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We detect variability of the Fe Ka emission line in the spectrum of X-raypulsar 4U 1626-67, correlated with changes in its luminosity and in the shapeof its pulse profile. Analysis of archival Chandra and RXTE observationsrevealed the presence of an intrinsically narrow Fe Ka emission line in thespectrum 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 highenergy grating of Chandra at the 98% confidence level, and its small intrinsicwidth, sigma=36.4 (-/+ 11.3,15.3)eV, suggests reflection off an accretion diskat the radius R~7.5(-/+ 3.8,8.2)x10^8cm assuming a Keplerian disk, viewed at aninclination angle of 20deg. This value is consistent with the radius of themagnetosphere of the pulsar, suggesting that the line originates near the inneredge 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 thepulse profile of the source from a distinct double peaked shape during the highluminosity state when the line was present, to a much more complex multi-peakstructure during the low luminosity state. We argue that the appearance of theline and the change in the shape of the pulse profile are correlated and arethe 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 highluminosity.

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