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Luminosity dependent change of the emission diagram in the X-ray pulsar 4U 1626-67

We detect variability of the Fe Ka emission line in the spectrum of X-ray pulsar 4U 1626-67, correlated with changes in its luminosity and in the shape of its pulse profile. Analysis of archival Chandra and RXTE observations revealed the presence of an intrinsically narrow Fe Ka emission line in the spectrum obtained during the source's current high luminosity period. However, the line was not present during an XMM-Newton observation seven years earlier, when the source was ~three times fainter. The line is resolved by the high energy grating of Chandra at the 98% confidence level, and its small intrinsic width, sigma=36.4 (-/+ 11.3,15.3)eV, suggests reflection off an accretion disk at the radius R~7.5(-/+ 3.8,8.2)x10^8cm assuming a Keplerian disk, viewed at an inclination angle of 20deg. This value is consistent with the radius of the magnetosphere of the pulsar, suggesting that the line originates near the inner edge of a disk that is truncated by the magnetic field of the neutron star. Timing analysis of the XMM-Newton and RXTE data revealed a major change in the pulse profile of the source from a distinct double peaked shape during the high luminosity state when the line was present, to a much more complex multi-peak structure during the low luminosity state. We argue that the appearance of the line and the change in the shape of the pulse profile are correlated and are the result of a major change in the emission diagram of the accretion column, from a pencil-beam pattern at low luminosity, to a fan-beam pattern at high luminosity.