692 research outputs found
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
Resonant Inverse Compton Scattering Spectra from Highly-magnetized Neutron Stars
Hard, non-thermal, persistent pulsed X-ray emission extending between 10 keV
and keV has been observed in nearly ten magnetars. For
inner-magnetospheric models of such emission, resonant inverse Compton
scattering of soft thermal photons by ultra-relativistic charges is the most
efficient production mechanism. We present angle-dependent upscattering spectra
and pulsed intensity maps for uncooled, relativistic electrons injected in
inner regions of magnetar magnetospheres, calculated using collisional
integrals over field loops. Our computations employ a new formulation of the
QED Compton scattering cross section in strong magnetic fields that is
physically correct for treating important spin-dependent effects in the
cyclotron resonance, thereby producing correct photon spectra. The spectral
cut-off energies are sensitive to the choices of observer viewing geometry,
electron Lorentz factor, and scattering kinematics. We find that electrons with
energies MeV will emit most of their radiation below 250 keV,
consistent with inferred turnovers for magnetar hard X-ray tails. More
energetic electrons still emit mostly below 1 MeV, except for viewing
perspectives sampling field line tangents. Pulse profiles may be singly- or
doubly-peaked dependent upon viewing geometry, emission locale, and observed
energy band. Magnetic pair production and photon splitting will attenuate
spectra to hard X-ray energies, suppressing signals in the Fermi-LAT band. The
resonant Compton spectra are strongly polarized, suggesting that hard X-ray
polarimetry instruments such as X-Calibur, or a future Compton telescope, can
prove central to constraining model geometry and physics.Comment: 43 pages, 12 figures; accepted for publication in ApJ; v3 fixes typos
and updates some reference
Resonant Compton Upscattering in High Field Neutron Stars
The extremely efficient process of resonant Compton upscattering by
relativistic electrons in high magnetic fields is believed to be a leading
emission mechanism of high field pulsars and magnetars in the production of
intense X-ray radiation. New analytic developments for the Compton scattering
cross section using Sokolov & Ternov (S&T) states with spin-dependent resonant
widths are presented. These new results display significant numerical
departures from both the traditional cross section using spin-averaged widths,
and also from the spin-dependent cross section that employs the Johnson &
Lippmann (J&L) basis states, thereby motivating the astrophysical deployment of
this updated resonant Compton formulation. Useful approximate analytic forms
for the cross section in the cyclotron resonance are developed for S&T basis
states. These calculations are applied to an inner magnetospheric model of the
hard X-ray spectral tails in magnetars, recently detected by RXTE and INTEGRAL.
Relativistic electrons cool rapidly near the stellar surface in the presence of
intense baths of thermal X-ray photons. We present resonant Compton cooling
rates for electrons, and the resulting photon spectra at various magnetospheric
locales, for magnetic fields above the quantum critical value. These
demonstrate how this scattering mechanism has the potential to produce the
characteristically flat spectral tails observed in magnetars.Comment: 2 pages, no 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
Young and middle age pulsar light-curve morphology: Comparison of Fermi observations with gamma-ray and radio emission geometries
Thanks to the huge amount of gamma-ray pulsar photons collected by the Fermi
Large Area Telescope since June 2008, it is now possible to constrain gamma-ray
geometrical models by comparing simulated and observed light-curve
morphological characteristics. We assumed vacuum-retarded dipole pulsar
magnetic field and tested simulated and observed morphological light-curve
characteristics in the framework of two pole emission geometries, Polar Cap
(PC), radio, and Slot Gap (SG), and Outer Gap (OG)/One Pole Caustic (OPC)
emission geometries. We compared simulated and observed/estimated light-curve
morphological parameters as a function of observable and non-observable pulsar
parameters. The PC model gives the poorest description of the LAT pulsar
light-curve morphology. The OPC best explains both the observed gamma-ray peak
multiplicity and shape classes. The OPC and SG models describe the observed
gamma-ray peak-separation distribution for low- and high-peak separations,
respectively. This suggests that the OPC geometry best explains the single-peak
structure but does not manage to describe the widely separated peaks predicted
in the framework of the SG model as the emission from the two magnetic
hemispheres. The OPC radio-lag distribution shows higher agreement with
observations suggesting that assuming polar radio emission, the gamma-ray
emission regions are likely to be located in the outer magnetosphere. The
larger agreement between simulated and LAT estimations in the framework of the
OPC suggests that the OPC model best predicts the observed variety of profile
shapes. The larger agreement between observations and the OPC model jointly
with the need to explain the abundant 0.5 separated peaks with two-pole
emission geometries, calls for thin OPC gaps to explain the single-peak
geometry but highlights the need of two-pole caustic emission geometry to
explain widely separated peaks.Comment: 28 pages, 20 figures, 8 tables; accepted for publication in Astronomy
and Astrophysic
Hard X-ray Quiescent Emission in Magnetars via Resonant Compton Upscattering
Non-thermal quiescent X-ray emission extending between 10 keV and around 150
keV has been seen in about 10 magnetars by RXTE, INTEGRAL, Suzaku, NuSTAR and
Fermi-GBM. For inner magnetospheric models of such hard X-ray signals, inverse
Compton scattering is anticipated to be the most efficient process for
generating the continuum radiation, because the scattering cross section is
resonant at the cyclotron frequency. We present hard X-ray upscattering spectra
for uncooled monoenergetic relativistic electrons injected in inner regions of
pulsar magnetospheres. These model spectra are integrated over bundles of
closed field lines and obtained for different observing perspectives. The
spectral turnover energies are critically dependent on the observer viewing
angles and electron Lorentz factor. We find that electrons with energies less
than around 15 MeV will emit most of their radiation below 250 keV, consistent
with the turnovers inferred in magnetar hard X-ray tails. Electrons of higher
energy still emit most of the radiation below around 1 MeV, except for
quasi-equatorial emission locales for select pulse phases. Our spectral
computations use a new state-of-the-art, spin-dependent formalism for the QED
Compton scattering cross section in strong magnetic fields.Comment: 5 pages, 2 figures, to appear in Proc. "Physics of Neutron Stars -
2017," Journal of Physics: Conference Series, eds. G. G. Pavlov, et al., held
in Saint Petersburg, Russia, 10-14 July, 201
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