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

We report on a serendipitous detection of an intense X-ray flare from the Tycho reference source on HD161084 during a Suzaku observation of the galactic center region for $\sim$ 20ks. The X-ray Imaging Spectrometer recorded a flare from this A1-type dwarf or subgiant star with a flux of $\sim$ 1.4 $\times$ 10 $^{-12}$ erg s $^{-1}$ cm $^{-2}$ (0.5-10keV) and a decay time scale of $\sim$ 0.5hr. The spectrum is hard with a prominent FeXXV K $\alpha$ emission line at 6.7keV, which is explained by a $\sim$ 5keV thin-thermal plasma model attenuated by a $\sim$ 1.4 $\times$ 10 $^{21}$ 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 a spectroscopic parallax distance of $\sim$ 530pc, the peak X-ray luminosity amounts to $\sim$ 1 $\times$ 10 $^{32}$ erg s $^{-1}$ (0.5-10keV). 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 nature of HD161084, 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 $\sim$ 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.