A B S T R A C TWe calculate self-consistent models of X-ray-irradiated accretion discs in close binary systems. We show that a point X-ray source powered by accretion and located in the disc plane cannot modify the disc structure, mainly because of the self-screening by the disc of its outer regions. As observations show that the emission of the outer disc regions in low-mass X-ray binaries is dominated by the reprocessed X-ray flux, accretion discs in these systems must be either warped or irradiated by a source above the disc plane, or both. We analyse the thermal-viscous stability of irradiated accretion discs and derive the stability criteria of such systems. We find that, contrary to the usual assumptions, the critical accretion rate below which a disc is unstable is rather uncertain because the correct formula describing irradiation is not well known.

We present outburst spectroscopy of GX 339-4 which may reveal the motion of its elusive companion star. N iii lines exhibit sharp emission components moving over ∼ 300 km s −1 in a single night. The most plausible interpretation of these components is that they are formed by irradiation of the companion star and the velocities indicate its orbital motion. We also detect motion of the wings of the He ii 4686Å line and changes in its morphology. No previously proposed period is consistent with periodic behavior of all of these measures. However, consistent and sensible solutions are obtained for periods around 1.7 days. For the best period, 1.7557 days, we estimate a mass function of 5.8 ± 0.5 M ⊙ . Even allowing for aliases, the 95 % confidence lower-limit on the mass function is 2.0 M ⊙ . GX 339-4 can therefore be added to the list of dynamical black hole candidates. This is supported by the small motion in the wings of the He ii line; if the compact object velocity is not larger than the observed motion then the mass ratio is q 0.08, similar to other systems harboring black holes. Finally, we note that the sharp components are not always present, but do seem to occur within a repeating phase range. This appears to migrate between our epochs of observation, and may indicate shielding of the companion star by a variable accretion geometry such as a warp.

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We observed the bright phase of the 2003 outburst of the Galactic black hole candidate H 1743−322 in Xrays 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 ± 400 km/s and a blue-shift of 700 ± 200 km/s, 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 4 r g (where r g = GM/c 2 ). Our models for the absorption geometry suggest that a combination of geometric effects and changing ionizing flux are required to account for the large changes in line flux observed between observations, and that the absorption likely occurs at a radius less than 10 4 r g for a 10 M ⊙ black hole. Viable models for the absorption geometry include cyclic absorption due to an accretion disk structure, absorption in a clumpy outflowing disk wind, or possibly a combination of these two. If the wind in H 1743−322 has unity filling factor, the highest implied mass outflow rate is 20% of the Eddington mass accretion rate. This wind may be a hot precursor to 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.

We describe observational evidence for a new kind of interacting-binary-star outburst that involves both an accretion instability and an increase in thermonuclear shell burning on the surface of an accreting white dwarf. We refer to this new type of eruption as a combination nova. In late 2000, the prototypical symbiotic star Z Andromedae brightened by roughly two magnitudes in the optical. We observed the outburst in the radio with the VLA and MERLIN, in the optical both photometrically and spectroscopically, in the far ultraviolet with F U SE, and in the X-rays with both Chandra and XM M . The two-year-long event had three distinct stages. During the first stage, the optical rise closely resembled an earlier, small outburst that was caused by an accretion-disk instability. In the second stage, the hot component ejected an optically thick shell of material. In the third stage, the shell cleared to reveal a white dwarf whose luminosity remained on the order of 10 4 L ⊙ for approximately one year. The eruption was thus too energetic to have been powered by accretion alone. We propose that the initial burst of accretion was large enough to trigger enhanced nuclear burning on the surface of the white dwarf and the ejection of an optically thick shell of material. This outburst therefore combined elements of both a dwarf nova and a classical nova. Our results have implications for the long-standing problem of producing shell flashes with short recurrence times on low-mass white dwarfs in symbiotic stars.

We observed the Galactic black hole GX 339−4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ksec during the decline of its 2002-2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT ≃ 0.6 keV) and power-law (Γ ≃ 2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe Kα emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The breadth of the line is sufficient to suggest that GX 339−4 may harbor a black hole with significant angular momentum. Absorption lines from H-like and He-like O, and He-like Ne and Mg are detected, as well as lines which are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to an intrinsic AGN-like warmabsorber geometry -perhaps produced by a disk wind in an extended disk-dominated state -may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650−500, and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes.

The distance to the black hole binary GX 339-4 remains a topic of debate. We examine high-resolution optical spectra of the Na D lines resolving the velocity structure along the line of sight. We find this to be complex, with at least nine components, mostly blue-shifted, spanning a velocity range of nearly 200 km s −1 . The presence of components with a large blue-shift rules out a nearby location and requires that the binary be located at or beyond the tangent point, implying a lower limit to the distance of ∼ 6 kpc. The presence of a significant red-shifted component at +30 km s −1 is even more intriguing as GX 339-4 also has a slightly positive systemic velocity, suggesting that the source, and this cloud, could be on the far side of the Galaxy, where the radial velocities due to Galactic rotation become positive again. If this is the case, we require a distance of ∼ 15 kpc. This is less secure than the 6 kpc lower limit however. We discuss the implications of these possible distances for the outburst and quiescent luminosities, and the nature of the companion star, and argue that a large distance is consistent with these characteristics. In particular, it would explain the non-detection of the companion star during the faintest states.

Optical spectra were obtained of the optical counterpart of the high-latitude (b^62¡) soft X-ray transient XTE J1118]480 near its quiescent state (R^18.3) with the new 6.5 m Multiple Mirror Telescope and the 4.2 m William Herschel Telescope. The spectrum exhibits broad, double-peaked emission lines of hydrogen (FWHM^2400 km s~1) arising from an accretion disk superposed with absorption lines of a late-type secondary star. Cross-correlation of the 27 individual spectra with late-type stellar template spectra reveals a sinusoidal variation in radial velocity with amplitude K \ 701^10 km s~1 and orbital period P \ 0.169930^0.000004 days. The mass function, 6.1^0.3 is a Ðrm lower limit on the M _ , mass of the compact object and strongly implies that it is a black hole. We estimate the spectral type of the secondary to be K7 VÈM0 V, and that it contributes 28%^2% of the light in the 5800È6400 The photometric period measured during the outburst is 0.5% longer than our orbital M _ . period and probably reÑects superhump modulations, as observed in some other soft X-ray transients. The estimated distance is d \ 1.9^0.4 kpc, corresponding to a height of 1.7^0.4 kpc above the Galactic plane. The spectroscopic, photometric, and dynamical results indicate that XTE J1118]480 is the Ðrst Ðrmly identiÐed black hole X-ray system in the Galactic halo.