Chandra Observations of Zeta Pup : Probing for Rotational Modulation of the X-Ray Flux and Hardness in a Supergiant O Star

Abstract

Hot stars such as Zeta Pup are strong X-ray emitters but the precise mechanism causing their X-ray emission is unclear. In this project we examine the temporal behavior of the X-ray emission from this star. If the X-rays are formed far out in the stellar wind, we would expect the X-ray output to fluctuate randomly. If the X-rays are formed close to the stellar surface (e.g. in magnetic structures), we would expect X-rays to vary clock-like on a rotational time scale. Recent work (Howarth and Stevens, “Time-series photometry of the O4 I(n)fp star ζ Puppis”) has identified a 1.78-day rotational period for this star, so we folded the data on this time scale to try to identify rotationally modulated variation. In addition to examining the overall X-ray photon count rate, we evenly divided the data into short-wavelength (“hard”) X-ray counts (from high-temperature gas) and long-wavelength (“soft”) X-ray counts (from low-temperature gas). This allowed us to construct the “hardness ratio” by calculating the difference between the hard and soft count rates and dividing by the total count rate. When examining the folded total, hard, soft, and hardness ratio data we found no case where there was unambiguous evidence for variation on the rotational time scale.