R. Iaria

Detection of a Hard Tail in the X-Ray Spectrum of the Z Source GX 349+2

We present the results of a BeppoSAX observation of the Z source GX 349+2 covering the energy range 0.1-200 keV. The presence of flares in the light curve indicates that the source was in the flaring branch during the BeppoSAX observation. We accumulated energy spectra separately for the nonflaring intervals and for the flares. In both cases, the continuum is well described by a soft blackbody (kTBB ~ 0.5 keV) and a Comptonized spectrum corresponding to electron temperature kTe ~ 2.7 keV, optical depth τ ~ 10 (for a spherical geometry), and seed-photon temperature kTW ~ 1 keV. All temperatures tend to increase during the flares. In the nonflaring emission, a hard tail dominates the spectrum above 30 keV. This can be fit by a power law with photon index ~2, contributing ~2% of the total source luminosity over the BeppoSAX energy range. A comparison with hard tails that are detected in some soft states of black hole binaries suggests that a similar mechanism could originate these components in black hole and neutron star systems.

Where May Ultrafast Rotating Neutron Stars Be Hidden

The existence of ultrafast rotating neutron stars (spin period P 1 ms) is expected on the basis of current models for the secular evolution of interacting binaries, although they have not been detected yet. Their formation depends on the quantity of matter accreted by the neutron star which, in turn, is limited by the mechanism of mass ejection from the binary. An efficient mass ejection can avoid the formation of ultrafast pulsars or their accretion-induced collapse to a black hole. We propose that significant reductions of the mass transfer rate may cause the switch-on of a radio pulsar phase, whose radiation pressure may be capable of ejecting out of the system most of the matter transferred by the companion. This can prevent, for long orbital periods and if a sufficiently fast spin has been reached, any further accretion, even if the original transfer rate is restored, thus limiting the minimum spin period attainable by the neutron star. We show that close systems (orbital periods Porb ~ 1 hr) are the only possible hosts for ultrafast spinning neutron stars. This could explain why ultrafast radio pulsars have not been detected so far, as the detection of pulsars with very short spin periods in close systems is hampered, in current radio surveys, by strong Doppler modulation and computational limitations.

A relativistically smeared spectrum in the neutron star X-ray binary 4U 1705−44: looking at the inner accretion disc with X-ray spectroscopy

Iron emission lines at 6.4-6.97 keV, identified with fluorescent Kα transitions, are among the strongest discrete features in the X-ray band. These are therefore one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper, we present a recent XMM―Newton observation of the X-ray burster 4U 1705-44, where we clearly detect a relativistically smeared iron line at about 6.7 keV, testifying with high statistical significance that the line profile is distorted by high-velocity motion in the accretion disc. As expected from disc reflection models, we also find a significant absorption edge at about 8.3 keV; this feature appears to be smeared, and is compatible with being produced in the same region where the iron line is produced. From the line profile, we derive the physical parameters of the inner accretion disc with large precision. The line is identified with the Kα transition of highly ionized iron, Fe xxv, the inner disc radius is R in = 14 ± 2 R g (where Rg is the Gravitational radius, GM/c 2 ), the emissivity dependence from the disc radius is r ―2.27±0.08 , the inclination angle with respect to the line of sight is i = 39° ± 1 °. Finally, the XMM―Newton spectrum shows evidences of other low-energy emission lines, which again appear broad and their profiles are compatible with being produced in the same region where the iron line is produced.

A Hard Tail in the X-Ray Broadband Spectrum of Circinus X-1 at the Periastron: A Peculiar Z Source

We report on the spectral analysis of the peculiar source Cir X-1 observed by the BeppoSAX satellite when the X-ray source was near the periastron. A flare lasting ~6 × 103 s is present at the beginning of the observation. The luminosity during the persistent emission is 1 × 1038 ergs s-1, while during the flare it is 2 × 1038 ergs s-1. We produced broadband (0.1-100 keV) energy spectra during the flare and the persistent emission. At low energies the continuum is well fitted by a model consisting of Comptonization of soft photons, with a temperature of ~0.4 keV, by electrons at a temperature of ~1 keV. After the flare, a power-law component with photon index ~3 is dominant at energies higher than 10 keV. This component contributes ~4% of the total luminosity. During the flare its addition is not statistically significant. An absorption edge at ~8.4 keV, with optical depth ~1, corresponding to the K edge of Fe XXIII-Fe XXV, and an iron emission line at 6.7 keV are also present. The iron-line energy is in agreement with the ionization level inferred from the absorption edge. The hydrogen column deduced from the absorption edge is ~1024 cm-2, 2 orders of magnitude larger than the low-energy absorption measured in this source. We calculated the radius of the region originating the Comptonized seed photons, RW ~ 150 km. We propose a scenario where RW (the Wien radius) is the inner disk radius, a highly ionized torus surrounds the accretion disk, and a magnetosphere is present up to RW. The absorption edge and the emission line could originate in the photoionized torus, while the Comptonized component originates in an inner region of the disk.

The spin and orbit of the newly discovered pulsar IGR J17480-2446

We present an analysis of the spin and orbital properties of the newly discovered accreting pulsar IGR J17480-2446, located in the globular cluster Terzan 5. Considering the pulses detected by the Rossi X-ray Timing Explorer at a period of 90.539645(2) ms, we derive a solution for the 21.27454(8) hr binary system. The binary mass function is estimated to be 0.021275(5) M o , indicating a companion star with a mass larger than 0.4 M ⊙ . The X-ray pulsar spins up while accreting at a rate of between 1.2 and 1.7 x 10 -12 Hz s -1 , in agreement with the accretion of disc matter angular momentum given the observed luminosity. We also report the detection of pulsations at the spin period of the source during a Swift observation performed ~2 d before the beginning of the RXTE coverage. Assuming that the inner disc radius lies in between the neutron star radius and the corotation radius while the source shows pulsations, we estimate the magnetic field of the neutron star to be within ~2 x 10 8 G and ~2.4 x 10 10 G. From this estimate, the value of the spin period and of the observed spin-up rate, we associate this source with the still poorly sampled population of slow, mildly recycled, accreting pulsars.

THE 0.1¨100 keV SPECTRUM OF LMC X-4 IN THE HIGH STATE: EVIDENCE FOR A HIGH-ENERGY CYCLOTRON ABSORPTION LINE

We report the spectral analysis of the X-ray pulsar LMC X-4 in its high state, out of eclipse, observed by BeppoSAX. During this observation no coherent pulsations are detected. The primary continuum is well described by a power law with a high-energy cutoff Ecutoff ~ Efold ~ 18 keV. The addition of a cyclotron absorption line at ~100 keV improves the fit significantly. The inferred magnetic moment is 1.1 × 1031 G cm3, in agreement with the value estimated, assuming that the neutron star is at the spin equilibrium, as has been proposed for this source. The remaining excess at low energies can be fitted by a Comptonization of soft photons by moderately hot electrons (kT ~0.9 keV) with an optical depth τ ~ 16. The seed photons for this Comptonization are consistent with blackbody emission from the accretion disk at the magnetospheric radius. Another possibility is to fit the soft excess with blackbody and thermal bremsstrahlung. In this case, the blackbody would originate from cold plasma at the magnetosphere while the bremsstrahlung component would be produced by the strong stellar wind from the companion star, ionized by the X-ray emission from the pulsar.

The Broadband Spectrum of MXB 1728–34 Observed by BeppoSAX

We 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 ~2 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 ~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 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is Rin ~ 20 km, where i is the inclination angle. The Comptonized component could originate in a spherical corona, with temperature ~10 keV and optical depth ~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. 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 (ξ ~ 280) and a solid angle of ~0.2 (in units of 2π) subtended by the reflector as seen from the corona.

A self-consistent approach to the hard and soft states of 4U 1705-44

Context. High-resolution spectroscopy has recently revealed in many low-mass X-ray binaries hosting a neutron star that the shape of the broad iron line observed in the 6.4-6.97 keV range is consistently well-fitted by a relativistically smeared line profile. Aims. The presence of other broad features, besides the iron line, together with a high S/N of the spectra offer the possibility of testing a self-consistent approach to the overall broadband reflection spectrum and evaluating the impact of the reflection component in the formation of the broadband X-ray spectra. Methods. We analyzed two XMM-Newton observations of the bright atoll source 4U 1705-44, which can be considered a prototype of the class of the persistent NS LMXBs showing both hard and soft states. The first observation was performed when the source was in a hard low flux state, the second during a soft, high-flux state. Both the spectra show broad iron emission lines. We fit the spectra using a two-component model, together with a reflection model specifically suited to the case of a neutron star, where the incident spectrum has a blackbody shape. Results. In the soft state, the reflection model, convolved with a relativistic smearing component, consistently describes the broad features present in the spectrum, and we find a clear relation between the temperature of the incident flux and the temperature of the harder X-ray component that we interpret as the boundary layer emission. In this state we find converging evidence that the boundary layer outer radius is � 2 times the neutron star radius. In the low flux state, we observe a change in the continuum shape of the spectrum with respect to the soft state. Still, the broad local emission features can be associated with a disk reflecting matter, but in a lower ionization state, and possibly produced in an accretion disk truncated at greater distance. Conclusions. Our analysis provides strong evidence that the reflection component in soft states of LMXBs comes from to hard X-ray thermal irradiation, which we identify with the boundary layer emission, also present in the continuum model. In the hard state, the broad iron line if also produced by reflection, and the continuum disk emission can be self-consistently accounted if the disk is truncated at a greater distance than the soft state.

Timing an accreting millisecond pulsar : Measuring the accretion torque in IGR j00291+5934

We performed a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of 2004 December. We corrected the arrival times of all the events for the orbital (Doppler) effects and performed a timing analysis of the resulting phase delays. In this way we are able to study, for the first time in this class of sources, the spin-up of a millisecond pulsar as a consequence of accretion torques during the X-ray outburst. The accretion torque gives us for the first time an independent estimate of the mass accretion rate onto the neutron star, which can be compared with the observed X-ray luminosity. We also report a revised value of the spin period of the pulsar.

DISK REFLECTION SIGNATURES IN THE SPECTRUM OF THE BRIGHT Z-SOURCE GX 340+0

We present the preliminary results of a 50 ks long XMM-Newton observation of the bright Z-source GX 340+0. In this Letter, we focus on the study of a broad asymmetric emission line in the Fe Kα energy band, whose shape is clearly resolved and compatible with a relativistically smeared profile arising from reflection on a hot accretion disk extending close to the central accreting neutron star. By combining temporal and spectral analysis, we are able to follow the evolution of the source along its horizontal branch. However, despite a significant change in the continuum emission and luminosity, the line profile does not show any strong correlated variation. This broad line is produced by recombination of highly ionized iron (Fe XXV) at an inferred inner radius close to 13R g, while the fit requires a high value for the outer disk radius. The inclination of the source is extremely well constrained at 35°, while the emissivity index is –2.50.