2009A&A...500..883S

We present an update of the orbital ephemeris of the binary X-ray pulsar Her X-1 and determine an improved value for the rate of orbital decay. In addition, we report the first measurement of the orbital eccentricity. We analyzed pulse timing data of Her X-1 from X-ray observations by RXTE (Rossi X-Ray Timing Explorer) and INTEGRAL over the period 1996-2007. Accurate pulse arrival times were determined from solar system bary-centered photon arrival times by generating pulse profiles averaged over appropriately short integration times. Applying pulse phase connection techniques, it was possible to determine sufficiently accurate local ephemeris data for seven observation periods distributed over 12 years. Combining the new local T_π/2 values with historical values from the literature we update the orbital ephemeris of Her X-1 to T_π/2=MJD 46359.871940(6) and P_orb=1.700167590(2)d and measure a continuous change of the orbital period of dP_orb/dt=-(4.85 ±0.13)x10^–11s/s. For the first time, a value for the eccentricity of the orbit of Her X-1 has been measured: ε=(4.2±0.8)x10^–4.