Effective area calibration of the reflection grating spectrometers of XMM-Newton. II. X-ray spectroscopy of DA white dwarfs

TL;DR

This study uses white dwarf spectra as calibration standards to accurately determine the effective area of X-ray spectrometers, achieving better than 5% accuracy in the lowest energy band.

Contribution

It demonstrates how white dwarf model atmospheres can calibrate X-ray instruments with high precision, refining previous measurements and constraining stellar atmospheric compositions.

Findings

White dwarf spectra can calibrate X-ray instruments with <5% uncertainty.

Best-fit models suggest pure hydrogen atmospheres for Sirius B and HZ 43A.

Helium abundance in Sirius B is constrained to be four times lower than previous estimates.

Abstract

White dwarf spectra have been widely used as a calibration source for X-ray and EUV instruments. The in-flight effective area calibration of the reflection grating spectrometers (RGS) of XMM-Newton depend upon the availability of reliable calibration sources. We investigate how well these white dwarf spectra can be used as standard candles at the lowest X-ray energies in order to gauge the absolute effective area scale of X-ray instruments. We calculate a grid of model atmospheres for Sirius B and HZ 43A, and adjust the parameters using several constraints until the ratio of the spectra of both stars agrees with the ratio as observed by the low energy transmission grating spectrometer (LETGS) of Chandra. This ratio is independent of any errors in the effective area of the LETGS. We find that we can constrain the absolute X-ray spectrum of both stars with better than 5 % accuracy. The best-fit model for both stars is close to a pure hydrogen atmosphere, and we put tight limits to the amount of helium or the thickness of a hydrogen layer in both stars. Our upper limit to the helium abundance in Sirius B is 4 times below the previous detection based on EUVE data. We also find that our results are sensitive to the adopted cut-off in the Lyman pseudo-continuum opacity in Sirius B. We get best agreement with a long wavelength cut-off. White dwarf model atmospheres can be used to derive the effective area of X-ray spectrometers in the lowest energy band. An accuracy of 3-4 % in the absolute effective area can be achieved.