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

Abstract

Compared to mass transfer in cataclysmic variables, the nature of accretion in symbiotic binaries in which red giants transfer material to 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, with its hard (δ-type) X-ray emission providing a view of its innermost accretion G. J. M. Luna et al.: RT Crucis; X-ray, UV, and optical observations. structures. In the past 20 years, RT Cru has experienced two similar optical brightening events, separated by ∼4000 days and with amplitudes of ∆V∼ 1.5 mag. After Swift became operative, the Burst Alert Telescope (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/X-Ray Telescope(XRT) + BAT + UltraViolet Optical Telescope (UVOT) (photometry) and 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 within the BAT band, indicate that the boundary layer of the accretion disk remained optically thin to its own radiation throughout the brightening 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 a WD symbiotic, the NuSTAR spectrum showed a Compton reflection hump at E> 10 keV, due to hard X-rays from the boundary layer reflecting off of the surface of the WD; the reflection amplitude was 0.77±0.21. The best fit spectral model, including reflection, gave a 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 not correspond to that observed in well-known dwarf nova.An alternative explanation for the brightening events could be that they are due 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 ∼4000 days. In either case, the constancy of the hard X-ray spectrum while the accretion rate rose suggests that the accretion-rate threshold between a mostly optically thin and thick boundary layer, in this object, may be higher than previously thought.

Accretion flows in nonmagnetic white dwarf binaries as observed in X-rays

“…[2.2,2.3] using advective hot flows). For example, multi-satellite X-ray observations of RT Cru indicate that the BL is optically thin and stays thin throughout a brightening event where the rate of accretion is increased to 6.7×10 −9 M ⊙ yr −1 (Luna et al, 2018a) (e.g., typically seen in DNe state changes). This observation also indicates a Compton reflection hump E>10 keV as detected in accreting (XRB) systems.…”

Long-term X-ray variability of the symbiotic system RT Cru based on Chandra spectroscopy

“…Although symbiotic stars mostly display soft or supersoft X-ray emission (see e.g. Muerset et al 1997), a few of them show hard X-ray emission, namely RT Cru (Luna & Sokoloski 2007;Kennea et al 2009;Eze 2014;Ducci et al 2016;Luna et al 2018), CH Cyg (Kennea et al 2009;Eze 2014), T CrB (Tueller et al 2005;Luna et al 2008;Kennea et al 2009;Eze 2014), CD-57 3057 (SS73 17) (Smith et al 2008;Kennea et al 2009;Eze et al 2010;Eze 2014), and MWC 560 (Stute & Sahai 2009). These hard X-ray emitting symbiotic systems pose a chal-⋆ E-mail: danehkar@umich.edu lenge to our understanding of accreting white dwarfs (WDs).…”

“…Furthermore, a high cooling flow temperature of kTmax = 51 +3 −2 keV was estimated from combined archival INTEGRAL γ-ray satellite and Swift/XRT data, in addition to kTmax = 79.9 +••• −19.9 keV from Suzaku data (Ducci et al 2016). More recently, Luna et al (2018) obtained a maximum post-shock temperature of kT = 53 ± 4 keV from spectral fitting analysis of Suzaku and NuSTAR+Swift observations, and suggested a WD mass of 1.25 ± 0.02M⊙. Previous X-ray observations of RT Cru have effectively measured its spectral features in the hard X-ray excess (≫ 1 keV), except for the recent Chandra observations in 2015 that we analyze here.…”

Features of X-Ray Radiation in Symbiotic Stars

“…et.al. [4] showed that the RT Cru object is a prototype of symbiotics without burning the shell on a dwarf; its hard type of X-ray radiation gives an idea of its most internal accretion structures. Over the past 20 years, RT Cru has experienced two similar events of increasing brightness, separated by an interval of 4000 days and with an amplitude of ΔV = 1.5 mag.…”

“…Spectral and temporal analysis of multiwave observations indicate that accretion continues through a disk that reaches the surface of a white dwarf. Moreover, the similarity of ultraviolet and x-ray fluxes indicate that the boundary layer of the accretion disk remained optically thin with respect to its own radiation during the increase in brightness when the accretion rate on WD increased to 6.7 × 10 -9 Mo / yr. [4] Among symbiotics, there are also white dwarfs with quasi-stable burning of the shell on their surface, although the origin of this burning is not yet clear. Probably in slow symbiotic, it is associated with past thermonuclear release.…”