Research Note

A detection of an anti-correlated hard X-ray lag in AM Herculis

¹ International Center for Astrophysics, Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea
e-mail: astrosriram@yahoo.co.in
² Tata Institute of Fundamental Research, Mumbai 400005, India

Received: 3 September 2010
Accepted: 26 October 2010

Abstract

Context. Earlier cross-correlation studies for AM Her were performed in various energy ranges from optical to X-ray, and it was suggested that it mostly shows a high level of correlation, but on occasion it shows a low level of correlation or even no correlation at all.

Aims. We aim to investigate the degree of correlation between soft (2−4 keV) and hard (9−20 keV) X-rays, and perform a 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 with 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 with regard to the origin of the hard X-ray lag and the anti-correlated intensity variation.