The Open UniversitySkip to content
 

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

Hynes, R.I.; Haswell, C.A.; Shrader, C.R.; Chen, W.; Horne, Keith; Harlaftis, E.T.; O'Brien, K.; Hellier, C. and Fender, R.P. (1998). The 1996 outburst of GRO J1655-40: the challenge of interpreting the multiwavelength spectra. Monthly Notices of the Royal Astronomical Society, 300(1) pp. 64–82.

DOI (Digital Object Identifier) Link: http://dx.doi.org/10.1046/j.1365-8711.1998.01835.x
Google Scholar: Look up in Google Scholar

Abstract

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.

Item Type: Journal Article
ISSN: 1365-2966
Keywords: ACCRETION, ACCRETION DISCS, BINARIES: CLOSE, STARS: INDIVIDUAL: NOVA SCO 1994 (GRO J1655-40), ULTRAVIOLET: STARS, X-RAYS: STARS
Academic Unit/Department: Science > Physical Sciences
Interdisciplinary Research Centre: Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)
Item ID: 11291
Depositing User: Astrid Peterkin
Date Deposited: 08 Aug 2008 14:23
Last Modified: 11 Jul 2013 15:25
URI: http://oro.open.ac.uk/id/eprint/11291
Share this page:

Actions (login may be required)

View Item
Report issue / request change

Policies | Disclaimer

© The Open University   + 44 (0)870 333 4340   general-enquiries@open.ac.uk