Detailed X-ray spectroscopy of AM Herculis with ASCA

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Title:		Detailed X-ray spectroscopy of AM Herculis with ASCA
Authors:		Ishida, Manabu; Matsuzaki, Keiichi; Fujimoto, Ryuichi; Mukai, Koji; Osborne, Julian P.
Affiliation:		AA(Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229, Japan ), AB(Faculty of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan ), AC(Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229, Japan ), AD(NASA/GSFC, Code 668, Greenbelt MD 20771, USA ), AE(Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH )
Publication:		Monthly Notices of the Royal Astronomical Society, Volume 287, Issue 3, pp. 651-662. (MNRAS Homepage)
Publication Date:		05/1997
Origin:		MNRAS
MNRAS Keywords:		ACCRETION, ACCRETION DISCS, BINARIES: CLOSE, STARS: INDIVIDUAL: AM HER, NOVAE, CATACLYSMIC VARIABLES, X-RAYS: STARS
Bibliographic Code:		1997MNRAS.287..651I

Abstract

We present the results of high-energy resolution X-ray spectroscopy of AM Herculis (Her) carried out with ASCA on 1993 September. Owing to the high sensitivity of the solid-state imaging spectrometer (SIS) down to ~0.4keV, we have detected the Wien tail of the soft blackbody component during the rotational maximum phase. The rotational intensity modulation of the soft blackbody and the hard bremsstrahlung are in phase, indicating that they originate from the same accretion pole. Using Extreme Ultraviolet Explorer (EUVE) data taken simultaneously with ASCA, we find the soft excess to be as great as 17+/-7 if we approximate the soft component observed by EUVE and ASCA with a single-temperature blackbody. From the blackbody luminosity, the fractional area is determined to be of order 10^-5. The spectrum during the rotational minimum phase is characterized by a strong fluorescent iron Kalpha emission with an equivalent width of ~1keV. This indicates that the hard X-ray emission during the rotational minimum phase consists entirely of scattered emission, and geometrical consideration suggests the pre-shock accretion column as a possible scattering site. The observed spectrum is softer than would be expected from optically thick scattering matter, and suggests that the column density of the scattering matter is not more than ~10^23cm^-2. The high-energy resolution of the SIS has enabled us to resolve the emission lines from hydrogenic and He-like Kalpha emission lines from iron. However, the emission lines from the other light elements are generally weaker than those seen in EX Hya, in which a significant post-shock cooling flow is found from the ASCA observation. The spectrum of AM Her in the bright phase is, on the other hand, consistent with a single-temperature optically thin thermal plasma spectrum with the abundance of 0.4^+0.2_-0.1. These facts indicate that most of the cooling part of the post-shock plasma is hidden from the observer in AM Her. A possible interpretation is that the post-shock plasma is highly inhomogeneous, and it penetrates the photosphere before it has cooled significantly.

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