Discovery of long term, large variability and very bright flare detection of the optical counterpart of the NVSS J004354+404634 radio source in the Messier 31 field: a new blazar candidate.

We report the discovery of the long term, large variability of the optical counterpart of the NVSS J004354+404634 radio source in the field of Messier 31, found during our survey of this galaxy. Our investigations indicate this is likely a new BL Lac blazar, undergoing a very bright flare.

Evidence of variability of the optical counterpart of the NVSS J004354+404634 radio source. The galaxy on the top is Messier 32.

Evidence of variability of the optical counterpart of the NVSS J004354+404634 radio source. The galaxy on the top is Messier 32.

The following info should be regarded as very preliminary: work in progress.

Last updates:

  • 6 Dec. 2023: 05.84 Dec. photometry added (R=17.8)

On 15 Oct. 2023, we published an announcement on the Nasa’s General Coordinates Network (GCN 34819) hopefully inspiring further follow-up.

On 1 Dec. 2023, we published a circular on the Nasa’s General Coordinates Network (GCN 35245), reporting the new bright state of the source.

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), 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)

Messier 31, as from the imaging setup used for the survey.

Messier 31, as from the imaging setup used for the survey.

Our first, unfiltered images of Messier 31 were taken on 10, 11 and 15 Aug. (several frames on each date). On 15 Aug. we compared the first and the last frame sets and 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 and other IDs) located within 0.5 arc-seconds from our astrometric position. The source is also reported as an optical object in the GSC2.4.2, Pan-STARRS DR1 and SDSS DR16 star catalogues (the latter reports R mags fainter than 22.5), much fainter than the brightness we measured, with no mention of variability. We labelled this transient source as VTP J004354.36+404634.0 within our survey program.

The source in our recent data vs Panstarss and SDSS archive data

The source in our recent data vs Panstarss and SDSS archive data

Needless to say, we were very intrigued by this hint of activity and measured the brightness of the optical counterpart on regular basis, 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
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
13.86 Oct.: 18.5
22.79 Oct.: 18.6
31.73 Oct.: 18.8
01.89 Nov.: 18.4
05.94 Nov.: 18.0
08.75 Nov.: 18.0
11.75 Nov.: 18.4
12.80 Nov.: 18.1
15.92 Nov.: 18.0
18.87 Nov.: 18.1
26.91 Nov.: 18.2 (bright Moon in the sky)
28.80 Nov.: 18.0
29.72 Nov.: 17.9
05.84 Dec.: 17.8
16.74 Dec.: 18.4
17.73 Dec.: 18.3
18.73 Dec.: 18.4

Errors are around 0.1 mag. for the brighter values and around 0.3 mags for the fainter ones.

* data taken via TelescopeLive.

Between 21 Aug. and 9 Sept. the telescope unit was not available.

Lightcurve of the VTP J004354.36+404634.0 new blazar candidate.

Lightcurve of the VTP J004354.36+404634.0 new blazar candidate.

Using the 19.82 Sept. images, we get the largest brightness variation, as in the animation below:

Brightness variation of the optical source between 10 Aug. and 19 Sept. 2023.

Brightness variation of the optical source between 10 Aug. and 19 Sept. 2023.

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.

CRTS data for the NVSS J004354+404634 source from Nov. 2006 to Oct. 2013.

CRTS data for the NVSS J004354+404634 source from Nov. 2006 to 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 around 5 mags, at least.

It is worth to note that the this radio source is also present in the Chandra and XMM X surveys.

Good references for the source we analyzed here the The Million Quasars (Milliquas) catalogue, version 7.2 (Flesch, 2021) and Millions of Optical Radio/X-ray Associations (MORX) v2 (Flesch, 2023): both report a redshift z = 0.1 (from literature or estimated), with a probability for this optical source to be a quasar = 62%. If a quasar, the amplitude of its brightness variation is quite large and brightness variations are too fast.

Another interesting reference for the corresponding X-ray source is “Classification of Swift and XMM-Newton sources (Tranin+, 2022)“, where authors indicate a distance of 0.79 Mpc (roughly the distance of  M31), adding it has a posterior probability to be a X-ray binary = 59% (and AGN = 40%).

The XMM-Newton spectral-fit redshift catalogue (Ruiz+, 2021) indicate a photometric redshift for the object z = 0.78.

Hints of a possible faint structure/background contamination around the bright source.

Hints of a possible faint structure/background contamination around the bright source.

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.

On 24 Sept. 2023, a spectrum was taken via the 1.82m telescope at Asiago Observatory, but it showed a low S/N ratio because of the very bright Moon and poor weather. Within these significant limitations, it is worth noting the absence of any strong feature. A better S/N spectrum is very desirable.

Our data, with large and fast brightness variations, as well as the absence of strong features in the spectrum (with all the limitations due to its low S/N) are suggestive of a blazar, in particular a BL Lac object, which is in fact dominated by synchrotron radiation (with no strong features), caught during a very bright flare.

I wish to thank dr. Lina Tomasella (INAF, Osservatorio Astronomico di Padova), dr. Makoto Uemura (Hiroshima University) and and Telescope Live for the precious support and fruitful discussion. 

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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