AstroNote 2025-216

A transient in the nearby galaxy NGC 1637, ASASSN-25cw, was discovered by K. Z. Stanek for the ASAS-SN team  on 2025-06-29 10:19:12 UT at g-Sloan = 13.3 and reported to the Transient Name Server  (TNS Astronomical Transient Report No. 260520) as AT 2025pht. The last ASAS-SN non detection was on 2025-04-06 17:16:48 UT  with a limiting magnitude of g-Sloan = 17. It was classified spectroscopically as a SN IIP by J. Strader (TNS Classification Report No. 21466) using the Goodman spectrograph for the SOAR telescope on 2025-07-03 10:18:06 UT. Using the ASAS-SN (Shapee et al. 2014, Ap J, 788, 48; Kochanek et al. 2017, PASP, 129, 104502) Sky Patrol forced photometry tool, 
we found that the supernova is fading. It has appeared in the southern sky at low elevations after the seasonal break and no observations are available close to the supernova explosion time or to the expected peak of the light curve.

Here we report on Las Cumbres Observatory (LCO) observations that provide an accurate SN 2025pht position and the discovery of the progenitor star candidate in pre-explosion HST (GO 9155) and JWST NIRCam and NIRI imaging programs (GO 3707 and  GO 4793). Our LCO imaging of NGC 1637 (z = 0.002392, from NED) in several epochs from July 4 -16 2025 using LCO 1-m telescopes and their Sinistro cameras in different LCO nodes (Cerro Tololo Inter-American Observatory, Siding Spring Observatory and Sutherland Observatory) have provided an improved position of SN 2025pht, after precise astrometric calibration of the LCO images using stars from the Gaia DR3 catalog. We measure a mean position of RA (J2000) = 04:41:28.872, Dec (J2000) = -02:51:55.84 with an error of 0.08" in both coordinates. The SN position is offset by 0.57" from the reported ASAS-SN position.

Using the JWST level 3 data products, we identify the supernova progenitor star candidate, that is visible in all the available NIRCam images and in the MIRI F770W one. The coordinates of this source in the JWST pipeline catalog of the NIRCam F150W observations of program GO 4793 are: RA (J2000) = 04:41:28.8748, Dec (J2000) = -02:51:55.825. The separation of this JWST NIRCam source from the LCO position of SN 2025pht is of 0.044 arcsec and there are no other JWST sources within 0.2 arcsec of the LCO SN position. We conclude that the JWST source is the progenitor star candidate of SN 2025pht. 

Using the JWST pipeline catalog, the total AB magnitude of the progenitor star candidate in the deeper NIRCam F150W image (of program GO 4793) is 22.099 +/- 0.004. A faint source is detected in the deeper HST WFPC2 images of program GO 9155 in the F555W and F814W bands at the reported JWST position. A more detailed analysis will be presented elsewhere. 

SN 2025pht is so far the brightest supernova of 2025. The other known supernova in NGC 1637, SN 1999em, was also the brightest one of 1999.

Additional multi-wavelength follow-up observations are encouraged.
 

This work makes use of observations from the Las Cumbres Observatory global telescope network, program IAC2025A-009 (SGLF).

This work presents results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC is provided by national institutions, in particular the institutions participating in the Gaia MultiLateral Agreement (MLA). The Gaia mission website is https://www.cosmos.esa.int/gaia. The Gaia archive website is https://archives.esac.esa.int/gaia.

This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. These observations are associated with HST program GO 9155. This work is based in part on observations made with the NASA/ESA/CSA James Webb Space Telescope. These observations are associated with JWST programmes GO 3707 and GO 4793. The HST and JWST data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST.