The young Be-star binary Circinus X-1

Cir X-1 is a young X-ray binary exhibiting X-ray flux changes of four orders of magnitude over several decades. It has been observed many times since the launch of the Chandra X-ray Observatory with high energy transmission grating spectrometer and each time the source gave us a vastly different look. At its very lowest X-ray flux we found a single 1.7 keV blackbody spectrum with an emission radius of 0.5 km. Since the neutron star in Cir X-1 is only few thousand years old we identify this as emission from an accretion column since at this youth the neutron star is assumed to be highly magnetized. At an X-ray flux of 1.8×10^-11 erg cm^-2 s^-1 this implies a moderate magnetic field of a few times of 10¹¹ G. The photoionized X-ray emission line properties at this low flux are consistent with B5-type companion wind. We suggest that Cir X-1 is a very young Be-star binary.