arXiv subjects

Luigi Stella

Publications and source records attributed to Luigi Stella.

At least 55 records · Page 3Linked to original sources

arXiv2014

Gravitational waves from massive magnetars formed in binary neutron star mergers

Binary neutron star (NS) mergers are among the most promising sources of gravitational waves (GWs), as well as candidate progenitors for short Gamma-Ray Bursts (SGRBs). Depending on the total initial mass of the system, and the NS equation of state (EOS), the post-merger phase can see a prompt collapse to a black hole, or the formation of a supramassive NS, or even a stable NS. In the case of post-merger NS (PMNS) formation, magnetic field amplification during the merger will produce a magnetar with a large induced mass quadrupole moment, and millisecond spin. If the timescale for orthogonalization of the magnetic symmetry axis with the spin axis is sufficiently short the NS will radiate its spin down energy primarily via GWs. Here we study this scenario for various outcomes of NS formation: we generalise the set of equilibrium states for a twisted torus magnetic configuration to include solutions that, at a fixed exterior dipole field, carry a larger magnetic energy reservoir; we hence compute their magnetic ellipticity and the strength of the expected GW signal as a function of the magnitude of the dipole and toroidal field. The relative number of GW detections from PMNSs and from binary NSs is a strong function of the NS equation of state (EOS), being higher (~ 1%) for the stiffest EOSs and negligibly small for the softest ones. For intermediate-stiffness EOSs, such as the n=4/7 polytrope recently used by Giacomazzo \& Perna or the GM1 used by Lasky et al., the relative fraction is ~0.3%; correspondingly we estimate a GW detection rate from stable PMNSs of ~ (0.1-1) yr$^{-1}$ with Advanced detectors, and of ~ (100-1000) yr$^{-1}$ with third generation detectors such as the Einstein Telescope. Measurement of such GW signal would provide strong constraints on the NS EOS and on the nature of the binary progenitors giving rise to SGRBs.

astro-ph.HE↗

arXiv2014

Fast variability from black-hole binaries

Currently available information on fast variability of the X-ray emission from accreting collapsed objects constitutes a complex phenomenology which is difficult to interpret. We review the current observational standpoint for black-hole binaries and survey models that have been proposed to interpret it. Despite the complex structure of the accretion flow, key observational diagnostics have been identified which can provide direct access to the dynamics of matter motions in the close vicinity of black holes and thus to the some of fundamental properties of curved spacetimes, where strong-field general relativistic effects can be observed.

astro-ph.HE↗

arXiv2013

XIPE: the X-ray Imaging Polarimetry Explorer

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017 but not selected. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus and two additional GPDs filled with pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 % at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil).

astro-ph.HE↗

arXiv2013

A variable absorption feature in the X-ray spectrum of a magnetar

Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly rotating, isolated neutron stars that sporadically undergo episodes of long-term flux enhancement (outbursts) generally accompanied by the emission of short bursts of hard X-rays. This behaviour can be understood in the magnetar model, according to which these sources are mainly powered by their own magnetic energy. This is supported by the fact that the magnetic fields inferred from several observed properties of AXPs and SGRs are greater than - or at the high end of the range of - those of radio pulsars. In the peculiar case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing parameters, whereas a strong field has been proposed to reside in the stellar interior and in multipole components on the surface. Here we show that the X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which depend strongly on the star's rotational phase. This line is interpreted as a proton cyclotron feature and its energy implies a magnetic field ranging from 2E14 gauss to more than 1E15 gauss.

astro-ph.HE↗

arXiv2012

Generation of strong magnetic fields by r-modes in millisecond accreting neutron stars: induced deformations and gravitational wave emission

Differential rotation induced by the r-mode instability can generate very strong toroidal fields in the core of accreting, millisecond spinning neutron stars. We introduce explicitly the magnetic damping term in the evolution equations of the r-modes and solve them numerically in the Newtonian limit, to follow the development and growth of the internal magnetic field. We show that the strength of the latter can reach large values, $B \sim 10^{14} $ G, in the core of the fastest accreting neutron stars. This is strong enough to induce a significant quadrupole moment of the neutron star mass distribution, corresponding to an ellipticity $|ε_B}| \sim 10^{-8}$. If the symmetry axis of the induced magnetic field is not aligned with the spin axis, the neutron star radiates gravitational waves. We suggest that this mechanism may explain the upper limit of the spin frequencies observed in accreting neutron stars in Low Mass X-Ray Binaries. We discuss the relevance of our results for the search of gravitational waves.

astro-ph.HE↗

arXiv2011

Emission geometry, radiation pattern, and magnetic topology of the magnetar XTE J1810-197 in its quiescent state

The return to the quiescent state of the Anomalous X-ray pulsar XTE J1810-197 following its 2003 outburst represents a unique opportunity to probe the surface emission properties of a magnetar. The quiescent emission of XTE J1810-197 is composed of two thermal components, one arising from the whole star surface, and the other from a small warm spot on it. By modeling the magnitude and shape of the pulse profile in narrow spectral bands, we have been able to constrain the physical characteristics and geometrical parameters of the system: the two angles that the line of sight and the spin axis make with respect to the warm spot axis (ψ and ξ respectively), the angular size of the spot, and the overall surface temperature distribution. Our modeling accounts for the general relativistic effects of gravitational redshift and light bending near the stellar surface, and allows for local anisotropic emission. We found that the surface temperature distribution on the neutron star is consistent with the expectations of a dipole magnetic field configuration; the local radiation requires a pencil-beamed emission pattern, suggesting the presence of a magnetized atmosphere. For a typical value of the radius, R=13 km, the viewing parameters (symmetric for an interchange between ψ and ξ), range from ψ=ξ=38 deg to (ψ,ξ)=(52 deg, 29 deg). These angles are consistent with those obtained by modeling the AXP in outburst, with uncertainty contours reduced by a factor of 2.5.

astro-ph.HE↗

arXiv2010

Stochastic background of gravitational waves emitted by magnetars

Two classes of high energy sources in our galaxy are believed to host magnetars, neutron stars whose emission results from the dissipation of their magnetic field. The extremely high magnetic field of magnetars distorts their shape, and causes the emission of a conspicuous gravitational waves signal if rotation is fast and takes place around a different axis than the symmetry axis of the magnetic distortion. Based on a numerical model of the cosmic star formation history, we derive the cosmological background of gravitational waves produced by magnetars, when they are very young and fast spinning. We adopt different models for the configuration and strength of the internal magnetic field (which determines the distortion) as well as different values of the external dipole field strength (which governs the spin evolution of magnetars over a wide range of parameters). We find that the expected gravitational wave background differs considerably from one model to another. The strongest signals are generated for magnetars with very intense toroidal internal fields ($\sim 10^{16}$ G range) and external dipole fields of $\sim 10^{14}$, as envisaged in models aimed at explaining the properties of the Dec 2004 giant flare from SGR 1806-20. Such signals should be easily detectable with third generation ground based interferometers such as the Einstein Telescope.

astro-ph.CO↗

arXiv2010

GRB Afterglows with Energy Injection from a spinning down NS

We investigate a model for the shallow decay phases of Gamma-ray Burst (GRB) afterglows discovered by Swift/XRT in the first hours following a GRB event. In the context of the fireball scenario, we consider the possibility that long-lived energy injection from a millisecond spinning, ultramagnetic neutron star (magnetar) powers afterglow emission during this phase. We consider the energy evolution in a relativistic shock subject to both radiative losses and energy injection from a spinning down magnetar in spherical symmetry. We model the energy injection term through magnetic dipole losses and discuss an approximate treatment for the dynamical evolution of the blastwave. We obtain an analytic solution for the energy evolution in the shock and associated lightcurves. To fully illustrate the potential of our solution we calculate lightcurves for a few selected X-ray afterglows observed by Swift and fit them using our theoretical lightcurves. Our solution naturally describes in a single picture the properties of the shallow decay phase and the transition to the so-called normal decay phase. In particular, we obtain remarkably good fits to X-ray afterglows for plausible parameters of the magnetar. Even though approximate, our treatment provides a step forward with respect to previously adopted approximations and provides additional support to the idea that a millisecond spinning (1-3 ms), ultramagnetic (B$\sim 10^{14}-10^{15}$ G) neutron star loosing spin energy through magnetic dipole radiation can explain the luminosity, durations and shapes of X-ray GRB afterglows.

astro-ph.HE↗

arXiv2010

Low-frequency oscillations in XTE J1550-564

We present the results of timing analysis of the low-frequency Quasi-Periodic Oscillation (QPO) in the Rossi X-Ray Timing Explorer data of the black hole binary XTE J1550--564 during its 1998 outburst. The QPO frequency is observed to vary on timescales between $\sim$100 s and days, correlated with the count rate contribution from the optically thick accretion disk: we studied this correlation and discuss its influence on the QPO width. In all observations, the quality factors ($ν_0$/FWHM) of the fundamental and second harmonic peaks were observed to be consistent, suggesting that the quasi-periodic nature of the oscillation is due to frequency modulation. In addition to the QPO and its harmonic peaks, a new 1.5$ν$ component was detected in the power spectra. This component is broad, with a quality factor of $\sim$0.6. From this, we argue what the peak observed at half the QPO frequency, usually referred to as "sub-harmonic" could be the fundamental frequency, leading to the sequence 1:2:3:4. We also studied the energy dependence of the timing features and conclude that the two continuum components observed in the power spectrum, although both more intense at high energies, show a different dependence on energy. At low energies, the lowest-frequency component dominates, while at high energies the higher-frequency one has a higher fractional rms. An interplay between these two components was also observed as a function of their characteristic frequency. In this source, the transition between low/hard state and hard-intermediate state appears to be a smooth process.

astro-ph.HE↗

arXiv2009

The first outburst of the new magnetar candidate SGR 0501+4516

We report here on the outburst onset and evolution of the new Soft Gamma Repeater SGR 0501+4516. We monitored the new SGR with XMM-Newton starting on 2008 August 23, one day after the source became burst-active, and continuing with 4 more observations, with the last one on 2008 September 30. Combining the data with the Swift-XRT and Suzaku data, we modelled the outburst decay over 160 days, and we found that the source flux decreased exponentially with a timescale of t_c=23.8 days. In the first XMM-Newton observation a large number of short X-ray bursts were observed, the rate of which decayed drastically in the following observations. We found large changes in the spectral and timing behavior of the source during the outburst, with softening emission as the flux decayed, and the non-thermal soft X-ray spectral component fading faster than the thermal one. Almost simultaneously to our XMM-Newton observations (on 2008 August 29 and September 2), we observed the source in the hard X-ray range with INTEGRAL, which clearly detected the source up to ~100keV in the first pointing, while giving only upper limits during the second pointing, discovering a variable hard X-ray component fading in less than 10 days after the bursting activation. We performed a phase-coherent X-ray timing analysis over about 160 days starting with the burst activation and found evidence of a strong second derivative period component (\ddot{P} = -1.6(4)x10^{-19} s/s^{-2}). Thanks to the phase-connection, we were able to study the the phase-resolved spectral evolution of SGR 0501+4516 in great detail. We also report on the ROSAT quiescent source data, taken back in 1992 when the source exhibits a flux ~80 times lower than that measured during the outburst, and a rather soft, thermal spectrum.

astro-ph.HE↗

arXiv2008

How rapidly do neutron stars spin at birth?

We have studied the X-ray properties of ageing historical core-collapse supernovae in nearby galaxies, using archival data from Chandra, XMM-Newton and Swift. We found possible evidence of a young X-ray pulsar in SN 1968D and in few other sources, but none more luminous than ~ a few 10^{37} erg/s. We compared the observational limits to the X-ray pulsar luminosity distribution with the results of Monte Carlo simulations for a range of birth parameters. We conclude that a pulsar population dominated by periods <~ 40 ms at birth is ruled out by the data.

astro-ph↗

arXiv2008

Short Gamma-Ray Bursts and Gravitational Waves from Dynamically Formed Merging Binaries

Merging binary systems consisting of two collapsed objects are among the most promising sources of high frequency gravitational wave, GW, signals for ground based interferometers. Double neutron star or black hole/neutron star mergers are also believed to give rise to short hard bursts, SHBs, a subclass of gamma ray bursts. SHBs might thus provide a powerful way to infer the merger rate of two-collapsed object binaries. Under the hypothesis that most SHBs originate from double neutron star or black hole/neutron star mergers, we outline here a method to estimate the incidence of merging events from dynamically formed binaries in globular clusters and infer the corresponding GW event rate that can be detected with Advanced LIGO/Virgo. In particular a sizeable fraction of detectable GW events is expected to be coincident with SHBs. The beaming and redshift distribution of SHBs are reassessed and their luminosity function constrained by using the results from recent SHBs observations. We confirm that a substantial fraction of SHBs goes off at low redshifts, where the merging of systems formed in globular clusters through dynamical interactions is expected.

astro-ph↗

arXiv2008

XMM-Newton observations of IGR J00291+5934: signs of a thermal spectral component during quiescence

We present X-ray observations of the transient accretion-powered millisecond pulsar IGR J00291+5934 during quiescence. IGR J00291+5934 is the first source among accretion powered millisecond pulsars to show signs of a thermal component in its quiescent spectrum. Fitting this component with a neutron star atmosphere or a black body model we obtain soft temperatures (~64 eV and ~110 eV, respectively). As in other sources of this class a hard spectral component is also present, comprising more than 60% of the unabsorbed 0.5-10 keV flux. Interpreting the soft component as cooling emission from the neutron star, we can conclude that the compact object can be spun up to milliscond periods by accreting only <0.2 solar masses.

astro-ph↗

arXiv2008

The general relativistic Poynting-Robertson effect

The general relativistic version is developed for Robertson's discussion of the Poynting-Robertson effect that he based on special relativity and Newtonian gravity for point radiation sources like stars. The general relativistic model uses a test radiation field of photons in outward radial motion with zero angular momentum in the equatorial plane of the exterior Schwarzschild or Kerr spacetime.

gr-qc↗

arXiv2008

Swift observations of SAX J1808.4-3658: monitoring the return to quiescence

The transient accreting millisecond pulsar SAX J1808.4-3658 has shown several outbursts to date but the transition from outburst to quiescence has never been investigated in detail. Thanks to the Swift observing flexibility, we monitored for the first time the decay to quiescence during the 2005 outburst. At variance with other transients, wide luminosity variations are observed. In addition, close to quiescence, SAX J1808.4-3658 seems to switch between two different states. We interpret them in terms of the accretion states accessible to a magnetized, fast rotating neutron star.

astro-ph↗

arXiv2007

How rapidly do neutron stars spin at birth? Constraints from archival X-ray observations of extragalactic supernovae

Traditionally, studies aimed at inferring the distribution of birth periods of neutron stars are based on radio surveys. Here we propose an independent method to constrain the pulsar spin periods at birth based on their X-ray luminosities. In particular, the observed luminosity distribution of supernovae poses a constraint on the initial rotational energy of the embedded pulsars, via the L_X-dot{E}_{rot} correlation found for radio pulsars, and under the assumption that this relation continues to hold beyond the observed range. We have extracted X-ray luminosities (or limits) for a large sample of historical SNe observed with Chandra, XMM and Swift, that have been firmly classified as core-collapse supernovae. We have then compared these observational limits with the results of Monte Carlo simulations of the pulsar X-ray luminosity distribution, for a range of values of the birth parameters. We find that a pulsar population dominated by millisecond periods at birth is ruled out by the data.

astro-ph↗

arXiv2007

Spectral modelling of the high energy emission of the magnetar 4U 0142+614

We present an empirical spectral modelling of the high energy emission of the anomalous X-ray pulsar 4U 0142+614, based on simultaneous Swift and INTEGRAL observations from X to gamma-ray energies. We adopted models contained in the XSPEC analysis package, as well as models based on recent theoretical studies, and restricted ourselves to those combinations of up to three components which produce a good fit while requiring the lowest number of free parameters. Only three models were found to fit satisfactorily the 0.5-250keV spectrum of 4U 0142+614: i) a ~0.4keV blackbody and two power-laws, ii) a resonant cyclotron scattering model plus a power-law and iii) two log-parabolic functions. We found that only the latter two models do not over-predict the infrared/optical emission observed simultaneously from this AXP, and only the log-parabolic functions can naturally account for the upper limits set by COMPTEL in the gamma-ray range. A possible interpretation of the two log-parabolae in terms of inverse Compton scattering of soft X-ray photons by very energetic particles is discussed.

astro-ph↗

arXiv2007

Newborn Magnetars as sources of Gravitational Radiation: constraints from High Energy observations of Magnetar Candidates

Soft Gamma Repeaters and the Anomalous X-ray Pulsars are believed to contain slowly spinning "magnetars". The enormous energy liberated in the 2004 Dece 27 giant flare from SGR 1806-20, together with the likely recurrence time of such events, points to an internal magnetic field strength ~ 10^{16} G. Such strong fields are expected to be generated by a coherent alpha-Omega dynamo in the early seconds after the Neutron Star formation, if its spin period is of a few milliseconds at most. A substantial deformation of the NS is caused by such fields and a newborn millisecond-spinning magnetar would thus radiate for a few days a strong gravitational wave signal. Such a signal may be detected with Advanced LIGO-class detectors up to the distance of the Virgo cluster, where ~ 1 magnetar per year are expected to form. Recent X-ray observations reveal that SNRs around magnetar candidates do not show evidence for a larger energy content than standard SNRs (Vink & Kuiper 2006). This is at variance with what would be expected if the spin energy of the young, millisecond NS were radiated away as electromagnetic radiation andd/or relativistic particle winds and, thus, transferred quickly to the expanding gas shell. We show here that these recent findings can be reconciled with the idea of magnetars being formed with fast spins, if most of their initial spin energy is radiated thorugh GWs. In particular, we find that this occurs for essentially the same parameter range that would make such objects detectable by Advanced LIGO-class detectors up to the Virgo Cluster.

astro-ph↗