A Detection of an Anti-correlated Hard X-ray Lag in AM Herculis

Original · EN

Context Earlier cross-correlation studies for AM Her were performed in various energy range from optical to X-ray and suggested that it mostly shows a high level of correlation but on occasion it shows a low level of correlation or uncorrelation. Aims To investigate the degree of correlation between soft (2-4 keV) and hard (9-20 keV) X-rays, we perform the cross-correlation study of the X-ray data sets of AM Her obtained with RXTE. Methods We cross-correlate the background-subtracted soft and hard X-ray light curves using the XRONOS program crosscor and fit a model to the obtained cross-correlation functions. Results We detect a hard X-ray lag of 192±33 s in a specific section of energy-dependent light curve, where the soft X-ray (2-4 keV) intensity decreases but the hard X-ray (9-20 keV) intensity increases. From a spectral analysis, we find that the X-ray emission temperature increases during the anti-correlated intensity variation. In two other observations, the cross-correlation functions show a low level of correlation, which is consistent with the earlier results performed in a different energy range. Conclusions We report a detection of an anti-correlated hard X-ray lag of 190 s from the proto-type polar AM Her. The hard X-ray lag is detected for the first time in the given energy range, and it is the longest lag among those reported in magnetic cataclysmic variables. We discuss the implications of our findings regarding the origin of the hard X-ray lag and the anti-correlated intensity variation.