8th HEAD Meeting, 8-11 September, 2004
Session 17 Neutron Stars and X-ray Binaries
Poster, Thursday, September 9, 2004, 9:00am-10:00pm

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[17.29] A Multiwavelength Search for the Donor in Cir X-1

W.I. Clarkson (The Open University), J.S. Clark (University College London), P.A. Charles, M.J. Coe (Southampton University), N Onyett (Sussex University)

We report on a joint IR and X-ray campaign to uncover the nature of the donor in the enigmatic X-ray binary Circinus X-1. With an eccentric 16-day orbit accompanied by enhanced accretion at periastron passage, weak neutron star external magnetic field and a highly relativistic bipolar outflow, Cir X-1 mixes properties from several classes of X-ray binary, making it a key object in our understanding of the accretion/outflow process. However, high uncertain extinction have made direct detection of the donor extremely difficult, frustrating attempts to constrain physical models of the system.

Our efforts to constrain the donor have thus been focused on wavelengths not forbidden by extinction. We have used the superb long-term lightcurve of the XTE/ASM to model the phase-dependent accretion flow of the system, finding for reasonable parameters the system is indeed likely to be highly eccentric. In a complementary study to that of Saz Parkinson et al. (2003), we have fit an X-ray ephemeris to the system, this time based on the occurrence of the X-ray dips which occur tightly clustered at periastron passage. Using IRIS2 on the Anglo Australian Telescope, we have obtained the highest-resolution near-IR spectra yet made of the system, covering the post-periastron outburst (when the accretion component is at a maximum) and an interval at apastron (when the donor should be at its most significant). Finally we will discuss some very recent simultaneous IRIS2 and XTE/PCA observations, which follow jointly the IR spectrum and X-ray variability of Cir X-1 through a complete apastron-periastron passage, in order to better constrain the accretion contamination and indentify the donor.

The author(s) of this abstract have provided an email address for comments about the abstract: w.i.clarkson@open.ac.uk

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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.