A Report on the X-ray Properties of the tau Sco Like Stars

TL;DR

This study investigates the X-ray properties of tau Sco-like magnetic OB stars, revealing diverse X-ray behaviors and confirming a hard X-ray component in one such star, which supports the connection between magnetic fields and X-ray emission.

Contribution

The paper provides new XMM-Newton observations of two tau Sco-like stars, highlighting the diversity of their X-ray properties and strengthening the link between magnetic fields and hard X-ray emission in these stars.

Findings

HD 63425 exhibits a substantial hard X-ray component.

HD 66665's X-ray properties remain inconclusive due to limited data.

The X-ray properties of tau Sco-like stars are diverse, with some showing hard X-ray spectra.

Abstract

An increasing number of OB stars have been shown to possess magnetic fields. Although the sample remains small, it is surprising that the magnetic and X-ray properties of these stars appear to be far less correlated than expected. This contradicts model predictions, which generally indicate that the X-rays from magnetic stars to be harder and more luminous than their non-magnetic counterparts. Instead, the X-ray properties of magnetic OB stars are quite diverse. $\tau$ Sco is one example where the expectations are better met. This bright main sequence, early B star has been studied extensively in a variety of wavebands. It has a surface magnetic field of around 500 G, and Zeeman Doppler tomography has revealed an unusual field configuration. Furthermore, tau Sco displays an unusually hard X-ray spectrum, much harder than similar, non-magnetic OB stars. In addition, the profiles of its UV P Cygni wind lines have long been known to possess a peculiar morphology. Recently, two stars, HD 66665 and HD 63425, whose spectral types and UV wind line profiles are similar to those of $\tau$ Sco, have also been determined to be magnetic. In the hope of establishing a magnetic field - X-ray connection for at least a sub-set of the magnetic stars, we obtained XMM-Newton EPIC spectra of these two objects. Our results for HD 66665 are somewhat inconclusive. No especially strong hard component is detected; however, the number of source counts is insufficient to rule out hard emission. longer exposure is needed to assess the nature of the X-rays from this star. On the other hand, we do find that HD 63425 has a substantial hard X-ray component, thereby bolstering its close similarity to tau Sco.