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 $\times$ 10-9 Msun yr^{-1} (d/2 kpc)^2. (Abridged abstract version)
A novel method is presented which can pin down the time the accretion disk re-established itself in the RS Oph system after it experienced a nova disruption. The method is based on the re-ionisation of the ejecta by photoionisation from the radiation released in the boundary layer from accretion.
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 ac…
The accretion process in a typical S-type symbiotic star, targeting~AG Draconis, is investigated through 3D hydrodynamical simulations using the FLASH code. Regardless of the wind velocity of the giant star, an accretion disk surrounding the white dwarf is always formed. In the wind models faster than the orbital velocity of the white dwarf, the disk size and accretion rate are consistent with the predictions under the Bondi-Hoyle-Lyttleton (BHL) condition. In slower wind models, unlike the BHL predictions, the disk size does not grow and the accretion rate increases to a considerably higher l…
We use a combined binary evolution code including dynamical effects to study nova eruptions in a symbiotic system. Following the evolution, over $\sim10^5$ years, of multiple consecutive nova eruptions on the surface of a $1.25M_\odot$ white dwarf (WD) accretor, we present a comparison between simulations of two types of systems. The first is the common, well known, cataclysmic variable (CV) system in which a main sequence donor star transfers mass to its WD companion via Roche-lobe overflow. The second is a detached, widely separated, symbiotic system in which an asymptotic giant branch donor…
A circumstellar corona is proposed for a strange quark-cluster star during an accretion phase, that could be essential to understand the observations of the puzzling symbiotic X-ray system, 4U 1700+24. The state of cold matter at supranuclear density is still an important matter of debate, and one of the certain consequences of strange star as the nature of pulsars is the self-bound on surface which makes extremely low-mass compact objects unavoidable. In principle, both the redshifted O VIII Ly-$α$ emission line and the change of the black-body radiation area could be understood naturally if …
The shock interaction and evolution of nova ejecta with a wind from a red giant star in a symbiotic binary system are investigated via three-dimensional hydrodynamics simulations. We specifically model the March 2010 outburst of the symbiotic recurrent nova V407~Cygni from the quiescent phase to its eruption phase. The circumstellar density enhancement due to wind-white dwarf interaction is studied in detail. It is found that the density-enhancement efficiency depends on the ratio of the orbital speed to the red giant wind speed. Unlike another recurrent nova, RS~Ophiuchi, we do not observe a …
Symbiotic stars show emission across the electromagnetic spectrum from a wide array of physical processes. At cm-waves both synchrotron and thermal emission is seen, often highly variable and associated with outbursts in the optical and X-rays. Most models of the radio emission include an ionized region within the dense wind of the red giant star, that is kept ionized by activity on the white dwarf companion or its accretion disk. In some cases there is on-going shell burning on the white dwarf due to its high mass accretion rate or a prior nova eruption, in other cases nuclear fusion occurs o…
Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of nine white dwarf symbiotics that were not previously known to be X-ray sources and one that had previously been detected as a supersoft X-ray source. The nine new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by approximately 30%. The Swift/XRT telescope detected all of the …
How are accretion discs affected by their outflows? To address this question for white dwarfs accreting from cool giants, we performed optical, radio, X-ray, and ultraviolet observations of the outflow-driving symbiotic star MWC 560 (=V694 Mon) during its 2016 optical high state. We tracked multi-wavelength changes that signalled an abrupt increase in outflow power at the initiation of a months-long outflow fast state, just as the optical flux peaked: (1) an abrupt doubling of Balmer absorption velocities; (2) the onset of a $20$ $μ$Jy/month increase in radio flux; and (3) an order-of-magnitud…
Active phases of some symbiotic binaries survive for a long time from years to decades. The accretion process onto a white dwarf (WD) sustaining long-lasting activity, and sometimes leading to collimated ejection, is not well understood. We present the repeated emergence of highly collimated outflows (jets) from the symbiotic prototype Z And during its 2008 and 2009-10 outbursts and suggest their link to the current long-lasting (from 2000) active phase. We monitored Z And with the high-resolution spectroscopy, multicolor $UBVR_{\rm C}$ and high-time-resolution photometry. The well-pronounced …
We study the dynamical effects of gravitational focusing by a binary companion on winds from late-type stars. In particular, we investigate the mass transfer and formation of accretion disks around the secondary in detached systems consisting of an AGB mass-losing star and an accreting companion. The presence of mass outflows is studied as a function of mass loss rate, wind temperature and binary orbital parameters. A 2-dimensional hydrodynamical model is used to study the stability of mass transfer in wind accreting symbiotic binary systems. In our simulations we use an adiabatic equation of …
Gravitationally focused wind accretion in binary systems consisting of an evolved star with a gaseous envelope and a compact accreting companion is a possible mechanism to explain mass transfer in symbiotic binaries. We study the mass accretion around the secondary caused by the strong wind from the primary late-type component using global three-dimensional hydrodynamic numerical simulations during quiescence and outburst stages. In particular, the dependence of the mass accretion rate on the mass-loss rate, wind parameters and phases of wind outburst development is considered. For a typical w…
We present an analysis of recent X-ray observations of the symbiotic star AG Peg. The X-ray emission of AG Peg as observed with Swift in 2015 shows considerable variability on time scale of days as variability on shorter time scales might be present as well. Analysis of the X-ray spectra obtained in 2013 and 2015 confirms that AG Peg is an X-ray source of class $β$ of the X-ray sources amongst the symbiotic stars. The X-ray emission of AG Peg as observed with ROSAT (1993 June) might well originate from colliding stellar winds (CSW) in binary system. On the other hand, the characteristics of th…
Symbiotic stars are interacting binaries with one of the longest orbital periods. Since they can contain a massive white dwarf with a high accretion rate they are considered a possible type Ia supernovae (SNe Ia) progenitors. Among symbiotic binaries there are systems containing a Mira donor, which can have orbital periods of a few tens of years and more. This subclass of symbiotic stars due to their very large separation usually was not considered promising SNe Ia progenitors. We analysed evolution of one of the well studied symbiotic star with a Mira donor, V407 Cyg. We showed that the stand…
Context: Luminosities of hot components in symbiotic binaries require accretion rates that are higher than those that can be achieved via a standard Bondi-Hoyle accretion. This implies that the wind mass transfer in symbiotic binaries has to be more efficient. Aims: We suggest that the accretion rate onto the white dwarfs (WDs) in S-type symbiotic binaries can be enhanced sufficiently by focusing the wind from their slowly rotating normal giants towards the binary orbital plane. Methods: We applied the wind compression model to the stellar wind of slowly rotating red giants in S-type symbiotic…
AG Dra is a symbiotic variable consisting of a metal poor, yellow giant mass donor under-filling its Roche lobe, and a hot accreting white dwarf, possibly surrounded by an optically thick, bright accretion disk which could be present from wind accretion. We constructed NLTE synthetic spectral models for white dwarf spectra and optically thick accretion disk spectra to model a FUSE spectrum of AG Dra, obtained when the hot component is viewed in front of the yellow giant. The spectrum has been de-reddened (E(B-V) = 0.05) and the model fitting carried out, with the distance regarded as a free pa…
We present high resolution spectroscopy of the yellow symbiotic star AG Draconis with ESPaDOnS at the {\it Canada-France-Hawaii Telescope}. Our analysis is focused on the profiles of Raman scattered \ion{O}{VI} features centered at 6825 Å and 7082 Å, which are formed through Raman scattering of \ion{O}{VI}$λλ$1032 and 1038 with atomic hydrogen. These features are found to exhibit double component profiles with conspicuously enhanced red parts. Assuming that the \ion{O}{vi} emission region constitutes a part of the accretion flow around the white dwarf, Monte Carlo simulations for \ion{O}{VI} l…
The optical light of the symbiotic binary BF Cygni during its last eruption after 2006 shows orbital variations because of an eclipse of the outbursting compact object. The first orbital minimum is deeper than the following ones. Moreover, the Balmer profiles of this system acquired additional satellite components indicating bipolar collimated outflow at one time between the first and second orbital minima. This behaviour is interpreted in the framework of the model of collimated stellar wind from the outbursting object. It is supposed that one extended disc-like envelope covering the accretio…
Any white dwarf or neutron star that accretes enough material from a red giant companion, such that this interaction can be detected at some wavelength, is currently termed Symbiotic Star (typical P(orb)=2-3 years). In the majority of ~400 known systems, the WD burns nuclearly at its surface the accreted material, and the resulting high temperature (T(eff)=10(^5)~K) and luminosity (L(hot)=10(^3)-10(^4) Lsun) allow ionization of a large fraction of the cool giant's wind, making such symbiotic stars easily recognizable through the whole Galaxy and across the Local Group. X-ray observations are n…
Symbiotic stars are long-period interacting binaries where the compact objects, most commonly a white dwarf, is embedded in the dense stellar wind of an evolved companion star. UV and soft X-ray emission of the accretion disk and nuclear burning white dwarf plays a major role in shaping the ionisation balance of the surrounding wind material and giving rise to the rich line emission. In this paper, we employ 2D photoionisation calculations based on Cloudy code to study the ionisation state of the circumbinary material in symbiotic systems and to predict their emission line spectra. Our simulat…
Estimates of the accretion rate in symbiotic recurrent novae (RNe) often fall short of theoretical expectations by orders of magnitude. This apparent discrepancy can be resolved if the accumulation of mass by the white dwarf (WD) is highly sporadic, and most observations are performed during low states. Here we use a reanalysis of archival data from the Digital Access to a Sky Century @Harvard (DASCH) survey to argue that the most recent nova eruption in symbiotic RN T CrB, in 1946, occurred during -- and was therefore triggered by -- a transient accretion high state. Based on similarities in …
We use K-band spectroscopy of the counterpart to the rapidly variable X-ray transient XMMU J174445.5-295044 to identify it as a new symbiotic X-ray binary. XMMU J174445.5-295044 has shown a hard X-ray spectrum (we verify its association with an Integral/IBIS 18-40 keV detection in 2013 using a short Swift/XRT observation), high and varying N$_H$, and rapid flares on timescales down to minutes, suggesting wind accretion onto a compact star. We observed its near-infrared counterpart using the Near-infrared Integral Field Spectrograph (NIFS) at Gemini-North, and classify the companion as ~ M2 III…
Symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In June 2015, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks -- like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between …
Assuming that soft X-ray sources in symbiotic stars result from strong thermonuclear runaways, and supersoft X-ray sources from weak thermonuclear runaways or steady hydrogen burning symbiotic stars, we investigate the Galactic soft and supersoft X-ray sources in symbiotic stars by means of population synthesis. The Galactic occurrence rates of soft X-ray sources and supersoft X-ray sources are from $\sim$ 2 to 20 $\rm yr^{-1}$, and $\sim$ 2 to 17 $\rm yr^{-1}$, respectively. The numbers of X-ray sources in symbiotic stars range from 2390 to 6120. We simulate the distribution of X-ray sources …
RT Cru belongs to the rare class of hard X-ray emitting symbiotics, whose origin is not yet fully understood. In this work, we have conducted a detailed spectroscopic analysis of X-ray emission from RT Cru based on observations taken by the Chandra Observatory using the Low Energy Transmission Grating (LETG) on the High-Resolution Camera Spectrometer (HRC-S) in 2015 and the High Energy Transmission Grating (HETG) on the Advanced CCD Imaging Spectrometer S-array (ACIS-S) in 2005. Our thermal plasma modeling of the time-averaged HRC-S/LETG spectrum suggests a mean temperature of $kT \sim 1.3$ ke…
T CrB is a symbiotic recurrent nova known to exhibit active phases, characterised by apparent increases in the hot component temperature and the appearance of flickering, i.e. changes in the observed flux on the time-scale of minutes. Historical UV observations have ruled out orbital variability as an explanation for flickering and instead suggest flickering is caused by variable mass transfer. We have analysed optical and X-ray observations to investigate the nature of the flickering as well as the active phases in T CrB. The spectroscopic and photometric observations confirm that the active …
High resolution observations in the region of the lines Halpha, He II 4686 and Hgamma of the spectrum of the symbiotic binary Z And were performed during its small-amplitude brightening at the end of 2002. The profiles of the hydrogen lines were double-peaked. These profiles give a reason to suppose that the lines can be emitted mainly by an optically thin accretion disc. The Halpha line is strongly contaminated by the emission of the envelope, therefore for consideration of accretion disc properties we use the Hgamma line. The Halpha line had broad wings which are supposed to be determined mo…
We present a far ultraviolet spectroscopic analysis of the slowest known symbiotic nova AG Peg (M3/4III giant + hot white dwarf; P = 818.4 days) which underwent a nova explosion in 1850 followed by a very slow decline that did not end until 1996, marking the beginning of quiescence. The 19 years of quiescence ended in June 2015, when AG Peg exhibited a Z And-type outburst with an optical amplitude of 1.5 magnitudes. We have carried out accretion disk and WD photosphere synthetic spectral modeling of a Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum obtained on June 5.618, 2003 during th…
We report the discovery and characterization of a new symbiotic star of the accreting-only variety, which we observed in the optical/near-infrared (NIR) with VLT/X-Shooter and in the X-rays/ultraviolet with Swift/UVOT+XRT. The new symbiotic star, THA 15-31, was previously described as a pre-main sequence star belonging to the Lupus~3 association. Our observations, ancillary data, and Gaia EDR3 parallax indicate that THA 15-31 is a symbiotic star composed of an M6III red giant and an accreting companion, is subject to E(B-V)=0.38 reddening, and is located at a distance of ~12 kpc and at 1.8 kpc…
We present photometric and spectral observations of the symbiotic star ZZ CMi. We detect intranight variability - flickering and smooth variations in U band. The amplitude of the flickering is about 0.10-0.20 mag in U band. In the B band the variability is lower, with amplitude less than 0.03 mag. We also detect variability in the H-alpha and H-beta emission lines, and find an indication for outflow with velocity of about 120-150 km/s. The results indicate that ZZ CMi is an accretion powered symbiotic containing an M4-M6 III cool component with an white dwarf resembling recurrent novae and jet…
In the symbiotic (or WD+RG) channel of the single degenerate scenario for type Ia supernovae (SNe Ia) the explosions occur a relatively long time after star formation. The birthrate from this channel would be too low to account for all observed SNe Ia were it not for some mechanism to enhance the rate of accretion on to the white dwarf. A tidally enhanced stellar wind, of the type which has been postulated to explain many phenomena related to giant star evolution in binary systems, can do this. Compared to mass-stripping this model extends the space of SNe Ia progenitors to longer orbital peri…
Symbiotic stars in which the symbiotic phenomenon is powered solely by accretion, often at an average rate that is higher than in cataclysmic variable stars, provide an important opportunity to diagnose boundary layers around disk-accreting white dwarfs. Here we investigate SU Lyncis, a recently discovered example of a purely accretion-powered symbiotic star, using the first reliable X-ray spectroscopy, obtained with NuSTAR, and UV photometry obtained with Swift. SU Lyn has hard, thermal, X-ray emission that is strongly affected by a variable local absorber - that has little impact on the UV e…
We report optical CCD photometry of the recently identified symbiotic star EF Aql. Our observations in Johnson V and B bands clearly show the presence of stochastic light variations with an amplitude of about 0.2 mag on a time scale of minutes. The observations point toward a white dwarf (WD) as the hot component in the system. It is the 11-th object among more than 200 symbiotic stars known with detected optical flickering. Estimates of the mass accretion rate onto the WD and the mass loss rate in the wind of the Mira secondary star lead to the conclusion that less than 1 per cent of the wind…
Various white-dwarf (WD) binary scenarios have been proposed trying to understand the nature and the diversity of Type Ia supernovae (SNe Ia). In this work, we study the evolution of carbon-oxygen WD -- red giant (RG) binaries (including the role of magnetic confinement) as possible SN Ia progenitors (the so-called symbiotic progenitor channel). Using the \textsc{mesa} stellar evolution code, we calculate the time dependence of the structure of the RG star, the wind mass loss, the Roche-lobe-overflow (RLOF) mass-transfer rate, the polar mass-accretion rate (in the case of magnetic confinement)…
LIN 358 and SMC N73 are two symbiotic binaries in the halo of the Small Magellanic Cloud, each composed of a hot white dwarf accreting from a cool giant companion. In this work, we characterize these systems using a combination of SED-fitting to the extant photometric data spanning a broad wavelength range (X-ray/ultraviolet to near-infrared), detailed analysis of the APOGEE spectra for the giant stars, and orbit fitting to high quality radial velocities from the APOGEE database. Using the calculated Roche lobe radius for the giant component and the mass ratio for each system, it is found that…
Hen 3-1341 is a symbiotic binary system consisting of a white dwarf and a red giant star that is one of about ten symbiotics that show hints of jets. The bipolar jets have been detected through displaced components of emission lines during its outburst from 1998 to 2004. These components disappeared when Hen 3-1341 reached quiescence. On February 23, 2012, Hen 3-1341 started a new outburst with the emergence of new bipolar jets on March 3, 2012. We observed Hen 3-1341 during quiescence with XMM-Newton in March 2010 with an effective exposure time of 46.8 ks and with Swift on March 8-11, 2012 a…
The Galactic Bulge Survey is a wide but shallow X-ray survey of regions above and below the Plane in the Galactic Bulge. It was performed using the Chandra X-ray Observatory's ACIS camera. The survey is primarily designed to find and classify low luminosity X-ray binaries. The combination of the X-ray depth of the survey and the accessibility of optical and infrared counterparts makes this survey ideally suited to identification of new symbiotic X-ray binaries in the Bulge. We consider the specific case of the X-ray source CXOGBS J173620.2-293338. It is coincident to within 1 arcsec with a ver…
Symbiotic stars are the longest orbital period interacting binaries, where nova-like outbursts are generated by the accretion of a high mass loss rate red giant wind onto a white dwarf companion. Long-term photometric monitoring surveys such as OGLE and MACHO are ideal platforms to identify nova-like events in symbiotic stars, however there are only a handful of known systems within the small footprint of these surveys. We introduce a systematic Halpha emission line object survey for new symbiotic stars covering 35 deg^2 towards the Galactic Bulge that combines deep 2dF/AAOmega spectroscopy wi…
The current $super-active$ state of the recurrent nova T CrB has been observed with unprecedented detail. Previously published observations provide strong evidence that this state is due to an enhancement of the flow of material through the accretion disk, which increased the optical depth of its most internal region, the boundary layer. $NuSTAR$ and $Swift$ observed T CrB in 2015 September, roughly halfway through the rise to optical maximum. In our analysis of these data, we have found that: $i$) the UV emission, as observed with $Swift$/UVOT in 2015, was already as bright as it became in 20…
We report photometric observations of the hidden symbiotic star SU Lyn in the optical bands. In five nights we detect a weak flickering in U band with amplitude of about 0.05 magnitudes. No intranight variations are found in B, V, g' and r' bands. This is one more indication that the secondary component is a white dwarf accreting at low accretion rate. We also searched for intranight variability of a dozen related object (RR Boo, RT Boo, AM Cyg, AG Peg, BF Cyg, NQ Gem, StHa190, V627 Cas, XX Oph, FS Cet and Y Gem) in which no variability above the observational errors is detected.
We analyze the last 125 years optical light curve of the symbiotic star AX Per through some remarkable correlations that we discovered in its power spectrum. The data were assembled from the literature and from the AAVSO database. A series of 6 major outbursts dominate the light curve. They are presented in the power spectrum as 13 harmonics of the fundamental frequency fa=1/Pa=1/23172 day-1. We refer to them as the "red" frequencies. Oscillations with the binary periodicity of the system Pb=1/fb=681.48 d are also seen in the light curve, with particularly large amplitudes during outbursts. Th…
