GCN Circular 34819

On 15 Aug. 2023, during a regular survey work on the Messier 31 (NGC 224) field, performed with the 0.25m-f/4.5 robotic unit part of the Virtual Telescope Project facility in Manciano, Italy, we discovered a bright object on several unfiltered images, located at RA,DEC=00 43 54.36, +40 46 34.0 (J2000.0 astrometry from the discovery image). On 15.07 Aug. 2023 we estimated it at R=18.2 (R-mags for the reference stars from the Gaia DR2 catalogue). We have found the source by comparing the 15 Aug. images with 10.91 Aug. ones, taken via the same equipment. On the 10.91 Aug. images, the source was at R=18.9. We labelled this source as VTP J004354.36+404634.0 within our survey program. 

We queried Simbad at the mentioned coordinates, concluding it is most likely an optical counterpart to a nearby ( r < 1”)  radio source NVSS J004354+404634. At the same position, Vizier reports a few entries for X-ray sources. 

SDSS (DR16) reports an optical source at two different epochs at the mentioned coordinates: 
2002.7634: g=22.6 and r=22.5 (errors: 0.2 mags)
2002.7584: g=23.8 and r=23.4 (errors: 0.9 and 0.6 mags, respectively) 

Pan-STARRS DR1 reports an optical source, too: 
11 July 2012: g=22.08 and r=21.24 (errors: 0.08 and 0.07 mags, respectively) 

On 22 Aug. 2023, the MASTER survey detected a mag. 18.2 (clear) object, reporting it as a probable nova in M31, with no follow-up:
https://www.wis-tns.org/object/2023qhf

The CRTS archival light curve shows evident optical variability in V-band between 2006 and 2013, never brighter than V=18.6 (with detection limit at about V=21). 

Our data, from 10 Aug. 2023 to 13 Oct. 2023 likely covers an unprecedented bright flare reaching R=17.6 on 19.82 Aug. 2023 (UT). The source has been brighter than R=19.0 for more than 60 days in a row now. Detailed photometry is provided below (dates/times are UT, year is 2023): 

10.91 Aug.: 18.9
11.91 Aug.: 18.7 
15.07 Aug.: 18.2 
20.88 Aug.: 18.3 
09.84 Sept.: 18.0 
10.82 Sept.: 18.0 
11.82 Sept.: 17.9 
15.86 Sept.: 18.0 
16.96 Sept.: 17.9 
19.82 Sept.: 17.6 
20.87 Sept.: 17.8 
23.82 Sept.: 17.9 (bright Moon in the sky) 
24.96 Sept.: 18.2 
27.08 Sept.: 18.7 (very bright Moon in the sky) 
01.76 Oct.: 18.4 
02.76 Oct.: 18.8 
03.77 Oct.: 18.6 
05.82 Oct.: 18.9 
06.78 Oct.: 18.6 
08.00 Oct.: 18.8
08.80 Oct.: 18.8 
09.77 Oct.: 18.9 
11.85 Oct.: 18.8 
12.85 Oct.: 18.7 
13.76 Oct.: 18.5 
  
Errors are around 0.1 mag. for the brighter values and around 0.3 mags for the fainter ones. 
  
The scenario summarized above suggests that the variable optical source we have discovered is the optical counterpart of a blazar. 
  
A spectrum taken on 24 Sept. 2023 upon our request at the Copernico telescope operated by INAF Osservatorio Astronomico di Padova in Asiago-Ekar under poor sky conditions, with a resulting low S/N ration, does not show obvious, strong features, this supporting our hypothesis of the blazar nature for this transient.
  
Multi-wavelength observations are strongly encouraged. 
  
We wish to thank Lina Tomasella, Osservatorio Astronomico di Padova, Makoto Uemura, Hiroshima University, and Telescope Live for the precious support and fruitful discussion. 
  
Details about our work are available here: 
https://www.virtualtelescope.eu/2023/08/16/discovery-of-variability-of-the-optical-counterpart-of-the-nvss-j004354404634-radio-source-in-the-messier-31-field/

Gianluca Masi
Virtual Telescope Project