ANTARES neutrino detection: A preliminary VLA catalogue of radio source components and their variability levels in the field

ATel #8034; A. Tetarenko, G. Sivakoff, A. Bahramian, C. O. Heinke (Alberta) G. Hallinan (Caltech) J. Miller-Jones (ICRAR - Curtin), A. Mioduszewski (NRAO), K. Mooley (Oxford)
on 10 Sep 2015; 15:49 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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Following the reported ANTARES neutrino detection and Swift detection of a possible X-ray counterpart (ATel #7987), NRAO performed service observations of the field using the Karl G. Jansky Very Large Array (VLA) from 22:19:34-23:19:22 UT on 2015 Sep 3 (MJD 57268.930-57268.972) and from 22:30:46-23:14:26 UT on Sep 4 (MJD 57269.938-57269.968). Each epoch spent 40 minutes on source, with the VLA in its most extended A configuration. Observations were taken in the 1-2 GHz (L) band so that the primary beam approximately covered the ANTARES 50% containment circle. The calibrators 3C286 and J1626-2951 were used to set the amplitude scale and determine the complex gains of the instrument, respectively. Instructions are online (http://science.nrao.edu/science/service-observing/service-observing) for downloading the NRAO calibrated visibility data. Our analysis below excludes additional RFI that was not caught by automatic flagging routines.

We created our catalogue from the image of the combined calibrated data sets, identifying 67 point source components (PSCs) and 30 extended source components (ESCs) within the primary beam (FWHM of 30’ at the middle of the L-Band). 32 of the 67 PSCs and all but one ESC were potentially consistent with emission from a previously known source (e.g., in the NED and SIMBAD databases) or optical/radio emission in DSSII/NVSS images. No PSC or ESC was consistent with the transient optical sources identified by Pan-STARRS (ATel #8027).

We measured the flux density of each source in the Sep 3 and 4 images separately, to search for variability between the days. Source positions, flux densities (peak flux densities for PSCs and integrated flux densities for ESCs) in the combined image, and the ratio of flux densities between observations are presented at http://astro.physics.ualberta.ca/sivakoff/ANTARES/Antares-L-Band-Sources.pdf and a primary-beam corrected image with a finding chart is included.

The median ratio of the flux density of components between Sep 3 and 4 was 0.923 (epoch_2/ epoch_1), with an estimated error of 0.014. This suggests that future analyses will need more accurate calibration. We note that the complex gain calibrator is slightly resolved, but has been modelled as a point source during calibration.

In agreement with ATel #7999, we detect a PSC at a position consistent with the X-ray position reported in ATel #7987 (P17 in our linked catalog). The fitted (J2000) coordinates of this source (P17) are:
RA: 16h 26m 02.1153 +/- 0.0033s
Dec: -27d 18' 14.143 +/- 0.115"
This position is consistent with the star, USNO-B1.0 0626-0501169, highlighted in ATel #7992, #7993, #7994, $7996, #7998, #8000, #8002, and #8006. We find flux densities of 0.113+/- 0.013 mJy, 0.088+/-0.018, and 0.140+/-0.019 mJy/beam, for the combined, Sep 3, and Sep 4 images, respectively. We do not significantly detect variability between the two observations; the flux density ratio is within 1.5 (1.7) sigma of 1 (0.9234).

We significantly detected variability (the flux ratio is neither consistent with 1.0 nor 0.9234 at the >3 sigma level) in 4 PSCs:

 
Src  RA                DEC              Flux_2015Sep03_/_2015Sep04 
P22  16h 25m 54.4828s -27d 26’ 36.604”  0.358+/-0.018 / 0.227+/-0.020 mJy/beam 
P23  16h 25m 50.9320s -27d 16’ 04.675”  0.873+/-0.018 / 0.722+/-0.018 mJy/beam 
P46  16h 25m 19.4137s -27d 24’ 34.449”  0.513+/-0.020 / 0.379+/-0.018 mJy/beam 
P49  16h 25m 15.0367s -27d 10’ 10.515”  0.510+/-0.021 / 0.370+/-0.017 mJy/beam 

Of these, P23 has no previously identified radio or optical emission, P22 is a marginal match to archival optical emission (DSSII) by eye, and P46 and P49 are a matches to archival radio emission (NVSS) by eye.

We thank NRAO for the rapid scheduling of these service-mode observations for the astronomical community.

Preliminary Catalogue and Finding Chart