Browsing Researchers in Statistics and Population Studies by Author "Koen, Chris"
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The analysis of indexed astronomical time series – X. Significance testing of O − C data
Koen, Chris (Oxford University Press, 2006)It is assumed that O − C (‘observed minus calculated’) values of periodic variable stars are determined by three processes, namely measurement errors, random cycletocycle jitter in the period, and possibly longterm ... 
The analysis of indexed astronomical time series – XI. The statistics of oversampled white noise periodograms
Koen, Chris (Oxford University Press, 2015)The distribution of the maxima of periodograms is considered in the case where the time series is made up of regularly sampled, uncorrelated Gaussians. It is pointed out that if there is no oversampling, then for large ... 
The analysis of indexed astronomical time series – XII. The statistics of oversampled Fourier spectra of noise plus a single sinusoid
Koen, Chris (Oxford University Press, 2015)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 ... 
The analysis of irregularly observed stochastic astronomical timeseries – I. Basics of linear stochastic differential equations
Koen, Chris (Oxford University Press, 2005)The theory of loworder linear stochastic differential equations is reviewed. Solutions to these equations give the continuous time analogues of discrete time autoregressive timeseries. Explicit forms for the power ... 
Confidence intervals for the correlation between the gammaray burst peak energy and the associated supernova peak brightness
Koen, Chris (Oxford University Press, 2009)A strong correlation between the gammaray burster peak energy and the peak luminosity of the associated supernova was discovered by Li for four GRBs. Despite the fact that the formal significance level of the correlation ... 
Correcting CCD photometry of stars for seeing effects
Koen, Chris (Oxford Univeristy Press, 2009)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, ... 
The detailed forms of the LMC Cepheid PL and PLC relations
Koen, Chris; Kanbur, S; Ngeow, C (Oxford University Press, 2007)Possible deviations from linearity of the Large Magellanic Cloud Cepheid period–luminosity (PL) and period–luminosity–colour (PLC) relations are investigated. Two data sets are studied, respectively from the Optical ... 
Detection of an increasing orbital period in the subdwarf B eclipsing system NSVS 14256825
Kilkenny, David; Koen, Chris (Oxford University Press, 2012)New timings of eclipses made in 2010 and 2011 are presented for the hot subdwarf B (sdB) eclipsing binary NSVS 14256825. Composed of an sdB star and a much cooler companion, with a period near 0.1104 days, this system ... 
A detection threshold in the amplitude spectra calculated from Kepler data obtained during K2 mission.
Baran, A.C.; Koen, Chris; Pokrzywka, B. (Oxford University Press, 2015)We present our analysis of simulated data in order to derive a detection threshold which can be used in the prewhitening process of amplitude spectra. In case of groundbased data of pulsating stars, this threshold is ... 
Estimation of the coherence time of stochastic oscillations from modest samples
Koen, Chris (Oxford University Press, 2012)‘Quasiperiodic’ or ‘solarlike’ oscillations can be described by three parameters – a characteristic frequency, a coherence time (or ‘quality factor’) and the variance of the random driving process. This paper is concerned ... 
Evidence for rapid evolution of periodic variations in an ultracool dwarf
Koen, Chris (Oxford University Press, 2006)The results of three short photometric monitoring runs on the L0 dwarf 2MASS J06050196−2342270, during three consecutive nights, are presented. The observations show the persistent presence of a 2.4h period, with an IC ... 
An extensive search for rapid optical variability in ultracool dwarfs
Koen, Chris (Oxford University Press, 2013)In this paper, a summary of optical timeseries photometry of 125 ultracool dwarfs is given. The observing strategy was to monitor each object continuously for 2–3 h in order to ascertain whether itwas rapidly variable. ... 
Fitting powerlaw distributions to data with measurement errors
Koen, Chris; Kondlo, L (Oxford University Press, 2009)If X, which follows a powerlaw distribution, is observed subject to Gaussian measurement error e, thenX+e is distributed as the convolution of the powerlawand Gaussian distributions. Maximumlikelihood estimation of ... 
Fitting sinusoids to observations of rotating spotted stars
Koen, Chris (Oxford University Press, 2007)It is assumed that K blocks (e.g. seasons) of observations are available, and the parameters characterizing a fixedfrequency sinusoidal variability (mean light level, amplitude, phase) are constant within each of the ... 
The “four sigma” limit for significant periodicities
Koen, Chris (Springer Verlag, 2010)Most of the pulsations which have been observed in sdB stars have very low amplitudes. Detection of modes from photometric observations is usually performed by noting the most prominent peaks in a periodogram or, ... 
Further observations of Hipparcos red stars and standards for UBV(RI)C photometry
Kilkenny, David; Koen, Chris; van Wyk, F.; Marang, F.; Cooper, D. (Oxford University Press, 2007)We present homogeneous and standardized UBV(RI)C JHK photometry for over 100 M stars selected from an earlier paper on the basis of apparent photometric constancy. L photometry has been obtained for stars brighter than ... 
HE 0230−4323 revisited: a new rapidly pulsating sdB star
Kilkenny, David; Koen, Chris; Worters, H. (Oxford University Press, 2010)HE 0230−4323 is a hot sdB star in a binary system. An earlier work demonstrated that the light curve of the system shows a strong (∼4 per cent) reflection effect and also appears to exhibit photometric variations of the ... 
HE0230–4323: an unusual pulsating hot subdwarf star
Koen, Chris (Oxford University Press, 2007)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 reflectioneffect and a ... 
IC and RC band timeseries observations of some bright ultracool dwarfs
Koen, Chris (Oxford University Press, 2005)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. ... 
Improved SAAO–2MASS photometry transformations
Koen, Chris; Marang, F.; Kilkenny, David; Jacobs, C. (Oxford University Press, 2007)Nearinfrared photometry of 599 stars is used to calculate transformations from the South African Astronomical Observatory (SAAO) JHK system to the TwoMicron AllSky Survey (2MASS) JHKS system. Both severalterm formal ...