1,721,471 research outputs found
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
Constraints on the neutron star magnetic field of the two X-ray transients SAX J1808.4-3658 and Aql X-1
Full text linkThe recently discovered coherent X-ray pulsations at a frequency of similar to400 Hz in SAX J1808.4-3658, together with a measure of the source luminosity in quiescence, allow us to put an upper limit on the neutron star magnetic field, that is B less than or equal to 5 x 10(8) Gauss, using simple considerations on the position of the magnetospheric radius during quiescent periods. Combined with the lower limit inferred from the presence of X-ray pulsations, this constrains the SAX J1808.4-3658 neutron star magnetic field in the quite narrow range (1-5) x 10(8) Gauss. Similar considerations applied to the case of Aql X-1 give a neutron star magnetic field lower than similar to 10(9) Gauss
Self-consistent study of the reflection component in 4U1705-44 with XMM-Newton, BeppoSAX and RXTE in the hard and soft state
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The Noether Symmetry Approach: Foundation and Applications: The Case of Scalar-Tensor Gauss–Bonnet Gravity
Full text linkWe sketch the main features of the Noether Symmetry Approach, a method to reduce and solve dynamics of physical systems by selecting Noether symmetries, which correspond to conserved quantities. Specifically, we take into account the vanishing Lie derivative condition for general canonical Lagrangians to select symmetries. Furthermore, we extend the prescription to the first prolongation of the Noether vector. It is possible to show that the latter application provides a general constraint on the infinitesimal generator xi, related to the spacetime translations. This approach can be used for several applications. In the second part of the work, we consider a gravity theory, including the coupling between a scalar field phi and the Gauss-Bonnet topological term G. In particular, we study a gravitational action containing the function F(G,phi) and select viable models by the existence of symmetries. Finally, we evaluate the selected models in a spatially flat cosmological background and use symmetries to find exact solutions
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
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
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