EVN observation reveals a radio flare in the changing-look AGN 1ES 1927+654

ATel #15382; Xiaolong Yang (SHAO), Prashanth Mohan (SHAO), Tao An (SHAO), Erin Kara (MIT), Jun Yang (OSO), Minfeng Gu (SHAO), Ran Wang (KIAA-PKU), Luic C. Ho (KIAA-PKU), Xiang Liu (XAO), Jiri Svododa (ASU-CZ), Luigi C. Gallo (SMU), Michael Loewenstein (UMD/GSFC), Ruancun Li (KIAA-PKU)
on 16 May 2022; 18:11 UT
Credential Certification: Xiaolong Yang (yangxl@shao.ac.cn)

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We report on recent high-resolution very long baseline interferometry (VLBI) observations of the changing-look AGN 1ES 1927+654 (z=0.017). The All Sky Automated Survey for SuperNovae (ASAS-SN) first reported an optical flare from the nuclear region of 1ES 1927+654 on 2018-03-03 (ATel #11391, Nicholls et al., 2018); this was accompanied by the emergence of broad Balmer lines in the optical spectrum (Trakhtenbrot et al., 2019) rendering a changing-look AGN (from Type 2 to Type 1). The Neutron star Interior Composition Explorer (NICER) observations of 1ES 1927+654 (on 2018-05-22) found an extremely soft X-ray spectrum and a continued decrease in the X-ray luminosity (ATel #12169, Kara et al., 2018) compared to archival data. This was followed by a NICER detection of increased X-ray luminosity beyond 1st July (ATel #12169, Kara et al., 2018) -- 4 months after the optical outburst. The dense optical/UV and X-ray monitoring observations (Trakhtenbrot et al., 2019; Ricci et al., 2021) support the changing-look nature of 1ES 1927+654. We have been monitoring this source in the radio band for ~ 5 years (Yang et al. In preparation) since 2018. The observations reveal a relatively faint radio emitting source with a flux density of 3 - 6 mJy between 2018 and March 2021. Our most recent target of opportunity European VLBI Network (EVN) observations of 1ES 1927+654 at 5 GHz were conducted on 23 October 2021 (from 15:00 to 19:00 UT); 14 telescopes participated in this observation. The target was phase-referenced to the nearby source J1933+6540 (RA: 19:33:57.3373, DEC: +65:40:16.828, J2000), which is 0.69 degrees away. The EVN data were correlated on 26 March 2022 at JIVE. Preliminary analysis indicates that the target is slightly resolved in this observation, just under a beam size of 3.31 * 1.32 milli-arcsec and has a peak at RA: 19:27:19.5400, DEC: +65:33:54.336 (J2000). The observation yielded a 5 GHz total flux density of 8.4+-0.3 mJy, two times higher than the 5 GHz VLBA observation on 15 March 2021, which is 4.04 +- 0.15 mJy (Yang et al. In preparation). It indicates the source has entered a brightening phase in the radio band. We calibrated additional data and confirmed the long-term radio flare activity. The increase of radio flux density follows the most recent optical and X-ray flares by ~700 and ~450 days, respectively. Further radio observations have been planned; we encourage multi-band follow-up observations during this opportune phase. We thank the EVN Program Committee Chair, Zsolt Paragi for supporting our ToO observation. We thank the Head of Science Operations, Bob Campbell, and the EVN Scheduler, Alastair Gunn for making this observation possible. The participating telescopes were Badary, Effelsberg, Irbene, Jodrell Bank, Kunming, Medicina, Onsala, Shanghai, Svetloe, Torun, Urumqi, Westerbork, Yebes and Zelenchukskaya. The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. Scientific results from data presented in this publication are derived from the following EVN project code: RY009.