A. D’Aì

Testing reflection features in 4U 1705−44 with XMM-Newton, BeppoSAX, and RXTE in the hard and soft states

We use data from the bright atoll source 4U 1705−44 taken with XMM-Newton, BeppoSAX, and RXTE both in the hard and in the soft state to perform a self-consistent study of the reflection component in this source. Although the data from these X-ray observatories are not simultaneous, the spectral decomposition is shown to be consistent among the different observations, when the source flux is similar. We have therefore selected observations performed at similar flux levels in the hard and soft states to study the spectral shape in these two states in a broad-band (0.1−200 keV) energy range, with good energy resolution, and using self-consistent reflection models. These reflection models provide a good fit for the X-ray spectrum both in the hard and in the soft state in the whole spectral range. We discuss the differences in the main spectral parameters we find in both states, providing evidence that the inner radius of the optically thick disk slightly recedes in the hard state.

Spin down during quiescence of the fastest known accretion-powered pulsar

We present a timing solution for the 598.89 Hz accreting millisecond pulsar, IGR J00291+5934, using Rossi X-ray Timing Explorer data taken during the two outbursts exhibited by the source on 2008 August and September. We estimate the neutron star spin frequency and we refine the system orbital solution. To achieve the highest possible accuracy in the measurement of the spin frequency variation experienced by the source in-between the 2008 August outburst and the last outburst exhibited in 2004, we re-analysed the latter considering the whole data set available. We find that the source spins down during quiescence at an average rate of u νsd = (−4.1 ± 1.2) × 10 −15 Hz s −1 . We discuss possible scenarios that can account for the long-term neutron star spin-down in terms of either magneto-dipole emission, emission of gravitational waves, and a propeller effect. If interpreted in terms of magneto-dipole emission, the measured spin down translates into an upper limit to the neutron star magnetic field, B < 3 × 10 8 G, while an upper limit to the average neutron star mass quadrupole moment of Q < 2 × 10 36 gc m 2 is set if the spin down is interpreted in terms of the

The Swift-BAT survey reveals the orbital period of three high-mass X-ray binaries

Aims. A growing number of previously hidden Galactic X-ray sources are now detected with recent surveys performed by the INTEGRAL and Swift satellites. Most of these new sources eluded past surveys due to their large local X-ray extinction and consequent low soft X-ray flux. The Swift-BAT performs daily monitoring of the sky in an energy band (15‐150 keV) which is only marginally affected by X-ray extinction, thus allowing for the search of long periodicities in the light curve and ident ification of the nature of the X-ray sources. Methods. We performed a period search using the folding technique in the Swift-BAT light curves of three INTEGRAL sources: IGR J05007−7047, IGR J13186−6257 and IGR J17354−3255. Their periodograms show significant peaks at 30: 77 �0: 01 d, 19: 99�0: 01 d and 8: 448�0: 002 d, respectively. We estimate the significance of these fe atures from the � 2 distribution of all the trials, finding a probability �1.5� 10 −4 that the detections occurred due to chance. We complement our analysis with the study of their broadband X-ray emission. Results. We identify the periodicities with the orbital periods of th e sources. The periods are typical for the wind accretors X-ray binaries and we support this identification showing that als o their energy spectra are compatible with an X-ray spectral emission characteristic of high-mass X-ray binaries. The spectrum of IGR J05007−704 that resides in the Large Magellanic Cloud, does not show any intrinsic local absorption, whereas the spectra of the Galactic sources IGR J17354−3255 and IGR J13186−6257 may be affected by a local absorber. The folded light curve for IGR J13186−6257 suggests a possible Be companion star. Conclusions.

The complex behaviour of the microquasar GRS 1915+105 in the ρ class observed with BeppoSAX - II. Time-resolved spectral analysis

Context. BeppoSAX observed GRS 1915+105 on October 2000 with a long pointing lasting about ten days. During this observation, the source was mainly in the ρ class characterized by bursts with a recurrence time of between 40 and 100 s. Aims. We identify five segments in the burst structure and accumulate the average spectra of these segments during each satellite orbit. We present a detailed spectral analysis aimed at determining variations that occur during the burst and understanding the physical process that produces them. Methods. We compare MECS, HPGSPC, and PDS spectra with several models. Under the assumption that a single model is able to fit all spectra, we find that the combination of a multi-temperature black-body disk and a hybrid corona is able to give a consistent physical explanation of the source behaviour. Results. Our measured variations in KTel, τ, KTin ,a ndRin appear to be either correlated or anti-correlated with the count rate in the energy range 1.6–10 keV. The strongest variations are detected along the burst segments: almost all parameters exhibit significant variations in the segments that have the highest fluxes (pulse) with the exception of Rin, which varies continuously and reaches a maximum just before the peak. The flux of the multi-temperature disk strongly increases in the pulse and simultaneously the corona contribution is significantly reduced. Conclusions. The disk luminosity increases in the pulse and the Rin − Tin correlation can be most successfully interpreted in term of the slim disk model. In addition, the reduction in the corona luminosity during the bursts might represent the condensation of the corona onto the disk.

Timing of the accreting millisecond pulsar IGR J17511-3057

Context. Timing analysis of accretion-powered millisecond pulsars (AMPs) is a powerful tool for probing the physics of compact objects. The recently discovered IGR J17511-3057 was the twelfth discovered of the 13 AMPs known. The Rossi XTE satellite provided an extensive coverage of the 25 days-long observation of the source outburst. Aims. Our goal is to investigate the complex interaction between the neutron star magnetic field and the accretion disk by determining the angular momentum exchange between them. The presence of a millisecond coherent flux modulation allows us to investigate this interaction from the study of pulse arrival times. To separate the neutron star proper spin frequency variations from other effects, a precise set of orbital ephemeris is mandatory. Methods. Using timing techniques, we analysed the pulse phase delays by fitting differential corrections to the orbital parameters. To remove the effects of pulse phase fluctuations, we applied the timing technique that had been already successfully applied to the case of an another AMP, XTE J1807-294. Results. We report a precise set of orbital ephemeris. We demonstrate that the companion star is a main-sequence star. We find pulse phase delay fluctuations on the first harmonic with a characteristic amplitude of about 0.05, similar to those also observed for the AMP XTE J1814-338. For the second time, an AMP shows a third harmonic detected during the entire outburst. The first harmonic phase delays exhibit a puzzling behaviour, while the second harmonic phase delays clearly spin-up. The third harmonic also shows a spin-up, although not highly significant (3σ c.l.). The presence of a fourth harmonic is also reported. If we assume that the second harmonic is a good tracer of the spin frequency of the neutron star, we infer a mean spin frequency derivative for this source of 1.65(18) × 10 −13 Hz s −1 . Conclusions. To interpret the pulse phase delays of the four harmonics, we apply the disk threading model, but obtain different and incompatible u M estimates for each harmonic. In particular, the phase delays of the first harmonic are heavily affected by phase noise, and consequently, on the basis of these data, it is not possible to derive a reliable estimate of u M. The second harmonic gives a u M consistent with the flux assuming that the source is at a distance of 6.3 kpc. The third harmonic gives a lower u M value, with respect to the first and second harmonic, and this would reduce the distance estimate to 3.6 kpc.

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.

X-ray spectroscopy of the ADC source X1822-371 with Chandra and XMM-Newton

Context. The eclipsing low-mass X-ray binary X1822-371 is the prototype of the accretion disc corona (ADC) sources. Its inclination angle (� 82.5 ◦ ) is high enough that flux from the neutron star is blocked by the edge-on accretion disc. Because the neutron star’s direct emission is hidden, its ADC emission is visible. The physical properties of the ADC in X1822-371 have been widely studied, but are still debated in literature. In light of the recent literature and of the results reported in this work we show that the ADC is optically thin. Aims. We analyse two Chandra observations and one XMM-Newton observation to study the discrete features in this source and their variation as a function of the orbital phase, deriving constraints on the temperature, density, and location of the plasma responsible for emission lines. Methods. The HETGS and XMM/Epic-pn observed X1822-371 for 140 and 50 ks, respectively. We extracted an averaged spectrum and five spectra from five selected orbital-phase intervals that are 0.04–0.25, 0.25–0.50, 0.50–0.75, 0.75–0.95, and, finally, 0.95–1.04; the orbital phase zero corresponds to the eclipse time. All spectra cover the energy band between 0.35 and 12 keV. Results. We confirm the presence of local neutral matter that partially covers the X-ray emitting region; the equivalent hydrogen column is 5 × 10 22 cm −2 and the covered fraction is about 60–65%. We identify several emission lines of He-like and H-like ions, and a prominent fluorescence iron line associated with a blending of Fe i-Fe xv resonant transitions. The transitions of He-like ions show that the intercombination dominates over the forbidden and resonance lines. The line fluxes are the highest during the orbital phases between 0.04 and 0.75. Conclusions. We discuss the presence of an extended, optically thin corona with optical depth of about 0.01 that scatters the X-ray photons from the innermost region into the line of sight. The photoionised plasma producing the O viii ,N eix ,N ex ,M gxi ,M gxii, Si xiii ,a nd Sixiv lines is placed in the bulge at the outer radius of the disc distant from the central source of 6 × 10 10 cm. The O vii and the fluorescence iron line are probably produced in the photoionised surface of the disc at inner radii. Finally, we suggest that the observed local neutral matter is the matter transferred by the companion star that was expelled from the system by the X-ray radiation pressure, which in turn originated in the accretion process onto the neutron star.

A relativistic iron emission line from the neutron star low-mass X-ray binary GX3+1

We present the results of a spectroscopic study of the Fe Kα emission of the persistent neutron-star atoll low-mass X-ray binary and type I X-ray burster GX 3+1 with the EPIC-PN on board XMM-Newton. The source shows a flux modulation over several years and we observed it during its fainter phase, which corresponds to an X-ray luminosity of LX ∼ 10 37 erg s −1 . When fitted with a two-component model, the X-ray spectrum shows broad residuals at ∼6−7 keV that can be ascribed to an iron Kα fluorescence line. In addition, lower energy features are observed at ∼3. 3k eV,∼3.9 keV and might originate from Ar XVIII and Ca XIX. The broad iron line feature is well fitted with a relativistically smeared profile. This result is robust against possible systematics caused by instrumental pile-up effects. Assuming that the line is produced by reflection from the inner accretion disk, we infer an inner disk radius of ∼25Rg and a disk inclination of 35 ◦ < i < 44 ◦ .

The discovery of the 401 Hz accreting millisecond pulsar IGR J17498-2921 in a 3.8 h orbit

We report on the detection of a 400.99018734(1) Hz coherent signal in the Rossi X-ray Timing Explorer (RXTE) light curves of the recently discovered X-ray transient, IGR J17498−2921. By analysing the frequency modulation caused by the orbital motion observed between August 13 and September 8, 2011, we derive an orbital solution for the binary system with a period of 3.8432275(3) h. The measured mass function, f (M2, M1,i) = 0.00203807(8) M� , allows to set a lower limit of 0.17 Mon the mass of the companion star, while an upper limit of 0.48 Mis set by imposing that the companion star does not overfill its Roche lobe. We observe a marginally significant evolution of the signal frequency at an average rate of −(6.3 ± 1.9) × 10 −14 Hz s −1 . The low statistical significance of this measurement and the possible presence of timing noise hampers a firm detection of any evolution of the neutron star spin. We also present an analysis of the spectral properties of IGR J17498−2921 based on the observations performed by the Swift-X-ray Telescope and the RXTE-Proportional Counter Array between August 12 and September 22, 2011. During most of the outburst, the spectra are modeled by a power-law with an index Γ ≈ 1.7−2, while values of ≈3 are observed as the source fades into quiescence.

Detailed study of the X-ray and optical/UV orbital ephemeris of X1822–371

Aims. Recent studies of the optical/UV and X-ray ephemerides of X1822-371 have found some discrepancies in the value of the orbital period derivative. Because of the importance of this value in constraining the system evolution, we comprehensively analyse all the available optical/UV/X eclipse times of this source to investigate the origin of these discrepancies. Methods. We collected all previously published X-ray eclipse times from 1977 to 2008, to which we added the eclipse time observed by Suzaku in 2006. This point is very important to cover the time gap between the last RXTE eclipse time (taken in 2003) and the most recent Chandra eclipse time (taken in 2008). Similarly we collected the optical/UV eclipse arrival times covering the period from 1979 to 2006, adding a further eclipse time taken on 1978 and updating previous optical/UV ephemeris. We compared the X-ray and the optical/UV ephemeris, and finally derived a new ephemeris of the source by combining the eclipse arrival times in the X-ray and optical/UV bands. Results. The X-ray eclipse time delays calculated with respect to a constant orbital period model display a clear parabolic trend, confirming that the orbital period of this source constantly increases at a rate of u Porb = 1.51(7) × 10 −10 s/s. Combining the X-ray and the optical/UV data sets, we find that u Porb = 1.59(9) × 10 −10 s/s, which is compatible with the X-ray orbital solution. We also investigate the possible presence of a delay of the optical/UV eclipse with respect to the X-ray eclipse, finding that this delay may not be constant in time. In particular, this variation is compatible with a sinusoidal modulation of the optical/UV eclipse arrival times with respect to the long-term parabolic trend. In this case, the optical/UV eclipse should lag the X-ray eclipse and the time-lag oscillate about an average value. Conclusions. We confirm that the orbital period derivative is three orders of magnitude larger than expected from conservative mass transfer driven by magnetic braking and gravitational radiation.