Discovery of Rapidly Moving Partial X-Ray Absorbers Within Gamma Cassiopeiae

Abstract

Gamma Cassiopeiae is an enigmatic Be star with unusually strong hard X-ray emission. The Suzaku observatory detected six rapid X-ray spectral hardening events called "softness dips" in a ∼100 ksec duration observation in 2011. All the softness dip events show symmetric softness ratio variations, and some of them have flat bottoms apparently due to saturation. The softness dip spectra are best described by either ∼40% or ∼70% partial covering absorption to kT ∼12 keV plasma emission by matter with a neutral hydrogen column density of ∼2−8×10 21 cm −2 , while the spectrum outside of these dips is almost free of absorption. This result suggests the presence of two distinct X-ray emitting spots in the γ Cas system, perhaps on a white dwarf companion with dipole mass accretion. The partial covering absorbers may be blobs in the Be stellar wind, the Be disk, or rotating around the white dwarf companion. Weak correlations of the softness ratios to the hard X-ray flux suggest the presence of stable plasmas at kT ∼0.9 and 5 keV, which may originate from the Be or white dwarf winds. The formation of a Be star and white dwarf binary system requires mass transfer between two stars; γ Cas may have experienced such activity in the past.

Suzaku and NuSTAR X-ray spectroscopy of γ Cassiopeiae and HD 110432

“…In the X-rays, however, γ Cas is quite anomalous. Previous observations (Mason et al 1976;Frontera et al 1987;White et al 1982;Murakami et al 1986;Horaguchi et al 1994;Haberl 1995;Smith et al 1998;Kubo et al 1998;Owens et al 1999;Robinson & Smith 2000;Robinson et al 2002;Smith et al 2004b;den Hartog et al 2006;Lopes de Oliveira et al 2010;Shrader et al 2015;Hamaguchi et al 2016) revealed an X-ray luminosity of ∼10 32−33 erg s −1 , the lack of drastic flux variations such as X-ray outbursts, the presence of the Fe K complex resolved into three emission lines from highly ionized Fe (Fe XXV Heα and Fe XXVI Lyα respectively at 6.7 and 7.0 keV) as well as quasi-neutral Fe (Fe I Kα at 6.4 keV). It has an exceptionally high plasma temperature beyond ∼10 keV, which is unseen in other classes of early-type stars.…”

π Aquarii is another γ Cassiopeiae object

“…2), the periodogram appears similar to that found for HD 110432 with Chandra (Torrejón et al 2012); it does not show a clear 1/ f trend, as detected notably in γ Cas by Lopes de Oliveira et al (2010), probably because of the larger noise in our data. This noise also prevents us from significantly detecting very short events associated with increased absorption (Hamaguchi et al 2016).…”

Let there be more variability in two γ Cas stars

“…The origin of this peculiar X-ray emission is still under debate, with two main scenarios under consideration: accretion onto a compact object (a white dwarf, see e.g. Hamaguchi et al 2016;Tsujimoto et al 2018, or a neutron star in a propeller regime, see e.g. Postnov et al 2017), or star-disk interactions (Smith et al 2016, and references ⋆ Based on spectra obtained with Aurélie at OHP and with FEROS and UVES at ESO, as well as on data collected with the SMEI, BRITE, and TESS space missions.…”