The analysis of indexed astronomical time series – X. Significance testing of O − C data

It is assumed that O − C (‘observed minus calculated’) values of periodic variable stars are determined by three processes, namely measurement errors, random cycle-to-cycle jitter in the period, and possibly long-term changes in the mean period. By modelling the latter as a random walk, the covariances of all O − C values can be calculated. The covariances can then be used to estimate unknown model parameters, and to choose between alternative models. Pseudo-residuals which could be used in model fit assessment are also defined. The theory is illustrated by four applications to spotted stars in eclipsing binaries.Web of Scienc

Predicting gamma-ray burster redshifts from their prompt emission properties

This paper presents a study of the relation between gamma-ray burster redshift and several of its prompt emission properties (spectral lag, light curve variability, peak energy, rise time and the peak bolometric flux Pbol). Simple predictors of log(1 + z) in terms of Pbol and the other indicators are derived. The typical scatter in the relations is 0.13 dex. An illustrative application to observations of GRB 090423 (z ∼ 8) is given.Web of Scienc

Multicolour time series photometry of 2MASS 1207−3932

Time series photometry in VRI of the young brown dwarf Two-Micron All-Sky Survey (2MASS) J1207334−393254 is presented. Variability in I is insignificant, but amplitudes up to ∼0.2 mag in R, and ∼0.55 mag in V, are seen within a single night. There is also microvariability on time-scales of an hour or so, which appears to be present in data from all the three filters. There may be a non-sinusoidal periodicity of about half a day in the V and R data. A likely explanation for the variability is the presence of accretion hot spots.Web of Scienc

The analysis of indexed astronomical time series – XII. The statistics of oversampled Fourier spectra of noise plus a single sinusoid

With few exceptions, theoretical studies of periodogram properties focus on pure noise time series. This paper considers the case in which the time series consists of noise together with a single sinusoid, observed at regularly spaced time points. The distribution of the periodogram ordinates in this case is shown to be of exponentially modified Gaussian form. Simulations are used to demonstrate that if the periodogram is substantially oversampled (i.e. calculated in a dense grid of frequencies), then the distribution of the periodogram maxima can be accurately approximated by a simple form (at least at moderate signal-to-noise ratios). This result can be used to derive a calculation formula for the probability of correct signal frequency identification at given values of the time series length and (true) signal-to-noise ratio. A set of curves is presented which can be used to apply the theory to, for example, asteroseismic data. An illustrative application to Kepler data is given.Web of Scienc

Multicolour time series photometry of the variable star 1SWASP J234401.81−212229.1

1SWASP J234401.81-212229.1 may be one of a handful of contact binaries comprising two Mdwarfs. Modelling of the available observations is complicated by the fact that the radiation of the eclipsing system is dominated by a third star, a K dwarf. New photometry, presented in this paper, strengthens this interpretation of the data. The existence of such systems will have implications for the statistical distributions of masses in hierarchical multiple star systems.Web of Scienc

Fitting sinusoids to observations of rotating spotted stars

It is assumed that K blocks (e.g. seasons) of observations are available, and the parameters characterizing a fixed-frequency sinusoidal variability (mean light level, amplitude, phase) are constant within each of the blocks. The paper is concerned with estimation when any combination of these parameters varies between blocks of observations. This allows observations subject to changes in mean light level, spot sizes and/or spot locations to be modelled. Objective choices between competing models, and the calculation of the standard errors of model parameters, are also dealt with. Illustrative applications to simulated and real data are given.Web of Scienc

Correcting CCD photometry of stars for seeing effects

Systematic variability in stellarmagnitudes, as derived from profile fitting to CCD images,may in some instances be due to variable seeing. It is suggested that this happens in cases where the stars are unresolved pairs, typically with sub-arcsecond separation between the components. It is shown that the fitting of suitable Generalised Additive Models to time series photometry can disentangle intrinsic stellar variability and seeing-induced brightness changes. It is possible that there will be a fixed seeing response associated with a given star which exhibits the effect: estimation of this response from several long photometric runs is demonstrated.Web of Scienc

Two large amplitude slowly pulsating hot subdwarf stars

The discovery observations of two new slowly pulsating subdwarf B stars (V1093 Her stars), BPS CS22890−74 and PB 7032, are presented. Both stars are highly multiperiodic, and have – for V1093 Her stars – large amplitudes. Published descriptions of the (wavelength) spectra of the stars conflict, but new spectra show that both are sdB stars with effective temperatures below 30 000 K. BPS CS22890−74 has a cool companion.Web of Scienc

HE0230–4323: an unusual pulsating hot subdwarf star

HE 0230−4323 is a known binary, consisting of a subdwarf star and a companion which is not observable in the optical. Photometric measurements reported in this paper have shown it to be both a reflection-effect and a pulsating variable. The dominant pulsation frequencies changed over the course of several nights of observing, from ∼32–39 d−1 to ∼8–16 d−1. Observations were obtained through B and V filters, and the variations in the two wavebands appear to be approximately 180◦ out of phase.Web of Scienc

IC and RC band time-series observations of some bright ultracool dwarfs

The results of more than 100 h of CCD photometric monitoring of 10 L dwarfs and two T dwarfs are presented. Reasonable evidence is shown for IC band variability in DENIS 0255−4700, DENIS 1705−0516 and 2M 2104−1037. Substantial brightening of the T dwarf binary Indi Bab was observed over the course of 3.6 h; it is unlikely that this could have been caused by surface spots. No unequivocal variability could be detected in the RC band.Web of Scienc