Title:Late-time Radio Brightening and Emergence of a Radio Jet in the Changing-look AGN 1ES 1927+654
Author(s): Eileen T. Meyer, Sibasish Laha, Onic I. Shuvo, Agniva Roychowdhury, David A. Green, Lauren Rhodes, Amelia M. Hankla, Alexander Philippov, Rostom Mbarek, Ari laor, Mitchell C. Begelman, Dev R. Sadaula, Ritesh Ghosh, Gabriele Bruni, Francesca Panessa, Matteo Guainazzi, Ehud Behar, Megan Masterson, Haocheng Zhang, Xiaolong Yang, Mark A. Gurwell, Garrett K. Keating, David Williams-Baldwin, Justin D. Bray, Emmanuel K. Bempong-Manful, Nicholas Wrigley, Stefano Bianchi, Federica Ricci, Fabio La Franca, Erin Kara, Markos Georganopoulos, Samantha Oates, Matt Nicholl, Main Pal, S. Bradley Cenko
First Author’s Institution: Department of Physics, University of Maryland, USA
Status: Published in the Astrophysical Journal Letters [open access]
First you see me, now you don’t
Supermassive black holes (SMBHs) are responsible for the most energetic events in the universe and continue to exhibit puzzling behaviors. Hosted at the centers of massive galaxies, some remain “quiescent”: relatively devoid of any gas and dust in its circumnuclear medium (CNM) and slowly accreting new material. SMBHs are notoriously messy eaters, meaning the more they consume, the more material “misses” and can be energetically launched away from the SMBH along its spin axis in the form of an astrophysical jet we can observe from Earth. These black holes are called “active galactic nuclei” (AGNs) and are found throughout the universe, usually classified by their orientation relative to us. In the rarest scenario, the jet is pointed directly at Earth and is observed as an extremely bright “quasar.” In most cases, depending on how “off-axis” (the angle between the jet and the Earth) we observe this jet, it can be classified as Type I or Type II in the Unified AGN Model, which have different properties due to how much of the dusty ring surrounding the AGN is in view. An important implication is that this basic underlying model cannot explain transitions between AGN types. In today’s article, the authors observed one exception to this model in the “Changing Look” AGN (CLAGN) 1ES 1927+654.
Although it’s long been known that this simple AGN unification framework does not explain all the observed phenomena in AGN, it does predict slow, multi-year sub-magnitude variation in the optical, UV, and X-ray luminosities. However, CLAGN are delineated by their orders-of-magnitude luminosity changes and apparent transition from one type to another. These drastic changes over weeks or months remain incompatible with this simple picture.
The target 1ES 1927+654 was first identified as a “changing look” candidate after an optical flare in December 2017 with a peak brightness of deltaV=5 magnitudes (~100x brighter). Most recently, a new X-ray component appeared in late 2022, which triggered Director’s Discretionary Time (DDT) follow-up observations using the NRAO Very Long Baseline Array (VLBA).
Figure 1: Four separate observations of K-band (22 GHz)Very Long Baseline Array (VLBA) imaging spanning roughly one year demonstrate the expansion of two oppositely-located radio sources plus the radio core (subtracted from the colormap and illustrated as contours on each image). The two unresolved radio components are expanding radially away from each other with an apparent velocity of around 30% the speed of light.
Launching a jet
Radio imaging of AGNs is a unique probe of their behavior since, unlike other observing wavelengths, it enables us to trace the highest-velocity outflows. The material launched undergoes shocks that generate synchrotron emission that we can observe at ~GHz frequencies with radio telescopes like the VLBA.
The authors of today’s paper observed the famous CLAGN 1ES 1927+654 four times with the VLBA in K-band (22 GHz) over a year, beginning approximately six months after the initial X-ray flare. The authors subtracted an “unresolved core” corresponding to a point source centered on the SMBH; see, e.g., the featured image of Cygnus A, an example of a many-thousands-of-years-old AGN. The resulting “residual images” are plotted in Figure 1. This sequence of four images demonstrates the radial expansion of two unresolved radio sources from the central source over a year. At the distance to this source (redshift z = 0.017; image scale 0.35 parsecs/milliarcsecond), the approximate apparent speed of these sources expanding from one another is 30% the speed of light. However, the authors point out that future monitoring will enable a more accurate kinematic analysis for a precise velocity measurement.
The directly measured apparent velocity of the expanding jet (and counterjet from the other axis) agreed remarkably well with several independent indirect estimates of the outflow. The apparent equal brightness of the two jets allows the authors to argue that the AGN’s orientation must be at least somewhat “off-axis” at >20 degrees or so. The structure is remarkably similar to that of “low-luminosity” compact symmetric objects (CSOs), which have been suggested to be powered by the disruption of a star by the SMBH in a “tidal disruption event” (TDE). The late-time radio rebrightening is also reminiscent of the behavior of optically-discovered TDEs. However, the original X-ray properties responsible for the event’s discovery remain highly differentiated from the TDE case. Further VLBA monitoring will enable a more detailed analysis of the jet’s evolution and allow us to begin sketching the conditions and requirements for understanding these closely related SMBH phenomena.