118 research outputs found
Deriving global structure of the Galactic Magnetic Field from Faraday Rotation Measures of extragalactic sources
We made use of the two latest sets of Rotational Measures (RMs) of
extra-galactic radio sources, namely the NRAO VLA Sky Survey otation Measures
Catalogue, and a compilation by Kronberg&Newton-McGee(2011), to infer the
global structure of the Galactic Magnetic Field (GMF). We have checked that
these two data sets are consistent with each other. Motivated by clear patterns
in the observed distribution of RMs over the sky, we considered GMF models
consisting of the two components: disk (spiral or ring) and halo. The
parameters of these components were determined by fitting different model field
geometries to the observed RMs. We found that the model consisting of a
symmetric (with respect to the Galactic plane) spiral disk and anti-symmetric
halo fits the data best, and reproduces the observed distribution of RMs over
the sky very well. We confirm that ring disk models are disfavored. Our results
favor small pitch angles around -5 degrees and an increased vertical scale of
electron distribution, in agreement with some previous studies. Based on our
fits, we identify two benchmark models suitable for studies of cosmic ray
propagation, including the ultra-high energies.Comment: 15 pages, 14 figures, 4 tables misprints corrected, presentation
improved generally matches the published versio
Large oxygen-isotope effect in Sr_{0.4}K_{0.6}BiO_{3}: Evidence for phonon-mediated superconductivity
Oxygen-isotope effect has been investigated in a recently discovered
superconductor Sr_{0.4}K_{0.6}BiO_{3}. This compound has a distorted perovskite
structure and becomes superconducting at about 12 K. Upon replacing ^{16}O with
^{18}O by 60-80%, the T_c of the sample is shifted down by 0.32-0.50 K,
corresponding to an isotope exponent of alpha_{O} = 0.40(5). This isotope
exponent is very close to that for a similar bismuthate superconductor
Ba_{1-x}K_{x}BiO_{3} with T_c = 30 K. The very distinctive doping and T_c
dependencies of alpha_{O} observed in bismuthates and cuprates suggest that
bismuthates should belong to conventional phonon-mediated superconductors while
cuprates might be unconventional supercondutors.Comment: 9 pages, 5 figure
New Constraints on the Galactic Halo Magnetic Field using Rotation Measures of Extragalactic Sources Towards the Outer Galaxy
We present a study of the Milky Way disk and halo magnetic field, determined
from observations of Faraday rotation measure (RM) towards 641 polarized
extragalactic radio sources in the Galactic longitude range 100-117 degs,
within 30 degs of the Galactic plane. For |b| < 15 degs, we observe a symmetric
RM distribution about the Galactic plane. This is consistent with a disk field
in the Perseus arm of even parity across the Galactic mid-plane. In the range
15<|b|<30 degs, we find median rotation measures of -15+/-4 rad/m^2 and -62+/-5
rad/m^2 in the northern and southern Galactic hemispheres, respectively. If the
RM distribution is a signature of the large-scale field parallel to the
Galactic plane, this suggests that the halo magnetic field toward the outer
Galaxy does not reverse direction across the mid-plane. The variation of RM as
a function of Galactic latitude in this longitude range is such that RMs become
more negative at larger |b|. This is consistent with an azimuthal magnetic
field of strength 2 microGauss (7 microGauss) at a height 0.8-2 kpc above
(below) the Galactic plane between the local and the Perseus spiral arm. We
propose that the Milky Way could possess spiral-like halo magnetic fields
similar to those observed in M51.Comment: 23 pages, 9 figures, 2 tables. Accepted for publication in ApJ.
Electronic version of Table 1 is available via email from the first autho
Search for Anisotropy of Ultra-High Energy Cosmic Rays with the Telescope Array Experiment
We study the anisotropy of Ultra-High Energy Cosmic Ray (UHECR) events
collected by the Telescope Array (TA) detector in the first 40 months of
operation. Following earlier studies, we examine event sets with energy
thresholds of 10 EeV, 40 EeV, and 57 EeV. We find that the distributions of the
events in right ascension and declination are compatible with an isotropic
distribution in all three sets. We then compare with previously reported
clustering of the UHECR events at small angular scales. No significant
clustering is found in the TA data. We then check the events with E>57 EeV for
correlations with nearby active galactic nuclei. No significant correlation is
found. Finally, we examine all three sets for correlations with the large-scale
structure of the Universe. We find that the two higher-energy sets are
compatible with both an isotropic distribution and the hypothesis that UHECR
sources follow the matter distribution of the Universe (the LSS hypothesis),
while the event set with E>10 EeV is compatible with isotropy and is not
compatible with the LSS hypothesis at 95% CL unless large deflection angles are
also assumed. We show that accounting for UHECR deflections in a realistic
model of the Galactic magnetic field can make this set compatible with the LSS
hypothesis.Comment: 10 pages, 9 figure
Radio precursors to neutron star binary mergings
We discuss a possible generation of radio bursts preceding final stages of
binary neutron star mergings which can be accompanied by short gamma-ray
bursts. Detection of such bursts appear to be advantageous in the low-frequency
radio band due to a time delay of ten to several hundred seconds required for
radio signal to propagate in the ionized intergalactic medium. This delay makes
it possible to use short gamma-ray burst alerts to promptly monitor specific
regions on the sky by low-frequency radio facilities, especially by LOFAR. To
estimate the strength of the radio signal, we assume a power-law dependence of
the radio luminosity on the total energy release in a magnetically dominated
outflow, as found in millisecond pulsars. Based on the planned LOFAR
sensitivity at 120 MHz, we estimate that the LOFAR detection rate of such radio
transients could be about several events per month from redshifts up to
in the most optimistic scenario. The LOFAR ability to detect such
events would crucially depend on exact efficiency of low-frequency radio
emission mechanism.Comment: 6 pages, 2 figures, Accepted for publication in Astrophysics & Space
Science. Largely extended version of ArXiv:0912.521
Complementarity of Galactic radio and collider data in constraining WIMP dark matter models
In this work we confront dark matter models to constraints that may be
derived from radio synchrotron radiation from the Galaxy, taking into account
the astrophysical uncertainties and we compare these to bounds set by
accelerator and complementary indirect dark matter searches. Specifically we
apply our analysis to three popular particle physics models. First, a generic
effective operator approach, in which case we set bounds on the corresponding
mass scale, and then, two specific UV completions, the Z' and Higgs portals. We
show that for many candidates, the radio synchrotron limits are competitive
with the other searches, and could even give the strongest constraints (as of
today) with some reasonable assumptions regarding the astrophysical
uncertainties.Comment: 22 pages, 12 figure
Limits on the speed of gravitational waves from pulsar timing
In this work, analyzing the propagation of electromagnetic waves in the field
of gravitational waves, we show the presence and significance of the so called
surfing effect for pulsar timing measurements. It is shown that, due to the
transverse nature of gravitational waves, the surfing effect leads to enormous
pulsar timing residuals if the speed of gravitational waves is smaller than
speed of light. This fact allows to place significant constraints on parameter
, which characterizes the relative deviation of the speed of
gravitational waves from the speed of light. We show that the existing
constraints from pulsar timing measurements already place stringent limits on
and consequently on the mass of graviton . These limits on
are three orders of magnitude stronger than the current constraints from
Solar System tests. The current constraints also allow to rule out massive
gravitons as possible candidates for cold dark matter in galactic halo. In the
near future, the gravitational wave background from extragalactic super massive
black hole binaries, along with the expected sub-microsecond pulsar timing
accuracy, will allow to achieve constrains of and
possibly stronger
Radio Remnants of Compact Binary Mergers - the Electromagnetic Signal that will follow the Gravitational Waves
The question "what is the observable electromagnetic (EM) signature of a
compact binary merger?" is an intriguing one with crucial consequences to the
quest for gravitational waves (GW). Compact binary mergers are prime sources of
GW, targeted by current and next generation detectors. Numerical simulations
have demonstrated that these mergers eject energetic sub-relativistic (or even
relativistic) outflows. This is certainly the case if the mergers produce short
GRBs, but even if not, significant outflows are expected. The interaction of
such outflows with the surround matter inevitably leads to a long lasting radio
signal. We calculate the expected signal from these outflows (our calculations
are also applicable to short GRB orphan afterglows) and we discuss their
detectability. We show that the optimal search for such signal should,
conveniently, take place around 1.4 GHz. Realistic estimates of the outflow
parameters yield signals of a few hundred Jy, lasting a few weeks, from
sources at the detection horizon of advanced GW detectors. Followup radio
observations, triggered by GW detection, could reveal the radio remnant even
under unfavorable conditions. Upcoming all sky surveys can detect a few dozen,
and possibly even thousands, merger remnants at any give time, thereby
providing robust merger rate estimates even before the advanced GW detectors
become operational. In fact, the radio transient RT 19870422 fits well the
overall properties predicted by our model and we suggest that its most probable
origin is a compact binary merger radio remnant
CORRELATIONS OF THE ARRIVAL DIRECTIONS OF ULTRA-HIGH ENERGY COSMIC RAYS WITH EXTRAGALACTIC OBJECTS AS OBSERVED BY THE TELESCOPE ARRAY EXPERIMENT
We search for correlations between the positions of extragalactic objects and the arrival directions of ultra-high energy cosmic rays (UHECRs) with primary energy E ??? 40 EeV as observed by the surface detector array of the Telescope Array (TA) experiment during the first 40 months of operation. We examine several public astronomical object catalogs, including the Veron-Cetty and Veron catalog of active galactic nuclei. We count the number of TA events correlated with objects in each catalog as a function of three parameters: the maximum angular separation between a TA event and an object, the minimum energy of the events, and the maximum redshift of the objects. We determine the combination of these parameters that maximizes the correlations, and we calculate the probability of having the same levels of correlations from an isotropic distribution of UHECR arrival directions. No statistically significant correlations are found when penalties for scanning over the above parameters and for searching in several catalogs are taken into account.open4
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