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Register a new userThe X-ray Halo of GX 13+1
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We present observations of the X-ray halo around the LMXB GX 13+1 from the Chandra X-ray telescope. The halo is caused by scattering in interstellar dust grains, and we use it to diagnose the line-of-sight position, size distribution, and density of the grains. Using the intrinsic energy resolution of Chandras ACIS CCDs and the recent calibration observation of the Chandra point spread function (PSF), we were able to extract the halo fraction as a function of energy and off-axis angle. We define a new quantity, the ``halo coefficient, or the total halo intensity relative to the source at 1 keV, and measure it to be $1.5^{+0.5}_{-0.1}$ for GX 13+1. We find a relationship between this value and the dust size, density, and hydrogen column density along the line of sight to GX 13+1. We also conclude that our data does not agree with ``fluffy dust models that earlier X-ray halo observations have supported, and that models including an additional large dust grain population are not supported by these data.
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We present the analysis of seven emph{Chandra} High Energy Transmission Grating Spectrometer and six simultaneous emph{RXTE} Proportional Counter Array observations of the persistent neutron star (NS) low-mass X-ray binary GX 13+1 on its normal and horizontal branches. Across nearly 10 years, GX 13+1 is consistently found to be accreting at $50-70$% Eddington, and all observations exhibit multiple narrow, blueshifted absorption features, the signature of a disk wind, despite the association of normal and horizontal branches with jet activity. A single absorber with standard abundances cannot account for all seven major disk wind features, indicating that multiple absorption zones may be present. Two or three absorbers can produce all of the absorption features at their observed broadened widths and reveal that multiple kinematic components produce the accretion disk wind signature. Assuming the most ionized absorber reflects the physical conditions closest to the NS, we estimate a wind launching radius of $7times10^{10}$ cm, for an electron density of $10^{12}$ cm$^{-3}$. This is consistent with the Compton radius and also with a thermally driven wind. Because of the sources high Eddington fraction, radiation pressure likely facilitates the wind launching.
We fit the observed high ionisation X-ray absorption lines in the neutron star binary GX13+1 with a full simulation of a thermal-radiative wind. This uses a radiation hydrodynamic code coupled to Monte Carlo radiation transfer to compute the observed line profiles from Hydrogen and Helium-like iron and Nickel, including all strong K{alpha} and K{beta} transitions. The wind is very strong as this object has a very large disc and is very luminous. The absorption lines from Fe K{alpha} are strongly saturated as the ion columns are large, so the line equivalent widths (EWs) depend sensitively on the velocity structure. We additionally simulate the lines including isotropic turbulence at the level of the azimuthal and radial velocities. We fit these models to the Fe xxv and xxvi absorption lines seen in the highest resolution Chandra third order HETGS data. These data already rule out the addition of turbulence at the level of the radial velocity of ~500 km/s. The velocity structure predicted by the thermal-radiative wind alone is a fairly good match to the observed profile, with an upper limit to additional turbulence at the level of the azimuthal velocity of ~100 km/s. This gives stringent constraints on any remaining contribution from magnetic acceleration.
We present analysis of RXTE--PCA observations of GX 1+4 between March 3, 2001 and January 31, 2003 together with the CGRO--BATSE X-ray flux and frequency derivative time series between 1991 and 1999. From the timing analysis of RXTE-PCA observations, we are able to phase connect pulse arrival times of the source within two different time intervals and obtain corresponding timing solutions. Using these pulse arrival times, we contribute to long term pulse frequency history of the source. We look for episodic correlations and anti-correlations between torque and X-ray luminosity using CGRO--BATSE X-ray flux and frequency derivative time series and find that correlation state of GX 1+4 seems to change on $sim$ 100-200 days long intervals. We estimate torque noise of the source and observe flickering noise ($f^{-1}$). We achieve to measure the longest observed timescale for a noise process among accretion powered X-ray pulsars by extending the noise estimate for a time scale ranging from 31 days to 44 years. Spectral analysis of individual RXTE-PCA observations indicates a significant correlation between iron line flux and unabsorbed X-ray flux. Pulse phase resolved spectra of the source indicate a broadening of iron line complex at the bin corresponding to the pulse minimum.
Broad, asymmetric, and red-skewed Fe Kalpha emission lines have been observed in the spectra of low-mass X-ray binaries hosting neutron stars (NSs) as a compact object. Because more than one model is able to describe these features, the explanation of where and how the red-skewed Fe lines are produced is still a matter of discussion. It is broadly accepted that the shape of the Fe Kalpha line is strongly determined by the special and general relativistic effects occurring in the innermost part of the accretion disk. In this relativistic framework, the Fe fluorescent lines are produced in the innermost part of the accretion disk by reflection of hard X-ray photons coming from the central source (corona and/or NS surface). We developed an alternative and nonrelativistic model, called the windline model, that is capable to describe the Fe line features. In this nonrelativistic framework, the line photons are produced at the bottom of a partly ionized outflow (wind) shell as a result of illumination by the continuum photons coming from the central source, and the red-skewness of the line profile is explained by repeated electron scattering of the photons in a diverging outflow. Because GX~13+1 is a well-known disk-wind source, it is a perfect target for testing the windline model and comparing it to the relativistic one. In order to access the goodness of the fit and distinguish between the two line models, we used the run-test statistical method in addition to the canonical $chi^2$ statistical method. The diskline and windline models both fit the asymmetric GX13+1 Fe line well. From a statistical point of view, for the two observations we analyzed, the run-test was not able to distinguish between the two Fe line models, at 5% significance level.
We present results obtained from a Suzaku observation of the accretion powered X-ray pulsar GX 1+4. Broad-band continuum spectrum of the pulsar was found to be better described by a simple model consisting of a blackbody component and an exponential cutoff power-law than the previously used compTT continuum model. Though the pulse profile had a sharp dip in soft X-rays ($<$10 keV), phase-resolved spectroscopy confirmed that the dimming was not due to increase in photoelectric absorption. Phase-sliced spectral analysis showed the presence of a significant spectral modulation beyond 10 keV except for the dip phase. A search for the presence of cyclotron resonance scattering feature in the Suzaku spectra yielded a negative result. Iron K-shell (K$_alpha$ and K$_beta$) emission lines from nearly neutral iron ions ($<$Fe III) were clearly detected in the source spectrum. A significant K$_alpha$ emission line from almost neutral Ni atoms was detected for the first time in this source. We estimated the iron abundance of $sim$80 % of the solar value and Ni/Fe abundance ratio of about two times of the solar value. We searched for a iron Ly$_alpha$ emission line and found a significant improvement in the spectral fitting by inclusion of this line.
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