Reaching thermal noise at ultra-low radio frequencies

Abstract

Context. Ultra-low frequency observations (< 100 MHz) are particularly challenging because they are usually performed in a low signal-to-noise ratio regime due to the high sky temperature and because of ionospheric disturbances whose effects are inversely proportional to the observing frequency. Nonetheless, these observations are crucial for studying the emission from low-energy populations of cosmic rays. Aims. We aim to obtain the first thermal-noise limited (∼1.5 mJy beam−1) deep continuum radio map using the Low Frequency Array’s Low Band Antenna (LOFAR LBA) system. Our demonstration observation targeted the galaxy cluster RX J0603.3+4214 (known as the Toothbrush cluster). We used the resulting ultra-low frequency (39–78 MHz) image to study cosmic-ray acceleration and evolution in the post shock region considering the presence of a radio halo. Methods. We describe the data reduction we used to calibrate LOFAR LBA observations. The resulting image was combined with observations at higher frequencies (LOFAR 150 MHz and VLA 1500 MHz) to extract spectral information. Results. We obtained the first thermal-noise limited image from an observation carried out with the LOFAR LBA system using all Dutch stations at a central frequency of 58 MHz. With eight hours of data, we reached an rms noise of 1.3 mJy beam−1 at a resolution of 18″ × 11″. Conclusions. The procedure we developed is an important step towards routine high-fidelity imaging with the LOFAR LBA. The analysis of the radio spectra shows that the radio relic extends to distances of 800 kpc downstream from the shock front, larger than what is allowed by electron cooling time. Furthermore, the shock wave started accelerating electrons already at a projected distance of < 300 kpc from the crossing point of the two clusters. These results may be explained by electrons being re-accelerated downstream by background turbulence, possibly combined with projection effects with respect to the radio halo.

Deep low-frequency radio observations of Abell 2256 I: The filamentary radio relic

“…The textbook example of radio relics, such as the Sausage and the Toothbrush, show clear spectral index gradients in downstream areas, reflecting the aging of the relativistic electron population, while the shock front propagates outward (van Weeren et al 2010(van Weeren et al , 2012aStroe et al 2013;Rajpurohit et al 2020b;Di Gennaro et al 2018;de Gasperin et al 2020). Even at such a high resolution, we do not find any clear uniform trends of the spectral index gradient across the entire relic in .…”

The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz

“…After the application of the calibrator solutions, the data for the target field required different steps of calibration to correct for differential ionospheric effects. For the target field calibration, we followed the procedure described in de Gasperin et al (2020). An initial directions independent (DI) calibration of the target field removed three systematic effects: the direction-averaged ionospheric delay, the Faraday rotation and beam variations with time and frequency on top of the LOFAR beam model.…”

The LOFAR LBA Sky Survey: Deep Fields

“…Only recently have we successfully implemented these and other directiondependent calibration and imaging algorithms to work at the lower frequencies of the LBA, leveraging advances in other software, including DPPP (van Diepen et al 2018) and WS-CLEAN (Offringa et al 2014). Together with a full understanding of the systematic effects in LOFAR data (de Gasperin et al 2018b(de Gasperin et al , 2019, the application of direction-dependent calibration has led to a number of high-resolution (∼ 15 arcsec), low-noise (∼ 1 mJy beam −1 ) images of several individual targets, including the Toothbrush cluster (de Gasperin et al 2020a), Abell 1758 (Botteon et al 2020) and the planetary system HD 80606 (de Gasperin et al 2020b). This has also allowed for the start of the first LOFAR low-frequency wide-area surveys with the LOFAR LBA Survey (LoLSS; de Gasperin et al 2021).…”