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Junichiro Miura, Masahiro Tsujimoto, Yohko Tsuboi, Yoshitomo Maeda, Yasuharu Sugawara, Katsuji Koyama, Shigeo Yamauchi, Suzaku Detection of an Intense X-Ray Flare from an A-Type Star, HD161084, Publications of the Astronomical Society of Japan, Volume 60, Issue sp1, 20 February 2008, Pages S49–S56, https://doi.org/10.1093/pasj/60.sp1.S49
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Abstract
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.
