T. Di Salvo

A multi-Lorentzian timing study of the atoll sources 4U 0614+09 and 4U 1728-34

We present the results of a multi-Lorentzian fit to the power spectra of two kilohertz quasi-periodic oscillation (QPO) sources: 4U 0614+09 and 4U 1728-34. This work was triggered by recent results of a similar fit to the black hole candidates (BHCs) GX 339-4 and Cyg X-1 by Nowak in 2000. We find that one to six Lorentzians are needed to fit the power spectra of our two sources. The use of exactly the same fit function reveals that the timing behavior of 4U 0614+09 and 4U 1728-34 is almost identical at luminosities that are about a factor of 5 different. As the characteristic frequency of the Lorentzians we use the frequency νmax, at which each component contributes most of its variance per log frequency, as proposed by Belloni, Psaltis, & van der Klis in 2001. When using νmax instead of the centroid frequency of the Lorentzian, the recently discovered hectohertz Lorentzian is practically constant in frequency. We use our results to test the suggestions by Psaltis, Belloni, & van der Klis in 1999 and Nowak in 2000 that the two Lorentzians describing the high-frequency end of the broadband noise in BHCs in the low state can be identified with the kilohertz QPOs in the neutron star low-mass X-ray binaries. The prediction for the neutron star sources is that if the two kilohertz QPOs are present, then these two high-frequency Lorentzians should be absent from the broadband noise. We find that when the two kilohertz QPOs are clearly present, the low-frequency part of the power spectrum is too complicated to draw immediate conclusions from the nature of the components detected in any one power spectrum. However, the relations we observe between the characteristic frequencies of the kilohertz QPOs and the band-limited noise, when compared to the corresponding relations in BHCs, hint toward the identification of the second highest frequency Lorentzian in the BHCs with the lower kilohertz QPO. They do not confirm the identification of the highest frequency Lorentzian with the upper kilohertz QPO.

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.

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 that assessed its quiescent luminosity and spectrum with a good signal-to-noise ratio. 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 with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of LX = 5 ? 1031 ergs s-1, 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 (? ~ 1.5) power law with only a minor contribution (10%) from a soft blackbody 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 an X-ray luminosity conversion efficiency of ? ~ 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 neutron star (M 1.7 M?).

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 broadband noise component, the relation appears more complex. Both of these frequencies 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.

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.

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 eective on-source time of100 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 dierent positions of the source in the CD/HID, two in the HB and three in the NB. These spectra are t to a model consisting of a disk blackbody, a Comptonization component, and two Gaussian emission lines at 1k eV and 6.6 keV, respectively. The addition of a hard power-law tail with photon index2, contributing1.5% of the source luminosity, improves the t of the spectra in the HB. We interpret the soft component as the emission from the inner accretion disk, with inner temperature, kTin, varying between0.8 and1.7 keV and inner radius, Rin, varying between26 and11 km (assuming an inclination angle of the system of 60). The Comptonization component is probably emitted by hot plasma (electron temperature kTe varying between 3a nd20 keV, optical depth 11 0:4, seed-photon temperature kTW 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 signicantly when the source moves from the HB to the NB. We discuss possible scenarios which can explain these changes.

The optical counterpart to SAX J1808.4-3658 in quiescence: Evidence of an active radio pulsar?

The optical counterpart of the binary millisecond X-ray pulsar SAXxa0J1808.4–3658u2000during quiescence was detected at

On the correlated spectral and timing properties of 4U 1636-53: An atoll source at high accretion rates

V = 21.5

XMM-Newton X-ray spectroscopy of the high-mass X-ray binary 4U 1700-37 at low flux

xa0mag by Homer etxa0al. (2001). This star shows a 6% semi-amplitude sinusoidal modulation of its flux at the orbital period of the system. It was proposed that the modulation arises from X-ray irradiation of the intrinsically faint companion by a remnant accretion disk, and that the bulk of the optical emission arises from viscous dissipation in the disk. The serious difficulty in this scenario lies in the estimate of the irradiating luminosity required to match the observational data, that is a factor

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

10{-}50