http://arxiv.org/abs/1506.02411
We detect variability of the Fe Kalpha 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 Kalpha 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 small intrinsic width of the line, sigma ~ 36 eV, as measured by the high energy grating of Chandra, suggests reflection off the outer accretion disk, at R ~ 1800Rs,assuming a Keplerian disk. This value is consistent with the truncation radius of the disk by the magnetic field of the neutron star, ~ 3 x 10^12 Gauss, known from cyclotron line measurements. 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.
F. Koliopanos and M. Gilfanov
Tue, 9 Jun 15
24/56
Comments: Submitted to MNRAS 8pages, 3 figures, 2 tables
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