AstroNote 2024-173

AT 2021hdr / ZTF21aaqqwsa (https://alerce.online/object/ZTF21aaqqwsa) was discovered on Mar 22, 2021 (MJD = 59295) in the ZTF public alert stream at RA = 21:24:00.341, Dec = +34:09:11.49 (J2000). It was reported to TNS by the ALeRCE broker (Munoz-Arancibia et al. 2021). It was spectroscopically classified as an AGN based on an optical spectrum taken on July 3rd 2022 (MJD = 59763) by ZTF using LT/SPRAT (Johansson et al. 2022).

AT 2021hdr / ZTF21aaqqwsa is located towards the nucleus of the Seyfert 1 galaxy 2MASX J21240027+3409114, also known as PBC J2123.9+3407 (z = 0.083, Parisi et al. 2014). Milky Way extinction coefficients towards this region at ZTF bands are A_g = 0.44 mag and A_r = 0.30 mag, using the dust reddening map of Schlafly & Finkbeiner (2011), the Fitzpatrick (1999) extinction law, and an extinction to reddening ratio Rv = 3.1.

We obtained PSF forced photometry on ZTF difference images (Masci et al. 2023) at the AT 2021hdr / ZTF21aaqqwsa location, including epochs from the start of the ZTF surveys to date (MJD = 58194 to 60489). We attach figures that display these light curves in both difference and total estimates (i.e. including the host contribution to light, obtained following Forster et al. 2021 Appendix A.3). In the following, all quoted magnitudes are MW-dust deattenuated AB magnitudes. We adopt a flat LCDM cosmology with parameters H0 = 70 km/s/Mpc, Omega_m = 0.3 and Omega_lambda = 0.7.

After an initial event in ZTF alerts on MJD = 59295 to 59297 (peak apparent magnitude 16.55 in r-band, no alerts in g-band), AT 2021hdr / ZTF21aaqqwsa has experienced successive re-brightenings in an oscillating fashion. From around MJD = 59730 to date, at least six re-brightening episodes have been noticed in ZTF data. Four of them have starting dates around MJD = 59730, 59820, 60140 and 60210, each lasting roughly 60 to 90 observed days and reaching peak apparent magnitudes 17.06 to 16.90 (16.42 to 16.32) in the g- (r-) band. These imply durations of roughly 55 to 80 rest-frame days and peak absolute magnitudes -20.82 to -20.98 (-21.46 to -21.56) in the g- (r-) band. Other events before 2024 in alerts occurred soon before or after seasonal gaps, making their starting and end dates more uncertain.

ZTF forced photometry reveals that, from its first observation (MJD = 58218) to around MJD = 59010, AT 2021hdr / ZTF21aaqqwsa had mean apparent magnitudes of 17.53 and 16.66 in g- and r- bands, respectively (absolute magnitudes -20.35 and -21.23), with a deviation from this baseline only by up to 0.11 mag. After that, it shows two long re-brightenings, with approximate dates from MJD = 59010 to 59440 (duration of roughly 400 rest-frame days, with peak absolute magnitudes of -20.56 and -21.33 in g- and r- bands, respectively; the first ZTF alert was received within this time range), and from MJD = 59440 to 59730 (roughly 270 rest-frame days, with peak magnitudes -20.70 and -21.40). Both time ranges include gaps when the source was not observed by ZTF, which prevents the characterization of these brightenings as single or multiple events. ZTF forced photometry also shows that, from around MJD = 59490 to date, all epochs have been brighter than the baseline in both g- and r- bands. Accounting only for the transient emission and after MJD = 59500, all epochs have been brighter than 21 uJy, reaching flux densities up to 264 and 300 uJy in g- and r- bands (corresponding to ν Lν values of 2.8e43 and 2.4e43 erg/s, respectively).

An oscillating pattern is also found in the evolution of the ZTF g-r color using difference imaging magnitudes (see attached figure). This behavior is clearly seen during the two brightening episodes from 2022 (starting dates around MJD = 59730 and 59820) and becomes noisier afterwards. From around MJD = 59730 to date, difference imaging g-r colors range between -0.08 to 0.90 mag, with the bluest (< 0.10 mag) colors occurring mostly during the first rise. A bluer-when-brighter trend is found for the transient component, same as when including the host contribution.

AT 2021hdr / ZTF21aaqqwsa then has shown long-lasting, ongoing transient emission spanning more than 1300 rest-frame days, after at least 730 rest-frame days of no variability above 0.11 mag amplitude in a Seyfert 1 nucleus. Multiwavelength follow up to investigate its nature has been conducted since late 2022. The distinctive variability displayed by AT 2021hdr / ZTF21aaqqwsa can be interpreted as a newly found transient in an AGN. Among the possible drivers of this emission is the tidal disruption of an infalling cloud onto a yet unconfirmed supermassive black hole binary (Goicovic et al. 2016).

This report is based on observations obtained with the Samuel Oschin 48-inch Telescope and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: M. J. Graham). We acknowledge the use of the ALeRCE broker, in particular its Web Interface and the ZTF Forced Photometry Notebook.