AstroNote 2023-197

AT2023mhs/ZTF23aaqdjhi was first detected by ZTF on MJD = 60125.300 at a magnitude of g = 18.78, rising from an upper limit of g > 20.48 two days prior.  The location of the transient is coincident with the nucleus of SDSS J134315.65+191500.9, a star-forming galaxy with a spectroscopic redshift of z=0.0482 and no evidence of AGN activity. The transient continued its sharp rise in subsequent detections, brightening by another 1.5 mag over the 1.89 days after first detection.  Four epochs of LT IO:O ugriz photometry reveal that the rise is continuing; photometry of the most recent epoch (MJD 60132.91) gives a magnitude of g = 15.96 +/- 0.07.  The colour is blue throughout (u-g ~ -0.3, g-r ~ -0.25).  Given the spectroscopic redshift of the host galaxy from SDSS (z = 0.04820), the inferred absolute magnitude is M_u = -21.1 (not accounting for host extinction).  Spectroscopy from the Palomar 60-inch telescope (SEDM), Liverpool Telescope (SPRAT), and Nordic Optical Telescope (ALFOSC) reveal only a featureless blue continuum.  Swift observations have been requested.

Considering the transient’s blue colour and high luminosity it is possible that it is a young fast-rising superluminous supernova, or a luminous tidal disruption event. Further follow-up and classification is encouraged.