Light Curve Changes Research Articles

Period study of TW Draconis

Context. TW Draconis is one of the most well known Algol-type eclipsing binaries. There is significant evidence for miscellaneous interacting physical processes in the system, which manifest themselves as for example period and light curve changes. We study time series data for the star accumulated over the past 150 years to detect changes in minima timings.

Observational evidence of interacting processes in the system TW Draconis

TW Draconis is one of the best studied Algol-type eclipsing binaries. There is significant evidence for miscellaneous interacting physical processes between binary components and these are manifested in period and light curve changes. We studied these processes on the basis of a long-time series of photometric and spectroscopic observations.

Multiperiodic Galactic field RR Lyrae stars in the ASAS catalogue

The All Sky Automated Survey (ASAS) monitors bright stars (8 < V < 14 mag) south of declination +28 ◦ . The ASAS Catalogue of Variable Stars (ACVS) presently contains 50 099 objects; among them are 2212 objects classified as RR Lyrae pulsating variables. We use ASAS photometric V-band data to search for multiperiodicity in those stars. We find that 73 of 1435 RRab stars and 49 of 756 RRc stars exhibit the Blazhko effect. We observe a deficiency of RRab Blazhko variables with main pulsation periods greater than 0.65 d. The Blazhko periods of RRc stars exhibit a strongly bimodal distribution. During our study we discovered the Blazhko effect with multiple periods in object ASAS 050747−3351.9 = SU Col. Blazhko periods of 89.3 and 65.8 d and a candidate of 29.5 d were identified with periodogram peaks near the first three harmonics of the main pulsation. These observations may inspire new models of the Blazhko effect, which has eluded a consistent theory since its discovery about one hundred years ago. Long-term light curve changes were found in 29 stars. We also found 19 Galactic double mode pulsators (RRd), of which four are new discoveries, raising the number of ASAS discoveries of such objects to 16, out of 27 known in the field of our Galaxy.

RR Lyrae stars in the southern globular cluster NGC 362

NGC 362 is a bright southern globular cluster for which no extensive variability survey has ever been done. Time-series CCD photometric observations have been obtained. Light curves have been derived with both profile fitting photometry and image subtraction. We developed a simple method to convert flux phase curves to magnitudes, which allows the use of empirical light curve shape vs. physical parameters calibrations. Using the RR Lyrae metallicity and luminosity calibrations, we have determined the relative iron abundances and absolute magnitudes of the stars. The color-magnitude diagram has been fitted with Yale-Yonsei isochrones to determine reddening and distance independently. For five RR Lyrae stars we obtained radial velocity measurements from optical spectra. We found 45 RR Lyr stars, of which the majority are new discoveries. About half of the RR Lyraes exhibit light curve changes (Blazhko effect). The RR Lyrae-based metallicity of the cluster is [Fe/H]=-1.16 +/- 0.25, the mean absolute magnitude of the RR Lyrae stars is M_V=0.82 +/- 0.04 mag implying a distance of 7.9 +/- 0.6 kpc. The mean period of RRab stars is 0.585 +/- 0.081 days. These properties place NGC 362 among the Oosterhoff type I globular clusters. The isochrone fit implies a slightly larger distance of 9.2 +/- 0.5 kpc and an age of 11 +/- 1 Gyr. We also found 11 eclipsing binaries, 14 pulsating stars of other types, including classical Cepheids in the SMC and 15 variable stars with no firm classification.

Low phase angle effects in photometry of trans-neptunian objects: 20000 Varuna and 19308 (1996 TO 66)

We present the results of photometric observations of trans-neptunian object 20000 Varuna, which were obtained during 7 nights in November 2004–February 2005. The analysis of new and available photometric observations of Varuna reveals a pronounced opposition surge at phase angles less than 0.1 deg with amplitude of 0.2 mag relatively to the extrapolation of the linear part of magnitude–phase dependence to zero phase angle. The opposition surge of Varuna is markedly different from that of dark asteroids while quite typical for moderate albedo Solar System bodies. We find an indication of variations of the scattering properties over Varuna's surface that could result in an increase of the lightcurve amplitude toward zero phase angle. It is shown that a similar phase effect can be responsible for lightcurve changes found for TNO 19308 (1996 TO 66) in 1997–1999.

X-ray outburst of 4U 0115+634 and ROTSE observations of its optical counterpart V635 Cas

ROTSE IIId (The Robotic Optical Transient Experiment) observations of X-ray binary system 4U 0115+634/V635 Cas obtained during 2004 June and 2005 January make possible, for the first time, to study the correlation between optical and type II X-ray outbursts. The X-ray outburst sharply enhanced after periastron passage where the optical brightness was reduced by 0.3 magnitude for a few days. We interpret the sharp reduction of optical brightness as a sign of mass ejection from the outer parts of the disc of the Be star. After this sharp decrease, the optical brightness healed and reached the pre X-ray outburst level. Afterwards, gradual decrease of the optical brightness followed a minimum then a gradual increase started again. Qualitatively, change of optical lightcurve suggests a precession of the Be star disc around a few hundred days. We also investigate the periodic signatures from the archival RXTE-ASM (Rossi X-ray Timing Explorer - All Sky Monitor) light curve covering a time span of $\sim 9$ years. We find significant orbital modulation in the ASM light curve during the type I X-ray outburst.

A first-overtone RR Lyrae star with cyclic period changes

A detailed light curve analysis is presented for a first-over tone RR Lyrae star, MACHO* J050918.712 695015.31, based on MACHO and OGLE-III observations. As a foreground object of the Large Magellanic Cloud, it gives an extraordinary opportunity to study an almost continuous, 12-year long dataset of a relatively bright (V � 15. 0) RRc star with rapid period change. Cyclic period modulation is suggested by the O C method, where the cycle length is about 8 years. With the available unique dataset, we could draw strong limits on other light curve changes that may be associated to the period modulation. We could exclude both multiple periodicity and amplitude modulation unambiguously. Any theoretical model should reproduce the observed lack of photometric modulations. Simple arguments are also given for possible hydromagnetic effects.

Colour changes in quasar light curves

The study of quasar variability has long been seen as a way to understanding the structure of the central engine of active galactic nuclei, and as a means of verifying the morphology of the standard model. Much work has already been done on the statistical properties of light curves monitored in one colour, and it is now possible to use these observations to test predictions of theoretical models. The addition of a second colour can add enormously to the power of such tests, and put tight constraints on the nature of the variations. In this paper a yearly monitoring programme of several hundred quasars in blue and red passbands covering 21 years is presented. The statistics of colour changes are examined for a 15 year period of homogeneous data with Fourier power spectrum analysis, in a form suitable for testing against theoretical predictions. The results of the Fourier analysis show that there is more power in blue light on all timescales than in the red. Examination of the light curves shows several different modes of colour change. However, if allowance is made for the effects of the underlying host galaxy, the variations become close to achromatie. There are however structural differences between red and blue light curves which cannot be accounted for in this way, and various modes of variability including disc instability and microlensing are examined to provide explanations for these features.

Long-Term X-ray Variability of Circinus X-1 as Observed by the RXTE All Sky Monitor

The luminous low mass X-ray binary Cir X-1 has been observed nearly continuously for about 5 years by the X-ray All Sky Monitor on board the RXTE satellite. We carried out a timing analysis on Cir X-1 with the RXTE data. We define the period from the X-ray data, comparing the period change over time with the best current ephemeris. with folded light curves of entire data set, characteristics of the system like long-term lightcurve changes, behavior of flares and dips and super-Eddington accretion were obtained. We also checked and identified the secondary flaring reported in radio band with X-ray data.

A new two-pole accretion polar: RX J1846.9+5538

We report the discovery of a new, bright (V 17 m ) AM Her system as the optical counterpart of the soft ROSAT All-Sky-Survey source RX J1846.9+5538 (=1RXS J184659.4+553834). Optical photometric and spectroscopic follow-up ob- servations reveal a single period of 128.7 min, consistent with a high degree of spin-orbit synchronization, and a low polar field strength (B < 20 MG) of the primary accretion region. The system was observed in optical intermediate and high states that dier by about 1 mag. These brightness variations were accompanied by a correlated change of the optical light curve, which we interpret as a switch between one- and two-pole accretion. This explanation is also supported by the X-ray light curves, which at two dierent epochs display emission from two equally bright accretion regions separated by160. Both spots pos- sess distinct spectral X-ray properties as seen from the X-ray hardness ratio, where the secondary accretion region appears significantly softer, thus probably indicating a higher field strength compared to the primary region. In all ROSAT pointings a deep dip is present during the primary flux maxima, very likely caused by absorption in one of the accretion streams.

Chromospherically Active Stars. XXI. The Giant, Single-lined Binaries HD 89546 and HD 113816

We have obtained spectroscopy and photometry of the chromospherically active, single-lined spectroscopic binaries HD 89546 and HD 113816. HD 89546 has a circular orbit with a period of 21.3596 days. Its primary has a spectral type of G9 III and is somewhat metal-poor with [Fe/H] ~ -0.5. HD 113816 has an orbit with a period of 23.6546 and a low eccentricity of 0.022. Its mass function is extremely small, 0.0007 M⊙, consistent with a very low inclination. The primary is a slightly metal-poor K2 III. A decade or more of photometric monitoring with an automatic telescope demonstrates that both systems display brightness variations due to rotational modulation of the visibility of photospheric star spots, as well as light-curve changes resulting from the redistribution of star spots by differential rotation and long-term changes in the filling factor of the spots. We determined rotation periods for each season when the observations were numerous enough. Our mean rotation periods of 21.3 and 24.1 days for HD 89546 and HD 113816, respectively, confirm that the giants in each system are synchronously rotating. The orbital elements and properties of the giant components of these two systems, including levels of surface magnetic activity, are quite similar. However, the two rotational inclinations are rather different, 57° for HD 89546 and 13° for HD 113816. Thus the latter giant is seen nearly pole on. We analyzed the light curves for similarities and differences that result from viewing these two systems from quite different inclinations.

Period and Light-Curve Changes in AP Leonis

We present a detailed study of the period and light curve of the eclipsing binary AP Leo. We have investigated the period variation of the system in detail using a method proposed by Kalimeris et al. (1994a,b; AAA 061.117.039, AAA 062.117.142). The orbital period of the system exhibits a periodic variation which contains four components with different frequencies. Our result is very different from that of other investigators. We compared light curves obtained by other groups, and found that the light curve of the system has changed considerably. Based on an analysis, we investigated the physical mechanisms which may underlie the variations of the period and the light curve, and obtained some new results. We have concluded that the rapid light variation is probably caused by a pulsation of the common envelope, and that the mechanism causing the pulsation may be mass transfer between the two components, or its variation. In addition, we have found that the mass transfer and magnetic activity may simultaneously play an important role in the variations in the orbital period and the light curve of the system. The paper concludes with a discussion of the evolution of the system.

The Later Evolution of the Contact Binaries AP Leo, AK Her, AB And and AM Leo

AbstractA detailed study of the periods and light curves of binaries AK Her, AP Leo, AB And and AM Leo is presented. Based on the study of the O – C curves, we find that the period variation of each system contains several components with different frequencies, and we suggest that the periodical variations in the periods are likely influenced by different mechanisms. Based on the study of the light curve changes, we find that the light curves exhibit two kinds of variations: rapid variations and long-term variations. We investigate the physical mechanisms which may underlie the variations of the period and the light curve of each system and obtain some new conclusions. According to the characteristics of the rapid light variation in these systems, we suggest that the rapid change in the light curve is probably caused by pulsation of the common envelope, and that the mechanism(s) causing the pulsation may be mass transfer through the inner Lagrangian point L1 or its variation. Finally, the evolutionary trends of these systems are discussed, and we suggest that these systems may be progenitors of cataclysmic variables.

Light Curve Changes in AH Cancri

AbstractLight curves of the W Ursae Majoris binary AH Cancri obtained in 1973 and 1988 show a remarkable change in shape. Earlier curves have rounded minima and eclipses appear to be partial; later curves have flat-bottomed minima indicating total eclipses. The 1973 curves are consistent with a system in overcontact with an orbital inclination of approximately 65°. Such a low inclination, however, is incompatible with the -duration flat-bottomed eclipse seen in the more recent light curves. Intrigued by this puzzle we decided to make new photometric observations. We present CCD photometry in V and R obtained with the Southeastern Association for Research in Astronomy (SARA) 0.9-m and Buehler Observatory 0.4-m telescopes. We solve the new light curves and find a high-inclination (i ≈ 84°), low-mass-ratio (q ≈ 0.145) model fits the observations reasonably well. In the post-1988 light curves, limb darkening causes eclipses of the cooler star to be deeper than those of the hotter star. Times of minima suggest a possible third body interaction.

Criteria of Classification of Long Period Variables Based on their Light Curve Changes

AbstractSome numerical parameters of long-term changes of light curves in Long Period Variables (LPVs) are obtained. Results of analysis of the sample of 53 LPVs and classification of these stars are discussed.

Multiwavelength Studies of Hercules X‐1 during Short High and Anomalous Low States: On‐again, Off‐again

We present results from multiwavelength observations of the Hercules X-1 system during a short high state (SHS) and during an anomalous low state (ALS). The magnitude of deviation from spin-up appears to be positively correlated with duration of the ALS. Such a correlation is consistent with an interpreta- tion of the ALS in terms of a change in mass accretion rate that causes the disk to tilt and twist beyond the normal deviations that cause the 35 day cycle. A larger deviation from the average results in a M0 stronger disruption of the disk and causes the disk to take longer to settle back to its ii normal ˇˇ 35 day behavior. Our modelwhich includes X-ray heating of the disk and companion star, shadowing of the X-ray —ux by the disk, and a contribution to the continuum emission from the accretion stream or hot spotcan consistently explain the observed changes in X-ray, ultraviolet (UV), and optical continuum light curves for both the SHS and ALS. The Hubble Space Telescope (HST ) and Space Telescope Imaging Spectrograph (STIS) observations presented here are the —rst UV observations of sufficient spectral and temporal resolution to construct Doppler tomograms of the line emission. Doppler tomo- grams of the UV emission lines during SHS and ALS show the majority of the emission coming from the surface of the companion star rather than from the accretion disk. Tomograms made after separating the N V emission lines into broad and narrow components suggests that while the narrow component is associated with emission from the companion star, the broad component may be associated with emis- sion from a distorted disk. The Doppler maps also show that heating over the inner face of HZ Her is not uniform and imply partial eclipse of the UV line emission by an accretion stream and/or hot spot. Subject headings: accretion, accretion disksbinaries: closepulsars: individual (Hercules X-1) ¨ ultraviolet: starsX-rays: stars On-line material: color —gures

Star-Spots Studies for Three Short Period RS CVn-Type Binaries

Light curve analysis for three short-period RS CVn-type binaries were performed, using: a) Fourier analysis techniques for light curve changes in the frequency domain; b) photometric curve fit by means of numerical quadratures to develop theoretical light curves appropriate to RS CVn stars. The two methods were applied to the systems ER Vul, BH Vir, and UV Psc. Improved physical and geometrical parameters are found by taking the average of the results measured by the above two methods.

Link between Mass-loss and Variability Type for AGB Stars?

We find that AGB stars separate in the 25–12 vs. 12-K color-color diagram according to their chemistry (O, S vs. C) and variability type (Miras vs. SRb/Lb). While discrimination according to the chemical composition is not surprising, the separation of Miras from SRb/Lb variables is unexpected.We show that “standard” steady-state radiatively driven models provide excellent fits to the color distribution of Miras of all chemical types. However, these models are incapable of explaining the dust emission from O-rich SRb/Lb stars. The models can be altered to fit the data by postulating different optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical condensation temperatures (800–1000 K), a possibility which is also supported by the detailed characteristics of LRS data. While such lower temperatures are required only for O- and S-rich SRb/Lb stars, they are also consistent with the colors of C-rich SRb/Lb stars.The absence of hot dust for SRb/Lb stars can be interpreted as a recent (order of 100 yr) decrease in the mass-loss rate. The distribution of O-rich SRb/Lb stars in the 25–12 vs. K-12 color-color diagram shows that the mass-loss rate probably resumes again, on similar time scales. It cannot be ruled out that the mass-loss rate is changing periodically on such time scales, implying that the stars might oscillate between the Mira and SRb/Lb phases during their AGB evolution as proposed by Kerschbaum et al. (1996). Such a possibility appears to be supported by recent HST images of the Egg Nebula obtained by Sahai et al. (1997), the discovery of multiple CO winds reported by Knapp et al. (1998), and long-term visual light-curve changes detected for some stars by Mattei (1998).

The changing X-ray light curves of the intermediate polar FO Aquarii

Comparison of five X-ray observations of the intermediate polar FO Aqr reveals that the morphology of the X-ray light curve changes considerably with time. In particular, power spectra of the 1988 Ginga and 1993 ASCA data reveal strong sideband and orbital variations, whereas the 1990 Ginga observation does not. This suggests that the amount of stream-fed accretion varies with epoch, and the system was accreting predominantly from a disc in 1990. In contrast to other intermediate polars, the X-ray spin-pulse profiles show significant variations between observations, ranging from relatively sinusoidal to sawtooth-shaped at medium energies. During the 1988 and 1990 observations a notch is visible at spin phase zero, due to the presence of an interpulse at phase 0.85, which is absent during the other observations. At lower energies a narrow pulse of emission is seen at spin phase 0.2. We interpret the pulse profile from the 1990 Ginga observation using a model for accretion from a disc on to a dipolar magnetic field, the axis of which is offset from the white dwarf centre by ~ 0.15 white dwarf radii. In order to account for the later occurrence of the hardness-ratio maximum in 1988 and 1993, we suggest that the accretion-rate profile changes so that accretion is favoured along the field lines which trail the magnetic pole. This also accounts for the disappearance of the interpulse and notch in 1993.

An interpretation of light curves of SV Camelopardalis by the inverse-problem method

The paper is devoted to the problem of the determination of the orbital and physical parameters of the active eclipsing binary SV Cam on the basis of the in- terpretation of photometric observations made by Patk os (1982) during the period 1973-1981. The problem is solved in two stages: by obtaining a synthetic light curve in the case when the parameters of the corresponding Roche model (Djura sevi c 1992a) are given a priori (direct prob- lem), and by determining the parameters of the model for which the best t between the synthetic light curve and the observations is achieved (inverse problem) (Djura sevi c 1992b). A total of 18 light curves are analysed in the framework of the Roche model, involving one and two spotted regions on the primary component of the system (Sp G3 V), for the temperature contrast between the spot- ted area and the surrounding photosphere As = Ts=T1 = 0:65. The basic parameters of the system and of the spot- ted areas are estimated. Throughout the whole set of the analysed light curves, a double spot model ts the observa- tions satisfactorily. A single spot model yields a poorer t, where the basic system parameters obtained by analysing the individual light curves show stronger variations about a mean value. That indicates that the single spot model cannot successfully reproduce the SV Cam light curve changes during the analysed period. According to the obtained results the spotted areas are formed at high latitudes and cover a signicant part of the stellar surface. No clear cyclicity of the system's activity is noted from the analysed observations. There are some indications that spotted areas at high latitudes (above 70 o ) correspond to an enhanced activity. Since the system's period is short (P =0 : d 59), the presence of spot- ted regions at high latitudes can be explained by the dy- namo mechanism for rapid rotators (Schussler & Solanski 1992). During the analysed period the spotted areas tend to fall into a specially active longitude sectors at high lat- itudes, near stellar polar regions. Due to a selection eect Send oprint requests to :G. Djura sevi c it is possible that a more extensive observational material would correct this result to some extent. The light curve analysis allowed an estimation of the system parameters and of the active spotted regions.