Watching an AGN launch a jet in real time in 1ES 1927+654
Eileen Meyer
In addition to the division of AGN based on radio power, we also distinguish between type I (broad-lined) and type II (narrow-lined) AGN, reflecting differences in both the AGN continuum strength and obscuration. The discovery of changing-look AGN, which exhibit major state changes over weeks to months has overturned our previous assumptions that such changes occur over far longer timescales (e.g. Trakhtenbrot et al., 2019; Komossa et al., 2020; Kokubo & Minezaki, 2020).
The formerly type-II radio-quiet AGN 1ES 1927+654 is widely considered one of the most unusual and extreme CL-AGN yet discovered (Trakhtenbrot et al., 2019; Ricci et al., 2021; Masterson et al., 2022). Initially classed as a rare ‘true’ or ‘naked’ type-2 AGN due to strong narrow forbidden lines, and the complete lack of broad emission lines or significant X-ray absorption (Tran et al., 2011; Gallo et al., 2013), 1ES 1927+654 went through an extreme changing-look (CL) event beginning December 2017 when the optical and UV fluxes brightened by four magnitudes over a few months, concurrent with the sudden appearance of broad emission lines. While the optical/UV lightcurve appears very similar to a tidal disruption event, the X-rays do not, presumably because the source is already an active AGN. The X-ray band has shown extreme variability during and after, with the X-ray corona completely vanashing shortly after the TDE-like event, then bouncing back to an extreme high (Eddington limit) state for over a year, before dropping back to it’s previous baseline. It has continued to show significant months-scale variability since, including a recent significant rise in very soft X-ray emission (Laha et al, 2025).
Initial radio monitoring of 1ES 1927+654 during and in the few years after its CL event was sparse, but as shown in Figure 1, fortuitous monitoring observations with the VLBA caught the source as it began to rise significantly in the radio in early 2023. A massive radio TOO campaign was triggered, with observations from EVN, the VLBA, e-Merlin, the VLA, and AMI. The rise traced by EVN+VLBA appears exponential with a characteristic timescale of only 44 days. This has been interpreted as the launch of a new mildly relativistic jet from this AGN, with a delay of 5 years from the CL event. Interestingly, this is a similar timescale to the late-time emergence of radio emission recently discovered in regular (non-AGN) TDEs (Cendes et al., 2023). This work, which included multi-band EVN observations was recently published in the ApJ Letters (Meyer et al., 2025).
The source has maintained a relatively steady radio emission with a spectrum reminiscent of gigahertz-peaked sources, for over 1 year without obvious signs of fading or further increase. The soft X-ray emission, which began rising a few months prior to the flare, may arise from shock-heated gas impacted by the jet; this scenario the radio jet may have been first screened by larger-scale and pre-existing hot gas before breaking through, consistent with the delayed but rapid radio brightening. Our most recent high-resolution VLBI shows bipolar radio extensions of similar brightness separated by approximately 0.5 mas or 0.15 pc, with a tentative expansion of separation speed of ∼0.3c. The resolved structures resemble a very low-luminosity and very young compact symmetric object (CSO). CSO are understood to be short-lived jets of age < 1000 years, and one hypothesis is that they are powered by TDE events (Readhead et al. 2024). If so, it may be that we managed to catch the birth of a CSO at the start of its 1000 year lifetime. Continued follow-up with high-resolution and multi-frequency radio observations will allow us to further constrain the kinematics and energetics of the outflow in this ever-changing and unique AGN.
The soft X-ray and radio light curves of 1ES 1927+654 since 2021. The top panel shows the soft (0.3–2 keV) flux observed by the Swift/XRT. The middle panel shows the total (log scale) VLBA/EVN flux in bands C, X, and K (5, 8.4, and 22.2/23.5 GHz, respectively), along with fluxes from lower-resolution AMI and e-MERLIN observations at 15.5 and 5 GHz, respectively. The bottom panel shows the evolution of the radio spectral index between 5 and 8.4 GHz (light red) and between 8.4 and 22 GHz (dark red); open circles denote two epochs of near-simultaneous observations for the lower band index. While the X-rays have shown considerable variability during the years since the late 2017 CL event, the radio remained quiescent in all bands until exhibiting an exponential rise over a few months in early 2023. The radio evolution since has shown only mild variability at or slightly below the peak radio flux density reached in 2023 June, with the exception of K band. (Figure and caption adapted from Meyer et al., 2025)
References:
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Link to the paper:
Eileen T. Meyer et al 2025 ApJL 979 L2
Contact:
Eileen Meyer, University of Maryland, Baltimore County, USA. Email: meyer@umbc.edu