ACT-T J061647-402140: a Strongly Variable, Flaring Source at 90, 150 and 220 GHz Positionally Coincident with the Transient Gamma-Ray Blazar, Fermi 0617-4026

ATel #12738; Sigurd Naess (Center for Computational Astrophysics, Flatiron Institute) on behalf of the ACT Collaboration
on 8 May 2019; 23:32 UT
Credential Certification: John P. Hughes (jph@physics.rutgers.edu)

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The Advanced Atacama Cosmology Telescope (ACT) Polarimeter (Henderson et al, JLTP 184, 772-779, 2016; Thornton et al, ApJS 227, 21, 2016) is an instrument for measuring intensity and linear polarization at 90, 150 and 220 GHz using the 6m ACT, located in Northern Chile in the Atacama Desert. The ACT full-width-at-half-maximum resolutions at 90, 150 and 220 GHz are 2.0 arcminute, 1.4 arcminute and 1.1 arcminute, respectively.

Between 2016 Aug 15 and 2018 Dec 31, ACT mapped a ~5000 sq deg. field centered on RA, DEC = (00:04:05, -40:00:00). The data were collected at night, with a twelve-day cadence as described in de Bernardis et al., SPIE 9910, 2016 (arXiv:1607.02120). A transient source, ACT-T J061647-402140, is found at RA = 06:16:47.0, Dec= -40:21:40.5 (J2000) in the co-added 150 GHz maps with an average flux of ~350 mJy at a signal-to-noise ratio of >100. The 1 sigma positional uncertainty is < 3 arcseconds in each coordinate. Unlike the case for other ACT sources of comparable flux density, this source does not appear in any other radio or sub-mm catalog. However, there is a plausible counterpart discovered as a transient gamma-ray source (Fermi J0617-4026, Atel #6912) which was subsequently classified as a blazar (WISE J061647.01-402142.8, Atel #6937) at a redshift of z=0.67. The ACT position is located 2 arcseconds from the WISE blazar, which is also coincident with a blue unresolved optical/UV source (cataloged as GALEXASC J061646.99-402142.5).

Data on the source have been collected in three epochs: [2016.6 - 2017.0], [2017.6 - 2018.1], and [2018.7 - 2018.9]. At 90 and 150 GHz, the source flux varied from < 50 mJy in the first epoch to > 650 mJy by the end of the second epoch, diminishing to ~ 350 mJy by the third epoch. The 220 GHz data for these maps are only available for the second and third epochs. The source flux at 220 GHz rose from 200 to 400 mJy during the second epoch, and fell to 300 mJy by the end of the third epoch. The spectral index of the source is -0.5 +/- 0.3 and does not vary strongly during the flare. The source polarization, averaged over the 2.5 years of ACT observations, was small, < 2 %. Additionally, there was no measurable polarization detected during the dozens of 11-minute long observations taken during each epoch.

We will publish refined information from the ACT mm-band data on this unusually highly flaring source after more detailed examination of those data.

We acknowledge the support of the National Science Foundation for the ACT project, and offer thanks to Anne Lähteenmäki and Merja Tornikoski for useful discussions of flaring radio sources.