We have studied the rapid X-ray time variability in 149 pointed observations with the Rossi X-Ray Timing Explorer (RXTE)'s Proportional Counter Array of the atoll source 4U 1636-53 in the banana ...state and, for the first time with RXTE, in the island state. We compare the frequencies of the variability components of 4U 1636-53 with those in other atoll and Z sources and find that 4U 1636-53 follows the universal scheme of correlations previously found for other atoll sources at (sometimes much) lower luminosities. Our results on the hectohertz QPO suggest that the mechanism that sets its frequency differs from that for the other components, while the amplitude-setting mechanism is common. A previously proposed interpretation of the narrow low-frequency QPO frequencies in different sources in terms of harmonic mode switching is not supported by our data or by previous data on other sources, and the frequency range that this QPO covers is found to be unrelated to spin, angular momentum, or luminosity.
We present the results of the timing and color analysis of more than two hundred RXTE/PCA observations of the bright black-hole transient GX 339–4 obtained during its 2002/2003 outburst. The ...color-intensity evolution of the system, coupled to the properties of its fast time variability, allow the identification of four separate states. Depending on the state, strong noise is detected, together with a variety of quasi-periodic oscillations at frequencies from 0.2 to 8 Hz. We present a characterization of the timing parameters of these states and compare them to what has been observed in other systems. These results, together with those obtained from energy spectra, point towards a common evolution of black-hole transients through their outbursts.
We report our analysis of a Chandra X-ray observation of the rich globular cluster Terzan 5, in which we detect 50 sources to a limiting 1.0-6 keV X-ray luminosity of 3 x 10 super(31) ergs s ...super(-1) within the half-mass radius of the cluster. Thirty-three of these have L sub(x) > 10 super(32) ergs s super(-1), the largest number yet seen in any globular cluster. In addition to the quiescent low-mass X-ray binary (LMXB; identified by Wijnands et al.), another 12 relatively soft sources may be quiescent LMXBs. We compare the X-ray colors of the harder sources in Terzan 5 to the Galactic center sources studied by Muno and collaborators and find the Galactic center sources to have harder X-ray colors, indicating a possible difference in the populations. We cannot clearly identify a metallicity dependence in the production of low-luminosity X-ray binaries in Galactic globular clusters, but a metallicity dependence of the form suggested by Jordan et al. for extragalactic LMXBs is consistent with our data.
ABSTRACT
On 2004 August 15, we observed a fast (shorter than 10 h) state transition in the bright black hole transient GX 339–4 simultaneously with Rossi X‐Ray Timing Explorer (RXTE) and INTEGRAL. ...This transition was evident both in timing and spectral properties. Combining the data from the Proportional Counter Array (PCA), the High‐Energy X‐ray Timing Experiment (HEXTE) and the Imager on Board the INTEGRAL Satellite (IBIS), we obtained good quality broad‐band (3–200 keV) energy spectra before and after the transition. These spectra indicate that the hard component steepened. Also, the high‐energy cut‐off that was present at ∼70 keV before the transition was not detected after the transition. This is the first time that an accurate determination of the broad‐band spectrum across such a transition has been measured on a short time‐scale. It shows that, although some spectral parameters do not change abruptly through the transition, the high‐energy cut‐off increases/disappears rather fast. These results constitute a benchmark on which to test theoretical models for the production of the hard component in these systems.
We present Chandra and XMM–Newton X-ray observations that monitor the neutron star cooling of the quasi-persistent neutron star X-ray transients KS 1731−260 and MXB 1659−29 for approximately 4 yr ...after these sources returned to quiescence from prolonged outbursts. In both sources the outbursts were long enough to significantly heat the neutron star crust out of thermal equilibrium with the core. We analyse the X-ray spectra by fitting absorbed neutron star atmosphere models to the observations. The results of our analysis strengthen the preliminary findings of Wijnands et al. that in both sources the neutron star crust cools down very rapidly suggesting it has a high heat conductivity and that the neutron star core requires enhanced core cooling processes. Importantly, we now detect the flattening of the cooling in both sources as the crust returns to thermal equilibrium with the core. We measure the thermal equilbrium flux and temperature in both sources by fitting a curve that decays exponentially to a constant level. The cooling curves cannot be fit with just a simple exponential decay without the constant offset. We find the constant bolometric flux and effective temperature components to be (9.2 ± 0.9) × 10−14 erg cm−2 s−1 and 70.0 ± 1.6 eV in KS 1731−260 and (1.7 ± 0.3) × 10−14 erg cm−2 s−1 and 51.6 ± 1.4 eV in MXB 1659−29. We note that these values are dependent on the assumed distance to the sources and the column density which was tied between the observations due to the low number of photons in the latter observations. However, importantly, the shape of the cooling curves is independent of the distance assumed. In addition, we find that the crust of KS 1731−260 cools faster than that of MXB 1659−29 by a factor of ∼2, likely due to different crustal properties. This is the first time that the cooling of a neutron star crust into thermal equilibrium with the core has been observed in such detail.
We observed the bright phase of the 2003 outburst of the Galactic black hole candidate H1743-322 in X-rays simultaneously with Chandra and RXTE on four occasions. The Chandra HETGS spectra reveal ...narrow, variable (He-like) Fe XXV and (H-like) Fe XXVI resonance absorption lines. In the first observation, the Fe XXVI line has a FWHM of 1800 c 400 km s super(-1) and a blueshift of 700 c 200 km s super(-1), suggesting that the highly ionized medium is an outflow. Moreover, the Fe XXV line is observed to vary significantly on a timescale of a few hundred seconds in the first observation, which corresponds to the Keplerian orbital period at approximately 10 super(4)r sub(g) (where r sub(g) = GM/c super(2)). Our models for the absorption geometry suggest that a combination of changing ionizing flux and geometric effects are required to account for the large changes in line flux observed between observations and that the absorption likely occurs at a radius between 10 super(2)r sub(g) and 10 super(4)r sub(g) for a 10 M sub(z) black hole. We suggest that the absorption occurs in an inhomogeneous accretion disk wind. If the wind in H1743-322 has unity filling factor, the highest implied mass outflow rate is 5% of the Eddington mass accretion rate. The observed wind may be a hotter, more ionized version of the Seyfert-like, outflowing warm absorber geometries recently found in the Galactic black holes GX 339-4 and XTE J1650-500. We discuss these findings in the context of ionized Fe absorption lines found in the spectra of other Galactic sources, and connections to warm absorbers, winds and jets in other accreting systems.
In this paper, we report on radio (Very Large Array and Austrialian Telescope Compact Array) and X-ray (RXTE, Chandra and Swift) observations of the outburst decay of the transient black hole ...candidate H 1743-322 in early 2008. We find that the X-ray light curve followed an exponential decay, levelling off towards its quiescent level. The exponential decay time-scale is ≈4 days and the quiescent flux corresponds to a luminosity of erg s−1. This together with the relation between quiescent X-ray luminosity and orbital period reported in the literature suggests that H 1743-322 has an orbital period longer than ≈10 h. Both the radio and X-ray light curve show evidence for flares. The radio–X-ray correlation can be well described by a power-law with index ≈0.18. This is much lower than the index of ≈0.6–0.7 found for the decay of several black hole transients before. The radio spectral index measured during one of the radio flares while the source is in the low–hard state is −0.5 ± 0.15, which indicates that the radio emission is optically thin. This is unlike what has been found before in black hole sources in the low–hard state. We attribute the radio flares and the low index for the radio–X-ray correlation to the presence of shocks downstream the jet flow, triggered by ejection events earlier in the outburst. We find no evidence for a change in X-ray power-law spectral index during the decay, although the relatively high extinction of NH≈ 2.3 × 1022 cm−2 limits the detected number of soft photons and thus the accuracy of the spectral fits.
In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748−676 started a transition from outburst to quiescence, after it actively accreted for more than 24 yr. In ...a previous work, we discussed Chandra and Swift observations obtained during the first 5 months of this transition. Here, we report on further X-ray observations of EXO 0748−676, extending the quiescent monitoring to 1.6 yr. Chandra and XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well fitted by a neutron star atmosphere model. An additional hard power-law tail is detected that changes non-monotonically over time, contributing between 4 and 20 per cent to the total unabsorbed 0.5-10 keV flux. The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal bolometric luminosity fades from ∼ 1 × 1034 to 6 × 1033 (D/7.4 kpc)2 erg s −1, whereas the inferred neutron star effective temperature decreases from ∼124 to 109 eV. We interpret the observed decay as cooling of the neutron star crust and show that the fractional quiescent temperature change of EXO 0748−676 is markedly smaller than observed for three other neutron star X-ray binaries that underwent prolonged accretion outbursts.
We have detected an x-ray nebula around the binary millisecond pulsar B1957+20. A narrow tail, corresponding to the shocked pulsar wind, is seen interior to the known Hα bow shock and proves the ...long-held assumption that the rotational energy of millisecond pulsars is dissipated through relativistic winds. Unresolved x-ray emission likely represents the shock where the winds of the pulsar and its companion collide. This emission indicates that the efficiency with which relativistic particles are accelerated in the postshock flow is similar to that for young pulsars, despite the shock proximity and much weaker surface magnetic field of this millisecond pulsar.
While iron emission lines are well studied in black hole systems, both in X-ray binaries and active galactic nuclei, there has been less of a focus on these lines in neutron star low-mass X-ray ...binaries (LMXBs). However, recent observations with Suzaku and XMM-Newton have revealed broad, asymmetric iron line profiles in three neutron star LMXBs, confirming an inner disk origin for these lines in neutron star systems. Here, we present a search for iron lines in six neutron star LMXBs. For each object we have simultaneous Chandra and RXTE observations at two separate epochs, allowing for both a high-resolution spectrum and broadband spectral coverage. Out of the six objects in the survey, we only find significant iron lines in two of the objects, GX 17+2 and GX 349+2. However, we cannot rule out that there are weak, broad lines present in the other sources. The equivalent width of the line in GX 17+2 is consistent between the two epochs, while in GX 349+2 the line equivalent width increases by a factor of ~3 between epochs as the source flux decreases by a factor of 1.3. This suggests that the disk is highly ionized, and the line is dominated by recombination emission. We find that there appears to be no specific locations in the long-term hardness-intensity diagrams where iron emission lines are formed, though more sources and further observations are required.
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