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VLA Radio Observations of the blazar TXS 0506+056 associated with the IceCube-170922A neutrino event

ATel #10861; A. J. Tetarenko, G. R. Sivakoff (UAlberta), A. E. Kimball (NRAO), and J. C.A. Miller-Jones (Curtin-ICRAR)
on 17 Oct 2017; 14:08 UT
Credential Certification: Alexandra Tetarenko (tetarenk@ualberta.ca)

Subjects: Radio, Neutrinos, AGN, Blazar, Quasar

We report VLA radio observations of the blazar TXS 0506+056, following its identification as the potential astrophysical origin of the extremely high energy neutrino event IceCube-170922A (GCN #21916). This blazar source has recently been reported to be in a flaring state by Fermi LAT and AGILE (ATel #10791, #10801), although HAWC (ATel #10802) and VERITAS (ATel #10833) reported no detection of its E>160 GeV and E= 300 GeV - 100 TeV emission, respectively. Followup observations at near-IR through X-ray wavelengths have also been reported (ATel #10792, #10794, #10830, #10831, #10838 #10840, #10844, #10845).

We observed TXS 0506+056 over four epochs with the VLA, 2017 Oct 5 (15:15:48-15:35:42 UTC), Oct 6 (08:26:24-08:46:18 UTC), Oct 9 (13:13:33-13:33:27 UTC), and Oct 12 (08:06:21-08:26:15 UTC). Through splitting the array into three sub-arrays, we simultaneously measure the source across three receiver sets S (2-4 GHz), C (4-8 GHz), and X bands (8-12 GHz). We centered our 1.024 GHz basebands at 2.5, 3.5, 5.25, 7.45, 9.0, and 11.0 GHz. The array was in the B configuration during all of our observations, with (10, 9 , and 8) antennas in each subarray selected so that all subarrays have similar beam patterns.

We significantly detect TXS 0506+056 in all bands/epochs. Through fitting a point source in the image plane we measure the following preliminary flux densities:

 
date     | mjd   | freq | flux (mJy) | err (mJy) 
20171005   58031    2.5   517.1        8.7 
20171005   58031    3.5   535.3        7.4 
20171005   58031   5.25   550.5        6.0 
20171005   58031   7.45   591.0       11.5 
20171005   58031   9.0    607.0       10.1 
20171005   58031  11.0    600.0       19.9 
20171006   58032   2.5    511.3        5.4 
20171006   58032   3.5    518.9        5.2 
20171006   58032   5.25   498.0        9.3 
20171006   58032   7.45   486.0       19.6 
20171006   58032   9.0    523.0       18.3 
20171006   58032   11.0   471.0       27.8 
20171009   58035   2.5    510.7        2.1 
20171009   58035   3.5    528.2        3.0 
20171009   58035   5.25   559.6        4.1 
20171009   58035   7.45   618.3        6.2 
20171009   58035   9.0    627.0        8.9 
20171009   58035  11.0    636.0       12.8 
20171012   58038   2.5    520.0        1.5 
20171012   58038   3.5    534.2        2.0 
20171012   58038   5.25   567.1        2.0 
20171012   58038   7.45   612.1        2.7 
20171012   58038   9.0    651.5       10.3 
20171012   58038  11.0    702.0       13.4 

Given these measurements, we find evidence for variability between epochs in all of our sampled bands, except for 2 GHz, where the variance in the data is much less than the measurement errors (fractional RMS of 1%, 5%, 10%, 9%, and 15% in the 3.5, 5.25, 7.45, 9.0, and 11.0 GHz bands).

The flux density is clearly rising with frequency on October 5, 9, and 12 (see figures, where solid lines guide the eye but do not represent fits to the data). We measure spectral indices, α, where Fν ∝ να of 0.12 ± 0.02, 0.15 ± 0.008, and 0.14 ± 0.005 in these epochs. This type of radio spectrum is typical of emission from a compact jet. However, on Oct 6 we see a different spectral index, -0.02 ± 0.01. This temporary flattening may be associated with decreased steady-state jet emission combined with the launching of a discrete, optically thin jet ejecta that undergoes strong, and rapid adiabatic losses (for the spectral index to have returned to ~0.15 on Oct 9). Alternatively, a change in the opening angle or the velocity profile, resulting in more/less boosting at different distances downstream in the jet, could be causing the change in the spectral slope.

We thank the NRAO for granting us DDT VLA time to observe this source, and the NRAO staff for rapidly executing our observations.