M33N 2011-10a (= M33N 2021-08a = M33N 2025-10a) is the First Confirmed Recurrent Nova in M33

ATel #17435; A. W. Shafter (SDSU), K. Hornoch (Astronomical Institute, Ondrejov, Czech Republic), Jingyuan Zhao, Ziyang Mai, Jianlin Xu, Hualin Li, Mi Zhang, Wenjie Zhou, Guoyou Sun, Jiangao Ruan, Xing Gao (Xingming Observatory)
on 5 Oct 2025; 16:45 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Allen W. Shafter (ashafter@sdsu.edu)

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The transient source PNV J01350197+3043202 was recently discovered in the outskirts of M33 and near the position of M33N 2021-08a (AT2021wfc) by the Popular Supernova Project (PSP) team at the Xingming Observatory. Subsequently, a spectrum of PNV J01350197+3043202 has been reported in ATel #17432, and is consistent with that of a nova eruption. Hereafter, we refer to the recent transient as M33N 2025-10a.

To establish whether the recent eruption is a recurrence of M33N 2021-08a, we have carefully registered images from the two eruptions: an ATLAS image for the recent eruption of M33N 2025-10a and a ZTF image of M33N 2021-08a. As shown in panel 1 of the comparison images linked below, we find that the positions of the novae are spatially coincident to within 0.5 arc second. Given the position of the nova in the outskirts of M33 (~15.7 arcmin from the center of the galaxy), the probability of a chance positional coincidence of two unrelated novae is negligibly small. Thus, we conclude that M33N 2021-08a and M33N 2025-10a are recurrent nova outbursts -- the first to be recognized in M33.

The discovery these recurrent nova outbursts has led us to examine archival images of M33 to search for additional eruptions. Perhaps not surprisingly, near the position of the recent outbursts, we have discovered a previously unknown nova in archival PTF images of M33 taken on 2011 Oct. 08.172 UT. The date of this earlier eruption is ~10 yr before that of the 2021 eruption, a little more than twice the 4.1-yr interval seen between the two most recent outbursts. As shown in panel 2 of the comparison images, this newly-discovered nova, which we designate as M33N 2011-10a, is indeed spatially coincident with the two more recent novae. In summary, there are now three known eruptions of the newly-identified M33 recurrent nova system, M33N 2011-10a, with additional searches of archival M33 data currently underway.

Finally, we report the following magnitudes of the recent eruption, M33N 2025-10a, that have been measured with ZTF and the 0.65m at Ondrejov (OND):

 
  Date [UT]     |  Mag  | Err  | Filter | Telescope 
 
2025 Oct 01.299 | 19.45 | 0.10 |   r    | ZTF 
2025 Oct 01.385 | 18.46 | 0.03 |   g    | ZTF 
2025 Oct 03.408 | 19.99 | 0.14 |   g    | ZTF 
2025 Oct 03.450 | 19.73 | 0.12 |   r    | ZTF 
2025 Oct 03.912 | 20.00 | 0.10 |   V    | OND 
2025 Oct 03.888 | 19.33 | 0.09 |   R    | OND 
2025 Oct 03.899 | 19.50 | 0.15 |   I    | OND 
 

As with all extragalactic novae, additional follow-up spectroscopic and photometric observations to determine a nova's spectroscopic class and rate of decline (speed class) are encouraged.

Comparison images: Left panel: 2021-08a (black) vs 2025-10a (white); Right panel: 2011-10a (black) vs 2025-10a (white)