Taking a break: paused accretion in the symbiotic binary RT Cru by A. Pujol et al. on Friday 25 November

Symbiotic binaries sometimes hide their symbiotic nature for significant

periods of time. There is mounting observational evidence that in those

symbiotics that are powered solely by accretion of red-giant's wind material

onto a white dwarf, without any quasi-steady shell burning on the surface of

the white dwarf, the characteristic emission lines in the optical spectrum can

vanish, leaving the semblance of an isolated red giant spectrum. Here we

present compelling evidence that this disappearance of optical emission lines

from the spectrum of RT Cru during 2019 was due to a decrease in the accretion

rate, which we derive by modeling the X-ray spectrum. This drop in accretion

rate leads to a lower flux of ionizing photons and thus to faint/absent

photoionization emission lines in the optical spectrum. We observed the white

dwarf symbiotic RT Cru with XMM-Newton and Swift in X-rays and UV and collected

ground-based optical spectra and photometry over the last 33 years. This

long-term coverage shows that during most of the year 2019, the accretion rate

onto the white dwarf was so low, $\dot{M}= (3.2\pm 0.06)\, \times$10$^{-11}$

$M_{\odot}$ yr$^{-1}$ (d/2.52 kpc)$^2$, that the historically detected hard

X-ray emission almost vanished, the UV flux faded by roughly 5 magnitudes, the

$U$, $B$ and $V$ flickering amplitude decreased, and the Balmer lines virtually

disappeared from January through March 2019. Long-lasting low-accretion

episodes as the one reported here may hamper the chances of RT Cru experiencing

nova-type outburst despite the high-mass of the accreting white dwarf.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.13193v1