ATCA finds a long-term radio outburst of PKS1424-418 coincident with optical/gamma-ray flaring activity

ATel #15536; Matthias Kadler (JMU Wuerzburg), Jamie Stevens (CSIRO Space and Astronomy), Roopesh Ojha (NASA HQ), Philip G. Edwards (CSIRO Space and Astronomy), and Florian Roesch (JMU Wuerzburg)
on 2 Aug 2022; 12:33 UT
Credential Certification: Matthias Kadler (matthias.kadler@astro.uni-wuerzburg.de)

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The blazar PKS1424-418 (z=1.522, White et al. 1988, ApJ, 327, 561) is currently showing an episode of rapid and bright flaring activity in the optical and gamma-ray band (ATel#15525, ATel#15527, ATel#15533). We report on contemporary radio observations of PKS1424-418, demonstrating that this high-energy flaring is associated with a long-term outburst in the radio band.

ATCA has monitored PKS1424-418 at multiple frequencies ranging from 2.1GHz to 40GHz since 2004. Most observations took place within the scope of the TANAMI/C1730 program to monitor gamma-ray bright blazars in the southern sky. In 2012-2015, the source showed a dramatic radio outburst , reaching almost 10Jy in the 13mm band. That outburst was found to occur in temporal and positional coincidence with multiwavelength flaring and a petaelectronvolt-energy neutrino event detected by IceCube and has been discussed as a possible origin of this neutrino event (Kadler et al. 2016, Nature Physics 12, 807).

Our monitoring observations show a new long-term increase of PKS1424-418 that started in late 2021 and is seen at all frequencies at 5.5GHz and above. The flux-density increase goes along with a strong inversion of the radio spectrum between 5.5GHz and higher frequencies. In contrast to this, the rising light curve exhibits a flat spectral index between the higher frequencies above 9GHz. As of Jun 25, 2022, the monotonic increase at the high frequencies has reached values of 4.81+-0.01 Jy at 9GHz, 4.90+-0.02 Jy at 16.7GHz, 4.73+-0.02 Jy at 21.2GHz, 4.02+-0.02 Jy at 38GHz, and 3.96+-0.02Jy at 40GHz, respectively.

Primary flux density calibration was performed against similar scans on PKS 1934-638 at all frequencies and epochs. Quoted uncertainties are statistical only. Systematic errors can be conservatively estimated to about 5% at the lower frequencies and about 10% at the higher frequencies.

The Australia Telescope Compact Array is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Gomeroi people as the traditional owners of the Observatory site.