1,230 research outputs found
Multifrequency study of a new Hybrid Morphology Radio Source
Hybrid Morphology Radio Sources (HyMoRS) are a class of radio galaxies having
the lobe morphology of a Fanaroff-Riley (FR) type I on one side of the active
nucleus and of a FR type II on the other. The origin of the different
morphologies between FR I and FR II sources has been widely discussed in the
past 40 years, and HyMoRS may be the best way to understand whether this
dichotomy is related to the intrinsic nature of the source and/or to its
environment. However, these sources are extremely rare (<1% of radio galaxies)
and only for a few of them a detailed radio study, that goes beyond the
morphological classification, has been conducted. In this paper we report the
discovery of one new HyMoRS; we present X-ray and multi-frequency radio
observations. We discuss the source morphological, spectral and polarisation
properties and confirm that HyMoRS are intrinsically bimodal with respect to
these observational characteristics. We notice that HyMoRS classification based
just on morphological properties of the source is hazardous.Comment: 8 pages, 9 figures, accepted MNRA
LLAGN and jet-scaling probed with the EVN
Accreting black holes on all mass scales (from stellar to supermassive)
appear to follow a nonlinear relation between X-ray luminosity, radio
luminosity and BH mass, indicating that similar physical processes drive the
central engines in X-ray binaries and active galactic nuclei (AGN). However, in
recent years an increasing number of BH systems have been identified that do
not fit into this scheme. These outliers may be the key to understand how BH
systems are powered by accretion. Here we present results from EVN observations
of a sample of low-luminosity AGN (LLAGN) with known mass that have unusually
high radio powers when compared with their X-ray luminosity.Comment: Presented at the 11th EVN Symposium, Bordeaux, France, 2012 October
9-12. Six pages, including a figure and a table. Final, accepted versio
Radio Observations of HD80606 Near Planetary Periastron: II. LOFAR Low Band Antenna Observations at 30-78 MHz
All the giant planets in the solar system generate radio emission via the
electron cyclotron maser instability, most notably giving rise to Jupiter's
decametric emissions. An interaction with the solar wind is at least partially
responsible for all of these solar system electron cyclotron masers. HD80606b
is a giant planet with a highly eccentric orbit, leading to predictions that
its radio emission may be enhanced substantially near periastron. This paper
reports observations with the Low Frequency Array (LOFAR) of HD80606b near its
periastron in an effort to detect radio emissions generated by an electron
cyclotron maser instability in the planet's magnetosphere. The reported
observations are at frequencies between 30 MHz and 78 MHz, and they are
distinguished from most previous radio observations of extrasolar planets by
two factors: (i) They are at frequencies near 50 MHz, much closer to the
frequencies at which Jupiter emits (< 40 MHz) and lower than most previously
reported observations of extrasolar planets; and (ii) Sensitivities of
approximately a few millijanskys have been achieved, an order of magnitude or
more below nearly all previous extrasolar planet observations below 100 MHz. We
do not detect any radio emissions from HD80606b and use these observations to
place new constraints on its radio luminosity. We also revisit whether the
observations were conducted at a time when it was super-Alfvenic relative to
the host star's stellar wind, which experience from the solar system
illustrates is a state in which an electron cyclotron maser emission can be
sustained in a planet's magnetic polar regions.Comment: 7 pages, 2 figures, accepted A&
Abell 1033: birth of a radio phoenix
Extended steep-spectrum radio emission in a galaxy cluster is usually
associated with a recent merger. However, given the complex scenario of galaxy
cluster mergers, many of the discovered sources hardly fit into the strict
boundaries of a precise taxonomy. This is especially true for radio phoenixes
that do not have very well defined observational criteria. Radio phoenixes are
aged radio galaxy lobes whose emission is reactivated by compression or other
mechanisms. Here, we present the detection of a radio phoenix close to the
moment of its formation. The source is located in Abell 1033, a peculiar galaxy
cluster which underwent a recent merger. To support our claim, we present
unpublished Westerbork Synthesis Radio Telescope and Chandra observations
together with archival data from the Very Large Array and the Sloan Digital Sky
Survey. We discover the presence of two sub-clusters displaced along the N-S
direction. The two sub-clusters probably underwent a recent merger which is the
cause of a moderately perturbed X-ray brightness distribution. A steep-spectrum
extended radio source very close to an AGN is proposed to be a newly born radio
phoenix: the AGN lobes have been displaced/compressed by shocks formed during
the merger event. This scenario explains the source location, morphology,
spectral index, and brightness. Finally, we show evidence of a density
discontinuity close to the radio phoenix and discuss the consequences of its
presence.Comment: accepted MNRA
Discovery of the supernova remnant G351.0-5.4
Context. While searching the NRAO VLA Sky Survey (NVSS) for diffuse radio
emission, we have serendipitously discovered extended radio emission close to
the Galactic plane. The radio morphology suggests the presence of a previously
unknown Galactic supernova remnant. An unclassified {\gamma}-ray source
detected by EGRET (3EG J1744-3934) is present in the same location and may stem
from the interaction between high-speed particles escaping the remnant and the
surrounding interstellar medium.
Aims. Our aim is to confirm the presence of a previously unknown supernova
remnant and to determine a possible association with the {\gamma}-ray emission
3EG J1744-3934.
Methods. We have conducted optical and radio follow-ups of the target using
the Dark Energy Camera (DECam) on the Blanco telescope at Cerro Tololo
Inter-American Observatory (CTIO) and the Giant Meterwave Radio Telescope
(GMRT). We then combined these data with archival radio and {\gamma}-ray
observations.
Results. While we detected the extended emission in four different radio
bands (325, 1400, 2417, and 4850 MHz), no optical counterpart has been
identified. Given its morphology and brightness, it is likely that the radio
emission is caused by an old supernova remnant no longer visible in the optical
band. Although an unclassified EGRET source is co-located with the supernova
remnant, Fermi-LAT data do not show a significant {\gamma}-ray excess that is
correlated with the radio emission. However, in the radial distribution of the
{\gamma}-ray events, a spatially extended feature is related with SNR at a
confidence level {\sigma}.
Conclusions. We classify the newly discovered extended emission in the radio
band as the old remnant of a previously unknown Galactic supernova: SNR
G351.0-5.4.Comment: 6 pages, 6 figures, accepted A&
The effect of the ionosphere on ultra-low-frequency radio-interferometric observations
Context. The ionosphere is the main driver of a series of systematic effects that limit our ability to explore the low-frequency (<1 GHz) sky with radio interferometers. Its effects become increasingly important towards lower frequencies and are particularly hard to calibrate in the low signal-to-noise ratio (S/N) regime in which low-frequency telescopes operate. Aims. In this paper we characterise and quantify the effect of ionospheric-induced systematic errors on astronomical interferometric radio observations at ultra-low frequencies (<100 MHz). We also provide guidelines for observations and data reduction at these frequencies with the LOw Frequency ARray (LOFAR) and future instruments such as the Square Kilometre Array (SKA). Methods. We derive the expected systematic error induced by the ionosphere. We compare our predictions with data from the Low Band Antenna (LBA) system of LOFAR. Results. We show that we can isolate the ionospheric effect in LOFAR LBA data and that our results are compatible with satellite measurements, providing an independent way to measure the ionospheric total electron content (TEC). We show how the ionosphere also corrupts the correlated amplitudes through scintillations. We report values of the ionospheric structure function in line with the literature. Conclusions. The systematic errors on the phases of LOFAR LBA data can be accurately modelled as a sum of four effects (clock, ionosphere first, second, and third order). This greatly reduces the number of required calibration parameters, and therefore enables new efficient calibration strategies
Radio Galaxy Zoo: Cosmological Alignment of Radio Sources
We study the mutual alignment of radio sources within two surveys, FIRST and
TGSS. This is done by producing two position angle catalogues containing the
preferential directions of respectively and extended
sources distributed over more than and square degrees. The
identification of the sources in the FIRST sample was performed in advance by
volunteers of the Radio Galaxy Zoo project, while for the TGSS sample it is the
result of an automated process presented here. After taking into account
systematic effects, marginal evidence of a local alignment on scales smaller
than is found in the FIRST sample. The probability of this happening
by chance is found to be less than per cent. Further study suggests that on
scales up to the alignment is maximal. For one third of the sources,
the Radio Galaxy Zoo volunteers identified an optical counterpart. Assuming a
flat CDM cosmology with , we
convert the maximum angular scale on which alignment is seen into a physical
scale in the range Mpc . This result supports recent
evidence reported by Taylor and Jagannathan of radio jet alignment in the
deg ELAIS N1 field observed with the Giant Metrewave Radio Telescope. The
TGSS sample is found to be too sparsely populated to manifest a similar signal
Searching for pulsars associated with the Fermi GeV excess
The Fermi Large Area Telescope has detected an extended region of GeV emission towards the Galactic Centre that is currently thought to be powered by dark matter annihilation or a population of young and/or millisecond pulsars. In a test of the pulsar hypothesis, we have carried out an initial search of a 20 deg2 area centred on the peak of the galactic centre GeV excess. Candidate pulsars were identified as a compact, steep spectrum continuum radio source on interferometric images and followed with targeted single-dish pulsation searches. We report the discovery of the recycled pulsar PSR 1751−2737 with a spin period of 2.23 ms. PSR 1751−2737 appears to be an isolated recycled pulsar located within the disc of our Galaxy, and it is not part of the putative bulge population of pulsars that are thought to be responsible for the excess GeV emission. However, our initial success in this small pilot survey suggests that this hybrid method (i.e. wide-field interferometric imaging followed up with single-dish pulsation searches) may be an efficient alternative strategy for testing whether a putative bulge population of pulsars is responsible for the GeV excess
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 cluster's 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.Comment: To appear in Space Science Reviews, 56 pages, 31 figures; For
associated data see http://galaxyclusters.co
The intracluster magnetic field in the double relic galaxy cluster Abell 2345
Magnetic fields are ubiquitous in galaxy clusters, yet their radial profile, power spectrum, and connection to host cluster
properties are poorly known. Merging galaxy clusters hosting diffuse polarized emission in the form of radio relics offer a
unique possibility to study the magnetic fields in these complex systems. In this paper, we investigate the intracluster magnetic
field in Abell 2345. This cluster hosts two radio relics that we detected in polarization with 1–2 GHz Jansky Very Large Array
observations. X-ray XMM–Newton images show a very disturbed morphology. We derived the rotation measure (RM) of five
polarized sources within ∼1 Mpc from the cluster centre applying the RM synthesis. Both, the average RM and the RM dispersion
radial profiles probe the presence of intracluster magnetic fields. Using the thermal electron density profile derived from X-ray
analysis and simulating a 3D magnetic field with fluctuations following a power spectrum derived from magneto-hydrodynamical
cosmological simulations, we build mock RM images of the cluster. We constrained the magnetic field profile in the eastern
radio relic sector by comparing simulated and observed RM images. We find that, within the framework of our model, the data
require a magnetic field scaling with thermal electron density as B(r) ∝ n e (r). The best model has a central magnetic field (within
a 200 kpc radius) of 2.8±0.1 μG. The average magnetic field at the position of the eastern relic is ∼ 0.3 μG, a factor 2.7 lower
than the equipartition estimate
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