Magnetic field strength in cosmic web filaments Carretti, Ettore; Vacca, V; O’Sullivan, S P ...
Monthly notices of the Royal Astronomical Society,
03/2022, Letnik:
512, Številka:
1
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ABSTRACT
We used the rotation measure (RM) catalogue derived from the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2) at 144 MHz to measure the evolution with redshift of the extragalactic RM ...(RRM: Residual RM) and the polarization fraction (p) of sources in low-density environments. We also measured the same at 1.4 GHz by cross-matching with the NRAO VLA Sky Survey RM catalogue. We find that RRM versus redshift is flat at 144 MHz, but, once redshift-corrected, it shows evolution at high significance. Also, p evolves with redshift with a decrement by a factor of ∼8 at z ∼ 2. Comparing the 144-MHz and 1.4-GHz data, we find that the observed RRM and p are most likely to have an origin local to the source at 1.4 GHz, while a cosmic web filament origin is favoured at 144 MHz. If we attribute the entire signal to filaments, we infer a mean rest-frame RRM per filament of RRM$_{\rm 0,f} = 0.71 \pm 0.07 \, \, \rm rad\, m^{-2}$ and a magnetic field per filament of Bf = 32 ± 3 nG. This is in agreement with estimates obtained with a complementary method based on synchrotron emission stacking, and with cosmological simulations if primordial magnetic fields are amplified by astrophysical source field seeding. The measurement of an RRM0, f supports the presence of diffuse baryonic gas in filaments. We also estimated a conservative upper limit of the filament magnetic turbulence of $\sigma _{\rm RRM_{\rm 0,f}} =0.039 \pm 0.001 \, \, \rm rad\, m^{-2}$, concluding that the ordered magnetic field component dominates in filaments.
Remnant radio-loud AGN in the Herschel-ATLAS field Mahatma, V H; Hardcastle, M J; Williams, W L ...
Monthly notices of the Royal Astronomical Society,
04/2018, Letnik:
475, Številka:
4
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Abstract
Only a small fraction of observed active galactic nuclei (AGN) display large-scale radio emission associated with jets, yet these radio-loud AGN have become increasingly important in models ...of galaxy evolution. In determining the dynamics and energetics of the radio sources over cosmic time, a key question concerns what happens when their jets switch off. The resulting ‘remnant' radio-loud AGN have been surprisingly evasive in past radio surveys, and therefore statistical information on the population of radio-loud AGN in their dying phase is limited. In this paper, with the recent developments of Low-Frequency Array (LOFAR) and the Very Large Array, we are able to provide a systematically selected sample of remnant radio-loud AGN in the Herschel-ATLAS field. Using a simple core-detection method, we constrain the upper limit on the fraction of remnants in our radio-loud AGN sample to 9 per cent, implying that the extended lobe emission fades rapidly once the core/jets turn off. We also find that our remnant sample has a wide range of spectral indices ($-1.5\leqslant \alpha ^{1400}_{150}\leqslant -0.5$), confirming that the lobes of some remnants may possess flat spectra at low frequencies just as active sources do. We suggest that, even with the unprecedented sensitivity of LOFAR, our sample may still only contain the youngest of the remnant population.
The Lockman Hole is a well-studied extragalactic field with extensive multi-band ancillary data covering a wide range in frequency, essential for characterizing the physical and evolutionary ...properties of the various source populations detected in deep radio fields (mainly star-forming galaxies and AGNs). In this paper, we present new 150-MHz observations carried out with the LOw-Frequency ARray (LOFAR), allowing us to explore a new spectral window for the faint radio source population. This 150-MHz image covers an area of 34.7 square degrees with a resolution of 18.6 x 14.7 arcsec and reaches an rms of 160 mu Jy beam super( -1) at the centre of the field. As expected for a low-frequency selected sample, the vast majority of sources exhibit steep spectra, with a median spectral index of ... The median spectral index becomes slightly flatter (increasing from ... with decreasing flux density down to S sub( 150) ~10 mJy before flattening out and remaining constant below this flux level. For a bright subset of the 150-MHz selected sample, we can trace the spectral properties down to lower frequencies using 60-MHz LOFAR observations, finding tentative evidence for sources to become flatter in spectrum between 60 and 150 MHz. Using the deep, multi-frequency data available in the Lockman Hole, we identify a sample of 100 ultra-steep-spectrum sources and 13 peaked-spectrum sources. We estimate that up to 21 per cent of these could have z > 4 and are candidate high-z radio galaxies, but further follow-up observations are required to confirm the physical nature of these objects. (ProQuest: ... denotes formulae/symbols omitted.)
The space-weather conditions that result from stellar winds significantly impact the habitability of exoplanets. The conditions can be calculated from first principles if the necessary boundary ...conditions are specified, namely the plasma density in the outer corona and the radial distance at which the plasma forces the closed magnetic field into an open geometry. Low frequency radio observations (
ν
≲ 200 MHz) of plasma and cyclotron emission from stars probe these magneto-ionic conditions. Here we report the detection of low-frequency (120–167 MHz) radio emission associated with the dMe6 star WX UMa. If the emission originates in WX UMa’s corona, we show that the closed field region extends to at least ≈10 stellar radii, which is about a factor of a few larger than the solar value, and possibly to ≳20 stellar radii. Our results suggest that the magnetic-field structure of M dwarfs is in between Sun-like and planet-like configurations, where compact over-dense coronal loops with X-ray emitting plasma co-exist with a large-scale magnetosphere with a lower plasma density and closed magnetic geometry.
We present the discovery of PSR J0250+5854, a radio pulsar with a spin period of 23.5 s. This is the slowest-spinning radio pulsar known. PSR J0250+5854 was discovered by the LOFAR Tied-Array All-Sky ...Survey (LOTAAS), an all-northern-sky survey for pulsars and fast transients at a central observing frequency of 135 MHz. We subsequently detected pulsations from the pulsar in the interferometric images of the LOFAR Two-meter Sky Survey, allowing for subarcsecond localization. This, along with a pre-discovery detection 2 years prior, allowed us to measure the spin-period derivative to be s s−1. The observed spin period derivative of PSR J0250+5854 indicates a surface magnetic field strength, characteristic age, and spindown luminosity of G, 13.7 Myr, and erg s−1, respectively, for a dipolar magnetic field configuration. This also places the pulsar beyond the conventional pulsar death line, where radio emission is expected to cease. The spin period of PSR J0250+5854 is similar to those of the high-energy-emitting magnetars and X-ray dim isolated neutron stars (XDINSs). However, the pulsar was not detected by the Swift/X-Ray Telescope in the energy band of 0.3-10 keV, placing a bolometric luminosity limit of erg s−1 for an assumed cm−2 and a temperature of 85 eV (typical of XDINSs). We discuss the implications of the discovery for models of the pulsar death line as well as the prospect of finding more similarly long-period pulsars, including the advantages provided by LOTAAS for this.
Abstract
We present LOw Frequency ARray observations of the Coma Cluster field at 144 MHz. The cluster hosts one of the most famous radio halos, a relic, and a low surface brightness bridge. We ...detect new features that allow us to make a step forward in the understanding of particle acceleration in clusters. The radio halo extends for more than 2 Mpc, which is the largest extent ever reported. To the northeast of the cluster, beyond the Coma virial radius, we discover an arc-like radio source that could trace particles accelerated by an accretion shock. To the west of the halo, coincident with a shock detected in the X-rays, we confirm the presence of a radio front, with different spectral properties with respect to the rest of the halo. We detect a radial steepening of the radio halo spectral index between 144 and 342 MHz, at ∼30′ from the cluster center, that may indicate a non-constant re-acceleration time throughout the volume. We also detect a mild steepening of the spectral index toward the cluster center. For the first time, a radial change in the slope of the radio–X-ray correlation is found, and we show that such a change could indicate an increasing fraction of cosmic-ray versus thermal energy density in the cluster outskirts. Finally, we investigate the origin of the emission between the relic and the source NGC 4789, and we argue that NGC 4789 could have crossed the shock originating the radio emission visible between its tail and the relic.
Coherent radio emission from stars can be used to constrain fundamental coronal plasma parameters, such as plasma density and magnetic field strength. It is also a probe of chromospheric and ...magnetospheric acceleration mechanisms. Close stellar binaries, such as RS Canum Venaticorum (RS CVn) systems, are particularly interesting as their heightened level of chromospheric activity and possible direct magnetic interaction make them a unique laboratory to study coronal and magnetospheric acceleration mechanisms. RS CVn binaries are known to be radio-bright but coherent radio emission has only conclusively been detected previously in one system. Here, we present a population of 14 coherent radio emitting RS CVn systems. We identified the population in the ongoing LOFAR Two Metre Sky Survey as circularly polarised sources at 144 MHz that are astrometrically associated with known RS CVn binaries. We show that the observed emission is powered by the electron cyclotron maser instability. We use numerical calculations of the maser’s beaming geometry to argue that the commonly invoked ‘loss-cone’ maser cannot generate the necessary brightness temperature in some of our detections and that a more efficient instability, such as the shell or horseshoe maser, must be invoked. Such maser configurations are known to operate in planetary magnetospheres. We also outline two acceleration mechanisms that could produce coherent radio emission, one where the acceleration occurs in the chromosphere and one where the acceleration is due to an electrodynamic interaction between the stars. We propose radio and optical monitoring observations that can differentiate between these two mechanisms.
The Beautiful Mess in Abell 2255 Botteon, A.; Brunetti, G.; van Weeren, R. J. ...
The Astrophysical journal,
07/2020, Letnik:
897, Številka:
1
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We present LOFAR observations of one of the most spectacular objects in the radio sky: Abell 2255. This is a nearby (z = 0.0806) merging galaxy cluster hosting one of the first radio halos ever ...detected in the intracluster medium (ICM). The deep LOFAR images at 144 MHz of the central ∼10 Mpc2 region show a plethora of emission on different scales, from tens of kiloparsecs to above megaparsec sizes. In this work, we focus on the innermost region of the cluster. Among the numerous interesting features observed, we discover remarkable bright and filamentary structures embedded in the radio halo. We incorporate archival WSRT 1.2 GHz data to study the spectral properties of the diffuse synchrotron emission and find a very complex spectral index distribution in the halo spanning a wide range of values. We combine the radio data with Chandra observations to investigate the connection between the thermal and nonthermal components by quantitatively comparing the radio and X-ray surface brightness and the spectral index of the radio emission with the thermodynamical quantities of the ICM. Despite the multitude of structures observed in the radio halo, we find that the X-ray and radio emission are overall well correlated. The fact that the steepest spectrum emission is located in the cluster center and traces regions with high entropy possibly suggests the presence of seed particles injected by radio galaxies that are spread in the ICM by the turbulence generating the extended radio halo.
ABSTRACT
We report extremely faint 144 MHz radio emission from two gravitationally lensed quasars, SDSS J1004+4112 (z = 1.730) and SDSS J2222+2745 (z = 2.803), using the LOFAR Two Metre Sky Survey ...(LoTSS) data release 2. After correcting for the lensing magnifications, the two objects have intrinsic flux-densities of 13 ± 2 and 58 ± 6 μJy, respectively, corresponding to 144 MHz rest-frame luminosities of 1023.2 ± 0.2 and 1024.42 ± 0.05 W Hz−1, respectively. In the case of SDSS J1004+4112, the intrinsic flux density is close to the confusion limit of LoTSS, making this radio source the faintest to be detected thus far at low frequencies, and the lowest luminosity known at z ≳ 0.65. Under the assumption that all of the radio emission is due to star-formation processes, the quasar host galaxies are predicted to have star-formation rates of $5.5^{+1.8}_{-1.4}$ and $73^{+34}_{-22}$ M⊙ yr−1, respectively. Further multiwavelength observations at higher angular resolution will be needed to determine if any of the detected radio emission is due to weak jets associated with the quasars.
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