| Popis: |
Hard X‐ray emission associated with white dwarfs (WDs) can be used to diagnose the presence of late‐type binary companions, mass accretion from companions, or physical processes with unknown origins. Since our previous systematic searches for hard X‐ray emission associated with WDs, the Galactic WD catalog has been augmented by >10,000 new WDs from the Sloan Digital Sky Survey and new X‐ray point source catalogs from XMM‐Newton and ROSAT have become available. Therefore, we have extended the search using the updated catalogs, and found 17 new cases of WDs associated with hard X‐ray emission. The 32 WDs associated with hard X‐ray emission, from the current and previous searches, can be divided into five categories: (1) binary WD with a coronal companion, (2) binary WD with mass transfer from a companion, (3) single hot WD with a hard X‐ray component peaking near 1 keV in addition to a soft photospheric component, (4) two PG 1159 stars with very faint X‐ray emission in the 0.9–2.0 keV band, and (5) two DA WDs whose photospheric emission component has a hard shoulder extending to 0.5–0.9 keV. The origin of the hard X‐ray emission in the latter three categories is not yet known. Deeper X‐ray observations with higher angular and spectral resolutions are needed to help us understand these WDs’ hard X‐ray emission. |
