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Register a new userConstraints on off-axis jets from stellar tidal disruption flares
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Many decades of observations of active galactic nuclei and X-ray binaries have shown that relativistic jets are ubiquitous when compact objects accrete. One could therefore anticipate the launch of a jet after a star is disrupted and accreted by a massive black hole. This birth of a relativistic jet may have been observed recently in two stellar tidal disruption flares (TDFs), which were discovered in gamma-rays by Swift. Yet no transient radio emission has been detected from the tens of TDF candidates that were discovered at optical to soft X-ray frequencies. Because the sample that was followed-up at radio frequencies is small, the non-detections can be explained by Doppler boosting, which reduces the jet flux for off-axis observers. And since the existing follow-up observation are mostly within ~10 months of the discovery, the non-detections can also be due to a delay of the radio emission with respect to the time of disruption. To test the conjecture that all TDFs launch jets, we obtained 5 GHz follow-up observations with the Jansky VLA of seven known TDFs. To avoid missing delayed jet emission, our observations probe 1-8 years since the estimated time of disruption. None of the sources are detected, with very deep upper limits at the 10 micro Jansky level. These observations rule out the hypothesis that these TDFs launched jets similar to radio-loud quasars. We also constrain the possibility that the flares hosted a jet identical to Sw 1644+57, the first and best-sampled relativistic TDF. We thus obtain evidence for a dichotomy in the stellar tidal disruption population, implying that the jet launching mechanism is sensitive to the parameters of the disruption.
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The discovery of jets from tidal disruption events (TDEs) rejuvenated the old field of relativistic jets powered by accretion onto supermassive black holes. In this Chapter, we first review the extensive multi-wavelength observations of jetted TDEs. Then, we show that these events provide valuable information on many aspects of jet physics from a new prospective, including the on-and-off switch of jet launching, jet propagation through the ambient medium, $gamma/$X-ray radiation mechanism, jet composition, and the multi-messenger picture. Finally, open questions and future prospects in this field are summarized.
Stars that pass within the Roche radius of a supermassive black hole will be tidally disrupted, yielding a sudden injection of gas close to the black hole horizon which produces an electromagnetic flare. A few dozen of these flares have been discovered in recent years, but current observations provide poor constraints on the bolometric luminosity and total accreted mass of these events. Using images from the Wide-field Infrared Survey Explorer (WISE), we have discovered transient 3.4 micron emission from several previously known tidal disruption flares. The observations can be explained by dust heated to its sublimation temperature due to the intense radiation of the tidal flare. From the break in the infrared light curve we infer that this hot dust is located ~0.1 pc from the supermassive black hole. Since the dust has been heated by absorbing UV and (potentially) soft X-ray photons of the flare, the reprocessing light curve yields an estimate of the bolometric flare luminosity. For the flare PTF-09ge, we infer that the most likely value of the luminosity integrated over frequencies at which dust can absorb photons is $8times 10^{44}$ erg/s, with a factor of 3 uncertainty due to the unknown temperature of the dust. This bolometric luminosity is a factor ~10 larger than the observed black body luminosity. Our work is the first to probe dust in the nuclei of non-active galaxies on sub-parsec scales. The observed infrared luminosity implies a covering factor ~1% for the nuclear dust in the host galaxies.
Recently, a high-energy muon neutrino event was detected in association with a tidal disruption event (TDE) AT2019dsg at the time about 150 days after the peak of the optical/UV luminosity. We propose that such a association could be interpreted as arising from hadronic interactions between relativistic protons accelerated in the jet launched from the TDE and the intense radiation field of TDE inside the optical/UV photosphere, if we are observing the jet at a moderate angle (i.e., approximately 10-30 degree) with respect to the jet axis. Such an off-axis viewing angle leads to a high gas column density in the line of sight which provides a high opacity for the photoionization and the Bethe-Heitler process, {and allows the existence of an intrinsic long-term X-ray radiation of comparatively high emissivity}. As a result, the cascade emission accompanying the neutrino production, which would otherwise overshoot the flux limits in X-ray and/or GeV band, is significantly obscured or absorbed. Since the jets of TDEs are supposed to be randomly oriented in the sky, the source density rate of TDE with an off-axis jet is significantly higher than that of TDE with an on-axis jet. Therefore, an off-axis jet is naturally expected in a nearby TDE being discovered, supporting the proposed scenario.
Supermassive black hole binaries (SMBHBs) are products of galaxy mergers, and are important in testing Lambda cold dark matter cosmology and locating gravitational-wave-radiation sources. A unique electromagnetic signature of SMBHBs in galactic nuclei is essential in identifying the binaries in observations from the IR band through optical to X-ray. Recently, the flares in optical, UV, and X-ray caused by supermassive black holes (SMBHs) tidally disrupting nearby stars have been successfully used to observationally probe single SMBHs in normal galaxies. In this Letter, we investigate the accretion of the gaseous debris of a tidally disrupted star by a SMBHB. Using both stability analysis of three-body systems and numerical scattering experiments, we show that the accretion of stellar debris gas, which initially decays with time $propto t^{-5/3}$, would stop at a time $T_{rm tr} simeq eta T_{rm b}$. Here, $eta sim0.25$ and $T_{rm b}$ is the orbital period of the SMBHB. After a period of interruption, the accretion recurs discretely at time $T_{rm r} simeq xi T_b$, where $xi sim 1$. Both $eta$ and $xi$ sensitively depend on the orbital parameters of the tidally disrupted star at the tidal radius and the orbit eccentricity of SMBHB. The interrupted accretion of the stellar debris gas gives rise to an interrupted tidal flare, which could be used to identify SMBHBs in non-active galaxies in the upcoming transient surveys.
Radio observations of tidal disruption events (TDEs) - when a star is tidally disrupted by a supermassive black hole (SMBH) - provide a unique laboratory for studying outflows in the vicinity of SMBHs and their connection to accretion onto the SMBH. Radio emission has been detected in only a handful of TDEs so far. Here, we report the detection of delayed radio flares from an optically-discovered TDE. Our prompt radio observations of the TDE ASASSN-15oi showed no radio emission until the detection of a flare six months later, followed by a second and brighter flare, years later. We find that the standard scenario, in which an outflow is launched briefly after the stellar disruption, is unable to explain the combined temporal and spectral properties of the delayed flare. We suggest that the flare is due to the delayed ejection of an outflow, perhaps following a transition in accretion states. Our discovery motivates observations of TDEs at various timescales and highlights a need for new models.
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