Collisional Ionization in the X-ray Spectrum of the Ultracompact Binary 4U 1626-67

Understand

We report on high-resolution X-ray spectroscopy of the ultracompact X-ray binary pulsar 4U 1626-67 with Chandra/HETGS acquired in 2010, two years after the pulsar experienced a torque reversal.

• The well-known strong Ne and O emission lines with Keplerian profiles are shown to arise at the inner edge of the magnetically-channeled accretion disk.
• We exclude a photoionization model for these lines based on the absence of sharp radiative recombination continua.
• Instead, we show that the lines arise from a collisional plasma in the inner-disk atmosphere, with $T\simeq 10^7$ K and $n_e \sim 10^{17}$ cm^(-3).