The influence of circumnuclear environment on the radio emission from TDE jets

Image: This computer-simulated image shows gas from a star that is ripped apart by tidal forces as it falls into a black hole. Credits: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz)

When a star in a galactic nucleus is deflected too close to the central supermassive black hole (BH), it can be torn apart by tidal forces. During this tidal disruption event (TDE), roughly half of the stellar debris remains bound to the BH, while the other half is flung outwards and unbound from the system. The bound material, following a potentially complex process of debris circularization accretes onto the BH, creating a luminous flare lasting months to years.


Dozens of stellar tidal disruption events (TDEs) have been identified at optical, UV and X-ray wavelengths. A small fraction of these, most notably Swift J1644+57, produce radio synchrotron emission, consistent with a powerful, relativistic jet shocking the surrounding circumnuclear gas.

Swift J1644+57 is the result of a truly extraordinary event - the awakening of a distant galaxy's dormant black hole as it shredded and consumed a star. The galaxy is so far away, it took the light from the event approximately 3.9 billion years to reach Earth.

Most galaxies, including our own, possess a central supersized black hole weighing millions of times the sun's mass. According to the new studies, the black hole in the galaxy hosting Swift J1644+57 may be twice the mass of the four-million-solar-mass black hole in the center of the Milky Way galaxy. As a star falls toward a black hole, it is ripped apart by intense tides. The gas is corralled into a disk that swirls around the black hole and becomes rapidly heated to temperatures of millions of degrees.

The innermost gas in the disk spirals toward the black hole, where rapid motion and magnetism create dual, oppositely directed "funnels" through which some particles may escape. Jets driving matter at velocities greater than 90 percent the speed of light form along the black hole's spin axis. In the case of Swift J1644+57, one of these jets happened to point straight at Earth.

Image: This illustration steps through the events that scientists think likely resulted in Swift J1644+57. Credits: NASA/Goddard Space Flight Center/Swift

The radio emission occurs when the outgoing jet slams into the interstellar environment. By contrast, the X-rays arise much closer to the black hole, likely near the base of the jet.

The dearth of similar non-thermal radio emission in the majority of TDEs may imply that powerful jet formation is intrinsically rare, or that the conditions in galactic nuclei are typically unfavorable for producing a detectable signal.

In a recent paper (Generozov et al. 2016) the authors explore the latter possibility by constraining the radial profile of the gas density encountered by a TDE jet using a one-dimensional model for the circumnuclear medium which includes mass and energy input from a stellar population. Near the jet Sedov radius of 10¹⁸ cm, they find gas densities in the range of n₁₈∼ 0.5−2000 cm⁻³ across a wide range of plausible star formation histories. Using one- and two-dimensional relativistic hydrodynamical simulations, they calculate the synchrotron radio light curves of TDE jets (as viewed both on and off-axis) across the allowed range of density profiles.

They find that bright radio emission would be produced across the plausible range of nuclear gas densities by jets as powerful as Swift J1644+57, and they quantify the relationship between the radio luminosity and jet energy. They use existing radio detections and upper limits to constrain the energy distribution of TDE jets. Radio follow up observations several months to several years after the TDE candidate will strongly constrain the energetics of any relativistic flow.

• Generozov et al. 2016 (preprint) - The influence of circumnuclear environment on the radio emission from TDE jets (arXiv)

• Researchers Detail How a Distant Black Hole Devoured a Star - (NASA)