1,139 research outputs found
Caracterização da expressão de genes MADS-box associados à dormência em gemas de macieira contrastantes em requerimento de frio.
Resumo
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
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
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
On the absence of radio halos in clusters with double relics
Pairs of radio relics are believed to form during cluster mergers, and are
best observed when the merger occurs in the plane of the sky. Mergers can also
produce radio halos, through complex processes likely linked to turbulent
re-acceleration of cosmic-ray electrons. However, only some clusters with
double relics also show a radio halo. Here, we present a novel method to derive
upper limits on the radio halo emission, and analyse archival X-ray Chandra
data, as well as galaxy velocity dispersions and lensing data, in order to
understand the key parameter that switches on radio halo emission. We place
upper limits on the halo power below the
correlation for some clusters, confirming that clusters with double relics have
different radio properties. Computing X-ray morphological indicators, we find
that clusters with double relics are associated with the most disturbed
clusters. We also investigate the role of different mass-ratios and
time-since-merger. Data do not indicate that the merger mass ratio has an
impact on the presence or absence of radio halos (the null hypothesis that the
clusters belong to the same group cannot be rejected). However, the data
suggests that the absence of radio halos could be associated with early and
late mergers, but the sample is too small to perform a statistical test. Our
study is limited by the small number of clusters with double relics. Future
surveys with LOFAR, ASKAP, MeerKat and SKA will provide larger samples to
better address this issue.Comment: 12 pages, 7 figures, MNRAS accepte
Radio Galaxy Zoo: The Distortion of Radio Galaxies by Galaxy Clusters
We study the impact of cluster environment on the morphology of a sample of
4304 extended radio galaxies from Radio Galaxy Zoo. A total of 87% of the
sample lies within a projected 15 Mpc of an optically identified cluster.
Brightest cluster galaxies (BCGs) are more likely than other cluster members to
be radio sources, and are also moderately bent. The surface density as a
function of separation from cluster center of non-BCG radio galaxies follows a
power law with index out to (Mpc), which
is steeper than the corresponding distribution for optically selected galaxies.
Non-BCG radio galaxies are statistically more bent the closer they are to the
cluster center. Within the inner (Mpc) of a cluster,
non-BCG radio galaxies are statistically more bent in high-mass clusters than
in low-mass clusters. Together, we find that non-BCG sources are statistically
more bent in environments that exert greater ram pressure. We use the
orientation of bent radio galaxies as an indicator of galaxy orbits and find
that they are preferentially in radial orbits. Away from clusters, there is a
large population of bent radio galaxies, limiting their use as cluster
locators; however, they are still located within statistically overdense
regions. We investigate the asymmetry in the tail length of sources that have
their tails aligned along the radius vector from the cluster center, and find
that the length of the inward-pointing tail is weakly suppressed for sources
close to the center of the cluster.Comment: 23 pages, 17 figures, 2 tables. Supplemental data files available in
The Astronomical Journal or contact autho
A powerful double radio relic system discovered in PSZ1 G108.18-11.53: Evidence for a shock with non-uniform Mach number?
Diffuse radio emission in the form of radio haloes and relics has been found in a number of merging galaxy clusters. These structures indicate that shock and turbulence associated with the merger accelerate electrons to relativistic energies. We report the discovery of a radio relic + radio halo system in PSZ1 G108.18-11.53 (z = 0.335). This cluster hosts the second most powerful double radio relic system ever discovered. We observed PSZ1 G108.18-11.53 with the Giant Meterwave Radio Telescope and the Westerbork Synthesis Radio Telescope. We obtained radio maps at 147, 323, 607 and 1380 MHz. We also observed the cluster with the Keck telescope, obtaining the spectroscopic redshift for 42 cluster members. From the injection index, we obtained the Mach number of the shocks generating the two radio relics. For the southern shock, we found M = 2.33-0.26+0.19, while the northern shock Mach number goes fromM= 2.20-0.14+0.07 in the north part down toM= 2.00-0.08+0.03 in the southern region. If the relation between the injection index and the Mach number predicted by diffusive shock acceleration theory holds, this is the first observational evidence for a gradient in the Mach number along a galaxy cluster merger shock
Tempo de resposta para brotação de gemas de macieira submetidas a diferentes ciclos diários de temperatura durante o período de dormência.
Particle re-acceleration and Faraday-complex structures in the RXC\ua0J1314.4-2515 galaxy cluster
Radio relics are sites of electron (re)acceleration inmerging galaxy clusters but the mechanism of acceleration and the topology of the magnetic field in and near relics are yet to be understood. We are carrying out an observational campaign on double relic galaxy clusters starting with RXC J1314.4-2515. With Jansky Very Large Array multiconfiguration observations in the frequency range 1-4 GHz, we perform both spectral and polarization analyses, using the rotation measure (RM) synthesis technique. We use archival XMM-Newton observations to constrain the properties of the shocked region. We discover a possible connection between the activity of a radio galaxy and the emission of the eastern radio relic. In the northern elongated arc of the western radio relic, we detect polarized emission with an average polarization fraction of 31 % at 3 GHz and we derive the Mach number of the underlying X-ray shock. Our observations reveal low levels of fractional polarization and Faraday-complex structures in the southern region of the relic, which point to the presence of thermal gas and filamentary magnetic field morphology inside the radio emitting volume. We measured largely different RM dispersion from the two relics. Finally, we use cosmological magnetohydrodynamical simulations to constrain the magnetic field, viewing angle, and to derive the acceleration efficiency of the shock. We find that the polarization properties of RXC J1314.4-2515 are consistent with a radio relic observed at 70 degrees with respect to the line of sight and that the efficient re-acceleration of fossil electrons has taken place
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