Magnetic fields in cosmic particle acceleration sources

We review here some magnetic phenomena in astrophysical particle accelerators associated with collisionless shocks in supernova remnants, radio galaxies and clusters of galaxies. A specific feature is that the accelerated particles can play an important role in magnetic field evolution in the objects. We discuss a number of CR-driven, magnetic field amplification processes that are likely to operate when diffusive shock acceleration (DSA) becomes efficient and nonlinear. The turbulent magnetic fields produced by these processes determine the maximum energies of accelerated particles and result in specific features in the observed photon radiation of the sources. Equally important, magnetic field amplification by the CR currents and pressure anisotropies may affect the shocked gas temperatures and compression, both in the shock precursor and in the downstream flow, if the shock is an efficient CR accelerator. Strong fluctuations of the magnetic field on scales above the radiation formation length in the shock vicinity result in intermittent structures observable in synchrotron emission images. Resonant and non-resonant CR streaming instabilities in the shock precursor can generate mesoscale magnetic fields with scale-sizes comparable to supernova remnants and even superbubbles. This opens the possibility that magnetic fields in the earliest galaxies were produced by the first generation Population III supernova remnants and by clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review

LOFAR M51 field low-frequency polarized sources

Item does not contain fulltextFITS image of Fig. 1 (LOFAR 150MHz total intensity image). 2) Catalog of 201 sources with flux density greater than 100mJy at 150MHz and located within 2.5 degrees of the center of M51. 3) Faraday cubes of 19 sources discussed in Sect. 5. (4 data files)

LOFAR M51 field low-frequency polarized sources

FITS image of Fig. 1 (LOFAR 150MHz total intensity image). 2) Catalog of 201 sources with flux density greater than 100mJy at 150MHz and located within 2.5 degrees of the center of M51. 3) Faraday cubes of 19 sources discussed in Sect. 5. (4 data files)

LOFAR reveals the giant: A low-frequency radio continuum study of the outflow in the nearby FR I radio galaxy 3C 31

We present a deep, low-frequency radio continuum study of the nearby Fanaroff-Riley class I (FR I) radio galaxy 3C 31 using a combination of LOw Frequency ARray (LOFAR; 30-85 and 115-178 MHz), Very Large Array (VLA; 290-420 MHz), Westerbork Synthesis Radio Telescope (WSRT; 609 MHz) and Giant Metre Radio Telescope (GMRT; 615 MHz) observations. Our new LOFAR 145-MHz map shows that 3C 31 has a largest physical size of 1.1Mpc in projection, which means 3C 31 now falls in the class of giant radio galaxies. We model the radio continuum intensitieswith advective cosmic ray transport, evolving the cosmic ray electron population and magnetic field strength in the tails as functions of distance to the nucleus. We find that if there is no in situ particle acceleration in the tails, then decelerating flows are required that depend on radius r as v α rβ (β ≈ -1). This then compensates for the strong adiabatic losses due to the lateral expansion of the tails. We are able to find selfconsistent solutions in agreement with the entrainment model of Croston &amp; Hardcastle, where the magnetic field provides ≈1/3 of the pressure needed for equilibrium with the surrounding intracluster medium. We obtain an advective time-scale of ≈190 Myr, which, if equated to the source age, would require an average expansion Mach number M≈ 5 over the source lifetime. Dynamical arguments suggest that instead either the outer tail material does not represent the oldest jet plasma or else the particle ages are underestimated due to the effects of particle acceleration on large scales.</p

Polarized point sources in LOTSS-HETDEX

Visibility data taken from LOTSS, imaged in polarization, and had RM synthesis applied. Resulting RM spectra were searched for polarization peaks. Detected peaks that were determined to not be foreground or instrumental effects were collected in this catalog. Source locations (for peak searches) were selected from TGSS-ADR1 (J/A+A/598/A78). Due to overlap between fields, some sources were detected multiple times, as recorded in the Ndet column. Polarized sources were cross-matched with the high-resolution LOTSS images (Shimwell+, in prep), and WISE and PanSTARRS images, which were used to determine the source classification and morphology. (1 data file)

Polarized point sources in LOTSS-HETDEX

Item does not contain fulltextVisibility data taken from LOTSS, imaged in polarization, and had RM synthesis applied. Resulting RM spectra were searched for polarization peaks. Detected peaks that were determined to not be foreground or instrumental effects were collected in this catalog. Source locations (for peak searches) were selected from TGSS-ADR1 (J/A+A/598/A78). Due to overlap between fields, some sources were detected multiple times, as recorded in the Ndet column. Polarized sources were cross-matched with the high-resolution LOTSS images (Shimwell+, in prep), and WISE and PanSTARRS images, which were used to determine the source classification and morphology. (1 data file)

NGC 891 LOFAR and AMI maps

The LOFAR observations of NGC 891 were done in interleaved mode, switching between scans on the calibrator 3C48 (at RA(J2000)=01:34:41.3, Dec(J2000)=+33:09:35and the target NGC 891. A total of 44 stations were used for this observation, of which 32 were core stations and 12 were remote stations. &lt;P /&gt;The AMI (Arcminute Microkelvin Imager) consists of two radio arrays, the Small Array (SA) and the Large Array (LA), located at the Mullard Radio Astronomy Observatory (Cambridge, UK). &lt;P /&gt;The SA data were taken as a single pointing with interleaved observations of the phase calibrator J0222+4302 and 3C286 used as a flux density and bandpass calibrator. The LA data were taken as a mosaic, consisting of seven pointings arranged on a hexagonal raster centred on NGC 891. Individual pointings were cycled between the pointing centers every 60s, switching to the phase calibrator J0222+4302 every 10min for 2min. &lt;P /&gt;(2 data files)

Stokes I image of M51

Item does not contain fulltextLow-frequency radio continuum observations (<300MHz) can provide valuable information on the propagation of low-energy cosmic ray electrons (CRE). Nearby spiral galaxies have hardly been studied in this frequency range because of the technical challenges of low-frequency radio interferometry. This is now changing with the start of operations of LOFAR. We aim to study the propagation of low-energy CRE in the interarm regions and the extended disk of the nearly face-on spiral galaxy Messier 51. We also search for polarisation in M51 and other extragalactic sources in the field. The grand-design spiral galaxy M51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarisation. This observation covered the frequencies between 115MHz and 175MHz with 244 subbands of 8 channels each, resulting in 1952 channels. This allowed us to use RM synthesis to search for polarisation. We produced an image of total emission of M51 at the mean frequency of 151MHz with 20'' resolution and 0.3mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300MHz so far. The integrated spectrum of total radio emission is described well by a power law, while flat spectral indices in the central region indicate thermal absorption. We observe that the disk extends out to 16kpc and see a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are greater at 151MHz, and the break is smoother at 151MHz than those observed at 1.4GHz. The arm-interarm contrast is lower at 151MHz than at 1400MHz, indicating propagation of CRE from spiral arms into interarm regions. The correlations between the images of radio emission at 151MHz and 1400MHz and the FIR emission at 70µm reveal breaks on scales of 1.4 and 0.7kpc, respectively. The total (equipartition) magnetic field strength decreases from about 28 µG in the central region to about 10 µG at 10 kpc radius. No significant polarisation was detected from M51, owing to severe Faraday depolarisation. Six extragalactic sources are detected in polarisation in the M51 field of 4.1°x4.1° size. Two sources show complex structures in Faraday space. Our main results, the scale lengths of the inner and outer disks at 151MHz and 1.4GHz, arm-interarm contrast, and the break scales of the radio-FIR correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by HII regions. The non-detection of polarisation from M51 at 151MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.nul