Orbital Decay, Spin-down, and Pulse-Phase--resolved Spectroscopy of LMC X-4 from GINGA and ROSAT Observations
We report pulse-timing and pulse-phase resolved spectrum analysis of LMC X-4 observations by the X-ray satellites Ginga in 1988 and ROSAT in 1991 extending over a combined energy range from 0.2 to 37 keV. The results, when combined with previous results of X-ray observations dating back to 1977, yield marginal evidence of orbital decay with an average rate of change in the orbital period of P_orb_/P_orb_ = (-5.3 +/- 2.7) x 10^-7^ yr^-1^. This low decay rate and the relatively small radius of the companion point to the conclusion that the companion is still in the hydrogen core-burning stage of its evolution. The pulse period increased between the two observations at an average rate of P_pulse_ = (1.91+/- 0.001) x 10^-3^ s yr^-1^. At other times, the spin period has both decreased and increased. The neutron star must therefore be in the condition of approximate spin equilibrium wherein its period is close to the Kepler period at the inner edge of the accretion disk. For a disk- accreting pulsar with the luminosity (~10^38.6^ ergs s^-1^) and spin period (~13.5 s) of LMC X-4, this condition implies that its magnetic dipole moment is exceptionally large, of the order of 10^31.5^ G cm^3^. The pulse-phase resolved combined pulse-height distributions are well fitted with a model spectrum consisting of the sum of four components, a Planck function (PF), thermal bremsstrahlung (TB), power law (PL), and iron K-line (FE), multiplied by a photoelectric absorption factor, an exponential high-energy cutoff and a cyclotron resonance attenuation factor. The pulse profiles of the TB and PL (E > 15 keV) energy fluxes are single-peaked with pulse fractions of ~0.12 and ~0.16, respectively, but ~180^deg^ different in phase. The Fe-line flux also exhibits a single peak pulse profile. Its pulse fraction is ~0.37, and its peak lags that of the FL peak by ~140^deg^. The PL (E > 15 keV) profile is attributed to a pencil-beam-like anisotropy in the emission and the TB profile to a fan-beam-like anisotropy with the condition that the sum of the spin axis inclination and the angle between the spin and magnetic dipole axes is less than 90^deg^. The PF component shows no significant pulse variation and is attributed to emission from the accretion disk.