AstroNote 2025-13

AT2024tvd was initially reported to the Transient Name Server as ZTF22aaigqsr (Sollerman et al. 2024; TNS discovery report 2024-3166) based on a detection on 2024 August 25 at g = 19.68 +/- 0.23 mag as part of the Zwicky Transient Facility high-cadence partnership survey. It was classified as a tidal disruption event (TDE) on 2024 October 14 based on long-lasting UV emission, proximity to the galaxy nucleus, and broad hydrogen and helium spectral lines in its optical spectra (Faris et al. 2024; TNS classification report 18377). The host of AT2024tvd appears to be a S0-type galaxy with a well-defined centroid. A historical SDSS spectrum gives a redshift of z=0.045. The galaxy nucleus coordinates given by the DESI Legacy Imaging Survey (LS), PS1, and SDSS are consistent with each other (within a distance of 0.1’’). The LS host centroid position is R.A. = 17:10:42.53, decl. = +28:50:14.29 (J2000). 

An assessment of the ZTF position of the transient reveals that AT2024tvd is offset from the nucleus of the host galaxy. The median position of ZTF alerts generated for AT2024tvd is R.A. = 17:10:42.57, decl. = +28:50:15.11, which is 0.98’’ away from the host galaxy centroid. This off-nuclear location was further validated by modeling of the multi-epoch ZTF imaging using the scene modeling code Scarlet2 (Ward et al. 2024; arXiv:2409.15427). 

Follow-up observations with the Swift satellite reveal bright UV and X-ray counterparts. The location of the UV source measured from UVOT images is consistent with the optical ZTF location and offset from the galaxy nucleus. Over the past three months, the XRT X-ray flux has generally increased from ~1e-13 erg/s/cm^2 to ~3e-12 erg/s/cm^2, with significant variability (i.e., at least a multiplicative factor of two in flux change) detected on short timescales (from 1.6 hours to a few days). Such a high luminosity (L_X ~ 1e+43 erg/s) and short-timescale variability are incompatible with a supernova origin, and has been previously observed in other TDEs, such as AT2022lri (Yao et al. 2024, ApJ, 976, 34). 

Observations with the Chandra X-ray Observatory confirm the off-nuclear location of the X-ray source and prove the physical association of the X-ray source with the UV/optical TDE. AT2024tvd was observed by Chandra under a DDT program (PI Yao) on 2024 November 18 for a total of 21.5 ks. After applying the astrometric shifts, the location of the X-ray transient is R.A. = 17:10:42.57, decl.=+28:50:15.14. This is 0.06’’ away from and consistent with the ZTF position, but 0.98’’ away from the galaxy nucleus. We note that combining systematic and statistical errors, 1-sigma Chandra astrometric uncertainty in R.A. is 0.71’’ and in decl. is 0.56’’. 

We conclude that AT2024tvd is the first off-nuclear TDE selected from modern sky surveys, opening the studies of offset massive black holes using TDEs. Further follow-up observations are planned.

We thank the Chandra team for rapidly approving and executing the X-ray observations.