We analyze the observations of the X-ray pulsar LMC X-4 performed by the INTEGRAL observatory and the All-Sky Monitor (ASM) of the RXTE observatory over a wide energy range. The observed hard X-ray flux from the source is shown to change by more than a factor of 50 (from ∼ 70 mCrab in the high state to ∼ 1.3 mCrab in the low state) on the time scale of the accretion-disk precession period, whose mean value for 1996-2004 was determined with a high accuracy, P prec = 30.275 ± 0.004 days. In the low state, a flare about 10 h in duration was detected from the source; the flux from the source increased by more than a factor of 4 during this flare. The shape of the pulsar's broadband spectrum is essentially invariable with its intensity; no statistically significant features associated with the possible resonance cyclotron absorption line were found in the spectrum of the source. c 2005 Pleiades Publishing Inc.Key words: pulsars, neutron stars, X-ray sources. * E-mail: st@hea.iki.rssi.ru
INTRODUCTIONThe X-ray pulsar LMC X-4 in the Large Magellanic Cloud (LMC) (the distance to the object is d = 50 kpc) is a high-mass binary with a pulsation period of ∼ 13.5 s (Kelley et al. 1983), in which the compact object is eclipsed by its optical companion, an O8 star of the 14th magnitude with a mass of 20 M ⊙ (Chevalier and Ilovaisky 1977), every ∼1.408 days (Li et al. 1978;Lang et al. 1981;Levine et al. 2000). Levine et al. (2000) provided the following orbital parameters for the binary: a x sini = 26.333 ± 0.019 light seconds, the eccentricity e < 0.003(2σ), and the epoch of the zero orbital phase T 0 =MJD51110.86571. Lang et al. (1981) pointed to the existence of a superorbital period in the binary, ∼ 30.5 days, within which the intensity of the source changes by a factor of about 60. These authors also assumed that this effect is produced by the blockage of direct X-rays by a precessing accretion disk tilted with respect to the orbital plane, much as is the case in the binary Her X-1 (Tananbaum et al. 1972). Based on the measurements of almost the same Xray flux observed from the source during eclipses in its low and high states, Woo et al. (1995) concluded that the intrinsic luminosity of the pulsar is constant, which also confirms the assumption made by Lang et al. (1981). Having analyzed the RXTE and GINGA observations, Paul and Kitamoto (2002) estimated the rate of decrease in the precession period of the accretion disk to beṖ ∼ −2 × 10 −5 s s −1 .The flaring activity of LMC X-4 has been widely discussed in the literature (see, e.g., Epstein et al. 1977;Skinner et al. 1980;Kelley et al. 1983;Levine et al. 1991). A flare with a duration from dozens of seconds to dozens of minutes is detected from the source during its high state, on average, once a day. However, there is also evidence (Woo et al. 1995) for the presence of bursts in the low state.In different periods, the pulsar LMC X-4 demonstrates both a spin-down and a spin-up, suggesting that its period is close to its equilibrium value. In this case, according to the ...