11,047 research outputs found
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
Rotational velocities of A-type stars II. Measurement of vsini in the northern hemisphere
This work is the second part of the set of measurements of vsini for A-type
stars, begun by Royer et al. (2002). Spectra of 249 B8 to F2-type stars
brighter than V=7 have been collected at Observatoire de Haute-Provence (OHP).
Fourier transforms of several line profiles in the range 4200--4600 A are used
to derive vsini from the frequency of the first zero. Statistical analysis of
the sample indicates that measurement error mainly depends on vsini and this
relative error of the rotational velocity is found to be about 5% on average.
The systematic shift with respect to standard values from Slettebak et al.
(1975), previously found in the first paper, is here confirmed. Comparisons
with data from the literature agree with our findings: vsini values from
Slettebak et al. are underestimated and the relation between both scales
follows a linear law: vsini(new) = 1.03 vsini(old) + 7.7. Finally, these data
are combined with those from the previous paper (Royer et al. 2002), together
with the catalogue of Abt & Morrell (1995). The resulting sample includes some
2150 stars with homogenized rotational velocities.Comment: 16 pages, includes 13 figures, accepted in A&
Cosmic-ray propagation properties for an origin in SNRs
We have studied the impact of cosmic-ray acceleration in SNR on the spectra
of cosmic-ray nuclei in the Galaxy using a series expansion of the propagation
equation, which allows us to use analytical solutions for part of the problem
and an efficient numerical treatment of the remaining equations and thus
accurately describes the cosmic-ray propagation on small scales around their
sources in three spatial dimensions and time. We found strong variations of the
cosmic-ray nuclei flux by typically 20% with occasional spikes of much higher
amplitude, but only minor changes in the spectral distribution. The locally
measured spectra of primary cosmic rays fit well into the obtained range of
possible spectra. We further showed that the spectra of the secondary element
Boron show almost no variations, so that the above findings also imply
significant fluctuations of the Boron-to-Carbon ratio. Therefore the commonly
used method of determining CR propagation parameters by fitting
secondary-to-primary ratios appears flawed on account of the variations that
these ratios would show throughout the Galaxy.Comment: Accepted for publication in Ap
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
Half-ordered state in the anisotropic Haldane-gap antiferromagnet NDMAP
Neutron diffraction experiments performed on the Haldane gap material NDMAP
in high magnetic fields applied at an angle to the principal anisotropy axes
reveal two consecutive field-induced phase transitions. The low-field phase is
the gapped Haldane state, while at high fields the system exhibits
3-dimensional long-range Neel order. In a peculiar phase found at intermediate
fields only half of all the spin chains participate in the long-range ordering,
while the other half remains disordered and gapped.Comment: 4 pages, 2 figures, submitted to Phys. Rev.
Cooperative ordering of gapped and gapless spin networks in CuFeGeO
The unusual magnetic properties of a novel low-dimensional quantum
ferrimagnet CuFeGeO are studied using bulk methods, neutron
diffraction and inelastic neutron scattering. It is shown that this material
can be described in terms of two low-dimensional quantum spin subsystems, one
gapped and the other gapless, characterized by two distinct energy scales.
Long-range magnetic ordering observed at low temperatures is a cooperative
phenomenon caused by weak coupling of these two spin networks.Comment: 4 pages, 4 figure
Absolute magnitudes and kinematics of CP stars from Hipparcos data
The position in the HR diagram and the kinematic characteristics of different
kinds of CP stars of the upper main sequence are obtained using the LM method
(Luri et al., 1996). Most of the CP stars are main sequence stars occupying the
whole width of the sequence. From a kinematic point of view, they belong to the
young disk population (ages < 1.5 Gyr). It has also been found that, on
kinematic grounds, the behaviour of lambda Bootis stars is similar to the one
observed for normal stars of the same spectral range. On the other hand, roAp
and noAp stars show the same kinematic characteristics. The peculiar velocity
distribution function has been decomposed into a sum of three dimensional
gaussians and the presence of Pleiades, Sirius and Hyades moving groups has
been clearly established. Finally, a small number of CP stars are found to be
high-velocity objects.Comment: 8 pages, 1 figure, to appear in: Proc. of the 26th workshop of the
European Working Group on CP stars, eds. P. North, A. Schnell and J.
Ziznovsky, Contrib. Astr. Obs. Skalnate Pleso Vol. 27, No
Fermi-LAT Observations of High- and Intermediate-Velocity Clouds: Tracing Cosmic Rays in the Halo of the Milky Way
It is widely accepted that cosmic rays (CRs) up to at least PeV energies are
Galactic in origin. Accelerated particles are injected into the interstellar
medium where they propagate to the farthest reaches of the Milky Way, including
a surrounding halo. The composition of CRs coming to the solar system can be
measured directly and has been used to infer the details of CR propagation that
are extrapolated to the whole Galaxy. In contrast, indirect methods, such as
observations of gamma-ray emission from CR interactions with interstellar gas,
have been employed to directly probe the CR densities in distant locations
throughout the Galactic plane. In this article we use 73 months of data from
the Fermi Large Area Telescope in the energy range between 300 MeV and 10 GeV
to search for gamma-ray emission produced by CR interactions in several high-
and intermediate-velocity clouds located at up to ~ 7 kpc above the Galactic
plane. We achieve the first detection of intermediate-velocity clouds in gamma
rays and set upper limits on the emission from the remaining targets, thereby
tracing the distribution of CR nuclei in the halo for the first time. We find
that the gamma-ray emissivity per H atom decreases with increasing distance
from the plane at 97.5% confidence level. This corroborates the notion that CRs
at the relevant energies originate in the Galactic disk. The emissivity of the
upper intermediate-velocity Arch hints at a 50% decline of CR densities within
2 kpc from the plane. We compare our results to predictions of CR propagation
models.Comment: Accepted for publication in the Astrophysical Journa
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