ATCA Observations of the New Gamma-ray Source PKS 1824-582

ATel #6076; Philip G. Edwards (CSIRO Astronomy and Space Science), Jamie Stevens (CSIRO Astronomy and Space Science), and Roopesh Ojha (NASA/GSFC/UMBC/CRESST)
on 17 Apr 2014; 03:33 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)

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The report of the Fermi-LAT detection of a new gamma ray source positionally coincident with PKS 1824-582 (ATel #6067), has been followed up with observations of this radio source using the Australia Telescope Compact Array (ATCA) between 22:37UT on 2014 April 14 and 00:40UT on 2014 April 15.

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 6A antenna configuration at the time of the observations, which provides the highest angular resolution. 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 1824-582 were 0.77 Jy at 5.5 GHz, 1.05 Jy at 9 GHz, 1.40 Jy at 17 GHz, and 1.39 Jy at 38 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 highly core dominated: there is no evidence of structure on the longest ATCA baselines.

PKS 1824-582 was observed in October/November 2005 as part of the AT20G survey (Murphy et al. 2010 MNRAS, 402, 2403), with catalogued flux densities of 829+/-41 mJy at 4.8 GHz, 801+/-40 mJy at 8.6 GHz, and 693+/-34 mJy at 20 GHz, yielding a spectral index alpha (where flux density is proportional to observing frequency raised to the power +alpha) of approximately -0.1 between the bands.

The observations reported here were not at historically high levels at the lowest frequency, but do surpass previously reported levels at the higher frequencies. The spectrum is very inverted: alpha = +0.4 at 5.5 GHz, +0.88 at 9.0 GHz, +0.11 at 17 GHz, and -0.45 at 38 GHz. These values strongly suggest PKS 1824-582 is correctly identified as the counterpart to the flaring gamma-ray source. Further observations are planned to follow the spectral evolution over the coming weeks.