NICER, Swift and ZTF identify further X-ray and optical brightening of the ongoing changing-look AGN AT2021fxu

ATel #15825; Yukta Ajay (IISER Tirupati), Dheeraj Pasham (MIT), Muryel Guolo (JHU)
on 24 Dec 2022; 17:52 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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AT2021fxu/ZTF21aalxxzn/Gaia22dgm is an optical transient identified by the Zwicky Transient Facility on 13 February 2021 09:06:19 UT, classified as an AGN at a redshift of 0.1097 (Newsome et al., 2022). It had previously been cataloged by the BAT AGN Spectroscopic Survey as an AGN (2MASX J13471874+0210579/SWIFT J1347.7+0212) (Koss et al., 2017). A comparison of the optical spectra from the Sloan Digital Sky Survey (SDSS) in 2001 and the Faulkes Telescope North (FTN) in 2022 revealed the appearance of previously absent broad emission lines (BELs) in the recent FTN spectrum. This suggests a shift in the spectral class of the AGN, from Type-II to Type-I, indicative of a changing-look (CL) AGN.

AT2021fxu showed an optical outburst in 2022, with an optical rise spanning 1 April 2022 to 27 May 2022, followed by an optical decline until 19 July 2022. During the optical decline, Swift/XRT and NICER carried out multiple observations of the source, and the X-ray spectra are consistent with a simple powerlaw model (powerlaw index ~1.8). No change in the neutral column density since 2011 implied that the spectral transition occurred due to a change in accretion rate, and not because of variable obscuration along our line-of-sight. Furthermore, the optical g- and r- magnitudes of the source remained approximately the same from 2000, until 2021, which confirmed that the optical outburst in 2022 coincided with the CL transition in the AGN, and that AT2021fxu is currently undergoing its spectral transition (see Ajay et al. 2022, RNAAS for more details).

After a decay of ~0.34 and ~0.2 magnitudes in the optical g- and r- over about 2 months after the first optical peak (until July 2022), the source showed further optical brightening reaching a second optical peak in November 2022, which was 0.12 magnitudes brighter than the first peak in May 2022. This motivated us to obtain additional Swift and NICER observations. Swift/XRT recorded a mean background-subtracted countrate of (0.104+/-0.009) cps on 11 December 2022, and (0.063+/-0.010) cps on 20 December 2022. Modeling the 0.3-10keV XRT spectrum from 11 December 2022 with the same powerlaw model as before, viz, TBabs*zTBabs*zashift(powerlaw) in XSPEC, gave a best-fit powerlaw index of 1.93+/-0.21. The observed 0.3-10 keV luminosity and flux from that XRT spectrum were (1.02+/-0.09)e+44 ergs/s and (3.36+0.49-0.27)e-12 ergs/cm^2/s, respectively (68% confidence values), which are higher by a factor of about 1.5 as compared to the previously observed XRT values after the outburst in April-May 2022. Similarly, we fit the 0.3-2 keV NICER spectrum (1.25+/-0.02cps) recorded between 16-17 December 2022 with the same powerlaw model and obtained a best-fit powerlaw index of (2.15+0.13-0.07), and 1.5x10^20 /cm^2 as an upper limit on the neutral column. These values are consistent with their respective measurements from the outburst in April-May 2022. The observed NICER 0.3-10 keV luminosity and flux are (7.19+0.62-1.28)e+43 ergs/s and (2.31+0.06-0.21)e-12 ergs/cm^2/s (68% confidence values), respectively, lower than the XRT values recorded about 5 days prior to NICER in December 2022.

These values suggest that the source has shown an overall increase in the observed X-ray flux following the new outburst (November 2022), and is variable in X-rays, but the overall X-ray spectral shape has remained the same. Further NICER and Swift monitoring observations are planned. We encourage optical spectroscopy and radio observations of this ongoing CL AGN.

References: Newsome M., Dgany Y., Arcavi I. et al. 2022, TNSCR, 2022-1320

Koss M., Trakhtenbrot B., Ricci C. et al. 2017 ApJ 850 74

Ajay Y., Guolo M., Pasham D. 2022 Res. Notes AAS 6 274 (DOI 10.3847/2515-5172/acacf1)