AstroNote 2024-177

ZTF24aaoxmyb/AT 2024lhc was first detected by ZTF on 2024 April 28.43 UT as a nuclear transient in a host galaxy (SDSS J165054.16+325236.2) with archival SDSS and DESI spectra indicating a massive quiescent galaxy lacking emission lines at redshift z=0.2045. A single pre-discovery detection on 2020-07-06 of a source at z=20.83 mag at the object coordinates was reported to TNS by Pan-STARRS (PS20nxv; Chambers et al. TNS AT Report 215646). However, inspection of forced photometry at the object coordinates by ZTF (Masci et al. 2023; arxiv: 2305.16279) and ATLAS (Shingles et al. 2021; AstroNote 2021-7) reveals no obvious previous variability or outbursts associated with the galaxy nucleus from 2016 to 2023. The transient has gradually brightened over the last 60 days in ZTF observations and appears to be nearing a maximum at g~19.2 mag (M_g ~ -20.8 mag). 

We obtained optical spectra (range 3500-10500 Angs) using the Kast spectrograph on the Shane 3 m telescope at Lick Observatory on 2024 June 14 and June 28 and with Keck-I/LRIS on 2024 July 2. Subtraction of the archival SDSS spectrum from the Lick and Keck spectra reveals a blue featureless continuum for the transient with no clear broad features detected, although there is a potential low-amplitude emission feature near 4600 Angs (rest frame).

Swift observations were obtained on 2024 June 21, 25, and 29th (PI: Guo). The source is well detected, with AB magnitudes near 19.0 in all three UVOT UV filters (W1, M2, and W2). In the first UVOT epoch, we measure magnitudes of UVW1=19.01 +/- 0.07 mag, UVM2=19.01 +/- 0.08, and UVW2=19.10 +/- 0.07 mag.

AT 2024lhc was detected by the X-ray Telescope (XRT) with an initial count-rate of 0.028 ct/s. During our observations we find clear evidence for variability of the X-ray source on a few day timescale.  The time-averaged XRT spectrum can be well described by an absorbed power-law (tbabs*zashift*powerlaw in Xspec) with power-law photon index Gamma = 1.96 +/- 0.26 (90% confidence interval) and no evidence for intrinsic absorption. The inferred 0.3-10 keV X-ray flux is (9.8, 3.9, 9.2) e-13 erg/s/cm2 on June 21, 25 and 29th, corresponding to luminous X-ray emission with Lx = (1.2, 0.5, 1.1) e+44 erg/s at the distance of the transient. 

The X-ray spectral index is much harder than seen in typical TDEs and indicates that the X-ray emission is not dominated by thermal emission from an inner accretion disk. With Lx ~ 1e+44 erg/s at ~60 days post discovery, this is among the most X-ray luminous TDEs. 

The roughly constant g-r colors over a long risetime, blue UV-optical colors (W2 – g ~ -0.2 mag), variable X-ray detection with XRT, apparently featureless optical spectrum, and lack of previous optical outbursts lead us to classify AT 2024lhc as a TDE-featureless. The high optical and X-ray luminosities and hard X-ray spectrum are unusual for typical optically selected TDEs, and raise the possibility of emission from an off-axis jet, although the source is a few orders of magnitude less luminous than Swift J1644+57 or AT2022cmc in the X-rays.

Further multiwavelength observations to better clarify the nature of this nuclear transient are planned.