M31N 2006-11d (= AT 2025wkx): Variable Star or Recurrent Nova in M31?

ATel #17371; A. W. Shafter (SDSU), Jingyuan Zhao, Jianlin Xu, Mi Zhang (Xingming Observatory), K. Hornoch, H. Kucakova (Astronomical Institute, Ondrejov, Czech Republic) T. A. Rector (U. Alaska, Anchorage), R. Fidrich (HAA/VSS)
on 5 Sep 2025; 10:44 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Allen W. Shafter (ashafter@sdsu.edu)

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The recently-identified M31 transient AT 2025wkx = PNV J00430561+4101301 was reported on 2025 Aug 31.05 UT at an unfiltered magnitude of ~20.5. The source was also detected on images taken with the 0.43m CDK reflector, Alnitak Remote Observatory at Nerpio, Spain starting on 2025 Aug 2.05 UT (continuing to the present) and by the Zwicky Transient Facility (ZTF) starting on 2025 Aug 17.43 UT (continuing to the present). Surprisingly, it is also visible on the Digital Sky Survey (DSS1) red image. However, it is invisible on several archival images taken with the 2.5-m INT at La Palma and with the 4-m Mayall at Kitt Peak (Massey et al. 2006, AJ, 131, 2478) down to 22.5 and 23 mag in I and R, respectively. Thus, the full amplitude of the transient is > 3 mag.

We noted that the position of the object is very near that of the nova candidate M31N 2006-11d (Rector et al. 2022, ApJ, 936, 117), and thus might be a recurrent nova system. In order to confirm this association, we have carefully registered the image of AT 2025wkx from 0.43-m Telescope at Alnitak Remote Observatories with the unpublished imaging data from Rector et al. 2022. The images of the two sources are spatially coincident to better than 0.5" as shown in the registered comparison image linked below. Given that the probability of a chance positional coincidence at a distance of ~15' from the center of M31 is extremely remote, we conclude that AT 2025wkx is indeed associated with the source identified earlier as M31N 2006-11d.

Assuming M31N 2006-11d was in fact a nova, the recent eruption would establish the system as a new recurrent nova system in M31. However, the slow photometric evolution of the source would be very unusual for a recurrent nova. Conversely, the photometric behavior is more reminiscent of a Long Period Variable (LPV) star in M31. To explore this possibility, we obtained additional photometry of the most recent brightening using the 0.65-m reflector at Ondrejov and found the following magnitudes: R = 19.9 +/- 0.2 on 2025 Sep 03.807 UT and I = 19.6 +/- 0.2 shortly thereafter on 2025 Sep 04.022 UT. A color of R-I ~ 0.3 is inconsistent with that expected for a LPV (which typically are much redder). Thus, the nature of the object remains elusive, and further observations, particularly spectroscopic observations, are strongly encouraged to firmly establish the nature of this object.

Comparison image: M31N 2006-11d is black, the recent PNV is white