1,843 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
Spitzer Observations of Centaurus A: Infrared Synchrotron Emission from the Northern Lobe
We present measurements obtained with the Spitzer Space Telescope in five
bands from 3.6-24 microns of the northern inner radio lobe of Centaurus A, the
nearest powerful radio galaxy. We show that this emission is synchrotron in
origin. Comparison with ultraviolet observations from GALEX shows that diffuse
ultraviolet emission exists in a smaller region than the infrared but also
coincides with the radio jet. We discuss the possibility, that synchrotron
emission is responsible for the ultraviolet emission and conclude that further
data are required to confirm this.Comment: 4 pages, accepted by ApJ
Deoxyuridine triphosphatase (dUTPase) expression and sensitivity to the thymidylate synthase (TS) inhibitorD9331
Uracil DNA misincorporation and misrepair of DNA have been recognized as important events accompanying thymidylate synthase (TS) inhibition. dUTPase catalyses the hydrolysis of dUTP to dUMP, thereby maintaining low intracellular dUTP. We have addressed the relationship between dUTPase expression and cellular sensitivity to TS inhibition in four human lung tumour cell lines. Sensitivity (5-day MTT assay) to the growth inhibitory effects of the non-polyglutamatable, specific quinazoline TS inhibitor ZD9331, varied up to 20-fold (IC 50 3–70 nM). TS protein expression correlated with TS activity (r2= 0.88 P= 0.05). Intracellular concentrations of drug following exposure to ZD9331 (1 μM, 24 h) varied by ~2-fold and dTTP pools decreased by > 80% in all cell lines. No clear associations across the cell lines between intracellular drug concentrations, TS activity/expression, or TTP depletion could be made. dUTPase activity varied 17-fold and correlated with dUTPase protein expression (r2= 0.94 P= 0.03). There was a striking variation in the amount of dUTP formed following exposure to ZD9331 (between 1.3 and 57 pmole 10–6cells) and was in general inversely associated with dUTPase activity. A large expansion in the dUTP pool was associated with increased sensitivity to a 24-h exposure to ZD9331 in A549 cells that have low dUTPase activity/expression. dUTPase expression and activity were elevated (approximately 3-fold) in two variants of a human lymphoblastoid cell line with acquired resistance to TS inhibitors, further suggesting an important role for this enzyme in TS inhibited cells. © 2000 Cancer Research Campaig
A LOFAR mini-survey for low-frequency radio emission from the nearest brown dwarfs
We have conducted a mini-survey for low-frequency radio emission from some of the closest brown dwarfs to the Sun with rapid rotation rates: SIMP J013656.5 +093347, WISEPC 150649.97+702736.0, and WISEPA J174124.26+255319.5.We have placed robust 3s upper limits on the flux density in the 111 – 169 MHz frequency range for these targets: WISE 1506: < 0:72 mJy; WISE 1741: < 0:87 mJy; SIMP 0136: < 0:66 mJy. At 8 hours of integration per target to achieve these limits, we find that systematic and detailed study of this class of object at LOFAR frequencies will require a substantial dedication of resources
Implications of pc and kpc jet asymmetry to the cosmic ray acceleration
We probe the role that the directional asymmetry, between relativistic
outflows and kilo-parsec scale jets, play in the acceleration of cosmic rays.
For this reason we use two powerful, nearby Active Galactic Nuclei (AGNs).
These radio galaxies are atypical compared to the usual AGN as they contain
ring-like features instead of hotspots. Our VLBI radio data have revealed a
substantial misalignment between their small and large scale jets. Taking into
account the overall information we have obtained about the AGNs themselves (VLA
and VLBI radio data at 18 cm) and their clusters (X-ray observations) our study
supports the present ideas of powerful radiogalaxies (radio quiet and radio
loud) being sources of cosmic rays as well as their ability to accelarate the
latter to ultra high energies.Comment: 4 pages, Conference HEPRO II
Remnant radio-loud AGN in the Herschel-ATLAS field
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 ⩽ α1400150 ⩽ −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 origins of X-ray emission from the hotspots of FRII radio sources
We use new and archival Chandra data to investigate the X-ray emission from a
large sample of compact hotspots of FRII radio galaxies and quasars from the 3C
catalogue. We find that only the most luminous hotspots tend to be in good
agreement with the predictions of a synchrotron self-Compton model with
equipartition magnetic fields. At low hotspot luminosities inverse-Compton
predictions are routinely exceeded by several orders of magnitude, but this is
never seen in more luminous hotspots. We argue that an additional synchrotron
component of the X-ray emission is present in low-luminosity hotspots, and that
the hotspot luminosity controls the ability of a given hotspot to produce
synchrotron X-rays, probably by determining the high-energy cutoff of the
electron energy spectrum. It remains plausible that all hotspots are close to
the equipartition condition.Comment: 49 pages, 16 figures. ApJ accepted. Revised version fixes a typo in
one of the Tables and corrects a statement about 3C27
MRC B0319-454: Probing the large-scale structure with a giant radio galaxy
We present an investigation of the relationships between the radio properties
of a giant radio galaxy MRC B0319-454 and the surrounding galaxy distribution
with the aim of examining the influence of intergalactic gas and gravity
associated with the large-scale structure on the evolution in the radio
morphology. Our new radio continuum observations of the radio source, with high
surface brightness sensitivity, images the asymmetries in the megaparsec-scale
radio structure in total intensity and polarization. We compare these with the
3-D galaxy distribution derived from galaxy redshift surveys. Galaxy density
gradients are observed along and perpendicular to the radio axis: the
large-scale structure is consistent with a model wherein the galaxies trace the
ambient intergalactic gas and the evolution of the radio structures are
ram-pressure limited by this associated gas. Additionally, we have modeled the
off-axis evolution of the south-west radio lobe as deflection of a buoyant jet
backflow by a transverse gravitational field: the model is plausible if
entrainment is small. The case study presented here is a demonstration that
giant radio galaxies may be useful probes of the warm-hot intergalactic medium
believed to be associated with moderately over dense galaxy distributions.Comment: 27 pages, 15 figures, accepted for publication in MNRA
The X-ray emission of the most luminous 3CR radio sources
We investigate the X-ray properties of the most luminous radio sources in the
3CR catalogue, in order to assess if they are similar to the most luminous
radio quiet quasars, for instance in the X-ray normalization with respect to
the optical luminosity, or in the distribution of the absorption column
density. We have selected the (optically identified) 3CR radio sources whose
178-MHz monochromatic luminosity lies in the highest factor-of-three bin. The 4
most luminous objects had already been observed in X rays. Of the remaining 16,
we observed with XMM-Newton 4 randomly chosen, optical type 1s, and 4 type 2s.
All targets have been detected. The optical-to-Xray spectral index, alphaox,
can be computed only for the type 1s and, in agreement with previous studies,
is found to be flatter than in radio quiet quasars of similar luminosity.
However, the Compton thin type 2s have an absorption corrected X-ray luminosity
systematically lower than the type 1s, by a factor which makes them consistent
with the radio quiet alphaox. Within the limited statistics, the Compton thick
objects seem to have a reflected component more luminous than the Compton thin
ones. The extra X-ray component observed in type 1 radio loud quasars is beamed
for intrinsic causes, and is not collimated by the absorbing torus as is the
case for the (intrinsically isotropic) disk emission. The extra component can
be associated with a relativistic outflow, provided that the flow opening angle
and the Doppler beaming factor are 1/5 - 1/7 radians.Comment: LaTex, 6 pages, 3 figures, 3 tables; accepted for publication in A&
PKS B1545-321: Bow shocks of a relativistic jet?
Sensitive, high resolution images of the double-double radio galaxy PKS
B1545-321 reveal detailed structure, which we interpret in the light of
previous work on the interaction of restarted jets with pre-existing relict
cocoons. We have also examined the spectral and polarization properties of the
source, the color distribution in the optical host and the environment of this
galaxy in order to understand its physical evolution. We propose that the
restarted jets generate narrow bow shocks and that the inner lobes are a
mixture of cocoon plasma reaccelerated at the bow shock and new jet material
reaccelerated at the termination shock. The dynamics of the restarted jets
implies that their hot spots advance at mildly relativistic speeds with
external Mach numbers of at least 5. The existence of supersonic hot spot Mach
numbers and bright inner lobes is the result of entrainment causing a reduction
in the sound speed of the pre-existing cocoon. The interruption to jet activity
in PKS B1545-321 has been brief - lasting less than a few percent of the
lifetime of the giant radio source. The host
galaxy is located at the boundary of a large scale filamentary structure, and
shows blue patches in color distribution indicative of a recent merger, which
may have triggered the Mpc-scale radio galaxy.Comment: 26 pages including 1 table and 16 figures. To appear in MNRA
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