47 research outputs found
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
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
Alfv\'en Polar Oscillations of Relativistic Stars
We study polar Alfv\'en oscillations of relativistic stars endowed with a
strong global poloidal dipole magnetic field. Here we focus only on the
axisymmetric oscillations which are studied by evolving numerically the
two-dimensional perturbation equations. Our study shows that the spectrum of
the polar Alfv\'{e}n oscillations is discrete in contrast to the spectrum of
axial Alfv\'{e}n oscillations which is continuous. We also show that the
typical fluid modes, such as the f and p modes, are not significantly affected
by the presence of the strong magnetic field.Comment: 10 pages, 5 figure
Validation of satellite OPEMW precipitation product with ground-based weather radar and rain gauge networks
Abstract. The Precipitation Estimation at Microwave Frequencies (PEMW) algorithm was developed at the Institute of Methodologies for Environmental Analysis of the National Research Council of Italy (IMAA-CNR) for inferring surface rain intensity (sri) from satellite passive microwave observations in the range from 89 to 190 GHz. The operational version of PEMW (OPEMW) has been running continuously at IMAA-CNR for two years. The OPEMW sri estimates, together with other precipitation products, are used as input to an operational hydrological model for flood alert forecast. This paper presents the validation of OPEMW against simultaneous ground-based observations from a network of 20 weather radar systems and a network of more than 3000 rain gauges distributed over the Italian Peninsula and main islands. The validation effort uses a data set covering one year (July 2011–June 2012). The effort evaluates dichotomous and continuous scores for the assessment of rain detection and quantitative estimate, respectively, investigating both spatial and temporal features. The analysis demonstrates 98% accuracy in correctly identifying rainy and non-rainy areas; it also quantifies the increased ability (with respect to random chance) to detect rainy and non-rainy areas (0.42–0.45 Heidke skill score) or rainy areas only (0.27–0.29 equitable threat score). Performances are better than average during summer, fall, and spring, while worse than average in the winter season. The spatial–temporal analysis does not show seasonal dependence except over the Alps and northern Apennines during winter. A binned analysis in the 0–15 mm h−1 range suggests that OPEMW tends to slightly overestimate sri values below 6–7 mm h−1 and underestimate sri above those values. With respect to rain gauges (weather radars), the correlation coefficient is larger than 0.8 (0.9). The monthly mean difference and standard deviation remain within ±1 and 2 mm h−1 with respect to rain gauges (respectively −2–0 and 4 mm h−1 with respect to weather radars)
Are there any stable magnetic fields in barotropic stars?
We construct barotropic stellar equilibria, containing magnetic fields with
both poloidal and toroidal field components. We extend earlier results by
exploring the effect of different magnetic field and current distributions. Our
results suggest that the boundary treatment plays a major role in whether the
poloidal or toroidal field component is globally dominant. Using time
evolutions we provide the first stability test for mixed poloidal-toroidal
fields in barotropic stars, finding that all these fields suffer instabilities
due to one of the field components: these are localised around the pole for
toroidal-dominated equilibria and in the closed-field line region for
poloidal-dominated equilibria. Rotation provides only partial stabilisation.
There appears to be very limited scope for the existence of stable magnetic
fields in barotropic stars. We discuss what additional physics from real stars
may allow for stable fields.Comment: 16 pages, 11 figures. Some minor revision from v1, including a new
figure; results unchanged. Now published in MNRA
Bayesian timing analysis of giant flare of SGR 1806-20 by RXTE PCA
By detecting high frequency quasi-periodic oscillations (QPOs) and estimating
frequencies of them during the decaying tail of giant flares from Soft
Gamma-ray Repeaters (SGRs) useful constraints for the equation of state (EoS)
of superdense matter may be obtained via comparison with theoretical
predictions of eigenfrequencies. We used the data collected by the Rossi X-Ray
Timing Explorer (RXTE/XTE) Proportional Counter Array (PCA) of a giant flare of
SGR 1806-20 on 2004 Dec 27 and applied a Bayesian periodicity detection method
(Gregory & Loredo, 1992) for the search of oscillations of transient nature. In
addition to the already detected frequencies, we found a few new frequencies
(f_{QPOs} ~ 16.9, 21.4, 36.4, 59.0, 116.3 Hz) of oscillations predicted by
Colaiuda et al. (2009) based on the APR_{14} EoS (Akmal et al., 1998) for SGR
1806-20.Comment: 5 pages, 7 figures, A&A accepte
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)
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
Sviluppo di strumenti tecnologici predittivi sanitario/meteo-ambientali per potenziare l’efficienza e la sostenibilità degli impianti di molluschicoltura: avvio del progetto FORESHELL
FORESHELL project is funded by the FLAG Costa Blu trough 2014-20 EMFF program of the Abruzzo Region. It is carrying out a pilot initiative to develop sanitary/weather-environmental predictive technological tools to enhance the efficiency and sustainability of the mussel farm in the Giulianova Maritime District. The hydrographic basins of the rivers close to the farm, Vibrata and Salinello, are constantly monitored trough the hydrological model (CHyM), to forecast occurrences of discharge peaks at the river mouth. Moreover, a specific sampling programme is established before and after severe weather events to determine the concentration of E. coli in freshwater at the river mouths and in mussels/seawater at the farming area. Until September 2021, there were 4 meteorological events that did not cause a peak discharge at the river mouth. Results did not show a significant increase of E. coli in the mussels. At the same time, the environmental parameters such as sea water temperature, salinity, chlorophyll a, sea currents and wave motion are acquired by the satellites and in situ probe. The web application for data visualisation is under construction, as well as the early warning signalling to the farmer. Furthermore, the growth of mussels is constantly monitored with biometric controls. In conclusion, the implementation of all phases of the FORESHELL project are progressing according to the timeline in order to develop innovative tools useful for the management of mussel farming area