X-ray and optical spectroscopic study of a ? Cassiopeiae analog source ? Aquarii

Abstract ? Cas analog sources are a subset of Be stars that emit intense and hard X-ray emission. Two competing ideas for their production mechanism (a) the magnetic activities star its disk (b) accretion from to an unidentified compact object. Among such sources, ? Aqr plays pivotal role as it is one only two spectroscopic binaries observed many orbital cycles three with brightness sufficient detailed spectroscopy. Bjorkman et al. (2002, ApJ, 573, 812) estimated secondary mass >2.0 M? optical spectroscopy, which would argue against object being white dwarf (WD). However, dynamical solution inconsistent evolutionary radial velocity measurement later work by Nazé (2019, A&A, 632, A23). We revisit this issue adding new data set NuSTAR observatory HIDES échelle spectrograph. found amplitude consistent A23), half claimed 812). Fixing primary, <1.4 over assumed range primary inclination angle. further constrained angle independently fitting spectra non-magnetic or accreting WD model under assumption indeed WD. The results match well. thus possibility should not be excluded Aqr.