Establishing HZ43 A, Sirius B, and RX J185635−3754 as soft X-ray standards: a cross-calibration between the Chandra LETG+HRC-S, the EUVE spectrometer, and the ROSAT PSPC

Context. The absolute calibration of space-borne instruments in the soft X-ray regime rests strongly on model spectra of hot white dwarfs. Aims. We analyze the Chandra LETG+HRC observations of the white dwarfs HZ43 A 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. Methods. We tie HZ43 A to the flux of RX J1856 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 RX J1856, tied to the respective optical fluxes. Results. The two-blackbody model for RX J1856 features a hot spot on a cooler star and yields kTspot = 62.8 ± 0.4 eV and kTstar = 32.3 ± 0.7 eV with a stellar angular radius as seen from infinity of 0.1371 ± 0.0010 km pc−1. For HZ43 A, our fit yields Teff=51126±660 K and log g=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 ≃ 0.6 keV. For Sirius B, we find Teff=24923±115 K for fixed log g=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. Conclusions. We have obtained improved parameters for RX J185635−3754, HZ43 A, 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.