Analysis of RXTE-PCA Observations of SMC X-1

TL;DR

This paper presents detailed timing and spectral analysis of SMC X-1 using RXTE-PCA data, revealing precise orbital parameters, long-term pulse period evolution, and spectral stability with flux-dependent absorption variations.

Contribution

It provides the first precise orbital eccentricity measurement, a 30-year pulse period history, and insights into flux-dependent spectral absorption in SMC X-1.

Findings

Orbital eccentricity measured as 0.00089(6).

Orbital decay rate consistent with previous studies.

Hydrogen column density varies with X-ray flux.

Abstract

We present timing and spectral analysis of RXTE-PCA observations of SMC X-1 between January 1996 and December 2003. From observations around 30 August 1996 with a time span of $\sim 6$ days, we obtain a precise timing solution for the source and resolve the eccentricity as 0.00089(6). We find an orbital decay rate of $\dot P_{orb}/P_{orb} =-3.402(7) \times 10^{-6}$ yr$^{-1}$ which is close to the previous results. Using our timing analysis and the previous studies, we construct a $\sim 30$ year long pulse period history of the source. We show that frequency derivative shows long (i.e. more than a few years) and short (i.e. order of days) term fluctuations. From the spectral analysis, we found that all spectral parameters except Hydrogen column density showed no significant variation with time and X-ray flux. Hydrogen column density is found to be higher as X-ray flux gets lower. This may be due to the increase in soft absorption when the pulsar is partially obscured as in Her X-1 or may just be an artifact of the tail of a soft excess in energy spectrum.