A multiphase Suzaku study of X-rays from τ Sco

Abstract

We obtained relatively high signal-to-noise X-ray spectral data of the early massive star tau Sco (B0.2V) with the Suzaku X-ray Imaging Spectrometer (XIS) instrument. This source displays several unusual features that motivated our study: (1) redshifted absorption in UV P Cygni lines to approximately +250 km s-1 suggestive of infalling gas, (2) unusually hard X-ray emission requiring hot plasma at temperatures in excess of 10 MK whereas most massive stars show relatively soft X-rays at a few MK, and (3) a complex photospheric magnetic field of open and closed field lines. In an attempt to understand the hard component better, X-ray data were obtained at six roughly equally spaced phases within the same epoch of tau Sco's 41 day rotation period. The XIS instrument has three operable detectors: XIS1 is back-illuminated with sensitivity down to 0.2 keV; XIS0 and XIS2 are front-illuminated with sensitivity only down to 0.4 keV and have an overall less effective area than XIS1. The XIS0 and XIS3 detectors show relatively little variability. In contrast, there is a approximate to ≈4 sigma detection of a approximate to ≈4% drop in the count rate of the XIS1 detector at one rotational phase. In addition, all three detectors show a approximate to ≈3% increase in count rate at the same phase. The most optimistic prediction of X-ray variability allows for a 40% change in the count rate, particularly near phases where we have pointings. Observed modulations in the X-ray light curve on the rotation cycle is an order of magnitude smaller than this, which places new stringent constraints on future modeling of this interesting magnetic massive star.