VLA observations reveal increasing brightness of 1WHSP J104516.2+275133, a potential source of IC190704A

ATel #12926; Alexandra J. Tetarenko (East Asian Observatory), Gregory R. Sivakoff (University of Alberta), Gregg W. Hallinan (Caltech), Amy Kimball (NRAO), James C. A. Miller-Jones (ICRAR/Curtin University), Kunal Mooley (Caltech)
on 11 Jul 2019; 22:16 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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Using the Fermi Gamma-ray Space telescope, the Fermi-LAT collaboration reported a new very-high-energy (>100GeV) gamma-ray source with a position that was consistent with the high-energy candidate neutrino IC190704A (GCN #24981, #24989). This gamma-ray source is also consistent with the blazar 1WHSP J104516.2+275133 (aka SDSS J104516.30+275133.4). This position is also consistent with a radio source detected by NRAO VLA Sky Survey, NVSS J104516+275136, whose integrated 1.4 GHz flux density on 1995 April 21 was 3.0+/-0.4 mJy (Condon et al. 1998, AJ, 115, 1693), as well as the Faint Images of the Radio Sky at Twenty-one cm survey, FIRST J104516.3+275133, whose peak 1.4 GHz flux density (co-added from observations from 1993 April to 1995 November) was 3.9 +/- 0.1 mJy (White et al. 1997, ApJ, 475, 479).

We have triggered multi-frequency radio monitoring of 1WHSP J104516.2+275133 with the Karl G. Jansky Very Large Array (VLA). Initial observations were taken on 2019 July 09 from 02:38 - 03:08 and July 10 from 02:34 - 03:04, when the VLA was in the BnA configuration. The weather conditions were appropriate for observations below 26 GHz. Using two subarrays, quasi-simultaneous data were taken at 2.00-3.00, 3.00-4.00, 4.75-5.75, 5.95-6.95, 8.00-9.00, 10.50-11.50, 12.00-13.00, and 14.00-15.00 GHz. We used 3C286 for flux and bandpass calibration, and J1125+2610 for the complex gain calibration. The synthesized beam size for our observations at 14-15 GHz of 1WHSP J104516.2+275133 was 0.57” x 0.31” at a PA of 64.9 deg.

We significantly detected an unresolved source coincident with 1WHSP J104516.2+275133 at all frequencies. Our J2000 position (from the 14-15 GHz image on July 09) was
RA = 10:45:16.3013 +/- 0.0012 s
Dec = +27:51:33.4955 +/- 0.0083 arcsec
where the astrometric errors are statistical in nature only. Systematic errors for typical calibrations with the VLA are 10% of the synthesized beam.

For each frequency range, we fit the data in the image plane, detecting peak flux densities (f_nu) of about 2.6 - 4.9 mJy, with decreasing flux density as frequency increases for each day. For a spectral index α given by f_nu ∝ nu^α, we find &alpha=-0.11+/-0.03. We note that the fluxes below are preliminary and that the errors are statistical in nature only. Further analysis will incorporate the final pad positions of telescopes in the BnA configuration, self-calibration, and checks for phase decoherence that can increasingly reduce flux density at increasing frequencies.

 
Central Frequency | f_nu,July 09  [mJy] | f_nu, July 10  [mJy] 
2.50              | 3.34 +/- 0.14       | 4.87 +/- 0.10 
3.50              | 3.10 +/- 0.06       | 4.09 +/- 0.07 
5.25              | 3.12 +/- 0.08       | 3.50 +/- 0.09 
7.45              | 3.11 +/- 0.11       | 3.03 +/- 0.08 
8.50              | 2.84 +/- 0.10       | 3.05 +/- 0.08 
11.0              | 2.82 +/- 0.11       | 2.92 +/- 0.11 
12.5              | 2.78 +/- 0.12       | 3.11 +/- 0.15 
14.5              | 2.60 +/- 0.11       | 2.69 +/- 0.16 

Although Swift XRT/UVOT followup of the source did not find a significant increase in activity from archival ROSAT data (ATel #12924), the radio flux has brightened both compared to archival data and between the two observations. The 2-4 GHz average peak flux densities of 3.14+/- 0.06 mJy and 4.36 +/- 0.06 mJy are noticeably brighter than observations at this frequency with the Very Large Array Sky Survey (Lacy et al. 2019, submitted, arXiv:1907.01981) on 2019 June 8, where the peak flux density was 2.29 +/- 0.13 mJy. This latter flux density is only accurate to 10-20% in their current quicklook status.

We will monitor this source over the next approximate 6 months. Continued multiwavelength observations of this source, where possible, are encouraged.

We thank the Very Large Array staff for helping us rapidly trigger these observations.