Standard interest profiles - Development of technical subjects Final report

NASA university program for technology transfer, and development of Standard Interest Profiles /SIP

The Broad Band Spectrum of MXB 1728-34 Observed by BeppoSAX

We report on the results of a broad band (0.1-100 keV) spectral analysis of the bursting atoll source MXB 1728-34 observed by the BeppoSAX satellite. Three bursts were present during this observation. The spectrum during the bursts can be fitted by a blackbody with a temperature of 2 keV. From the bursts we also estimate a distance to the source of 5.1 kpc. MXB 1728-34 was in a rather soft state during the BeppoSAX observation. The persistent spectrum is well fitted by a continuum consisting of a soft blackbody emission and a comptonized spectrum. We interpreted the soft component as the emission from the accretion disk. Taking into account a spectral hardening factor of 1.7, we estimated that the inner disk radius is $R_{\rm in} \sqrt{\cos i} \sim 20$ km, where i is the inclination angle. The comptonized component could originate in a spherical corona, with temperature of 10 keV and optical depth of 5, surrounding the neutron star. A broad gaussian emission line at 6.7 keV is observed in the spectrum, probably emitted in the ionized corona or in the inner part of the disk. Another emission line is present at 1.66 keV.Comment: 12 pages, accepted by Ap

Chandra Observation of the Persistent Emission from the Dipping Source XB 1916-053

We present the results of a 50 ks long Chandra observation of the dipping source XB 1916-053. During the observation two X-ray bursts occurred and the dips were not present at each orbital period. From the zero-order image we estimate the precise X-ray coordinates of the source with a 90% uncertainty of 0.6''. In this work we focus on the spectral study of discrete absorption features, during the persistent emission, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We detect, for the first time in the 1st-order spectra of XB 1916-053, absorption lines associated to Ne X, Mg XII, Si XIV, and S XVI, and confirm the presence of the Fe XXV and Fe XXVI absorption lines with a larger accuracy with respect to the previous XMM EPIC pn observation. Assuming that the line widths are due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines are produced in a photoionized absorber distant from the neutron star 4 x 10^{10} cm, near the disk edge.Comment: 20 pages, 10 figures, submitted to ApJ on 2005-09-22, accepted by ApJ on 2006-05-0

METODO INNOVATIVO PER CREARE UN LAYER NANOSTRUTTURATO HONEYCOMB DI TITANIA, AUTOGENERATO SULLE SUPERFICI DI MATERIALI A BASE TITANIO, DESTINATI AL DOMINIO MEDICALE E CHIRURGICO DEGLI IMPIANTI DENTALI, DELLE PROTESI ORTOPEDICHE E DELLE OSTEOSINTESI.

NANOTECNOLOGIA, TITANIO, PROTESI ORTOPEDICHE, OSTEOSINTESI, NANOSTRUTTUR

On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states

Most of the spectra of neutron star low mass X-ray binaries (NS LMXBs), being them persistent or transient, are characterized by the presence of a strong thermal Comptonization bump, thought to originate in the transition layer (TL) between the accretion disk and the NS surface. The observable quantities which characterize this component dominating the emission below 30 keV, are the spectral index alpha and the rollover energy, both related to the electron temperature and optical depth of the plasma. Starting from observational results on a sample of NS LMXBs in different spectral states, we formulate the problem of X-ray spectral formation in the TL of these sources. We predict a stability of the thermal Comptonization spectral index in different spectral states if the energy release in the TL is much higher than the intercepted flux coming from the accretion disk. We use an equation for the energy balance and the radiative transfer diffusion equation for a slab geometry in the TL, to derive a formula for the thermal Comptonization index alpha. We show that in this approximation the TL electron temperature kTe and optical depth tau_0 can be written as a function of the energy flux from the disk intercepted by the corona (TL) and that in the corona itself Qdisk/Qcor, in turn leading to a relation alpha=f(Qdisk/Qcor), with alpha ~ 1 when Qdisk/Qcor <<1. We show that the observed spectral index alpha for the sample of sources here considered lies in a belt around 1 +/- 0.2 a part for the case of GX 354--0. Comparing our theoretical predictions with observations, we claim that this result, which is consistent with the condition Qdisk/Qcor <<1, can give us constraints on the accretion geometry of these systems, an issue that seems difficult to be solved using only the spectral analysis method.Comment: 7 pages, 3 figures, accepted for publication in A&

Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram

We analyze a large collection of RXTE archive data from April 1997 to August 2003 of the bright X-ray source Scorpius X-1 in order to study the broadband spectral evolution of the source for different values of the inferred mass accretion rate by studying energy spectra from selected regions in the Z-track of its Color-Color Diagram. A two-component model, consisting of a soft thermal component interpreted as thermal emission from an accretion disk and a thermal Comptonization component, is unable to fit the whole 3--200 keV energy spectrum at low accretion rates. Strong residuals in the highest energy band of the spectrum require the addition of a third component that can be fitted with a power-law component, that could represent a second thermal Comptonization from a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. We discuss the physical implications derived from the results of our analysis, with a particular emphasis on the hardest part of the X-ray emission and its possible origins.Comment: 18 pages. Accepted for publication in Ap

Discovery of periodic dips in the light curve of GX 13+1: the X-ray orbital ephemeris of the source

The bright low-mass X-ray binary (LMXB) GX 13+1 is one of the most peculiar Galactic binary systems. A periodicity of 24.27 d with a formal statistical error of 0.03 d was observed in its power spectrum density obtained with RXTE All Sky Monitor (ASM) data spanning 14 years. Starting from a recent study, indicating GX 13+1 as a possible dipping source candidate, we systematically searched for periodic dips in the X-ray light curves of GX 13+1 from 1996 up to 2013 using RXTE/ASM, and MAXI data to determine for the first time the X-ray orbital ephemeris of GX 13+1. We searched for a periodic signal in the ASM and MAXI light curves, finding a common periodicity of 24.53 d. We folded the 1.3-5 keV and 5-12.1 keV ASM light curves and the 2-4 and 4-10 keV MAXI light curves at the period of 24.53 d finding a periodic dip. To refine the value of the period we used the timing technique dividing the ASM light curve in eight intervals and the MAXI light curve in two intervals, obtaining four and two dip arrival times from the ASM and MAXI light curves, respectively. We improved the X-ray position of GX 13+1 using a recent Chandra observation. The new X-ray position is discrepant by \sim 7\arcsec from the previous one, while it is compatible with the infrared and radio counterpart positions. We detected an X-ray dip, that is totally covered by the Chandra observation, in the light curve of GX 13+1 and showed, a-posteriori, that it is a periodic dip. We obtained seven dip arrival times from ASM, MAXI, and Chandra light curves. We calculated the delays of the detected dip arrival times with respect to the expected times for a 24.52 d periodicity. Fitting the delays with a linear function we find that the orbital period and the epoch of reference of GX 13+1 are 24.5274(2) days and 50,086.79(3) MJD, respectively.(Abridged)Comment: 12 pages, including 16 figures. Accepted for publication in A&

Modeling Evacuation Risk Using a Stochastic Process Formulation of Mesoscopic Dynamic Network Loading

One of the actions usually conducted to limit exposure to a hazardous event is the evacuation of the area that is subject to the effects of the event itself. This involves modifications both to demand (a large number of users all want to move together) and to supply (the transport network may experience changes in capacity, unusable roads, etc.). In order to forecast the traffic evolution in a network during an evacuation, a natural choice is to adopt an approach based on Dynamic Traffic Assignment (DTA) models. However, such models typically give a deterministic prediction of future conditions, whereas evacuations are subject to considerable uncertainty. The aim of the present paper is to describe an evacuation approach for decision support during emergencies that directly predicts the time-evolution of the probability of evacuating users from an area, formulated within a discrete-time stochastic process modelling framework. The approach is applied to a small artificial case as well as a real-life network, where we estimate users' probabilities to reach a desired safe destination and analyze time dependent risk factors in an evacuation scenario

Spectral analysis of LMC X-2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source

We present the results of the analysis of an archival observation of LMC X-2 performed with XMM/Newton. The spectra taken by high-precision instruments have never been analyzed before. We find an X-ray position for the source that is inconsistent with the one obtained by ROSAT, but in agreement with the Einstein position and that of the optical counterpart. The correlated spectral and timing behaviour of the source suggests that the source is probably in the normal branch of its X-ray color-color diagram. The spectrum of the source can be fitted with a blackbody with a temperature 1.5 keV plus a disk blackbody at 0.8 keV. Photoelectric absorption from neutral matter has an equivalent hydrogen column of 4 x 10^{20} cm^{-2}. An emission line, which we identify as the O VIII Lyman alpha line, is detected, while no feature due to iron is detected in the spectrum. We argue that the emission of this source can be straightforwardly interpreted as a sum of the emission from a boundary layer between the NS and the disc and a blackbody component coming from the disc itself. Other canonical models that are used to fit Z-sources do not give a satisfactory fit to the data. The detection of the O VIII emission line (and the lack of detection of lines in the iron region) can be due to the fact that the source lies in the Large Magellanic Cloud.Comment: Accepted for publication by Astronomy & Astrophysic

Chandra X-ray spectroscopy of a clear dip in GX 13+1

The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an accreting neutron star. It shows highly ionized absorption features, with a blueshift of $\sim$ 400 km s$^{-1}$ and an outflow-mass rate similar to the accretion rate. Many other X-ray sources exhibit warm absorption features, and they all show periodic dipping behavior at the same time. Recently, a dipping periodicity has also been determined for GX 13+1 using long-term X-ray folded light-curves, leading to a clear identification of one of such periodic dips in an archival Chandra observation. We give the first spectral characterization of the periodic dip of GX 13+1 found in this archival Chandra observation performed in 2010. We used Chandra/HETGS data (1.0-10 keV band) and contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broadband X-ray spectrum. We adopted different spectral models to describe the continuum emission and used the XSTAR-derived warm absorber component to constrain the highly ionized absorption features. The 1.0-25 keV continuum emission is consistent with a model of soft accretion-disk emission and an optically thick, harder Comptonized component. The dip event, lasting $\sim$ 450 s, is spectrally resolved with an increase in the column density of the neutral absorber, while we do not find significant variations in the column density and ionization parameter of the warm absorber with respect to the out-of-dip spectrum. We argue that the very low dipping duty-cycle with respect to other sources of the same class can be ascribed to its long orbital period and the mostly neutral bulge, that is relatively small compared with the dimensions of the outer disk radius.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and Astrophysic