Suzaku Detection of an Intense X-Ray Flare from an A-type Star HD161084

TL;DR

This paper reports the detection of a rare, intense X-ray flare from the A-type star HD 161084, suggesting it is likely an interacting binary with elevated magnetic activity, which is unusual for such stars.

Contribution

First detection of an intense X-ray flare from HD 161084, indicating it is likely an interacting binary with high magnetic activity, challenging previous understanding of A-type stars.

Findings

X-ray flare with flux of 1.4x10^{-12} erg s^{-1} cm^{-2}

Peak X-ray luminosity of 1x10^{32} erg s^{-1}

Flux during flare is 100 times higher than quiescent level

Abstract

We report a serendipitous detection of an intense X-ray flare from the Tycho reference source HD 161084 during a Suzaku observation of the Galactic Center region for 20 ks. The X-ray Imaging Spectrometer (XIS) recorded a flare from this A1-type dwarf or subgiant star with a flux of 1.4x10^{-12} erg s^{-1} cm^{-2} (0.5--10 keV) and a decay time scale of 0.5 hr. The spectrum is hard with a prominent Fe XXV K alpha emission line at 6.7 keV, which is explained by a 5 keV thin-thermal plasma model attenuated by a 1.4x10^{21} cm^{-2} extinction. The low extinction, which is consistent with the optical reddening, indicates that the source is a foreground star toward the Galactic Center region. Based on the spectroscopic parallax distance of 530 pc, the peak X-ray luminosity amounts to 1x10^{32} erg s^{-1} (0.5--10 keV). This is much larger than the X-ray luminosity of ordinary late-type main-sequence stars, and the X-ray emission is unattributable to a hidden late-type companion that comprises a wide binary system with the A-star. We discuss possible natures of HD 161084 and suggest that it is most likely an interacting binary with elevated magnetic activity in the companion such as the Algol-type system. The flux detected by Suzaku during the burst is 100 times larger than the quiescent level measured using the archived XMM-Newton and Chandra data. The large flux amplification makes this star a unique example among sources of this class.