23 research outputs found
Untangling cosmic magnetic fields: Faraday tomography at metre wavelengths with LOFAR
14 pages, 6 figures. Accepted for publication in "The Power of Faraday Tomography" special issue of GalaxiesThe technique of Faraday tomography is a key tool for the study ofmagnetised plasmas in the new era of broadband radio-polarisation observations. In particular, observations at metre wavelengths provide significantly better Faraday depth accuracies compared to traditional centimetre-wavelength observations. However, the effect of Faraday depolarisationmakes the polarised signal very challenging to detect at metre wavelengths (MHz frequencies). In this work, Faraday tomography is used to characterise the Faraday rotation properties of polarised sources found in data from the LOFAR Two-Metre Sky Survey (LoTSS). Of the 76 extragalactic polarised sources analysed here, we find that all host a radio-loud AGN (Active Galactic Nucleus). The majority of the sources (~64%) are large FRII radio galaxies with a median projected linear size of 710 kpc and median radio luminosity at 144 MHz of 4 × 10 26 W Hz -1 (with ~13% of all sources having a linear size > 1 Mpc). In several cases, both hotspots are detected in polarisation at an angular resolution of ~20'. One such case allowed a study of intergalactic magnetic fields on scales of 3.4 Mpc. Other detected source types include an FRI radio galaxy and at least eight blazars. Most sources display simple Faraday spectra, but we highlight one blazar that displays a complex Faraday spectrum, with two close peaks in the Faraday dispersion function.Peer reviewe
The 15-43 GHz parsec-scale circular polarization of 41 active galactic nuclei
We present the results of parsec-scale circular-polarization measurements based on Very Long Baseline Array data for a number of radio-bright, core-dominated active galactic nuclei obtained simultaneously at 15, 22 and 43 GHz. The degrees of circular polarization m(c) for the Very Long Baseline Interferometry (VLBI) core region at 15 GHz are similar to the values reported earlier at this wavelength, with typical values of a few tenths of a per cent. We find that mc as often rises as falls with increasing frequency between 15 and 22 GHz, while the degree of circular polarization at 43 GHz is in all cases higher than at 22 and 15 GHz. This behaviour seems contrary to expectations, since the degree of circular polarization from both synchrotron radiation and the Faraday conversion of linear to circular polarization - the two main mechanisms considered thus far in the literature - should decrease towards higher frequencies if the source is homogeneous. The increase in mc at 43 GHz may be due to the presence of regions of both positive and negative circular polarization with different frequency dependences ( but decreasing with increasing frequency) on small scales within the core region; alternatively, it may be associated with the intrinsic inhomogeneity of a Blandford-Konigl like jet. In several objects, the detected circular polarization appears to be near, but not coincident with, the core, although further observations are needed to confirm this. We find several cases of changes in sign with frequency, most often between 22 and 43 GHz. We find tentative evidence for transverse structure in the circular polarization of 1055+018 and 1334-127, that is consistent with their being generated by either the synchrotron mechanism or the Faraday conversion in a helical magnetic field. Our results confirm the earlier finding that the sign of the circular polarization at a given observing frequency is generally consistent across epochs separated by several years or more, suggesting stability of the magnetic-field orientation in the innermost jets
LOFAR Deep Fields: Probing faint Galactic polarised emission in ELAIS-N1
We present the first deep polarimetric study of Galactic synchrotron emission
at low radio frequencies. Our study is based on 21 observations of the European
Large Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field using the
Low-Frequency Array (LOFAR) at frequencies from 114.9 to 177.4 MHz. These data
are a part of the LOFAR Two-metre Sky Survey Deep Fields Data Release 1. We
used very low-resolution () Stokes QU data cubes of this release. We
applied rotation measure (RM) synthesis to decompose the distribution of
polarised structures in Faraday depth, and cross-correlation RM synthesis to
align different observations in Faraday depth. We stacked images of about 150
hours of the ELAIS-N1 observations to produce the deepest Faraday cube at low
radio frequencies to date, tailored to studies of Galactic synchrotron emission
and the intervening magneto-ionic interstellar medium. This Faraday cube covers
of the sky and has a noise of in polarised intensity. This is an improvement in noise
by a factor of approximately the square root of the number of stacked data
cubes (), as expected, compared to the one in a single data cube
based on five-to-eight-hour observations. We detect a faint component of
diffuse polarised emission in the stacked cube, which was not detected
previously. Additionally, we verify the reliability of the ionospheric Faraday
rotation corrections estimated from the satellite-based total electron content
measurements to be of . We also demonstrate that
diffuse polarised emission itself can be used to account for the relative
ionospheric Faraday rotation corrections with respect to a reference
observation.Comment: 15 pages, 15 figures, accepted for publication in A&
Metal-THINGS: On the metallicity and ionization of ULX sources in NGC 925
We present an analysis of the optical properties of three Ultra Luminous
X-ray (ULX) sources identified in NGC 925. We use Integral field unit data from
the George Mitchel spectrograph in the context of the Metal-THINGS survey. The
optical properties for ULX-1 and ULX-3 are presented, while the spaxel
associated with ULX-2 had a low S/N, which prevented its analysis. We also
report the kinematics and dimensions of the optical nebula associated with each
ULX using ancillary data from the PUMA Fabry-Perot spectrograph. A BPT analysis
demonstrates that most spaxels in NGC 925 are dominated by star-forming
regions, including those associated with ULX-1 and ULX-3. Using the resolved
gas-phase metallicities, a negative metallicity gradient is found, consistent
with previous results for spiral galaxies, while the ionization parameter tends
to increase radially throughout the galaxy. Interestingly, ULX-1 shows a very
low gas metallicity for its galactocentric distance, identified by two
independent methods, while exhibiting a typical ionization. We find that such
low gas metallicity is best explained in the context of the high-mass X-ray
binary population, where the low-metallicity environment favours active Roche
lobe overflows that can drive much higher accretion rates. An alternative
scenario invoking accretion of a low-mass galaxy is not supported by the data
in this region. Finally, ULX-3 shows both a high metallicity and ionization
parameter, which is consistent with the progenitor being a highly-accreting
neutron star within an evolved stellar population region.Comment: Accepted by Ap
The extraordinary linear polarisation structure of the southern Centaurus A lobe revealed by ASKAP
We present observations of linear polarisation in the southern radio lobe of
Centaurus A, conducted during commissioning of the Australian Square Kilometre
Array Pathfinder (ASKAP) telescope. We used 16 antennas to observe a 30 square
degree region in a single 12 hour pointing over a 240 MHz band centred on 913
MHz. Our observations achieve an angular resolution of arcseconds
(480 parsecs), a maximum recoverable angular scale of 30 arcminutes, and a
full-band sensitivity of 85 \muupJy beam. The resulting maps of
polarisation and Faraday rotation are amongst the most detailed ever made for
radio lobes, with of order 10 resolution elements covering the source. We
describe several as-yet unreported observational features of the lobe,
including its detailed peak Faraday depth structure, and intricate networks of
depolarised filaments. These results demonstrate the exciting capabilities of
ASKAP for widefield radio polarimetry.Comment: 10 pages, 6 figures. Accepted in "The Power of Faraday Tomography"
special issue of Galaxie