Pulse phase dependence of emission lines in the X-ray pulsar 4U 1626-67

Abstract

We present results from a pulse phase resolved spectroscopy of the complex emission lines around 1 keV in the unique accretion powered X-ray pulsar 4U 1626-67 using observation made with the XMM-Newton in 2003. In this source, the red and blue shifted emission lines and the line widths measured earlier with Chandra suggest their accretion disk origin. Another possible signature of lines produced in accretion disk can be a modulation of the line strength with pulse phase. We found the line fluxes to have pulse phase dependence, making 4U 1626-67 only the second pulsar after Her X-1 to show such variability. The O~VII line at 0.568~keV from 4U 1626-67 varied by a factor of $\sim$4, stronger than the continuum variability, that support their accretion disk origin. The line flux variability may appear due to variable illumination of the accretion disk by the pulsar or more likely, a warp like structure in the accretion disk. We also discuss some further possible diagnostics of the accretion disk in 4U 1626-67 with pulse phase resolved emission line spectroscopy.