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

¹ Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: beuermann@astro.physik.uni-goettingen.de
² MPI für Extraterrestrische Physik, 85748 Garching, Germany e-mail: burwitz@mpe.mpg.de
³ Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany e-mail: rauch@astro.uni-tuebingen.de

Received: 22 April 2006
Accepted: 29 June 2006

Abstract

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 44-48 Å 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 eV and eV with a stellar angular radius as seen from infinity of km pc^-1. For HZ43 A, our fit yields T_eff = K and (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 keV. For Sirius B, we find T_eff = K for fixed . The calibration of the short-wavelength EUVE spectrometer differs from that of the LETG+HRC-S by %. 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.