145 research outputs found
CCOs and the hidden magnetic field scenario
CCOs are X-ray sources lying close the center of supernova remnants, with
inferred values of the surface magnetic fields significantly lower (less than
about 1e11 G) than those of standard pulsars. In this paper, we revise the
hidden magnetic field scenario, presenting the first 2D simulations of the
submergence and reemergence of the magnetic field in the crust of a neutron
star. A post-supernova accretion stage of about 1e-4-1e-3 solar masses over a
vast region of the surface is required to bury the magnetic field into the
inner crust. When accretion stops, the field reemerges on a typical timescale
of 1-100 kyr, depending on the submergence conditions. After this stage, the
surface magnetic field is restored close to its birth values. A possible
observable consequence of the hidden magnetic field is the anisotropy of the
surface temperature distribution, in agreement with observations of several of
these sources. We conclude that the hidden magnetic field model is viable as
alternative to the anti-magnetar scenario, and it could provide the missing
link between CCOs and the other classes of isolated neutron stars.Comment: 7 pages, 7 figures, MNRA
Magnetic fields in neutron stars
This work aims at studying how magnetic fields affect the observational
properties and the long-term evolution of isolated neutron stars, which are the
strongest magnets in the universe. The extreme physical conditions met inside
these astronomical sources complicate their theoretical study, but, thanks to
the increasing wealth of radio and X-ray data, great advances have been made
over the last years. A neutron star is surrounded by magnetized plasma, the
so-called magnetosphere. Modeling its global configuration is important to
understand the observational properties of the most magnetized neutron stars,
magnetars. On the other hand, magnetic fields in the interior are thought to
evolve on long time-scales, from thousands to millions of years. The magnetic
evolution is coupled to the thermal one, which has been the subject of study in
the last decades. An important part of this thesis presents the
state-of-the-art of the magneto-thermal evolution models of neutron stars
during the first million of years, studied by means of detailed simulations.
The numerical code here described is the first one to consistently consider the
coupling of magnetic field and temperature, with the inclusion of both the
Ohmic dissipation and the Hall drift in the crust.Comment: PhD thesis defended in University of Alicante, 20 September 201
Triggering magnetar outbursts in 3D force-free simulations
In this letter, we present the first 3D force-free general relativity
simulations of the magnetosphere dynamics related to the magnetar
outburst/flare phenomenology. Starting from an initial dipole configuration, we
adiabatically increase the helicity by twisting the footprints of a spot on the
stellar surface and follow the succession of quasi-equilibrium states until a
critical twist is reached. Twisting beyond that point triggers instabilities
that results in the rapid expansion of magnetic field lines, followed by
reconnection, as observed in previous axi-symmetric simulations. If the
injection of magnetic helicity goes on, the process is recurrent, periodically
releasing a similar amount of energy, of the order of a few % of the total
magnetic energy. From our current distribution, we estimate the local
temperature assuming that dissipation occurs mainly in the highly resistive
outermost layer of the neutron star. We find that the temperature smoothly
increases with injected twist, being larger for spots located in the tropical
regions than in polar regions, and rather independent of their sizes. After the
injection of helicity ceases, the magnetosphere relaxes to a new stable state,
in which the persistent currents maintain the footprints area slightly hotter
than before the onset of the instability.Comment: 6 pages, 5 figure
Spectral features in isolated neutron stars induced by inhomogeneous surface temperatures
The thermal X-ray spectra of several isolated neutron stars display
deviations from a pure blackbody. The accurate physical interpretation of these
spectral features bears profound implications for our understanding of the
atmospheric composition, magnetic field strength and topology, and equation of
state of dense matter. With specific details varying from source to source,
common explanations for the features have ranged from atomic transitions in the
magnetized atmospheres or condensed surface, to cyclotron lines generated in a
hot ionized layer near the surface. Here we quantitatively evaluate the X-ray
spectral distortions induced by inhomogeneous temperature distributions of the
neutron star surface. To this aim, we explore several surface temperature
distributions, we simulate their corresponding general relativistic X-ray
spectra (assuming an isotropic, blackbody emission), and fit the latter with a
single blackbody model. We find that, in some cases, the presence of a spurious
'spectral line' is required at a high significance level in order to obtain
statistically acceptable fits, with central energy and equivalent width similar
to the values typically observed. We also perform a fit to a specific object,
RX J0806.4-4123, finding several surface temperature distributions able to
model the observed spectrum. The explored effect is unlikely to work in all
sources with detected lines, but in some cases it can indeed be responsible for
the appearance of such lines. Our results enforce the idea that surface
temperature anisotropy can be an important factor that should be considered and
explored also in combination with more sophisticated emission models like
atmospheres.Comment: 11 pages, 7 figures; accepted for publication in MNRA
Population synthesis of isolated Neutron Stars with magneto--rotational evolution
We revisit the population synthesis of isolated radio-pulsars incorporating
recent advances on the evolution of the magnetic field and the angle between
the magnetic and rotational axes from new simulations of the magneto-thermal
evolution and magnetosphere models, respectively. An interesting novelty in our
approach is that we do not assume the existence of a death line. We discuss
regions in parameter space that are more consistent with the observational
data. In particular, we find that any broad distribution of birth spin periods
with s can fit the data, and that if the alignment angle is
allowed to vary consistently with the torque model, realistic magnetospheric
models are favoured compared to models with classical magneto-dipolar radiation
losses. Assuming that the initial magnetic field is given by a lognormal
distribution, our optimal model has mean strength with width .
However, there are strong correlations between parameters. This degeneracy in
the parameter space can be broken by an independent estimate of the pulsar
birth rate or by future studies correlating this information with the
population in other observational bands (X-rays and -rays).Comment: 10 pages, 9 figures, submitted and accepted to MNRAS, comments
welcom
Compact formulae, dynamics and radiation of charged particles under synchro-curvature losses
We consider the fundamental problem of charged particles moving along and
around a curved magnetic field line, revising the synchro-curvature radiation
formulae introduced by Cheng and Zhang (1996). We provide more compact
expressions to evaluate the spectrum emitted by a single particle, identifying
the key parameter that controls the transition between the curvature-dominated
and the synchrotron-dominated regime. This parameter depends on the local
radius of curvature of the magnetic field line, the gyration radius, and the
pitch angle. We numerically solve the equations of motion for the emitting
particle by considering self-consistently the radiative losses, and provide the
radiated spectrum produced by a particle when an electric acceleration is
balanced by its radiative losses, as it is assumed to happen in the outer gaps
of pulsar's magnetospheres. We compute the average spectrum radiated throughout
the particle trajectory finding that the slope of the spectrum before the peak
depends on the location and size of the emission region. We show how this
effect could then lead to a variety of synchro-curvature spectra. Our results
reinforce the idea that the purely synchrotron or curvature losses are, in
general, inadequate to describe the radiative reaction on the particle motion,
and the spectrum of emitted photons. Finally, we discuss the applicability of
these calculations to different astrophysical scenarios.Comment: 9 pages, 5 figures, 2 tables. Accepted for publication in MNRAS main
journal. References update
- …