Characterization of the decametre sky at subarcminute resolution

TL;DR

This study presents a calibration method for decameter radio observations that significantly improves resolution and sensitivity, enabling the detection of rare fossil plasma sources and advancing low-frequency radio astronomy.

Contribution

The paper introduces a novel ionospheric correction strategy for decameter radio imaging, achieving an order of magnitude enhancement in resolution and sensitivity over previous studies.

Findings

Identified four fossil plasma sources in the surveyed region.

Achieved 45 arcsecond resolution at 16-30 MHz with 12 mJy/beam sensitivity.

Detected steep-spectrum radio sources indicating old, re-energized plasma.

Abstract

The largely unexplored decameter radio band (10-30 MHz) provides a unique window for studying a range of astronomical topics, such as auroral emission from exoplanets, inefficient cosmic ray acceleration mechanisms, fossil radio plasma, and free-free absorption. The scarcity of low-frequency studies is mainly due to the severe perturbing effects of the ionosphere. Here we present a calibration strategy that can correct for the ionosphere in the decameter band. We apply this to an observation from the Low Frequency Array (LOFAR) between 16 to 30 MHz . The resulting image covers 330 square degrees of sky at a resolution of 45", reaching a sensitivity of 12 mJy/beam. Residual ionospheric effects cause additional blurring ranging between 60 to 100". This represents an order of magnitude improvement in terms of sensitivity and resolution compared to previous decameter band observations. In the region we surveyed, we have identified four fossil plasma sources. These rare sources are believed to contain old, possibly re-energised, radio plasma originating from previous outbursts of active galactic nuclei. At least three of them are situated near the center of low-mass galaxy clusters. Notably, two of these sources display the steepest radio spectral index among all the sources detected at 23 MHz. This indicates that fossil plasma sources constitute the primary population of steep-spectrum sources at these frequencies, emphasising the large discovery potential of ground-based decameter observations.