Radio relics in PSZ2 G096.88+24.18: A connection with pre-existing plasma

Giant radio relics are arc-like structures of diffuse, non-thermal synchrotron radiation that trace shock waves induced by galaxy cluster mergers. The particle (re-)acceleration mechanism producing such radio relics is unclear. One major open question is whether relics can be formed directly from a population of thermal seed electrons, or if pre-existing relativistic seed electrons are required. In some cases AGN can provide such a population of sub-GeV electrons. However, it is unclear how common this connection is. In this paper we present LOFAR 140 MHz and VLA L-band radio observations, as well as Chandra data of PSZ2 G096.88+24.18, a merging galaxy cluster system hosting a pair of radio relics. A large patch of diffuse emission connects a bright radio galaxy with one of the relics, likely affecting the properties of the relic. We find that the most plausible explanation for the connection is that the merger shock wave has passed over an AGN lobe. The shock passing over this seed population of electrons has led to an increased brightness in the relic only in the region filled with seed electrons.

Investigating ionospheric calibration for LOFAR 2.0 with simulated observations

A number of hardware upgrades for the Low-Frequency Array (LOFAR) are currently under development. These upgrades are collectively referred to as the LOFAR 2.0 upgrade. The first stage of LOFAR 2.0 will introduce a distributed clock signal and allow for simultaneous observation with all the low-band and high-band antennas of the array. Our aim is to provide a tool for accurate simulations of LOFAR 2.0. We present a software to simulate LOFAR and LOFAR 2.0 observations, which includes realistic models for all important systematic effects such as the first and second order ionospheric corruptions, time-variable primary-beam attenuation, station based delays and bandpass response. The ionosphere is represented as a thin layer of frozen turbulence. Furthermore, thermal noise can be added to the simulation at the expected level. We simulate a full 8-hour simultaneous low- and high-band antenna observation of a calibrator source and a target field with the LOFAR 2.0 instrument. The simulated data is calibrated using readjusted LOFAR calibration strategies. We examine novel approaches of solution-transfer and joint calibration to improve direction-dependent ionospheric calibration for LOFAR. We find that the calibration of the simulated data behaves very similarly to a real observation and reproduces characteristic properties of LOFAR data such as realistic solutions and image quality. We analyze strategies for direction-dependent calibration of LOFAR 2.0 and find that the ionospheric parameters can be determined most accurately when combining the information of the high-band and low-band in a joint calibration approach. In contrast, the transfer of total electron content solutions from the high-band to the low-band shows good convergence but is highly susceptible to the presence of non-ionospheric phase errors in the data.

The LOFAR LBA Sky Survey I. survey description and preliminary data release

LOFAR is the only radio telescope that is presently capable of high-sensitivity, high-resolution (<1 mJy/b and <15) observations at ultra-low frequencies (<100 MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range 42-66 MHz, at a resolution of 15 arcsec and at a sensitivity of 1 mJy/b. Here we outline the survey strategy, the observational status, the current calibration techniques, and briefly describe several scientific motivations. We also describe the preliminary public data release. The preliminary images were produced using a fully automated pipeline that aims to correct all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, are not yet corrected, the images presented in this work are still 10 times more sensitive than previous surveys available at these low frequencies. The preliminary data release covers 740 sqdeg around the HETDEX spring field region at a resolution of 47 with a median noise level of 5 mJy/b. The images and the catalogue with 25,247 sources are publicly released. We demonstrate that the system is capable of reaching an rms noise of 1 mJy/b and the resolution of 15 once direction-dependent effects are corrected for. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique dataset that can be used for a wide range of science topics such as: the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, and from AGN activity.

Reaching thermal noise at ultra-low radio frequencies: the Toothbrush radio relic downstream of the shock front

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 to study the emission from low-energy populations of cosmic rays. We aim to obtain the first thermal-noise limited (~ 1.5 mJy/beam) deep continuum radio map using the LOFAR Low Band Antenna (LBA) system. Our demonstration observation targeted the galaxy cluster RX J0603.3+4214 (the Toothbrush cluster). We used the resulting ultra-low frequency (58 MHz) image to study cosmic-ray acceleration and evolution in the post shock region, as well as their relation with the presence of a radio halo. We describe the data reduction we have used to calibrate LOFAR LBA observations. The resulting image is combined with observations at higher frequencies (LOFAR 150 MHz and VLA 1500 MHz) to extract spectral information. 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 8 hours of data, we reached an rms noise of 1.3 mJy/beam at a resolution of 18 x 11. The procedure we have developed is an important step forward 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 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 can be explained if electrons are reaccelerated downstream by background turbulence possibly combined with projection effects.

The relation between the diffuse X-ray luminosity and the radio power of the central AGN in galaxy groups

Our understanding of how AGN feedback operates in galaxy clusters has improved in recent years owing to large efforts in multi-wavelength observations and hydrodynamical simulations. However, it is much less clear how feedback operates in galaxy groups, which have shallower gravitational potentials. In this work, using very deep VLA and new MeerKAT observations from the MIGHTEE survey, we compiled a sample of 247 X-ray selected galaxy groups detected in the COSMOS field. We have studied the relation between the X-ray emission of the intra-group medium and the 1.4 GHz radio emission of the central radio galaxy. For comparison, we have also built a control sample of 142 galaxy clusters using ROSAT and NVSS data. We find that clusters and groups follow the same correlation between X-ray and radio emission. Large radio galaxies hosted in the centres of groups and merging clusters increase the scatter of the distribution. Using statistical tests and Monte-Carlo simulations, we show that the correlation is not dominated by biases or selection effects. We also find that galaxy groups are more likely than clusters to host large radio galaxies, perhaps owing to the lower ambient gas density or a more efficient accretion mode. In these groups, radiative cooling of the ICM could be less suppressed by AGN heating. We conclude that the feedback processes that operate in galaxy clusters are also effective in groups.

Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies

The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (<100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15 arcsec with a high dynamic range and good image fidelity. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna (LBA) system. We reduced the datasets and obtained an image for each A-team source. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further.

Searching for the largest bound atoms in space

(abridged) Radio recombination lines (RRLs) at frequencies $ u$ < 250 MHz trace the cold, diffuse phase of the ISM. Next generation low frequency interferometers, such as LOFAR, MWA and the future SKA, with unprecedented sensitivity, resolution, and large fractional bandwidths, are enabling the exploration of the extragalactic RRL universe. We observed the radio quasar 3C 190 (z~1.2) with the LOFAR HBA. In reducing this data for spectroscopic analysis, we have placed special emphasis on bandpass calibration. We devised cross-correlation techniques to significantly identify the presence of RRLs in a low frequency spectrum. We demonstrate the utility of this method by applying it to existing low-frequency spectra of Cassiopeia A and M 82, and to the new observations of 3C 190. RRLs have been detected in the foreground of 3C 190 at z = 1.12355 (assuming a carbon origin), owing to the first detection of RRLs outside of the local universe (first reported in Emig et al. 2019). Towards the Galactic supernova remnant Cas A, we uncover three new detections: (1) C$epsilon$-transitions ($Delta$n = 5) for the first time at low radio frequencies, (2) H$alpha$-transitions at 64 MHz with a FWHM of 3.1 km/s, the most narrow and one of the lowest frequency detections of hydrogen to date, and (3) C$alpha$ at v$_{LSR}$ = 0 km/s in the frequency range 55-78 MHz for the first time. Additionally we recover C$alpha$, C$beta$, C$gamma$, and C$delta$ from the -47 km/s and -38 km/s components. In the nearby starburst galaxy, M 82, we do not find a significant feature. Our current searches for RRLs in LOFAR observations are limited to narrow (< 100 km/s) features, owing to the relatively small number of channels available for continuum estimation. Future strategies making use of larger contiguous frequency coverage would aid calibration to deeper sensitivities and broader features.

Diffuse Radio Emission from Galaxy Clusters

In a growing number of galaxy clusters diffuse extended radio sources have been found. These sources are not directly associated with individual cluster galaxies. The radio emission reveal the presence of cosmic rays and magnetic fields in the intracluster medium (ICM). We classify diffuse cluster radio sources into radio halos, cluster radio shocks (relics), and revived AGN fossil plasma sources. Radio halo sources can be further divided into giant halos, mini-halos, and possible `intermediate sources. Halos are generally positioned at cluster center and their brightness approximately follows the distribution of the thermal ICM. Cluster radio shocks (relics) are polarized sources mostly found in the clusters periphery. They trace merger induced shock waves. Revived fossil plasma sources are characterized by their radio steep-spectra and often irregular morphologies. In this review we give an overview of the properties of diffuse cluster radio sources, with an emphasis on recent observational results. We discuss the resulting implications for the underlying physical acceleration processes that operate in the ICM, the role of relativistic fossil plasma, and the properties of ICM shocks and magnetic fields. We also compile an updated list of diffuse cluster radio sources which will be available on-line http://galaxyclusters.com. We end this review with a discussion on the detection of diffuse radio emission from the cosmic web.

The first detection of radio recombination lines at cosmological distances

Recombination lines involving high principal quantum numbers populate the radio spectrum in large numbers. Low-frequency (<1 GHz) observations of radio recombination lines (RRLs) primarily from carbon and hydrogen offer a new, if not unique, way to probe cold, largely atomic gas and warm, ionised gas in other galaxies. Furthermore, RRLs can be used to determine the physical state of the emitting regions, such as temperature and density. These properties make RRLs, potentially, a powerful tool of extragalactic ISM physics. At low radio frequencies, its conceivable to detect RRLs out to cosmological distances when illuminated by a strong radio continuum. However, they are extremely faint (tau ~ 1e-3 -- 1e-4) and have so far eluded detection outside of the local universe. With LOFAR observations of the radio quasar 3C 190 (z=1.1946), we aim to demonstrate that the ISM can be explored out to great distances through low-frequency RRLs. We report the detection of RRLs in the frequency range 112--163 MHz in the spectrum of 3C 190. Stacking 13 a-transitions with principal quantum numbers n=266-301, a peak 6sigma feature of optical depth, tau(peak) = (1.0 +- 0.2) x 1e-3 and FWHM = 31.2 +/- 8.3 km/s was found at z=1.124. This corresponds to a velocity offset of -9965 km/s with respect to the systemic redshift of 3C 190. We consider three interpretations of the origin of the RRL emission: an intervening dwarf-like galaxy, an AGN-driven outflow, and the inter-galactic medium. We argue that the RRLs most likely originate in a dwarf-like galaxy (M ~ 1e9 Msun) along the line of sight, although we cannot rule out an AGN-driven outflow. We do find the RRLs to be inconsistent with an inter-galactic medium origin. With this detection, we have opened up a new way to study the physical properties of cool, diffuse gas out to cosmological distances.

A radio spectral index map and catalogue at 147-1400 MHz covering 80 per cent of the sky

The radio spectral index is a powerful probe for classifying cosmic radio sources and understanding the origin of the radio emission. Combining data at 147 MHz and 1.4 GHz from the TIFR GMRT Sky Survey (TGSS) and the NRAO VLA Sky Survey (NVSS), we produced a large-area radio spectral index map of ~80 per cent of the sky (Dec > -40 deg), as well as a radio spectral index catalogue containing 1,396,515 sources, of which 503,647 are not upper or lower limits. Almost every TGSS source has a detected counterpart, while this is true only for 36 per cent of NVSS sources. We released both the map and the catalogue to the astronomical community. The catalogue is analysed to discover systematic behaviours in the cosmic radio population. We find a differential spectral behaviour between faint and bright sources as well as between compact and extended sources. These trends are explained in terms of radio galaxy evolution. We also confirm earlier reports of an excess of steep-spectrum sources along the galactic plane. This corresponds to 86 compact and steep-spectrum source in excess compared to expectations. The properties of this excess are consistent with normal non-recycled pulsars, which may have been missed by pulsation searches due to larger than average scattering along the line of sight.