Interplanetary scintillation studies with the Murchison Widefield Array – II. Properties of sub-arcsecond compact sources at low radio frequencies

Abstract

We report the first astrophysical application of the technique of wide-field Interplanetary Scintillation (IPS) with the Murchison Widefield Array (MWA). This powerful technique allows us to identify and measure sub-arcsecond compact components in lowfrequency radio sources across large areas of sky without the need for long-baseline interferometry or ionospheric calibration. We present the results of a five-minute observation of a 30×30 deg 2 MWA field at 162 MHz with 0.5 second time resolution. Of the 2550 continuum sources detected in this field, 302 (12 per cent) show rapid fluctuations caused by IPS. We find that at least 32% of bright low-frequency radio sources contain a sub-arcsec compact component that contributes over 40% of the total flux density. Perhaps surprisingly, peaked-spectrum radio sources are the dominant population among the strongly-scintillating, low-frequency sources in our sample. While gamma-ray AGN are generally compact, flat-spectrum radio sources at higher frequencies, the 162 MHz properties of many of the Fermi blazars in our field are consistent with a compact component embedded within more extended low-frequency emission. The detection of a known pulsar in our field shows that the wide-field IPS technique is at the threshold of sensitivity needed to detect new pulsars using image plane analysis, and scaling the current MWA sensitivity to that expected for SKA-low implies that large IPS-based pulsar searches will be feasible with SKA. Calibration strategies for the SKA require a better knowledge of the space density of compact sources at low radio frequencies, which IPS observations can now provide.

Interplanetary scintillation studies with the Murchison Wide-field Array – IV. The hosts of sub-arcsecond compact sources at low radio frequencies

“…Freq Kapahi et al (1998b) and (80% galaxies, 20% QSOs) Baker et al (1999) Chhetri et al (2018a), and compact sources are defined to be those with a normalised scintillation index (NSI) ≥ 0.9, meaning that 90% of the low-frequency flux density arises from a sub-arcsec compact component. The lower S/N cutoff adopted for the expanded compact sample means that this sample contains more faint sources and so has a lower median flux density.…”

“…An expanded sample of strongly-scintillating sources, also from Table 1 of Chhetri et al (2018a), was selected to have normSNR ≥8.0 and NSI ≥0.90. We were able to relax the S/N cut for these compact sources, since the high level of scintillation they show can be reliably detected even for weaker sources.…”

“…Since strong IPS is only seen for sources that are dominated by a compact component less than ∼ 0.5 arcsec in size, this technique makes it possible to identify large numbers of subarcsecond radio sources efficiently without the need for VLBI observations. Chhetri et al (2018a) applied this technique to a five-E-mail:elaine.sadler@sydney.edu.au (EMS) minute observation of a 900 deg 2 MWA field at 162 MHz with 0.5 s time resolution, and found that 12% of the strong continuum sources in this field showed rapid fluctuations caused by IPS. Interestingly, Chhetri et al (2018a) showed that the compact source population at low frequencies is dominated by peaked-spectrum radio sources, and that many of the flat-spectrum QSOs that are compact at frequencies of a few GHz show extended structure at 160 MHz.…”

“…Chhetri et al (2018a) applied this technique to a five-E-mail:elaine.sadler@sydney.edu.au (EMS) minute observation of a 900 deg 2 MWA field at 162 MHz with 0.5 s time resolution, and found that 12% of the strong continuum sources in this field showed rapid fluctuations caused by IPS. Interestingly, Chhetri et al (2018a) showed that the compact source population at low frequencies is dominated by peaked-spectrum radio sources, and that many of the flat-spectrum QSOs that are compact at frequencies of a few GHz show extended structure at 160 MHz. All the sources studied by Chhetri et al (2018a) are also catalogued in the 72-231 MHz MWA GLEAM survey (Wayth et al 2015;Hurley-Walker et al 2017).…”

“…Interestingly, Chhetri et al (2018a) showed that the compact source population at low frequencies is dominated by peaked-spectrum radio sources, and that many of the flat-spectrum QSOs that are compact at frequencies of a few GHz show extended structure at 160 MHz. All the sources studied by Chhetri et al (2018a) are also catalogued in the 72-231 MHz MWA GLEAM survey (Wayth et al 2015;Hurley-Walker et al 2017).…”