Collissional Ionisation in the X-Ray Spectrum of the Ultra-compact Binary 4U 1626-67
Abstract
4U 1626-67 is an ultracompact binary pulsar with a pulse period of 7.7 sec and an orbital period of 40 min. Its X-ray spectrum exhibits strong Doppler line pairs from highly ionized Ne and O. In 2008 the pulsar underwent another episode of torque reversal since the one in the early 1990. We observed the X-ray source one year after the event with the HETG spectrometer onboard Chandra. While the light curve before torque reversal is featureless, it now shows enhanced flaring similar to observations before the first reversal episode. The spectrum for the first time shows a narrow Fe K fluorescence line. We show while a photo-ionized plasma cannot fit the Ne and O Doppler line emissions, a collisionally ionized plasma provides a better description of the data. Based on this new interpretation of the Doppler line emissions, we propose that magnetospheric accretion shocks and shocked matter moving towards the magnetospheric poles provide a more consistent description than the ionized disk paradigm. We also compare this spectrum to the more recent assessment of the Suzaku spectrum after torque reversal.
- Publication:
-
39th COSPAR Scientific Assembly
- Pub Date:
- July 2012
- Bibcode:
- 2012cosp...39.1735S