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1

A New Indirect Method of Determining Density of Cometary Nuclei

Paradowski M. L.

Acta Astronomica

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2022

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Vol. 72, No. 2

141--159

EN

A new indirect method for determining the bulk density of cometary nuclei is presented. This method has been applied to 136 comets taken from the literature. The method uses the calibration function in the form a1=10.88526+3.39275×a2 and the equation logDN=a2-0.13H, where DN is the nucleus' diameter [km], and H is the comets' absolute total magnitude (coma + nucleus). To obtain the nuclear bulk density [kg/m3] the formula d=(6/π)×10a where a=a1-3a2-9 is used. The mean nuclear bulk density of the 136 comets is 520±10 kg/m3. The relationship between the nuclear bulk density and the origin of the comets is found. For the 40 long-period comets the mean density is 470±10 kg/m3, whereas for the 94 short-period comets from Jupiter family it is 540±10 kg/m3. The accuracy of the calibration function is 1.5%.

2

Photometric and Spectroscopic Investigations of the Galactic Field RRc Candidates V764 Mon and HY Com

Benkő J. M., Sódor Á., Pál A.

Acta Astronomica

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2021

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Vol. 71, No. 1

55--72

EN

By analyzing photometric and spectroscopic time series, we show that the pulsator V764 Mon, assumed to be the brightest RR Lyr star in the sky, is in fact a rapidly rotating δ Sct star with an unusually long dominant period (P1=0.29 d). Our spectroscopy confirmed the binarity of V764 Mon discovered by the Gaia satellite. In the case of HY Com, a bona fide RRc star, we present its first complete radial velocity curve. Additionally, we found that the star continues its strong phase variation reported before.

3

Empirical Scaling Relations for the Photospheric Magnetic Elements of the Flaring and Non-Flaring Active Regions

Moradhaseli M. A., Javaherian M., Fathalian N., Safari H.

Acta Astronomica

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2021

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Vol. 71, No. 2

163--188

EN

Here, we analyze magnetic elements of the solar active regions (ARs) observed in the line-of-sight magnetograms (the 6173 Å FeI line) recorded with the Solar Dynamics Observatory (SDO)/Hel\-ioseismic and Magnetic Imager (HMI). The Yet Another Feature Tracking Algorithm (YAFTA}) was employed to analyze the statistical properties of these features (e.g., filling factor, magnetic flux, and lifetime). Magnetic features were extracted from the areas of 180o×180o inside the flaring AR (NOAA 12443) for November 3-5, 2015 and non-flaring AR (NOAA 12446) for November 4-6, 2015. The mean filling factor of polarities was found to be about 0.49 for the flaring AR, while this value was 0.08 for the non-flaring AR. Time series of the filling factors of the negative and positive polarities for the flaring AR showed anti-correlated behavior (with the Pearson value of -0.80). However, there was a strong positive correlation (with the Pearson value of 0.95) for the non-flaring AR. A power-law function was fitted to the frequency distributions of flux (F), size (S), and lifetime ($T$). Power exponents of the distributions of flux, size, and lifetime for the flaring AR were found to be -2.36±0.27, -3.11±0.17, and -1.70±0.29, respectively, while for the non-flaring AR: -2.53±0.20, -3.42±0.21, and -1.61±0.19, respectively. The code detected a magnetic element with the maximum flux of 23.54×1020 Mx. The maximum size of detected patches was found to be about 300 Mm2. The most long-lived patch in the flaring AR belonged to an element with a lifetime of 2208 min. We showed that S, F, and T for patches in the flaring AR follow empirical scaling relations: S∼F0.66±0.01, F∼T0.48±0.04, and S∼T0.32±0.02, respectively. For patches in the non-flaring AR, we obtained S∼F0.64±0.02, F∼T0.37±0.06, and S∼T0.23±0.03, respectively. The comparisons indicated that correlations between parameters of F and T, and also, S and T for the flaring AR, are larger than those of the non-flaring AR. The scaling law relation between the flux growth rate of positive polarities and their size indicates a strong correlation of more than 0.7 in both ARs.

4

A Survey Length for AGN Variability Studies

Kozłowski S.

Acta Astronomica

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2021

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Vol. 71, No. 2

103--112

EN

The damped random walk (DRW) process is one of the most commonly used and simplest stochastic models to describe variability of active galactic nuclei (AGN). An AGN light curve can be converted to just two DRW model parameters - the signal decorrelation timescale τ and the asymptotic amplitude SF∞. In principle, these two model parameters may be correlated with the physical parameters of AGN. By simulation means, we have recently shown that in order to measure the decorrelation timescale accurately, the experiment or the light curve length must be at least 10 times the underlying decorrelation timescale. In this paper, we investigate the origin of this requirement and find that typical AGN light curves do not sufficiently represent the intrinsic stationary process. We simulated extremely long (10 000τ) AGN light curves using DRW, and then measured the variance and the mean of short light curves spanning 1-1000τ. We modeled these light curves with DRW to obtain both the signal decorrelation timescale τ and the asymptotic amplitude SF∞. The variance in light curves shorter than ≈30τ is smaller than that of the input process, as estimated by both a simple calculation from the light curve and by DRW modeling. This means that while the simulated stochastic process is intrinsically stationary, short light curves do not adequately represent the stationary process. Since the variance and timescale are correlated, underestimated variances in short light curves lead to underestimated timescales as compared to the input process. It seems, that a simulated AGN light curve does not fully represent the underlying DRW process until its length reaches even ≈30 decorrelation timescales. Modeling short AGN light curves with DRW leads to biases in measured parameters of the model - the amplitude being too small and the timescale being too short.

5

A Detection Threshold in the Amplitude Spectra Calculated from TESS Time-Series Data

Baran A. S., Koen C.

Acta Astronomica

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2021

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Vol. 71, No. 2

113--121

EN

We present results of time-series data simulation. We aimed at estimating the threshold used for detecting signals in amplitude spectra, calculated from simulating TESS photometry of up to one year duration. We selected the threshold at a false alarm probability FAP=0.1% and derived S/N ratios between 4.6 and 5.7 depending on the data cadence and coverage. We also provide a formula to estimate the threshold for any FAP adopted and a given number of data points. Our result confirms that, to avoid spurious detection, space-based photometry may require substantially higher S/N than that typically being employed for ground-based data.

6

Identifying Microlensing Events Using Neural Networks

Mróz P.

Acta Astronomica

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2020

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Vol. 70, No. 3

169--180

EN

Current gravitational microlensing surveys are observing hundreds of millions of stars in the Galactic bulge - which makes finding rare microlensing events a challenging tasks. In almost all previous works, microlensing events have been detected either by applying very strict selection cuts or manually inspecting tens of thousands of light curves. However, the number of microlensing events expected in the future space-based microlensing experiments forces us to consider fully-automated approaches. They are especially important for selecting binary-lens events that often exhibit complex light curve morphologies and are otherwise difficult to find. There are no dedicated selection algorithms for binary-lens events in the literature, which hampers their statistical studies. Here, we present two simple neural-network-based classifiers for detecting single and binary microlensing events. We demonstrate their robustness using OGLE-III and OGLE-IV data sets and show they perform well on microlensing events detected in data from the Zwicky Transient Facility (ZTF). Classifiers are able to correctly recognize ≈98% of single-lens events and 80-85% of binary-lens events.

7

The Cluster AgeS Experiment (CASE). Detecting Aperiodic Photometric Variability with the Friends of Friends Algorithm

Rozyczka M., Narloch W., Pietrukowicz P., Thompson I. B., Pych W., Poleski R.

Acta Astronomica

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2018

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Vol. 68, No. 1

63--78

EN

We adapt the friends of friends algorithm to the analysis of light curves, and show that it can be succesfully applied to searches for transient phenomena in large photometric databases. As a test case we search OGLE-III light curves for known dwarf novae. A single combination of control parameters allows us to narrow the search to 1% of the data while reaching a ≈90% detection efficiency. A search involving ≈2% of the data and three combinations of control parameters can be significantly more effective - in our case a 100% efficiency is reached. The method can also quite efficiently detect semi-regular variability. In particular, 28 new semi-regular variables have been found in the field of the globular cluster M22, which was examined earlier with the help of periodicity-searching algorithms.

8

Predicted Microlensing Events by Nearby Very-Low-Mass Objects: Pan-STARRS DR1 vs. Gaia DR2

Nielsen M. B., Bramich D. M.

Acta Astronomica

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2018

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Vol. 68, No. 4

351--370

EN

Microlensing events can be used to directly measure the masses of single field stars to a precision of ≈1–10%. The majority of direct mass measurements for stellar and sub-stellar objects typically only come from observations of binary systems. Hence microlensing provides an important channel for direct mass measurements of single stars. The Gaia satellite has observed ≈1.7 billion objects, and analysis of the second data release has recently yielded numerous event predictions for the next few decades. However, the Gaia catalog is incomplete for nearby very-low-mass objects such as brown dwarfs for which mass measurements are most crucial. We employ a catalog of very-low-mass objects from Pan-STARRS data release 1 (PDR1) as potential lens stars, and we use the objects from Gaia data release 2 (GDR2) as potential source stars. We then search for future microlensing events up to the year 2070. The Pan-STARRS1 objects are first cross-matched with GDR2 to remove any that are present in both catalogs. This leaves a sample of 1718 possible lenses. We fit MIST isochrones to the Pan-STARRS1, AllWISE and 2MASS photometry to estimate their masses. We then compute their paths on the sky, along with the paths of the GDR2 source objects, until the year 2070, and search for potential microlensing events. Source-lens pairs that will produce a microlensing signal with an astrometric amplitude of greater than 0.131 mas, or a photometric amplitude of greater than 0.4 mmag, are retained.

9

An Almanac of Predicted Microlensing Events for the 21st Century

Bramich D. M., Nielsen M. B.

Acta Astronomica

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2018

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Vol. 68, No. 3

183--203

EN

Using Gaia data release 2 (GDR2), we present an almanac of 2509 predicted microlensing events, caused by 2130 unique lens stars, that will peak between July 25, 2026 and the end of the century. This work extends and completes a thorough search for future microlensing events initiated by Bramich and Nielsen using GDR2. The almanac includes 161 lenses that will cause at least two microlensing events each. A few highlights are presented and discussed, including: (i) an astrometric microlensing event with a peak amplitude of ≈9.7 mas, (ii) an event that will probe the planetary system of a lens with three known planets, and (iii) an event (resolvable from space) where the blend of the lens and the minor source image will brighten by a detectable amount (≈2 mmag) due to the appearance of the minor source image. All of the predicted microlensing events in the almanac will exhibit astrometric signals that are detectable by observing facilities with an angular resolution and astrometric precision similar to, or better than, that of the Hubble Space Telescope (e.g., NIRCam on the James Webb Space Telescope), although the events with the most extreme source-to-lens contrast ratios may be challenging. Ground-based telescopes of at least 1 m in diameter can be used to observe many of the events that are also expected to exhibit a photometric signal.

10

Cool Spot and Flare Activities of a RS CVn Binary KIC 7885570

Kunt M., Dal H. A.

Acta Astronomica

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2017

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Vol. 67, No. 4

345--362

EN

We present here the results of our studies on the physical nature and chromospheric activity of a RS CVn binary KIC 7885570 based on the Kepler Mission data. Assuming the primary component temperature, 6530 K, the temperature of the secondary component was found to be 5732±4 K. The mass ratio of the components (q) was found to be 0.43±0.01, while the inclination (i) of the system - 80.6°±0.1°. Additionally, the data were separated into 35 subsets to model the sinusoidal variation due to the rotational modulation, using the SpotModel program, as the light curve analysis indicated the chromospherically active secondary component. It was found that there are generally two spotted areas, whose radii, longitudes and latitudes are rapidly changing, located around the latitudes of +50° and +90° on the active component. Moreover, 113 flares were detected and their parameters were computed from the available data. The One Phase Exponential Association function model was derived from the parameters of these flares. Using the regression calculations, the Plateau value was found to be 1.9815±0.1177, while the half-life value was computed as 3977.2 s. In addition, the flare frequency (N1) - the flare number per hour, was estimated to be 0.00362 h-1, while flare frequency (N2 - the flare-equivalent duration emitted per hour, was computed as 0.00001. Finally, the times of eclipses were computed for 278 minima of the light curves, whose analysis indicated that the chromosphere activity nature of the system causes some effects on these minima times. Comparing the chromospheric activity patterns with the analogues of the secondary component, it is seen that the magnetic activity level is remarkably low. However, it is still at the expected level according to the B-V color index of 0.643 mag for the secondary component.

11

Robust Filtering of Artifacts in Difference Imaging for Rapid Transients Detection

Klencki J., Wyrzykowski Ł., Kostrzewa-Rutkowska Z., Udalski A.

Acta Astronomica

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2016

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Vol. 66, No. 1

15--29

EN

Real-time analysis and classification of observational data collected within synoptic sky surveys is a huge challenge due to constant growth of data volumes. Machine learning techniques are often applied in order to perform this task automatically. The current bottleneck of transients detection in most surveys is the process of filtering numerous artifacts from candidate detection. We present a new method for automated artifact filtering based on hierarchical unsupervised classifier employing Self-Organizing Maps (SOMs). The system accepts 97% of real transients and removes 97.5% of artifacts when tested on the OGLE-IV Transient Detection System. The improvement of the artifacts filtering allows for single-frame-based detection of transients within OGLE-IV, which now alerts on transient discoveries in less than 15 minutes from the image acquisition.

12

Analysis of Photometric Uncertainties in the OGLE-IV Galactic Bulge Microlensing Survey Data

Skowron J., Udalski A., Kozłowski S., Szymański M. K., Mróz P., Wyrzykowski Ł., Poleski R., Pietrukowicz P., Ulaczyk K., Pawlak M., Soszyński I.

Acta Astronomica

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2016

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Vol. 66, No. 1

1--13

EN

We present a statistical assessment of both, observed and reported, photometric uncertainties in the OGLE-IV Galactic bulge microlensing survey data. This dataset is widely used for the detection of variable stars, transient objects, discovery of microlensing events, and characterization of the exo-planetary systems. Large collections of RR Lyr stars and Cepheids discovered by the OGLE project toward the Galactic bulge provide light curves based on this dataset. We describe the method of analysis, and provide the procedure, which can be used to update preliminary photometric uncertainties, provided with the light curves, to the ones reflecting the actual observed scatter at a given magnitude and for a given CCD detector of the OGLE-IV camera. This is of key importance for data modeling, in particular, for the correct estimation of the goodness of fit.

13

Spurious Frequencies in the Kepler Short Cadence Data

Baran A. S.

Acta Astronomica

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2013

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Vol. 63, No. 2

203--224

EN

We present our search for artifacts in the Kepler short cadence data using a commonly known Fourier technique. We analyzed data on a monthly basis searching for a possible correlation between artifacts and the events attributed to the spacecraft as potential sources of the spurious frequencies. We defined a peak to be an artifact if it shows in at least two, yet preferentially most of the stars, during a given month. Besides the commonly known "long cadence comb" we found a periodic appearance of another two combs, one single artifact and very strange wide artifacts roaming between 10 c/d and 35 c/d. These artifacts evolve on a yearly basis (four of Kepler's rolls) and we may only speculate that their sources are in the reaction wheels since they are the only moving parts, or temperature variation. The orientation of the spacecraft is likely excluded from the possible sources. More resources are needed to provide a definite explanation of the artifacts.

14

A Simple Method of Correcting Magnitudes for the Errors Introduced by Atmospheric Refraction

Kruszewski A., Semeniuk I.

Acta Astronomica

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2003

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Vol. 56, No. 3

241--248

EN

We show that the errors due to atmospheric refraction are present in the magnitudes determined with the Difference Images Analysis method. In case of single, unblended stars the size of the effect agrees with the theoretical prediction. But when the blending is strong, what is quite common in a dense field, then the effect of atmospheric refraction can be strongly amplified to the extent that some cases of apparently variable stars with largest amplitudes of variations are solely due to refraction. We present a simple method of correcting for this kind of errors.

15

The Optical Gravitational Lensing Experiment. Difference Image Analysis of LMC and SMC Data. The Method

Żebruń K., Soszyński I., Woźniak P. R.

Acta Astronomica

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2001

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Vol. 51, No. 4

303--316

EN

We describe the Difference Image Analysis (DIA) algorithms and software used to analyze four years (1997-2000) of OGLE-II photometric monitoring of the Magellanic Clouds, the calibration, the photometric error analysis and the search for variable stars. A preliminary analysis of photometric errors is based on the field LMC_SC2. A full catalog of more than 68 000 variable stars is presented in a separate publication.

16

Difference Image Analysis of the OGLE-II Bulge Data. I. The Method

Woźniak P. R.

Acta Astronomica

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2000

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Vol. 50, No. 4

421--450

EN

We present an implementation of the difference image photometry based on the Alard and Lupton optimal PSF matching algorithm. The most important feature distinguishing this method from the ones using Fourier divisions is that equations are solved in real space and the knowledge of each PSF is not required for determination of the convolution kernel. We evaluate the method and software on 380 GB of OGLE-II bulge microlensing data obtained in 1997-1999 observing seasons. The error distribution is Gaussian to better than 99% with the amplitude only 17% above the photon noise limit for faint stars. Over the entire range of the observed magnitudes the resulting scatter is improved by a factor of 2-3 compared to DoPhot photometry, currently a standard tool for massive stellar photometry in microlensing searches. For testing purposes the photometry of ≈4600 candidate variable stars and sample difference image data are provided for BUL_SC1 field. In the candidate selection process, very few assumptions have been made about the specific types of flux variations, which makes this data set well suited for general variability studies, including the development of the classification schemes.