[Paper Review] BeppoSAX broad-band observations of Gamma Cassiopeiae

Alan Owens, T. Oosterbroek+4 more|arXiv (Cornell University)|May 19, 1999

Astrophysical Phenomena and Observations1 references3 citations

TL;DR

This study presents BeppoSAX broad-band X-ray observations of the Be star Gamma Cassiopeiae from July 1998, finding a 12.5 keV optically thin thermal plasma model with residual features at 0.3 keV and 1 keV. The data support an accreting white dwarf interpretation, with a 6×10³² erg/s luminosity in the 2–10 keV band and consistent optical magnitudes indicating a stable Be phase.

ABSTRACT

We report broad-band X-ray measurements of the Be star Gamma Cassiopeiae by the BeppoSAX X-ray astronomy satellite. The observations took place on 1998 July, 18-23. The 0.1-200 keV X-ray spectrum is reasonably well fit by an optically thin thermal plasma model of temperature 12.5 +/- 0.6 keV with significant residuals around 0.3 keV and 1 keV. The former is interpreted as the variable soft component reported by ROSAT, although there is no evidence for variability at the 5 % level. For a blackbody interpretation, the fitted temperature is 100 +320 -13 eV, in agreement with the ROSAT value of 200 +/- 10 eV. However, a MEKAL interpretation gives a significantly lower temperature of (48 +/- 11 eV). The fitted abundances are about half solar values, in agreement with previous measurements. At higher energies, the spectrum does not require non-thermal components and the observation of a line at 6.8 keV supports the ASCA interpretation of the source as an accreting white dwarf. Assuming a source distance of 188 pc, the bolometric luminosity in the 2-10 keV band is 6 10^32 erg/s. Simultaneous optical measurements by the Wendelstein Observatory near Munich, indicate that the source continues to be in a late but rather normal Be phase, with no obvious signs of a transition to the Be-shell phase. The measured magnitudes at B, V and R wavelengths of 2.18 +/- 0.06, 2.23 +/- 0.02 and 2.36 + /- 0.03, respectively, confirm this.

Motivation & Objective

• To characterize the broad-band X-ray spectrum of Gamma Cassiopeiae using BeppoSAX data.
• To investigate the nature of the soft X-ray excess and absorption features at 0.3 keV and 1 keV.
• To determine whether the X-ray emission arises from a thermal plasma or non-thermal processes.
• To assess the source's distance and bolometric luminosity in the 2–10 keV band.
• To correlate X-ray behavior with optical state, particularly whether the source is transitioning to a Be-shell phase.

Proposed method

• Acquired broad-band X-ray data from the BeppoSAX satellite over 18–23 July 1998, covering 0.1–200 keV.
• Fitted the X-ray spectrum using an optically thin thermal plasma model (MEKAL code) to derive temperature and abundance parameters.
• Compared blackbody and MEKAL model fits to assess soft X-ray component interpretation.
• Analyzed spectral residuals around 0.3 keV and 1 keV to identify possible emission or absorption features.
• Used a source distance of 188 pc to calculate bolometric luminosity in the 2–10 keV band.
• Correlated X-ray results with simultaneous optical photometry from the Wendelstein Observatory to assess the Be star's optical state.

Experimental results

Research questions

• RQ1 What is the dominant X-ray emission mechanism in Gamma Cassiopeiae across the 0.1–200 keV energy range?
• RQ2 Do the spectral residuals at 0.3 keV and 1 keV indicate a variable soft X-ray component or intrinsic absorption features?
• RQ3 Is the X-ray spectrum better described by a thermal plasma model or a non-thermal component?
• RQ4 Does the observed 6.8 keV emission line support the interpretation of an accreting white dwarf?
• RQ5 Is Gamma Cassiopeiae undergoing a transition to a Be-shell phase, based on simultaneous optical and X-ray data?

Key findings

• The 0.1–200 keV X-ray spectrum is best fit by an optically thin thermal plasma model with a temperature of 12.5 ± 0.6 keV.
• Residuals at 0.3 keV and 1 keV are consistent with a variable soft X-ray component, though no significant variability was detected at the 5% level.
• A blackbody fit yields a temperature of 100 +320/-13 eV, while a MEKAL model gives a cooler temperature of 48 ± 11 eV.
• Elemental abundances are approximately half solar, consistent with previous measurements.
• The 6.8 keV emission line supports the ASCA interpretation of an accreting white dwarf as the X-ray source.
• The bolometric luminosity in the 2–10 keV band is 6 × 10³² erg/s, assuming a distance of 188 pc.
• Simultaneous optical photometry shows magnitudes of B = 2.18 ± 0.06, V = 2.23 ± 0.02, and R = 2.36 ± 0.03, confirming the source remains in a late but normal Be phase.