471 research outputs found
Quiescent thermal emission from neutron stars in LMXBs
We monitored the quiescent thermal emission from neutron stars in low-mass
X-ray binaries after active periods of intense activity in x-rays (outbursts).
The theoretical modeling of the thermal relaxation of the neutron star crust
may be used to establish constraints on the crust composition and transport
properties, depending on the astrophysical scenarios assumed. We numerically
simulated the thermal evolution of the neutron star crust and compared them
with inferred surface temperatures for five sources: MXB 1659-29, KS 1731-260,
EXO 0748-676, XTE J1701-462 and IGR J17480-2446. We find that the evolution of
MXB 1659-29, KS 1731-260 and EXO 0748-676 can be well described within a deep
crustal cooling scenario. Conversely, we find that the other two sources can
only be explained with models beyond crustal cooling. For the peculiar emission
of XTE J1701-462 we propose alternative scenarios such as residual accretion
during quiescence, additional heat sources in the outer crust, and/or thermal
isolation of the inner crust due to a buried magnetic field. We also explain
the very recent reported temperature of IGR J17480-2446 with an additional heat
deposition in the outer crust from shallow sources.Comment: 19 pages, 32 figures, 2 Append., revised version accepted for
publication in Astronomy & Astrophysic
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
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The relevance of ambipolar diffusion for neutron star evolution
We study ambipolar diffusion in strongly magnetised neutron stars, with
special focus on the effects of neutrino reaction rates and the impact of a
superfluid/superconducting transition in the neutron star core. For
axisymmetric magnetic field configurations, we determine the deviation from
equilibrium induced by the magnetic force and calculate the velocity of
the slow, quasi-stationary, ambipolar drift. We study the temperature
dependence of the velocity pattern and clearly identify the transition to a
predominantly solenoidal flow. For stars without superconducting/superfluid
constituents and with a mixed poloidal-toroidal magnetic field of typical
magnetar strength, we find that ambipolar diffusion proceeds fast enough to
have a significant impact on the magnetic field evolution only at low core
temperatures, K. The ambipolar diffusion timescale
becomes appreciably shorter when fast neutrino reactions are present, because
the possibility to balance part of the magnetic force with pressure gradients
is reduced. We also find short ambipolar diffusion timescales in the case of
superconducting cores for K, due to the reduced interaction
between protons and neutrons. In the most favourable scenario, with fast
neutrino reactions and superconducting cores, ambipolar diffusion results in
advection velocities of several km/kyr. This velocity can substantially
reorganize magnetic fields in magnetar cores, in a way that can only be
confirmed by dynamical simulations.Comment: 14 pages, 11 figures, version accepted for publication in MNRA
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
L'Assecat de les fruites: idees generals i la seva possibilitat d'utilització com a recurs didàctic
La conservació dels aliments és una de les preocupacions de la societat. Una de les tècniques més antigues és l'assecat, que començà sent una activitat artesanal i avui és un procés industrial rigorós, amb processos interessants que convé conèixer. En aquest treball es mostren els fonaments d'aquesta tècnica de conservació i alguns experiments que es poden proposar als estudiants.Foods preservation is one of the concerns of human society. One of the techniques that has been used for centuries is drying, but what started as an artisanal activity is today a very rigorous industrial process that hides several interesting
procedures. This work shows the scientific basis of this technique and some experiments that can be proposed to students
Quasi-periodic oscillations in superfluid, relativistic magnetars with nuclear pasta phases
We study the torsional magneto-elastic oscillations of relativistic superfluid magnetars and explore the effects of a phase transition in the crust–core interface (nuclear pasta) which results in a weaker elastic response. Exploring various models with different extension of nuclear pasta phases, we find that the differences in the oscillation spectrum present in purely elastic modes (weak magnetic field) are smeared out with increasing strength of the magnetic field. For magnetar conditions, the main characteristic and features of models without nuclear pasta are preserved. We find, in general, two classes of magneto-elastic oscillations which exhibit a different oscillation pattern. For Bp 5 × 1014 G. We do not find any evidence of fundamental pure crustal modes in the low-frequency range (below 200 Hz) for Bp ≥ 1014 G.AP acknowledges support from the European Union under the Marie Sklodowska Curie Actions Individual Fellowship, grant agreement no 656370. This work is supported in part by the Spanish MINECO grant AYA2015-66899-C2-2-P, the programme PROMETEOII-2014-069 (Generalitat Valenciana), and by the NewCompstarCOST action MP1304
Magnetars: Properties, Origin and Evolution
Magnetars are neutron stars in which a strong magnetic field is the main energy source. About two dozens of magnetars, plus several candidates, are currently known in our Galaxy and in the Magellanic Clouds. They appear as highly variable X-ray sources and, in some cases, also as radio and/or optical pulsars. Their spin periods (2–12 s) and spin-down rates (∼10−13–10−10 s s−1) indicate external dipole fields of ∼1013−15 G, and there is evidence that even stronger magnetic fields are present inside the star and in non-dipolar magnetospheric components. Here we review the observed properties of the persistent emission from magnetars, discuss the main models proposed to explain the origin of their magnetic field and present recent developments in the study of their evolution and connection with other classes of neutron stars.The work of SM has been partially supported through the agreement ASI-INAF I/037/12/0. JAP acknowledges support of the Spanish national grant AYA 2013-42184-P and of the New Compstar COST action MP1304. AM acknowledges support of an Australian Research Council Discovery Project grant
Are pulsars born with a hidden magnetic field?
The observation of several neutron stars in the centre of supernova remnants and with significantly lower values of the dipolar magnetic field than the average radio-pulsar population has motivated a lively debate about their formation and origin, with controversial interpretations. A possible explanation requires the slow rotation of the protoneutron star at birth, which is unable to amplify its magnetic field to typical pulsar levels. An alternative possibility, the hidden magnetic field scenario, considers the accretion of the fallback of the supernova debris on to the neutron star as responsible for the submergence (or screening) of the field and its apparently low value. In this paper, we study under which conditions the magnetic field of a neutron star can be buried into the crust due to an accreting, conducting fluid. For this purpose, we consider a spherically symmetric calculation in general relativity to estimate the balance between the incoming accretion flow and the magnetosphere. Our study analyses several models with different specific entropy, composition, and neutron star masses. The main conclusion of our work is that typical magnetic fields of a few times 1012 G can be buried by accreting only 10−3–10−2 M⊙, a relatively modest amount of mass. In view of this result, the central compact object scenario should not be considered unusual, and we predict that anomalously weak magnetic fields should be common in very young (< few kyr) neutron stars.This work has been supported by the Spanish MINECO grants AYA2013-40979-P and AYA2013-42184-P and by the Generalitat Valenciana (PROMETEOII-2014-069)
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