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Astron. Astrophys. 339, L21-L24 (1998)

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2. ROSAT Observations

V Sge has been the target of three dedicated pointed PSPC and HRI observations (one of these splits into 3 separate observation intervals), and in addition is in the field of view of another PSPC observation (Table 1). The results of these observations are quite diverse: V Sge has not been detected during the ROSAT all-sky survey in 1990 and a long ROSAT HRI pointing in April 1994, but has been detected during all other observations, even in a much shorter HRI observation. Thus, V Sge shows strong X-ray variability with an amplitude of a factor of 140. In addition, the X-ray spectral characteristics during two ROSAT PSPC pointings obtained 1 yr apart show a remarkable difference: at one occasion V Sge has a `supersoft' X-ray spectrum, at another occasion the spectrum is very hard.


Table 1. ROSAT observations of V Sge Notes: (1) The letter after the observation ID number gives the ROSAT detector: P = PSPC, H = HRI. (2) Count rates in the corresponding detector in the 0.1-2.4 keV range (PSPC: channels 11-240). Upper limits are 3[FORMULA] confidence level. Note the different PSPC to HRI count rate conversion factors of 2.7:1 and 7.8:1 for hard and soft spectrum sources. (3) Hardness ratios with HR1 = [FORMULA] and HR2 = [FORMULA], where [FORMULA] keV), [FORMULA] keV), [FORMULA] keV), and [FORMULA] keV) are the counts in the given energy range. (4) Distance between best-fit X-ray and optical position. For the optical position [FORMULA], [FORMULA] has been used as determined from the second generation DSS. This position differs from the SIMBAD position by [FORMULA] and [FORMULA].

The diversity of X-ray measurements looks more ordered when it is compared with the optical brightness of V Sge. This binary system is included in the RoboScope program of automatic long-term monitoring the results of which led to the classification of three distinct optical states: bright state (V[FORMULA]11 mag), intermediate state (V[FORMULA]11-12 mag) and faint state (V[FORMULA]12 mag) (Robertson et al. 1997). We have combined the optical lightcurve obtained by these observations with data collected in the VSOLJ database (www.kusastro.kyoto-u.ac.jp/vsnet/) and plotted these in Fig. 1 together with the times of the ROSAT observations. This suggests that during optical bright state V Sge is a hard, but rather faint X-ray source, while during optical faint state V Sge is a more luminous and very soft X-ray source. During the intermediate optical state also the X-ray spectrum is intermediate with respect to the very soft and hard spectrum.

[FIGURE] Fig. 1. Optical light curve of V Sge with data from Robertson et al. 1997 (red dots) and VSOLJ (Web; green triangles). Vertical dashes mark the times of ROSAT observations. The lower panels show blow-ups around the ROSAT observations which are characterized by two vertical lines marking the start and the end of the ROSAT exposure. The dotted lines denote the boundaries of the three optical states.

To obtain an idea about the X-ray spectral parameters during the soft X-ray state, we fit the Nov. 1992 PSPC spectrum with a solar-abundance LTE [FORMULA] white dwarf atmosphere model (Van Teeseling et al. 1994). The [FORMULA] contours are shown in Fig. 2. The [FORMULA] contour suggests a temperature [FORMULA] K and a bolometric luminosity [FORMULA] erg s-1 (with [FORMULA] kpc), but lower temperatures and higher luminosities are still acceptable within the 90% confidence contour. If we require that the soft X-ray absorbing column is at least [FORMULA] cm-2 as derived from the 2200 Å absorption dip ([FORMULA]; Verbunt 1987), we find [FORMULA] K and [FORMULA] erg s-1. With [FORMULA] cm-2, only for [FORMULA]200 000 K a luminosity of [FORMULA] erg s-1 is reached. It is possible, however, that because of the very high orbital inclination the soft X-ray absorption is much larger than the ultraviolet absorption. A similar discrepancy is known for CAL 87 (cf. Hutchings et al. 1995; Parmar et al. 1997). If we relax the absorption constraint and assume that V Sge is a SSB with [FORMULA] erg s-1, we find [FORMULA] K and a radius [FORMULA] cm consistent with a white dwarf. However, because of a very high orbital inclination, the white dwarf may be completely obscured from view by the accretion disk rim, in which case the observable luminosity (from X-rays scattered into the line of sight) may be much less than [FORMULA] erg s-1. We note that there is no significant modulation of the soft X-rays on the orbital period.

[FIGURE] Fig. 2. Top: [FORMULA] and 90% confidence contours (solid lines) of a [FORMULA] fit of solar-abundance LTE [FORMULA] model spectra to the Nov. 1992 ROSAT PSPC spectrum, when V Sge was in an intermediate optical state and had a very soft X-ray spectrum. Dashed lines denote contours of constant absorbing column [FORMULA]/[FORMULA] cm-2, dotted lines indicate white dwarf radii in units of [FORMULA] cm. The radius and luminosity have been scaled to a distance of 1 kpc. Bottom: Comparison of the normalized HRI channel distribution of the photons during the May 1994 (dotted line; soft state) and Oct. 1994 (solid line; hard state) observations.

A factor of 45 increase in HRI count rate occurred within less than three weeks in April/May 1994 during which the optical brightness decreased and V Sge eventually became a very soft X-ray source. Though the ROSAT observations during this optical state transition have been performed with the HRI, the grossly different spectral shapes are easy to recognize (Fig. 2, bottom).

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Online publication: September 30, 1998