Black hole fireworks: tidal disruption events light up SMBHs

Title: Late-time Radio Flares in Tidal Disruption Events

Author(s): Tatsuya Matsumoto and Tsvi Piran

First Author’s Institution: Department of Astronomy, Kyoto University

Status: Accepted to the Astrophysical Journal [open access]

Hungry (and loud) black holes

Tidal disruption events (TDEs) arise when a star wanders too close to a supermassive black hole (SMBH) 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 SMBH into the “circumnuclear medium,” or the region immediately surrounding the SMBH. We typically discover TDEs from the optical emission resulting from the initial disruption, which lasts several weeks. However, TDEs are known to be multi-wavelength events visible across the electromagnetic spectrum. Before optical TDEs were discovered, almost all of the TDEs were found in the X-ray, where the formation of an accretion disk around the SMBH may be powering some high-energy activity. On the other end of the spectrum, the radio properties of TDEs have proven to be unique. Today’s paper aims to explain the radio light curves of TDEs.

The radio emission from TDEs is caused by the material that survives the disruption of the star and is ejected away from the SMBH. This stellar material runs into the ambient density surrounding the SMBH, 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 <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 SMBH after the disruption, we do not understand how the SMBH launched this material. For example, SMBHs in active galactic nuclei (AGN) 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 are 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 TDEs do not originate from an underlying homogeneous population and that a spectrum of disruption scenarios results in many different ejecta geometries.

Today’s paper uses the non-relativistic approach to model the TDE scenario. The authors model a shock quasi-spherically propagating through a radially decreasing density circumnuclear medium that reaches a constant density interstellar medium (ISM). 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 how steep the spectral index of the radial density profile is of the circumnuclear medium, the light curve will fall and eventually reach a minimum at the Bondi radius of the SMBH. At this point, the radial density profile becomes flat (i.e., constant density ISM), 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 and decreases indefinitely.

Figure 1: The fifth figure in today’s paper displays 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.

How does this model compare to some real scenarios? The authors of today’s paper select two well-known events from the literature and gather radio observations to compare with their light curves. When comparing AT2019dsg and AT2020vwl, the double-peaked feature is evident in both light curves, as seen in Figure 5 of today’s paper. The authors note that while the rapid t3 initial rise is well-explained for both sources, other radio-loud TDEs, 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.

Astrobite edited by Archana Aravindan

Featured Image Credit: Carl Knox / OzGrav, ARC Centre of Excellence for Gravitational Wave Discovery, Swinburne University of Technology

About 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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