087081

We propose joint NuSTAR-XMM-Newton observations of a periodically variable quasar recently found from the Dark Energy Survey openParDESclosePar. The target is unique because its periodicity is highly significant and robust thanks to the sensitive DES imaging and a virgul20-yr long time baseline by combining DES with SDSS. The high sensitivity and long baseline are both crucial to rejecting false positives caused by quasars stochastic red noise variability. The proposed X-ray observations will test and distinguish two generic but different sets of models of circumbinary accretion disk simulations openPara central cavity vs. minidisksclosePar comma which predict dramatically different X-ray fluxes. The results will provide an independent and complementary test for the binary black hole hypothesis to explain the periodicity.