plateau phase; (ii) termination of the wind can produce sudden steep decays; (iii) mild variations in the wind luminosity can produce short-duration afterglow flares.
We present and analyze the optical/UV and X-ray observations of a nearby tidal disruption event (TDE) candidate AT2019azh, spanning from 30 d before to ~ 250 d after its early optical peak. The X-rays show a late brightening by a factor of ~ 30-100 around 250 days after discovery, while the UV/opticals continuously decayed. The early X-rays show two flaring episodes of variation, temporally uncorrelated with the early UV/opticals. We found a clear sign of X-ray hardness evolution, i.e., the source is harder at early times, and becomes softer as it brightens later. The drastically different temporal behaviors in X-rays and UV/opticals suggest that the two bands are physically distinct emission components, and probably arise from different locations. These properties argue against the reprocessing of X-rays by any outflow as the origin of the UV/optical peak. The full data are best explained by a two-process scenario, in which the UV/optical peak is produced by the debris stream-stream collisions during the circularization phase; some low angular momentum, shocked gas forms an early, low-mass accretion disk which emits the early X-rays. The major body of the disk is formed after the circularization finishes, whose enhanced accretion rate produces the late X-ray brightening. AT2019azh is a strong case of TDE whose emission signatures of stream-stream collision and delayed accretion are both identified.
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