R. I. Hynes

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.

Modeling the low state spectrum of the x-ray nova xte j1118+480

Based on recent multiwavelength observations of the new X-ray nova XTE J1118+480, we can place strong constraints on the geometry of the accretion flow in which a low/hard-state spectrum, characteristic of an accreting black hole binary, is produced. We argue that the absence of any soft blackbody-like component in the X-ray band implies the existence of an extended hot optically thin region, with the optically thick cool disk truncated at some radius Rtr 55RSchw. We show that such a model can indeed reproduce the main features of the observed spectrum: the relatively high optical to X-ray ratio, the sharp downturn in the far-UV band, and the hard X-ray spectrum. The absence of the disk blackbody component also underscores the requirement that the seed photons for thermal Comptonization be produced locally in the hot flow, e.g., via synchrotron radiation. We attribute the observed spectral break at 2 keV to absorption in a warm, partially ionized gas.

Complete and simultaneous spectral observations of the black hole X-Ray nova XTE J1118+480

The X-ray nova XTE J1118+480 suffers minimal extinction (b = 62 degrees) and therefore represents an outstanding opportunity for multiwavelength studies. Hynes et al. (2000) conducted the first such study, which was centered on 2000 April 8 using UKIRT, EUVE, HST and RXTE. On 2000 April 18, the Chandra X-ray Observatory obtained data coincident with a second set of observations using all of these same observatories. A 30 ks grating observation using Chandra yielded a spectrum with high resolution and sensitivity covering the range 0.24-7 keV. Our near-simultaneous observations cover approximately 80% of the electromagnetic spectrum from the infrared to hard X-rays. The UV/X-ray spectrum of XTE J1118+480 consists of two principal components. The first of these is an approximately 24 eV thermal component which is due to an accretion disk with a large inner disk radius: > 35 Schwarzschild radii. The second is a quasi power-law component that was recorded with complete spectral coverage from 0.4-160 keV. A model for this two-component spectrum is presented in a companion paper by Esin et al. (2001).

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.

The X-Ray Transient XTE J1118+480: Multiwavelength Observations of a Low-State Minioutburst

We present multiwavelength observations of the newly discovered X-ray transient XTE J1118+480 obtained in the rising phase of the 2000 April outburst. This source is located at unusually high Galactic latitude and in a very low absorption line of sight. This made the first Extreme Ultraviolet Explorer (EUVE) spectroscopy of an X-ray transient outburst possible. Together with our Hubble Space Telescope, Rossi X-Ray Timing Explorer, and United Kingdom Infrared Telescope data, this gives unprecedented spectral coverage. We find the source in the low hard state. The flat IR-UV continuum appears to be a combination of optically thick disk emission and possibly synchrotron, while at higher energies (including EUV), a typical low hard state power law is seen. EUVE observations reveal no periodic modulation, suggesting an inclination low enough that no obscuration by the disk rim occurs. We discuss the nature of the source and this outburst and conclude that it may be more akin to minioutbursts seen in GRO J0422+32 than to a normal X-ray transient outburst.

Multiwavelength observations revealing the evolution of the outburst of the black hole XTE J1118+480

We report multiwavelength observations of the soft X-ray transient XTE J1118+480, which we observed with UKIRT, Hubble Space Telescope (HST), RXTE, Extreme Ultraviolet Explorer (EUVE) and many other instruments and facilities. Adding radio (Ryle Telescope, VLA), submillimetre (JCMT) and X-ray (Chandra and SAX) data from the literature, we assembled the most complete spectral energy distribution (SED) of this source yet published. We followed the evolution of this source for 1 yr, including six observations performed during the outburst, and one observation at the end of the outburst. Because of the unusually high galactic latitude of XTE J1118+480, it suffers from very low extinction, and its SED is nearly complete, including extreme ultraviolet observations. XTE J1118+480 exhibits an unusually low low/hard state (estimated inner radius of 350R s ) and a strong non-thermal contribution in the radio to optical domain, which is likely to be due to synchrotron emission. We discuss the interstellar column density and show that it is low, between 0.80 and 1.30 x 10 2 0 cm - 2 . We analyse the evolution of the SED during the outburst, including the contributions from the companion star, the accretion disc, the outflow, and relating irradiation and variability of the source in different bands to the SED. We find no significant spectral variability during the outburst evolution, consistent with the presence of a steady outflow. An analysis of its outflow to accretion energy ratio suggests that the microquasar XTE J1118+480 is analogous to radio-quiet quasars. This, combined with the inverted spectrum from radio to optical, makes XTE J 1118+480 very similar to other microquasar sources, e.g. GRS 1915+105 and GX 339-4 in their low/hard state. We model the high-energy emission with a hot disc model, and discuss different accretion models for the broad-band spectrum of XTE J1118+480.

The Evolving Accretion Disc in the Black Hole X-ray Transient XTE J1859+226 ⋆

ABSTRACT We present HST, RXTE, and UKIRT observations of the broad band spectra of theblack hole X-ray transient XTE J1859+226 during the decline from its 1999–2000outburst. Our UV spectra define the 2175˚A interstellar absorption feature very welland based on its strength we estimate E(B − V) = 0.58 ± 0.12. Hence we dereddenour spectra and follow the evolution of the spectral energy distribution on the declinefrom outburst. We find that the UV and optical data, and the X-ray thermal com-ponent when detectable, can be fit with a simple blackbody model of an accretiondisc heated by internal viscosity and X-ray irradiation, and extending to close to thelast stable orbit around the black hole, although the actual inner radius cannot bewell constrained. During the decline we see the disc apparently evolving from a modelwith the edge dominated by irradiative heating towards one where viscous heatingis dominant everywhere. The outer disc radius also appears to decrease during thedecline; we interpret this as evidence of a cooling wave moving inwards and discuss itsimplications for the disc instability model. Based on the normalisation of our spectralfits we estimate a likely distance range of 4.6–8.0kpc, although a value outside of thisrange cannot securely be ruled out.Key words: accretion, accretion discs – binaries: close – stars: individual: XTEJ1859+226

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.

Detection of superhumps in XTE J1118+480 approaching quiescence

We present the results of our monitoring of the halo black hole soft X-ray transient (SXT) XTE J1118+480 during its decline to quiescence. The system has decayed 0.5 mag from 2000 December to its present near-quiescent level at R ≃ 18.65 (2001 June). The ellipsoidal light curve is distorted by an additional modulation that we interpret as a superhump of P s h = 0.17049(1)d i.e. 0.3 per cent longer than the orbital period. This implies a disc precession period P p r e c ∼ 52 d. After correcting the average phase-folded light curve for veiling, the amplitude difference between the minima suggests that the binary inclination angle lies in the range i = 71-82°. However, we urge caution in the interpretation of these values because of residual systematic contamination of the ellipsoidal light curve by the complex form of the superhump modulation. The orbital-mean Ha profiles exhibit clear velocity variations with ∼500 km s - 1 amplitude. We interpret this as the first spectroscopic evidence of an eccentric precessing disc.