ATCA Detection of a radio flare in the blazar PKS 0903-57 currently exhibiting a hard gamma-ray flare

ATel #13638; Jamie Stevens (CSIRO Astronomy and Space Science), Philip G. Edwards (CSIRO Astronomy and Space Science), Roopesh Ojha (NASA/GSFC/UMBC/CRESST), and Matthias Kadler (Uni. of Wuerzburg)
on 15 Apr 2020; 20:55 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)

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Following the Fermi-LAT detection of the strongest recorded gamma-ray flare in the blazar PKS 0903-57 with several high energy photons likely associated with it (ATel#13604; ATel#13599) the Australia Telescope Compact Array (ATCA) observed this radio source on 2020 April 02. Intense gamma-ray activity was also reported by AGILE (ATel#13602). H.E.S.S has made a significant detection (ATel#13632) of this object.

The ATCA observed bands centered at 5.5, 9.0, 17, 19, 38, and 40 GHz, each with a bandwidth of 2 GHz. The array was in the H168 antenna configuration at the time of the observations, which results in a resolution about a third lower than the highest angular resolution possible with ATCA. A snapshot of ~10 minutes duration was made at each frequency. Primary flux-density calibration was performed against similar scans on PKS 1934-638 at all frequencies.

The resulting flux densities for PKS 0903-57 were 1.64 Jy at 5.5 GHz, 1.54 Jy at 9 GHz, 1.45 Jy at 17 GHz, 1.42 Jy at 19 GHz, 1.43 at 38 GHz, and 1.49 at 40 GHz. The (one-sigma) errors in these values are estimated to be 4% for frequencies below 30 GHz, 11% for frequencies above 30 GHz, and these are dominated by systematic effects. The source is core dominated, but there is evidence of structure on the longest ATCA baselines.

PKS 0903-57 is periodically observed by ATCA. The most recent previous observations at each band and the corresponding flux densities were 1.14 Jy at 5.5 GHz (2015 Nov 23), 0.93 Jy at 9 GHz (2015 Nov 23), 1.37 Jy at 17 GHz (2019 May 08), 1.35 Jy at 19 GHz (2019 May 08), 1.05 at 38 GHz (2016 May 31), and 1.05 at 40 GHz (2016 May 31). Other, older, archival data are at similar lower levels in all bands.

Hence, the observations reported here represent the highest recorded radio fluxes in all observed bands. The spectral index alpha = -0.13, between 5.5 and 9.0 GHz, -0.10 between 9 and 17/19 GHz, and 0.02 between 17/19 and 38/40 GHz (where flux density is proportional to observing frequency raised to the power +alpha). Further observations are planned to follow the spectral evolution over the coming weeks.

The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO.