Luminous binary supersoft X-ray sources: Optical colors and absolute magnitudes

Abstract

An analysis of the color indices and absolute magnitudes M V 0 of the supersoft X-ray binary sources (SSXBs), the V Sge-type stars (VSs), the symbiotic SSXBs, and the classical novae in the soft X-ray phase ispresented. This approach can help comparing the properties and configuration of the reprocessing medium in the individual systems. The range of M V 0 of the individual objects is very large, from M V 0 = 8.5 to M V 0 = -4, most systems having M V 0 < 2. The objects with the orbital period P o r b < 4 days tend to possess brighter M V 0 with the increasing P o r b . The same trend is apparent also for M U 0 . There is no systematic difference between the classical SSXBs and the VSs, as regards their M V 0 and M U 0 . The soft X-ray phase of the novae V 1974 Cyg and V 382 Vel occurred at significantly brighter M V 0 than those of most other SSXBs while M V 0 of the recurrent nova U Sco was comparable to the SSXBs of the comparable P o r b . We showed that the luminosity of the reprocessing medium in the symbiotic SSXBs does not increase much with P o r b , when compared with the luminosity of the classical SSXBs and VSs; this speaks against large disks in these symbiotics. The classical SSXBs and the VSs have colors very similar each to other and form closed groups in the color-color diagrams. A Balmer jump occurs frequently in emission as inferred from the colors. The correlation between the bolometric luminosity, determined from the X-ray spectra and M V 0 or M U 0 of all kinds of the SSXBs is rather weak, but resolvable, and is more prominent for M U 0 . The implications for the reprocessing medium are discussed.