V2051 Oph Research Articles

Indirect evidence for short period magnetic cycles in W UMa stars

Complex period variations of five W UMa type binaries (AB And, OO Aql, DK Cyg, V566 Oph, U Peg) were investigated by analyzing their O--C diagrams, and several common features were found. Four of the five systems show secular period variations at a constant rate on the order of |dP / dt 1/P | ~ 10E-7 1/yr. In the case of AB And, OO Aql, and U Peg a high-amplitude, nearly one-century long quasi-sinusoidal pattern was also found. It might be explained as light-time effect, or by some magnetic phenomena, although the mathematical, and consequently the physical, parameters of these fits are very problematic, as the obtained periods are very close to the length of the total data range. The most interesting feature of the studied O--C diagrams is a low amplitude (2-4 x 10E-3 d) modulation with a period around 18--20 yr in four of the five cases. This phenomenon might be indirect evidence of some magnetic cycle in late-type overcontact binaries as an analog to the observed activity cycles in RS CVn systems.

High-speed, energy-resolved STJ observations of the AM Her system V2301 Oph

We present high time-resolution optical energy-resolved photometry of the eclipsing cataclysmic variable V2301 Oph made using the ESA S-Cam detector, an array of photon counting super-conducting tunnel junction (STJ) devices with intrinsic energy resolution. Three eclipses were observed, revealing considerable variation in the eclipse shape, particularly during ingress. The eclipse shape is shown to be understood in terms of AM Her accretion via a bright stream, with very little contribution from the white dwarf photosphere and/or hotspot. About two thirds of the eclipsed light arises in the threading region. Variation in the extent of the threading region can account for most of the variations observed between cycles. Spectral fits to the data reveal a 10 000 K blackbody continuum with strong, time-varying emission lines of hydrogen and helium. This is the first time that stellar emission lines have been detected in the optical band using a non-dispersive photon-counting system.

On the Overcontact Binary V2388 Oph and An Improved M–R Relation for Ltcbs

On the Overcontact Binary V2388 Oph and An Improved M–R Relation for Ltcbs

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.

ChandraObservation of V426 Ophiuchi: Weighing the Evidence for a Magnetic White Dwarf

We report the results of a 45 ks Chandra observation of the cataclysmic variable (CV) V426 Ophiuchus. The high-resolution spectrum from the high-energy transmission grating spectrometer is most consistent with a cooling flow model, placing V426 Oph among the group of CVs including U Gem and EX Hya. An uninterrupted light curve was also constructed, in which we detect a significant 4.2 hr modulation together with its first harmonic at 2.1 hr. Reanalysis of archival Ginga and ROSAT X-ray light curves also reveals modulations at periods consistent with 4.2 and/or 2.1 hr. Furthermore, optical photometry in V, simultaneous with the Chandra observation, indicates a modulation anticorrelated with the X-ray, and later more extensive R-band photometry finds a signal at ~2.1 hr. The earlier reported X-ray periods at ~0.5 and 1 hr appear to be only transient and quasi-periodic in nature. In contrast, the 4.2 hr period or its harmonic is stable and persistent in X-ray/optical data from 1988 to 2003. This periodicity is clearly distinct from the 6.85 hr orbit and could be due to the spin of the white dwarf. If this is the case, V426 Oph would be the first long-period intermediate polar with a ratio Pspin/Porb of 0.6. However, this interpretation requires unreasonable values of magnetic field strength and mass accretion rate.

Eclipse Mapping of the Flickering Sources in the Dwarf Nova V2051 Ophiuchi

We report an eclipse-mapping analysis of an ensemble of light curves of the dwarf nova V2051 Oph, with the aim to study the spatial distribution of its steady-light and flickering sources. The data are combined to derive the orbital dependence of the steady-light and the flickering components at two different brightness levels, named the and states. The differences in brightness are caused by long-term variations in the mass transfer rate from the secondary star. The white dwarf is hardly affected by the long-term changes; its flux increases by only 10% from the faint to the bright state, whereas the disk flux raises by a factor of 2. Eclipse maps of the steady light show asymmetric brightness distributions with enhanced emission along the ballistic stream trajectory, clear evidence of gas stream overflow. Comparison between the steady-light maps of the faint and bright states suggests that the quiescent disk responds to changes in mass transfer rate in a homologous way. The ability to separate the orbital dependence of the low- and high-frequency flickering components allows us to identify the existence of two different and independent sources of flickering in V2051 Oph. The low-frequency flickering arises mainly in the overflowing gas stream and is associated with the mass transfer process. It maximum emission occurs at the position of closest approach of the gas stream to the white dwarf, and its spatial distribution changes in response to variations in the mass transfer rate. The high-frequency flickering originates in the accretion disk, showing a radial distribution similar to that of the steady-light maps and no evidence of emission from the hot spot, gas stream, or white dwarf. This disk flickering component has a relative amplitude of about 3% of the steady disk light, independent of disk radius and brightness state. If the disk flickering is caused by fluctuations in the energy dissipation rate induced by magnetohydrodynamic turbulence, its relative amplitude leads to a viscosity parameter αcool 0.1–0.2 at all radii for the quiescent disk. This value seems uncomfortably high to be accommodated by the disk-instability model.

An analysis of the light curves of the overcontact binary system V2388 Ophiuchi

We present four seasons' (2000-2003) ground-based photometry of the short-period contact binary V2388 Oph. The system is the brighter component of visual binary HIP 87655. The magnitude difference between the visual companion and the eclipsing pair was estimated to be 1. m 19, 1. m 09 and 1. 09 in B, V and R bandpasses, respectively. The light curves in BVR are solved by the WD code. Combining the parameters found by the light curve analysis with those of the radial velocity solution we derived the masses and radii of the star components as: M1 = 1.80(2) M� , M2 = 0.34(1) M� , R1 = 2.60(2) R� , R2 = 1.30(1) R� . The primary component appears to be more evolved in the mass-radius, mass-luminosity planes and also in the HR diagram. It lies near the terminal-age main-sequence, which agrees well with the position of deeper-contact A-type systems. The large fill-out factor of 0.65 does also support this classification.

Radial Velocity Studies of Close Binary Stars. IX.

Radial-velocity measurements and sine-curve fits to the orbital velocity variations are presented for the eighth set of ten close binary systems: AB And, V402 Aur, V445 Cep, V2082 Cyg, BX Dra, V918 Her, V502 Oph, V1363 Ori, KP Peg, V335 Peg. Half of the systems (V445 Cep, V2082 Cyg, V918 Her, V1363 Ori, V335 Peg) were discovered photometrically by the Hipparcos mission and all systems are double-lined (SB2) contact binaries. The broadening function method permitted improvement of the orbital elements for AB And and V502 Oph. The other systems have been observed for radial velocity variations for the first time; in this group are five bright (V<7.5) binaries: V445 Cep, V2082 Cyg, V918 Her, KP Peg and V335 Peg. Several of the studied systems are prime candidates for combined light and radial-velocity synthesis solutions.

Hard X-ray Observations of Magnetic Cataclysmic Variables

AbstractHard X-ray light curves and spectral parameters from our analysis of X-ray data of five AM Her type systems – V2301 Oph, V1432 Aql, EP Draconis, GG Leonis, &amp; V834 Cen, and one intermediate polar – TV Col, observed using the Rossi X — ray Timing Explorer (RXTE) satellite are presented. A new improved ephemeris has been derived for V2301 Oph using the mid-eclipse timings. Average intensity variations, without any change of shape of the light curve or hardness ratio, are observed on timescales of a few days to a few months in V2301 Oph. V1432 Aql shows erratic variations on a timescale of a day, at least two sharp dips near orbital phases 0.35 and 0.5, and a total eclipse. Hard X-ray eclipses are also reported in EP Dra and GG Leo. V834 Cen shows intensity variations on yearly timescale and is found to be in a low state in 2002. In TV Col, a binary orbital modulation at 5.5h, in addition to the spin period of 1910s, is reported for the first time. Maximum spectral temperatures in Polars have been determined and used to estimate the masses of the white dwarfs.

High Temporal Resolution Spectroscopic Observations of the Flare Star V1054 Oph

We present the results of a high temporal resolution spectroscopic monitoring of the flare (UV Cet type) star V1054 Oph (Wolf 630AB), classified as a dM3.5e visual binary system. Intermediate resolution spectra have been taken during four nights (2–5 April 2001) using the IDS spectrograph of the 2.5 m Isaac Newton Telescope (INT) (La Palma, Canary Islands, Spain). The V1054 Oph spectra show very strong emission lines even in its quiescent state. The analysis of the temporal evolution of the observed emission lines (from Hβ to H11 and the Ca II H and K lines) reveals four strong flares and several weak flares. We have studied in detail the behaviour of the chromospheric lines during the different phases (pre-flare, impulsive and gradual decay) of these flares. The observed flares last from ∼25 to 95 min. The equivalent width of the Hβ line changes by a factor up to ∼2.3. Broad wings and asymmetric (red-shifted) lines are observed as well.

The photometric observations and analysis of the eclipsing binar V566 Ophiuchi

The system of V566 Oph was observed photometrically in B and V filters in1999 at the Birouni Observatory, Shiraz, Iran. Two light curves were obtained and have been analyzed separately of Wilson-Devinney code. New geometrical and physical elements for the system are given. The absolute dimensions of the binary have been derived by combining the photometric solution with spectroscopic data of the system. Also, the period of the system is calculated.

Chandra X-Ray Results on V426 Ophiuchi

AbstractFrom a 45ks Chandra observation of V42G Oph we have obtained high-resolution X-ray spectra at moderate signal-to-noise, and a, good quality, uninterrupted lightcurve. The spectra are reasonably fit with a cooling flow model, similar to EX Hya and U Gem. Our analysis of the Chandra and additional X-ray/optical lightcurves reveals a persistent modulation at 4.2 hr from 1988 to 2003, likely the white dwarf spin period indicating an intermediate polar nature for V426 Oph.

Cyclical period changes in the dwarf novae V2051 Oph and V4140 Sgr

We report the identification of cyclical changes in the orbital period of the eclipsing dwarf novae V2051 Ophiuchi and V4140 Sagitarii. We used sets of white dwarf mideclipse timings to construct observed-minus-calculated diagrams covering, respectively, 25 and 16 years of observations. The V2051 Oph data present cyclical variations that can be fitted by a linear plus sinusoidal function with period 22±2 yr and amplitude 17±3 s. The statistical significance of this period by an F-test is larger than 99.9 per cent. The V4140 Sgr data present cyclical variations of similar amplitude and period 6.9 ± 0.3 yr which are statistically significant at the 99.7 per cent level. We derive upper limits for secular period changes of | u P| < 3 × 10 12 and | u P| < 1.8 × 10 11 , respectively for V2051 Oph and V4140 Sgr. We combined our results with those in the literature to construct a diagram of the amplitude versus period of the modulation for a sample of 11 eclipsing cataclysmic variables (CVs). If the cyclical period changes are the consequence of a solar-type magnetic activity cycle in the secondary star, then magnetic activity is a widespread phenomenon in CVs, being equally common among long- and short-period systems. This gives independent evidence that the magnetic field (and activity) of the secondary stars of CVs do not disappear when they become fully convective. We also find that the fractional cycle period changes of the short-period CVs are systematically smaller than those of the long-period CVs.

Superhumps in Cataclysmic Binaries. XXIV. Twenty More Dwarf Novae

ABSTRACTWe report precise measures of the orbital and superhump period in 20 more dwarf novae. For 10 stars, we report new and confirmed spectroscopic periods—signifying the orbital period Po—as well as the superhump period Psh. These are GX Cas, HO Del, HS Vir, BC UMa, RZ Leo, KV Dra, KS UMa, TU Crt, QW Ser, and RZ Sge. For the remaining 10, we report a medley of Po and Psh measurements from photometry; most are new, with some confirmations of previous values. These are KV And, LL And, WX Cet, MM Hya, AO Oct, V2051 Oph, NY Ser, KK Tel, HV Vir, and RX J1155.4−5641.Periods, as usual, can be measured to high accuracy, and these are of special interest since they carry dynamical information about the binary. We still have not quite learned how to read the music, but a few things are clear. The fractional superhump excess ε[= (Psh - Po)/Po] varies smoothly with Po. The scatter of the points about that smooth curve is quite low, and can be used to limit the intrinsic scatter in M1, the white dwarf mass, and the mass‐radius relation of the secondary. The dispersion in M1 does not exceed 24%, and the secondary‐star radii scatter by no more than 11% from a fixed mass‐radius relation. For the well‐behaved part of ε(Po) space, we estimate from superhump theory that the secondaries are 18% ± 6% larger than theoretical ZAMS stars. This affects some other testable predictions about the secondaries: at a fixed Po, it suggests that the secondaries are (compared with ZAMS predictions) 40% ± 14% less massive, 12% ± 4% smaller, 19% ± 6% cooler, and less luminous by a factor of 2.5(7). The presence of a well‐defined mass‐radius relation, reflected in a well‐defined ε(Po) relation, strongly limits effects of nuclear evolution in the secondaries.

A Recent Spectroscopic Study of V841 Ophiuchi

Extensive time-resolved, spectrophotometric observations of the Hα and He I λ6678 emission-line profiles in the old nova V841 Oph are presented and analyzed. The orbital period of this system was measured as 0.60144 (±56) days. Spectral signatures from the companion star were detected in the continuum around Hα and constraints on stellar masses and orbital parameters are derived on the basis of a statistical analysis of the emission and absorption radial velocities. Doppler tomography of the emission profiles is performed and disk radial emissivity functions are computed for Hα and He I lines.

The systemic velocities of four long-period cataclysmic variable stars

Although a large number of orbital periods of cataclysmic variable stars (CVs) have been measured, comparison of period and luminosity distributions with evolutionary theory is affected by strong selection effects. A test has been discovered which is independent of these selection effects and is based upon the kinematics of CVs (Kolb & Stehle, 1996). If the standard models of evolution are correct then long-period (P_orb > 5 hrs) CVs should be typically less than 1.5 Gyr old, and their line-of-sight velocity dispersion ($\sigma_\gamma$) should be small. We present results from a pilot study which indicate that this postulate is indeed true. Four long-period dwarf novae (EM Cyg, V426 Oph, SS Cyg and AH Her) were observed over a complete orbit, in order that accurate radial velocities be obtained. We find values of -1.7, 5.4, 15.4 and 1.8 km/s with uncertainties of order 3 km/s, referred to the dynamical Local Standard of Rest (LSR), leading to a dispersion of ~ 8 km/s. Calculation of a 95 per cent confidence interval gives the result 4 < \sigma_\gamma < 28 km/s compared to a prediction of 15 km/s. We also have an improved determination of mass donor spectral type, K_2 and q for the four systems.

Superhumps in Cataclysmic Binaries. XXIII. V442 Ophiuchi and RX J1643.7+3402

ABSTRACTWe report the results of long observing campaigns on two novalike variables: V442 Ophiuchi and RX J1643.7+3402. These stars have high‐excitation spectra, complex line profiles signifying mass loss at particular orbital phases, and similar orbital periods (respectively, 0.12433 and 0.12056 days). They are well‐credentialed members of the SW Sex class of cataclysmic variables. Their light curves are also quite complex. V442 Oph shows periodic signals with periods of 0.12090(8) and 4.37(15) days, and RX J1643.7+3402 shows similar signals at 0.11696(8) and 4.05(12) days. We interpret these short and long periods, respectively, as a “negative superhump” and the wobble period of the accretion disk. The superhump could then possibly arise from the heating of the secondary (and structures fixed in the orbital frame) by inner‐disk radiation, which reaches the secondary relatively unimpeded since the disk is not coplanar. At higher frequencies, both stars show another type of variability: quasi‐periodic oscillations with a period near 1000 s. Underlying these strong signals of low stability may be weak signals of higher stability. Similar quasi‐periodic oscillations, and negative superhumps, are quite common features in SW Sex stars. Both can in principle be explained by ascribing strong magnetism to the white dwarf member of the binary; and we suggest that SW Sex stars are borderline AM Herculis binaries, usually drowned by a high accretion rate. This would provide an ancestor channel for AM Hers, whose origin is still mysterious.

A classical nova, V2487 Oph 1998, seen in x-rays before and after its explosion.

Classical nova explosions are very energetic and frequent phenomena caused by explosive hydrogen burning on top of an accreting white dwarf. Observations of the recent nova V2487 Oph 1998 by the X-ray Multi-Mirror satellite (XMM-Newton) provide evidence that accretion (probably on a magnetic white dwarf) was reestablished as early as 2.7 years after the explosion. In addition, positional correlation with a source previously discovered by the Röntgen Satellite (ROSAT) in 1990 suggests that the site of a nova explosion had been seen in x-rays before the outburst.

The disc evolution of V2051 Oph on decline from superoutburst

We present an Eclipse Mapping analysis of ten eclipses taken during decline from superoutburst of the dwarf nova V2051 Oph. On decline from superoutburst the disc cools down considerably from nearly 50 000 K in the intermediate disc (∼0.2 RL1) near maximum to about 25 000 K at the end of our observing run, i.e. within four days. The average mass accretion rate through the disc drops in the same time from 1018 g s−1 to below 1017 g s−1. While in some maps the brightness temperature follows the steady-state model, in others the temperature profile shows flattenings and/or an indication of an inward travelling cooling front with a speed of approximately −0.12 km s−1, possibly a reflected heating front with a speed of +1.8 km s−1 and a newly reflected cooling front with the same speed as the first. Such a scenario has been predicted but has not been observed before. Furthermore, we see a prograde precession of the enlarged disc with a precession period of about 52.5 h in very good agreement with the independently determined superhump period observed by Kiyota and Kato. At the same time, the uneclipsed component — presumably a disc wind — decreases significantly in strength during decline from superoutburst.

Radial Velocity Studies of Close Binary Stars. VI.

Radial velocity measurements and sine-curve fits to the orbital velocity variations are presented for the sixth set of 10 close binary systems: SV Cam, EE Cet, KR Com, V410 Cyg, GM Dra, V972 Her, ET Leo, FS Leo, V2388 Oph, and II UMa. All systems except FS Leo are double-lined spectroscopic binaries. The type of FS Leo is unknown, while SV Cam is a close, detached binary; all remaining systems are contact binaries. Eight binaries (all except SV Cam and V401 Cyg) are the recent photometric discoveries of the Hipparcos satellite project. Five systems, EE Cet, KR Com, V401 Cyg, V2388 Oph, and II UMa, are members of visual/spectroscopic triple systems. We were able to observe EE Cet separately from its companion, but in the remaining four triple systems we could separate the spectral components only through the use of the broadening-function approach. Several of the studied systems are prime candidates for combined light and radial velocity synthesis solutions.