SS Cyg Research Articles

The matter-flow structure in the SS Cyg system in its quiescent state from comparisons of observational and synthetic Doppler tomograms

Doppler tomograms are constructed for the quiescent state of the SS Cyg system based on Hβ and Hγ spectral-line observations carried out in August 2006 with the 2-m telescope at Terskol Peak. Gasdynamical simulations combined with the Doppler tomograms enable identification of the main features of the flow. Comparisons of synthetic tomograms with observations indicate that an accretion disk is present in the quiescent system. In the tomograms, the luminosity is maximum at the arms of the spiral tidal shock at the shock front due to the interaction between the gas of the circum-binary envelope and material in the stream issuing from the Lagrangian point L1 (the “hot line”), and in the region behind the bow shock due to the motion of the accretor and disk in the gas of the envelope. The contribution of this last element results in appreciable asymmetry of the tomograms.

ADAFs, accretion discs and outbursts in compact binaries

I discuss the status of the soft X-ray transient model. First, I discuss and then compare with observations the assumption that the geometrically thin disc evaporates into an ADAF. Second, I address the problems created by the recent determinations of the distance to SS Cyg, according to which the disc instability model does not apply to this famous dwarf-nova, thus casting doubt on the application of this model to any system at all.

SS Cygni Outburst Predictors and Long Term Quasi-periodic Behavior

ABSTRACTWe report null results on a 2 year photometric search for outburst predictors in SS Cyg. Observations in Johnson V and Cousins I were obtained almost daily for multiple hours per night for two observing seasons. The accumulated data are put through various statistical and visual analysis techniques, but we fail to detect any outburst predictors. However, analysis of 102 years of AAVSO archival visual data has led to the detection of a correlation between a long term quasi-periodic feature at around 1000–2000 days in length and an increase in outburst rate.

Global Disk Oscillation Modes in Cataclysmic Variables and Other Newtonian Accretors

Diskoseismology, the theoretical study of small adiabatic hydrodynamical global perturbations of geometrically thin, optically thick accretion disks around black holes (and other compact objects), is a potentially powerful probe of the gravitational field. For instance, the frequencies of the normal mode oscillations can be used to determine the elusive angular momentum parameter of the black hole. The general formalism developed by diskoseismologists for relativistic systems can be readily applied to the Newtonian case of cataclysmic variables (CVs). Some of these systems (e.g., the dwarf nova SS Cygni) show rapid oscillations in the UV with periods of tens of seconds and high coherence. In this paper we assess the possibility that these dwarf nova oscillations (DNOs) are diskoseismic modes. Besides its importance in investigating the physical origin of DNOs, the present work could help us to answer the following question. To what extent are the similarities in the oscillation phenomenology of CVs and X-ray binaries indicative of a common physical mechanism?

Synthetic NLTE accretion disc spectra for the dwarf nova SS Cygni during an outburst cycle

Dwarf nova outbursts result from enhanced mass transport through the accretion disc of a cataclysmic variable system. We assess the question of whether these outbursts are caused by an enhanced mass transfer from the late-type main sequence star onto the white dwarf (so-called mass transfer instability model, MTI) or by a thermal instability in the accretion disc (disc instability model, DIM). We compute non-LTE models and spectra of accretion discs in quiescence and outburst and construct spectral time sequences for discs over a complete outburst cycle. We then compare our spectra to published optical spectroscopy of the dwarf nova SS Cygni. In particular, we investigate the hydrogen and helium line profiles that are turning from emission into absorption during the rise to outburst. The evolution of the hydrogen and helium line profiles during the rise to outburst and decline clearly favour the disc-instability model. Our spectral model sequences allow us to distinguish inside-out and outside-in moving heating waves in the disc of SS Cygni, which can be related to symmetric and asymmetric outburst light curves, respectively.

The dwarf nova SS Cygni: what is wrong?

Context.Since the Fine Guiding Sensor (FGS) on the Hubble Space Telescope (HST) was used to measure the distance to SS Cyg to be pc, it became apparent that at this distance the disc instability model fails to explain the absolute magnitude during outburst. It remained, however, an open question whether the model or the distance have to be revised. Recent observations led to a revision of the system parameters of SS Cyg and seem to be consistent with a distance of pc.

The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni

The dwarf nova SS Cygni is a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg with the Hobby-Eberly Telescope (HET). Fits of synthetic spectra for Roche-lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio: K_{K} = 162.5 +/- 1.0 km/s and q= M_{K} /M_{wd} = 0.685 +/- 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 +/- 0.075 of the total flux at 5500 angstroms. Taking the weighted average of our results with previously published results obtained using similar techniques, we find <K_{K}> = 163.7 +/- 0.7 km/s and <q> = 0.683 +/- 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations we limit the orbital inclination to the range 45 deg. <= i <= 56 deg. The derived masses of the K star and white dwarf are M_{K} = 0.55 +/- 0.13 M_sun and M_{wd} = 0.81 +/- 0.19 M_sun, where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10% to 50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for Zero-Age Main-Sequence stars; nor does it follow the ZAMS mass/spectral-type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted.

Suzaku Observations of the Dwarf Nova SS Cyg in Quiescence and Outburst

Suzaku Observations of the Dwarf Nova SS Cyg in Quiescence and Outburst

The Nature of the Secondary Star in the Black Hole X-Ray Transient V616 Mon (=A0620-00)

We have used NIRSPEC on Keck II to obtain $K$-band spectroscopy of the low mass X-ray binary V616 Mon (= A0620$-$00). V616 Mon is the proto-typical soft x-ray transient containing a black hole primary. As such it is important to constrain the masses of the binary components. The modeling of the infrared observations of ellipsoidal variations in this system lead to a derived mass of 11.0 M$_{\sun}$ for the black hole. The validity of this derivation has been called into question due to the possiblity that the secondary star's spectral energy distribution is contaminated by accretion disk emission (acting to dilute the variations). Our new $K$-band spectrum of V616 Mon reveals a late-type K dwarf secondary star, but one that has very weak $^{\rm 12}$CO absorption features. Comparison of V616 Mon with SS Cyg leads us to estimate that the accretion disk supplies only a small amount of $K$-band flux, and the ellipsoidal variations are not seriously contaminated. If true, the derived orbital inclination of V616 Mon is not greatly altered, and the mass of the black hole remains large. A preliminary stellar atmosphere model for the $K$-band spectrum of V616 Mon reveals that the carbon abundance is approximately 50% of the solar value. We conclude that the secondary star in V616 Mon has either suffered serious contamination from the accretion of supernova ejecta that created the black hole primary, or it is the stripped remains of a formerly more massive secondary star, one in which the CNO cycle had been active.

20-100 keV properties of cataclysmic variables detected in the INTEGRAL/IBIS survey

Analysis of INTEGRAL/IBIS survey observations has revealed that the rare intermediate polar and asynchronous polar cataclysmic variables are consistently found to emit in the 20-100 keV energy band, whereas synchronous polars and the common non-magnetic CVs rarely do so. From the correlation of a candidate INTEGRAL/IBIS survey source list with a CV catalogue, 15 CV detections by IBIS have been established including a new INTEGRAL source IGR J06253+7334. The properties of these sources and 4 additional CV candidates are discussed in the context of their 20-100 keV emission characteristics and we conclude that the INTEGRAL mission is an important tool in the detection of new magnetic CV systems. Furthermore, analysis of the time-averaged spectra of CVs detected by INTEGRAL indicate that although there is little difference between the spectral slopes of the different sub-types, intermediate polars may be considerably more luminous than polars in the soft gamma-ray regime. We also present the detection of an unusual high-energy burst from V1223 Sgr discovered by inspection of the IBIS light-curve. Additionally, we have compared the IBIS and optical AAVSO light-curves of SS Cyg and extracted IBIS spectra during single periods of optical outburst and quiescence. We find that the 20-100 keV flux is an order of magnitude greater during optical quiescence. This is in agreement with previous studies which show that the hard X-ray component of SS Cyg is suppressed during high accretion states

Optical spectroscopy of the dwarf nova U Geminorum

U Gem is unique in having a direct measurement of K1 = 107 +- 2 km/s, Long et al. 1999). We present high-resolution optical spectra of the dwarf nova U Gem in quiescence taken to test the accuracy to which the HST value can be recovered from optical data. We find that, even with data of very high S/N we cannot recover Long et al.'s value to better than about 20% by any method. Contamination by neighbouring emission lines seems a likely culprit. Our data reveal a number of new features: Doppler tomograms show emission at low velocity, close to the centre of mass, and a transient, narrow absorption feature is seen in the Balmer lines near the line centres at the time of eclipse. We suggest that stellar prominences, as previously invoked for the dwarf novae IP Peg and SS Cyg in outburst, may explain both of these features. The He II 4686.75 A line emission is dominated by the gas stream/disc impact region. Two distinct spots are seen in Doppler maps, the first being very narrow and showing a velocity close to that of the accretion disc in the impact region, and the second much broader and located between the velocities of the (ballistic) stream and the (Keplerian) disc. We present tentative evidence of weak spiral structure. We find no evidence of stream-disc overflow in the system. Our data suggests an inclination angle > 70 degrees, adding to the evidence of a puzzle in the mass of U Gem's white dwarf. The mass donor is clearly seen in the Doppler maps, with emission concentrated towards its poles, and mainly on the side facing the white dwarf. This suggests irradiation with shielding by the disc from which we estimate an H/R ratio between 0.15 and 0.25.

Study of Fe Kα lines in Nonmagnetic Cataclysmic Variables UsingChandraHEG Data

Results from a study of Fe Kα emission lines for a sample of six nonmagnetic cataclysmic variables (CVs) using high-resolution X-ray data from the Chandra High Energy Transmission Grating (HETG) are presented. Two of the sources, SS Cyg and U Gem, are observed in both quiescent and outburst states, whereas V603 Aql, V426 Oph, WX Hyi, and SU UMa are observed only in quiescence. The fluorescent Fe line is prominent in V603 Aql, V426 Oph, and SS Cyg during quiescence, indicating the presence of a conspicuous reflection component in these sources. The observed equivalent width of the fluorescent Fe line is consistent with reflection from a white dwarf surface that subtends a 2π solid angle at the X-ray source. During the outburst in SS Cyg, the fluorescent line is redshifted by about 2300 km s-1. The Fe XXV triplet at 6.7 keV is found to be dominant in all sources. The value of the G ratio derived from the Fe XXV triplet indicates that the plasma is in collisional ionization equilibrium during the quiescent state. The Fe XXV line is significantly broadened in U Gem and SS Cyg during the outbursts compared to quiescence, indicating the presence of high-velocity material near the white dwarf during the outburst. The ratio of Fe XXVI/XXV indicates a higher ionization temperature during quiescence than in outburst in U Gem and SS Cyg.

X-ray spectral properties of non-magnetic cataclysmic variables using Chandra HETG

Results from a study of high resolution spectra obtained with the Chandra X-ray observatory for a sample of 6 Cataclysmic Variables (CVs) are presented. A global fit approach has been employed to obtain the spectral characteristics of the sources. The line-rich high-resolution spectra of these sources clearly indicate multi-temperature nature of the emitting plasma. Multi-temperature APEC models describe the spectra very well. Detection of significantly broad emission lines, indicates the presence of high velocity gas in SS Cyg and U Gem during the optical outbursts.

Far‐Ultraviolet Spectroscopy of the Dwarf Novae SS Cygni and WX Hydri in Quiescence

We present time-resolved FUV spectra of the dwarf novae SS Cyg and WX Hyi in quiescence from observations using the Hopkins Ultraviolet Telescope on the Astro-1 and Astro-2 space shuttle missions and the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Both dwarf novae are characterized by blue continua that extend to the Lyman limit punctuated by broad emission lines including transitions of O VI, N V, Si IV, and C IV. The continuum of WX Hyi can be fitted with a white dwarf model with physically reasonable model parameters, but neither system actually shows unambiguous signatures of white dwarf emission. The shape and flux of the spectrum of SS Cyg cannot be self-consistently reconciled with a white dwarf providing all of the FUV continuum flux. Combination white dwarf/disk or white dwarf/optically thin plasma models improve the fit but still do not give physically reasonable model parameters for a quiescent dwarf nova. Assuming that the UV emission lines arise from the disk, the line shapes indicate that surface fluxes fall roughly as R-2 in both systems. Fits to the double-peaked line profiles in SS Cyg indicate that the FUV line-forming region is concentrated closer to the white dwarf than that of the optical lines and provide no evidence of a hole in the inner disk. Although the flux from SS Cyg was relatively constant during all of our observations, WX Hyi showed significant variability during the GHRS observations. In WX Hyi, the line and continuum fluxes are (with the exception of He II) highly correlated, indicating a link between the formation mechanisms of the line and continuum regions.

Cataclysmic and Close Binaries in Star Clusters. V. Erupting Dwarf Novae, Faint Blue Stars, X‐Ray Sources, and the Classical Nova in the Core of M80

Large populations of cataclysmic variables (CVs) in globular clusters have long been predicted, but the number of absolutely certain cluster CVs known in globular clusters is still less than 10. HST and Chandra observers have recently found dozens of CV candidates in several populous globular clusters. Confirmation and characterization of these candidates are extremely difficult; thus, identification of unambiguous CVs remains important. We have searched all archival HST images of the dense globular cluster M80 for erupting dwarf novae (DNe) and to check the outburst behaviors of two very blue objects first identified a decade ago. Two new erupting DNe were found in eight searched epochs, making M80 a record holder for erupting DNe. The quiescent classical nova in M80 varies by no more than a few tenths of a magnitude on timescales of minutes to years, and a similar faint, blue object varies by a similar amount. Simulations and completeness tests indicate that there are at most three erupting DNe like SS Cyg and at most nine U Gem-like DNe in M80. Either this very dense cluster contains about an order of magnitude fewer CVs than theory predicts, or most M80 CVs are extremely faint and/or erupt very infrequently like WZ Sge. We have detected a sequence of 54 objects running parallel to the main sequence and several tenths of a magnitude blueward of it. These blue objects are significantly more centrally concentrated than the main-sequence stars, but not as centrally concentrated as the blue stragglers. We suggest that these objects are white dwarf-red dwarf binaries and that some are the faint CV population of M80.

X‐Ray Observations of the Bright Old Nova V603 Aquilae

We report on our Chandra and RXTE observations of the bright old nova, V603 Aql, performed in 2001 April, supplemented by our analysis of archival X-ray data on this object. We find that the RXTE data are contaminated by the Galactic Ridge X-ray emission. After accounting for this effect, we find a high level of aperiodic variability in the RXTE data, at a level consistent with the uncontaminated Chandra data. The Chandra HETG spectrum clearly originates in a multi-temperature plasma. We constrain the possible emission measure distribution of the plasma through a combination of global and local fits. The X-ray luminosity and the spectral shape of V603 Aql resemble those of SS Cyg in transition between quiescence and outburst. The fact that the X-ray flux variability is only weakly energy dependent can be interpreted by supposing that the variability is due to changes in the maximum temperature of the plasma. The plasma density is likely to be high, and the emission region is likely to be compact. Finally, the apparent overabundance of Ne is consistent with V603 Aql being a young system.

The complete set ofASCAX-ray observations of non-magnetic cataclysmic variables

We present the complete set of thirty four ASCA observations of non-magnetic cataclysmic variables. Timing analysis reveals large X-ray flux variations in dwarf novae in outburst (Z Cam, SS Cyg and SU UMa) and orbital modulation in high inclination systems (including OY Car, HT Cas, U Gem, T Leo). We also found episodes of unusually low accretion rate during quiescence (VW Hyi and SS Cyg). Spectral analysis reveals broad temperature distributions in individual systems, with emission weighted to lower temperatures in dwarf novae in outburst. Absorption in excess of interstellar values is required in dwarf novae in outburst, but not in quiescence. We also find evidence for sub-solar abundances and X-ray reflection in the brightest systems. LS Peg, V426 Oph and EI UMa have X-ray spectra that are distinct from the rest of the sample and all three exhibit candidate X-ray periodicities. We argue that they should be reclassified as intermediate polars. In the case of V345 Pav we found that the X-ray source had been previously misidentified.

Fireballs, Flares, and Flickering: A Semianalytic, LTE, Explosive Model from Accretion Disks to Supernovae

We derive simple analytic expressions for the continuum light curves and spectra of flaring and flickering events that occur over a wide range of astrophysical systems. We compare these results to data taken from the cataclysmic variable SS Cygni and also from SN 1987A, deriving physical parameters for the material involved. Fits to the data indicate a nearly time-independent photospheric temperature arising from the strong temperature dependence of opacity when hydrogen is partially ionized.

Rapid Keck Spectroscopy of Cataclysmic Variables

Abstract We present an analysis of Rapid Keck Spectroscopy of the CVs AM Her (polar) and SS Cyg (dwarf nova). We decompose the spectra into constant and variable components and identify different types of variability in AM Her with different characteristic timescales. The variable flickering component of the accretion disc flux md the observational characteristics of a small flare in SS Cyg are isolated.

Excess mid-infrared emission in cataclysmic variables

We present a search for excess mid-infrared emission due to circumbinary (CB) material in the orbital plane of cataclysmic variables (CVs). Our motivation stems from the fact that the strong braking exerted by a CB disc on the binary system could explain several puzzles in our current understanding of CV evolution. Since theoretical estimates predict that the emission from a CB disc can dominate the spectral energy distribution (SED) of the system at λ > 5 μm, we obtained simultaneous visible to mid-infrared (mid-IR) SEDs for eight systems. We report detections of SS Cyg at 11.7 μm and AE Aqr at 17.6 μm, both in excess of the contribution from the secondary star. In AE Aqr, the IR likely originates from synchrotron-emitting clouds propelled by the white dwarf. In SS Cyg, we argue that the observed mid-IR variability is difficult to reconcile with simple models of CB discs and we consider free–free emission from a wind. In the other systems, our mid-IR upper limits place strong constraints on the maximum temperature of a putative CB disc. The results show that if any sizeable CB discs are present in these systems, they must be self-shadowed or perhaps dust-free, with the peak thermal emission shifted to far-IR wavelengths.