1,721,030 research outputs found
Circinus X-1: An X-ray chameleon
No full textWe briefly discuss lightcurves, colour-colour diagram and spectra of the peculiar X-ray binary Circinus X-1, which periodically changes its X-ray colour. The data were obtained mainly with the Wide Field Cameras aboard BeppoSAX
THE POWER SPECTRA AS A PROBE OF SOME PHYSICAL FEATURES OF X-RAY BINARIES
No full textWe have simulated, using a Monte Carlo method, intensity time series representing the time behaviour of some high mass X ray binaries. Taking into account some physical models of the accretion processes, we have parametrized different physical conditions on the source. The resulting time series have been analysed by Fast Fourier Transform techniques. Using different sets of parameters we have been able to generate power spectra of different shapes. Some preliminary results of these simulations and a comparison with an observed spectrum of a galactic X ray binary are discussed
Implication of the thermal origin of the soft excess in Seyfert 1 galaxies
No full textWe present a study of the soft X-ray properties of a sample of 15 Seyfert 1 observed with ASCA. The spectrum of 11 objects in the sample shows a soft excess over a single power-law model. We found that the best interpretation of this excess is a thermal radiation coming from the innermost region of an accretion disc. Indeed the use of a black body model gives a good fit in 9 sources out of the 11 that show the excess. However the temperatures obtained from the best fit are higher than those expected if the emission comes from a region just outside the last stable orbit around a non-rotating black hole (Schwarzschild black hole). The temperatures obtained seem to indicate that the emission region is closer than three Schwarzschild radii, suggesting the presence of a Kerr (rotating) black hole
The two-component X-ray broadband spectrum of X Persei observed by BeppoSAX
No full textWe report temporal and broadband (0.1-200 keV) spectral analysis of the Be/X-ray binary X Persei observed by the Narrow Field Instruments (NFI) on board the BeppoSAX satellite. The source luminosity is similar to 1.2 x 10(34) ergs s(-1) in the energy range 0.1-10 keV and similar to 2.4 x 10(34) ergs s(-1) in the range 0.1-200 keV. The source shows pulsations from 0.1 keV up to 80 keV. No variations of the pulse profile with energy are visible. The barycentric pulse period is 837.376 +/- 0.026 s, in agreement with the secular spin-down observed since 1978. The 0.1-10 keV energy spectrum can be well fitted by a power law plus high-energy cutoff, in agreement with previous observations, although at higher energies a hard excess is visible. The whole energy range spectrum can be fitted by adding to the previous model another power law with a high-energy cutoff and a low-energy turnover. A possible interpretation of this two-component spectrum could be given in terms of a high-energy component produced by a nonthermal and partially Comptonized cyclotron emission, and a soft component produced by radiation partially thermalized by the atmosphere. In this model, the second high-energy cutoff (similar to 65 keV) gives an estimate of the cyclotron energy and the magnetic field strength. The value obtained, B similar to 5.6 x 10(12) G, is in agreement with the estimate of the accretion torque theory. The spectrum can also be fitted by adding to the low-energy component a high-temperature thermal bremsstrahlung
A hard tail in the broadband spectrum of the dipper XB 1254-690
No full textWe report on the results of spectral analysis of the dipping source XB 1254-690 observed by the BeppoSAX satellite. We find that the X-ray dips are not present during the BeppoSAX observation, in line with recent RXTE results. The broadband (0.1-100 keV) energy spectrum is well fitted by a three-component model consisting of a multicolor disk blackbody with an inner disk temperature of similar to0.85 keV, a Comptonized spectrum with an electron temperature of similar to2 keV, and bremsstrahlung at a temperature of similar to 20 keV. Adopting a distance of 10 kpc and taking into account a spectral hardening factor of similar to1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is R-in(cos i)(1/2) similar to 11 km, where i is the inclination angle of the system with respect to the line of sight. The Comptonized component could originate in a spherical corona or boundary layer, surrounding the neutron star, with optical depth similar to 19. The bremsstrahlung emission, contributing similar to4% of the total luminosity, probably originates in an extended accretion disk corona with radius similar to 10(10) cm. In this scenario, we calculated that the optical depth of this region is similar to0.71 and its mean electron density is N-e similar to 2.7 x 10(14) cm(-3). This last component might also be present in other low- mass X-ray binaries, but, because of its low intrinsic luminosity, it is not easily observable. We also find an absorption edge at similar to1.27 keV with an optical depth of similar to0.15. Its energy could correspond to the L edge of Fe XVII or K edge of Ne X or neutral Mg
The broadband spectrum of MXB 1728-34 observed by BeppoSAX
No full textWe report on the results of a broadband (0.1-100 keV) spectral analysis of the bursting atoll source MXB 1728-34 (4U 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 similar to2 keV. The radius of the blackbody emitting region is compatible with the radius of the neutron star if we correct for the difference between the observed color temperature and the effective temperature. From the bursts we also estimate a distance to the source of similar to5.1 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 similar to1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is R(in)root cosi similar to 20 km, where i is the inclination angle. The Comptonized component could originate in a spherical corona, with temperature similar to 10 keV and optical depth similar to5, surrounding the neutron star. A broad Gaussian emission line at similar to6.7 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 similar to1.66 keV. No reflection component is detected with high statistical significance, probably because of the low temperature of the corona in this state of the source. If the iron emission line is caused by reflection of the Comptonized spectrum by the accretion disk, it requires a ionized disk (xi similar to 280) and a solid angle of similar to 0.2 (in units of 2 pi) subtended by the reflector as seen from the corona
A Ginga observation of the X-ray pulsar 4U 0352+30
No full text4U 0352+30 (X Persei) is a low-luminosity binary X-ray pulsar with a pulse period of 835 s. We present timing and spectral analysis of a Ginga observation of X Persei from 1990 January 26 to January 29. The observation shows the peculiar spectral behavior of X Persei: the pulse-averaged hardness ratio exhibits a sharp hardening episode at phase minimum of the light curve. In order to explain the shape of the observed hardness ratio, we discuss the possible geometry of the emitting region. Simple models of fan emission from a hollow accretion column can reproduce the qualitative features of the observed light curve and hardness ratio
Harmonic coupling of the red noise in X-ray pulsars
No full textThe power spectra of X-ray pulsars often show the presence of a red-noise component. This noise is produced by aperiodic variability believed to be associated with instabilities that seem to occur in accretion hows onto compact objects. In this paper we discuss how, independently of the details of the physical processes that generate these instabilities, a careful analysis of the power spectra can furnish some constraints on the distance from the stellar surface at which the sudden energy release associated with the instabilities occurs. In particular, any aperiodic variability coming from the accretion flow funneled toward the magnetic poles should be modulated at the pulsar spin period (coupling). We show how, in the power spectra, this coupling results in a broadening at the base of the harmonics. To investigate this effect, we have adopted a mathematical description of the noise in order to produce simulated light curves and the resulting power spectra. A comparison of power spectra from simulations with real data allows the detection or exclusion of the broadening effect. As an application of this method we have compared simulated power spectra with one obtained from a Ginga observation of the X-ray pulsar SMC X-1. For this source the coupling effect is evident
Spectral evolution of circinus X-1 along its orbit
Full text linkWe report on the spectral analysis of Circinus X-1 observed by the ASCA satellite in 1998 March along one orbital period. The luminosity of the source (in the 0.1-100 keV band) ranges from 2.5 x 10(38) ergs s(-1) at the periastron (orbital phase 0.01) to 1.5 x 10(38) ergs s(-1) at orbital phase 0.3. From the spectral analysis and the light curve, we argue that Cir X-1 shows three states along the orbital evolution. The first state is at the orbital phase interval 0.97-0.3 : the luminosity becomes super-Eddington, and a strong flaring activity is present. In this state a shock could form in the inner region of the system because of the super-Eddington accretion rate, producing an outflow of ionized matter whose observational signature could be the prominent absorption edge at similar to8.7 keV observed in the energy spectrum at these phases. In the second state, corresponding to the orbital phase interval between 0.3 and 0.7, the accretion rate is sub-Eddington, and we observe a weaker outflow, with a smaller hydrogen column: the absorption edge is now at similar to8.3 keV with an optical depth a factor of 2.5-6 smaller. The third state corresponds to the orbital phase interval 0.78-0.97. In this state the best-fit model to the spectrum requires the presence of a partial covering component, indicating that the emission from the compact object is partially absorbed by neutral matter, probably the atmosphere of the companion star and/or the accreting matter from the companion
BeppoSAX observations of EXO 0531-66 in outburst and X Persei
No full textWe report temporal and spectral analysis of two Be/X-ray systems observed by the BeppoSAX NFI. EXO 0531-66 was in outburst during the BeppoSAX observation: it is not a well known source because it is not detectable during the low states and the outbursts are unforeseeable and at temporal distances of years. X Persei is a well known source, but the broad band spectrum, obtained for the first time from the BeppoSAX observation, revealed an unexpected complexity
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