Enhanced optical emission from the Seyfert 1.9 galaxy IC 3599 and a late report of its potential missed brightening in 2020

ATel #17483; Patrik Milan Veres (Ruhr-University, Bochum)
on 7 Nov 2025; 00:07 UT
Credential Certification: Patrik Milan Veres (veres@astro.ruhr-uni-bochum.de)

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We report the recent optical brightening of the Seyfert 1.9 galaxy IC 3599. The AGN was detected by Swift (ATel #17479 D. Grupe) in a bright X-ray state with soft spectrum.

We requested forced photometry data using the Zwicky Transient Facility (ZTF) Forced-Photometry Service following the recommendations outlined at https://irsa.ipac.caltech.edu/data/ZTF/docs/forcedphot.pdf. It shows an optical brightening (in r-band) between 20th of October and 26th of October this year. The source has not been observed since 26th of October. We note that the nucleus of IC 3599 is associated with the known transient ZTF18aahsnat. A light curve of it is available in the AleRCE ZTF Explorer at https://alerce.online/object/ZTF18aahsnat.

IC 3599 is known as a repeating partial TDE candidate in an AGN with X-ray flares detected in the past. Its X-ray flares occurred around 1990 and 2010 (in the latter year an optical flare was detected as well) and the three-outbursts model implied a flare in 2000 which was largely missed due to sparse observations (see more detail in [1]). A bright state was expected around 2020 which did not happen [2]. We note that the ZTF differential light curve hints enhanced optical emission around 2020.

Especially optical spectroscopic follow-up observation of IC 3599 is encouraged.

References: [1] S. Campana et al 2015, A&A 581, A17 [2] Dirk Grupe et al 2024, ApJ 969 98

The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham).

The extinction-corrected differential light curve (PSF-fit) and the light curve zoomed around 2020