Suzaku Observations of Cyg X-1

TL;DR

This paper reports on Suzaku and RXTE observations of Cyg X-1, analyzing spectral features, absorption effects, and comparing models, revealing insights into the source's low flux, hard state behavior, and spectral components.

Contribution

It introduces simultaneous Suzaku/RXTE observations of Cyg X-1, addressing data analysis challenges and comparing spectral models, including phenomenological and physical Comptonization models.

Findings

Both narrow and broad Fe line components are necessary in spectral fits.

Suzaku data show evolution due to wind absorption at orbital phase 0.

Physical Comptonization models fit the data slightly better than phenomenological models.

Abstract

We present highlights from a series of four simultaneous Suzaku/RXTE observations of the black hole candidate Cyg X-1. We briefly summarize several key results from our decade long RXTE monitoring campaign. We then comment on challenges of analyzing the Suzaku data, i.e., improving the aspect correction beyond that of the existing tools, and quantitatively assessing pileup. All of our Suzaku observations (one, by design) occurred at or very near orbital phase 0 (superior conjunction), and hence show evolution in color-color diagrams due to X-ray absorption by material from the wind of the secondary. We present simple partial absorption models for this evolution. We then compare the Suzaku and RXTE data, and explicitly divide the Fe line region into narrow and broad components. Both are required for the Suzaku data, and are seen to be consistent with the RXTE data. These Suzaku observations occurred near historically hard, low flux states. We present fits of the broad band spectra with a simple phenomenological broken powerlaw model, as well as a more physically motivated Comptonization model. Whereas the former class of models described nearly all of the RXTE campaign better than any physical model, here the latter model is slightly more successful. The Comptonization model, however, exhibits little evidence for a soft disk component, which formally corresponds to a small, inner disk radius. Whether this is physical, due to unmodeled absorption, or is a calibration issue, remains an open question.