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Astron. Astrophys. 327, 245-251 (1997)

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5. The X-ray spectrum

Given the very low spectral resolution of the ROSAT PSPC and the modest number of observed photons, it is difficult to identify the source of the hard X-ray spectrum. The simplest measures of the shape of the spectrum are hardness ratios, which are measures of the relative number of hard versus soft photons. The standard ROSAT hardness ratio HR1 was between 0.6 and 0.9 during the RASS and pointed PSPC observations (Fig. 2), i.e. indicative of a relatively hard spectrum, and neither data set shows any sign of orbital variations in HR1.

Fitting the observed average spectrum with a single optically thin (Raymond-Smith) component yields a H column density [FORMULA] of [FORMULA] cm-2 (consistent with the maximum expected galactic extinction of [FORMULA] cm-2 and the absence of any 2200 Å interstellar absorption feature), and a very poorly determined temperature of [FORMULA] keV. Though a high-temperature component is needed to produce a hard X-ray spectrum, the fit has significant residuals around 0.6 to 1 keV, suggesting the presence of line emission from a much cooler gas. A meaningful two-component fit (with 5 parameters: one H column density, two emission measures, and two temperatures) is not possible. Therefore, we arbitrarily fixed the temperature of the hot component at 10 keV or 20 keV and fit the remaining 4 parameters: the results can be seen in Table 2 and Fig. 6 (left graph). The emission measures of the "warm" component are not significantly affected by the choice of [FORMULA]. A "banana diagram" showing the confidence region for the two parameters [FORMULA] and [FORMULA] is shown in Fig. 7. An analysis of the (poorer) RASS data yielded similar results.


Table 2. Spectral fits to the PSPC data

[FIGURE] Fig. 6a and b. Spectral fits to the ROSAT PSPC data (dashed lines are the sub-components). Left: a two-component Raymond-Smith model. Right: a constant pressure accretion column model. See the text for details.
[FIGURE] Fig. 7. "Banana" diagram showing the 50,60,70, and 80% probability contours for the interstellar H column density and the temperature of the cool component in a two-temperature Raymond-Smith fit to the ROSAT PSPC spectrum assuming a hot-component temperature of 10 (solid lines) or 20 keV (dashed lines). The plus marks show best-fit positions. See the text for details.

The relative difficulty in finding [FORMULA] (and hence [FORMULA]) is most easily explained by a wide spread in the temperatures of the X-ray emitting regions. The choice of a two- or more-component plasma is, however, very arbitrary. A much more meaningful model for the spectrum of an AM Her star is the radiation produced in a magnetic accretion column in which a wide range of temperatures naturally exists. In systems with low magnetic field strengths like V2301 Oph, bremsstrahlung emission should dominate the cooling, and the structure of the accretion column can be derived analytically (Hoshi 1973; Aizu 1973). We fit the PSPC spectrum using the simplest possible model: an accretion column with constant pressure (e.g. Frank, King, and Rayne 1992; pp. 142-144). The shock temperature


is held fixed, as is the (not particularly relevant) base temperature [FORMULA]. The column was split up into 9 sections (the maximum number allowed within EXSAS: a total of only 20 parameters can be used) with fixed mean relative emission measures and temperatures (weighted by the bremsstrahlung emissivities) ranging from 0.0083 to 0.6961 and 0.02 to 23 keV, respectively. The only fitted quantities are the interstellar absorption and the total emission measure (Table 2). With N [FORMULA] cm-2 and [FORMULA], this physically-motivated model for the X-ray spectrum not only yields similar results for the amount of interstellar absorption but is statistically as good as the two-component model (Fig. 6).

There are not enough photons in the narrow "dip" phases to make a formal fit to the covering factor and absorption column densities. The factor of [FORMULA] reduction in the flux could be explained by an additional column density of typically [FORMULA] cm-2, but would have produced a more pronounced "glitch" in the HR2 hardness-ratio light curve (Fig. 3). A better explanation is partial covering by a very optically thick component which totally blocks out a fraction of the emission from the underlying X-ray source. The covering factor would then be roughly 0.5-0.8 and would require [FORMULA] cm-2.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998