Neutral absorber dips in the periodic burster LMXB XB 1323-619 from Suzaku

We present results of an observation with Suzaku of the dipping, periodic bursting low mass X-ray binary XB 1323-619. Using the energy band 0.8 - 70 keV, we show that the source spectrum is well-described as the emission of an extended accretion disk corona, plus a small contribution of blackbody emission from the neutron star. The dip spectrum is well-fitted by the progressive covering model in which the extended ADC is progressively overlapped by the absorbing bulge of low ionization state in the outer accretion disk and that dipping is basically due to photoelectric absorption in the bulge. An energy-independent decrease of flux at high energies (20 - 70 keV) is shown to be consistent with the level of Thomson scattering expected in the bulge. An absorption feature at 6.67 keV (Fe XXV) is detected in the non-dip spectrum and other possible weak features. In dipping, absorption lines of medium and highly ionized states are seen suggestive of absorption in the ADC but there is no evidence that the lines are stronger than in non-dip. We show that the luminosity of the source has changed substantially since the Exosat observation of 1985, increasing in luminosity between 1985 and 2003, then in 2003 - 2007 falling to the initial low value. X-ray bursting has again become periodic, which it ceased to do in its highest luminosity state, and we find that the X-ray bursts exhibit both the fast decay and later slow decay characteristic of the rp burning process. We present arguments against the recent proposal that the decrease of continuum flux in the dipping LMXB in general can be explained as absorption in an ionized absorber rather than in the bulge in the outer disk generally accepted to be the site of absorption.Comment: 12 pages, 6 figures, Astronomy and Astrophysics in pres

Boundary layer emission and Z-track in the color-color diagram of luminous LMXBs

We demonstrate that Fourier-frequency resolved spectra of atoll and Z- sources are identical, despite significant difference in their average spectra and luminosity (by a factor of ~10-20). This result fits in the picture we suggested earlier, namely that the f> 1 Hz variability in luminous LMXBs is primarily due to variations of the boundary layer luminosity. In this picture the frequency resolved spectrum equals the boundary layer spectrum, which therefore can be straightforwardly determnined from the data. The obtained so boundary layer spectrum is well approximated by the saturated Comptonization model, its high energy cut-off follows kT~2.4 keV black body. Its independence on the global mass accretion rate lends support to the theoretical suggestion by Inogamov &Sunyaev (1999) that the boundary layer is radiation pressure supported. With this assumption we constrain the gravity on the neutron star surface and its mass and radius. Equipped with the knowledge of the boundary layer spectrum we attempt to relate the motion along the Z-track to changes of physically meaningful parameters. Our results suggest that the contribution of the boundary layer to the observed emission decreases along the Z-track from conventional ~50% on the horizontal branch to a rather small number on the normal branch. This decrease can be caused, for example, by obscuration of the boundary layer by the geometrically thick accretion disk at Mdot ~ Mdot_Edd. Alternatively, this can indicate significant change of the structure of the accretion flow at Mdot ~ Mdot_ Edd and disappearance of the boundary layer as a distinct region of the significant energy release associated with the neutron star surface.Comment: 9 pages, 7 figures, Accepted in A&

Is there a compact companion orbiting the late O-type binary star HD 164816?

We present a multi-wavelength (X-ray, $\gamma$-ray, optical and radio) study of HD 194816, a late O-type X-ray detected spectroscopic binary. X-ray spectra are analyzed and the X-ray photon arrival times are checked for pulsation. In addition, newly obtained optical spectroscopic monitoring data on HD 164816 are presented. They are complemented by available radio data from several large scale surveys as well as the \emph{FERMI} $\gamma$-ray data from its \emph{Large Area Telescope}. We report the detection of a low energy excess in the X-ray spectrum that can be described by a simple absorbed blackbody model with a temperature of $\sim$ 50 eV as well as a 9.78 s pulsation of the X-ray source. The soft X-ray excess, the X-ray pulsation, and the kinematical age would all be consistent with a compact object like a neutron star as companion to HD 164816. The size of the soft X-ray excess emitting area is consistent with a circular region with a radius of about 7 km, typical for neutron stars, while the emission measure of the remaining harder emission is typical for late O-type single or binary stars. If HD 164816 includes a neutron star born in a supernova, this supernova should have been very recent and should have given the system a kick, which is consistent with the observation that the star HD 164816 has a significantly different radial velocity than the cluster mean. In addition we confirm the binarity of HD 164816 itself by obtaining an orbital period of 3.82 d, projected masses $m_1 {\rm sin}^{3} i$ = 2.355(69) M$_\odot$, $m_2 {\rm sin}^{3} i$ = 2.103(62) M$_\odot$ apparently seen at low inclination angle, determined from high-resolution optical spectra.Comment: Accepted for publication by MNRAS, 11 pages, 6 figures, 4 table

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.Comment: 11 pages, including 2 figures. Accepted for publication in ApJ Letter

Discovery of X-ray burst triplets in EXO 0748-676

[Abridged] Type-I X-ray bursts are thermonuclear flashes that take place on the surface of accreting neutron stars. The wait time between consecutive bursts is set by the time required to accumulate the fuel needed to trigger a new burst; this is at least one hour. Sometimes secondary bursts are observed, approximately 10 min after the main burst. These short wait-time bursts are not yet understood. We observed the low-mass X-ray binary and X-ray burster EXO 0748-676 with XMM-Newton for 158 h, during 7 uninterrupted observations lasting up to 30 h each. We detect 76 X-ray bursts. Most remarkably, 15 of these bursts occur in burst triplets, with wait times of 12 min between the three components of the triplet. We also detect 14 doublets with similar wait times between the two components of the doublet. The characteristics of the bursts indicate that possibly all bursts in this system are hydrogen-ignited, in contrast with most other frequent X-ray bursters in which bursts are helium-ignited, but consistent with the low mass accretion rate in EXO 0748-676. Possibly the hydrogen ignition is the determining factor for the occurrence of short wait-time bursts.Comment: 23 pages, 16 figures, accepted for publication in A&

Discovery of type-I X-ray bursts from the low-mass X-ray binary 4U 1708-40

We report the discovery of type-I X-ray bursts from the low-mass X-ray binary 4U 1708-40 during the 100 ks observation performed by BeppoSAX on 1999 August 15-16. Six X-ray bursts have been observed. The unabsorbed 2-10 keV fluxes of the bursts range from ~ (3-9)x10^(-10) erg cm^(-2)s^(-1). A correlation between peak flux and fluence of the bursts is found, in agreement with the behaviour observed in other similar sources. There is a trend of the burst flux to increase with the time interval from the previous burst. From the value of the persistent flux we infer a mass accretion rate Mdot~7x10^(-11) Msun/yr, that may correspond to the mixed hydrogen/helium burning regime triggered by thermally unstable hydrogen. We have also analysed a BeppoSAX observation performed on 2001 August 22 and previous RXTE observations of 4U 1708-40, where no bursts have been observed; we found persistent fluxes of more than a factor of 7 higher than the persistent flux observed during the BeppoSAX observation showing X-ray bursts.Comment: accepted for publication in MNRA

Optical design and performance simulations for the 1.49 keV beamline of the BEaTriX X-ray facility

The BEaTriX (Beam Expander Testing X-ray) facility, now operational at INAF-Brera Astronomical Observatory, will represent a cornerstone in the acceptance roadmap of Silicon Pore Optics (SPO) mirror modules, and will so contribute to the final angular resolution of the ATHENA X-ray telescope. By expansion and collimation of a microfocus X-ray source via a paraboloidal mirror, a monochromation stage, and an asymmetric crystal, BEaTriX enables the full-aperture illumination of an SPO mirror module with a parallel, monochromatic, and broad (140 mm × 60 mm) X-ray beam. The beam then propagates in a 12 m vacuum range to image the point spread function of the mirror module, directly on a focal plane camera. Currently the 4.51 keV beamline, based on silicon crystals, is operational in BEaTriX. A second beamline at 1.49 keV, which requires a separate paraboloidal mirror and organic crystals (ADP) for beam expansion, is being realized. As for monochromators, the current design is based on asymmetric quartz crystals. In this paper, we show the current optical design of the 1.49 keV beamline and the optical simulations carried out to predict the achievable performances in terms of beam collimation, intensity, and uniformity. In the next future, the simulation activity will allow us to determine manufacturing and alignment tolerances for the optical components

X-ray tests of the ATHENA mirror modules in BEaTriX: from design to reality

The BEaTriX (Beam Expander Testing X-ray) facility is now operative at the INAF-Osservatorio Astronomico Brera (Merate, Italy). This facility has been specifically designed and built for the X-ray acceptance tests (PSF and Effective Area) of the ATHENA Silicon Pore Optics (SPO) Mirror Modules (MM). The unique setup creates a parallel, monochromatic, large X-ray beam, that fully illuminates the aperture of the MMs, generating an image at the ATHENA focal length of 12 m. This is made possible by a microfocus X-ray source followed by a chain of optical components (a paraboloidal mirror, 2 channel cut monochromators, and an asymmetric silicon crystal) able to expand the X-ray beam to a 6 cm × 17 cm size with a residual divergence of 1.5 arcsec (vertical) × 2.5 arcsec (horizontal). This paper reports the commissioning of the 4.5 keV beam line, and the first light obtained with a Mirror Module

First light of BEaTriX, the new testing facility for the modular X-ray optics of the ATHENA mission

Aims: The Beam Expander Testing X-ray facility (BEaTriX) is a unique X-ray apparatus now operated at the Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Brera (OAB), in Merate, Italy. It has been specifically designed to measure the point spread function (PSF) and the effective area (EA) of the X-ray mirror modules (MMs) of the Advanced Telescope for High-ENergy Astrophysics (ATHENA), based on silicon pore optics (SPO) technology, for verification before integration into the mirror assembly. To this end, BEaTriX generates a broad, uniform, monochromatic, and collimated X-ray beam at 4.51 keV. The beam collimation is better than a few arcseconds, ensuring reliable tests of the ATHENA MMs, in their focus at a 12 m distance. Methods: In BEaTriX, a micro-focus X-ray source with a titanium anode is placed in the focus of a paraboloidal mirror, which generates a parallel beam. A crystal monochromator selects the 4.51 keV line, which is expanded to the final size by a crystal asymmetrically cut with respect to the crystalline planes. An in-house-built Hartmann plate was used to characterize the X-ray beam divergence, observing the deviation of X-ray beams from the nominal positions, on a 12-m-distant CCD camera. After characterization, the BEaTriX beam has the nominal dimensions of 170 mm × 60 mm, with a vertical divergence of 1.65 arcsec and a horizontal divergence varying between 2.7 and 3.45 arcsec, depending on the monochromator setting: either high collimation or high intensity. The flux per area unit varies from 10 to 50 photons/s/cm2 from one configuration to the other. Results: The BEaTriX beam performance was tested using an SPO MM, whose entrance pupil was fully illuminated by the expanded beam, and its focus was directly imaged onto the camera. The first light test returned a PSF and an EA in full agreement with expectations. As of today, the 4.51 keV beamline of BEaTriX is operational and can characterize modular X-ray optics, measuring their PSF and EA with a typical exposure of 30 min. Another beamline at 1.49 keV is under development and will be integrated into the current equipment. We expect BEaTriX to be a crucial facility for the functional test of modular X-ray optics, such as the SPO MMs for ATHENA