42 research outputs found
Coupled polar-axial magnetar oscillations
We study coupled axial and polar axisymmetric oscillations of a neutron star
endowed with a strong magnetic field, having both poloidal and toroidal
components. The toroidal component of the magnetic field is driving the
coupling between the polar and axial oscillations. The star is composed of a
fluid core as well as a solid crust. Using a two dimensional general
relativistic simulation and a magnetic field B = 10^16 G, we study the change
in the polar and axial spectrum caused by the coupling. We find that the axial
spectrum suffers a dramatic change in its nature, losing its continuum
character. In fact, we find that only the 'edges' of the continua survive,
generating a discrete spectrum. As a consequence the crustal frequencies, that
in our previous simulation could be absorbed by the continua, if they were
embedded inside it, are now long living oscillations. They may lose their
energy only in the very special case that they are in resonance with the
'edges' of the continua.Comment: 12 pages, 3 figures. Revised version accepted in MNRA
On the Quasi-Periodic Oscillations of Magnetars
We study torsional Alfv\'en oscillations of magnetars, i.e., neutron stars
with a strong magnetic field. We consider the poloidal and toroidal components
of the magnetic field and a wide range of equilibrium stellar models. We use a
new coordinate system (X,Y), where ,
and is the radial component of the magnetic
field. In this coordinate system, the 1+2-dimensional evolution equation
describing the quasi-periodic oscillations, QPOs, see Sotani et al. (2007), is
reduced to a 1+1-dimensional equation, where the perturbations propagate only
along the Y-axis. We solve the 1+1-dimensional equation for different boundary
conditions and open magnetic field lines, i.e., magnetic field lines that reach
the surface and there match up with the exterior dipole magnetic field, as well
as closed magnetic lines, i.e., magnetic lines that never reach the stellar
surface. For the open field lines, we find two families of QPOs frequencies; a
family of "lower" QPOs frequencies which is located near the X-axis and a
family of "upper" frequencies located near the Y-axis. According to Levin
(2007), the fundamental frequencies of these two families can be interpreted as
the turning points of a continuous spectrum. We find that the upper frequencies
are constant multiples of the lower frequencies with a constant equaling 2n+1.
For the closed lines, the corresponding factor is n+1 . By these relations, we
can explain both the lower and the higher observed frequencies in SGR 1806-20
and SGR 1900+14.Comment: 8 pages, 7 figure
Relativistic models of magnetars: structure and deformations
We find numerical solutions of the coupled system of Einstein-Maxwell's
equations with a linear approach, in which the magnetic field acts as a
perturbation of a spherical neutron star. In our study, magnetic fields having
both poloidal and toroidal components are considered, and higher order
multipoles are also included. We evaluate the deformations induced by different
field configurations, paying special attention to those for which the star has
a prolate shape. We also explore the dependence of the stellar deformation on
the particular choice of the equation of state and on the mass of the star. Our
results show that, for neutron stars with mass M = 1.4 Msun and surface
magnetic fields of the order of 10^15 G, a quadrupole ellipticity of the order
of 10^(-6) - 10^(-5) should be expected. Low mass neutron stars are in
principle subject to larger deformations (quadrupole ellipticities up to
10^(-3) in the most extreme case). The effect of quadrupolar magnetic fields is
comparable to that of dipolar components. A magnetic field permeating the whole
star is normally needed to obtain negative quadrupole ellipticities, while
fields confined to the crust typically produce positive quadrupole
ellipticities.Comment: 25 pages, 9 figures, submitted to MNRA
Relativistic models of magnetars: the twisted-torus magnetic field configuration
We find general relativistic solutions of equilibrium magnetic field
configurations in magnetars, extending previous results of Colaiuda et al.
(2008). Our method is based on the solution of the relativistic Grad-Shafranov
equation, to which Maxwell's equations can be reduced in some limit. We obtain
equilibrium solutions with the toroidal magnetic field component confined into
a finite region inside the star, and the poloidal component extending to the
exterior. These so-called twisted-torus configurations have been found to be
the final outcome of dynamical simulations in the framework of Newtonian
gravity, and appear to be more stable than other configurations. The solutions
include higher order multipoles, which are coupled to the dominant dipolar
field. We use arguments of minimal energy to constrain the ratio of the
toroidal to the poloidal field.Comment: 13 pages, 12 figures. Minor changes to match the version published on
MNRA
Gravitational wave emission from a magnetically deformed non-barotropic neutron star
A strong candidate for a source of gravitational waves is a highly
magnetised, rapidly rotating neutron star (magnetar) deformed by internal
magnetic stresses. We calculate the mass quadrupole moment by perturbing a
zeroth-order hydrostatic equilibrium by an axisymmetric magnetic field with a
\emph{linked poloidal-toroidal structure}. In this work, we do \emph{not}
require the model star to obey a barotropic equation of state (as a realistic
neutron star is not barotropic), allowing us to explore the hydromagnetic
equilibria with fewer constraints. We derive the relation between the ratio of
poloidal-to-total field energy and ellipticity and briefly
compare our results to those obtained using the barotropic assumption. Then, we
present some examples of how our results can be applied to astrophysical
contexts. First, we show how our formulae, in conjunction with current
gravitational wave (non-)detections of the Crab pulsar and the Cassiopeia A
central compact object (Cas A CCO), can be used to constrain the strength of
the internal toroidal fields of those objects. We find that, for the Crab
pulsar (whose canonical equatorial dipole field strength, inferred from spin
down, is T) to emit detectable gravitational radiation, the
neutron star must have a strong toroidal field component, with maximum internal
toroidal field strength T; for gravitational
waves to be detected from the Cas A CCO at 300 Hz, T, whereas detection at 100 Hz would require T. Using our results, we also show how the gravitational wave signal
emitted by a magnetar immediately after its birth (assuming it is born rapidly
rotating, with ) makes such a newborn magnetar a stronger
candidate for gravitational wave detection than, for example, an SGR giant
flare.Comment: 15 pages, 8 figures, 2 table
A meteorological–hydrological regional ensemble forecast for an early-warning system over small Apennine catchments in Central Italy
Abstract. The weather forecasts for precipitation have considerably improved in recent years thanks to the increase of computational power.
This allows for the use of both a higher spatial resolution and the parameterization schemes specifically developed for representing sub-grid scale physical processes at high resolution. However, precipitation estimation is still affected by errors that can impact the response of hydrological models. To the aim of improving the hydrological forecast and the characterization of related uncertainties, a regional-scale meteorological–hydrological ensemble is presented. The uncertainties in the precipitation forecast and how they propagate in the hydrological model are also investigated. A meteorological–hydrological offline coupled ensemble is built to forecast events in a complex-orography terrain where catchments of different sizes are present. The Best Discharge-based Drainage (BDD; both deterministic and probabilistic) index, is defined with the aim of forecasting hydrological-stress conditions and related uncertainty.
In this context, the meteorological–hydrological ensemble forecast is implemented and tested for a severe hydrological event which occurred over Central Italy on 15 November 2017, when a flood hit the Abruzzo region with precipitation reaching 200 mm (24 h)−1 and producing damages with a high impact on social and economic activities. The newly developed meteorological–hydrological ensemble is compared with a high-resolution deterministic forecast and with the observations (rain gauges and radar data) over the same area. The receiver operating characteristic (ROC) statistical indicator shows how skilful the ensemble precipitation forecast is with respect to both rain-gauge- and radar-retrieved precipitation. Moreover, both the deterministic and probabilistic configurations of the BDD index are compared with the alert map issued by Civil Protection Department for the event showing a very good agreement.
Finally, the meteorological–hydrological ensemble allows for an estimation of both the predictability of the event a few days in advance and the uncertainty of the flood. Although the modelling framework is implemented on the basins of the Abruzzo region, it is portable and applicable to other areas
Overview of the first HyMeX Special Observation Period over Italy: observations and model results
Abstract. The Special Observation Period (SOP1), part of the HyMeX campaign (Hydrological cycle in the Mediterranean Experiments, 5 September–6 November 2012), was dedicated to heavy precipitation events and flash floods in the western Mediterranean, and three Italian hydro-meteorological monitoring sites were identified: Liguria–Tuscany, northeastern Italy and central Italy. The extraordinary deployment of advanced instrumentation, including instrumented aircrafts, and the use of several different operational weather forecast models, including hydrological models and marine models, allowed an unprecedented monitoring and analysis of high-impact weather events around the Italian hydro-meteorological sites. This activity has seen strong collaboration between the Italian scientific and operational communities. In this paper an overview of the Italian organization during SOP1 is provided, and selected Intensive Observation Periods (IOPs) are described. A significant event for each Italian target area is chosen for this analysis: IOP2 (12–13 September 2012) in northeastern Italy, IOP13 (15–16 October 2012) in central Italy and IOP19 (3–5 November 2012) in Liguria and Tuscany. For each IOP the meteorological characteristics, together with special observations and weather forecasts, are analyzed with the aim of highlighting strengths and weaknesses of the forecast modeling systems, including the hydrological impacts. The usefulness of having different weather forecast operational chains characterized by different numerical weather prediction models and/or different model set up or initial conditions is finally shown for one of the events (IOP19)
Gravitational waves from single neutron stars: an advanced detector era survey
With the doors beginning to swing open on the new gravitational wave
astronomy, this review provides an up-to-date survey of the most important
physical mechanisms that could lead to emission of potentially detectable
gravitational radiation from isolated and accreting neutron stars. In
particular we discuss the gravitational wave-driven instability and
asteroseismology formalism of the f- and r-modes, the different ways that a
neutron star could form and sustain a non-axisymmetric quadrupolar "mountain"
deformation, the excitation of oscillations during magnetar flares and the
possible gravitational wave signature of pulsar glitches. We focus on progress
made in the recent years in each topic, make a fresh assessment of the
gravitational wave detectability of each mechanism and, finally, highlight key
problems and desiderata for future work.Comment: 39 pages, 12 figures, 2 tables. Chapter of the book "Physics and
Astrophysics of Neutron Stars", NewCompStar COST Action 1304. Minor
corrections to match published versio
Strongly magnetized pulsars: explosive events and evolution
Well before the radio discovery of pulsars offered the first observational
confirmation for their existence (Hewish et al., 1968), it had been suggested
that neutron stars might be endowed with very strong magnetic fields of
-G (Hoyle et al., 1964; Pacini, 1967). It is because of their
magnetic fields that these otherwise small ed inert, cooling dead stars emit
radio pulses and shine in various part of the electromagnetic spectrum. But the
presence of a strong magnetic field has more subtle and sometimes dramatic
consequences: In the last decades of observations indeed, evidence mounted that
it is likely the magnetic field that makes of an isolated neutron star what it
is among the different observational manifestations in which they come. The
contribution of the magnetic field to the energy budget of the neutron star can
be comparable or even exceed the available kinetic energy. The most magnetised
neutron stars in particular, the magnetars, exhibit an amazing assortment of
explosive events, underlining the importance of their magnetic field in their
lives. In this chapter we review the recent observational and theoretical
achievements, which not only confirmed the importance of the magnetic field in
the evolution of neutron stars, but also provide a promising unification scheme
for the different observational manifestations in which they appear. We focus
on the role of their magnetic field as an energy source behind their persistent
emission, but also its critical role in explosive events.Comment: Review commissioned for publication in the White Book of
"NewCompStar" European COST Action MP1304, 43 pages, 8 figure