2,324 research outputs found
Modeling the non-recycled Fermi gamma-ray pulsar population
We use Fermi Gamma-ray Space Telescope detections and upper limits on
non-recycled pulsars obtained from the Large Area Telescope (LAT) to constrain
how the gamma-ray luminosity L depends on the period P and the period
derivative \dot{P}. We use a Bayesian analysis to calculate a best-fit
luminosity law, or dependence of L on P and \dot{P}, including different
methods for modeling the beaming factor. An outer gap (OG) magnetosphere
geometry provides the best-fit model, which is L \propto P^{-a} \dot{P}^{b}
where a=1.36\pm0.03 and b=0.44\pm0.02, similar to but not identical to the
commonly assumed L \propto \sqrt{\dot{E}} \propto P^{-1.5} \dot{P}^{0.5}. Given
upper limits on gamma-ray fluxes of currently known radio pulsars and using the
OG model, we find that about 92% of the radio-detected pulsars have gamma-ray
beams that intersect our line of sight. By modeling the misalignment of radio
and gamma-ray beams of these pulsars, we find an average gamma-ray beaming
solid angle of about 3.7{\pi} for the OG model, assuming a uniform beam. Using
LAT-measured diffuse fluxes, we place a 2{\sigma} upper limit on the average
braking index and a 2{\sigma} lower limit on the average surface magnetic field
strength of the pulsar population of 3.8 and 3.2 X 10^{10} G, respectively. We
then predict the number of non-recycled pulsars detectable by the LAT based on
our population model. Using the two-year sensitivity, we find that the LAT is
capable of detecting emission from about 380 non-recycled pulsars, including
150 currently identified radio pulsars. Using the expected five-year
sensitivity, about 620 non-recycled pulsars are detectable, including about 220
currently identified radio pulsars. We note that these predictions
significantly depend on our model assumptions.Comment: 26 pages, 10 figures, Accepted by ApJ on 8 September 201
Upper Limits On Periodic, Pulsed Radio Emission from the X-Ray Point Source in Cassiopeia A
The Chandra X-ray Observatory recently discovered an X-ray point source near
the center of Cassiopeia A, the youngest known Galactic supernova remnant. We
have conducted a sensitive search for radio pulsations from this source with
the Very Large Array, taking advantage of the high angular resolution of the
array to resolve out the emission from the remnant itself. No convincing
signatures of a dispersed, periodic source or of isolated dispersed pulses were
found, whether for an isolated or a binary source. We derive upper limits of 30
and 1.3 mJy at 327 and 1435 MHz for the phase-averaged pulsed flux density from
this source. The corresponding luminosity limits are lower than those for any
pulsar with age less than 10^4 years. The sensitivities of our search to single
pulses were 25 and 1.0 Jy at 327 and 1435 MHz. For comparison, the Crab pulsar
emits roughly 80 pulses per minute with flux densities greater than 100 Jy at
327 MHz and 8 pulses per minute with flux densities greater than 50 Jy at 1435
MHz. These limits are consistent with the suggestion that the X-ray point
source in Cas A adds to the growing number of neutron stars which are not radio
pulsars.Comment: accepted by ApJ Letter
PSR J1829+2456: a relativistic binary pulsar
We report the discovery of a new binary pulsar, PSR J1829+2456, found during
a mid-latitude drift-scan survey with the Arecibo telescope. Our initial timing
observations show the 41-ms pulsar to be in a 28-hr, slightly eccentric, binary
orbit. The advance of periastron, omegadot = 0.28 +/- 0.01 deg/yr is derived
from our timing observations spanning 200 days. Assuming that the advance of
periastron is purely relativistic and a reasonable range of neutron star masses
for PSR J1829+2456 we constrain the companion mass to be between 1.22 Msun and
1.38 Msun, making it likely to be another neutron star. We also place a firm
upper limit on the pulsar mass of 1.38 Msun. The expected coalescence time due
to gravitational-wave emission is long (~60 Gyr) and this system will not
significantly impact upon calculations of merger rates that are relevant to
upcoming instruments such as LIGO.Comment: Accepted MNRAS, 5 pages, 3 figure
GaBoDS: The Garching-Bonn Deep Survey - III. Lyman-Break Galaxies in the Chandra Deep Field South
We present first results of our search for high-redshift galaxies in deep CCD
mosaic images. As a pilot study for a larger survey, very deep images of the
Chandra Deep Field South (CDFS), taken withWFI@MPG/ESO2.2m, are used to select
large samples of 1070 U-band and 565 B-band dropouts with the Lyman-break
method. The data of these Lyman-break galaxies are made public as an electronic
table. These objects are good candidates for galaxies at z~3 and z~4 which is
supported by their photometric redshifts. The distributions of apparent
magnitudes and the clustering properties of the two populations are analysed,
and they show good agreement to earlier studies. We see no evolution in the
comoving clustering scale length from z~3 to z~4. The techniques presented here
will be applied to a much larger sample of U-dropouts from the whole survey in
near future.Comment: 11 pages, 11 figures, replaced with version accepted by A&A. Minor
changes and tabular appendix with LBG catalogues. Version with full
resolution figures available at
http://www.astro.uni-bonn.de/~hendrik/2544.pd
A VLA Search for the Geminga Pulsar: A Bayesian Limit on a Scintillating Source
We derive an upper limit of 3 mJy (95% confidence) for the flux density at
317 MHz of the Geminga pulsar (J0633+1746). Our results are based on 7 hours of
fast-sampled VLA data, which we averaged synchronously with the pulse period
using a period model based on CGRO/EGRET gamma-ray data. Our limit accounts for
the fact that this pulsar is most likely subject to interstellar scintillations
on a timescale much shorter than our observing span. Our Bayesian method is
quite general and can be applied to calculate the fluxes of other scintillated
sources. We also present a Bayesian technique for calculating the flux in a
pulsed signal of unknown width and phase.
Comparing our upper limit of 3 mJy with the quoted flux density of Geminga at
102 MHz, we calculate a lower limit to its spectral index of 2.7. We discuss
some possible reasons for Geminga's weakness at radio wavelengths, and the
likelihood that many of the unidentified EGRET sources are also radio-quiet or
radio-weak Geminga-like pulsars.Comment: 27 pages, including figures. Published in Ap
Pulsar Parallaxes at 5 GHz with the Very Long Baseline Array
We present the first pulsar parallaxes measured with phase-referenced pulsar
VLBI observations at 5 GHz. Due to the steep spectra of pulsars, previous
astrometric measurements have been at lower frequencies. However, the strongest
pulsars can be observed at 5 GHz, offering the benefit of lower combined
ionospheric and tropospheric phase errors, which usually limit VLBI astrometric
accuracy. The pulsars B0329+54, B0355+54 and B1929+10 were observed for 7
epochs spread evenly over 2 years. For B0329+54, large systematic errors lead
to only an upper limit on the parallax (pi < 1.5 mas). A new proper motion and
parallax were measured for B0355+54 (pi = 0.91 +- 0.16 mas), implying a
distance of 1.04+0.21-0.16 kpc and a transverse velocity of 61+12-9 km/s. The
parallax and proper motion for B1929+10 were significantly improved (pi = 2.77
+- 0.07 mas), yielding a distance of 361+10-8 pc and a transverse velocity of
177+4-5 km/s. We demonstrate that the astrometric errors are correlated with
the angular separation between the phase reference calibrator and the target
source, with significantly lower errors at 5 GHz compared to 1.6 GHz. Finally,
based on our new distance determinations for B1929+10 and B0355+54, we derive
or constrain the luminosities of each pulsar at high energies. We show that,
for thermal emission models, the emitting area for X-rays from PSR B1929+10 is
roughly consistent with the canonical size for a heated polar cap, and that the
conversion of spin-down power to gamma-ray luminosity in B0355+54 must be low.
The new proper motion for B1929+10 also implies that its progenitor is unlikely
to have been the binary companion of the runaway O-star zeta-Ophiuchi.Comment: 8 pages, including 3 figures and 3 tables; emulateapj; ApJ submitte
Microarcsecond VLBI pulsar astrometry with PSR II. parallax distances for 57 pulsars
We present the results of PSR, a large astrometric project targeting
radio pulsars using the Very Long Baseline Array (VLBA). From our astrometric
database of 60 pulsars, we have obtained parallax-based distance measurements
for all but 3, with a parallax precision of typically 40 as and
approaching 10 as in the best cases. Our full sample doubles the number of
radio pulsars with a reliable (5) model-independent distance
constraint. Importantly, many of the newly measured pulsars are well outside
the solar neighbourhood, and so PSR brings a near-tenfold increase in the
number of pulsars with a reliable model-independent distance at kpc.
Using our sample along with previously published results, we show that even the
most recent models of the Galactic electron density distribution model contain
significant shortcomings, particularly at high Galactic latitudes. When
comparing our results to pulsar timing, two of the four millisecond pulsars in
our sample exhibit significant discrepancies in the estimates of proper motion
obtained by at least one pulsar timing array. With additional VLBI observations
to improve the absolute positional accuracy of our reference sources and an
expansion of the number of millisecond pulsars, we will be able to extend the
comparison of proper motion discrepancies to a larger sample of pulsar
reference positions, which will provide a much more sensitive test of the
applicability of the solar system ephemerides used for pulsar timing. Finally,
we use our large sample to estimate the typical accuracy attainable for
differential astrometry with the VLBA when observing pulsars, showing that for
sufficiently bright targets observed 8 times over 18 months, a parallax
uncertainty of 4 as per arcminute of separation between the pulsar and
calibrator can be expected.Comment: updated to version accepted by ApJ: 30 pages, 20 figures, 9 table
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Oncogenic R132 IDH1 Mutations Limit NADPH for De Novo Lipogenesis through (D)2-Hydroxyglutarate Production in Fibrosarcoma Sells.
Neomorphic mutations in NADP-dependent isocitrate dehydrogenases (IDH1 and IDH2) contribute to tumorigenesis in several cancers. Although significant research has focused on the hypermethylation phenotypes associated with (D)2-hydroxyglutarate (D2HG) accumulation, the metabolic consequences of these mutations may also provide therapeutic opportunities. Here we apply flux-based approaches to genetically engineered cell lines with an endogenous IDH1 mutation to examine the metabolic impacts of increased D2HG production and altered IDH flux as a function of IDH1 mutation or expression. D2HG synthesis in IDH1-mutant cells consumes NADPH at rates similar to de novo lipogenesis. IDH1-mutant cells exhibit increased dependence on exogenous lipid sources for in vitro growth, as removal of medium lipids slows growth more dramatically in IDH1-mutant cells compared with those expressing wild-type or enzymatically inactive alleles. NADPH regeneration may be limiting for lipogenesis and potentially redox homeostasis in IDH1-mutant cells, highlighting critical links between cellular biosynthesis and redox metabolism
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