G. Lavagetto

Timing of the accreting millisecond pulsar XTE J1814−338

We present a precise timing analysis of the accreting millisecond pulsar XTE J1814-338 during its 2003 outburst, observed by RXTE. A full orbital solution is given for the first time; Doppler effects induced by the motion of the source in the binary system were corrected, leading to a refined estimate of the orbital period, P orb = 15 388.7229(2) s, and of the projected semimajor axis, a sin i/c = 0.390633(9) light-second. We could then investigate the spin behaviour of the accreting compact object during the outburst. We report here a refined value ·of the spin frequency (v = 314.356 108 79(1) Hz) and the first estimate of the spin frequency derivative of this source while accreting [ν = (-6.7 ± 0.7) x 10 -14 Hz s -1 ]. This spin-down behaviour arises when both the fundamental frequency and the second harmonic are taken into consideration. We discuss this in the context of the interaction between the disc and the quickly rotating magnetosphere, at accretion rates sufficiently low to allow a threading of the accretion disc in regions where the Keplerian velocity is slower than the magnetosphere velocity. We also present indications of a jitter of the pulse phases around the mean trend, which we argue results from movements of the accreting hotspots in response to variations of the accretion rate.

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.

A Broad Iron Line in the Chandra High Energy Transmission Grating Spectrum of 4U 1705-44

We present the results of a Chandra 30 ks observation of the low-mass X-ray binary and atoll source 4U 1705-44. Here we concentrate on the study of discrete features in the energy spectrum at energies below ~3 keV, as well as on the iron Kα line, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. Below 3 keV, three narrow emission lines are found at 1.47, 2.0, and 2.6 keV. The 1.47 and 2.6 keV lines are probably identified with Lyα emission from Mg XII and S XVI, respectively. The identification of the feature at ~2.0 keV is uncertain because of the presence of an instrumental feature at the same energy. The iron Kα line at ~6.5 keV is found to be intrinsically broad (FWHM ~ 1.2 keV); its width can be explained by reflection from a cold accretion disk extending down to ~15 km from the neutron star center or by Compton broadening in the external parts of a hot (~2 keV) Comptonizing corona. We finally report here precise X-ray coordinates of the source.We present preliminary results from a Chandra 30 ks observation of the atoll source 4U 1705-44. In particular we concentrate on the study of the iron K alpha line, using the HEG spectrometer. The iron K alpha line at ~ 6.6 keV is found to be intrinsically broad ( FWHM ~ 1.7 keV); its width can be explained by reflection from a cold accretion disk extending down to ~ 17 km from the neutron star center or by Compton broadening in the hotter ( ~ 3 - 4 keV) corona.

Spin up and phase fluctuations in the timing of the accreting millisecond pulsar XTE J1807-294

We have performed a timing analysis of the 2003 outburst of the accreting X-ray millisecond pulsar XTE J1807–294 as observed by the Rossi X-Ray Timing Explorer. Using recently refined orbital parameters, we report for the first time a precise estimate of the spin frequency and of the spin frequency derivative. The phase delays of the pulse profile show a strong erratic behavior superposed on what appears to be a global spin-up trend. The erratic behavior of the pulse phases is strongly related to rapid variations of the light curve, making it very difficult to fit these phase delays with a simple formula. As in previous cases, we therefore separately analyze the phase delays of the first harmonic and of the second harmonic of the spin frequency, finding that the phases of the second harmonic are far less affected by the erratic behavior. Under the hypothesis that the second-harmonic pulse phase delays are a good tracer of the spin frequency evolution, we give for the first time an estimate of the spin frequency derivative for this source. XTE J1807–294 shows a clear spin-up of = 2.5(7) × 10−14 Hz s−1 (1 σ confidence level). The majority of the uncertainty in the value of the spin-up rate is due to the uncertainties in the source position on the sky. We discuss the effect of this systematic error on the spin frequency and its derivative.

Broadband Spectral Evolution of Scorpius X-1 along Its Color-Color Diagram

We analyze a large collection of RXTE archive data from 1997 April to 2003 August of the bright X-ray source Scorpius X-1 in order to study the broadband spectral evolution of the source for different values of the inferred mass accretion rate by studying energy spectra from selected regions in the Z track of its color-color diagram (CD). A two-component model, consisting of a soft thermal component interpreted as thermal emission from an accretion disk and a thermal Comptonization component, is unable to fit the whole 3-200 keV energy spectrum at low accretion rates. Strong residuals in the highest energy band of the spectrum require the addition of a third component that can be fitted with a power-law component, which could represent a second thermal Comptonization from a much hotter plasma, or a hybrid thermal/nonthermal Comptonization. The presence of this hard emission in Sco X-1 has been previously reported, however, without a clear relation with the accretion rate. We show, for the first time, that there exists a common trend in the spectral evolution of the source, where the spectral parameters change in correlation with the position of the source in the CD. In particular, using a hybrid Comptonization model, we show that the power supplied to the nonthermal distribution can be as high as half of the total hard power injected in heating the electron distribution. We discuss the physical implications derived from the results of our analysis, with a particular emphasis on the hardest part of the X-ray emission and its possible origins.

Chandra Observation of Cir X-1 near the Periastron Passage: Evidence for an X-Ray Jet?

We present the results of a 25 ks long Chandra observation of the peculiar source Cir X-1 near the periastron passage. We report precise X-ray coordinates of the source, which were compatible with the optical and radio counterpart coordinates. We focus on the study of the detected emission features using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We detect emission lines associated with Mg XII, Si XIII, Si XIV, S XV, S XVI, Ar XVII, Ar XVIII, Ca XIX, Ca XX, Fe XXV, and Fe XXVI, showing an average redshift of 470 km s−1. The most intense emission features can be fitted with two lines; this is more evident for the 6.6 keV emission feature, which shows a double-peaked structure. We propose that the redshifted and blueshifted lines of Mg XII, Si XIV, S XVI, and Fe XXV can be interpreted as emission from a blue and red beam of an X-ray jet, finding that the angle between the line of sight and the jet direction is 92° and the jet velocity is 0.08c.

The BeppoSAX 0.1–18 keV spectrum of the bright atoll source GX 9+1: an indication of the source distance

We report the results of a long, 350 ks, BeppoSAX observation of the bright atoll source GX 9+1 in the 0.12-18 keV energy range. During this observation GX 9+1 showed a large count rate variability in its lightcurve. From its color-color diagram we selected six zones and extracted the source energy spectrum from each zone. We find that the model, composed of a blackbody plus a Comptonized component absorbed by an equivalent hydrogen column of ~1.4 ? 1022 cm-2, fits the spectra in the energy range 1-18 keV well; however, below 1 keV a soft excess is present. We find that the spectrum of GX 9+1, in the 0.12-18 keV energy range, is well fitted by the model above, if we use an equivalent hydrogen column of ~0.8 ? 1022 cm-2, together with several absorption edges from ionized matter (O VII, O VIII, Ne IX, Ar XVII, and Ca XX) and an absorption line from Si XIV. From the study of these features we deduce that the electron density of the plasma and the equivalent hydrogen column density associated with the ionized matter is ~1012 cm-3 and ~1023 cm-2, respectively, at a distance from the central object of r ~ 1011 cm, while at larger distances the equivalent hydrogen column density associated to the ionized matter decreases down to ~1022 cm-2. The value of the equivalent hydrogen column of ~0.8 ? 1022 cm-2 associated to the interstellar matter implies that GX 9+1 is at a distance of 5 kpc, and not 8.5-10 kpc as usually assumed. This result implies that the atoll source GX 9+1 does not emit at the Eddington limit, as supposed until now, but has a luminosity of 6 ? 1037 erg/s in the 0.1-18 keV energy range, typical of LMXBs belonging to the bright atoll class.

Chandra observation of the Big Dipper X 1624–490

We present the results of a 73 ks long Chandra observation of the dipping source X 1624-490. During the observation a complex dip lasting 4 hours is observed. We analyse the persistent emission detecting, for the first time in the 1st-orde r spectra of X 1624-490, an absorption line associated to Caxx. We confirm the presence of the Fexxv Kand Fexxvi Kabsorption lines with a larger accuracy with respect to a previous XMM observation. Assuming that the line widths are due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines have been produced in a photoionized absorber between the coronal radius and the outer edge of the accretion disk.

Chandra Observation of the Persistent Emission from the Dipping Source XB 1916–053

We present the results of a 50 ks long Chandra observation of the dipping source XB 1916-053. During the observation two X-ray bursts occurred, and the dips were not present at each orbital period. From the zeroth-order image we estimate the precise X-ray coordinates of the source with a 90% uncertainty of 06. In this work we focus on the spectral study of discrete absorption features, during the persistent emission, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We detect, for the first time in the first-order spectra of XB 1916-053, absorption lines associated with Ne X, Mg XII, Si XIV, and S XVI and confirm the presence of the Fe XXV and Fe XXVI absorption lines with a greater accuracy than the previous XMM EPIC pn observation. Assuming that the line widths are due to a bulk motion or a turbulence associated with the coronal activity, we estimate that the lines are produced in a photoionized absorber 4 × 1010 cm from the neutron star , near the disk edge.

The different fates of a low-mass X-ray binary - I. Conservative mass transfer

We study the evolution of a low mass x-ray binary coupling a binary stellar evolution code with a general relativistic code that describes the behavior of the neutron star. We assume the neutron star to be low–magnetized (B � 10 8 G). In the systems investigated in this paper, our computations show that during the binary evolution the companion transfers as much as 1 M⊙ to the neutron star, with an accretion rate of � 10 −9 M⊙/yr. This is sufficient to keep the inner rim of the accretion disc in contact with the neutron star surface, thus preventing the onset of a propeller phase capable of ejecting a significant fraction of the matter transferred by the companion. In this scenario we find that, for neutron stars governed by equations of state from soft up to moderately stiff, an accretion induced collapse to a black hole is almost unavoidable. The collapse to a black hole can occur either during the accretion phase or after the end of the mass transfer when the neutron star is left in a supramassive sequence. In this last case the collapse is driven by energy losses of the fast spinning magneto-dipole rotator (pulsar). For extremely supramassive neutron stars these energy losses cause a spin up. As a consequence the pulsar will have a much shorter lifetime than that of a canonical, spinning down radio pulsar. This complex behavior strongly depends on the equation of state for ultra-dense matter and therefore could be used to constrain the internal structure of the neutron star. In the hypothesis that the r-modes of the neutron star are excited during the accretion process, the gravitational waves emisson limits the maximum spin attainable by a NS to roughly 2 ms. In this case, if the mass transfer is conservative, the collapse to a black hole during the accretion phase is even more common since the maximum mass achievable before the collapse to a black hole during accretion is smaller due to the limited spin frequency.