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

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 structures. In the past 20 yr, RT Cru has experienced two similar optical brightening events, separated by ~4000 days and with amplitudes of ΔV ~ 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 WD 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 × 10−9M⊙ 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.

The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binary\nstars is quite soft. Several symbiotic WDs, however, produce strong X-ray\nemission at energies greater than ~20 keV. The Swift BAT instrument has\ndetected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RT\nCru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detected\nX-rays out to greater than 50 keV at a > 5 sigma confidence level. Combining\ndata from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT\nCru, T CrB, and CD -57 3057 are well described by emission from a\nsingle-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9\nkeV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright\nsoft component. For all 4 systems, the spectra suffer high levels of absorption\nfrom material that both fully and partially covers the source of hard X-rays.\nThe XRT data did not show any of the rapid, periodic variations that one would\nexpect if the X-ray emission were due to accretion onto a rotating, highly\nmagnetized WD. The X-rays were thus more likely from the accretion-disk\nboundary layer around a massive, non-magnetic WD in each binary. The X-ray\nemission from RT Cru varied on timescales of a few days. This variability is\nconsistent with being due to changes in the absorber that partially covers the\nsource, suggesting localized absorption from a clumpy medium moving into the\nline of sight. The X-ray emission from CD -57 3057 and T CrB also varied during\nthe 9 months of Swift observations, in a manner that was also consistent with\nvariable absorption.\n

We describe Chandra High-Energy Transmission Grating Spectrometer observations of RT Cru, the first of a new sub-class of symbiotic stars that appear to contain white dwarfs (WDs) capable of producing hard X-ray emission out to greater than 50 keV. The production of such hard X-ray emission from the objects in this sub-class (which also includes CD -57 3057, T CrB, and CH Cyg) challenges our understanding of accreting WDs. We find that the 0.3 -- 8.0 keV X-ray spectrum of RT Cru emanates from an isobaric cooling flow, as in the optically thin accretion-disk boundary layers of some dwarf novae. The parameters of the spectral fit confirm that the compact accretor is a WD, and they are consistent with the WD being massive. We detect rapid, stochastic variability from the X-ray emission below 4 keV. The combination of flickering variability and a cooling-flow spectrum indicates that RT Cru is likely powered by accretion through a disk. Whereas the cataclysmic variable stars with the hardest X-ray emission are typically magnetic accretors with X-ray flux modulated at the WD spin period, we find that the X-ray emission from RT Cru is not pulsed. RT Cru therefore shows no evidence for magnetically channeled accretion, consistent with our interpretation that the Chandra spectrum arises from an accretion-disk boundary layer.

Symbiotic stars are interacting binary systems consisting of a hot star, typically a white dwarf, and a cool red giant companion. The radiation from the hot star partially ionizes the wind from the cool star, resulting in a characteristic combination of sharp nebular lines and stellar molecular absorption bands in the optical spectrum. Most of the emission lines are readily identifiable with common atoms and ions. However, roughly half of all known symbiotic stars exhibit two strong, broad emission lines at ll6825 and 7082 (D. A. Allen 1980, MNRAS, 190, 75) which defied identification for a number of years. H. M. Schmid (1989, AA W. A. Fiebelman, F. C. Bruhweiler, & S. Johanssson 1991, ApJ, 373, 649; P. S. Li & S. A. Leah 1997, ApJ, 484, 424). H. M. Schmid & H. Schild (1994, AA T. J. Harries & I. D. Howarth 1997, AA K. W. Lee & H. W. Lee 1997, MNRAS, 287, 211). The derived efficiencies for Z And, V1016 Cyg, AG Dra, and EG And imply ionization geometries –4.0, in qualiX ∼ 0.4 H tative agreement with other studies (H. Murset et al. 1991, AA H. M. Schmid et al. 1999, A&A, 348, 950). The relative mass-loss rates of Z And, V1016 Cyg, AG Dra, and EG And based on their Raman efficiencies are not generally in agreement with those derived from radio measurements (E. R. Seaquist, M. Krogulec, & A. R. Taylor 1993, ApJ, 410, 260). Since both the Raman line intensity and polarization profiles are very sensitive to the mass-loss rate (T. J. Harries & I. D. Howarth 1997, A&AS, 121, 15), detailed studies of these profiles may provide another useful method to examine the mass-loss rates of cool red giants in symbiotic systems. At present, Raman scattering is found to occur only in symbiotic stars, with the exception of the young planetary nebula NGC 7027. D. Pequignot et al. (1997, A&A, 323, 217) observed a Raman scattered He ii line in the optical spectrum of NGC 7027. This unusual planetary nebula appears to have two central stars, one of which is either a main-sequence star or subdwarf and the other a white dwarf star, and has been discussed as a possible post-symbiotic system (G. A. Gurzadyan 1997, The Physics and Dynamics of Planetary Nebulae [New York: Springer]). Spectroscopic similarities between bipolar, proto–planetary nebulae and symbiotic stars have lead many researchers to speculate on possible evolutionary links between the two types of objects (H. W. Lee & M. G. Park 1999, ApJ, 515, L89, and references therein). The binarity of symbiotic stars is well-established while that of bipolar proto–planetary nebulae remains a matter of debate. Nonetheless, it remains an exciting potential that Raman scattering may ultimately reveal evolutionary links between symbiotic stars and bipolar, proto–planetary nebulae.

2S 0918-549 is a low-mass X-ray binary (LMXB) with a low optical to X-ray flux ratio. Probably it is an ultracompact binary with an orbital period shorter than 60 min. Such binaries cannot harbor hydrogen rich donor stars. As with other (sometimes confirmed) ultracompact LMXBs, 2S 0918-549 is observed to have a high neon-to-oxygen abundance ratio (Juett et al. 2001, ApJ, 560, L59) which has been used to argue that the companion star is a CO or ONe white dwarf. However, types-I X-ray bursts have been observed from several of these systems implying the presence of hydrogen or helium on the neutron star surface. In this paper, we argue that the companion star in 2S 0918-549 is a helium white dwarf. We first present a type-I X-ray burst from 2S 0918-549 with a long duration of 40 min. We show that this burst is naturally explained by accretion of pure helium at the inferred accretion rate of ∼0.01 times the Eddington accretion rate. At higher accretion rates of ∼0.1 Eddington, hydrogen is required to explain long duration bursts. However, at low rates the long duration is due to the large amount of helium that accumulates prior to the burst. We show that it is possible to form a helium white dwarf donor in an ultracompact binary if accretion starts during the first ascent of the giant branch, when the core is predominantly made of helium. Furthermore, this scenario naturally explains the high neon-to-oxgen ratio, without requiring a CO or ONe white dwarf companion. The only observational aspect of 2S 0918-549 that we cannot explain is the absence of helium lines in the optical spectrum. Model calculations of optical accretion disk spectra need to be carried out in order to obtain limits on the helium abundance.

We have carried out an ultraviolet spectroscopic study of the hot white dwarf in the eclipsing S-type symbiotic variable RW Hydrae (Porb = 370.2 days). Our study used orbital phase–resolved IUE archival low-resolution and high-resolution spectra and Hubble Space Telescope archival spectra, including all available images obtained at inferior conjunction (when the accreting hot white dwarf is in front of the red giant). This system is one of less than a handful of symbiotic variables in which the photospheric continuum from the hot white dwarf is the dominant source of light in the far-UV. We have found N V features characteristic of an outflow with velocity of approximately 170 km s-1, almost certainly associated with outflow from the white dwarf, as they are present only in those phases in which the white dwarf is visible. We have also performed the first model atmosphere synthetic spectral fit to the white dwarf–dominated far-UV continuum and found that the best fit was provided by a model with solar abundances, log g = 8, and a temperature of 41,000 K, considerably cooler than estimates of the temperature based on Zanstra techniques. We cannot exclude the possibility that the hot component is hidden by an accretion curtain and that our temperature does not characterize the accreting white dwarf's true photospheric temperature. For a measured red giant wind mass-loss rate of 1 × 10-7 M⊙ yr-1 and a wind accretion efficiency of 7%, we estimate = 7 × 10-9 M⊙ yr-1. The heating at this rate of accretion cannot maintain the white dwarf surface temperature even close to what we have derived. At the surface luminosity implied by our fitting, the required accretion rate for steady nuclear burning is a factor of 20 lower than the above accretion rate. Based on these results, we conclude that the hot component of RW Hya is a young (lower limit age of 1.6 Myr), accretion-heated white dwarf, very likely with nuclear shell burning, on its final cooling track.

Theoretical predictions of the population of Galactic symbiotic stars (SySts) are highly inconsistent with the current known population. Despite intense effort over the past decades, observations are still far below the predictions. The majority of known SySts so far are identified based on selection criteria established in the optical regime. The recent discovery of SU Lyn with very faint optical emission lines uncloaked a subgroup of SySts with accreting-only white dwarfs. In this particular case, the luminous red giant may overshadow the dimmed white dwarf companion. A new approach to search for this subgroup of SySts is presented, employing GALEX UV and 2MASS/AllWISE IR photometry. The FUV-NUV colour index is an indicator, direct or indirect, for the presence of hot compact companions. The cross-match of the Catalogue of Variable Stars III obtained from the All-Sky Automated Survey for SuperNovae (ASAS-SN) with the GALEX, 2MASS, and AllWISE catalogues result in a sample of 814 potential SySt candidates. From them, 105 sources have photometric measurements from both FUV and NUV bands and 35 exhibit FUV-NUV<1, similar to what it is expected from known SySts. Five known SySts are recovered, while two new genuine SySts are discovered in spectroscopic follow-up observations after the detection of the typical emission lines.

Symbiotic stars are a heterogeneous class of interacting binaries. Among them, RT Cru has been classified as prototype of a subclass that is characterised by hard X-ray spectra extending past ~20 keV. We analyse ~8.6 Ms of archival INTEGRAL data collected in the period 2003-2014, ~140 ks of Swift/XRT data, and a Suzaku observation of 39 ks, to study the spectral X-ray emission and investigate the nature of the compact object. Based on the 2MASS photometry, we estimate the distance to the source of 1.2-2.4 kpc. The X-ray spectrum obtained with Swift/XRT, JEM-X, IBIS/ISGRI, and Suzaku data is well fitted by a cooling flow model modified by an absorber that fully covers the source and two partial covering absorbers. Assuming that the hard X-ray emission of RT Cru originates from an optically thin boundary layer around a non-magnetic white dwarf, we estimated a mass of the WD of about 1.2 M_Sun. The mass accretion rate obtained for this source might be too high for the optically thin boundary layer scenario. Therefore we investigate other plausible scenarios to model its hard X-ray emission. We show that, alternatively, the observed X-ray spectrum can be explained with the X-ray emission from the post-shock region above the polar caps of a magnetised white dwarf with mass ~0.9-1.1 M_Sun.

view Abstract Citations (33) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of the optical spectrum of HM Sagittae : 1977-1982. Stauffer, J. R. Abstract High-resolution spectra have been obtained for HM Sge and V1016 Cygni, two prominent members of the RR Tel family of symbiotic stars. Gaussian fits were made to the emission lines present in those spectra in order to attempt a kinemaic separation of the different line-emitting regions expected in those stars. The dominant emission lines are attributed to an expanding, biconical shell created by the interaction of a high-velocity wind from the white dwarf with a preexisting, low velocity wind from the cool star. Twenty eight low-resolution spectra of HM Sge were obtained during the period 1977 June through 1982 June. Relative fluxes for emission lines in the 3800 - 8300 Å region have been derived from those spectra. Prior to 1979 August, the effective temperature of the hot star was less than 70,000K. By 1980 May, the temperature of the hot star had increased to 160,000K. A plausible model for the system is that the 1975 outburst of HM Sge resulted from a hydrogen flash in the accretion envelope of the white dwarf. The 1979 transition occurred when the wind from the white dwarf completed removal of that envelope, exposing the hotter core of the white dwarf. Publication: The Astrophysical Journal Pub Date: May 1984 DOI: 10.1086/162043 Bibcode: 1984ApJ...280..695S Keywords: Companion Stars; High Resolution; Red Giant Stars; Stellar Spectra; Visible Spectrum; White Dwarf Stars; Astronomical Catalogs; Emission Spectra; Line Spectra; Radiant Flux Density; Spectral Line Width; Stellar Envelopes; Stellar Evolution; Stellar Models; Stellar Winds; Astrophysics full text sources ADS | data products SIMBAD (2)

The 6.4 keV iron emission line is typically created by irradiation of the neutral (or low ionized) iron by a hard X-ray source. Whereas the 6.7 and 7.0 keV emission lines are mainly produced by photoionization and collisional excitation in hot plasma, the 6.4 keV fluorescence line is typically a signature of either reflection from an accretion disc or absorption. We have surveyed the emission using a collection of Suzaku observations of hard X-ray emitting symbiotic stars (hSSs) to better understand the geometry of these systems. We find that they do not seem to have a single geometry, and that while absorption-induced fluorescence leads to some emission in three of the hSSs in our study, CH Cyg, T CrB and RT Cru, there are strong hints that significant 6.4 keV emission arises in the accretion disc irradiated by the hard X-rays from the boundary layer between the accretion disc and hot white dwarf in one of our sources, SS73 17. The 6.7 and 7.0 keV lines, however, are largely produced by collisional excitation in the vicinity of the compact white dwarf.

Hubble Space Telescope spectra obtained in 2010 and 2011, three and four years after the large amplitude dwarf nova outburst of V455 And, were combined with optical photometry and spectra to study the cooling of the white dwarf, its spin, and possible pulsation periods after the outburst. The modeling of the ultraviolet (UV) spectra show that the white dwarf temperature remains ~600 K hotter than its quiescent value at three years post outburst, and still a few hundred degrees hotter at four years post outburst. The white dwarf spin at 67.6 s and its second harmonic at 33.8 s are visible in the optical within a month of outburst and are obvious in the later UV observations in the shortest wavelength continuum and the UV emission lines, indicating an origin in high temperature regions near the accretion curtains. The UV light curves folded on the spin period show a double-humped modulation consistent with two-pole accretion. The optical photometry two years after outburst shows a group of frequencies present at shorter periods (250-263 s) than the periods ascribed to pulsation at quiescence, and these gradually shift toward the quiescent frequencies (300-360 s) as time progresses past outburst. The most surprising result is that the frequencies near this period in the UV data are only prominent in the emission lines, not the UV continuum, implying an origin away from the white dwarf photosphere. Thus, the connection of this group of periods with non-radial pulsations of the white dwarf remains elusive.

Context. A considerable number of asymptotic giant branch (AGB) stars exhibit UV excess and/or X-ray emission that indicates a binary companion. AGB stars are so bright that they easily outshine their companions. This almost prevents their identification. Y Gem has been known for some decades to be an AGB star that is bright in the far-UV and X-rays, but it is unclear whether its companion is a main-sequence star or a white dwarf (WD) in a symbiotic system (SySt). Aims. Our goal is to uncover the true nature of Y Gem, which will help us to study the possible misidentified population of SySts. Methods. Multiwavelength IR, optical, UV, and X-ray observations were analyzed to investigate the properties of the stellar components and the accretion process in Y Gem. In particular, an optical spectrum of Y Gem is presented here for the first time, while X-ray data are interpreted by means of reflection models produced by an accretion disk and material in its vicinity. Results. The optical spectrum exhibits the typical sawtooth-shaped features of molecular absorptions in addition to narrow recombination and forbidden emission lines. The emission lines and the analysis of the extinction-corrected UV spectrum suggest a hot component with Teff ≈ 60 000 K, L = 140 L⊙, and R = 0.11 R⊙ that very likely is an accreting WD. The late component is found to be an 1.1 M⊙ AGB star with Teff = 3350 K and R = 240 R⊙. Conclusions. Using IR, optical, UV, and X-ray data, we found that Y Gem is an S-type SySt whose compact component is accreting at an estimated mass-accretion rate of Ṁacc = 2.3 × 10−7 M⊙ yr−1. At this accretion rate, the accreting WD has reached the stable and steady burning phase in which no recurrent events are expected.

The Symbiotic star (SySt) is a long-period interacting binary system, typically consisting of a white dwarf (WD) and a red giant surrounded by a nebula. These systems are natural astrophysical laboratories for investigating binary star evolution. In this paper, we identified nine SySts from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) DR10 low-resolution spectra survey, seven of which were previously known, while two are newly identified. Initially, we selected LAMOST spectra exhibiting typical SySt emission lines (e.g., H α , H β , H γ , and He II). Subsequently, we utilized the distribution of known SySts on the Hertzsprung–Russell diagram to select SySt candidates, and visually inspected their spectra. Ultimately, we classified all nine as S-type SySts using the J − H versus H − Ks diagram. Additionally, based on multiband photometric data from GALEX, Gaia, Two Micron All Sky Survey, ALLWISE, and several X-ray catalogs, we found 12 accreting-only SySt (acc-SySt) candidates, characterized by concurrent ultraviolet and infrared excess and accretion process. Furthermore, we estimated the WD temperatures by fitting their observed spectral energy distributions using a combination of the Kurucz stellar atmosphere model and the Koester WD model. We compared the accretion rates of acc-SySt candidates and confirmed SySts, and found they have similar accretion rate distributions, providing evidence that these acc-SySt candidates constitute bona fide SySts.

Hard X-ray-emitting (δ-type) symbiotic binaries, which exhibit a strong hard X-ray excess, have posed a challenge to our understanding of accretion physics in degenerate dwarfs. RT Cru, which is a member of the δ-type symbiotics, shows stochastic X-ray variability. Timing analyses of X-ray observations from XMM-Newton and NuSTAR, which we consider here, indicate hourly fluctuations, in addition to a spectral transition from 2007 to a harder state in 2012 seen with Suzaku observations. To trace the nature of X-ray variability, we analyze the multimission X-ray data using principal component analysis (PCA), which determines the spectral components that contribute most to the flickering behavior and the hardness transition. The Chandra HRC-S/LETG and XMM-Newton EPIC-pn data provide the primary PCA components, which may contain some variable emission features, especially in the soft excess. Additionally, the absorbing column (first order with 50%), along with the source continuum (20%), and a third component (9%)—which likely accounts for thermal emission in the soft band—are the three principal components found in the Suzaku XIS1 observations. The PCA components of the NuSTAR data also correspond to the continuum and possibly emission features. Our findings suggest that the spectral hardness transition between the two Suzaku observations is mainly due to changes in the absorbing material and X-ray continuum, while some changes in the thermal plasma emission may result in flickering-type variations.

About 20% of the white dwarfs possess a magnetic field that may be detected by the splitting and/or polarization of their spectral lines. As they cool, the effective temperatures of the white dwarfs become so low that no spectral lines can be seen in the visible wavelength range. If their atmospheres are not polluted by the debris of a planetary system, these cool white dwarfs have featureless optical spectra. Until quite recently, very little was known about the incidence of magnetic fields in these objects. However, when observed with polarimetric techniques, a significant number of featureless white dwarfs reveal strong magnetic fields in their optical continuum spectra. Measuring the occurrence rate and strength of magnetic fields in old white dwarfs may help us to understand how these fields are generated and evolve. We report the results of an ongoing survey of cool white dwarfs with the high-precision broad-band polarimeter DIPOL-UF, which is deployed at the Nordic Optical Telescope on La Palma, Spain. This survey has led to the firm discovery of 13 new cool magnetic white dwarfs in the solar neighborhood so far, including six new detections that we report in this paper.