Showing papers by "Luigi Stella published in 2014"
TL;DR: In this article, the authors present a systematic analysis of the fast time variability properties of the transient black hole binary GRO J1655-40, based on the complete set of RossiXTE observations.
Abstract: We present a systematic analysis of the fast time variability properties of the transient black hole binary GRO J1655-40, based on the complete set of RossiXTE observations. We demonstrate that the frequencies of the quasi-periodic oscillations and of the broad band noise components and their variations match accurately the strong field general relativistic frequencies of particle motion in the close vicinity of the innermost stable circular orbit, as predicted by the relativistic precession model. We obtain high precision measurements of the black hole mass (M = (5.31 +/- 0.07) M solar masses, consistent with the value from optical/NIR observations) and spin (a/M = 0.290 +/- 0.003), through the sole use of X-ray timing.
190 citations
TL;DR: In this article, the relativistic precession model was used to determine the spin of the BH binary XTE J1550-564 obtained by the Rossi X-ray Timing Explorer.
Abstract: We present a systematic analysis of the complete set of observations of the black hole (BH) binary XTE J1550-564 obtained by the Rossi X-ray Timing Explorer. We study the fast time variability properties of the source and determine the spin of the black hole through the relativistic precession model. Similarly to what is observed in the BH binary GRO J1655-40, the frequencies of the QPOs and broad band noise components match the general relativistic frequencies of particle motion close to the compact object predicted by the relativistic precession model. The combination of two simultaneously observed quasi-periodic oscillation (QPO) frequencies together with the dynamical BH mass from optical/infrared observations yields a spin equal to a = 0.34 +/- 0.01, consistent with previous determinations from X-ray spectroscopy. Based on the derived BH parameters, the low frequency QPO emission radii vary from about 30 gravitational radii to the innermost stable orbit for this spin (about 5 gravitational radii), where they sharply disappear as observed for the case of GRO J1655-40.
90 citations
TL;DR: In this paper, the authors studied the magnetic ellipticity of a twisted torus magnetic configuration for binary neutron star (NS) mergers and derived the expected GW signal as a function of the relative magnitude of the dipolar and toroidal field components.
Abstract: 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, the post-merger phase can be characterized by a prompt collapse to a black hole, or by the formation of a supramassive NS, or even a stable NS. In the latter cases of post-merger NS (PMNS) formation, magnetic field amplification during the merger will produce a magnetar and induce a mass quadrupole moment in the newly formed NS. If the timescale for orthogonalization of the magnetic symmetry axis with the spin axis is smaller than the spindown time, the NS will radiate its spin down energy primarily via GWs. Here we study this scenario for the various outcomes of NS formation: we generalize the set of equilibrium states for a twisted torus magnetic configuration to include solutions that, for the same external dipolar field, carry a larger magnetic energy reservoir; we hence compute the magnetic ellipticity for such configurations, and the corresponding strength of the expected GW signal as a function of the relative magnitude of the dipolar and toroidal field components. The relative number of GW detections from PMNSs and from binary NSs is a very 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 detectors of third generation such as the Einstein Telescope. Measurement of such GW signal would provide constraints on the NS equation of state and, in connection with a SGRB, on the nature of the binary progenitors giving rise to these events. Subject headings:
83 citations
INAF1
TL;DR: In this paper, the authors review the current observational standpoint for black-hole binaries and survey models that have been proposed to interpret it, and identify key observational diagnostics which can provide direct access to the dynamics of matter motions in close vicinity of black holes and thus to the fundamental properties of curved spacetimes, where strong-field general relativistic effects can be observed.
Abstract: 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.
74 citations
TL;DR: In this article, the relativistic precession model was used to test general relativity against those alternative theories of gravity which predict deviations from the classical theory in the strong-field regime, and showed that detection of quasi-periodic oscillations with the expected sensitivity of the proposed ESA M-class mission LOFT would set the most stringent constraints on the parameter space of this theory.
Abstract: Quasi-Periodic Oscillations (QPOs) observed in the X-ray flux emitted by accreting black holes, are associated to phenomena occurring near the horizon. Future very large area X-ray instruments will be able to measure QPO frequencies with very high precision, thus probing this strong-field region. By using the relativistic precession model, we show the way in which QPO frequencies could be used to test general relativity against those alternative theories of gravity which predict deviations from the classical theory in the strong-field regime. We consider one of the best motivated strong-curvature corrections to general relativity, namely the Einstein-Dilaton-Gauss-Bonnet theory, and show that a detection of QPOs with the expected sensitivity of the proposed ESA M-class mission LOFT would set the most stringent constraints on the parameter space of this theory.
70 citations
TL;DR: In this article, the authors focus on the return to quiescence following the 2010 outburst of the source Aql X-1, which was monitored thanks to 11 pointed observations taken with XMM-Newton, Chandra and Swift.
Abstract: Aql X-1 is the most prolific low-mass X-ray binary transient hosting a neutron star. In this paper we focus on the return to quiescence following the 2010 outburst of the source. This decay was monitored thanks to 11 pointed observations taken with XMM-Newton, Chandra and Swift. The decay from outburst to quiescence is very fast, with an exponential decay characteristic time-scale of ∼2 d. Once in quiescence the X-ray flux of Aql X-1 remained constant, with no further signs of variability or decay. The comparison with the only other well-monitored outburst from Aql X-1 (1997) is tale-telling. The luminosities at which the fast decay starts are fully compatible for the two outbursts, hinting at a mechanism intrinsic to the system and possibly related to the neutron star rotation and magnetic field (i.e. the propeller effect). In addition, for both outbursts, the decay profiles are also very similar, likely resulting from the shut-off of the accretion process on to the neutron star surface. Finally, the quiescent neutron star temperatures at the end of the outbursts are well consistent with one another, suggesting a hot neutron star core dominating the thermal balance. Small differences in the quiescent X-ray luminosity among the two outbursts can be attributed to a different level of the power-law component.
61 citations
TL;DR: In this article, a detailed analysis of the XMM-Newton data and focus on the timing and spectral variability of the source was performed, where the authors interpreted the variability observed from IGR(J18245-2452 in terms of a hiccup accretion phase, during which the accretion of material from the inner boundary of the Keplerian disk is reduced by the onset of centrifugal inhibition of accretion, possibly causing the launch of outflows.
Abstract: The source IGR J18245-2452 is the fifteenth discovered accreting millisecond X-ray pulsar and the first neutron star to show direct evidence of a transition between accretion- and rotation-powered emission states. These swings provided the strongest confirmation to date of the pulsar recycling scenario. During the two XMM-Newton observations that were carried out while the source was in outburst in April 2013, IGR J18245-2452 displayed a unique and peculiar X-ray variability. In this work, we report on a detailed analysis of the XMM-Newton data and focus on the timing and spectral variability of the source. In the 0.4-11 keV energy band, IGR J18245-2452 continuously switched between lower and higher intensity states, with typical variations in flux by factor of ~100 on time scales as short as a few seconds. These variations in the source intensity were sometimes accompanied by dramatic spectral hardening, during which the X-ray power-law photon index varied from Γ = 1.7 to Γ = 0.9. The pulse profiles extracted at different count-rates, hardnesses, and energies also showed a complex variability. These phenomena were never observed in accreting millisecond X-ray pulsars, at least not on such a short time-scale. Fast variability was also found in the 5.5 and 9 GHz ATCA radio observations that were carried out for about 6 h during the outburst. We interpret the variability observed from IGR J18245-2452 in terms of a hiccup accretion phase, during which the accretion of material from the inner boundary of the Keplerian disk is reduced by the onset of centrifugal inhibition of accretion, possibly causing the launch of outflows. Changes across accretion and propeller regimes have been long predicted and reproduced by magnetohydrodynamic simulations of accreting millisecond X-ray pulsars, but have never observed to produce as extreme a variability as that shown by IGR J18245-2452.
58 citations
TL;DR: The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final downselection for a launch slot in 2022-2024 as mentioned in this paper.
Abstract: The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final downselection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supranuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m2 effective area, 2-30 keV, 240 eV spectral resolution, 1° collimated field of view) and a Wide Field Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.
46 citations
TL;DR: The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024 as mentioned in this paper.
Abstract: The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.
14 citations
TL;DR: In this article, the authors studied the effect of magnetic field amplification on the strength of the expected GW signal as a function of the magnitude of the dipole and toroidal field.
Abstract: 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.
12 citations
TL;DR: In this article, the authors compared the radial profiles extracted from data images and PSF simulations, carefully considering various issues related to the uncertain calibration of the HRC PSF at sub-arcsecond scales.
Posted Content•
TL;DR: In this article, the authors compared the radial profiles extracted from data images and PSF simulations, carefully considering various issues related with the uncertain calibration of the HRC PSF at sub-arcsecond scales.
Abstract: Diffuse radio emission was detected around the soft gamma-ray repeater SGR 1806-20, after its 2004 powerful giant flare. We study the possible extended X-ray emission at small scales around SGR 1806-20, in two observations by the High Resolution Camera Spectrometer (HRC-S) on board of the Chandra X-ray Observatory: in 2005, 115 days after the giant flare, and in 2013, during quiescence. We compare the radial profiles extracted from data images and PSF simulations, carefully considering various issues related with the uncertain calibration of the HRC PSF at sub-arcsecond scales. We do not see statistically significant excesses pointing to an extended emission on scales of arcseconds. As a consequence, SGR 1806-20 is compatible with being point-like in X-rays, months after the giant flare, as well as in quiescence.
TL;DR: In this article, the signal produced by an emitting point-like specific source moving in a Schwarzschild spacetime under the influence of such a radiation field was analyzed. But the results were not compared with those for a spot moving on a circular geodesic in the Schwarzschild metric.
Abstract: The motion of matter immersed in a radiation field is affected by radiation drag, as a result of scattering or absorption and re-emission. The resulting friction-like drag, also known as Poynting-Robertson effect, has been recently studied in the general relativistic background of the Schwarzschild and Kerr metric, under the assumption that all photons in the radiation field possess the same angular momentum. We calculate here the signal produced by an emitting point-like specific source moving in a Schwarzschild spacetime under the influence of such a radiation field. We derive the flux, redshift factor and solid angle of the hot spot as a function of (coordinate) time, as well as the time-integrated image of the hot spot as seen by an observer at infinity. The results are then compared with those for a spot moving on a circular geodesic in a Schwarzschild metric.
TL;DR: In this paper, an absorption feature, the properties of which strongly depend on the pulse phase, has been recently discovered in the X-ray spectrum of the soft gamma repeater SGR0418+5729 and its energy implies a magnetic field ranging from 2× 1014 G to more than 1015 G.
Abstract: An absorption feature, the properties of which strongly depend on the pulse phase, has been recently discovered in the X-ray spectrum of the soft gamma repeater SGR0418+5729. If interpreted as a proton cyclotron line, its energy implies a magnetic field ranging from 2× 1014 G to more than 1015 G, which confirms the magnetar interpretation for this source and provides us with the most direct measurement of the magnetic field intensity of an isolated neutron star. The lower value of the dipole field inferred from the timing parameters for SGR 0418+5729 (B = 6× 1012 G) requires that the high magnetic field responsible for the observed feature resides in a strong multi-polar component located close to the neutron star surface, in agreement with the predictions of the magnetar model. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, the motion of a test particle in a stationary, axially and reflection symmetric spacetime of a central compact object, as affected by interaction with a test radiation field of the same symmetries, was studied.
Abstract: We study the motion of a test particle in a stationary, axially and reflection symmetric spacetime of a central compact object, as affected by interaction with a test radiation field of the same symmetries. Considering the radiation flux with fixed but arbitrary (non-zero) angular momentum, we extend previous results limited to an equatorial motion within a zero-angular-momentum photon flux in the Kerr and Schwarzschild backgrounds. While a unique equilibrium circular orbit exists if the photon flux has zero angular momentum, multiple such orbits appear if the photon angular momentum is sufficiently high.
TL;DR: The Large Observatory for X-ray Timing (LOFT) is one of five ESA M3 candidate missions as mentioned in this paper, which is designed to determine the properties of ultradense matter by reconstructing its Equation of State through neutron star mass and radius measurements.
Abstract: LOFT, the Large Observatory For X-ray Timing, is one of five ESA M3 candidate missions. It will address the Cosmic Vision theme: "Matter under Extreme Conditions". By coupling for the first time a huge collecting area for the detection of X- ray photons with CCD-quality spectral resolution (15 times bigger in area than any previously flown X-ray instrument and >100 times bigger for spectroscopy than any similar-resolution instrument), the instruments onboard LOFT have been designed to (i) determine the properties of ultradense matter by reconstructing its Equation of State through neutron star mass and radius measurements of unprecedented accuracy; (ii) measure General Relativity effects in the strong field regime in the stationary spacetimes of neutron stars and black holes of all masses down to a few gravitational radii. Besides the above two themes, LOFT's observations will be devoted to "observatory science", providing new insights in a number of research fields in high energy astrophysics (e.g. Gamma-ray Bursts). The assessment study phase of LOFT, which ended in September 2013, demonstrated that the mission is low risk and the required Technology Readiness Level can be easily reached in time for a launch by the end of 2022.
21 Mar 2014
TL;DR: The IGR J11014-6103 source as mentioned in this paper is a hard X-ray source with an elongated pulsar wind nebula formed by a neutron star escaping supersonically its parent supernova remnant SNR MSH 11-61A.
Abstract: IGR J11014-6103 is a hard X-ray source discovered by INTEGRAL. Follow-up X-ray and radio observations revealed an elongated pulsar wind nebula formed by a neutron star escaping supersonically its parent supernova remnant SNR MSH 11-61A. The pulsar also emits highly collimated jets extending perpendicularly to the direction of motion. The jet has a continuous helical structure extending up to more than 10 parsecs. IGR J11014-6103 is a laboratory to study jet ejection in the wind of a pulsar and to constrain the core collapse supernova mechanism responsible for the observed pulsar kick velocity in excess of 1000 km/s.