Measured flux densities with the South Pole Telescope at 95, 150, 220 GHz for compact source SPT-S J061647-402147, a possible millimeter-wavelength counterpart of the transient gamma-ray blazar, Fermi 0617-4026

ATel #12837; W. Everett (University of Colorado, Boulder) on behalf of the SPT Collaboration
on 4 Jun 2019; 17:56 UT
Credential Certification: Wendeline Everett (wendeline.everett@colorado.edu)

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We report flux densities measured in three bands centered at 95, 150, and 220 GHz in the SPT-SZ survey for source SPT-S J061647-402147, positionally consistent with ACT-T J061647-402140, for which fluxes were reported recently in Atel #12738. This source is considered to be a possible millimeter-wavelength counterpart of the transient gamma-ray source, Fermi 0617-4026 (Atel #6912), confirmed to be a blazar via follow-up observations (WISE J061647.01-402142.8, Atel #6937). The SPT-SZ observations were taken between 2008 - 2011 on the South Pole Telescope (Carlstrom et al., 2011), with beams well-described by Gaussian functions with FWHM of 1.7, 1.2 and 1.0 arcmin for 95, 150 and 220 GHz, respectively. The survey is composed of 19 fields observed independently; the field containing SPT-S J061647-402147 was observed in 2010 between Sept. 12 - Oct 3rd and in 2011 between Sept. 19 - Oct. 28th. Coadded maps from all observations of the field yield measured flux densities of 41.2 +2.5/-2.2, 32.8 +/- 1.6, and 32.5 +/- 5.6 mJy at 95, 150, and 220 GHz, respectively. The best-fit source location in SPT data is RA = 06:16:46.8, Dec. = -40:21:47.0 (J2000), within 5 arcsec of the reported WISE position and 7 arcsec of the reported ACT position. These are consistent with the source position uncertainties in the SPT-SZ compact source catalog, namely 4.6 arcsec in declination and 6.0 arcsec in RA at this declination, calculated by comparing locations of the brightest sources in each field with AT20G. Other than the positional cross-match with the transient gamma-ray source discovered by Fermi-LAT (Fermi 0617-4026), we find no cross-matches for this source with external catalogs in radio, IR, or X-ray. We note that we do cross-match the SPT-SZ catalog with the WISE AllWISE source catalog, but we restrict to sources with flux densities greater than 5 mJy in the WISE W4 band in order to mitigate source cross-match confusion within the SPT beam, and WISE J061647.01-402142.8 has a measured flux density below this threshold. In comparison with flux densities reported by ACT in three epochs, [2016.6 - 2017.0], [2017.6 - 2018.1], and [2018.7 - 2018.9], where the source flux in 90 and 150 GHz flared from below 50 mJy in the first epoch to above 650 mJy in the second and reduced to 350 mJy in the third (Atel #12738), fluxes detected by SPT indicate the source was in a relatively non-flaring phase in 2010 - 2011, with the flux increasing by a factor of 15-20 between 2010 - 2011 and 2017 - 2018. We note that the lack of cross-matches in radio catalogs including PMN (observed primarily in 1990, with a detection threshold of roughly 50 mJy, Wright et al., 1994), SUMSS (observing from 1997 - 2007, with a detection threshold of roughly 6 mJy, Mauch et al., 2003), and AT20G (observing from 2004 - 2008, with a detection threshold of 40 mJy, Murphy et al., 2010), potentially indicate that this source may have an inverted spectrum or may have undergone more complicated flaring behavior in the last several decades. Of the roughly 500 sources in the SPT-SZ catalog with flux densities as bright or brighter than SPT-S J061647-402147 and spectral indices consistent with the source of emission being synchrotron, only six have no cross-match counterpart in SUMSS and two (including SPT-S J061647-402147) have no cross-matches in any of the external catalogs we check.