Astronomy and Astrophysics – Astronomy
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997mnras.287..651i&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 287, Issue 3, pp. 651-662.
Astronomy and Astrophysics
Astronomy
26
Accretion, Accretion Discs, Binaries: Close, Stars: Individual: Am Her, Novae, Cataclysmic Variables, X-Rays: Stars
Scientific paper
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.
Fujimoto Ryuichi
Ishida Manabu
Matsuzaki Keiichi
Mukai Koji
Osborne Julian Paul
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