Low Brightness State Research Articles

VY Scl-type cataclysmic variable SDSS J154453.60+255348.8: stellar and disc parameters

ABSTRACT We present a new study of the eclipsing cataclysmic variable SDSS J154453.60+255348.8 to determine the object’s nature and its system parameters together with the probe of the accretion flow structure in the system. Based on analyses of new simultaneous time-resolved photometric and spectroscopic observations of SDSS J154453.60+255348.8 and using our light-curve modelling techniques and the Doppler tomography method, we found that the system contains a white dwarf with the mass of MWD = 0.62(7) M⊙ and an evolved red dwarf as a secondary. The system inclination is close to 90° and the mass ratio is q = 0.49(2). The secondary has an effective temperature T2 = 3400(40) K and a radius about 1.35(15) times larger than a zero-age main-sequence star with similar mass. From observation of the system in high- and low-brightness states, we conclude that SDSS J154453.60+255348.8 is a long-orbital period VY Scl-type system. The accretion disc in the high state is about two times less than the truncation radius, and is completely missing during the low state of the system.

Short-duration accretion states of Polars as seen in TESS and ZTF data

ABSTRACT Polars are highly magnetic cataclysmic variables which have been long observed to have both high and low brightness states. The duration of these states has been previously seen to vary from a number of days up to years. Despite this, these states and their physical origin have not been explained in a consistent manner. We present observations of the shortest duration states of a number of Polars observed by ZTF and TESS. This has allowed us to determine that short-duration states are a relatively common feature across the population of Polars. Furthermore, we have been able to generalize the model of star-spot migration to explain both short-lived high and low states in Polars by incorporating the interaction between the magnetic field of the white dwarf and that of the star spots.

Insights from multi-wavelength observations during high and low states of non-magnetic CVs

Close binaries containing a white dwarf accreting from a disk that receives material from a late stellar companion show an interesting variety of behaviors that depend on their accretion rates and the changes in this rate. The highest accretion rates are evident during the bright states of novalikes, as well as the superoutbursts of the shortest period dwarf novae followed by normal outbursts and the Z Cam systems. While the normal outbursts and superoutbursts of dwarf novae can be understood from the standpoint of disk and tidal instabilities, the changes in rates in novalikes when they enter low brightness states and the cause of extremely high rates for the systems with orbital periods between 3 and 4 h remain elusive. This paper highlights some recent insights and continuing problems found from X-ray, UV and optical observations of these high and low states, as well as the prospects for increased understanding from the anticipated future ground and space missions.

Polarization angle swings in blazars: The case of 3C 279(Corrigendum)

Over the past few years, several occasions of large, continuous rotations of the electric vector position angle (EVPA) of linearly polarized optical emission from blazars have been reported. These events are often coincident with high energy gamma-ray flares and they have attracted considerable attention, as they could allow one to probe the magnetic field structure in the gamma-ray emitting region of the jet. The flat-spectrum radio quasar 3C279 is one of the most prominent examples showing this behaviour. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C279, yielding exceptionally well-sampled flux density and polarization curves that cover a period of 2008-2012. Several large EVPA rotations are identified in the data. We introduce a quantitative measure for the EVPA curve smoothness, which is then used to test a set of simple random walk polarization variability models against the data. 3C279 shows different polarization variation characteristics during an optical low-flux state and a flaring state. The polarization variation during the flaring state, especially the smooth approx. 360 degrees rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes. We conclude that during the two different optical flux states, two different processes govern the polarization variation, possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity.

Dynamical masses of a nova-like variable on the edge of the period gap

We present the first dynamical determination of the binary parameters of an eclipsing SW Sextantis star in the 3–4 h orbital period range during a low state. We obtained time-resolved optical spectroscopy and photometry of HS 0220+0603 during its 2004–2005 low-brightness state, as revealed in the combined Small & Moderate Aperture Research Telescope System, IAC80 and M1 Group long-term optical light curve. The optical spectra taken during primary eclipse reveal a secondary star spectral type of M5.5 ± 0.5 as derived from molecular band-head indices. The spectra also provide the first detection of a DAB white dwarf in a cataclysmic variable. By modelling its optical spectrum we estimate a white dwarf temperature of 30 000 ± 5000 K. By combining the results of modelling the white dwarf eclipse from ULTRACAM light curves with those obtained by simultaneously fitting the emission- and absorption-line radial velocity curves and I-band ellipsoidal light curves, we measure the stellar masses to be M1 = 0.87 ± 0.09 M⊙ and M2 = 0.47 ± 0.05 M⊙ for the white dwarf and the M dwarf, respectively, and an inclination of the orbital plane of i ≈ 79°. A radius of 0.0103 ± 0.0007 R⊙ is obtained for the white dwarf. The secondary star in HS 0220+0603 is likely too cool and undersized for its mass.

MLS110213:022733+130617: a new eclipsing polar above the period gap

This study confirms MLS110213:022733+130617 as a new eclipsing polar. We performed optical spectroscopic, polarimetric and photometric follow-up of this variable source identified by the Catalina Real Time Transient Survey. Using the mid-eclipse times, we estimated an orbital period of 3.787 h, which is above the orbital period gap of the cataclysmic variables (CVs). There are nine other known polars with longer orbital periods, and only two of them are eclipsing. We identified high- and low-brightness states and high polarization modulated with the orbital period. The spectra are typical of polars, with strong high ionization emission lines and inverted Balmer decrement. The He ii 4686 Å line is as strong as H β. We modelled the photometric and polarimetric bright-state light curves using the cyclops code. Our modelling suggests an extended emitting region on the white dwarf (WD) surface, with a mean temperature of 9 keV and B in the range 18–33, although the possibility that it could be a two-pole accretor cannot yet be ruled out. The WD mass estimated from the shock temperature is 0.67 M⊙. The derived parameters are consistent with the eclipse profile. The distance was estimated as 406 ± 54 pc using the period–luminosity–colours method. MLS110213 populates a rare sub-group of polars, near the upper limit of the period distribution, important to understand the evolution of mCVs.

Analyzing polarization swings in 3C 279

Quasar 3C 279 is known to exhibit episodes of optical polarization angle rotation. We present new, well-sampled optical polarization data for 3C 279 and introduce a method to distinguish between random and deterministic electric vector position angle (EVPA) variations. We observe EVPA rotations in both directions with different amplitudes and find that the EVPA variation shows characteristics of both random and deterministic cases. Our analysis indicates that the EVPA variation is likely dominated by a random process in the low brightness state of the jet and by a deterministic process in the flaring state.

A photometric study of the polar MT draconis in 2005–2009

We have obtained data on various brightness states of the polar MT Dra over five years of observations, including the first multicolor photometry for this close binary. We confirm the known orbital period, which has remained constant over 17 years, which is unusual for cataclysmic binaries. Our observations in October 2006 demonstrated a transition of the polar to its low brightness state within one day.

The accreting white dwarfs in VY Scl nova-like variables

Accurate distances for nova-like variables offer the possibility of extracting information on nova-like accretion rates during high states of optical brightness and on their underlying accretion-heated white dwarfs during intermediate and low brightness states. The modeling technique which is employed is discussed and a representative example, the novalike variable KR Aur, is presented. Although KR Aur was in a fainter high state when its far ultraviolet spectrum was obtained, roughly one-half of its FUV radiation is due to the light of an accretion disk and the other half is contributed by a hot white dwarf with Teff = 29, 000±2, 000K. However, this best-fit solution corresponds to a distance of 180 pc which was an early distance estimate due to Patterson (1984).

Asynchronous polar BY Cam: periods, quasiperiods, accretion modes and magnetic field structure

The asynchronous polar BY Cam in the low-brightness state displays observational evidence of the quadrupole component magnetic field of a magnetic white dwarf.

Magnetic field structure and accretion regimes of the asynchronous polar by Cam

Series of photometric CCD observations of the asynchronous polar BY Cam in a low accretion state (R = 14m–16m) were made on the K-380 telescope at the Crimean Astrophysical Observatory (CrAO) over 100 hours in the course of 31 nights during 2004–2005. A period of P1 = 0.137120±0.000002 days was found for the variations in the brightness, along with less significant periods of P2 = 0.139759±0.000003 and P3 = 0.138428±0.000002 days, where P2 and P3 are obviously the orbital and rotation periods, while the dominant period P1 is the sideband period. A modulation in the brightness and an amplitude of 0.137 days in the oscillations at the orbital-rotational beat period (synodic cycle) of 14.568±0.003 day are found for the first time. The profile of the modulation period is four humped. This indicates that the magnetic field has a quadrupole component, which shows up well during the low brightness state. Accretion takes place simultaneously into two or three accretion zones, but at different rates. The times of the times of maxima for the main accretion zone vary with the phase of the beat period. Three types of variation of this sort are distinguished: linear, discontinuous, and chaotic, which indicate changes in the accretion regimes. At synodic phases 0.25 and 0.78 the bulk of the stream switches by 180°, and at phase 0.55, by ∼75°. At phases of 0.25–0.55 and 0.55–0.78, the O-C shift with a period of 0.1384 days, which can be explained by a retrograde shift of the main accretion zone relative to the magnetic pole and/or a change in the angle between the field lines and the surface of the white dwarf owing to the asynchronous rotation. For phases of 0.78–1.25 the motion of the accretion zone is quite chaotic. It is found that synchronization of the components occurs at a rate of less than dProt/Prot∼10−9 day/day.

The Low State Temperature Distribution and First Chemical Abundances of White Dwarfs in Polars

AbstractDuring the low optical brightness states of AM Her systems (polars) when accretion has declined to a very low value, the underlying magnetic white dwarf photosphere can be modelled without the complication of thermal bremstrahlung and cyclotron emission from the luminous accretion column. The far ultraviolet spectra can be modelled with high gravity Solar composition photospheres. In this way, I present new temperatures and the first chemical abundance estimates for the white dwarfs in three selected polars from the IUE NEWSIPS archive. For the white dwarf in V834 Cen with Teff = 16,000K, Si/H = 0.1 Solar, C/H = 0.5 Solar, for BY Cam, Teff = 17,000K, Si/H = 0.1, C/H = 5 Solar and for RX J1313-32, Teff = 22,000K, Si/H = 0.1 Solar, C/H = 0.1 Solar. The temperature distribution of 24 white dwarfs in polars with known temperatures above and below the period gap is compared with the distribution of the white dwarf temperatures in dwarf novae during quiescence. In both cases, the magnetic white dwarfs in polars are significantly cooler than the non-magnetics. For all CV white dwarfs, magnetic and non-magnetic with Teff < 12,500K, 91% of the objects (10 out of 11) are magnetics in polars. This suggests that long term accretion heating and cooling of white dwarfs in polars differs from the effects of long term accretion in non-magnetic disk accretors.

Features of the Energy Distribution in the Spectrum of the Symbiotic Star CH Cyg in Different Brightness States

The results of spectrophotometric observations of CH Cyg in an active phase and during emergence from it are given: the spectral energy distribution in the 3225-7500 A range with 23 A resolution, the absolute monochromatic illuminances at eight selected wavelengths in the 3225-8015 A, and the emission fluxes in the Hα and Hβ lines. The features of the energy distribution in the stellar spectrum and the observed forms of variability, different between the short-wavelength and red parts of the spectrum, are determined by the instability of both of the system's components. In the active phase, CH Cyg's emission in the observed spectral range can be represented as the total emission from a cool M6 III giant and a hot hydrogen gas with a temperature Te = 10,000-15,000 K. In the low-brightness state, the star's energy distribution fully corresponds to that of a cool giant, the spectrum of which varies from M6 III to M7 III. The photometric variability of the cool component in the λ 8015 A range is 0m.3-0m.4. An increase in the brightness of the cool giant preceded a bright outburst of the hot component.

White Dwarfs in Cataclysmic Variables

ABSTRACTPrior to the last 15 years, the literature on cataclysmic variables (CVs) contained little on the properties of the underlying white dwarf accreter other than estimates of their masses. They were regarded simply as nondescript potential wells for studies of accretion disk structure and stability. Estimates of their masses and associated core compositions were discussed, but only in the context of thermonuclear runaway theory. With the advent of space spectroscopy, especially HST, IUE, EUVE, and HUT, direct spectroscopic observations of exposed white dwarfs in CVs were carried out during dwarf nova quiescence or low brightness states of nova‐like variables and magnetic CVs when accretion rates were very low. This review covers new insights and physical properties of white dwarfs in nonmagnetic and magnetic CVs, including surface temperatures, heating and cooling measurements, rotational velocities, CV white dwarf masses (including Einstein redshift masses), photospheric chemical abundances (including composition relics of ancient novae), accretion belts, the physics of accretion heating, and long‐term CV evolution. For an ensemble of 37 CV degenerates with secure temperatures, the average Teff=20,800 K, for nonmagnetic CV degenerates Teff=24,100 K, and for magnetic CV degenerates Teff=16,400 K. The lowest Teff values are associated with magnetic CVs and with systems whose orbital periods are less than 2 hr. Thermal e‐folding times of the white dwarf envelope in response to accretion heating are in the range 6–600 days, photospheric chemical abundances range from moderately subsolar to greatly above solar, and rotational velocities lie within the range 50 km s−1<Vsin i<1200 km s−1. If pre‐CV white dwarfs had time to cool to 10-3L⊙ prior to the onset of Roche lobe overflow, then their average lower limit lifetime is 2.5 × 108 yr, and they have been accretion heated to an average luminosity log (L/L⊙) = 1.90 or, on average, by ∼11,000 K since the end of their pre‐CV evolution phase.

HST and ground-based eclipse observations of V2051 Ophiuchi: binary parameters

We report on high-speed eclipse photometry of the dwarf nova V2051 Oph while it was in a low brightness state, at B ≃ 16.2 mag. In comparison with the average IUE spectra, the ultraviolet continuum and emission lines appear reduced by factors of, respectively, ≃ 4 and ≃ 5. Flickering activity is mostly suppressed and the light curve shows the eclipse of a compact white dwarf at the disc centre which contributes ≃ 60 per cent of the total light at 3900–4300 Å. We use measurements of contact phases in the eclipse light curve to derive the binary geometry and to estimate masses and relevant dimensions. We find a mass ratio of q = 0.19 ± 0.03 and an inclination of i = 83 ± 2°. The masses of the component stars are M1 = 0.78 ± 0.06 M⊙ and M2 = 0.15 ± 0.03 M⊙. Our photometric model predicts K1 = 83 ± 12 km s−1 and K2 = 436 ± 11 km s−1. The predicted value of K1 is in accordance with the velocity amplitude obtained from the emission lines after a correction for asymmetric line emission in the disc is made. The secondary of V2051 Oph is significantly more massive than the secondaries of the other ultrashort period dwarf novae. V2051 Oph is probably a relatively young system, with a secondary star that has not had enough time to evolve out of thermal equilibrium.

MV Lyrae – since Parenago to the present time

Abstract The photometry of the novalike variable UMV Lyrae carried out in the years 1951–1996 is performed in the light curve. The evidence of two steady brightness levels lasting about 10 or more years is shown. The peculiarities of the low brightness state which begin in 1995 are described in more detail.

Rotationally Broadened Synthetic Spectra for HST and IUE Observations of Exposed White Dwarfs in Cataclysmic Variables11Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5‐26555.

ABSTRACTIn many dwarf novae during quiescence, the underlying white dwarf dominates the flux in the far ultraviolet. By observing these exposed degenerate stars with the Hubble Space Telescope Goddard High Resolution Spectrograph and Faint Object Spectrograph (HST GHRS and FOS), the white‐dwarf rotation rates can be determined or constrained by fitting synthetic spectra broadened by rotation. Using TLUSTY and SYNSPEC, we present a grid of rotationally broadened white‐dwarf synthetic spectra, convolved with the resolution of the HST GHRS (G140L, G160M), FOS (G130H), and IUE low‐resolution short‐wavelength prime (SWP) spectra, computed for log g = 8, solar composition, Teff = 15,000, 25,000, and 50,000 K, and rotational velocities of 0–5000 km s−1. We examine the predicted appearance of accreted solar‐composition white‐dwarf atmospheres in the context of determining or constraining the white‐dwarf rotational velocities. We present model grids that demonstrate this capability. We present the first application of these grids to the SWP low‐resolution spectra of the white dwarf in the nova‐like variable TT Ari, exposed during the very low brightness state of that system. We also rule out the possibility that the absence of Lyα absorption in many cataclysmic variables in quiescence (specifically those containing white dwarfs with Teff = 15,000–20,000 K) could be due to rotational velocities of the white dwarf near breakup.

Long-Term Brightness Changes of Two CVs

AbstractThe long-term light curves of the AM Her type binary BY Cam and the nova-like variable PG 2133+115 are presented. The BVR observations were carried out at a 0.5 m telescope equipped with a high sensitive TV tube superisocon (Abramenko et al. 1978), the UBV photoelectric data were obtained at 0.6 and 1.25 m telescopes and the photographic estimates were made from the negatives obtained at the 0.4 m astrograph in Crimea.Both stars show high and low brightness states. They show similar behaviour in some respects: the amplitude between high and low states was about two magnitudes in B, but the duration of a low state was approximately two months for BY Cam and 1… 2 years for PG 2133+115.

QPOS of By Cam in Low and High States

AbstractWe present the analysis of BV photometry of BY Cam based on observations carried out in Crimea. The long QPOs in brightness were detected in both high and low brightness states. Their typical periods, equal to 6, 12 and 24 m, are found. The amplitude of the QPOs depends on the phase of Pbeat.

&lt;italic&gt;ROSAT&lt;/italic&gt; observations of AM Her stars in low or intermediate states

We present data for four AM Her stars which were observed in an intermediate or low brightness state, We show that as the luminosity of the blackbody component decreases so does the energy balance ratio L(bb,bol)/(L(tb,bol)+ L(cyc)). Indeed, AM Her in an intermediate state shows a soft X-ray deficiency over the standard shock model of Lamb & Masters and does not follow the correlation between the coupling radius and the energy balance ratio found by Ramsay et al. for systems in a high state. ST LMi in a low state shows a much reduced bremsstrahlung temperature compared to its high state. We hypothesize that as the overall accretion rate decreases the fraction of the gas which shocks above the surface of the white dwarf becomes smaller. Instead, much of the accreting gas cools via Coulomb collisions and cyclotron emission. At low accretion rates, the thermalized component of the accretion energy emitted at the surface of the white dwarf is too cool to be observed in the ROSAT band above the cut-off imposed by interstellar absorption.