arXiv subjects

T. Di Salvo

Publications and source records attributed to T. Di Salvo.

At least 181 records · Page 10Linked to original sources

arXiv2002

High Energy Radiation from Neutron Star Binaries

This paper surveys our current knowledge of the hard X-ray emission properties of old accreting neutron stars in low mass X-ray binaries. Hard X-ray components extending up to energies of a few hundred keV have been clearly detected in sources of both the Atoll and Z classes. The presence and characteristics of these hard components are discussed in relation to source properties and state. An overall anticorrelation between the fraction luminosity in hard X-rays and mass accretion rate is apparent over different sources spanning a large range of luminosities as well as individual source undergoing state changes. Evidence for a second, yet unknown, parameter controlling the hard X-ray emission is emerging. We draw a parallel with the spectral properties of X-ray binaries hosting a stellar mass accreting black hole, and conclude that, at a merely phenomenological level, there appears to be a close analogy between the spectral properties of black hole candidates in their high and intermediate states and Z-sources. We briefly mention models that have been proposed for the hard X-ray emission of neutron star low mass X-ray binaries and comment on perspectives in the INTEGRAL era.

astro-ph↗

arXiv2002

An XMM-Newton study of the 401 Hz accreting pulsar SAX J1808.4-3658 in quiescence

SAX J1808.4-3658 is a unique source being the first Low Mass X-ray Binary showing coherent pulsations at a spin period comparable to that of millisecond radio pulsars. Here we present an XMM-Newton observation of SAX J1808.4-3658 in quiescence, the first which assessed its quiescent luminosity and spectrum with good signal to noise. XMM-Newton did not reveal other sources in the vicinity of SAX J1808.4-3658 likely indicating that the source was also detected by previous BeppoSAX and ASCA observations, even if with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of L_X=5x10^{31} erg/s, a relatively low value compared with other neutron star soft X-ray transient sources. At variance with other soft X-ray transients, the quiescent spectrum of SAX J1808.4-3658 was dominated by a hard (Gamma~1.5) power law with only a minor contribution (<10%) from a soft black body component. If the power law originates in the shock between the wind of a turned-on radio pulsar and matter outflowing from the companion, then a spin-down to X-ray luminosity conversion efficiency of eta~10^{-3} is derived; this is in line with the value estimated from the eclipsing radio pulsar PSR J1740-5340. Within the deep crustal heating model, the faintness of the blackbody-like component indicates that SAX J1808.4-3658 likely hosts a massive neutronstar (M>1.7 solar masses).

astro-ph↗

arXiv2002

A BeppoSAX Observation of KS 1731-260 in its Quiescent State: Constraints on the NS Magnetic Field

We report here the results of a 90 ks BeppoSAX observation of the low mass X-ray binary and atoll source KS 1731-260 during a quiescent phase. From this observation we derive a source X-ray luminosity of \sim 10^{33} ergs/s (for a source distance of 7 kpc). If the neutron star is spinning at a period of a few milliseconds, as inferred from the nearly-coherent oscillations observed during type-I X-ray bursts, the quiescent X-ray luminosity constrains the neutron star magnetic field strength to less than \sim 10^9 G. If the quiescent emission originates from the release of thermal energy accumulated through deep crust heating in the previous accretion phase, then the measured quiescent luminosity requires a neutron star mass of more than 1.7 M_\odot or a very long outbust recurrence time (\sim 10^3 yr). The former possibility would be in agreement with the expectation of the recycling scenario, namely that the neutron star has accreted a significant amount of mass, enough to spin it up to millisecond periods.

astro-ph↗

arXiv2002

On the Spectral Evolution of Cygnus X-2 along its Color-Color Diagram

We report on the results of a broad band (0.1-200 keV) spectral study of Cyg X-2 using two BeppoSAX observations taken in 1996 and 1997, respectively, for a total effective on-source time of ~100 ks. The color-color (CD) and hardness-intensity (HID) diagrams show that the source was in the horizontal branch (HB) and normal branch (NB) during the 1996 and 1997 observation, respectively. Five spectra were selected around different positions of the source in the CD/HID, two in the HB and three in the NB. These spectra are fit to a model consisting of a disk blackbody, a Comptonization component, and two Gaussian emission lines at ~1 keV and ~6.6 keV, respectively. The addition of a hard power-law tail with photon index ~2, contributing ~1.5% of the source luminosity, improves the fit of the spectra in the HB. We interpret the soft component as the emission from the inner accretion disk, with inner temperature, k T_in, varying between 0.8 and 1.7 keV and inner radius, R_in, varying between 26 and 11 km (assuming an inclination angle of the system of 60 degrees). The Comptonization component is probably emitted by hot plasma (electron temperature k T_e varying between 3 and ~20 keV, optical depth τ~ 11-0.4, seed-photon temperature k T_W ~ 1-2.4 keV) surrounding the NS. The changes in the parameters of the blackbody component indicate that the inner rim of the disk approaches the NS surface when the source moves from the HB to the NB, i.e. as the (inferred) mass accretion rate increases. The parameters of the Comptonized component also change significantly when the source moves from the HB to the NB. We discuss possible scenarios which can explain these changes.

astro-ph↗

arXiv2001

Study of the Cir X--1 Broad Band Spectrum at Orbital Phases Close to the Apoastron

We report on the results of a BeppoSAX (1.8--200 keV) observation of the peculiar X-ray binary source Circinus X--1 (Cir X--1) at the orbital phases between 0.61 and 0.63. We find that three components are needed to fit the broad band spectrum: a blackbody component, at a temperature of $\sim 0.6$ keV, a Comptonized component, with a seed-photon temperature of $\sim 1.2$ keV, electron temperature of $\sim 6$ keV and optical depth of $\sim 1.7$, and a power-law component dominating the spectrum at energies higher than 20 keV. We interpret the blackbody as the emission from the accretion disk, while the Comptonized component probably comes from a corona surrounding the inner part of the system. This spectrum is different from that observed at the periastron (Iaria et al. 2001a) because of the presence of the blackbody component. We discuss the implications of this difference and the presence of the power-law component.

astro-ph↗

arXiv2001

Where May Ultra-Fast Rotating Neutron Stars Be Hidden?

The existence of ultra-fast rotating neutron stars (spin period P < 1 ms) is expected on the basis of current models for the secular evolution of interacting binaries, though 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 ultra-fast 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 P_orb \sim 1 hr are the only possible hosts for ultra-fast spinning neutron stars. This could explain why ultra-fast 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.

astro-ph↗

arXiv2001

The BeppoSAX 0.1 - 100 keV Spectrum of the X-ray Pulsar 4U 1538-52

We report the results of temporal and spectral analysis performed on the X-ray pulsar 4U 1538-52 observed by BeppoSAX. We obtained a new estimate of the spin period of the neutron star P=528.24 \pm 0.01 s (corrected for the orbital motion of the X-ray source): the source is still in the spin-up state, as since 1988. The pulse profile is double peaked, although significant variations of the relative intensity of the peaks with energy are present. The broad band (0.12-100 keV) out-of-eclipse spectrum is well described by an absorbed power law modified by a high energy cutoff at \sim 16 keV (e-folding energy \sim 10 keV) plus an iron emission line at \sim 6.4 keV. A cyclotron line at \sim 21 keV is present. The width of the line is consistent with thermal Doppler broadening at the temperature of the exponential cutoff. We searched for the presence of the second harmonic, previously reported for this source. We found no evidence of lines at \sim 42 keV, although an absorption feature at 51 keV seems to be present (at 99% confidence level). A soft excess, modelled by a blackbody with a temperature of \sim 0.08 keV could be present, probably emitted by the matter at the magnetosphere. We also performed a spectral analysis during the X-ray eclipse. The spectral evolution during the eclipse can be well described by a progressive covering of the primary Comptonization spectrum that is scattered into the line of sight. During the deep eclipse this spectrum also softens, suggesting that the dust-scattered component becomes important. An alternative, more complex model, with an emission iron line and scattered components (as the one that has been used to fit the eclipse of Centaurus X-3), also gives a good fit of the deep-eclipse data.

astro-ph↗

arXiv2001

Spectral Evolution of Circinus X-1 Along its Orbit

We report on the spectral analysis of Circinus X-1 observed by the ASCA satellite in March 1998 along one orbital period. The luminosity of the source (in the 0.1-100 keV band) ranges from $2.5 \times 10^{38}$ erg s$^{-1}$ at the periastron (orbital phase 0.01) to $1.5 \times 10^{38}$ erg s$^{-1}$ at orbital phase 0.3. From the spectral analysis and the lightcurve 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 becames super-Eddington and a strong flaring activity is present. In this state a shock could form in the inner region of the system due to the super-Eddington accretion rate, producing an outflow of ionized matter whose observational signature could be the prominent absorption edge at $\sim 8.7$ 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 smaller hydrogen column: the absorption edge is now at $\sim 8.3$ keV with an optical depth a factor of 2.5 to 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.

astro-ph↗

arXiv2001

Partially Absorbed Comptonization Spectrum from the Nearly Edge-on Source X 1822-371

We report the results of a spectral analysis over the range 0.1-200 keV performed on the dipping source X 1822-371 observed by BeppoSAX. We find the best fit to the continuum using a partially covered Comptonization model, due to scattering off soft seed photons by electrons at a temperature of ~4.8 keV, without the presence of any soft blackbody emission. The equivalent hydrogen column obtained for the absorbed component is ~4.5 10^{22} cm^{-2}, an order of magnitude larger than the Galactic absorption for this source, and the covering fraction is ~71%. Because the inclination angle of X 1822-371 to the line of sight is ~85^\circ, this model gives a reasonable scenario for the source: the Comptonized spectrum could come from an extended accretion disk corona (ADC), probably the only region that can be directly observed due to the high inclination. The excess of matter producing the partial covering could be close to the equatorial plane of the system, above the outer disk, occulting the emission from the inner disk and the inner part of the ADC. An iron emission line is also present at ~6.5 keV with an equivalent width of ~150 eV. We argue that this strong iron line cannot be explained as reflection of the Comptonized spectrum by the accretion disk. It is probably produced in the ADC. An emission line at ~1.9 keV (with an equivalent width of ~54 eV) and an absorption edge at \~8.7 keV (with an optical depth of ~0.1) are also required to fit this spectrum. These features are probably produced by highly ionized iron (Fe XXIV) present in the outer part of the ADC, where the plasma density is \~10^{11}-10^{12} cm^{-3} and ionized plasma is present.

astro-ph↗

arXiv2001

The 0.1-100 keV Spectrum of LMC X-4 in the High State: Evidence for a High Energy Cyclotron Absorption Line

We report on 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 (E_cutoff ~ E_fold ~ 18 keV). The addition of a cyclotron absorption line at ~100 keV improves the fit significantly. The inferred magnetic moment is 1.1 10^{31} Gauss cm^3, in agreement with the value estimated assuming that the neutron star is at the spin equilibrium, as it 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 black body emission from the accretion disk at the magnetospheric radius. Another possibility is to fit the soft excess with black body and thermal bremsstrahlung. In this case the black body would originate from cold plasma at the magnetosphere while the bremsstrahlung component may be produced by the strong stellar wind from the companion star, ionized by the X-ray emission from the pulsar.

astro-ph↗

arXiv2001

Where Have All the Submillisecond Pulsars Gone?

The existence of pulsars with spin period below one millisecond is expected, though they have not been detected up to now. Their formation depends on the quantity of matter accreted from the companion which, in turn, is limited by the mechanism of mass ejection from the binary. Mass ejection must be efficient, at least in some cases, in order to produce the observed population of moderately fast spinning millisecond pulsars. First we demonstrate, in the framework of the widely accepted recycling scenario, using a population synthesis approach, that a significant number of pulsars with spin period below one millisecond is expected. Then we propose that significant variations in the mass-transfer rate may cause, in systems with orbital periods larger than 1 hr, the switch-on of a radio pulsar whose radiation pressure is capable of ejecting out of the system most of the matter transferred by the companion and prevent any further accretion. We show how this mechanism could dramatically alter the binary evolution since the mechanism that drives mass overflow from the inner Lagrangian point is still active while the accretion is inhibited. Moreover we demonstrate that the persistency of this ``radio ejection'' phase depends on the binary orbital period, demonstrating that close systems (orbital periods lower than 1 hr) are the only possible hosts for ultra fast spinning neutron stars. This could explain why submillisecond pulsars have not been detected so far, as current radio surveys are hampered by computational limitations with respect to the detection of very short spin period pulsars in short orbital period binaries.

astro-ph↗

arXiv2001

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 non-flaring intervals and the flares. In both cases the continuum is well described by a soft blackbody ($k T_{BB} \sim 0.5$ keV) and a Comptonized spectrum corresponding to an electron temperature of $k T_e \sim 2.7$ keV, optical depth $τ\sim 10$ (for a spherical geometry), and seed photon temperature of $k T_W \sim 1$ keV. All temperatures tend to increase during the flares. In the non-flaring emission a hard tail dominates the spectrum above 30 keV. This can be fit by a power law with photon index $\sim 2$, contributing $\sim 2%$ of the total source luminosity over the BeppoSAX energy range. A comparison with hard tails 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.

astro-ph↗

arXiv2000

A Hard Tail in the Broad Band Spectrum of the Dipper XB 1254-690

We 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 broad band (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 ~0.85 keV, a comptonized spectrum with an electron temperature of ~2 keV, and a bremsstrahlung at a temperature of ~20 keV. Adopting a distance of 10 kpc and 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 $R_{\rm in} \sqrt{\cos i} \sim 11$ 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 ~19. The bremsstrahlung emission, contributing ~4% of the total luminosity, probably originates in an extended accretion disk corona with radius $\sim 10^{10}$ cm. In this scenario we calculated that the optical depth of this region is ~0.71 and its mean electron density is $N_e \sim 2.7 \times 10^{14}$ 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 ~1.27 keV with an optical depth of ~0.15. Its energy could correspond to the L-edge of Fe XVII, or K-edge of Ne X or neutral Mg.

astro-ph↗

arXiv2000

Probing the Inner Region of Cyg X-1 in the Low/Hard State through its X-ray Broad Band Spectrum

We present the broad band X-ray spectrum of Cyg X-1 in the low/hard state as observed by the instruments on board of BeppoSAX. The spectrum spans from 0.1 to 200 keV, allowing the total accretion luminosity to be observed, rather than extrapolated, corresponding to ~2 per cent of the Eddington limit for a 10 Solar mass black hole. The broad bandpass allows us to determine the continuum shape with great accuracy. Simple models of Compton up-scattering of seed photons from the accretion disk do not adequately match the spectrum. At low energies an additional continuum component is required, giving a complex soft excess which extends up to ~4 keV, in line with previous results from ASCA. Moreover we clearly detect a reflected component from the accretion disk which is smeared, probably because of relativistic and Doppler effects. The reflecting material is not strongly ionized and does not subtend a large solid angle as seen from the corona. The inner radius of the disk, that depends on the inclination of the system, is most probably between 10 and 70 gravitational radii (Rg). An unsmeared reprocessed component, probably originating from the companion star or the outer disk, could also be present. In this case, the inner radius of the disk, as inferred from the smeared reflection, is smaller, between 6 and 20 Rg.

astro-ph↗

arXiv2000

The Discovery of a State Dependent Hard Tail in the X-ray Spectrum of the Luminous Z-source GX 17+2

We report results of a BeppoSAX (0.1-200 keV) observation of the Z-type low mass X-ray binary GX 17+2. The source was on the so-called Horizontal and Normal branches. Energy spectra were selected based on the source position in the X-ray hardness-intensity diagram. The continuum could be fairly well described by the sum of a ~0.6 keV blackbody, contributing ~10% of the observed 0.1-200 keV flux, and a Comptonized component, resulting from upscattering of \~1 keV seed photons by an electron cloud with temperature of ~3 keV and optical depth of ~10. Iron K-line and edge were also present at energies 6.7 and 8.5 keV, respectively. In the spectra of the Horizontal branch a hard tail was clearly detected at energies above ~30 keV. It could be fit by a power law of photon index ~2.7, contributing ~8% of the source flux. This component gradually faded as the source moved towards the Normal branch, where it was no longer detectable. We discuss the possible origin of this component and the similarities with the spectra of Atoll sources and black hole X-ray binaries.

astro-ph↗

arXiv2000

The Composite Broad Band Spectrum of Cir 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 $\sim 6 \times 10^3$ s is present at the beginning of the observation. The luminosity during the persistent emission is $1 \times 10^{38}$ erg s$^{-1}$, while during the flare is $2 \times 10^{38}$ erg s$^{-1}$. We produced broad band (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 $\sim 0.4$ keV, by electrons at a temperature of $\sim 1$ keV. Out of the flare a power-law component, with photon index $\sim 3$, is dominant at energies higher than 10 keV. This component contributes to $\sim 4%$ of the total luminosity. During the flare its addition is not statistically significant. An absorption edge at $\sim 8.4$ keV, with optical depth $\sim 1$, corresponding to the K-edge of Fe XXIII-XXV, and an iron emission line at 6.7 keV are also present. The iron line energy is in agreement with the ionization level of the absorption edge. The hydrogen column deduced from the absorption edge is $\sim 10^{24}$ cm$^{-2}$, two order of magnitude larger than the absorption measured in this source. We calculated the radius of the region originating the comptonized seed photons, $R_W \sim 150$ km. We propose a scenario where $R_W$ is the inner disk radius, a highly ionized torus surrounds the accretion disk and a magnetosphere is large up to $R_W$. 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.

astro-ph↗

arXiv2000

Study of the Temporal Behavior of 4U 1728-34 as a Function of its Position in the Color-Color Diagram

We study the timing properties of the bursting atoll source 4U 1728-34 as a function of its position in the X-ray color-color diagram. In the island part of the color-color diagram (corresponding to the hardest energy spectra) the power spectrum of 4U 1728-34 shows several features such as a band-limited noise component present up to a few tens of Hz, a low frequency quasi-periodic oscillation (LFQPO) at frequencies between 20 and 40 Hz, a peaked noise component around 100 Hz, and one or two QPOs at kHz frequencies. In addition to these, in the lower banana (corresponding to softer energy spectra) we also find a very low frequency noise (VLFN) component below ~1 Hz. In the upper banana (corresponding to the softest energy spectra) the power spectra are dominated by the VLFN, with a peaked noise component around 20 Hz. We find that the frequencies of the kHz QPOs are well correlated with the position in the X-ray color-color diagram. For the frequency of the LFQPO and the break frequency of the broad-band noise component the relation appears more complex. These frequencies both increase when the frequency of the upper kHz QPO increases from 400 to 900 Hz, but at this frequency a jump in the values of the parameters occurs. We interpret this jump in terms of the gradual appearance of a QPO at the position of the break at high inferred mass accretion rate, while the previous LFQPO disappears. Simultaneously, another kind of noise appears with a break frequency of ~7 Hz, similar to the NBO of Z sources. The 100 Hz peaked noise does not seem to correlate with the position of the source in the color-color diagram, but remains relatively constant in frequency. This component may be similar to several 100 Hz QPOs observed in black hole binaries.

astro-ph↗

arXiv2000

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.

astro-ph↗