Showing papers by "Istituto Universitario Di Studi Superiori Di Pavia published in 2008"

An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays.

Abstract: Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

Experimental Verification of Viscous Damping Modeling for Inelastic Time History Analyzes

Abstract: Inelastic time history analyzes (ITHA) are recognized as the most suitable tool for describing the behavior of structures under seismic actions. It is clear that the reliability of ITHA results is strongly dependent on the modeling choices. One of the aspects still debated, in spite of a large amount of research, is the modeling of viscous damping forces. In the case of a single degree of freedom (SDOF) structure, the damping not represented by hysteretic response is commonly assumed proportional to the square root of the system stiffness: typically in the research papers it is not declared whether the initial or tangent-stiffness is considered. Recent research has shown that initial or tangent-stiffness proportional damping can give very different results in terms of energy dissipation, and peak response displacement. In this work, experimental and numerical tests performed to detect the most appropriate model are described and discussed.

The 2008 May burst activation of SGR 1627-41

Abstract: In 2008 May, the soft gamma-ray repeater (SGR) SGR 1627-41 resumed its bursting activity after nearly a decade of quiescence. After detection of a bright burst, Swift pointed its X-ray telescope in the direction of the source in less than five hours and followed it for over five weeks. In this Letter, we present an analysis of the data from these Swift observations and an XMM-Newton one performed when SGR 1627-41 was still in a quiescent state. The analysis of the bursts detected with Swift/Burst Alert Telescope shows that their temporal and spectral properties are similar to those found in previous observations of SGR 1627-41 and other SGRs. The maximum peak luminosity of the bursts was similar to 2 x 10(41) erg s(-1). Our data show that the outburst was accompanied by a fast flux enhancement and by a hardening of the spectrum with respect to the persistent emission.

Deep Infrared Observations of the Puzzling Central X-Ray Source in RCW 103* **

Abstract: The object 1E 161348–5055 (1E 1613) is a pointlike, soft X-ray source shining at the center of the 2000 yr old supernova remnant (SNR) RCW 103. It features a puzzling 6.67 hr periodicity and dramatic variability over a timescale of a few years. This, coupled with a young age and the lack of an obvious optical counterpart, makes 1E 1613 a unique source among all compact objects associated with SNRs. It could either be the first low-mass X-ray binary system discovered inside a SNR or a peculiar isolated magnetar with an extremely slow spin period. Analysis of archival VLT ISAAC and HST NICMOS infrared observations unveils a very crowded field. A few sources are positionally consistent with the refined X-ray error region that we derived from the analysis of 13 Chandra observations. To shed light on the nature of 1E 1613, we have performed deep IR observations of the field with the NACO instrument at the VLT, searching for variability. None of the candidates show clear modulation at 6.67 hr or have significant long-term variability. Moreover, none of the candidates stand out for peculiar colors with respect to the bulk of the field sources. We find no compelling reasons to associate any of the candidates with 1E 1613. On one hand, it is very unlikely that one of the candidates is a low-mass companion star to 1E 1613. On the other hand, if the X-ray source is an isolated magnetar surrounded by a fallback disk, we cannot exclude that the IR counterpart is hidden among the candidates. If none of the potential counterparts are linked to the X-ray source, 1E 1613 will remain undetected in the IR down to Ks > 22.1, which will make its interpretation as an accreting binary system rather problematic.

Quantum Vacuum Friction in highly magnetized neutron stars

Abstract: In this letter we calculate the energy loss of a highly magnetized neutron star due to Quantum Vacuum Friction (QVF). Taking into account one-loop corrections in the effective Heisenberg-Euler Lagrangian of the light-light interaction, we derive an analytic expression for QVF allowing us to take into account a magnetic field at the surface of the star as high as 1011 T. In the case of magnetars, with magnetic fields above the QED critical field, we show that the QVF is the dominating energy loss process. This has important consequences, in particular for the inferred value of the magnetic field. This also indicates the need for independent measurements of magnetic field, energy loss rate, and the braking index in order to fully characterize magnetars.

Traffic, crowds, and swarms

Abstract: The interest in complex systems, namely systems of several individuals interacting in a nonlinear manner, has seen, in recent years, a remarkable increase. Their collective behavior is difficult to be understood and the model is based only on the description of the dynamics of a few individual entities. This interest is due to a raising awareness that many systems in nature are of this kind and that cannot be successfully modelled by traditional methods used for systems of the inert matter. Moreover, an increasing number of applications in technology, economy, and social sciences resemble such systems, given the high number of elements and the complex interactions among them. See for instance in fields such as transportation and communication networks, or social and economics interchanges. This special issue is devoted to modelling and simulation of the dynamics of crowds, swarms, and vehicular traffic on roads and networks of roads. The contributions are focused on the study of complex systems, where the overall dynamics is determined by individual interactions, while modelling of individual dynamics does not straightforwardly lead to the mathematical description of the collective dynamics. As known, different representations and related mathematical modelling approaches can be developed corresponding to the microscopic and macroscopic scales. Moreover, the statistical representation over the microscopic state delivered by suitable generalizations of the mathematical kinetic theory or by master equation methods for multi-agent systems are often used. It is also well understood that none of the above representations is fully appropriate considering that the number of interacting entities in crowds, swarms, and vehicular traffic is not large enough to justify either the continuum mechanics approximation, namely at the macroscopic scale, or the statistical representation

Psr j0737-3039 : interacting pulsars in x-rays

Abstract: We present the results of a ~230 ks long X-ray observation of the relativistic double-pulsar system PSR J0737–3039 obtained with the XMM-Newton satellite in 2006 October. We confirm the detection in X-rays of pulsed emission from PSR J0737–3039A (PSR A), mostly ascribed to a soft nonthermal power-law component (Γ ~ 3.3) with a 0.2-3 keV luminosity of ~1.9 × 1030 erg s−1 (assuming a distance of 500 pc). For the first time, pulsed X-ray emission from PSR J0737–3039B (PSR B) is also detected in part of the orbit. This emission, consistent with thermal radiation with temperature kBT 30 eV and a bolometric luminosity of ~1032 erg s−1, is likely powered by heating of PSR B's surface caused by PSR A's wind. A hotter (~130 eV) and fainter (~5 × 1029 erg s−1) thermal component, probably originating from backfalling particles heating polar caps of either PSR A or PSR B, is also required by the data. No signs of X-ray emission from a bow shock between PSR A's wind and the interstellar medium or PSR B's magnetosphere are present. The upper limit on the luminosity of such a shock component (~1029 erg s−1) constrains the wind magnetization parameter σM of PSR A to values greater than 1.

The probability distribution of returns in the exponential Ornstein–Uhlenbeck model

Abstract: We analyze the problem of the analytical characterization of the probability distribution of financial returns in the exponential Ornstein–Uhlenbeck model with stochastic volatility. In this model the prices are driven by a geometric Brownian motion, whose diffusion coefficient is expressed through an exponential function of an hidden variable Y governed by a mean-reverting process. We derive closed-form expressions for the probability distribution and its characteristic function in two limit cases. In the first one the fluctuations of Y are larger than the volatility normal level, while the second one corresponds to the assumption of a small stationary value for the variance of Y. Theoretical results are tested numerically by intensive use of Monte Carlo simulations. The effectiveness of the analytical predictions is checked via a careful analysis of the parameters involved in the numerical implementation of the Euler–Maruyama scheme and is tested on a data set of financial indexes. In particular, we discuss results for the German DAX30 and Dow Jones Euro Stoxx 50, finding a good agreement between the empirical data and the theoretical description.

Chandra astrometry sets a tight upper limit to the proper motion of SGR 1900+14

Abstract: The soft gamma-ray repeater (SGR) SGR 1900+14 lies a few arcminutes outside the edge of the shell supernova remnant (SNR) G42.8+0.6. A physical association between the two systems has been proposed - for this and other SGR-SNR pairs - based on the expectation of high space velocities for SGRs in the framework of the magnetar model. The large angular separation between the SGR and the SNR center, coupled with the young age of the system, suggest a test of the association with a proper motion measurement. We used a set of three Chandra/ACIS observations of the field spanning 5 years to perform accurate relative astrometry in order to measure the possible angular displacement of the SGR as a function of time. Our investigation sets a 3-sigma upper limit of 70 mas/yr to the overall proper motion of the SGR. Such a value argues against an association of SGR 1900+14 with G42.8+0.6 and adds further support to the mounting evidence for an origin of the SGR within a nearby, compact cluster of massive stars.

Unveiling the nature of RX J0002+6246 with XMM–Newton★

Abstract: The X-ray source RXJ0002+6246 was discovered close to the supernova remnant CTB1 in aROSAT observation performed in 1992. The source phenomenology (soft spectrum, apparentlack of counterparts, possible pulsations at 242 ms, hints for surrounding diffuse emission)led to interpret it as an isolated neutron star in a new supernova remnant. We have analysedan archival XMM-Newtonobservation performed in 2001. The source coordinates, as com-puted on the XMM-Newtonimages, coincide with those of a bright source listed in opticaland infrared catalogues. The X-ray spectrum is well described by an optically thin plasmamodel. No fast pulsations are seen, nor clear evidence of a supernova remnant associated tothe source. Thus, we conclude that RXJ0002+6246 is not an isolated neutron star, but theX-ray counterpart of the bright optical/infrared source, most likely a F7 spectral class starlocated at about 0.2kpc.Key words: X-rays: individual (RXJ0002+6246, PSRJ0002+6246) – X-rays: stars – ISM:individual (G117.7+0.6) – supernova remnants.

Gamma Rays and Neutron Stars

Abstract: Satellite and ground observations provide new insights into gamma-ray emissions from neutron stars.

XMM-Newton discovery of 2.6 s pulsations in the soft gamma-ray repeater SGR 1627-41

Abstract: After nearly a decade of quiescence, the soft gamma-ray repeater SGR 1627-41 reactivated on 2008 May 28 with a bursting episode followed by a slowly decaying enhancement of its persistent emission. To search for the still unknown spin period of this SGR taking advantage of its high flux state, we performed on 2008 September 27-28 a 120 ks long X-ray observation with the XMM-Newton satellite. Pulsations with P = 2.594578(6) s were detected at a higher than 6-sigma confidence level, with a double-peaked pulse profile. The pulsed fraction in the 2-12 keV range is 19% +/- 3% and 24% +/- 3% for the fundamental and the second harmonic, respectively. The observed 2-10 keV flux is 3.4E-13 erg/cm^2/s, still a factor of ~ 5 above the quiescent pre-burst-activation level, and the spectrum is well fitted by an absorbed power law plus blackbody model (photon index Gamma = 0.6, blackbody temperature kT = 0.5 keV, and absorption nH = 1.2E+23 cm^-2). We also detected a shell of diffuse soft X-ray emission which is likely associated with the young supernova remnant G337.0-0.1.