783 research outputs found
Modeling the broadband persistent emission of magnetars
In this paper, we discuss our first attempts to model the broadband
persistent emission of magnetars within a self consistent, physical scenario.
We present the predictions of a synthetic model that we calculated with a new
Monte Carlo 3-D radiative code. The basic idea is that soft thermal photons
(e.g. emitted by the star surface) can experience resonant cyclotron
upscattering by a population of relativistic electrons threated in the twisted
magnetosphere. Our code is specifically tailored to work in the
ultra-magnetized regime; polarization and QED effects are consistently
accounted for, as well different configurations for the magnetosphere. We
discuss the predicted spectral properties in the 0.1-1000 keV range, the
polarization properties, and we present the model application to a sample of
magnetars soft X-ray spectra.Comment: 14 pages, 7 figures, to be published in Advances in Space Research.
Proceedings of the conference "Frontieres of Space Astrophysics, Neutron
Stars & Gamma Ray Bursts", Cairo/Alexandria, 30 March- 4 April 200
Extensive population synthesis of isolated neutron stars with field decay
We perform population synthesis studies of different types of neutron stars
taking into account the magnetic field decay. For the first time, we confront
our results with observations using {\it simultaneously} the Log N -- Log S
distribution for nearby isolated neutron stars, the Log N -- Log L distribution
for magnetars, and the distribution of radio pulsars in the --
diagram. We find that our theoretical model is consistent with all sets of data
if the initial magnetic field distribution function follows a log-normal law
with and . The
typical scenario includes about 10% of neutron stars born as magnetars,
significant magnetic field decay during the first million years of a NS life.
Evolutionary links between different subclasses may exist, although robust
conclusions are not yet possible.
We apply the obtained field distribution and the model of decay to study
long-term evolution of neuton stars till the stage of accretion from the
interstellar medium. It is shown that though the subsonic propeller stage can
be relatively long, initially highly magnetized neutron stars ( G) reach the accretion regime within the Galactic lifetime if their
kick velocities are not too large. The fact that in previous studies made 10
years ago, such objects were not considered results in a slight increase of the
Accretor fraction in comparison with earlier conclusions. Most of the neutron
stars similar to the Magnificent seven are expected to become accreting from
the interstellar medium after few billion years of their evolution. They are
the main predecestors of accreting isolated neutron stars.Comment: 4 pages, conference "Astrophysics of Neutron Stars - 2010" in honor
of M. Ali Alpar, Izmir, Turke
The Galactic centre pulsar population
The recent discovery of a magnetar in the Galactic centre region has allowed
Spitler et al. to characterize the interstellar scattering in that direction.
They find that the temporal broadening of the pulse profile of the magnetar is
substantially less than that predicted by models of the electron density of
that region. This raises the question of what the plausible limits for the
number of potentially observable pulsars - i.e., the number of pulsars beaming
towards the Earth - in the Galactic centre are. In this paper, using reasonable
assumptions - namely, (i) the luminosity function of pulsars in the Galactic
centre region is the same as that in the field, (ii) the region has had a
constant pulsar formation rate, (iii) the spin and luminosity evolution of
magnetars and pulsars are similar, and (iv) the scattering in the direction of
the Galactic centre magnetar is representative of the entire inner parsec - we
show that the potentially observable population of pulsars in the inner parsec
has a conservative upper limit of 200, and that it is premature to
conclude that the number of pulsars in this region is small. We also show that
the observational results so far are consistent with this number and make
predictions for future radio pulsar surveys of the Galactic centre.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS Letter
Magnetars: the physics behind observations
Magnetars are the strongest magnets in the present universe and the
combination of extreme magnetic field, gravity and density makes them unique
laboratories to probe current physical theories (from quantum electrodynamics
to general relativity) in the strong field limit. Magnetars are observed as
peculiar, burst--active X-ray pulsars, the Anomalous X-ray Pulsars (AXPs) and
the Soft Gamma Repeaters (SGRs); the latter emitted also three "giant flares,"
extremely powerful events during which luminosities can reach up to 10^47 erg/s
for about one second. The last five years have witnessed an explosion in
magnetar research which has led, among other things, to the discovery of
transient, or "outbursting," and "low-field" magnetars. Substantial progress
has been made also on the theoretical side. Quite detailed models for
explaining the magnetars' persistent X-ray emission, the properties of the
bursts, the flux evolution in transient sources have been developed and
confronted with observations. New insight on neutron star asteroseismology has
been gained through improved models of magnetar oscillations. The long-debated
issue of magnetic field decay in neutron stars has been addressed, and its
importance recognized in relation to the evolution of magnetars and to the
links among magnetars and other families of isolated neutron stars. The aim of
this paper is to present a comprehensive overview in which the observational
results are discussed in the light of the most up-to-date theoretical models
and their implications. This addresses not only the particular case of magnetar
sources, but the more fundamental issue of how physics in strong magnetic
fields can be constrained by the observations of these unique sources.Comment: 81 pages, 24 figures, This is an author-created, un-copyedited
version of an article submitted to Reports on Progress in Physic
Ultra-High Energy Cosmic Rays Detected by Auger and AGASA:Corrections for Galactic Magnetic Field Deflections, Source Populations, and Arguments for Multiple-Components
The origin and composition of Ultra-High Energy Cosmic Ray Events (UHECRs)
are under debate. Here we improve constraints on the source population(s) and
compositions of UHECRs by accounting for UHECR deflections within existing
Galactic magnetic field models (GMFs). We used Monte Carlo simulations for
UHECRs detected by the Pierre Auger Observatory and AGASA in order to determine
their outside-the-Galaxy arrival directions, and compared these with Galactic
and extragalactic sources. The simulations, which used UHECR compositions from
protons to Iron and seven models of the ordered GMF, include uncertainties in
the GMF and a turbulent magnetic field. The correlation between UHECRs and
nearby extended radiogalaxies (Nagar & Matulich 2008) remains valid, even
strengthened, within several GMF models. Both the nearest radiogalaxy CenA, and
the nearest radio-extended BL Lac, CGCG 413-019, are likely sources of multiple
UHECRs. The correlation appears to be linked to the presence of the extended
radio source rather than a tracer of an underlying population. It is possible,
but unlikely, that all UHECRs originate in the nearby radiogalaxy CenA. For
light UHECRs about a third of UHECRs can be "matched" to nearby galaxies with
extended radio jets. The remaining UHECRs could also be explained as
originating in extended radiogalaxies if one has at least one of: a large UHECR
mean free path, a high cluster and/or intergalactic magnetic field, a heavy
composition for two-thirds of the detected UHECRs. Several UHECRs have
trajectories which pass close to Galactic magnetars and/or microquasars.
If extended radiogalaxies are, or trace, UHECR sources, the most consistent
models for the ordered GMF are the BS-S and BS-A models; the GMF models of Sun
et al. 2008 are acceptable if a dipole component is added.Comment: to appear in A&
Gravitational waves from resolvable massive black hole binary systems and observations with Pulsar Timing Arrays
Massive black holes are key components of the assembly and evolution of
cosmic structures and a number of surveys are currently on-going or planned to
probe the demographics of these objects and to gain insight into the relevant
physical processes. Pulsar Timing Arrays (PTAs) currently provide the only
means to observe gravitational radiation from massive black hole binary systems
with masses >10^7 solar masses. The whole cosmic population produces a
stochastic background that could be detectable with upcoming Pulsar Timing
Arrays. Sources sufficiently close and/or massive generate gravitational
radiation that significantly exceeds the level of the background and could be
individually resolved. We consider a wide range of massive black hole binary
assembly scenarios, we investigate the distribution of the main physical
parameters of the sources, such as masses and redshift, and explore the
consequences for Pulsar Timing Arrays observations. Depending on the specific
massive black hole population model, we estimate that on average at least one
resolvable source produces timing residuals in the range ~5-50 ns. Pulsar
Timing Arrays, and in particular the future Square Kilometre Array (SKA), can
plausibly detect these unique systems, although the events are likely to be
rare. These observations would naturally complement on the high-mass end of the
massive black hole distribution function future surveys carried out by the
Laser Interferometer Space Antenna (LISA)Comment: 12 pages, 10 figures, accepted for publication in MNRAS. Results
revised (differences within a factor of two) after a bug in the code for
generating the timing residuals has been fixe
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