A Hard and Variable X-Ray Emission From the Massive Emission-Line Star Hd 157832

et al. 2018

A&A

Context. A growing number of Be and Oe stars, named the γ Cas stars, are known for their unusually hard and intense X-ray emission. This emission could either trace accretion by a compact companion or magnetic interaction between the star and its decretion disk. Aims. To test these scenarios, we carried out a detailed optical monitoring of HD 45314, the hottest member of the class of γ Cas stars, along with dedicated X-ray observations on specific dates. Methods. High-resolution optical spectra were taken to monitor the emission lines formed in the disk, while X-ray spectroscopy was obtained at epochs when the optical spectrum of the Oe star was displaying peculiar properties. Results. Over the last four years, HD 45314 has entered a phase of spectacular variations. The optical emission lines have undergone important morphology and intensity changes including transitions between single-and multiple-peaked emission lines as well as shell events, and phases of (partial) disk dissipation. Photometric variations are found to be anti-correlated with the equivalent width of the Hα emission. Whilst the star preserved its hard and bright X-ray emission during the shell phase, the X-ray spectrum during the phase of (partial) disk dissipation was significantly softer and weaker. Conclusions. The observed behaviour of HD 45314 suggests a direct association between the level of X-ray emission and the amount of material simultaneously present in the Oe disk as expected in the magnetic star-disk interaction scenario.

Section: X-ray Spectral Analysismentioning

confidence: 93%

Intriguing X-ray and optical variations of the γ Cassiopeiae analog HD 45314

Rauw

Nazé

et al. 2018

A&A

“…As remarked in the discussion of Figure 3, variations of the soft X-ray band flux over long timescales are another matter. First, a factor of three variation was found for γ Cas itself (S04, SLM12) and then over 14 years for the γ Cas analog HD 157832 (Lopes de Oliveira & Motch, 2011). The γ Cas case is particularly edifying, once again, because the changes in the soft X-ray spectrum can be modeled by variations of more than 300× in the local N H b absorption column for 1 4 of the X-ray site(s).…”

Section: The 70-day Cyclical and Annualized Signalsmentioning

confidence: 99%

“…Of the known γ Cas stars HD 157832 is the latest spectral type, B1.5 Ve (possibly B2) according to a detailed analysis of its optical spectrum by Lopes de Oliveira & Motch (2011). Modeling of XMM/EPIC data suggests a thermal, two-component model, with kT hot ≈ 11.25 keV and kT warm ≈ 2.3 keV.…”

Section: Hd 157832mentioning

confidence: 99%

The X-ray emission of theγCassiopeiae stars

Advances in Space Research

Oliveira

Motch

2016

Self Cite

Long considered as the "odd man out" among X-ray emitting Be stars, γ Cas (B0.5e IV) is now recognized as the prototype of a class of stars that emit hard thermal X-rays. Our classification differs from the historical use of the term "γ Cas stars" defined from optical properties alone. The luminosity output of this class contributes significantly to the hard X-ray production in massive stars in the Galaxy. The γ Cas stars have light curves showing variability on a few broadly-defined timescales and spectra indicative of an optically thin plasma consisting of one or more hot thermal components. By now 9-13 Galactic ≈B0-1.5e main sequence stars are judged to be members or candidate members of the γ Cas class. Conservative criteria for this designation are for a ≈B0-1.5e III-V star to have an X-ray luminosity of 10 32 -10 33 ergs s −1 , a hot thermal spectrum containing the short wavelength Lyα Fe XXV and Fe XXVI lines and the fluorescence FeK feature all in emission. If thermality cannot be demonstrated, for example from either the presence of these Lyα lines or curvature of the hard continuum of the spectrum of an X-ray active Be star, we call them γ Cas candidates. We discuss the history Email addresses: masmith@noao.edu (Myron A. Smith), rlopes@ufs.br/ (R. Lopes de Oliveira), christian.motch@unistra.fr (C. Motch) Preprint submitted to Advances in Space ResearchJanuary 21, 2016 of the discovery of the complicated characteristics of the variability in the optical, UV, and X-ray domains, leading to suggestions for the physical cause of the production of hard X-rays. These include scenarios in which matter from the Be star accretes onto a degenerate secondary star and interactions between magnetic fields on the Be star and its decretion disk. The greatest aid to the choice of the causal mechanism is the temporal correlations of X-ray light curves and spectra with diagnostics in the optical and UV wavebands. We show why the magnetic star-disk interaction scenario is the most tenable explanation for the creation of hard X-rays on these stars.

“…Recently several members of a new "γ Cas class" of X-ray emitters have been discovered with spectral types near B0.5, but one, HD 157832, is classified as late as B1.5e (Lopes de Oliveira & Motch 2011). The first and also brightest of these "analogs" is the B1e star HD 110432 (Smith & Balona 2006; "SB"), also known as BZ Cru.…”

mentioning

confidence: 99%

CHARACTERIZATION OF THE X-RAY LIGHT CURVE OF THE γ Cas-LIKE B1e STAR HD 110432

Oliveira

Motch

2012

ApJ

HD 110432 (BZ Cru; B1Ve) is the brightest member of a small group of "γ Cas analogs" that emit copious hard X-ray flux, punctuated by ubiquitous "flares." To characterize the X-ray time history of this star, we made a series of six RXTE multi-visit observations in 2010 and an extended observation with the XMM-Newton in 2007. We analyzed these new light curves along with three older XMM-Newton observations from 2002-2003. Distributed over five months, the RXTE observations were designed to search for long X-ray modulations over a few months. These observations indeed suggest the presence of a long cycle with P ≈ 226 days and an amplitude of a factor of two. We also used Xray light curves constructed from XMM-Newton observations to characterize the lifetimes, strengths, and interflare intervals of 1615 flare-like events in the light curves. After accounting for false positive events, we infer the presence of 955 (2002)(2003) and 386 (2007) events we identified as flares. Similarly, as a control we measured the same attributes for an additional group of 541 events in XMM-Newton light curves of γ Cas, which after a similar correction yielded 517 flares. We found that the flare properties of HD 110432 are mostly similar to our control group. In both cases the distribution of flare strengths are best fit with log-linear relations. Both the slopes of these distributions and the flaring frequencies themselves exhibit modest fluctuations. We discovered that some flares in the hard X-ray band of HD 110432 were weak or unobserved in the soft band and vice versa. The light curves also occasionally show rapid curve drop offs that are sustained for hours. We discuss the existence of the long cycle and these flare properties in the backdrop of two rival scenarios to produce hard X-rays, a magnetic star-disk interaction and the accretion of blobs onto a secondary white dwarf.