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 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 = and HR2 = , where keV), keV), keV), and keV) are the counts in the given energy range. (4) Distance between best-fit X-ray and optical position. For the optical position , has been used as determined from the second generation DSS. This position differs from the SIMBAD position by and .
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 (V11 mag), intermediate state (V11-12 mag) and faint state (V12 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.
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 white dwarf atmosphere model (Van Teeseling et al. 1994). The contours are shown in Fig. 2. The contour suggests a temperature K and a bolometric luminosity erg s-1 (with 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 cm-2 as derived from the 2200 Å absorption dip (; Verbunt 1987), we find K and erg s-1. With cm-2, only for 200 000 K a luminosity of 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 erg s-1, we find K and a radius 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 erg s-1. We note that there is no significant modulation of the soft X-rays on the orbital period.
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).
© European Southern Observatory (ESO) 1998
Online publication: September 30, 1998