2006A&A...458..541B

The absolute calibration of space-borne instruments in the soft X-ray regime rests strongly on model spectra of hot white dwarfs. We analyze the Chandra LETG+HRC observations of the white dwarfs HZ43A and Sirius B and of the neutron star RX J185635-3754 with the aim of resolving current uncertainties in the soft X-ray spectral fluxes and photospheric parameters of the three stars. We apply the derived photon spectra to a cross-calibration of the LETG+HRC-S with the short-wavelength EUVE spectrometer and the ROSAT PSPC. We tie HZ43A to the flux of RXJ1856 in the 44-48Å range and perform a simultaneous least squares fit to the LETG+HRC spectra of the three stars. This allows us to determine an internally consistent set of spectral energy distributions and an empirically derived wavelength-dependent correction to the LETG+HRC-S effective area. We employ NLTE model atmospheres calculated with TMAP for the white dwarfs and a two-blackbody model for RXJ1856, tied to the respective optical fluxes. The two-blackbody model for RXJ1856 features a hot spot on a cooler star and yields kT_spot=62.8±0.4eV and kT_star=32.3±0.7eV with a stellar angular radius as seen from infinity of 0.1371±0.0010km/pc. For HZ43A, our fit yields T_eff=51126±660K and logg=7.90±0.080(cgs) with anti-correlated errors (1-σ) which include not only the statistical but also the systematic uncertainties of the fit. HZ43AB displays a previously detected bremsstrahlung component with a temperature kT≃0.6keV. For Sirius B, we find T_eff=24923±115K for fixed logg=8.6. The calibration of the short-wavelength EUVE spectrometer differs from that of the LETG+HRC-S by 15±7%. The ROSAT PSPC is found to be correctly calibrated within a few percent and reports of a major miscalibration are unfounded. We have obtained improved parameters for RX J185635-3754, HZ43A, and Sirius B which fit the observations from the optical to the soft X-ray regime. Our approach allows us to quote their absolute spectral fluxes at selected wavelengths which may aid the calibration of other space-borne instruments.