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Register a new userProgressive Covering of the Accretion Disk Corona during Dipping in the LMXB XB 1916-053
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Added by
Monika Balucinska-Church
Publication date
1998
fields
Physics
and research's language is
English
Authors
R. Morley
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Results are reported for analysis of the extensive Rosat observation of the dipping low mass X-ray binary XB 1916-053. Dipping is 100% deep showing that the emission regions are completely covered by the absorber. A good fit to the non-dip spectrum is obtained using a model consisting of a blackbody with kT_BB = 1.95 +0.74 -0.34 keV and a power law with photon index 2.32 +/- 0.04. These components are identified with emission from the neutron star, and Comptonized emission from an extended accretion disk corona (ADC). Dip spectra are well-fitted by rapid absorption of the blackbody, and progressive covering of the extended component, as the absorber moves across the source, with a covering fraction that increases smoothly from zero to ~1.0. Progressive covering shows that the Comptonized emission region is extended, consistent with it originating in the accretion disk corona. The strong unabsorbed component in the dip spectra is well-modelled as the uncovered part of the Comptonized emission at all stages of dipping. There is no detectable change in the low energy cut-off of the spectrum in dipping which supports the identification of the unabsorbed part of the spectrum with the uncovered part of the ADC emission. The absorbed part of the ADC emission is rapidly removed from the 0.1 - 2.0 keV band of the PSPC, which therefore selects only the uncovered part of the emission, and so the spectral evolution in dipping as viewed by the PSPC depends only on the covering fraction, determined by the geometric overlap between the source and absorber.
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Context: XB 1916-053 is a low mass X-ray binary system (LMXB) hosting a neutron star (NS) and showing periodic dips. The spectrum of the persistent emission was modeled with a blackbody component having a temperature between 1.31 and 1.67 keV and with a Comptonization component with an electron temperature of 9.4 keV and a photon index $Gamma$ between 2.5 and 2.9. The presence of absorption features associated with highly ionized elements suggested the presence of partially ionized plasma in the system. Aims: In this work we performed a study of the spectrum of XB 1916-053, which aims to shed light on the nature of the seed photons that contribute to the Comptonization component. Methods: We analyzed three Suzaku observations of XB 1916-053: the first was performed in November 2006 and the others were carried out in October 2014. We extracted the persistent spectra from each observation and combined the spectra of the most recent observations, obtaining a single spectrum with a higher statistic. We also extracted and combined the spectra of the dips observed during the same observations. Results: On the basis of the available data statistics, we infer that the scenario in which the corona Comptonizes photons emitted both by the innermost region of the accretion disk and the NS surface is not statistically relevant with respect to the case in which only photons emitted by the NS surface are Comptonized. We find that the source is in a soft spectral state in all the analyzed observations. We detect the K$alpha$ absorption lines of ion{Fe}{xxv} and ion{Fe}{xxvi}, which have already been reported in literature, and for the first time the K$beta$ absorption lines of the same ions. We also detect an edge at 0.876 keV, which is consistent with a ion{O}{viii} K absorption edge. (Abridged)
We report on the long term monitoring of X-ray dips from the ultracompact low-mass X-ray binary (LMXB) XB 1916-053. Roughly one-month interval observations were carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996, during which the source varied between dim, hard states and more luminous, soft states. The dip spectra and dip lightcurves were compared against both the broadband luminosity and the derived mass accretion rate Mdot. The dips spectra could be fitted by an absorbed blackbody plus cut-off power law non-dip spectral model, with additional absorption ranging from 0 to >100 x 10^22 cm^-2. The amount of additional blackbody absorption was found to vary with the source luminosity. Our results are consistent with an obscuration of the inner disk region by a partially ionized outer disk. The size of the corona, derived from the dip ingress times, was found to be ~10^9 cm. The corona size did not correlate with the coronal temperature, but seemed to increase when Mdot also increased. We discuss our findings in the context of an evaporated accretion disk corona model and an ADAF-type model.
We report the discovery of narrow Fe XXV and Fe XXVI K alpha X-ray absorption lines at 6.65 and 6.95 keV in the persistent emission of the dipping low-mass X-ray binary (LMXB) XB 1916-053 during an XMM-Newton observation performed in September 2002. In addition, there is marginal evidence for absorption features at 1.48 keV, 2.67 kev, 7.82 keV and 8.29 keV consistent with Mg XII, S XVI, Ni XXVII K alpha and Fe XXVI K beta transitions, respectively. Such absorption lines from highly ionized ions are now observed in a number of high inclination (ie. close to edge-on) LMXBs, such as XB 1916-053, where the inclination is estimated to be between 60-80 degrees. This, together with the lack of any orbital phase dependence of the features (except during dips), suggests that the highly ionized plasma responsible for the absorption lines is located in a cylindrical geometry around the compact object. Using the ratio of Fe XXV and Fe XXVI column densities, we estimate the photo-ionization parameter of the absorbing material to be 10^{3.92} erg cm s^{-1}. Only the Fe XXV line is observed during dipping intervals and the upper-limits to the Fe XXVI column density are consistent with a decrease in the amount of ionization during dipping intervals. This implies the presence of cooler material in the line of sight during dipping. We also report the discovery of a 0.98 keV absorption edge in the persistent emission spectrum. The edge energy decreases to 0.87 keV during deep dipping intervals. The detected feature may result from edges of moderately ionized Ne and/or Fe with the average ionization level decreasing from persistent emission to deep dipping. This is again consistent with the presence of cooler material in the line of sight during dipping.
The dipping source XB 1916-053 is a compact binary system with an orbital period of 50 min harboring a neutron star. Using ten new {it Chandra} observations and one {it Swift/XRT} observation, we are able to extend the baseline of the orbital ephemeris; this allows us to exclude some models that explain the dip arrival times. The Chandra observations provide a good plasma diagnostic of the ionized absorber and allow us to determine whether it is placed at the outer rim of the accretion disk or closer to the compact object. From the available observations we are able to obtain three new dip arrival times extending the baseline of the orbital ephemeris from 37 to 40 years. From the analysis of the dip arrival times we confirm an orbital period derivative of $dot{P}=1.46(3) times 10^{-11}$ s s$^{-1}$. We show that the $dot{P}$ value and the luminosity values are compatible with a mass accretion rate lower than 10% of the mass transfer rate. We show that the mass ratio $q=m_2/m_1$ of 0.048 explains the apsidal precession period and the nodal precession period. The observed absorption lines are associated with the presence of ion{Ne}{x}, ion{Mg}{xii}, ion{Si}{xiv}, ion{S}{xvi,} and ion{Fe}{xxvi} ions. We observe a redshift in the absorption lines between $1.1 times 10^{-3}$ and $1.3 times 10^{-3}$. By interpreting it as gravitational redshift, as recently discussed in the literature, we find that the ionized absorber is placed at a distance of $10^8$ cm from the neutron star with a mass of 1.4 M$_{odot}$ and has a hydrogen atom density greater than $10^{15}$ cm$^{-3}$. (Abstract abridged)
The very small accretion disks in ultra-compact X-ray binaries (UCXBs) are special laboratories in which to study disk accretion and outflows. We report on three sets of new (250 ks total) and archival (50 ks) Chandra/HETG observations of the dipping neutron-star X-ray binary 4U 1916$-$053, which has an orbital period of $Psimeq 50$~minutes. We find that the bulk of the absorption in all three spectra originates in a disk atmosphere that is redshifted by $vsimeq 220-290$ $text{km}$ $text{s}^{-1}$, corresponding to the gravitational redshift at radius of $R sim 1200$ $GM/{c}^{2}$. This shift is present in the strongest, most highly ionized lines (Si XIV and Fe XXVI), with a significance of 5$sigma$. Absorption lines observed during dipping events (typically associated with the outermost disk) instead display no velocity shifts and serve as a local standard of rest, suggesting that the redshift is intrinsic to an inner disk atmosphere and not due to radial motion in the galaxy or a kick. In two spectra, there is also evidence of a more strongly redshifted component that would correspond to a disk atmosphere at $R sim 70$ $GM/{c}^{2}$; this component is significant at the 3$sigma$ level. Finally, in one spectrum, we find evidence of disk wind with a blue shift of $v = {-1700}^{+1700}_{-1200}$ $text{km}$ $text{s}^{-1}$. If real, this wind would require magnetic driving.
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