Effects of the photo-ionized absorber on the continuum spectra of EXO 0748–676

We report on the analysis of XMM-Newton archival data of EXO 0748–676. We studied changes of the continuum spectra due to the presence of photo-ionized plasma on the line of sight. We show that the ionization degree of the plasma could change largely during the X-ray bursts and the dips. These changes can significantly modify the soft-band spectrum, which was in fact observed from EXO 0748–676. We discuss the effect of the photo-ionized plasma on the continuum spectra in comparison with a frequently used model such as partial covering absorption.     

A new eclipsing low-mass X-ray binary with a giant companion

We detected hard X-ray emission from the unidentified Galactic bulge source 1RXS J175721.2-304405 with ASCA. The observed absorption column, flux and power-law index led us to consider that 1RXS J175721.2-304405 may be a new low-mass X-ray binary located near the Galactic center. Furthermore, the X-ray light-curve shows a step-function-like time variability, which is likely due to the occultation of a companion star. Future follow-up observations by missions such as ASTROSAT may reveal a periodic eclipse from 1RXS J175721.2-304405 if it is covered long enough. Since the long orbital period suggests a giant companion, follow-up observations will give firm evidence that 1RXS J175721.2-304405 is a new and rare eclipsing low-mass X-ray binary with a giant companion.     

Possible Suzaku detection of non-thermal X-ray signals from a rotating magnetized white dwarf

Although rotating neutron stars (NSs) have been regarded as being textbook examples of astrophysical particle acceleration sites for decades, details of the acceleration mechanism remain a mystery; for example, we cannot yet observationally distinguish “polar cap” models from “outer gap” models. To solve the model degeneracy, it is useful to study similar systems with much different physical parameters. Strongly magnetized white dwarfs (WDs) are ideal for this purpose, because they have essentially the same system geometry as NSs, but differ largely from NSs in the system parameters, including the size, magnetic field, and the rotation velocity, with the induced electric field expected to reach 10¹³–10¹⁴ eV. Based on this idea, the best candidate among WDs, AE Aquarii, was observed with the fifth Japaneses X-ray satellite, Suzaku. The hard X-ray detector (HXD) on-board Suzaku has the highest sensitivity in the hard X-ray band over 10 keV. A marginal detection in the hard X-ray band was achieved with the HXD, and was separated from the thermal emission. The flux corresponds to about 0.02% of its spin-down energy. If the signal is real, this observation must be a first case of the detection of non-thermal emission from WDs.