Taking a break: Paused accretion in the symbiotic binary RT Cru

Abstract

Symbiotic binaries sometimes hide their symbiotic nature for significant periods of time. There is mounting observational evidence that, in symbiotics that are powered solely by the accretion of the 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 in 2019 was due to a decrease in the accretion rate, which we derived by modeling the X-ray spectrum. This drop in accretion rate leads to a lower flux of ionizing photons and thus to faint or 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 obtained over the last 33 yr. This long-term coverage shows that, during most of the year 2019, the accretion rate onto the white dwarf was so low, Ṁ = (3.2 ± 0.06) × 10−11 M⊙ yr−1 (d/2.52 kpc)2, that the historically detected hard X-ray emission almost vanished, the UV flux faded by roughly 5 mag, the U, B, and V flickering amplitude decreased, and the Balmer lines virtually disappeared from 2019 January through March. Long-lasting low-accretion episodes such as the one reported here may hamper the chances of RT Cru experiencing a nova-type outburst despite the high mass of the accreting white dwarf.