Understanding supersoft X-ray sources with eROSITA observations

• Applicants: Privatdozent Dr. Jochen Greiner; Professor Dr. Klaus Werner
• Subject Area: Astrophysics and Astronomy
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 446281683

Luminous supersoft X-ray sources (SSS) were established as a distinct class of objects by the ROSAT X-ray observatory about 30 years ago. Observationally they are distinguished by their very soft X-ray spectra with blackbody temperatures from 200,000 to 1 million Kelvin and luminosities log(L) = 37-38 [erg/s]. About 100 SSS are known, many of them in the Magellanic Clouds (MCs) and other nearby galaxies, but far more sources are theoretically predicted. The standard model is a white dwarf (WD) accreting mass at high rate from a donor such that the mass-accretion rate is about equal to the rate of hydrogen burning on the WD surface. More than half of the SSS are transient sources with the majority being post-outburst novae. Despite large progress since the 1990s to understand the nature of the SSS, many puzzles remain. New insights are now possible with the all-sky survey being performed by the German X-ray observatory eROSITA during four years since the end of 2019. eROSITA provides (i) better sensitivity than ROSAT, (ii) lower energy cutoffs and better spectral resolution than the imagers on Chandra, XMM-Newton, and Swift observatories, and (iii) a longer survey coverage than ROSAT. Based on this progress, we have set several objectives for our proposed project. Some of them are the following. (*) Discover more SSS in the MCs to understand the difference between observed and predicted population size. (*) Discover SSS over a broader temperature range to conclude on WD mass distribution and relation to novae. (*) Distinguish closely related source types by distinguishing potential spectral differences and measuring accurate luminosities. (*) Much better map the nova population to understand recurrence times and duration of burning phases, which theoretically are widely debated. (*) Establish stringent constraints for chemical evolution and yields that are predicted by SSS and nova models. (*) Develop advanced model atmospheres for WDs, for the emission from boundary layers and from irradiated accretion disks. In the proposed project, we will combine our complementary expertise to use the eROSITA mission for a substantial advance of our understanding of these enigmatic SSS.

• DFG Programme: Research Grants