Updating the orbital ephemeris of Her X-1; rate of decay and eccentricity of the orbit

TL;DR

This paper updates the orbital parameters of Her X-1, measures its orbital decay rate, and reports the first measurement of its orbital eccentricity using 12 years of X-ray pulse timing data.

Contribution

It provides the first measurement of Her X-1's orbital eccentricity and an improved orbital ephemeris based on extensive pulse timing analysis.

Findings

Orbital period decay rate: -(4.85 +/- 0.13) x 10^-11 s/s

Updated orbital ephemeris: T90 and Porb values

First measurement of orbital eccentricity: (4.2 +/- 0.8) x 10^-4

Abstract

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 have 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 T90 values with historical values from the literature we update the orbital ephemeris of Her X-1 to T90 = MJD 46359.871940(6) and Porb = 1.700167590(2) d and measure a continuous change of the orbital period of dPorb/dt = -(4.85 +/- 0.13) x 10-11 s/s. For the first time, a value for the eccentricity of the orbit of Her X-1 is measured to be e = (4.2 +/- 0.8) x 10-4.