Abstract for Proposal 090454

Searching for QPO in non-AGN bright nuclear transients discovered by eROSITA

Quasi-periodic oscillations (QPOs) are powerful tools to study the imprint of relativistic effects on the inner regions of the accretion flow around BHs. However until now, they have predominantly been found in X-ray binaries, and still rare in SMBH-accreting systems. During the past year of eROSITA's all sky survey, we serendipitously detected QPOs in one of only two bright non-AGN nuclear transients that were followed-up with long exposure XMM-Newton observations. This may indicate a high QPO detection fraction in these X-ray bright transients. In order to search for new QPOs and significantly enlarge the number of QPOs detected in SMBH-accreting systems, we ask for XMM-Newton ToO observations of 2 non-AGN bright X-ray nuclear transients that will be discovered by eROSITA during AO21.




Details on Observing Strategy and Trigger Criteria
Trigger cirteria: we propose EPIC ToO observations of 2 non-AGN bright nuclear X-ray transients to be discovered during the eROSITA all-sky survey in AO21. Targets will be selected from the sample of new, bright (f_0.2-2.3 keV>2E10-12 erg/s/cm$^2) X-ray sources found by comparing observations performed in eRASS6 (June--November 2021) and eRASS7 (until June 2023) to previous eRASS. They should also show variability in the per scan eRASS light curve. The source must have no previous X-ray detection and show an X-ray brightening by at least a factor of 10 relative to a previous upper flux detection limit. The counterpart should be a galaxy at redshift z < 0.15 not known to be an AGN (e.g., from WISE colors, SDSS, etc.) and without significant historical variability at UV/optical/near-IR wavelengths. Sources selected following these criteria will furthermore be vetted through multi-wavelength observations available in guaranteed time to the proposing team. Reaction time: the X-ray flux of those transients can decrease significantly within months. we thus request a reaction time of up to two weeks after we trigger. Observing strategy: each of the targets will be observed once with exposure time of 120ks.