Radio characterization of the candidate source for neutrino event IceCube-201115A

ATel #14196; Gabriele Bruni (INAF-IAPS), Loredana Bassani (INAF-OAS), Angela Bazzano (INAF-IAPS), Mariateresa Fiocchi (INAF-IAPS), Francesca Panessa (INAF-IAPS), Pietro Ubertini (INAF-IAPS)
on 18 Nov 2020; 22:02 UT
Credential Certification: Gabriele Bruni (gabriele.bruni@inaf.it)

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We conducted an archival search for radio data to characterize the candidate source for the recent neutrino event IceCube-201115A (GCN #28889), identified as PKS 1256+018 in ATel #14177. Very Large Array (VLA) observations on this source were conducted on September 25, 1999, in A-configuration, at 1.45 GHz and 8.49 GHz (project code AJ0268). We downloaded images available on the NRAO VLA Archive Survey (NVAS, (c) AUI/NRAO, http://archive.nrao.edu/nvas/ ) database. The angular resolution is 2.6x1.4 arcsec at 1.4 GHz, while 0.4x0.2 arcsec at 8.49 GHz, with an RMS of 390 uJy/beam and 130 uJy/beam, respectively.

In the 1.4 GHz image, a resolved double, symmetrical morphology is visible, with a bright component towards NE (392+/-20 mJy), and a fainter one towards SW (151+/-8 mJy). This morphology and flux ratio (~2.5) is consistent with the one reported in ATel #14177 making use of Very Large Array Sky Survey (VLASS, Gordon et al. 2020) data (2-4 GHz) from April 2019, at very similar resolution (2.9x2.1 arcsec). We can also confirm that the optical counterpart SDSS J125908.28+013605.2 lies right between the two radio components, suggesting a radio galaxy classification for this source, following its morphology.

The 8.49 GHz image has a factor x6 better angular resolution, and shows an emission spot (9.5+/-0.5 mJy) roughly corresponding to the position of the NE component maximum at 1.4 GHz, and two fainter spots (1.1+/-0.1 mJy and 1.0+/-0.1 mJy) located in the brightest emission region of the SW component detected at 1.4 GHz. No statistically significant emission is detected in correspondence of the optical counterpart, where the supposed radio core should lie. We thus report a 3-sigma upper limit of 0.264 mJy/beam in that position.

Considering the redshift reported for the optical counterpart in ATel #14177 (z=0.62), we can estimate a projected linear size of 42 kpc from the 1.4 GHz VLA image (6 arcsec from component to component, at a physical scale of 7 kpc/arcsec). The total radio power as calculated from the VLA image at 1.4 GHz is 10^26.87 W/Hz. In order to estimate the spectral index for the two components, we re-imaged the 8.4 GHz data in AIPS with a restoring beam equal to the 1.4 GHz image, and applying a uv-taper to match the two observations uv-coverage. In this way, we recovered a flux density of 46+/-5 mJy for the NE component and 51+/-5 mJy for the SW one, resulting in a spectral index of -1.2 and -0.6, respectively (S_nu proportional to nu^alpha, where alpha is the spectral index). These values are consistent with the steep (<-0.5) one expected for the lobes of a radio galaxy.

The source is also detected in the Faint Images of the Radio Sky at Twenty-Centimeter (FIRST, Becker et al. 1995) and NRAO Very Large Array Sky Survey (NVSS, Condon et al. 1998) at 1.4 GHz, although the lower resolution (~5 and ~45 arcsec, respectively) can barely resolve the source in FIRST, while it does not allow to resolve it at all in NVSS. As reported in ATel #14177, the source is also detected and resolved in the first epoch of VLASS: fro those data we measured a flux density of 198+/-30 mJy and 60+/-9 mJy for the NE and SW components, respectively.

Finally, the Wide-field Infrared Survey Explorer (WISE, Wright et al. 2010) catalogue reports the following values at the coordinates of the SDSS counterpart mentioned before: W1=15.56, W2=15.945, W3=12.596, W4=8.803. These result in the following colors W1-W2=-0.385, W2-W3=3.349, W3-W4=3.793. The latter values place the source outside the Blazar stripe identified by Massaro et al. 2012 (ApJ, 750, 138). This, together with the discussed radio properties, is at odds with the previous BL Lac classification mentioned in ATel #14177, and based on previous works.

GRAL research group website reporting the discussed radio images