11,589 research outputs found
High Energy Neutrinos and Photons from Curvature Pions in Magnetars
We discuss the relevance of the curvature radiation of pions in strongly
magnetized pulsars or magnetars, and their implications for the production of
TeV energy neutrinos detectable by cubic kilometer scale detectors, as well as
high energy photons.Comment: 19 pages, 4 figures, to appear in JCA
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
Approximation of Rough Functions
For given and , we establish
the existence and uniqueness of solutions , to the
equation where , , and . Solutions include well-known nowhere differentiable functions such as
those of Bolzano, Weierstrass, Hardy, and many others. Connections and
consequences in the theory of fractal interpolation, approximation theory, and
Fourier analysis are established.Comment: 16 pages, 3 figure
Cascade model of gamma-ray bursts: Power-law and annihilation-line components
If, in a neutron star magnetosphere, an electron is accelerated to an energy of 10 to the 11th or 12th power eV by an electric field parallel to the magnetic field, motion of the electron along the curved field line leads to a cascade of gamma rays and electron-positron pairs. This process is believed to occur in radio pulsars and gamma ray burst sources. Results are presented from numerical simulations of the radiation and photon annihilation pair production processes, using a computer code previously developed for the study of radio pulsars. A range of values of initial energy of a primary electron was considered along with initial injection position, and magnetic dipole moment of the neutron star. The resulting spectra was found to exhibit complex forms that are typically power law over a substantial range of photon energy, and typically include a dip in the spectrum near the electron gyro-frequency at the injection point. The results of a number of models are compared with data for the 5 Mar., 1979 gamma ray burst. A good fit was found to the gamma ray part of the spectrum, including the equivalent width of the annihilation line
Discovery of a Spin-Down State Change in the LMC Pulsar B0540-69
We report the discovery of a large, sudden, and persistent increase in the
spin-down rate of B0540-69, a young pulsar in the Large Magellanic Cloud, using
observations from the Swift and RXTE satellites. The relative increase in the
spin-down rate of 36% is unprecedented for B0540-69. No accompanying change in
the spin rate is seen, and no change is seen in the pulsed X-ray emission from
B0540-69 following the change in the spin-down rate. Such large relative
changes in the spin-down rate are seen in the recently discovered class of
'intermittent pulsars', and we compare the properties of B0540-69 to such
pulsars. We consider possible changes in the magnetosphere of the pulsar that
could cause such a large change in the spin-down rate.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
Self-consistent models for Coulomb heated X-ray pulsar atmospheres
Calculations of accreting magnetized neutron star atmospheres heated by the gradual deceleration of protons via Coulomb collisions are presented. Self consistent determinations of the temperature and density structure for different accretion rates are made by assuming hydrostatic equilibrium and energy balance, coupled with radiative transfer. The full radiative transfer in two polarizations, using magnetic cross sections but with cyclotron resonance effects treated approximately, is carried out in the inhomogeneous atmospheres
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
Light-curve modelling constraints on the obliquities and aspect angles of the young Fermi pulsars
In more than four years of observation the Large Area Telescope on board the
Fermi satellite has identified pulsed -ray emission from more than 80
young pulsars, providing light curves with high statistics. Fitting the
observations with geometrical models can provide estimates of the magnetic
obliquity and aspect angle , yielding estimates of the
radiation beaming factor and luminosity. Using -ray emission geometries
(Polar Cap, Slot Gap, Outer Gap, One Pole Caustic) and radio emission geometry,
we fit -ray light curves for 76 young pulsars and we jointly fit their
-ray plus radio light curves when possible. We find that a joint radio
plus -ray fit strategy is important to obtain (, )
estimates that can explain simultaneous radio and -ray emission. The
intermediate-to-high altitude magnetosphere models, Slot Gap, Outer Gap, and
One pole Caustic, are favoured in explaining the observations. We find no
evolution of on a time scale of a million years. For all emission
geometries our derived -ray beaming factors are generally less than one
and do not significantly evolve with the spin-down power. A more pronounced
beaming factor vs. spin-down power correlation is observed for Slot Gap model
and radio-quiet pulsars and for the Outer Gap model and radio-loud pulsars. For
all models, the correlation between -ray luminosity and spin-down power
is consistent with a square root dependence. The -ray luminosities
obtained by using our beaming factors not exceed the spin-down power. This
suggests that assuming a beaming factor of one for all objects, as done in
other studies, likely overestimates the real values. The data show a relation
between the pulsar spectral characteristics and the width of the accelerator
gap that is consistent with the theoretical prediction for the Slot Gap model.Comment: 90 pages, 80 figures (63 in Appendices), accepted for publication in
Astronomy and Astrophysic
An iron(ii) spin-crossover metallacycle from a back-to-back bis-[dipyrazolylpyridine]
The syntheses of 4-mercapto-2,6-di(pyrazol-1-yl)pyridine (bppSH) and bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide (bppSSbpp) are reported. In contrast to previously published “back-to-back” bis-[2,6-di(pyrazol-1-yl)pyridine] derivatives, which form coordination polymers with transition ions that are usually insoluble, bppSSbpp yields soluble oligomeric complexes with iron(II) and zinc(II). Mass spectrometry and DOSY data show that [{Fe(μ-bppSSbpp)}n]2n+ and [{Zn(μ-bppSSbpp)}n]2n+ form tetranuclear metallacycles in nitromethane solution (n = 4), although 1H NMR and conductivity measurements imply the iron compound may undergo more fragmentation than its zinc congener. Both [{Fe(bppSH)2]2+ and [{Fe(μ-bppSSbpp)}n]2n+ exhibit thermal spin-crossover in CD3NO2 solution, with midpoint temperatures near 245 K. The similarity of these equilibria implies there is little cooperativity between the iron centres in the metallacyclic structures
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