An illustration of an accretion disk forming around a supermassive black hole in the wake of a tidal disruption event.
Adapted from NASA/Swift/Aurore Simonnet, Sonoma State University
Adapted from NASA/Swift/Aurore Simonnet, Sonoma State University
Title: Late-Time Radio Flares in Tidal Disruption Events
Authors: Tatsuya Matsumoto and Tsvi Piran
First Author’s Institution: Kyoto University
Status: Published in ApJ
Hungry (and Loud) Black Holes
Tidal disruption events arise when a star wanders too close to a supermassive black hole that then exerts a tidal force
across the star, shredding it. These events are relatively rare: we
have only discovered a few hundred. When the star is disrupted, about
two-thirds of the material remains bound. The remaining material is
ejected from the supermassive black hole into the “circumnuclear
medium,” or the region immediately surrounding the supermassive black
hole. We typically discover tidal disruption events from the optical
emission resulting from the initial disruption, which lasts several
weeks. However, tidal disruption events are known to be multi-wavelength events
visible across the electromagnetic spectrum. Before optical tidal
disruption events were discovered, almost all of the tidal disruption
events were found in the X-ray, where the formation of an accretion disk around the supermassive black hole may be powering some high-energy activity. On the other end of the spectrum, the radio properties of tidal disruption events have proven to be unique. Today’s article aims to explain the radio light curves of tidal disruption events.
The radio emission from tidal disruption events is caused by the material that survives the disruption of the star and is ejected away from the supermassive black hole. This stellar material runs into the ambient density surrounding the supermassive black hole, causing shocks inside the material. These shocks give rise to synchrotron radiation, an emission caused by free electrons in a plasma spiraling around magnetic field lines. Directly related to the density and energy of the material, the synchrotron radiation is emitted across the radio spectrum, typically at frequencies lower than 10 GHz, making it an excellent choice for instruments like the Very Large Array.
Second Peak, Second Life?
Although we know about a third of the material from the star is ejected away from the supermassive black hole after the disruption, we do not understand how the black hole launches this material. For example, supermassive black holes in active galactic nuclei can launch powerful relativistic jets as they accrete massive amounts of material. Or, in a less energetic scenario, a jet does not have to be launched, and the outflows could be in all directions and essentially non-relativistic. In yet another situation, the delayed formation of an accretion disk may induce a relativistic jet to be launched much later than the initial disruption. To complicate matters further, it is almost certain that tidal disruption events do not originate from an underlying homogeneous population and that a spectrum of disruption scenarios results in many different ejecta geometries.
Today’s article uses the non-relativistic approach to model the tidal disruption event scenario. The authors model a shock quasi-spherically propagating first through a circumnuclear medium with a radially decreasing density and then through an interstellar medium with constant density. Using a standard set of code and modeling packages for synchrotron emission, they produce light curves for what this model should look like. In this model, there are two peaks caused by differing effects. The radio emission is “self-absorbed” in the first peak and transitions to optically thin, eventually peaking. By measuring the peak frequency and luminosity, we can estimate the radius of the outflow and local circumnuclear medium density. Then, depending on the spectral index of the circumnuclear medium’s radial density profile, the light curve will fall and eventually reach a minimum at the Bondi radius of the supermassive black hole. At this point, the radial density profile becomes flat (i.e., constant density interstellar medium), and the radio light curve will rise again as the shock wave sweeps up material. The brightness will continue to increase until the swept-up mass is comparable to the mass from the original ejected outflow. After the second peak, the radio brightness decreases indefinitely.
How does this model compare to some real scenarios? The authors of today’s article select two well-known events from the literature and gather radio observations to compare with their modeled light curves. When comparing AT2019dsg and AT2020vwl, the double-peaked feature is evident in both light curves, as seen in Figure 1. The authors note that while the rapid t3 initial rise is well explained for both sources, other radio-loud tidal disruption events, such as AT2018hyz, rise even faster like t5 and thus are better candidates for relativistic models. The authors state that further observations at even later times will enable improvements to this model and constrain their parameters.
The radio emission from tidal disruption events is caused by the material that survives the disruption of the star and is ejected away from the supermassive black hole. This stellar material runs into the ambient density surrounding the supermassive black hole, causing shocks inside the material. These shocks give rise to synchrotron radiation, an emission caused by free electrons in a plasma spiraling around magnetic field lines. Directly related to the density and energy of the material, the synchrotron radiation is emitted across the radio spectrum, typically at frequencies lower than 10 GHz, making it an excellent choice for instruments like the Very Large Array.
Second Peak, Second Life?
Although we know about a third of the material from the star is ejected away from the supermassive black hole after the disruption, we do not understand how the black hole launches this material. For example, supermassive black holes in active galactic nuclei can launch powerful relativistic jets as they accrete massive amounts of material. Or, in a less energetic scenario, a jet does not have to be launched, and the outflows could be in all directions and essentially non-relativistic. In yet another situation, the delayed formation of an accretion disk may induce a relativistic jet to be launched much later than the initial disruption. To complicate matters further, it is almost certain that tidal disruption events do not originate from an underlying homogeneous population and that a spectrum of disruption scenarios results in many different ejecta geometries.
Today’s article uses the non-relativistic approach to model the tidal disruption event scenario. The authors model a shock quasi-spherically propagating first through a circumnuclear medium with a radially decreasing density and then through an interstellar medium with constant density. Using a standard set of code and modeling packages for synchrotron emission, they produce light curves for what this model should look like. In this model, there are two peaks caused by differing effects. The radio emission is “self-absorbed” in the first peak and transitions to optically thin, eventually peaking. By measuring the peak frequency and luminosity, we can estimate the radius of the outflow and local circumnuclear medium density. Then, depending on the spectral index of the circumnuclear medium’s radial density profile, the light curve will fall and eventually reach a minimum at the Bondi radius of the supermassive black hole. At this point, the radial density profile becomes flat (i.e., constant density interstellar medium), and the radio light curve will rise again as the shock wave sweeps up material. The brightness will continue to increase until the swept-up mass is comparable to the mass from the original ejected outflow. After the second peak, the radio brightness decreases indefinitely.
How does this model compare to some real scenarios? The authors of today’s article select two well-known events from the literature and gather radio observations to compare with their modeled light curves. When comparing AT2019dsg and AT2020vwl, the double-peaked feature is evident in both light curves, as seen in Figure 1. The authors note that while the rapid t3 initial rise is well explained for both sources, other radio-loud tidal disruption events, such as AT2018hyz, rise even faster like t5 and thus are better candidates for relativistic models. The authors state that further observations at even later times will enable improvements to this model and constrain their parameters.
Figure 1: The late-time C-band (6 GHz) radio light curves of AT2019dsg and AT2020vwl. These sources have some of the best data quality and quantity in the literature. The double-peaked feature of our authors’ model is evident in the light curves of both events. Credit: Matsumoto & Piran 2024
Original astrobite edited by Archana Aravindan
About the author, Will Golay:
I am a graduate student in the Department of Astronomy at Harvard University and the Center for Astrophysics | Harvard & Smithsonian, advised by Edo Berger. I study radio emission from transient astrophysical objects like tidal disruption events.
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