Power density spectrum of NGC 5548 and the nature of its variability

We derive power density spectra in the optical and X-ray band in the timescale range from several years down to $\sim$ a day. We suggest that the optical power density spectrum consists of two separate components: long timescale variations and short timescale variations, with the dividing timescale around 100 days. The shape of the short timescale component is similar to X-ray power density spectrum which is consistent with the interpretation of short timescale optical variations being caused by X-ray reprocessing. We show that the observed optical long timescale variability is consistent with thermal pulsations of the accretion disc.Comment: Accepted for publication in The Monthly Notices of the Royal Astronomical Societ

Time-resolved multicolour photometry of bright B-type variable stars in Scorpius

The first two of a total of six nano-satellites that will constitute the BRITE-Constellation space photometry mission have recently been launched successfully. In preparation for this project, we carried out time-resolved colour photometry in a field that is an excellent candidate for BRITE measurements from space. We acquired 117 h of Stromgren uvy data during 19 nights. Our targets comprised the Beta Cephei stars Kappa and Lambda Sco, the eclipsing binary Mu 1 Sco, and the variable super/hypergiant Zeta 1 Sco. For Kappa Sco, a photometric mode identification in combination with results from the spectroscopic literature suggests a dominant (l, m) = (1, -1) Beta Cephei-type pulsation mode of the primary star. The longer period of the star may be a rotational variation or a g-mode pulsation. For Lambda Sco, we recover the known dominant Beta Cephei pulsation, a longer-period variation, and observed part of an eclipse. Lack of ultraviolet data precludes mode identification for this star. We noticed that the spectroscopic orbital ephemeris of the closer pair in this triple system is inconsistent with eclipse timings and propose a refined value for the orbital period of the closer pair of 5.95189 +/- 0.00003 d. We also argue that the components of the Lambda Sco system are some 30% more massive than previously thought. The binary light curve solution of Mu 1 Sco requires inclusion of the irradiation effect to explain the u light curve, and the system could show additional low amplitude variations on top of the orbital light changes. Zeta 1 Sco shows long-term variability on a time scale of at least two weeks that we prefer to interpret in terms of a variable wind or strange mode pulsations.Comment: 7 pages, 7 figures, 3 Tables, accepted by A&

Efficient analysis in planet transit surveys

With the growing number of projects dedicated to the search for extrasolar planets via transits, there is a need to develop fast, automatic, robust methods with a statistical background in order to efficiently do the analysis. We propose a modified analysis of variance (AoV) test particularly suitable for the detection of planetary transits in stellar light curves. We show how savings of labor by a factor of over 10 could be achieved by the careful organization of computations. Basing on solid analytical statistical formulation, we discuss performance of our and other methods for different signal-to-noise and number of observations.Comment: 7 pages, to be published in MNRAS, downloadable software from http://www.camk.edu.pl/~alex/#softwar

Photometric study of the variable star population in the globular cluster NGC 6397

We present the results of a photometric survey for variable stars in the central region of the nearby globular cluster NGC 6397.Time series photometry was obtained for 30 variable objects. The sample includes 12 new objects, of which 6 show periodic lightcurves and 2 are eclipsing binaries of unknown period. Six variables possess certain and three possess likely X-ray counterparts detected with the Chandra observatory. Among them four are cataclysmic variables and one is a foreground eclipsing binary. The cataclysmic variable CV2 exhibited a likely dwarf nova type outburst in May 2003. The cataclysmic variable CV3 was observed at 18.5<V<20.0 during 5 observing runs, but went into a low state in May 2003 when it reached V>22. We have found that thelight curve of the optical companion to the millisecond pulsar PSRJ1740-5340 exhibits noticeable changes of its amplitude on a time scale of a few months. A shallow eclipse with Delta_V=0.03 mag was detected in one of the cluster turnoff stars suggesting the presence of a large planet or brown dwarf in orbit.Comment: 8 pages, 9 figures, submitted to MNRAS on 21 April 200

Assessing statistical significance of periodogram peaks

The least-squares (or Lomb-Scargle) periodogram is a powerful tool which is used routinely in many branches of astronomy to search for periodicities in observational data. The problem of assessing statistical significance of candidate periodicities for different periodograms is considered. Based on results in extreme value theory, improved analytic estimations of false alarm probabilities are given. They include an upper limit to the false alarm probability (or a lower limit to the significance). These estimations are tested numerically in order to establish regions of their practical applicability.Comment: 7 pages, 6 figures, 1 table; To be published in MNRA

The Lick Planet Search: Detectability and Mass Thresholds

We analyse 11 years of precise radial velocities for 76 solar type stars from the Lick survey. Eight stars in this sample have previously reported planetary-mass companions, all with mass (m sin i) less than 8 Jupiter masses (MJ). For the stars without a detected companion, we place upper limits on possible companion mass. For most stars, we can exclude companions with m sin i > 0.7 MJ (a/AU)^1/2 for orbital radii a < 5 AU. We use our results to interpret the observed masses and orbital radii of planetary-mass companions. For example, we show that the finite duration of the observations makes detection of Jupiter mass companions more and more difficult for orbital radii beyond 3 AU. Thus it is possible that the majority of solar type stars harbor Jupiter-mass companions much like our own, and if so these companions should be detectable in a few years. To search for periodicities, we adopt a "floating-mean" periodogram, which improves on the traditional Lomb-Scargle periodogram by accounting for statistical fluctuations in the mean of a sampled sinusoid. We discuss in detail the normalization of the periodogram, an issue which has been of some debate in the literature.Comment: To appear in the Astrophysical Journal (50 pages, LaTeX, including 11 figures

On the orbital period of the cataclysmic variable RZ Leonis

In this research note we present a time-resolved study of the Balmer emission lines of RZ Leo. From the analysis of the radial velocities we find an orbital period of 0.07651(26) d. This is in excellent agreement with the photometrically determined periods in quiescence and during the early stages of superoutburst. A comparison of the recently determined superhump period gives an excess of ~0.03, which is a typical value for an SU UMa star of this period.Comment: 3 pages, 6 figures, A&A, accepte

CCD Photometry of Faint Variable Stars in the Globular Cluster NGC 6752

We present the results of a photometric survey for variable stars in the field of the nearby globular cluster NGC 6752. The cluster was monitored in 1996 and 1997 for a total of 54 hours with 3 different CCD cameras mounted on the 1.0-m Swope telescope. Eleven new variables were identified: 3 SX Phe stars, 7 contact binaries and 1 candidate detached eclipsing binary. All 3 SX Phe variables are likely members of the cluster while only 1 out of the 7 contact binaries is a potential cluster member. As a by-product of our survey we obtained UBV photometry for a large sample of stars in the cluster field. Two stars with U-B \approx -1.0 and V=19.3 and V=20.6 were identified. They lie along the extended horizontal branch of the cluster, and are likely to be faint sdB stars from NGC 6752.Comment: 18 pages, LaTex, 9 figures (Fig. 1 not available), accepted for publication in the Astronomical Journa