Speaker
Description
Her X-1 is an intermediate (or low) mass X-ray binary pulsar that exhibits a ~35-day super-orbital modulation whose origin-disc precession or neutron-star free precession-remains debated. Earlier work reported stochastic variations without a clear long-term trend. Using $\sim$50 years of data from CGRO/BATSE, RXTE/ASM, Swift/BAT, Fermi/GBM, and MAXI/GSC supplemented with archival measurements, we studied the long-term evolution of this modulation. Three methods are applied: epoch-folding searches for constant periods, $\chi^2-$ maximisation in the period-period-derivative space, and timing of main-on peaks via an O $-$ C analysis. All approaches consistently show a long term decrease in the super-orbital period at a rate of $\sim 10^{-5}$ s s$^{-1}$ , with additional short-timescale variabilities superposed on this trend. The possible mechanisms for the observed decay of the super-orbital period will be discussed.