548 research outputs found
One blind and three targeted searches for (sub)millisecond pulsars
We conducted one blind and three targeted searches for millisecond and
submillisecond pulsars. The blind search was conducted within 3deg of the
Galactic plane and at longitudes between 20 and 110deg. It takes 22073
pointings to cover this region, and 5487 different positions in the sky. The
first targeted search was aimed at Galactic globular clusters, the second one
at 24 bright polarized and pointlike radiosources with steep spectra, and the
third at 65 faint polarized and pointlike radiosources. The observations were
conducted at the large radiotelescope of Nancay Observatory, at a frequency
near 1400 MHz. Two successive backends were used, first a VLBI S2 system,
second a digital acquisition board and a PC with large storage capacity
sampling the signal at 50 Mb/s on one bit, over a 24-MHz band and in one
polarization. The bandwidth of acquisition of the second backend was later
increased to 48 MHz and the sampling rate to 100 Mb/s. The survey used the
three successive setups, with respective sensitivities of 3.5, 2.2, and 1.7
mJy. The targeted-search data were obtained with the third setup and reduced
with a method based on the Hough transform, yielding a sensitivity of 0.9 mJy.
The processing of the data was done in slightly differed time by
soft-correlation in all cases. No new short-period millisecond pulsars were
discovered in the different searches. To better understand the null result of
the blind survey, we estimate the probability of detecting one or more
short-period pulsars among a given Galactic population of synthetic pulsars
with our setup: 25% for the actual incomplete survey and 79% if we had
completed the whole survey with a uniform nominal sensitivity of 1.7 mJy. The
alternative of surveying a smaller, presumably more densely populated, region
with a higher sensitivity would have a low return and would be impractical at a
transit instrument. (abridged)Comment: accepted for publication in Astronomy & Astrophysic
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. II: anisotropy
We investigate the effects of stochasticity in the spatial and temporal
distribution of supernova remnants on the anisotropy of cosmic rays observed at
Earth. The calculations are carried out for different choices of the diffusion
coefficient D(E) for propagation in the Galaxy. The propagation and spallation
of nuclei are taken into account. At high energies we assume that
, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars with and without
accounting for the spiral structure of the Galaxy. Our calculations allow us to
determine the contribution to anisotropy resulting from both the large scale
distribution of SNRs in the Galaxy and the random distribution of the nearest
remnants. The naive expectation that the anisotropy amplitude scales as D(E) is
shown to be an oversimplification which does not reflect in the predicted
anisotropy for any realistic distribution of the sources. The fluctuations in
the anisotropy pattern are dominated by nearby sources, so that predicting or
explaining the observed anisotropy amplitude and phase becomes close to
impossible. We find however that the very weak energy dependence of the
anisotropy amplitude below GeV and the rise at higher energies, can
best be explained if the diffusion coefficient is . Faster
diffusion, for instance with , leads in general to an exceedingly
large anisotropy amplitude. The spiral structure introduces interesting trends
in the energy dependence of the anisotropy pattern, which qualitatively reflect
the trend seen in the data. For large values of the halo size we find that the
anisotropy becomes dominated by the large scale regular structure of the source
distribution, leading indeed to a monotonic increase of with energy.Comment: 21 Pages, to appear in JCA
Predictive computational phenotyping and biomarker discovery using reference-free genome comparisons
The antibiotic susceptibility data for the Clostridium difficile genomes. (XLSX 20.2 kb
Highly Ionized Collimated Outflow from HE 0238 - 1904
We present a detailed analysis of a highly ionized, multiphased and
collimated outflowing gas detected through O V, O VI, Ne VIII and Mg X
absorption associated with the QSO HE 0238 - 1904 (z_em ~ 0.629). Based on the
similarities in the absorption line profiles and estimated covering fractions,
we find that the O VI and Ne VIII absorption trace the same phase of the
absorbing gas. Simple photoionization models can reproduce the observed N(Ne
VIII), N(O VI) and N(Mg X) from a single phase whereas the low ionization
species (e.g. N III, N IV, O IV) originate from a different phase. The measured
N(Ne VIII)/N(O VI) ratio is found to be remarkably similar (within a factor of
~ 2) in several individual absorption components kinematically spread over ~
1800 km/s. Under photoionization this requires a fine tuning between hydrogen
density (nH) and the distance of the absorbing gas from the QSO. Alternatively
this can also be explained by collisional ionization in hot gas with T >
10^{5.7} K. Long-term stability favors the absorbing gas being located outside
the broad line region (BLR). We speculate that the collimated flow of such a
hot gas could possibly be triggered by the radio jet interaction.Comment: Minor revision (accepted for publication in MNRAS letter
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
A low escape fraction of ionizing photons of L>L* Lyman break galaxies at z=3.3
We present an upper limit for the relative escape fraction (f_{esc}^{rel}) of
ionizing radiation at z~3.3 using a sample of 11 Lyman Break Galaxies (LBGs)
with deep imaging in the U band obtained with the Large Binocular Camera,
mounted on the prime focus of the Large Binocular Telescope. We selected 11
LBGs with secure redshift in the range 3.27<z<3.35, from 3 independent fields.
We stacked the images of our sources in the R and U band, which correspond to
an effective rest-frame wavelength of 1500\AA and 900\AA respectively,
obtaining a limit in the U band image of >=30.7(AB)mag at 1 sigma. We derive a
1 sigma upper limit of f_{esc}^{rel}~5%, which is one of the lowest values
found in the literature so far at z~3.3. Assuming that the upper limit for the
escape fraction that we derived from our sample holds for all galaxies at this
redshift, the hydrogen ionization rate that we obtain (Gamma_{-12}<0.3 s^{-1})
is not enough to keep the IGM ionized and a substantial contribution to the UV
background by faint AGNs is required. Since our sample is clearly still limited
in size, larger z~3 LBG samples, at similar or even greater depths are
necessary to confirm these results on a more firm statistical basis.Comment: 15 pages, 2 figures, 1 table, accepted for publication in Ap
Lyman alpha emission from the first galaxies: Signatures of accretion and infall in the presence of line trapping
The formation of the first galaxies is accompanied by large accretion flows
and virialization shocks, during which the gas is shock-heated to temperatures
of K, leading to potentially strong fluxes in the Lyman alpha line.
Indeed, a number of Lyman alpha blobs has been detected at high redshift. In
this letter, we explore the origin of such Lyman alpha emission using
cosmological hydrodynamical simulations that include a detailed model of atomic
hydrogen as a multi-level atom and the effects of line trapping with the
adaptive mesh refinement code FLASH. We see that baryons fall into the center
of a halo through cold streams of gas, giving rise to a Lyman alpha luminosity
of at least at , similar to observed Lyman
alpha blobs. We find that a Lyman alpha flux of emerges from the envelope of the halo rather than its center,
where the photons are efficiently trapped. Such emission can be probed in
detail with the upcoming James Webb Space Telescope (JWST) and will constitute
an important probe of gas infall and accretion.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS LETTER
He II Ly{\beta} Gunn-Peterson Absorption: New HST Observations, and Theoretical Expectations
Observations of He II Ly{\alpha} Gunn-Peterson troughs have proved to be a
valuable probe of the epoch of helium reionization at z~3. Since this optical
depth can become unmeasurably large even for modest He II fractions, various
alternate techniques have been proposed to push to higher redshift, and among
the more promising is looking at higher order Lyman-series troughs. We here
report four new observations of the He II Ly{\beta} trough, including new data
on the only sightline with a prior Ly{\beta} observation. However, the
effective optical depth ratio {\tau}_eff,{\beta}/{\tau}_eff,{\alpha} is not
simply predicted by (f_{\beta} {\lambda}_{\beta})/(f_{\alpha}
{\lambda}_{\alpha})=0.16, and we analyze cosmological simulations to find that
the correct ratio for helium at z~3 is ~0.35. In one case we infer
{\tau}_eff,{\alpha} > 8.8, strong evidence that helium was not fully reionized
at z=3.2--3.5, in agreement with previous measurements suggesting a later
completion of reionization.Comment: ApJ accepted. 25 pages, 3 figures, 4 table
Search for Pairs of Isolated Radio Pulsars - Components in Disrupted Binary Systems
We have developed a method for analyzing the kinematic association of
isolated relativistic objects - possible remnants of disrupted close binary
systems. We investigate pairs of fairly young radio pulsars with known proper
motions and estimated distances (dispersion measures) that are spaced no more
than 2-3 kpc apart. Using a specified radial velocity distribution for these
objects, we have constructed 100-300 thousand trajectories of their possible
motion in the Galactic gravitational field on a time scale of several million
years. The probabilities of their close encounters at epochs consistent with
the age of the younger pulsar in the pair are analyzed. When these
probabilities exceed considerably their reference values obtained by assuming a
purely random encounter between the pulsars under consideration, we conclude
that the objects may have been gravitationally bound in the past. As a result,
we have detected six pulsar pairs (J0543+2329/J0528+2200,
J1453-6413/J1430-6623, J2354+6155/J2321+6024, J1915+1009/J1909+1102,
J1832-0827/J1836-1008, and J1917+1353/J1926+1648) that are companions in
disrupted binary systems with a high probability. Estimates of their kinematic
ages and velocities at binary disruption and at the present epoch are provided
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