Title: An untargeted search for radio-emitting tidal disruption events in the VAST Pilot survey
Authors: Hannah Dykaar, Maria R. Drout, B. M. Gaensler, David L. Kaplan, Tara Murphy, Assaf Horesh, Akash Anumarlapudi, Dougal Dobie, Laura N. Driessen, Emil Lenc, and Adam J. Stewart
First Author’s Institution: David A. Dunlap Department of Astronomy and Astrophysics, Dunlap Institute for Astronomy and Astrophysics, University of Toronto
Status: Published in ApJ (Open Access)
The supermassive black holes in the center of most galaxies are notoriously, and predictably, violent actors in the universe. While some, classified as Active Galactic Nuclei (AGN, or AGNi, in the plural form), act like a drain on their host galaxies, swallowing anything and everything that falls into them, even dormant black holes will react destructively when provoked. Orbit too closely, and any galactic nucleus will break you apart like a first-year chemistry student bumping an unsuspecting beaker off the lab bench. As the ill-fated star falls into the black hole, the system will briefly glow across the electromagnetic spectrum. When and where these mishaps, known as Tidal Disruption Events (TDEs, shown in Figure 1), occur, as well as the exact physical processes causing the brief glow, are not well understood. TDEs have been detected overwhelmingly in non-AGN galaxies which are calming down after an era of intense star formation, and current models of the TDEs occurence rate disagree with observations. We expect to see more types of galaxies, such as AGNi, to host TDEs at similar rates, but we don’t – however, we might just be looking in the wrong places, or rather, with the wrong set of eyes. Traditionally, TDEs have been identified by their optical, UV, or x-ray emission, but AGNi are surrounded with dust, which absorbs this light on its way to us. However, at radio wavelengths, the issue of dust obscuration fades, allowing us to uncover the TDEs that may be hiding. While radio emission has been observed from known TDEs, identifying TDEs in the radio comes with a major hurdle, presented by the pesky AGNi themselves; they are famously variable in radio emission, and serve as pretty convincing TDE imposters.
Searching for TDEs at radio wavelengths
Today’s authors decide to take on this challenge, armed with data from the Variable and Slow Transients (VAST) pilot survey, which observes large swathes of the sky at regular intervals to track variability on the order of days to months. VAST is optimized for observing TDEs, but unfortunately, it is also excellent at finding AGNi. How do we know what to look for, and how can we distinguish a TDE from an AGN? Easy, we can just identify characteristics common to all the known radio-emitting TDEs in the VAST field of view – all one of them, that is. Surely, that won’t do. Instead, our authors simulate the evolution of TDEs as seen by VAST, which can only catch discrete snapshots of light at a specific radio wavelength. Their models of TDE radio emission assume one of three cases – either the TDE produces a relativistic jet directed at us (on-axis), directed away from us (off-axis), or none at all – which determines the shape of the light curve, as shown in Figure 2.
From these simulations, the authors identify three overarching characteristics that wannabe TDEs must exhibit: first, they must be variable, signaling the flare of activity as the star crashes into the black hole; second, the flare should be sufficiently bright compared to the galaxy’s normal brightness; and third, the flare must last for more than one observation, to ensure it is not a spurious detection. Additionally, the authors find that the peak brightness of the TDE must be double the typical galaxy brightness to effectively rule out AGN imposters, which do not tend to vary this drastically, as shown in Figure 3. Lastly, the TDE must actually occur near the center of a galaxy (the black hole locale), as confirmed by optical or infrared survey catalogs. In the VAST pilot survey, 12 sources meet these criteria.
Following up on TDE candidates in other wavelengths
The authors next subject these TDE candidates to thorough multi-wavelength scrutiny using archival survey data. First, they investigated whether they are associated with gamma-ray bursts (GRBs), which are extremely luminous and energetic events which may accompany TDEs. Unfortunately, gamma-rays are easily absorbed, making them notoriously difficult to trace back to their sources (after all, the journey of a gamma-ray through light-years of dust and gas to the Earth is not unlike Odysseus’ return to Ithaca, and we all know how many made that journey unscathed). They found that all 12 sources were coincident with a GRB, but they were also coincident with multiple GRBs (which is unlikely to be physical), as were randomized, TDE-free regions of the VAST sky. In other words, the GRB association is inconclusive. Contemporary optical and infrared observations of the candidates revealed no corresponding flares, which leads to more questions. Are the sources simply too far away for their optical and infrared flares to be discernible, or could dust absorption be at play? Additionally, nearly all candidates maintained an increased radio flux after the TDE flare. This may indicate that the TDE occurred within an AGN as it was transitioning to a higher radio flux state, that the TDE was followed by intense star formation, or both.
By comparing their candidates to the expected observational manifestations of their TDE models, the authors conclude that the candidate sources are consistent with TDEs which have relativistic jets. They also independently constrain the incidence rate of TDEs, which agrees with current theory. As our window into the variable radio universe expands with future observations, such as with the ongoing VAST survey, we will have a growing population of such radio-detected TDEs to study, and the ability to distinguish them from regular AGNi will be ever more valuable in our quest to understand them.
