Speaker
Description
The first all-sky X-ray survey was performed by the ROSAT X-ray observatory in the 0.1$-$2.4 keV energy range (Trümper 1982). It was not until almost 30 years later that an all-sky X-ray survey was to be carried out again with the SRG/eROSITA X-ray mission. Between December 2019 and December 2021 four all-sky surveys were completed. The sensitivity of eROSITA at soft energies (about 0.1 keV) is not as good as ROSAT's but a larger effective area makes up for this. Only the hotter isolated white dwarfs ($T_{eff} > 20000$ K) are observed in X-rays.
Taking the current cumulative eROSITA all-sky data from the surveys 1$-$4, which are limited at lower energies to 0.2 keV, and determining the hardness ratios we found about 38000 soft sources with [(0.5keV-2.3keV) $-$ (0.2keV-0.5keV)]/[(0.2keV-0.5 keV)+(0.5keV-2.3keV)] $\leq$0.94. From this sample, about 700 have matches with the Gaia white dwarf catalogue (Gentile Fusillo et al. 2021) and about 270 have a probability of more than 90 percent being a white dwarf.
To improve these findings eROSITA data will be processed to a lower energy limit of about 0.1 keV to create a flux limited sample of isolated white dwarfs observed with eROSITA.
