ROSAT Observations of Scattered X-Rays from LMC X-4 in Its Low State

Details ROSAT Observations of Scattered X-Rays from LMC X-4 in Its Low State ROSAT Observations of Scattered X-Rays from LMC X-4 in Its Low State

Aug 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...449..880w&link_type=abstract

Astrophysical Journal v.449, p.880

Astronomy and Astrophysics

Astronomy

14

Accretion, Accretion Disks, Stars: Binaries: Eclipsing, Stars: Individual Alphanumeric: Lmc X-4, X-Rays: Stars

Scientific paper

Observations of the eclipsing accretion-powered high-mass X-ray binary LMC X-4 are described that confirm the idea that the low state of its 30.3 day period cyclical intensity variation is caused by a periodic blockage of the line of sight by a precessing accretion disk that is tilted with respect to the orbital plane of the binary. The principal evidence consists of the fact that the intensity and spectrum of scattered X-rays measured during total eclipse of the neutron star by the primary star are approximately the same in both the high and low states of the 30.3 day cycle. Therefore, the low state must be caused not by a decrease in the luminosity of the X-ray source but, rather, by total attenuation in intervening matter. Differences between the spectra observed in and out of eclipse in the high and in the low states are also consistent with the precessing disk hypothesis. The brightest of several flares detected in the low state had a peak intensity, which implies a source luminosity (0.2-2.5 keV) of 4 × 1039 ergs s-1 if the measured flux is interpreted as scattered and fluorescent radiation from the primary star's surface illuminated by the unblocked X-ray source. This peak luminosity is comparable to that previously reported for a flare observed in the high state. The flare also showed marginal evidence of the pulsar modulation, which is not inconsistent with the degree of spread in path lengths of the scattered radiation. The data also show that the column density to LMC X-4, derived by spectrum fits in both the low and high states, is about 4 times larger than the interstellar neutral hydrogen column density toward the LMC, possibly as a result of a circumstellar accumulation of matter from the wind of the primary.

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