Delayed X-ray brightening of the nuclear transient AT2022exr/ZTF22aadgefj

ATel #15574; Muryel Guolo (JHU), Suvi Gezari (STScI), Dheeraj Pasham (MIT), Erica Hammerstein (UMd), Yuhan Yao (Caltech)
on 25 Aug 2022; 21:14 UT
Credential Certification: Muryel Guolo (mguolop1@jhu.edu)

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The nuclear transient AT2022exr/ZTF22aadgefj was first detected by ATLAS (Tonry et al., 2022) and later classified as a TDE due to its persistent blue optical colors, UV detection, and characteristic TDE emission features in follow-up optical spectra (Hammerstein et al., 2022). However, late time ZTF data show a second peak in the optical, unlike classical TDE, but similar to the nuclear transient AT2019avd (Malyali et al., 2021). X-rays were not detected during the two optical peaks.

On 22 August 2022 (MJD 58912), Swift/XRT detected the source with a background-corrected count rate of 5.4 +/- 0.5 x10^(-2) ct/s (0.3-10.0 keV). The spectrum is soft, with essentially all counts below 1keV. We fit the spectrum containing 94 counts with a thermal disk model, assuming Galactic absorption (N_H ~ 5.8x10^20 cm^-2), and the reported redshift z = 0. 096 (TBabs*zashift*diskbb, in XSPEC), and using cash statistics. The best-fitting temperature is 0.11 +/- 0.01 keV. The 0.3-10 keV observed (unabsorbed) flux is 1.8 +/- 0.2 x 10^-12 (3.7 +/- 0.4 x 10^-12) erg/cm^2/s, which at the luminosity distance of the source (445 Mpc) represents an unabsorbed luminosity of 8.7x10^43 erg/s. This value is typical for X-ray TDEs. Assuming the same spectrum, we measured a 3 sigma upper flux limit of 2.2x10^-13 erg/cm^2/s (6.6x10^-3 ct/s) on 12 August 2022 (MJD 59802). This represents a factor of at least 10 increase in ten days.

This X-ray detection occurred ~140 days after the first optical peak and ~25 days after the second one. This adds AT2022exr to the growing list of optically-selected nuclear transients with delayed X-ray brightening. We have requested for additional Swift and NICER monitoring. Follow-up is encouraged especially in the radio band.

Tonroy et al., 2022, TNS Astronomical Transient Report No. 142163.
Hammerstein et. 2022, TNS Classification Report No. 12383.
Malyali, A., Rau, A., Merloni, A., et al. 2021, A&A, 647, A9.