INTEGRAL Picture Of the Month
September 2023

(Click to download full resolution)

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 again.

back to the POM archive