AARTFAAC flux density calibration and Northern hemisphere catalogue at 60 MHz

Abstract

We present a method for calibrating the flux density scale for images generated by the Amsterdam ASTRON Radio Transient Facility And Analysis Centre (AART-FAAC). AARTFAAC produces a stream of all-sky images at a rate of one second in order to survey the Northern Hemisphere for short duration, low frequency transients, such as the prompt EM counterpart to gravitational wave events, magnetar flares, blazars, and other as of yet unobserved phenomena. Therefore, an independent flux density scaling solution per image is calculated via bootstrapping, comparing the measured apparent brightness of sources in the field to a reference catalogue. However, the lack of accurate flux density measurements of bright sources below 74 MHz necessitated the creation of the AARTFAAC source catalogue, at 60 MHz, which contains 167 sources across the Northern Hemisphere. Using this as a reference results in a sufficiently high number of detected sources in each image to calculate a stable and accurate flux scale per one second snapshot, in real-time.

Constraining coherent low-frequency radio flares from compact binary mergers

“…This facility samples the sky on 1 s timescales using the inner-most 6 LOFAR core stations and 16 subbands (AARTFAAC-6; a total of 3.12 MHz of processed bandwidth centred around 60 MHz), with plans to upgrade to AARTFAAC-12 with the addition of 6 more core stations and another 16 subbands. The current AARTFAAC-6 configuration can reach a 3σ sensitivity of < 63 Jy over 90% of the Northern Hemisphere (Kuiack et al 2019). Doubling both the number of antennas and the bandwidth for AARTFAAC-12, will lead to a typical 3σ sensitivity of 3.3 Jy (private communication).…”

Apparent radio transients mapping the near-Earth plasma environment

“…If using these we do find that a source's variation is atypical we flag it as being of interest. We already studied the variations due to the quiescent ionosphere and noise quite extensively when constructing the AARTFAAC catalogue (Kuiack et al 2018), and when studying PSR B0950+08 (Kuiack et al 2020). Bottom line of our findings was that the measurement noise does not play a dominant role in the sources we detect almost by definition, because we set the detection threshold at 5-8 times the rms.…”

“…Data are streamed directly from 288 low band antennas (LBA) in the six central stations of the Low Frequency Array (LOFAR; van Haarlem et al 2013), the so called "superterp," and processed in parallel to LOFAR observations, employing an independent correlator, and subsequent calibration and imaging pipelines on a dedicated compute and storage cluster (Prasad et al 2014). (Kuiack et al 2018). The image stream can then be saved to disk for off-line transient analysis, and/or streamed to the AART-FAAC website 1 for live data quality inspection and public outreach.…”

Degree-Scale Galactic Radio Emission at 122 MHz around the North Celestial Pole with LOFAR-AARTFAAC

“…The correlator subsystem, located at the Centre for Information Technology (CIT) at the University of Groningen (the Netherlands), is a GPU based correlator that maximally handles 576 input signal streams per polarization (1152 total streams), and produces XX, XY, YX, YY correlations for all receiver pairs for every frequency channel with 1-second integration. The output correlations can either be dumped as raw correlations on disks or routed to the AART-FAAC real-time calibration and imaging pipeline for transient detection (Prasad et al 2016;Kuiack et al 2019). Readers are referred to van Haarlem et al (2013) and Prasad et al (2016) for detailed information about LOFAR and AARTFAAC system design and observing capabilities.…”