Delayed Iron Lines in Centaurus X‐3

Abstract

We present the study of the aperiodic time variation of the binary X-ray pulsar Centaurus X-3 observed with the Rossi X-Ray Timing Explorer (RXTE). We extracted light curves from six energy bands and calculated cross-spectral functions between the energy band containing the iron K lines (iron band) and the remaining bands. We discovered, for the first time, that the temporal variation of iron band emission is delayed as compared with that of the other energy X-rays by 0.39 ± 0.10 ms (1 σ error). We also discovered that the time difference shows a general trend such that the higher energy X-rays are advanced in comparison with the lower energy X-rays with the exception of the iron band. Assuming that the iron emission lines are fluorescent lines in origin and are emitted by circumstellar matter, we derived 5.6 ± 1.5 ms for the time delay of the temporal variation of the iron line from the average temporal variation in the energy band above the iron absorption edge, taking into account the energy spectral shape and the photoelectric absorption cross section. This result leads to a determination of (1.7 ± 0.5) × 108 cm for the distance between the original X-ray source and the reprocessor, which emits the iron lines. The general trend of the hard X-ray advance is considered to be related to the X-ray emission mechanism on the neutron star.