The X-ray Emission ofγCassiopeiae During the 2020–2021 disc eruption

Abstract

Context. γ Cas is known for its unusually hard and intense X-ray emission. This emission could trace accretion by a compact companion, wind interaction with a hot sub-dwarf companion, or magnetic interaction between the star and its Be decretion disc. Aims. These various scenarios should lead to diverse dependences of the hard X-ray emission on disc density. To test these scenarios, we collected X-ray observations of γ Cas during an episode of enhanced disc activity that took place around January 2021. Methods. We investigate the variations in the disc properties using time series of dedicated optical spectroscopy and existing broadband photometry. Equivalent widths and peak velocity separations are measured for a number of prominent emission lines. Epochdependent Doppler maps of the Hα, Hβ, and He  λ 5876 emission lines are built to characterise the emission regions in velocity space. We analyse four XMM-Newton observations obtained between January 2021 and January 2022 at key phases of the episode of enhanced disc activity. Archival XMM-Newton, Chandra, MAXI, and RXTE-ASM data are also used to study the long-term correlation between optical and X-ray emission. Results. Optical spectroscopy unveils a clear increase in the radial extent of the emission regions during the episode of enhanced disc activity, whilst no increase in the V-band flux is recorded. Our Doppler maps do not reveal any stable feature in the disc resulting from the putative action of the companion on the outer parts of the Be disc. Whilst the hard X-ray emission is found to display the usual level and type of variability, no specific increase in the hard emission is observed in relation to the enhanced disc activity. However, at two occasions, including at the maximum disc activity, the soft X-ray emission of γ Cas is strongly attenuated, suggesting more efficient obscuration by material from a large flaring Be disc. In addition, there is a strong correlation between the long-term variations in the X-ray flux and the optical variations in the V-band photometry. Conclusions. The observed behaviour of γ Cas suggests no direct link between the properties of the outer regions of the Be disc and the hard X-ray emission, but it favours a link between the level of X-ray emission and the properties of the inner part of the Be disc. These results thus disfavour an accretion or colliding wind scenario.

Fluorescent Fe K line emission of γ Cas stars

“…To convert the fluxes into luminosities, we adopted a distance of 190 pc (Nazé & Motch 2018). The fluorescent Fe Kα line observed in the XMM-Newton and Chandra spectra of γ Cas has an EW in the range between 34 and 59 eV (Rauw et al 2022).…”

“…The X-ray spectrum of γ Cas is described well by a combination of up to four optically thin thermal plasma components, with the hottest plasma providing the dominant contribution. For this hot component, Rauw et al (2022) determined a mean plasma temperature of kT = 12.5 keV and a mean flux, corrected for absorption by the interstellar medium, of 1.5 × 10 −10 erg cm −2 s −1 in the 2.0-10.0 keV energy band. To convert the fluxes into luminosities, we adopted a distance of 190 pc (Nazé & Motch 2018).…”

“…In low-eccentricity Be binary systems, the circumstellar disk is expected to be truncated by gravitational interaction with the companion at the 3:1 resonance radius (Okazaki & Negueruela 2001). In the case of γ Cas, this resonance radius corresponds to about 176 R ⊙ (Rauw et al 2022), which we adopted as the outer radius of the Keplerian disk.…”

“…The unusually hard and bright X-ray emission of the B0.5 IVe star γ Cassiopeiae has been a puzzle ever since its discovery (see Smith et al 2016 for a review and Rauw et al 2022 for a recent observational study). Over the last two decades, a number of similar objects have been identified (e.g.…”

Двойное Комптоновское Рассеяние Фотона Атомным Ионом

“…В качестве объекта исследования взят гелиеподобный ион атома неона (Ne 8+ , заряд ядра Z = 10, конфигурация и терм основного состояния [0] = 1s 2 [ 1 S 0 ]). Выбор обусловлен сферической симметрией основного состояния иона Ne 8+ , его доступностью в газовой фазе при проведении высокоточных экспериментов, например, с рентгеновским лазером на свободных электронах [13] и ярко выраженным присутствием его (r, i, f )-триплета Kα-линий в наблюдаемых спектрах рентгеновской эмиссии от горячих астрофизических объектов [14,15].…”