AstroNote 2023-195

The optical transient AT2019aalc/ZTF19aaejtoy has been suggested to be a tidal disruption event (TDE) candidate (Veres et al. 2023) and also potentially the host of the IceCube neutrino event IC191119A (van Velzen et al. 2021). Recently, it was reported to re-brighten in the optical band 4 years after its first flare in April 2019 (Veres et al. 2023).

We gathered archival multiwavelength observations on this source to investigate its nature.

We started by obtaining Zwicky Transient Facility (ZTF) absolute-photometry (i.e., not differential-photometry) from the ZTF forced photometry server (Masci et al. 2019). The light curve shows variability even before the first flare, discovered by ZTF in January 2019 (MJD ~ 58500). Furthermore, the g-band flux density at the peak of the first flare (~2.59e-15 cgs) is less than a factor of 2 higher than the mean pre-flare flux density (~ 1.60e-15 cgs).

We also obtained neoWISE infra-red (IR) photometry (from MJD ~56600 to ~59600). Clear IR variability is present in both W1 and W2 bands before the first flare. The IR W1-W2 color before the first flare was ~ 0.5 mag (Vega system); such color is within the 90% completeness AGN selection range by Assef et al. 2013, and indicates the presence of a hot dust `torus', even before the flare.

The X-ray properties of the source were recently reported by Pasham (ATEL #16118), who found that a typical AGN-like power-law (with index ~2.6) model is a better fit to the X-ray spectrum than a thermal model (diskbb) usually observed in TDEs. The observed flux variability in the X-rays was also less than a factor of 2.

Archival Sloan Digital Sky Survey (SDSS) taken in 2008 (MJD 54562), i.e., much before the first flare, shows clear Halpha, Hbeta, and Hgamma broad emission lines, as well as strong narrow forbidden lines, such as [O III] and [N II], with apparent high [OIII]/Hb and [N II]/Ha. All clear characteristics of type I AGN, in agreement with the classification of the event by van Velzen et al. (2021) as an AGN flare.

The source has also shown radio emission before the first flare (van Velzen et al. 2021), which similarly favors an active nucleus and not a quiescent galaxy previous to the 2019 flare.

In summary, the overall multiwavelength properties of this object are all reminiscent of a typical Seyfert 1 AGN, including the mild variability, which can be well explained by known red noise present in AGN. None of the properties requires a TDE scenario to explain AT2019aalc or its rebrightening. 

* Veres el al. 2023, AstroNote 2023-194

* van Velzen et al., arXiv e-prints, arXiv:2111.09391 (2021)

* Assef R. J., et al., 2013, ApJ, 772, 26

* Pasham 2023 (ATEL #16118)