High-resolution X-ray spectra of the compact binary supersoft X-ray source CAL 87

¹ School of Physics and Electrical Engineering, Liupanshui Normal University, Liupanshui, Guizhou 553004, China
² Qiannan Normal University for Nationalities, Duyun 558000, China
³ Qiannan Key Laboratory of Radio Astronomy, Guizhou Province, Duyun 558000, China
⁴ Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 201210, China

Received: 23 March 2024
Accepted: 23 July 2024

Abstract

In this study we present an analysis of the archival X-ray data of the eclipsing supersoft X-ray binary CAL 87 observed with the Chandra Advanced CCD Imaging Spectrometer (ACIS) camera and Low Energy Transmission Grating (LETG) in 2001 August and with XMM-Newton in 2003 April. The high-resolution X-ray spectra are almost unchanged on the two different dates. The average unabsorbed X-ray luminosity during the exposure was 4.64 − 5.46 × 10³⁶ ergs s⁻¹ in 2001 and 4.54 − 4.82 × 10³⁶ ergs s⁻¹ in 2003, with prominent and redshifted emission lines, mostly of nitrogen, oxygen, iron, and argon, contributing to at least 30% of the X-ray flux. The continuum X-ray flux is at least an order of magnitude too low for a hot hydrogen-burning white dwarf (WD). However, the continuum flux is consistent with Thomson-scattering reflecting about 5% of the light of a hydrogen-burning WD with effective temperature of 800 000 K and a mass of ∼1.2 M_⊙. It has been noted that a large Thomson-scattering corona explains the X-ray eclipse of CAL 87, in which the size of the eclipsed region is found to be on the order of a solar radius. The emission lines originate in an even more extended region beyond the eclipsed central X-ray source; the emission spectrum is very complex, with unusual line ratios.