PROBING THE MYSTERIES OF THE X-RAY BINARY 4U 1210-64 WITH ASM, PCA, MAXI, BAT, AND SUZAKU

Abstract

4U 1210-64 has been postulated to be a high-mass X-ray binary powered by the Be mechanism. X-ray observations with Suzaku, the ISS Monitor of All-sky X-ray Image (MAXI), and the Rossi X-ray Timing Explorer Proportional Counter Array (PCA) and All Sky Monitor (ASM) provide detailed temporal and spectral information on this poorly understood source. Long-term ASM and MAXI observations show distinct high and low states and the presence of a 6.7101 ± 0.0005 day modulation, interpreted as the orbital period. Folded light curves reveal a sharp dip, interpreted as an eclipse. To determine the nature of the mass donor, the predicted eclipse half-angle was calculated as a function of inclination angle for several stellar spectral types. The eclipse half-angle is not consistent with a mass donor of spectral type B5 V; however, stars with spectral types B0 V or B0-5 III are possible. The best-fit spectral model consists of a power law with index Γ = 1.85⁺⁰.⁰⁴₋₀.₀₅ and a high-energy cutoff at 5.5 ± 0.2 keV modified by an absorber that fully covers the source as well as partially covering absorption. Emission lines from S xvi Kα, Fe Kα, Fe xxv Kα, and Fe xxvi Kα were observed in the Suzaku spectra. Out of eclipse, the Fe Kα line flux was strongly correlated with unabsorbed continuum flux, indicating that the Fe i emission is the result of fluorescence of cold dense material near the compact object. The Fe i feature is not detected during eclipse, further supporting an origin close to the compact object.