P. A. Charles

X-RAY IRRADIATION IN LOW-MASS BINARY SYSTEMS

We 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.

A Long, Hard Look at the Low/Hard State in Accreting Black Holes

We present the first results of coordinated multiwavelength observations of the Galactic black hole GX 339-4 in a canonical low/hard state, obtained during its 2004 outburst. XMM-Newton observed the source for two revolutions, or approximately 280 ks; RXTE monitored the source throughout this long stare. The resulting data offer the best view yet obtained of the inner accretion flow geometry in the low/hard state, which is thought to be analogous to the geometry in low-luminosity active galactic nuclei. The XMM-Newton spectra clearly reveal the presence of a cool accretion disk component and a relativistic Fe K emission line. The results of fits made to both components strongly suggest that a standard thin disk remains at or near to the innermost stable circular orbit, at least in bright phases of the low/hard state. These findings indicate that potential links between the inner disk radius and the onset of a steady compact jet, and the paradigm of a radially recessed disk in the low/hard state, do not hold universally. The results of our observations can best be explained if a standard thin accretion disk fuels a corona that is closely related to, or consistent with, the base of a compact jet. In a brief examination of archival data, we show that Cygnus X-1 supports this picture of the low/hard state. We discuss our results within the context of disk-jet connections and prevailing models for accretion onto black holes.

Dynamical Evidence for a Black Hole in GX 339–4

We present outburst spectroscopy of GX 339-4 that 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 4686 A 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.

Chandra/high energy transmission grating spectrometer spectroscopy of the galactic black hole GX 339-4: A relativistic iron emission line and evidence for a Seyfert-like warm absorber

We observed the Galactic black hole GX 339-4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ks 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 (Gamma~=2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe Kalpha 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- and He-like O and He-like Ne and Mg are detected, as well as lines that 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 warm-absorber 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 and Interstellar Sight Line to GX 339?4

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 blueshifted, spanning a velocity range of nearly 200 km s-1. The presence of components with a large blueshift 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 redshifted 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, as well as the nature of the companion star, and argue that a large distance explains these characteristics. In particular, it would explain the nondetection of the companion star during the faintest states.

Simultaneous chandra and rxte spectroscopy of the microquasar H1743-322 : Clues to disk wind and jet formation from a variable ionized outflow

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 ? 400 km s-1 and a blueshift of 700 ? 200 km s-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 104rg (where rg = GM/c2). 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 102rg and 104rg for a 10 M? 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.

The Halo Black Hole X-Ray Transient XTE J1118+480*

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 M☉, is a firm lower limit on the 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 V-M0 V, and that it contributes 28% ± 2% of the light in the 5800-6400 A region on 2000 November 20, increasing to 36% ± 2% by 2001 January 4 as the disk faded. Photometric observations (R-band) with the Instituto de Astrofisica de Canarias 0.8 m telescope reveal ellipsoidal light variations of full amplitude 0.2 mag. Modeling of the light curve gives a large mass ratio (M1/M2 ~ 20) and a high orbital inclination (i = 81° ± 2°). Our combined fits yield a mass of the black hole in the range M1 = 6.0-7.7 M☉ (90% confidence) for plausible secondary star masses of M2 = 0.09-0.5 M☉. The photometric period measured during the outburst is 0.5% longer than our orbital period and probably reflects 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 first firmly identified black hole X-ray system in the Galactic halo.

The mass of x-ray Nova Scorpii 1994 GRO J1655-40

We have obtained high- and intermediate-resolution optical spectra of the black hole candidate Nova Scorpii 1994 in 1998 May/June, when the source was in complete (X-ray) quiescence. We measure the radial velocity curve of the secondary star and obtain a semi-amplitude of K2=215.5 ± 2.4 km s-1, which is 6 per cent lower than the only previously determined value. This new value for K2 thus reduces the binary mass function to f(M) = 2.73 ± 0.09 M⊙. Using only the high-resolution spectra, we constrain the rotational broadening of the secondary star, v sin i, to lie in the range 82.9–94.9 km s-1 (95 per cent confidence) and thus constrain the binary mass ratio to lie in the range 0.337–0.436 (95 per cent confidence). We can also combine our results with published limits for the binary inclination to constrain the mass of the compact object and secondary star to the ranges 5.5–7.9 and 1.7–3.3 M⊙ respectively (95 per cent confidence). Finally, we report on the detection of the lithium resonance line at 6707.8 A, with an equivalent width of 55 ± 8 m A.

Bright radio emission from an ultraluminous stellar-mass microquasar in M 31

A subset of ultraluminous X-ray sources (those with luminosities of less than 1040 erg s−1; ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of ∼5–20, probably by means of an accretion disk. The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 1039 erg s−1. The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.

Evidence of black hole spin in GX 339-4: XMM-Newton/EPIC-pn and RXTE spectroscopy of the very high state

We have analyzed spectra of the Galactic black hole GX 339-4 obtained through simultaneous 76 ks XMM-Newton/EPIC-pn and 10 ks Rossi X-Ray Timing Explorer observations during a bright phase of its 2002-2003 outburst. An extremely skewed, relativistic Fe Kalpha emission line and ionized disk reflection spectrum are revealed in these spectra. Self-consistent models for the Fe Kalpha emission-line profile and disk reflection spectrum rule out an inner disk radius compatible with a Schwarzschild black hole at more than the 8 sigma level of confidence. The best-fit inner disk radius of (2-3)rg suggests that GX 339-4 harbors a black hole with a>=0.8-0.9 (where rg=GM/c2 and a=cJ/GM2, and assuming that reflection in the plunging region is relatively small). This confirms indications for black hole spin based on a Chandra spectrum obtained later in the outburst. The emission line and reflection spectrum also rule out a standard power-law disk emissivity in GX 339-4 a broken power-law form with enhanced emissivity inside ~6rg gives improved fits at more than the 8 sigma level of confidence. The extreme red wing of the line and the steep emissivity require a centrally concentrated source of hard X-rays that can strongly illuminate the inner disk. Hard X-ray emission from the base of a jet-enhanced by gravitational light-bending effects-could create the concentrated hard X-ray emission; this process may be related to magnetic connections between the black hole and the inner disk. We discuss these results within the context of recent results from analyses of XTE J1650-500 and MCG -6-30-15, and of models for the inner accretion flow environment around black holes.