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Session 60 - Multiple Stars.
Display session, Wednesday, June 12
The intense (10^38 erg s^-1) supersoft (\lambda_max\sim300\,ÅX-ray sources may be as numerous as the better-known Eddington limited 1--10 keV X-ray binaries, but simply difficult to detect in our own galaxy due to photoelectric absorption. Although many workers believe that the degenerate secondary in these systems is an accreting white dwarf, both neutron stars and black holes have also been suggested as companions. The supersoft LMC X-ray source CAL 87, for example, has been inferred to contain a black hole, based on optical spectroscopy, which has implied an F--G spectral type for the secondary, and a high mass function.
We report high resolution imaging of the CAL 87 field using the Hubble Space Telescope WFPC2. The field has already been known to be optically complex, as an unrelated companion 0.9'' distant is seen in ground-based images, and has been suspected to interfere with both photometry and spectroscopy of the system. We easily resolve this object, and also discover an even closer (0.65'') somewhat fainter additional companion. UBVR photometry of CAL 87 and the two resolved close companions shows that the neighboring stars severely contaminate ground-based observations of CAL 87 during its eclipse; only 40% of the total V and R light comes from the X-ray counterpart at eclipse minimum. At least one of the unrelated companions has the colors of a G star, and thus presumably has spectral features identical to those attributed in previous contaminated ground-based spectra to the secondary of the X-ray source. This contamination probably explains the lack of detectable radial velocity variations in ground-based spectra. Further, the spectral type of the X-ray source secondary, and thus the inferred mass of the primary, will need to be carefully reconsidered. Past evidence for a black hole in CAL 87 is weakened by these observations.
This work has been supported by NASA through grant number GO-06135.01-94A from the Space Telescope Science Institute,
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