Discovery, photometry and spectroscopic classification of SN 2025umq as Type Ia supernova

ATel #17361; Filipp Romanov (amateur astronomer, complimentary member of the AAVSO), Jacob Wise (LJMU), Christoffer Fremling (Caltech)
on 27 Aug 2025; 21:06 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Christoffer Fremling (fremling@caltech.edu)

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On 2025-08-17 (midtime of the first image 17.627016 UT) F. D. Romanov discovered the supernova (SN) candidate in the galaxy SDSS J004819.14+075856.8 at 5x300 sec. images (at a distance of approximately 12 arcsec. from the edges of the images) obtained with the Luminance filter at his request remotely using the telescope T59 (0.51-m f/6.8 reflector + CCD) of iTelescope.Net in Siding Spring Observatory (Australia); it was designated AT 2025umq (TNS) = PSN J00481888+0759006 (CBAT TOCP). The measured magnitude was 19.6 +/- 0.3 in the stacked image compared to G magnitudes of nearby stars from Gaia DR2 catalogue (Gaia Collaboration et al. 2018), measured position (J2000.0): 00:48:18.884 +07:59:00.56.

F. D. Romanov remotely observed AT 2025umq using the IO:O camera on the 2-m Liverpool Telescope (Steele et al. 2004) with Sloan g', r' and i' filters and measured the magnitudes (aperture photometry):

JD / Magnitude / Error / Filter / Exposure
2460910.6791454400 20.74 0.10 g' 2x60s.
2460911.6596276271 21.07 0.15 g' 3x60s.
2460906.6590004745 20.38 0.13 r' 5x60s.
2460910.6815234604 20.63 0.15 r' 2x60s.
2460911.6624643635 20.49 0.12 r' 3x60s.
2460910.6838803818 20.66 0.13 i' 2x60s.
2460911.6652745022 21.08 0.15 i' 3x60s.

The magnitudes of comparison stars were taken from Pan-STARRS DR1 catalogue (Chambers et al. 2016). Romanov also measured the position from the Liverpool Telescope photographs: 00:48:18.875 +07:59:00.67.

The spectrum of AT 2025umq (PSN J00481888+0759006) was obtained during 3400 seconds (PATAG proposal AZ22A01; PI: F. Romanov) on 2025-08-20, midtime 03:06:46 UT, remotely using the low resolution (approximately R = 350 = 18 Å) SPRAT (Piascik et al. 2014) spectrograph on the Liverpool Telescope with the red-optimised mode (grating option).

The same image shows spectra of both the SN candidate and SDSS J004819.14+075856.8: the host galaxy spectrum https://www.wis-tns.org/system/files/uploaded/F.%20D.%20Romanov/tns_2025umq_atrep_21875_F._D._Romanov.png shows H-alpha emission, and also sodium (Na D) absorption features, this allowed to narrow down the redshift to z = 0.159.

Comparison (using the software package "Next Generation SuperFit" (NGSF): Goldwasser et al. 2022) of the spectrum of AT 2025umq to templates of previously classified transients at a redshift of z = 0.159, finding it to be consistent with a fading SN Ia: the typical feature caused by high velocity iron is visible between approximately 5000 A and 5500 A; and silicon absorption feature is visible at approximately 7100 A.

The ZTF light curve (presented online https://alerce.online/object/ZTF25abexekl, but photometric data were not reported to TNS) shows the first g-band detection on 2025-07-25 and the peak magnitude of about +20 near August 10, 2025. Calculation with K-correction shows the absolute magnitude of about -19.6, this also matches a SN Ia.

Based on the above, we classify AT 2025umq as Type Ia supernova in the galaxy SDSS J004819.14+075856.8. The spectrum of SN 2025umq has been uploaded to the Transient Name Server: https://www.wis-tns.org/system/files/uploaded/F.%20D.%20Romanov/tns_2025umq_atrep_21875_F._D._Romanov_0.png

Stacked (3x60 sec. images with g', r' and i' filters; 0.30"/px, north is up, east is left) color image of SN 2025umq for 2025-08-24 03:53:55 UT midtime is available at the link: https://www.wis-tns.org/system/files/uploaded/F.%20D.%20Romanov/tns_2025umq_atrep_267781_F._D._Romanov.jpg

F. D. Romanov is grateful to iTelescope.Net for giving him some complimentary points for observing time with their remote telescopes.The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University (LJMU) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council.