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

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2. X-ray observations

We obtained X-ray observations of V2301 Oph using the ROSAT satellite, both from the PSPC All-Sky Survey (RASS; Voges 1993) and from pointed PSPC and HRI observations (see the log of observations in Table 1). V2301 Oph is a fairly hard but bright X-ray source. All data were reduced using standard EXSAS procedures within the MIDAS data reduction environment.


Table 1. Log of observations

2.1. ROSAT All-Sky Survey

The orbital light curve of V2301 Oph pieced together from the RASS data (Beuermann and Thomas 1993) is shown in Fig. 1 (circles with error bars). V2301 Oph was fairly hard during the RASS (HR1=+0.8) but still showed an unusually high 0.1-3 cts s-1 in the ROSAT PSPC, making it one of the brightest X-rays sources in the CV sky at the time. Unfortunately, no data were available during the eclipse. Although there is considerable variation from orbit to orbit, an orbital hump around phases 0.5-1.1 can be seen. Most of the variation at a given orbital phase is due to orbit-to-orbit variations and the odd phase-coverage of the survey data rather than marked differences in count rates within an orbit. Thus, the orbital "hump" is real but its amplitude is variable. Similar orbital humps are seen in most AM Hers and are produced when the accretion shock on the white dwarf shows its maximal area ("bright phases"; King & Shaviv 1984).

[FIGURE] Fig. 1. Orbital light curve of V2301 Oph taken from the ROSAT survey data (circles) and the PSPC pointed data (solid line)

2.2. ROSAT PSPC observations

We obtained ROSAT PSPC pointed observations centered on two eclipses on 1993 September 10 and 11 (Table 1). The maximum count-rate was smaller than that seen in the RASS data: roughly 0.6 cts s-1 in the pointed versus 2-3 cts s-1 in the survey data. The binned light curve constructed from a total of 5490 s of data using the orbital ephemeris of BRB is shown in Fig. 2 (lower graph). The eclipse of the X-ray emission region expected for the estimated inclination of 80 [FORMULA] (BRB) is clearly visible and is total (the noise during mid-eclipse is due to the subtraction of the soft X-ray background). As in the RASS data, a broad orbital hump is apparent, but there is also a clear dip in the hump around orbital phase 0.87-0.90 which reaches down to about 20% of the hump level, presumedly due to photoelectric absorption by material along the line-of-sight to the emission region at that particular phase. The X-ray spectrum of V2301 Oph is hard and does not change in shape with orbital phase (Fig. 3). The weighted means of the standard ROSAT hardness-ratios HR1 and HR2 are [FORMULA] and [FORMULA], respectively. Thus, not only is there no sign of the optically thick soft spectrum component typical of most AM Hers, but there are also no spectral changes around the eclipse or during the dips.

[FIGURE] Fig. 2. Mean phased ROSAT PSPC and HRI light curves (histograms) showing the eclipse of the white dwarf, the orbital "hump" around phases 0.75-1.1, the variable "dips" around phases 0.8-0.9, and sinusoidal fits to the non-dip, non-eclipse light curves (solid lines).
[FIGURE] Fig. 3. The ROSAT PSPC hardness light curves showing that V2301 Oph is a hard source with practically no orbital spectrum modulation. Bottom: HR1, measuring the relative flux in the 0.1-0.4 and 0.4-2 keV bands. Top: HR2 (the 0.4-0.5 and 0.5-2 keV bands).

2.3. ROSAT HRI observations

We obtained ROSAT HRI pointed observations on 1995 Mar 25 and 26. During 5 OBI's, a total of 9349 s of accepted time was obtained. The binned light curve constructed using the orbital ephemeris of BRB is shown in Fig. 2 (upper graph): as with the PSPC data, the total eclipse of the X-ray emission region and the dipping behavior (this time around phase 0.9) is obvious. Comparisons of the mean PSPC and HRI light curves as well as the individual unphased HRI light curves clearly show that the dips are due to individual absorption events scattered around phases 0.8-0.9.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998