Swift reveals the eclipsing nature of the high mass X-ray binary IGR~J16195-4945

IGR J16195-4945 is a hard X-ray source discovered by INTEGRAL during the Core Program observations performed in 2003. We analyzed the X-ray emission of this source exploiting the Swift-BAT survey data from December 2004 to March 2015, and all the available Swift-XRT pointed observations. The source is detected at a high significance level in the 123-month BAT survey data, with an average 15-150 keV flux of the source of ~1.6 mCrab. The timing analysis on the BAT data reveals with a significance higher than 6 standard deviations the presence of a modulated signal with a period of 3.945 d, that we interpret as the orbital period of the binary system. The folded light curve shows a flat profile with a narrow full eclipse lasting ~3.5% of the orbital period. We requested phase-constrained XRT observations to obtain a more detailed characterization of the eclipse in the soft X-ray range. Adopting resonable guess values for the mass and radius of the companion star, we derive a semi-major orbital axis of ~31 R_sun, equivalent to ~1.8 times the radius of the companion star. From these estimates and from the duration of the eclipse we derive an orbital inclination between 55 and 60 degrees. The broad band time-averaged XRT+BAT spectrum is well modeled with a strongly absorbed flat power law, with absorbing column N_H=7x 10^22 cm^(-2) and photon index Gamma=0.5, modified by a high energy exponential cutoff at E_cut=14 keV.Comment: 5 pages, 5 figures, 2 tables. Published on MNRA

Swift-XRT 6-year monitoring of the ultraluminous X-ray source M33-X8

The long term evolution of ULX with their spectral and luminosity variations in time give important clues on the nature of ULX and on the accretion process that powers them. We report here the results of a Swift-XRT 6-year monitoring campaign of the closest example of a persistent ULX, M33 X-8, that extends to 16 years the monitoring of this source in the soft X-rays. The luminosity of this source is a few 10^39 erg/s, marking the faint end of the ULX luminosity function. We analysed the set of 15 observations collected during the Swift monitoring. We searched for differences in the spectral parameters at different observing epochs, adopting several models commonly used to fit the X-ray spectra of ULX. The source exhibits flux variations of the order of 30%. No significant spectral variations are observed along the monitoring. The average 0.5-10 keV spectrum can be well described by a thermal model, either in the form of a slim disk, or as a combination of a Comptonized corona and a standard accretion disk.Comment: 6 pages, 4 figures, 2 tables. Paper published in A&

Temporal features of LS I +61∘^{\circ}303 in hard X-rays from the Swift/BAT survey data

We study the long-term spectral and timing behaviour of LS I +61$^{\circ}$303 in hard X-rays (15--150 keV) using $\sim$10 years of survey data from the $Swift$ Burst Alert Telescope (BAT) monitor. We focus on the detection of long periodicities known to be present in this source in multiple wavelengths. We clearly detect three periods: the shorter one at 26.48 days is compatible with the orbital period of the system; the second, longer, periodicity at 26.93 days, is detected for the first time in X-rays and its value is consistent with an analogous temporal feature recently detected in the radio and in the gamma-ray waveband, and we associate it with a modulation caused by a precessing jet in this system. Finally, we find also evidence of the long-term periodicity at $\sim$1667 d, that results compatible with a beat frequency of the two close, and shorter, periodicities. We discuss our results in the context of the multi-band behaviour of the physical processes of this source.Comment: 5 pages, 8 figures. Published in MNRA

The complex time behaviour of the microquasar GRS 1915+105 in the \rho-class observed with BeppoSAX. III: The hard X-ray delay and limit cycle mapping

The microquasar GRS1915+105 was observed by BeppoSAX in October 2000 for about ten days while the source was in \rho-mode, which is characterized by a quasi-regular type I bursting activity. This paper presents a systematic analysis of the delay of the hard and soft X-ray emission at the burst peaks. The lag, also apparent from the comparison of the [1.7-3.4] keV light curves with those in the [6.8-10.2] keV range, is evaluated and studied as a function of time, spectral parameters, and flux. We apply the limit cycle mapping technique, using as independent variables the count rate and the mean photon rate. The results using this technique were also cross-checked using a more standard approach with the cross-correlation methods. Data are organized in runs, each relative to a continuous observation interval. The detected hard-soft delay changes in the course of the pointing from about 3 s to about 10 s and presents a clear correlation with the baseline count rate.Comment: accepted for publication in A&

Chandra X-ray spectroscopy of a clear dip in GX 13+1

The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an accreting neutron star. It shows highly ionized absorption features, with a blueshift of $\sim$ 400 km s$^{-1}$ and an outflow-mass rate similar to the accretion rate. Many other X-ray sources exhibit warm absorption features, and they all show periodic dipping behavior at the same time. Recently, a dipping periodicity has also been determined for GX 13+1 using long-term X-ray folded light-curves, leading to a clear identification of one of such periodic dips in an archival Chandra observation. We give the first spectral characterization of the periodic dip of GX 13+1 found in this archival Chandra observation performed in 2010. We used Chandra/HETGS data (1.0-10 keV band) and contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broadband X-ray spectrum. We adopted different spectral models to describe the continuum emission and used the XSTAR-derived warm absorber component to constrain the highly ionized absorption features. The 1.0-25 keV continuum emission is consistent with a model of soft accretion-disk emission and an optically thick, harder Comptonized component. The dip event, lasting $\sim$ 450 s, is spectrally resolved with an increase in the column density of the neutral absorber, while we do not find significant variations in the column density and ionization parameter of the warm absorber with respect to the out-of-dip spectrum. We argue that the very low dipping duty-cycle with respect to other sources of the same class can be ascribed to its long orbital period and the mostly neutral bulge, that is relatively small compared with the dimensions of the outer disk radius.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and Astrophysic