INTEGRAL Picture Of the Month
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INTEGRAL helps to measure torque reversal in HMXB 4U 1538-522
High-mass X-ray Binaries (HMXB) consist of a compact object (Neutron star
of Black Hole) and a giant companion star from which matter is accreted.
These systems show complex high-energy emission produced by dissipating
kinetic energy of the accreted material within the strong gravitational
potential of the compact object and complex absorption, e.g. within the
accretion stream, the stellar wind of the companion star.
4U 1538-522 is an eclipsing, wind-fed persistent HMXB with an orbital period
of ~3.7 days including a 0.6 d long eclipse phase and source inclination of
67 +/-1 degrees and a low-mass neutron star as compact object (exact mass
depending on the unknown eccentricity of the orbit). The spin period of the
neutron star, around 530 seconds, is not constant, and the image shows the
spin period measured by several observatories, including INTEGRAL, since 1976.
Period measurements are much less dependent on calibration than spectral
measurements, and the period evolution with different observatories is highly
reliable. INTEGRAL measured a period of ~526 seconds which is shorter than the
last BATSE observations but longer than recent FERMI observations. The FERMI
period measurements then show a torque reversal, i.e., the period becomes longer
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