Artist's impression of a tidal disruption event — the ripping apart of a star by a black hole
Credit: NASA/JPL-Caltech
Title: An Untargeted Search for Radio-Emitting Tidal Disruption Events in the VAST Pilot Survey
Authors: Hannah Dykaar et al.
First Author’s Institution: University of Toronto
Status: Published in ApJ
supermassive black holes in the centers of most galaxies are notoriously, and predictably, violent actors in the universe. While some, classified as
active galactic nuclei, 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.
If an ill-fated star falls into a 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 galaxies that do not have active
galactic nuclei and are calming down after an era of intense star
formation, and current models of the TDE occurrence rate disagree with
observations. We expect to see more types of galaxies, such as those
with active galactic nuclei, that 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.
Figure 1: An artist’s impression of a tidal disruption event observed with X-ray and optical telescopes. Credit:
X-ray:
NASA/CXC/Queen’s Univ. Belfast/M. Nicholl et al.; Optical/IR:
PanSTARRS, NSF/Legacy Survey/SDSS; Illustration: Soheb Mandhai / The
Astro Phoenix; Image Processing: NASA/CXC/SAO/N. Wolk
Traditionally, TDEs have been identified by their optical, ultraviolet, or X-ray
emission, but active galactic nuclei are surrounded by dust, which
absorbs light at these wavelengths 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 active galactic nuclei themselves; they
are famously variable in radio emission, and they can 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 swaths 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 active galactic nuclei. How do we know what to look for, and
how can we distinguish a TDE from an active galactic nucleus? 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. The presence or absence of a jet, and its direction, determine the
shape of the light curve, as shown in Figure 2.
Figure 2: This figure shows the change in radio brightness over time we expect
to see from a galaxy during a TDE given different models. The shape of
the radio flare depends strongly on whether the TDE results in a
relativistic jet, and if so, whether the jet points toward us (on-axis)
or not (off-axis). These simulated light curves were used to establish
criteria for TDE candidacy, and compared with observations from the
final sample to constrain the incidence rate of TDEs and likelihood of
different jet geometries. Credit: Dykaar et al. 2024
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 active galactic
nucleus 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.
Figure 3: To distinguish TDEs from active galactic nucleus imposters, the
authors kept only sources that exhibited one dominant peak in their
radio flux, shown by the blue windows. Sources with secondary peaks
(shown by the purple windows) that were much smaller than the primary
peak were allowed, as the secondary peak could reasonably be due to
ambient active galactic nucleus activity. However, multiple comparable
peaks are indicative of only intrinsic active galactic nucleus
fluctuations, not a TDE. Credit: Dykaar et al. 2024
Following Up on TDE Candidates at Other Wavelengths
The authors next subject these TDE candidates to thorough
multi-wavelength scrutiny using archival survey data. First, they
investigated whether the candidates are associated with
gamma-ray bursts, which are extremely luminous and energetic events that 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 Earth is
not unlike
Odysseus’s return to Ithaca, and we all know how many made
that journey unscathed.)
The authors found that all 12 sources were coincident with a gamma-ray burst, but all 12 sources were also coincident with
multiple
gamma-ray bursts (which is unlikely to be physical), as were
randomized, TDE-free regions of the VAST sky. In other words, the
gamma-ray burst 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 active galactic nucleus 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 that have relativistic jets.
They also independently constrain the TDE incidence rate, 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 active galactic nuclei
will be ever more valuable in our quest to understand them.
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About the author, Chloe Klare:
I’m a PhD student in Astronomy and Astrophysics at Penn State, with a physics doctoral minor. In my research, I’m looking for newly evolving synchrotron jets in active galactic nuclei (in the radio!).
