X-ray and optical spectroscopic study of a gamma Cassiopeiae analog source pi Aquarii by Masahiro Tsujimoto et al. on Monday 21 November

Gamma Cas analog sources are a subset of Be stars that emit intense and hard

X-ray emission. Two competing ideas for their X-ray production mechanism are

(a) the magnetic activities of the Be star and its disk and (b) the accretion

from the Be star to an unidentified compact object. Among such sources, Pi Aqr

plays a pivotal role as it is one of the only two spectroscopic binaries

observed for many orbital cycles and one of the three sources with X-ray

brightness sufficient for detailed X-ray spectroscopy. Bjorkman et al. (2002)

estimated the secondary mass > 2.0 Mo with optical spectroscopy, which would

argue against the compact object being a white dwarf (WD). However, their

dynamical mass solution is inconsistent with an evolutionary solution and their

radial velocity measurement is inconsistent with later work by Naze et al.

(2019). We revisit this issue by adding a new data set with the NuSTAR X-ray

observatory and the HIDES echelle spectrograph. We found that the radial

velocity amplitude is consistent with Naze et al. (2019), which is only a half

of that claimed by Bjorkman et al. (2002). Fixing the radial velocity amplitude

of the primary, the secondary mass is estimated as < 1.4 Mo over an assumed

range of the primary mass and the inclination angle. We further constrained the

inclination angle and the secondary mass independently by fitting the X-ray

spectra with a non-magnetic or magnetic accreting WD model under the assumption

that the secondary is indeed a WD. The two results match well. We thus argue

that the possibility of the secondary being a WD should not be excluded for pi

Aqr.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.10803v1