AstroNote 2024-202

For the past month we have continued monitoring AT2024kmq (z=0.192), which was initially identified as a fast, red, and luminous (g-band M=-20 mag) optical transient in an early-type galaxy (lack of any emission lines implying low star formation; see AstroNote 2024-155). 

In the optical band the source has rebrightened, with a slow rise (~1 month), blue colors and current g-band m = 18 mag (M = -22 mag). VLA X-band (10 GHz) radio observations on 14 June revealed a detection at 0.114 +/- 0.006 mJy coincident with the nucleus of the host galaxy, implying a high radio luminosity of 10^39 erg/s. Swift observations from 19 July revealed luminous X-ray emission, at L(0.3–10 keV) ~ 4.5x10^43 erg/s, as well as a bright UV source at uvm2 = 18.36 mag (AB). The UV and optical SED can be described by a blackbody with a temperature of Tbb ~ 2x10^4 K. Optical spectroscopy from the Double Beam Spectrograph (DBSP) on the Palomar 200-inch telescope on 11 July showed a featureless blue continuum. The ZTF optical transient position is slightly offset from the host nucleus (0.5”) but this is consistent with previous ZTF median alerts positions for AGN outbursts and TDEs (see attached figure), so together with the radio position we consider it consistent with being a nuclear transient.

The properties of the second peak of this source and the host galaxy are most similar to the population of optically over-luminous (Mpeak < -20.5 mag) and featureless TDEs (Hammerstein et al. 2023, Yao et al. 2023). An initial red and fast-fading optical peak has only been previously observed in the case of AT2022cmc, a jetted TDE (Andreoni & Coughlin et al. 2022, Pasham et al. 2023). The presence of both features may support the suggested link between featureless TDEs and relativistic jets (Andreoni & Coughlin et al. 2022). 

ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.