K. O'Brien

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.

The remarkable rapid X-ray, ultraviolet, optical and infrared variability in the black hole XTE J1118+480

The transient black-hole binary XTE J1118+480 exhibited dramatic rapid variability at all wavelengths which were suitably observed during its 2000 April–July outburst. We examine time-resolved X-ray, ultraviolet, optical and infrared data spanning the plateau phase of the outburst. We find that both X-ray and infrared bands show large amplitude variability. The ultraviolet and optical variability is more subdued, but clearly correlated with that seen in the X-rays. The ultraviolet, at least, appears to be dominated by the continuum, although the lines are also variable. Using the X-ray variations as a reference point, we find that the ultraviolet (UV) variability at long wavelengths occurs later than that at short wavelengths. Uncertainty in the Hubble Space Telescope timing prohibits a determination of the absolute lag with respect to the X-rays, however. The transfer function is clearly not a delta-function, exhibiting significant repeatable structure. For the main signal we can rule out an origin in reprocessing on the companion star – the lack of variation in the lags is not consistent with this, given a relatively high orbital inclination. Weak reprocessing from the disc and/or companion star may be present, but is not required, and another component must dominate the variability. This could be variable synchrotron emission correlated with X-ray variability, consistent with our earlier interpretation of the infrared (IR) flux as due to synchrotron emission rather than thermal disc emission. In fact, the broad-band energy distribution of the variability from IR to X-rays is consistent with expectations of optically thin synchrotron emission. We also follow the evolution of the low-frequency quasi-periodic oscillation in X-rays, UV, and optical. Its properties at all wavelengths are similar, indicating a common origin.

The 1996 outburst of GRO J1655-40: the challenge of interpreting the multiwavelength spectra

We report on the results of a multiwavelength campaign to observe the soft X-ray transient (SXT) and superluminal jet source GRO J1655-40 in outburst using HSTRXTECGRO together with ground-based facilities. This outburst was qualitatively quite different from other SXT outbursts and from previous outbursts of this source. The onset of hard X-ray activity occurred very slowly, over several months, and was delayed relative to the soft X-ray rise. During this period, the optical fluxes declined steadily. This apparent anticorrelation is not consistent with the standard disc instability model of SXT outbursts, nor is it expected if the optical output is dominated by reprocessed X-rays, as in persistent low-mass X-ray binaries. Based on the strength of the 2175-Angstroms interstellar absorption feature we constrain the reddening to be E(B-V)=1.2+/-0.1, a result which is consistent with the known properties of the source and with the strength of interstellar absorption lines. Using this result we find that our dereddened spectra are dominated by a component peaking in the optical, with the expected nu^1/3 disc spectrum seen only in the ultraviolet. We consider possible interpretations of this spectrum in terms of thermal emission from the outer accretion disc and/or secondary star, both with and without X-ray irradiation, and also as non-thermal optical synchrotron emission from a compact self-absorbed central source. In addition to the prominent Heii 4686-Angstroms line, we see Bowen fluorescence lines of Niii and Oiii, and possible P Cygni profiles in the ultraviolet resonance lines, which can be interpreted in terms of an accretion disc wind. The X-ray spectra broadly resemble the high-soft state commonly seen in black hole candidates, but evolve through two substates. Taken as a whole, the outburst data set cannot readily be interpreted by any standard model for SXT outbursts. We suggest that many of the characteristics could be interpreted in the context of a model combining X-ray irradiation with the limit-cycle disc instability, but with the added ingredient of a very large disc in this long-period system.

Echoes in X-ray binaries

We present a method of analysing the correlated X-ray and optical/UV variability in X-ray binaries, using the observed time delays between the X-ray driving light curves and their reprocessed optical echoes. This allows us to determine the distribution of reprocessing sites within the binary. We model the time-delay transfer functions by simulating the distribution of reprocessing regions, using geometrical and binary parameters. We construct best-fitting time-delay transfer functions, showing the regions in the binary responsible for the reprocessing of X-rays. We have applied this model to observations of the soft X-ray transient GRO J1655-40. We find that the optical variability lags the X-ray variability with a mean time delay of 19.3±2.2 s. This means that the outer regions of the accretion disc are the dominant reprocessing site in this system. On fitting the data to a simple geometric model, we derive a best-fitting disc half-opening angle of , which is similar to that observed after the previous outburst by Orosz & Bailyn. This disc thickening has the effect of almost entirely shielding the companion star from irradiation at this stage of the outburst.

Echoes from an irradiated disc in GRO J1655-40

We demonstrate correlated rapid variability between the optical/UV and X-ray emission for the first time in a soft X-ray transient, GRO J1655–40: Hubble Space Telescope (HST) light curves show similar features to those seen by the Rossi X-ray Timing Explorer (RXTE), but with mean delay of up to 10–20 s. We interpret the correlations as the result of reprocessing of X-rays into optical and UV emission, with a delay owing to finite light travel time, and thus perform echo mapping of the system. The time-delay distribution has a mean of 14.6 ± 1.4 s and dispersion (i.e. the standard deviation of the distribution) of 10.5 ± 1.9 s at binary phase 0.4. Hence we identify the reprocessing region as the accretion disc rather than the mass donor star.

Emission-line oscillations in the dwarf nova V2051 Ophiuchi

We have detected coherent oscillations, at multiple frequencies, in the line and continuum emission of the eclipsing dwarf nova V2051 Ophiuchi using the 10m Keck II telescope. Our own novel data acquisition system allowed us to obtain very fast spectroscopy using a continuous readout of the CCD on the LRIS spectrograph. This is the first time that dwarf nova oscillations are detected and resolved in the emission lines. The accretion disc is highly asymmetric with a stronger contribution from the blue-shifted side of the disc during our observations. The disc extends from close to the white dwarf out to the outer regions of the primary Roche lobe. Continuum oscillations at 56.12s and its first harmonic at 28.06 s are most likely to originate on the surface of a spinning white dwarf with the fundamental period corresponding to the spin period. Balmer and Helium emission lines oscillate with a period of 29.77s at a mean amplitude of 1.9%. The line kinematics as well as the eclipse constraints indicate an origin in the accretion disc at a radius of 12 R_wd. The amplitude of the emission line oscillation modulates (0-4%) at a period of 488s, corresponding to the Kepler period at R=12 R_wd. This modulation is due to the beating between the white dwarf spin and the orbital motion in the disc. The observed emission line oscillations cannot be explained by a truncated disc as in the intermediate polars. The observations suggest a non-axisymmetric bulge in the disc, orbiting at 12R_wd, is required. The close correspondence between the location of the oscillations and the circularisation radius of the system suggests that stream overflow effects may be of relevance

Spectroscopy of the optical counterpart to Ser X-1

We present William Herschel Telescope (WHT) and Very Large Telescope (VLT) spectroscopy of MM Ser, the optical counterpart to Ser X-1. We deblend the red spectra of the two close stars identified by Wachter and show that the brighter of the two is responsible for the Ha and He I emission, hence confirming that this is the true counterpart of the X-ray source. We also identify several He II and N III lines in the blue spectrum. The isolated emission lines are all remarkably narrow, with FWHM 200-300 km s -1 . The Bowen blend has structure suggesting that the individual components are also narrow. These narrow lines could be from the disc if the binary inclination is quite low, or they could come from a more localized region such as the heated face of the companion star. Several interstellar lines are detected and indicate that the reddening is moderate, and consistent with the neutral hydrogen column density inferred in X-rays.

High-speed Keck II and RXTE spectroscopy of Cygnus X-2 – I. Three X-ray components revealed by correlated variability

We have performed simultaneous X-ray and optical spectroscopic observations of the low-mass X-ray binary Cygnus X-2. We have used a new data system attached to the Low Resolution Imaging Spectrograph (LRIS) instrument on Keck II to obtain spectra with a mean time resolution of 72.075 ms, simultaneous with pointed X-ray observations using the Proportional Counter Array (PCA) onboard RXTE. In this paper, we have analysed the variability in both wavebands on time-scales of 16 s. During our observations Cygnus X-2 covered all three branches of the Z-curve, allowing us to study how the changes in X-ray spectral state affect the optical emission. As the optical flux rises, the X-ray intensity first rises on the horizontal branch (0 < Sz < 1) but then falls on the normal branch (1 < Sz < 2) and flaring branch (2 < Sz < 3), where Sz is a rank number characterizing the position on the Z-curve. This linear increase in the optical flux with Sz indicates the optical flux is a good predictor of the accretion rate (possibly normalized by its own long-term average) inferred from the Z-state Sz. We have used this correlation to decompose the total X-ray count rate into three distinct spectral components.