X-ray, UV, and optical observations of the accretion disk and boundary layer in the symbiotic star RT Cru.

LUNA, G.J.M; MUKAI, K.; SOKOLOSKI, J. L.; LUCY, A. B.; CUSUMANO, G.; SEGRETO, A. ; JAQUE ARANCIBIA, M. ; NUÑEZ, N. E; PUEBLA, R. E.; NELSON, T.; WALTER, F.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2018

0004-6361

Compared to mass transfer in cataclysmic variables, the nature of accretion in symbiotic binaries in which red giants transfer materialto white dwarfs (WDs) has been difficult to uncover. The accretion flows in a symbiotic binary are most clearly observable, however,when there is no quasi-steady shell burning on the WD to hide them. RT Cru is the prototype of such non-burning symbiotics, withits hard (δ-type) X-ray emission providing a view of its innermost accretion structures. In the past 20 years, RT Cru has experiencedtwo, similar optical brightening events, separated by ∼4000 days and with amplitudes of ∆V∼ 1.5 mag. After Swift became operative,the BAT detector revealed a hard X-ray brightening event almost in coincidence with the second optical peak.Spectral and timing analyses of multi-wavelength observations that we describe here, from NuSTAR, Suzaku, Swift/XRT+BAT+UVOTand optical photometry and spectroscopy, indicate that accretion proceeds through a disk that reaches down to the white dwarf surface.The scenario in which a massive, magnetic WD accretes from a magnetically truncated accretion disk is not supported. For example,none of our data show the minute-time-scale periodic modulations (with tight upper limits from X-ray data) expected from a spinning,magnetic WD. Moreover, the similarity of the UV and X-ray fluxes, as well as the approximate constancy of the hardness ratio withinthe BAT band, indicate that the boundary layer of the accretion disk remained optically thin to its own radiation throughout thebrightening event, during which the rate of accretion onto the WD increased to 6.7×10 −9 M ⊙ yr −1 (d/2 kpc) 2 . For the first time from aWD symbiotic, the NuSTAR spectrum showed a Compton reflection hump at E> 10 keV, due to hard X-rays from the boundary layerreflecting off of the surface of the WD; the reflection amplitude was 0.77±0.21. The best fit spectral model, including reflection, gavea maximum post-shock temperature of kT =53±4 keV, which implies a WD mass of 1.25±0.02 M ⊙ .Although the long-term optical variability in RT Cru is reminiscent of dwarf-novae type outbursts, the hard X-ray behavior does notcorrespond to that observed in well-known dwarf nova. An alternative explanation for the brightening events could be that they aredue to an enhancement of the accretion rate as the WD travels through the red giant wind in a wide orbit, with a period of about ∼4000days. In either case, the constancy of the hard X-ray spectrum while the accretion rate rose suggests that the accretion-rate thresholdbetween a mostly optically thin and thick boundary layer, in this object, may be higher than previously thought.