AstroNote 2025-201

The optical transient AT2025mkn was first reported to the Transient Name Server by ATLAS (internal name ATLAS2025frs) on 2025-05-29, and was also detected and reported by ZTF later on the same day (internal name ZTF25aasjeza).  The first reported detection of the transient is on 2025-05-25, by ZTF.  It has been classified as a “SN” on TNS (Hinkle 2025) based on a blue and largely featureless spectrum from SNIFS. The transient is offset by 0.83 arcseconds from the nearest object visible in reference imaging of the field, a red r~19.4 mag galaxy seen in Pan-STARRS and Legacy Survey (LS) images.  This galaxy has a spectroscopic redshift (provided by the DESI collaboration) of z=0.420.

Follow-up spectroscopy was acquired using the Low-Resolution Imaging Spectrometer (LRIS) on Keck I on 2025-06-24. The reduced spectrum is dominated by the light of the nearby galaxy, and shows several narrow absorption features at z=0.420, including Ca H&K, Mg I b, H beta, and Na I D.  However, several additional narrow absorption lines are also clearly detected at wavelengths consistent with absorbing systems at substantially higher redshifts, including Mg II, Mg I, and Fe II at z=1.256 and 1.371. These lines are also seen in the SNIFS spectrum posted to TNS.

The transient exhibited a 4- to 7-day rise time in the observer frame, with very blue colors (g-r = -0.4 +/- 0.2 mag) during the initial rise.  The peak magnitude in ZTF observations was g=19.2 on 2025-05-29 (although the true peak of the light curve may have been 1-3 days later). It then began fading rapidly, and quickly evolved from blue to red colors over the next 20 days; follow-up photometry from the Palomar 60-inch telescope from 2025-06-29 gives a magnitude of r~21.3 and a color of g-r = +0.5 mag. Photometry from NOT and Wendelstein confirm the trend in color evolution.

If the highest redshift in the LRIS spectrum is adopted, a peak absolute magnitude of -24.8 mag (at a rest-frame wavelength of 2050 Angstroms) is inferred, with a rest-frame rise time (after correcting for time dilation) of about two days. The multiple redshifts observed along the line of sight and the extraordinarily high inferred luminosity (for a transient source with a thermal color) suggests that the event could be magnified and that the z=0.420 object is a foreground lensing galaxy. The color evolution and the (relatively) slow time scale of the lightcurve are incompatible with it being a GRB afterglow.

Three epochs of Swift target-of-opportunity observations were obtained. The start dates were June 12, 13, and 18, corresponding to 18-24 days after the first detection. All observations resulted in non-detections in X-rays, with a 3-sigma upper limit on the count rate of 0.005 ct/s, corresponding to a flux upper limit of a few x 10^-13 erg/cm²/s. 

In order to assess the consistency of the lensing hypothesis, we retrieved science-ready images from PanSTARRS, DESI Legacy Imaging Survey, 2MASS and WISE and measured the brightness of the foreground galaxy using LAMBDAR (Wright et al. 2016). Fitting the spectral energy distribution with Prospector (Johnson et al. 2021) assuming a linear exponential star-formation history, the Chabrier IMF and the Calzetti attenuation model, we measure a stellar mass of log M/M_sun = 11.4 +/- 0.2 (the error indicates the 1 sigma uncertainty), neglecting contamination from the background galaxies. The large stellar mass is broadly consistent with the observed image offset, when compared with known galaxy lensed systems.

JWST follow-up observations (PI Goobar) are scheduled for the July 7-28 period, and should allow us to detect potential multiple images. The early blue and fast nature of the transient is unusual, but may originate from an LFBOT with a magnification factor of ~10.  It could also be due to a core-collapse supernova with a magnification factor of ~100 seen in the rest-frame UV, although the lack of spectral features is unexpected in the latter scenario. VLA and Chandra observations are being requested via DDT to test the former hypothesis.