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Register a new userVariations in the dip properties of the low-mass X-ray binary XB 1254-69 observed with XMM-Newton and INTEGRAL
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We have analysed data from five XMM-Newton observations of XB 1254-69, one of them simultaneous with INTEGRAL, to investigate the mechanism responsible for the highly variable dips durations and depths seen from this low-mass X-ray binary. Deep dips were present during two observations, shallow dips during one and no dips were detected during the remaining two observations. At high (1-4 s) time resolution ``shallow dips are seen to include a few, very rapid, deep dips whilst the ``deep dips consist of many similar very rapid, deep, fluctuations. The folded V-band Optical Monitor light curves obtained when the source was undergoing deep, shallow and no detectable dipping exhibit sinusoid-like variations with different amplitudes and phases. We fit EPIC spectra obtained from persistent or dip-free intervals with a model consisting of disc-blackbody and thermal comptonisation components together with Gaussian emission features at 1 and 6.6 keV modified by absorption due to cold and photo-ionised material. None of the spectral parameters appears to be strongly correlated with the dip depth except for the temperature of the disc blackbody which is coolest (kT ~ 1.8 keV) when deep dips are present and warmest (kT ~ 2.1 keV) when no dips are detectable. We propose that the changes in both disc temperature and optical modulation could be explained by the presence of a tilted accretion disc in the system. We provide a revised estimate of the orbital period of 0.16388875 +/- 0.00000017 day.
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We report on two XMM-Newton observations of the low-mass X-ray binary X 1254-690. During an XMM-Newton observation of the low-mass X-ray binary in 2001 January a deep X-ray dip was seen while in a second observation one year later no dips were evident. The 0.5-10 keV EPIC spectra from both non-dipping intervals are very similar being modeled by a disk-blackbody and a power-law continuum with additional structure around 1 keV and narrow absorption features at 7.0 keV and 8.2 keV which are identified with the K alpha and K beta absorption lines of Fe XXVI. The low-energy structure may be modeled as a 175 eV (sigma) wide emission line at ~0.95 keV. This feature is probably the same structure that was modeled as an absorption edge in an earlier BeppoSAX observation. The absorption line properties show no obvious dependence on orbital phase and are similar in both observations suggesting that the occurrence of such features is not directly related to the presence of dipping activity. Narrow Fe absorption features have been observed from the two superluminal jet sources GRO J1655-40 and GRS 1915+105, and the four low-mass X-ray binaries GX 13+1, MXB 1658-298, X 1624-490 and X 1254-690. Since the latter 3 sources are dipping sources, which are systems viewed close to the accretion disk plane, and the two microquasars are thought to be viewed at an inclination of ~70 degrees, this suggests that these features are more prominent when viewed at high-inclination angles. This, together with the lack of any orbital dependence, implies a cylindrical geometry for the absorbing material.
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.
We present photometric and spectroscopic observations of the low mass X-ray binary GR Mus (XB 1254-690), and find strong evidence for the presence of a negative superhump with a period that is 2.4+/-0.3% shorter than the orbital. This provides further support that GR Mus indeed harbours a precessing accretion disk (with a period of 6.74+/-0.07 day) that has retrograde precession and is completely tilted out of the orbital plane along its line of nodes. This tilt causes a large fraction of the gas in the accretion stream to either over- or underflow the accretion disk instead of hitting the disk rim, and could be a feature of all low mass X-ray binaries with characteristics similar to GR Mus (i.e. the so-called atoll sources). Furthermore, we also find marginal evidence for the presence of a positive superhump, suggesting that the accretion disk in GR Mus is eccentric due to tidal resonances. If true, than the relationship between the positive superhump period excess and the mass ratio (q) provides a constraint of q=M_donor/M_NS=0.33-0.36. Together with the radial velocity semi-amplitude measurements of the compact object, and previous modeling of the inclination we obtain a mass for the neutron star of 1.2<M_NS/M_sun<1.8 (95% confidence).
4U 0614+09 is a low-mass X-ray binary with a weakly magnetized neutron star primary. It shows variability on time scales that range from years down to ~0.8 milliseconds. Before the Chandra and XMM-Newton era, emission features around 0.7 keV have been reported from this source, but recent Chandra observations failed to detect them. Instead, these observations suggest an overabundance of Ne in the absorbing material, which may be common to ultracompact (P_{orb} simless 1 hour) systems with a neon-rich degenerate dwarf secondary. We observed 4U 0614+09 with XMM-Newton in March 2001. Here we present the energy spectra, both from the RGS and EPIC cameras, and the Fourier power spectra from EPIC high-time resolution light curves, which we use to characterize the spectral state of the source.
We report the discovery of narrow X-ray absorption features from the two dipping low-mass X-ray binary 4U 1916-053 and X 1254-690 during XMM-Newton observations. The features detected are identified with resonant scattering absorption lines of highly ionized iron (Fe XXV and Fe XXVI). Resonant absorption features are now observed in a growing number of low-mass X-ray binaries (LMXBs): the two superluminal jet sources GRS 1915+105 and GRO J1655-40, the bright LMXB GX 13+1 and the four dipping sources MXB 1658-298, X 1624-490, 4U 1916-053 and X 1254-690. The early hypothesis that their origin could be related to the presence of superluminal jets is thus ruled out. Ionized absorption features may be common characteristics of accreting systems. Furthermore, their presence may depend on viewing angle, as suggested by their detection in dippers which are viewed close to the disk plane, and by the fact that GRS 1915+105, GRO J1655-40 and GX 13+1, although not dippers, are suspected to be also viewed at high inclination.
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