Discovery of long term, large variability of the optical counterpart of the NVSS J004354+404634 radio source in the Messier 31 field.
We report the discovery of the long term, large variability of the optical counterpart of the NVSS J004354+404634 radio source, probably an active galactic nucleus (AGN: blazar or quasar), in the field of Messier 31, found during our survey of this galaxy.
Important note: while we’ve extensively checked the available sources in literature, we are still looking for details we may have missed to addess the real nature and story of this source.
So, the following info should be regarded as very preliminary.
- 21 Sept.: 20.87 Sept. photometry added (R=17.8)
- 20 Sept.: 19.82 Sept. photometry added, maximum brightness so far (R=17.6).
Early on Aug. 2023, we started dedicated surveys monitoring the fields of the Messier 31 and Messier 33 galaxies, the largest members, with our own Milky Way, of the so-called Local Group. This effort is motivated by the chance to discover nova explosions in those galaxies, thanks to the large field of view of our new 250mm-f/4.5 astrograph and the excellent, dark skies of Manciano, in the Tuscan Maremma, where our robotic facility is.
These efforts made possible for us to discover several, previously unknown variable stars (we released data for two of them in M33 field, others will follow soon), as well as two nova candidates, in one case only a few hours after it was already reported.
It is worth to note that scanning images of such a huge area of the sky by eye is an hard task: the field of view is 1.8 x 1.2 square degrees and the image is deep (limiting magnitude around 20), with plenty of structures from Messier 31 (see the image below, processed from our data by the DeepLab team)
Our first, unfiltered images of Messier 31 were taken on 10, 11 and 15 Aug. (several frames on each date): comparing the first and the last ones, we spotted a star-like transient at the following J2000.0 position:
RA: 00 43 54.36
Decl.: +40 46 34.0
On 10.91 Aug. 2023, we estimated the source at R=18.9, while on 15.07 Aug. it was at R=18.2, easy to spot by blinking the images. Please note: images were taken with no filters, but we used for the reference stars their R-mags from the Gaia DR2 catalogue.
Immediately, we queried the SIMBAD service at Université de Strasbourg/CNRS, retrieving X and radio data about a source (NVSS J004354+404634) located within 0.5 arc-seconds from our astrometric position. The source is also reported as an optical object in the GSC2.4.2 and SDSS star catalogues, much fainter than the brightness we measured, with no mention of variability.
Needless to say, we were very intrigued by this hint of activity and measured the brightness of this optical counterpart on several nights up to 19 Sept., getting the following photometry (assuming R band):
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
Using the 19.82 Sept. images, we get the largest brightness variation, as in the animation below:
The data above clearly demonstrate at least a mid-term variability of the mentioned optical counterpart of NVSS J004354+404634. To confirm our discovery, we also queried the CRTS database, providing photometric data from 1 Nov. 2006 to 25 Oct. 2013.
This plot not only confirms our findings, but makes clear this source exhibits a long-term optical variability. Assuming data from the SDSS and GSC catalogues, we estimate the real range in R band should be at least 4 mags.
It is worth to note that the this radio source is also present in the Chandra and XMM X surveys. A good reference for the source we analyzed here is Millions of Optical Radio/X-ray Associations (MORX) v2 (Flesch, 2023), where a redshift z=0.1 is provided, with a probability for this source to be a quasar=64%. If a quasar, the amplitude of its variation is quite large.
We also mention that on 22 Aug. the MASTER survey spotted a transient at the position of our source on 22 Aug.,assuming it was a nova in M31: of course, it is not a nova, as our findings clearly show.
Most likely, this is an active galactic nucleus (AGN), that is a quasar or blazar, with extreme variability: the existence in literature of a redshift of z=0.1 makes this option the most probable. Chances are, too, this could be a dwarf nova in our Milky Way, with a high duty cycle (number of days in outburst divided by the total number of days observed, including negative detections).
I wish to thank dr. Taichi Kato (Kyoto University), dr. Makoto Uemura (Hiroshima University), dr. Lina Tomasella (INAF, Osservatorio Astronomico di Padova) and dr. Brian Skiff (Lowell Observatory) for the fruitful discussion and inputs.
We will continue monitoring the optical behavior of this interesting target, s well as checking the literature for details we missed so far.
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