A multi-mission investigation of EXO0748-676: evidence for complex secular changes in the accretion structure of this system

Details A multi-mission investigation of EXO0748-676: evidence for complex secular changes in the accretion structure of this system A multi-mission investigation of EXO0748-676: evidence for complex secular changes in the accretion structure of this system

Mar 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aas...186.4806t&link_type=abstract

American Astronomical Society, 186th AAS Meeting, #48.06; Bulletin of the American Astronomical Society, Vol. 27, p.879

Astronomy and Astrophysics

Astronomy

Scientific paper

We present data from two ASCA observations (1993 March 17.92-18.12 and 1993 May 7.24-8.09) of the high inclination (i ~ 80(deg) ), dipping, eclipsing Low-mass X-ray Binary EXO0748-676. We have examined the intensity selected dipping and quiescent (non-dipping,non-eclipsing, non-bursting) behavior of EXO0748-676 in these observations. Over the interval of these observations the persistent luminosity of the source decreased by approximately a factor of 5. ``Low-state'' data indicate that we have observed the source near its historical minimicrons emission state. The low state quiescent/dipping data show several unusual and outstanding properties: (1) there is no time wherein the intrinsic persistent spectrum is free of some amount of variable column, (2) the dipping morphology of EXO0748-676 is remarkable; there exists a previously un-noted broad (FWHM Delta Phi ~ 0.15) dipping feature centered near Phi = 0.45, and (3) a previously unparameterized soft (0.4-1 keV) ``excess'' exists in low-state dipping spectra. This soft excess is well fit by either a Raymond-Smith plasma or by a thermal bremsstrahlung spectral model (kT ~ 0.15 keV). Combined archival and ASCA datasets show that the changing accretion structure in this source may play a significant role in the modulation of the long-term brightness of this source.

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