14,011 research outputs found
Probing millisecond pulsar emission geometry using light curves from the Fermi Large Area Telescope
An interesting new high-energy pulsar sub-population is emerging following
early discoveries of gamma-ray millisecond pulsars (MSPs) by the Fermi Large
Area Telescope (LAT). We present results from 3D emission modeling, including
the Special Relativistic effects of aberration and time-of-flight delays and
also rotational sweepback of B-field lines, in the geometric context of polar
cap (PC), outer gap (OG), and two-pole caustic (TPC) pulsar models. In contrast
to the general belief that these very old, rapidly-rotating neutron stars (NSs)
should have largely pair-starved magnetospheres due to the absence of
significant pair production, we find that most of the light curves are best fit
by TPC and OG models, which indicates the presence of narrow accelerating gaps
limited by robust pair production -- even in these pulsars with very low
spin-down luminosities. The gamma-ray pulse shapes and relative phase lags with
respect to the radio pulses point to high-altitude emission being dominant for
all geometries. We also find exclusive differentiation of the current gamma-ray
MSP population into two MSP sub-classes: light curve shapes and lags across
wavebands impose either pair-starved PC (PSPC) or TPC / OG-type geometries. In
the first case, the radio pulse has a small lag with respect to the single
gamma-ray pulse, while the (first) gamma-ray peak usually trails the radio by a
large phase offset in the latter case. Finally, we find that the flux
correction factor as a function of magnetic inclination and observer angles is
typically of order unity for all models. Our calculation of light curves and
flux correction factor for the case of MSPs is therefore complementary to the
"ATLAS paper" of Watters et al. for younger pulsars.Comment: 51 pages, 23 figures, 3 tables; low-resolution figures; accepted for
publication by Ap
Pair production rates in mildly relativistic, magnetized plasmas
Electron-positron pairs may be produced by either one or two photons in the presence of a strong magnetic field. In magnetized plasmas with temperatures kT approximately sq mc, both of these processes may be important and could be competitive. The rates of one-photon and two-photon pair production by photons with Maxwellian, thermal bremsstrahlung, thermal synchrotron and power law spectra are calculated as a function of temperature or power law index and field strength. This allows a comparison of the two rates and a determination of the conditions under which each process may be a significant source of pairs in astrophysical plasmas. It is found that for photon densities n(gamma) or = 10 to the 25th power/cu cm and magnetic field strengths B or = 10 to the 12th power G, one-photon pair production dominates at kT approximately sq mc for a Maxwellian, at kT approximately 2 sq mc for a thermal bremsstrahlung spectrum, at all temperatures for a thermal synchrotron spectrum, and for power law spectra with indices s approximately 4
Population Synthesis of Normal Radio and Gamma-ray Pulsars Using Markov Chain Monte Carlo Techniques
We present preliminary results of a pulsar population synthesis of normal
pulsars from the Galactic disk using a Markov Chain Monte Carlo method to
better understand the parameter space of the assumed model. We use the Kuiper
test, similar to the Kolmogorov-Smirnov test, to compare the cumulative
distributions of chosen observables of detected radio pulsars with those
simulated for various parameters. Our code simulates pulsars at birth using
Monte Carlo techniques and evolves them to the present assuming initial
spatial, kick velocity, magnetic field, and period distributions. Pulsars are
spun down to the present, given radio and gamma-ray emission characteristics,
filtered through ten selected radio surveys, and a {\it Fermi} all-sky
threshold map. Each chain begins with a different random seed and searches a
ten-dimensional parameter space for regions of high probability for a total of
one thousand different simulations before ending. The code investigates both
the "large world" as well as the "small world" of the parameter space. We apply
the K-means clustering algorithm to verify if the chains reveal a single or
multiple regions of significance. The outcome of the combined set of chains is
the weighted average and deviation of each of the ten parameters describing the
model. While the model reproduces reasonably well the detected distributions of
normal radio pulsars, it does not replicate the predicted detected
distribution of {\it Fermi} pulsars. The simulations do not produce sufficient
numbers of young, high- pulsars in the Galactic plane.Comment: 4 pages, 2 figures, The proceedings from the Pulsar Conference:
Electromagnetic Radiation from Pulsars and Magnetars will be published in the
Astronomical Society of the Pacific Conference Serie
The role of beam geometry in population statistics and pulse profiles of radio and gamma-ray pulsars
We present results of a pulsar population synthesis study that incorporates a
number of recent developments and some significant improvements over our
previous study. We have included the results of the Parkes multi-beam pulsar
survey in our select group of nine radio surveys, doubling our sample of radio
pulsars. We adopted with some modifications the radio beam geometry of
Arzoumanian, Chernoff & Cordes (2002). For the -ray beam, we have
assumed the slot gap geometry described in the work of Muslimov & Harding
(2003). To account for the shape of the distribution of radio pulsars in the
diagram, we continue to find that decay of the magnetic field on a
timescale of 2.8 Myr is needed. With all nine surveys, our model predicts that
EGRET should have seen 7 radio-quiet (below the sensitivity of these radio
surveys) and 19 radio-loud -ray pulsars. AGILE (nominal sensitivity
map) is expected to detect 13 radio-quiet and 37 radio-loud -ray
pulsars, while GLAST, with greater sensitivity is expected to detect 276
radio-quiet and 344 radio-loud -ray pulsars. When the Parkes multi-beam
pulsar survey is excluded, the ratio of radio-loud to radio-quiet -ray
pulsars decreases, especially for GLAST. The decrease for EGRET is 45%,
implying that some fraction of EGRET unidentified sources are radio-loud
-ray pulsars. In the radio geometry adopted, short period pulsars are
core dominated. Unlike the EGRET -ray pulsars, our model predicts that
when two -ray peaks appear in the pulse profile, a dominant radio core
peak appears in between the -ray peaks. Our findings suggest that
further improvements are required in describing both the radio and -ray
geometries.Comment: 39 pages, 13 eps figures, accepted for publication in ApJ, April 1,
200
Constraining pulsar gap models with the light-curve and flux properties of the gamma-ray pulsar population
We compare population synthesis results for inner and outer magnetosphere
emission models with the various characteristics measured in the first LAT
pulsar catalogue for both the radio-loud and radio-weak or radio-quiet
gamma-ray pulsars. We show that all models fail to reproduce the observations:
for each model there is a lack of luminous and energetic objects that suggest a
non dipolar magnetic field structure or spin-down evolution. The large
dispersion that we find in the simulated gamma-ray luminosity versus spin-down
power relation does not allow to use the present trend seen in the Fermi data
to distinguish among models. For each model and each Fermi detected pulsar, we
have generated light curves as a function of obliquity and inclination angles.
The theoretical curves were fitted to the observed one, using a
maximum-likelihood approach, to derive the best-fit orientations and to compare
how well each model can reproduce the data. Including the radio light-curve
gives an additional key constraint to restrict the orientation spaceComment: 4 pages, 3 figures, to appear in the proceedings of the Pulsar 2010
Conference, Italy, 10 - 15 October 201
The use of honey in healing a recalcitrant wound following surgical treatment of hidradenitis suppurativa
Ancient civilizations used honey to heal wounds. Despite the rediscovery of honey by modern physicians1 its use in conventional medicine, unlike in complementary medicine, remains limited. Much anecdotal evidence, some clinical observations, some animal models and some randomised controlled trials support the efficacy of honey in managing wounds2,3 , but few detailed descriptions of the use of honey in healing difficult surgical wounds have previously been published
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