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.

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‐Å 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 ν1/3 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 He ii 4686‐Å line, we see Bowen fluorescence lines of N iii and O iii, 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.

We report ASCA Solid State Imaging Spectrometer (SIS) X-ray observations of the O stars 60ri and 2 Ori. The energy resolution of the SIS allows us to resolve features iv. the O star X-ray spectra which are not apparent in spectra obtained by X-ray spectrometers with lower energy resolution. SIS spectra from both stars show evidence of line emission, suggesting the thermal nature of the X-ray source. However, the observed line strengths are different for the two stars. The observed stellar X-ray spectra are not well described by isothermal models although absorbed thermal emission models with two or more temperatures can provide an adequate fit to the data. For both stars we present evidence of absorbing columns significantly larger than the known ISM columns, indicative of absorption by a circumstellar medium, presumably the stellar winds. In addition, the ). Ori spectrum shows the presence of emission at energies > 3 keV which is not seen in the 60ri spectrum.

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