Quiescent Dwarf Novae Research Articles

A Serendipitous Observation of the Recently Discovered Cataclysmic Variable TUVO-21acq

The recently discovered non-magnetic cataclysmic variable TUVO-21acq was serendipitously observed in a quiescent state during an XMM-Newton observing campaign for the starburst galaxy NGC 4945. Data from this campaign was combined with archival serendipitous XMM observations to examine its X-ray and UV/optical characteristics. TUVO-21acq was found to have X-ray flux F X(0.4–3.0 keV) ∼ 1.3 × 10−14 erg s−1 cm−2 and features similar to other quiescent dwarf novae, with X-ray luminosity L X(0.4–3.0 keV) ∼ 8 × 1030 erg s−1 and a white dwarf mass MWD=0.78−0.27+0.37M⊙ . Its UV/optical spectrum was variable between observations, possibly due to changes in the accretion disk or the visibility of the bright spot.

The Accreting White Dwarfs in Cataclysmic Variables

Accreting white dwarfs (WDs) in cataclysmic variables (CVs) provide crucial insights about the accretion of mass and angular momentum in all types of binaries, including accreting NSs and BHs. Accreting WDs are the critical component in the single degenerate pathway to SNe Ia, along with the double degenerate merger pathway, they are the standard candles of cosmology proving that the universe is accelerating and the existence of dark energy. Another key question is whether the WD in a CV can grow in mass despite the mass loss due to thousands of nova explosions in its lifetime. Angular momentum loss drives CV evolution and accreting WDs offer critically needed WD masses from Gaia distances and reliable surface temperatures to derive the most accurate accretion rates. We review the studies on accreting WDs, including WD masses, accurate rotational velocities and chemical abundances of elements. Most of the progress that has been made is based upon Hubble Space Telescope spectroscopy and FUSE spectroscopy in the UV spectral region during dwarf nova quiescence and the low states of novalike variables, when the accreting WD dominates the UV spectral range.

The Galactic Bulge Diffuse Emission in Broadband X-Rays with NuSTAR

Abstract The diffuse hard X-ray emission that fills the Galactic center, bulge, and ridge is believed to arise from unresolved populations of X-ray binary systems. However, the identity of the dominant class of accreting objects in each region remains unclear. Recent studies of Fe line properties and the low-energy (<10 keV) X-ray continuum of the bulge indicate a major population fraction of nonmagnetic cataclysmic variables (CVs), in particular quiescent dwarf novae (DNe). This is in contrast to previous high-energy (>10 keV) X-ray measurements of the bulge and ridge, which indicate a dominant population of magnetic CVs, in particular intermediate polars. In addition, NuSTAR broadband measurements have uncovered a much heavier intermediate polar population in the central ∼100 pc than previously assumed, raising the possibility that some fraction of this population extends further from the center. Here we use NuSTAR’s large aperture for unfocused photons and its broadband X-ray range to probe the diffuse continuum of the inner ∼1°–3° of the Galactic bulge. This allows us to constrain possible multitemperature components of the spectrum, such as could indicate a mixture of soft and hard populations. Our emissivity is consistent with previous hard X-ray measurements in the bulge and ridge, with the diffuse X-ray luminosity tracing the stellar mass. The spectrum is well described by a single-temperature thermal plasma with kT ≈ 8 keV, with no significant emission above 20 keV. This supports that the bulge is dominated by quiescent DNe; we find no evidence of a significant intermediate polar population in the hard X-ray band.

X-ray properties of dwarf nova EY Cyg and the companion star using an XMM-Newton observation

We present the X-ray analysis of dwarf nova EY Cyg using the 45 ks XMM-Newton observatory archival data obtained in quiescence. We find orbital modulations in X-rays. We simultaneously fitted EPIC pn, MOS1 and MOS2 data using a model for interstellar medium absorption (tbabs) and a multi-temperature plasma emission model with a power-law distribution of temperatures (CEVMKL) as expected from low accretion rate quiescent dwarf novae. The XMM-Newton EPIC spectra of the source yields a maximum temperature kTmax∼14.9-2.2+3.3 keV with an unabsorbed X-ray flux and luminosity of (1.8–2.0) ×10-12 ergs−1 cm−1 and (8.7–9.7) ×1031 ergs−1, respectively, in the energy range 0.1–50 keV. There is 3–4 sigma excess at energies below 0.5 keV, we model the excess using MEKAL, POWERLAW and BBODY models and favor the model MEKAL which is physical. According to previous studies, the secondary in this system is thought to be a K-type star which may radiate in the soft X-ray region. The fit with an additive MEKAL model gives a temperature of kT ∼ 0.1 keV with an unabsorbed X-ray flux and luminosity of (2.7–8.8) ×10-14 ergs−1 cm−1 and (1.3–4.2) ×1030 ergs−1, respectively, for the companion star. Based on the results from the timing and spectral analysis, we highly suggest that the secondary of EY Cyg is a K-type star.

Perspectives of monitoring of cataclysmic variables in the very soft X‐ray band

Some components and important regions of X‐ray spectra of cataclysmic variables (CVs) are present only in the very soft band with energy E < 1.5 keV, which is beyond the region of sensitivity of most available X‐ray monitors. We show that a population study of a large ensemble of such systems with various states of activity that will be observable by eROSITA (on board the SRG satellite) will be a suitable method of investigation even with its very sampled data. We demonstrate that the hardness ratios of such X‐ray spectra show specific positions and patterns of the individual types of CVs in the hardness vs. hardness diagrams. Quiescent dwarf novae and intermediate polars constitute a long tail extending from a group of the high‐state polars. Interstellar absorption plays only a marginal role in this respect. A complicated distribution of CVs in these diagrams thus mainly reflects the complexity of the emitting regions, the absorbing regions intrinsic to the source, and the emission mechanisms in various CVs and CV types. Although this is a case study using incomplete data as examples, it demonstrates serendipitous scientific potentials of the eROSITA archives.

Impact of convection and resistivity on angular momentum transport in dwarf novae

The eruptive cycles of dwarf novae are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to imply enhanced angular momentum transport in the accretion disk during outburst. This is measured by the stress to pressure ratio α, with α ≈ 0.1 required in outburst compared to α ≈ 0.01 in quiescence. Such an enhancement in α has recently been observed in simulations of turbulent transport driven by the magneto-rotational instability (MRI) when convection is present, without requiring a net magnetic flux. We independently recover this result by carrying out PLUTO magnetohydrodynamic (MHD) simulations of vertically stratified, radiative, shearing boxes with the thermodynamics and opacities appropriate to dwarf novae. The results are robust against the choice of vertical boundary conditions. The thermal equilibrium solutions found by the simulations trace the well-known S-curve in the density-temperature plane that constitutes the core of the disk thermal-viscous instability model. We confirm that the high values of α ≈ 0.1 occur near the tip of the hot branch of the S-curve, where convection is active. However, we also present thermally stable simulations at lower temperatures that have standard values of α ≈ 0.03 despite the presence of vigorous convection. We find no simple relationship between α and the strength of the convection, as measured by the ratio of convective to radiative flux. The cold branch is only very weakly ionized so, in the second part of this work, we studied the impact of non-ideal MHD effects on transport. Ohmic dissipation is the dominant effect in the conditions of quiescent dwarf novae. We include resistivity in the simulations and find that the MRI-driven transport is quenched (α ≈ 0) below the critical density at which the magnetic Reynolds number Rm ≤ 104. This is problematic because the X-ray emission observed in quiescent systems requires ongoing accretion onto the white dwarf. We verify that these X-rays cannot self-sustain MRI-driven turbulence by photo-ionizing the disk and discuss possible solutions to the issue of accretion in quiescence.

Dual behavior of the toroidal magnetic field versus the Rossby wave instability

The Rossby wave instability (RWI) theory has been considered as one of the top topics in astrophysics due to the fact that it deals with some ambiguous questions, such as the angular momentum transport in the poorly ionized regions of the protoplanetary discs. Based on the theoretical and simulation works, two important factors in the study of the RWI are the viscosity and magnetic field, which are directly connected to each other because the large-scale toroidal magnetic fields are produced by the magnetohydrodynamic (MHD) turbulence. Therefore, it is essential to consider more details about the toroidal magnetic field both in the steady state and perturbation state. In this paper, the role of the strength and gradient of the toroidal magnetic field is investigated on the RWI at the dead zone in the regions of bump. The obtained results show that the gradient of the toroidal magnetic field or its strength have a major role in the RWI occurrence, which have received relatively less attention in previous works. Also, the role of the gradient of the toroidal magnetic field in the RWI is important even in the weak magnetic fields. Hence, the obtained results are very different from what we previously expected, and it seems crucial to research and develop this issue in the theoretical and simulation works. This paper can be helpful on the study of the angular momentum transport in the cold accretion discs, such as accretion discs in quiescent dwarf novae or around the white-dwarf primary.

Fe LINE DIAGNOSTICS OF CATACLYSMIC VARIABLES AND GALACTIC RIDGE X-RAY EMISSION

ABSTRACT The properties of the Galactic Ridge X-ray Emission (GRXE) observed in the 2–10 keV band place fundamental constraints on various types of X-ray sources in the Milky Way. Although the primarily discrete origin of the emission is now well established, the responsible populations of these sources remain uncertain, especially at relatively low fluxes. To provide insights into this issue, we systematically characterize the Fe emission line properties of the candidate types of the sources in the solar neighborhood and compare them with those measured for the GRXE. Our source sample includes 6 symbiotic stars, 16 intermediate polars (IPs), 3 polars, 16 quiescent dwarf novae, and 4 active binaries (ABs); they are all observed with the Suzaku X-ray Observatory. The data of about one-fourth of these sources are analyzed for the first time. We find that the mean equivalent width (EW6.7) of the 6.7 keV line and the mean 7.0/6.7 keV line ratio are 107 ± 16.0 eV and 0.71 ± 0.04 for IPs and 221 ± 135 eV and 0.44 ± 0.14 for polars, respectively, which are all substantially different from those (490 ± 15 eV and 0.2 ± 0.08) for the GRXE. Instead, the GRXE values are better agreed by the EW6.7 (438 ± 84.6 eV) and the ratio (0.27 ± 0.06) observed for the DNe. We further find that the EW6.7 is strongly correlated with the 2–10 keV luminosity of the DNe, which can be characterized by the relation EW 6.7 = ( 438 ± 95 eV ) ( L / 10 31 erg s − 1 ) ( − 0.31 ± 0.15 ) ?> . Accounting for this correlation, the agreement can be improved further, especially when the contributions from other class sources to the GRXE are considered, which all have low EW6.7 values. We conclude that the GRXE mostly consists of typically faint but numerous DNe, plus ABs, while magnetic cataclysmic variables are probably mainly the high-flux representatives of the responsible populations and dominate the GRXE only in harder energy bands.

Photometry of the long period dwarf nova MU Centauri

Even among the brighter cataclysmic variables an appreciable number of objects exist about which not much is known. One of them, MU Cen, was observed as part of a small project to better characterize these neglected systems. The temporal variations of the brightness of MU Cen during quiescence were studied in order to find clues to the structure of the system and its behavior on time scales of hours and shorter. Light curves observed in white light at a time resolution of a few seconds and with a duration of several hours, obtained in six nights and spanning a total time base of five months, were investigated using different time series analysis tools, as well as model fits. The light curve of MU Cen is dominated by ellipsoidal variations of the secondary star. The refined orbital period is Porb=0.341883 days. Model fits permit to constrain the temperature of the secondary star to ∼5000 K and the orbital inclination to 50° ≤ i ≤ 65°. The latter result permits estimates of the component masses which are probably somewhat smaller that derived in previous publications. A second persistent period of P2=0.178692 days was also identified. Its origin remains unclear. As all cataclysmic variables, MU Cen exhibits flickering, however, on a rather low level. Its frequency behavior is normal for quiescent dwarf novae. There are indications that the individual flickering events are not always independent but can lead to effects reminiscent of quasi-periodic oscillations.

CATACLYSMIC VARIABLES IN THE SUPERBLINK PROPER MOTION SURVEY

We have discovered a new high proper motion cataclysmic variable (CV) in the SUPERBLINK proper motion survey, which is sensitive to stars with proper motions greater than 40 mas/yr. This CV was selected for follow-up observations as part of a larger search for CVs selected based on proper motions and their NUV-V and V-K$_{s}$ colors. We present spectroscopic observations from the 2.4m Hiltner Telescope at MDM Observatory. The new CV's orbital period is near 96 minutes, its spectrum shows the double-peaked Balmer emission lines characteristic of quiescent dwarf novae, and its V magnitude is near 18.2. Additionally, we present a full list of known CVs in the SUPERBLINK catalog.

A flickering study of nova-like systems KR Aur and UU Aqr

We present a study of the flickering activity in two nova-like systems, KR Aur and UU Aqr. We applied a statistical model of flickering simulations in accretion discs based on turbulent angular momentum transport between two adjacent rings with an exponential distribution of the turbulence dimension scale. The model is based on a steady-state disc model, which is satisfied in the case of hot ionized discs of nova-like cataclysmic variables. Our model successfully fits the observed power-density spectrum of KR Aur with the disc parameter α= 0.10–0.40 and an inner-disc truncation radius in the range Rin= 0.88–1.67 × 109 cm. The exact values depend on the mass-transfer rate in the sense that α decreases and Rin increases with mass-transfer rate. In any case, the inner-disc radius found for KR Aur is considerably smaller than those for quiescent dwarf novae, as predicted by the disc instability model. On the other hand, our simulations fail to reproduce the power-density spectrum of UU Aqr. A tantalizing explanation involves the possible presence of spiral waves, which are expected in UU Aqr because of its low mass ratio but not in KR Aur. In general our model predicts the observed concentration of flickering in the central disc. We explain this by the radial dependence of the angular-momentum gradient.

SUZAKUOBSERVATIONS OF THE DWARF NOVA V893 SCORPII: THE DISCOVERY OF A PARTIAL X-RAY ECLIPSE

V893 Sco is an eclipsing dwarf nova that had attracted little attention from X-ray astronomers until it was proposed as the identification of an Rossi X-Ray Timing Explorer all-sky slew survey (XSS) source. Here, we report on the pointed X-ray observations of this object using Suzaku. V893 Sco was in quiescence at the time, as indicated by the coordinated optical photometry we obtained at the South African Astronomical Observatory. Our Suzaku data show V893 Sco to be X-ray bright, with a highly absorbed spectrum. Most importantly, we have discovered a partial X-ray eclipse in V893 Sco. This is the first time that a partial eclipse is seen in X-ray light curves of a dwarf nova. Our preliminary simulations demonstrate that the partial X-ray eclipse can be in principle reproduced if the white dwarf in V893 Sco is partially eclipsed. Higher quality observations of this object have the potential to place significant constraints on the latitudinal extent of the X-ray emission region and thereby discriminating between an equatorial boundary layer and a spherical corona. The partial X-ray eclipse therefore makes V893 Sco a key object in understanding the physics of accretion in quiescent dwarf nova.

HELIUM CORE WHITE DWARFS IN CATACLYSMIC VARIABLES

Binary evolution predicts a population of helium core (M < 0.5 Msol) white dwarfs (WDs) that are slowly accreting hydrogen-rich material from low mass main sequence or brown dwarf donors with orbital periods less than four hours. Four binaries are presently known in the Milky Way that will reach such a mass-transferring state in a few Gyr. Despite these predictions and observations of progenitor binaries, there are still no secure cases of helium core WDs among the mass-transferring cataclysmic variables (CVs). This led us to calculate the fate of He WDs once accretion begins at a rate Mdot < 1e-10 Msol/yr set by angular momentum losses. We show here that the cold He core temperatures (T_c < 1e7 K) and low Mdot result in ~ 1e-3 Msol of accumulated H-rich material at the onset of the thermonuclear runaway. Shara and collaborators noted that these large accumulated masses may lead to exceptionally long classical nova (CN) events. For a typical donor star of 0.2 Msol, such binaries will only yield a few hundred CNe, making these events rare amongst all CNe. We calculate the reheating of the accreting WD, allowing a comparison to the measured WD effective temperatures in quiescent dwarf novae and raising the possibility that WD seismology may be the best way to confirm the presence of a He WD. We also find that a very long (> 1000 yr) stable burning phase occurs after the CN outburst, potentially explaining enigmatic short orbital period supersoft sources like RX J0537-7034 (P_orb = 3.5 hr) and 1E 0035.4-7230 (P_orb = 4.1 hr).

Spectra of accretion discs around white dwarfs

We present the spectra of accretion discs around white dwarfs calculated with an improved and updated version of Shaviv and Wehrse [Shaviv, G., Wehrse, R., 1991. A&A 251, 117] model. The new version includes line opacities and convective energy transport and can be used to calculate the spectra of hot discs in bright systems (nova-like variables or dwarf novae in outburst) as well as the spectra of cold accretion discs in quiescent dwarf novae.

XMM-NewtonObservations of the Cataclysmic Variable GW Librae

XMM-Newton observations of the accreting, pulsating white dwarf in the quiescent dwarf nova GW Librae were conducted to determine whether the nonradial pulsations present in previous UV and optical data affect the X-ray emission. The nonradial pulsations are evident in the simultaneous Optical Monitor data but are not detected in the X-ray with an upper limit on the pulsation amplitude of 0.092 mag. The best fits to the X-ray spectrum are with a low-temperature diffuse gas model or a multitemperature cooling flow model with a strong O VIII line, similar to other short-period dwarf novae, but with a lower temperature range than evident in normal short-period dwarf novae. The lack of pulsations and the spectrum likely indicate that the boundary layer does not extend to the surface of the white dwarf.

Orbital Period of the Dwarf Nova RXS J053234.9+62475511Based on observations obtained at the MDM Observatory, operated by Dartmouth College, Columbia University, Ohio State University, and the University of Michigan.

ABSTRACTWe report spectroscopy of the newly discovered SU Ursae Majoris dwarf nova identified with the X‐ray source RXS J053234.9+624755. Radial velocities of the Hα emission line in the quiescent state give an orbital period of 0.05620(4) days (80.93 minutes), which is among the shortest for SU UMa stars with determined periods. We also report UBVI magnitudes of the quiescent dwarf nova and surrounding stars. Using a previous measurement of the superhump period, we find the fractional superhump excess ϵ to be 0.016(4), which is not atypical of dwarf novae in this period range.

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.

X‐Ray Observations of the Boundary Layer in Dwarf Novae at Low Accretion Rates

We present a spectral analysis of XMM-Newton data of 10 dwarf novae, 9 of which were observed during a quiescent state. The X-ray spectra indicate the presence of a hot, optically thin plasma with a temperature distribution consistent with a simple, isobaric cooling flow. The likely origin of the X-ray emission is cooling plasma in the boundary layer settling onto the white dwarf. Using a cooling flow model, we derive the temperatures, accretion rates, rotational velocities, and elemental abundances of the X-ray-emitting gas. We discuss the implications of our findings for the structure of the boundary layer. A comparison of X-ray and ultraviolet luminosities finds no evidence of underluminous boundary layers in any of the quiescent dwarf novae. The X-ray spectrum of EI UMa differs significantly from those of the other objects, showing a strong fluorescent Fe Kα line and a strong O VII line triplet. Based on the observational evidence, we argue that EI UMa is most likely an intermediate polar and not, as previously thought, a dwarf nova.

Far‐Ultraviolet Observations of the Dwarf Nova VW Hydri in Quiescence

We present a 904-1183 A spectrum of the dwarf nova VW Hyi taken with the Far Ultraviolet Spectroscopic Explorer during quiescence, 11 days after a normal outburst, when the underlying white dwarf accreter is clearly exposed in the far-ultraviolet. However, model fitting shows that a uniform-temperature white dwarf does not reproduce the overall spectrum, especially at the shortest wavelengths. A better approximation to the spectrum is obtained with a model consisting of a white dwarf and a rapidly rotating belt. The white dwarf component accounts for 83% of the total flux, has a temperature of 23,000 K, a v sin i = 400 km s-1, and a low carbon abundance. The best-fit accretion belt component accounts for 17% of the total flux, with a temperature of about 48,000-50,000 K and a rotation rate Vrot sin i around 3000-4000 km s-1. The requirement of two components in the modeling of the spectrum of VW Hyi in quiescence helps to resolve some of the differences in interpretation of ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of a second component (and its exact nature) in VW Hyi itself is still relatively uncertain, given the lack of better models for spectra of the inner disk in a quiescent dwarf nova.

X-ray Emission from Galactic Plane

We report several important results obtained from recent Galactic X-ray survey observations, in particular ASCA Galactic center and plane surveys and our Chandra deep survey on the (l,b) ≈ (255, 00) region. Strong hard X-ray diffuse components are observed from Galactic ridge, center and bulge, and they have both thermal and non-thermal spectral components. Dozens of discrete and extended sources have been discovered on the Galactic plane, which also indicate thermal and/or non-thermal X-ray energy spectra. They are often associated with radio sources and are considered to be SNR candidates. Most of the hard X-ray point sources in the outer part of the Galactic plane are considered to be background AGNs, while fraction of the Galactic hard X-ray sources (such as quiescent dwarf novae) increases toward the Galactic center. Most of the soft X-ray sources on the Galactic plane are presumably nearby active stars.