DRAFT
2025-03-04 23:38:07
Type: Object/s-Discovery/Classification
Bibcode: 2025TNSAN..78....1S
AT2025dhz in the NGC5128 (Centaurus A) : joint detections with ATLAS and BlackGEM indicate an intrinsically faint and rising transient
Authors: F. Stoppa (Oxford), M. Mlangeni (UCT/SAAO), N. Blagorodnova (ICCUB), S. J. Smartt (Oxford/QUB), K. W. Smith (Oxford/QUB), J. Tweddle, S. Srivastav (Oxford), S. Barocci-Faul (U Hamburg), P. J. Groot (Radboud/SAAO/UCT)
Keywords: Nova, Supernova
Abstract:
Here we report the joint ATLAS/BlackGEM discovery of the transient AT2025dhz (ATLAS25cji / BGEM J132606.24-425832.49) in the galaxy NGC5128 with an ATLAS cyan magnitude m_c = 19.36 +/- 0.2 and BlackGEM q magnitude m_q = 20.09 +/- 0.1. At a distance of 3.66 Mpc, the transient was discovered on MJD = 60738.09 (UT 2025-03-04.09) with an absolute magnitude of M_c = -8.8 (assuming m-M = 27.86 and A_g = 0.36 and A_r = 0.25). The last non-detection from ATLAS forced photometry was on MJD = 60736.2 and showed a limiting magnitude of 19.33. The last non-detection in BlackGEM was on MJD= 60736.82 showing a limiting magnitude of 21.05. Photometry indicates that the transient is rising. A nova is the most likely candidate, although a very young supernova cannot be ruled out. We recommend spectroscopic classification for confirmation.

We report the discovery of a new transient source, most likely a nova in the galaxy NGC5128 (Centaurus A). We discovered ATLAS25cji (AT2025dhz) on MJD 60738.09  (UT 2025-03-04.09), at m_c = 19.36 +/- 0.2. There was no detection by ATLAS on MJD 60736.2 (UT 2025-03-02.2). AT2025dhz is offset by 156.2 arcsec north and 424.0 arcsec east from NGC5128, which is at z =  0.001825 or d = 3.66 Mpc (mean of Cepheids, SBF, and TRGB), implying an absolute magnitude of M_c = -8.8 (assuming m-M = 27.86 and A_g = 0.36 and A_r = 0.25). 

We confirmed the discovery with BlackGEM images taken before ATLAS on MJD= 60737.35 which indicate a mag of q = 20.09 +/- 0.1. While the ATLAS c and BlackGEM q bands have different bandpasses, the difference indicates a rising transient. With a rise rate of around 0.9 mag per day, it is most likely a nova. A very young supernova is not yet ruled out and rapid follow-up observations are encouraged. There is no obvious massive stellar progenitor in HST WFC3 images, hence if it is a supernova it would most likely be a SN Ia. 

Show current TNS values
Catalog Name Reported RA Reported DEC Reported Obj-Type Reported Redshift Host Name Host Redshift Remarks TNS RA TNS DEC TNS Obj-Type TNS Redshift
TNS 2025dhz 13:26:06.249 -42:58:32.49 13:26:06.249 -42:58:32.49 NA/Unknown 0.0018

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