X‐Ray and Ultraviolet Orbital Phase Dependence in LMC X‐3

The black-hole binary LMC X-3 is known to be variable on time scales of daysto years. We investigate X-ray and ultraviolet variability in the system as afunction of the 1.7 day binary phase using a 6.4 day observation with the RossiX-ray Timing Explorer (RXTE) from December 1998. An abrupt 14% flux decrease,lasting nearly an entire orbit, is followed by a return to previous fluxlevels. This behavior occurs twice, at nearly the same binary phase, but it isnot present in consecutive orbits. When the X-ray flux is at lower intensity, aperiodic amplitude modulation of 7% is evident in data folded modulo theorbital period. The higher intensity data show weaker correlation with phase.This is the first report of X-ray variability at the orbital period of LMC X-3.Archival RXTE observations of LMC X--3 during a high flux state in December1996 show similar phase dependence. An ultraviolet light curve obtained withthe High Speed Photometer aboard the Hubble Space Telescope shows orbitalmodulation consistent with that in the optical, caused by the ellipsoidalvariation of the spatially deformed companion. The X-ray spectrum of LMC X-3 can be acceptably represented by aphenomenological disk-black-body plus a power law. Changes in the spectrum ofLMC X-3 during our observations are compatible with earlier observations duringwhich variations in the 2-10 keV flux are tracked closely by the disk geometryspectral model parameter.Comment: 11 pages, 7 figures, ApJ in pres