M. Váradi

University of Geneva, Genève, Geneva, Switzerland

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Publications (29)55.18 Total impact

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    ABSTRACT: We propose a robust principal component analysis (PCA) framework for the exploitation of multi-band photometric measurements in large surveys. Period search results are improved using the time series of the first principal component due to its optimized signal-to-noise ratio.The presence of correlated excess variations in the multivariate time series enables the detection of weaker variability. Furthermore, the direction of the largest variance differs for certain types of variable stars. This can be used as an efficient attribute for classification. The application of the method to a subsample of Sloan Digital Sky Survey Stripe 82 data yielded 132 high-amplitude Delta Scuti variables. We found also 129 new RR Lyrae variables, complementary to the catalogue of Sesar et al., 2010, extending the halo area mapped by Stripe 82 RR Lyrae stars towards the Galactic bulge. The sample comprises also 25 multiperiodic or Blazhko RR Lyrae stars.
    Monthly Notices of the Royal Astronomical Society 03/2012; 424(4). · 5.52 Impact Factor
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    ABSTRACT: CZ Lac was observed with a photometer attached to the 60-cm telescope of the Konkoly Observatory, Svabhegy, Budapest, through B and V filters on seven nights in 1967 (table A1), and 2 seasons between 2004 and 2005. We observed CZ Lac with the same, refurbished and automated 60-cm telescope equipped with a Wright Instruments 750x1100 CCD camera (field of view 17x24arcmin) in 2004 and 2005. About 29 000 frames were obtained in BV(RI)C bands. The data of the two seasons spanned 146 and 84d. (8 data files).
    VizieR Online Data Catalog. 09/2011;
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    ABSTRACT: CCD observations were obtained with the automated 60cm telescope of the Konkoly Observatory, Svabhegy, Budapest, equipped with a Wright Instruments 750x1100 CCD camera and BVIC filters. Measurements were taken on 81 nights between 2007 July and 2008 September. (4 data files).
    VizieR Online Data Catalog. 04/2011; 739:70350.
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    ABSTRACT: ABSTRACTA thorough analysis of the multicolour CCD observations of the RRab-type variable, CZ Lacertae, is presented. The observations were carried out in two consecutive observing seasons in 2004 and 2005 within the framework of the Konkoly Blazhko Survey of bright, northern, short-period RRab variables. The O − C variation of CZ Lac indicated that a significant period decrease took place just around the time of the CCD observations. Our data gave a unique opportunity to study the related changes in the pulsation and modulation properties of a Blazhko star in detail. Two different period components (≈ 14.6 and ≈ 18.6 d) of the Blazhko modulation were identified. Both modulation components had similar strength. The periods and amplitudes of the modulations changed significantly from the first season to the next, while the mean pulsation amplitude decreased slightly. The modulation frequencies were in a 5:4 resonance ratio in the first observing season, and then the frequencies shifted in opposite directions, and their ratio was close to the 4:3 resonance in the next season. The interaction of the two modulations caused beating with a period of 74 d in the first season, which resembled the 4-yr-long cycle of the ≈ 40-d modulation of RR Lyr. The mean values of the global physical parameters and their changes with the Blazhko phase of both modulation components were determined by the inverse photometric method.
    Monthly Notices of the Royal Astronomical Society 11/2010; 411(3):1585 - 1596. · 5.52 Impact Factor
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    ABSTRACT: Our goal is to assess Gaia's performance on the period recovery of short period (p < 2 hours) and small amplitude variability. To reach this goal first we collected the properties of variable stars that fit the requirements described above. Then we built a database of synthetic light-curves with short period and low amplitude variability with time sampling that follows the Gaia nominal scanning law and with noise level corresponding to the expected photometric precision of Gaia. Finally we performed period search on the synthetic light-curves to obtain period recovery statistics. This work extends our previous period recovery studies to short period variable stars which have non-stationary Fourier spectra. Comment: 6 pages, 2 figures, The proceedings of the "Gaia: at the frontiers of astrometry" conference will be published by EDP Sciences in the EAS Series. Editors: C. Turon, F. Arenou and F. Meynadier
    EAS Publications Series 11/2010;
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    ABSTRACT: We present more than 1000-day long photometry of EY Draconis in BV(RI)C passbands. The changes in the light curve are caused by the spottedness of the rotating surface. Modelling of the spotted surface shows that there are two large active regions present on the star on the opposite hemispheres. The evolution of the surface patterns suggests a flip-flop phenomenon. Using Fourier analysis, we detect a rotation period of P_rot=0.45875d, and an activity cycle with P~350d, similar to the 11-year long cycle of the Sun. This cycle with its year-long period is the shortest one ever detected on active stars. Two bright flares are also detected and analysed.
    Astronomische Nachrichten 01/2010; · 1.40 Impact Factor
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    ABSTRACT: The ESA Gaia mission will provide a multi-epoch database for a billion of objects, including variable objects that comprise stars, active galactic nuclei and asteroids. We highlight a few of Gaia’s properties that will benefit the study of variable objects, and illustrate with two examples the work being done in the preparation of the data processing and object characterization. The first example relates to the analysis of the nearly simultaneous multi-band data of Gaia with the Principal Component Analysis techniques, and the second example concerns the classification of Gaia time series into variability types. The results of the ground-based processing of Gaia’s variable objects data will be made available to the scientific community through the intermediate and final ESA releases throughout the mission.
    EAS Publications Series 12/2009; 45:161 - 166.
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    ABSTRACT: The Gaia satellite, to be launched in 2012, will offer an unprecedented survey of the whole sky down to magnitude 20. The multi-epoch nature of the mission provides a unique opportunity to study variable sources with their astrometric, photometric, spectro-photometric and radial velocity measurements. Many tens of millions of classical variable objects are expected to be detected, mostly stars but also QSOs and asteroids. The high number of objects observed by Gaia will enable statistical studies of populations of variable sources and of their properties. But Gaia will also allow the study of individual objects to some depth depending on their variability types, and the identification of potentially interesting candidates that would benefit from further ground based observations by the scientific community. Within the Gaia Data Processing and Analysis Consortium (DPAC), which is subdivided into 9 Coordination Units (CU), one (CU7) is dedicated to the variability analysis. Its goal is to provide information on variable sources for the Gaia intermediate and final catalogue releases. Comment: 4 pages, 1 figure, Annual meeting of the SF2A
    12/2009;
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    ABSTRACT: A systematic survey to establish the true incidence rate of the Blazhko modulation among short-period, fundamental-mode, Galactic field RR Lyrae stars has been accomplished. The Konkoly Blazhko Survey (KBS) was initiated in 2004. Since then more than 750 nights of observation have been devoted to this project. A sample of 30 RRab stars was extensively observed, and light-curve modulation was detected in 14 cases. The 47% occurrence rate of the modulation is much larger than any previous estimate. The significant increase of the detected incidence rate is mostly due to the discovery of small-amplitude modulation. Half of the Blazhko variables in our sample show modulation with so small amplitude that definitely have been missed in the previous surveys. We have found that the modulation can be very unstable in some cases, e.g. RY Com showed regular modulation only during one part of the observations while during two seasons it had stable light curve with abrupt, small changes in the pulsation amplitude. This type of light-curve variability is also hard to detect in other Survey's data. The larger frequency of the light-curve modulation of RRab stars makes it even more important to find the still lacking explanation of the Blazhko phenomenon. The validity of the [Fe/H](P,phi_{31}) relation using the mean light curves of Blazhko variables is checked in our sample. We have found that the formula gives accurate result for small-modulation-amplitude Blazhko stars, and this is also the case for large-modulation-amplitude stars if the light curve has complete phase coverage. However, if the data of large-modulation-amplitude Blazhko stars are not extended enough (e.g. < 500 data points from < 15 nights), the formula may give false result due to the distorted shape of the mean light curve used. Comment: Accepted for publication in MNRAS, 14 pages, 7 Figures
    Monthly Notices of the Royal Astronomical Society 08/2009; · 5.52 Impact Factor
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    ABSTRACT: A long-term project, aiming at systematic search for variable stars in Galactic Open Clusters (OCs), was started at the Geneva Observatory in 2002. We have been observing regularly a sample of twenty-seven Galactic Open Clusters in the U, B, V Geneva filters (hereafter U, B, V). The goal is to identify and to study their variable stars, as well as the connection between the variable stars in a cluster and the cluster properties. We present the status of this work in progress, and show preliminary results for one of these clusters, IC 4651.
    Proceedings of the International Astronomical Union 07/2009; 5:87 - 89.
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    ABSTRACT: We analyzed the frequency domain of time series of simulated ZZ Ceti light-curves to investigate the detectability and period recovery performance of short period variables (periods < 2 hours) for the Gaia mission. In our analysis, first we used a non-linear ZZ Ceti light-curves simulator code to simulate the variability of ZZ Ceti stars (we assumed stationary power spectra over five years). Second we used the Gaia nominal scanning law and the expected photometric precision of Gaia to simulate ZZ Ceti time series with Gaia's time sampling and photometric errors. Then we performed a Fourier analysis of these simulated time series. We found that a correct period can be recovered in ~65% of the cases if we consider Gaia per CCD time series of a G ~ 18 magnitude multiperiodic ZZ Ceti star with 5%-10% light-curve variation. In the pre-whitened power spectrum a second correct period was also recovered in ~26% of the cases. Comment: 3 pages, 2 figures, To appear in the proceedings of Stellar Pulsation: Challenges for theory and observations Conference, 2009, Santa Fe, NM, USA, edited by J. Guzik and A. Cox
    07/2009;
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    ABSTRACT: A long-term project, aiming at systematic search for variable stars in Galactic Open Clusters, was started at the Geneva Observatory in 2002. We have been observing regularly a sample of twenty-seven Galactic Open Clusters in the U, B, V Geneva filters. The goal is to identify and to study their variable stars, as well as the connection between the variable stars in a cluster and the cluster properties. We present the status of this work in progress, and show preliminary results for one of these clusters, IC 4651. Comment: To appear in the proceedings of Stellar Pulsation: Challenges for theory and observations Conference, Santa Fe, NM, USA
    07/2009;
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    ABSTRACT: ABSTRACTDM Cyg, a fundamental mode RRab star, was observed in the 2007 and 2008 seasons in the framework of the Konkoly Blazhko Survey. Very small amplitude light-curve modulation was detected with 10.57 d modulation period. The maximum brightness and phase variations do not exceed 0.07 mag and 7 min, respectively. In spite of the very small amplitude of the modulation, beside the frequency triplets characterizing the Fourier spectrum of the light curve, two quintuplet components were also identified. The accuracy and the good phase coverage of our observations made it possible to analyse the light curves at different phases of the modulation separately. Utilizing the inverse photometric Baade–Wesselink method, we could detect very small systematic changes in the global mean physical parameters of DM Cyg during its Blazhko cycle. The detected changes are similar to what we have already found for a large modulation amplitude Blazhko variable, MW Lyrae. The amplitudes of the detected changes in the physical parameters of DM Cyg are only about 10 per cent of those that have been found in MW Lyr. This is in accordance with its small modulation amplitude being about one-tenth of the modulation amplitude of MW Lyr.The pulsation period of DM Cyg has been increasing by a rate of β= 0.091 d Myr−1 during the hundred-year time base of the observations. Konkoly archive photographic observations indicate that when the pulsation period of the variable was shorter by Δppuls= 5 × 10−6 d the modulation period was longer by Δpmod= 0.066 d than today.
    Monthly Notices of the Royal Astronomical Society 07/2009; 397(1):350 - 360. · 5.52 Impact Factor
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    ABSTRACT: The major part of the observations were obtained with the automated 60-cm telescope of the Konkoly Observatory, Svabhegy, Budapest, equipped with a Wright Instruments 750x1100 CCD camera. Measurements were taken on 177 nights between 2006 May and 2007 July. CCD observations with the 60-cm telescope of the Michigan State University equipped with an Apogee Ap47p CCD camera and with the 1-m RCC telescope of the Konkoly Observatory equipped with a Princeton Instruments VersArray 1300B CCD camera were obtained on six and three additional nights in 2006 July and in 2007 August, respectively. Johnson-Cousins BVIC filters were used in all the observations. (6 data files).
    VizieR Online Data Catalog. 03/2009;
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    ABSTRACT: The analysis of the multicolour photometric observations of MW Lyr, a large modulation amplitude Blazhko variable, shows for the first time how the mean global physical parameters vary during the Blazhko cycle. About 1-2 percent changes in the mean radius, luminosity and surface effective temperature are detected. The mean radius and temperature changes are in good accordance with pulsation model results, which show that these parameters do indeed vary within this order of magnitude if the amplitude of the pulsation changes significantly. We interpret the phase modulation of the pulsation to be a consequence of period changes. Its magnitude corresponds exactly what one expects from the detected changes of the mean radius assuming that the pulsation constant remains the same during the modulation. Our results indicate that during the modulation the pulsation remains purely radial, and the underlying mechanism is most probably a periodic perturbation of the stellar luminosity with the modulation period.
    Monthly Notices of the Royal Astronomical Society 12/2008; · 5.52 Impact Factor
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    ABSTRACT: We have obtained the most extensive and most accurate photometric data of a Blazhko variable MW Lyr during the 2006-2007 observing seasons. The data within each 0.05 phase bin of the modulation period (P_m=1/f_m) cover the entire light cycle of the primary pulsation period (P_0=1/f_0), making possible a very rigorous and complete analysis. The modulation period is found to be 16.5462 d, which is about half of that was reported earlier from visual observations. Previously unknown features of the modulation have been detected. Besides the main modulation frequency f_m, sidelobe modulation frequencies around the pulsation frequency and its harmonics appear at +/- 2 f_m, +/- 4 f_m, and +/- 12.5 f_m separations as well. Residual signals in the prewhitened light curve larger than the observational noise appear at the minimum-rising branch-maximum phase of the pulsation, which most probably arise from some stochastic/chaotic behaviour of the pulsation/modulation. The Fourier parameters of the mean light curve differ significantly from the averages of the Fourier parameters of the observed light curves in the different phases of the Blazhko cycle. Consequently, the mean light curve of MW Lyrae never matches its actual light variation. The Phi_21, Phi_31 phase differences in different phases of the modulation show unexpected stability during the Blazhko cycle. A new phenomenological description of the light curve variation is defined that separates the amplitude and phase (period) modulations utilising the phase coherency of the lower order Fourier phases. Comment: Accepted for publication in MNRAS. 15 pages, 14 figures, and 7 printed tables (2 of them available in electronic form)
    Monthly Notices of the Royal Astronomical Society 07/2008; · 5.52 Impact Factor
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    ABSTRACT: Multicolour CCD observations of three RRab stars are presented.
    Information Bulletin on Variable Stars 07/2008; 5844:1.
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    ABSTRACT: Extended BV(RI)c photometric data of the double-mode RR Lyrae star BS Comae are presented. The observations were obtained with the automated 60 cm telescope of Konkoly Observatory, Svabhegy, Budapest, equipped with a Wright 750x1100 CCD, between 2005 March 1 and 2006 April 17. BS Com magnitudes are measured relative to BD+24 2598 by aperture photometry. (4 data files).
    VizieR Online Data Catalog. 04/2008;
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    ABSTRACT: We derive the basic physical parameters of the field double-mode RR Lyrae star BSCom from its observed periods and the requirement of consistency between the pulsational and evolutionary constraints. By using the current solar-scaled horizontal branch evolutionary models of Pietrinferni et al. and our linear non-adiabatic purely radiative pulsational models, we get M/Msolar = 0.698 +/- 0.004, log(L/Lsolar) = 1.712 +/- 0.005, Teff = 6840 +/- 14K, [Fe/H] = -1.67 +/- 0.01, where the errors are standard deviations assuming uniform age distribution along the full range of uncertainty in age. The last two parameters are in a good agreement with the ones derived from the observed BVIC colours and the updated ATLAS9 stellar atmosphere models. We get Teff = 6842 +/- 10K, [Fe/H] = -1.58 +/- 0.11, where the errors are purely statistical ones. It is remarkable that the derived parameters are nearly independent of stellar age at early evolutionary stages. Later stages, corresponding to the evolution towards the asymptotic giant branch, are most probably excluded because the required high temperatures are less likely to satisfy the constraints posed by the colours. We also show that our conclusions are only weakly sensitive to non-linear period shifts predicted by current hydrodynamical models.
    Monthly Notices of the Royal Astronomical Society 01/2008; 386:521-530. · 5.52 Impact Factor
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    ABSTRACT: Extended BV(RI)C CCD observations of SS Cnc, a short-period RRab star, are presented. Nearly 1400 data points in each band have been obtained, spanning over 79 days during the spring of 2005. The star exhibits light-curve modulation, the so-called Blazhko effect, with small amplitude (B maximum brightness varies by 0.1 mag) and with the shortest modulation period (5.309 days) ever observed. In the Fourier spectrum of the V light curve, the pulsation frequency components are detected up to the 24th harmonic order, and modulation sidelobe frequencies with significantly asymmetric amplitudes are seen up to the 15th and 9th orders for the lower and higher frequency components, respectively. A detailed comparison of the modulation behavior of SS Cnc and RR Gem, two recently discovered small-amplitude, short-modulation-period Blazhko stars, is presented. The modulation frequency (fm) appears in the Fourier spectrum of both stars with similar amplitude. We also demonstrate that the modulation frequencies have basically different properties from those of the pulsation and modulation sidelobe frequencies, indicating that the physics behind these frequency components is not the same. The discovery of small amplitude modulations of RRab stars cautions that the large photometric surveys (MACHO and OGLE) may seriously underestimate the number of modulated RR Lyrae stars.
    The Astronomical Journal 12/2007; 132(1):61. · 4.97 Impact Factor

Publication Stats

238 Citations
55.18 Total Impact Points

Institutions

  • 2008–2012
    • University of Geneva
      • Department of Astronomy
      Genève, Geneva, Switzerland
  • 2010
    • Pontifical Catholic University of Chile
      • Instituto de Astrofísica
      Santiago, Region Metropolitana de Santiago, Chile
  • 2008–2010
    • Hungarian Academy of Sciences
      Budapeŝto, Budapest, Hungary
  • 2007
    • Konkoly Observatory
      Budapeŝto, Budapest, Hungary