Genetic-Algorithm-based Light Curve Optimization Applied to Observations of the W UMa star BH Cas

I have developed a procedure utilizing a Genetic-Algorithm-based optimization scheme to fit the observed light curves of an eclipsing binary star with a model produced by the Wilson-Devinney code. The principal advantages of this approach are the global search capability and the objectivity of the final result. Although this method can be more efficient than some other comparably global search techniques, the computational requirements of the code are still considerable. I have applied this fitting procedure to my observations of the W UMa type eclipsing binary BH Cassiopeiae. An analysis of V-band CCD data obtained in 1994/95 from Steward Observatory and U- and B-band photoelectric data obtained in 1996 from McDonald Observatory provided three complete light curves to constrain the fit. In addition, radial velocity curves obtained in 1997 from McDonald Observatory provided a direct measurement of the system mass ratio to restrict the search. The results of the GA-based fit are in excellent agreement with the final orbital solution obtained with the standard differential corrections procedure in the Wilson-Devinney code.Comment: 9 pages, 2 figures, 2 tables, uses emulateapj.st

The Density of Coronal Plasma in Active Stellar Coronae

We have analyzed high-resolution X-ray spectra of a sample of 22 active stars observed with the High Energy Transmission Grating Spectrometer on {\em Chandra} in order to investigate their coronal plasma density. Densities where investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII. While Si XIII lines in all stars of the sample are compatible with the low-density limit, Mg XI lines betray the presence of high plasma densities ($> 10^{12}$ cm$^{-3}$) for most of the sources with higher X-ray luminosity ($> 10^{30}$ erg/s); stars with higher $L_X$ and $L_X/L_{bol}$ tend to have higher densities at high temperatures. Ratios of O VII lines yield much lower densities of a few $10^{10}$ cm$^{-3}$, indicating that the ``hot'' and ``cool'' plasma resides in physically different structures. Our findings imply remarkably compact coronal structures, especially for the hotter plasma emitting the Mg XI lines characterized by coronal surface filling factor, $f_{MgXI}$, ranging from $10^{-4}$ to $10^{-1}$, while we find $f_{OVII}$ values from a few $10^{-3}$ up to $\sim 1$ for the cooler plasma emitting the O VII lines. We find that $f_{OVII}$ approaches unity at the same stellar surface X-ray flux level as solar active regions, suggesting that these stars become completely covered by active regions. At the same surface flux level, $f_{MgXI}$ is seen to increase more sharply with increasing surface flux. These results appear to support earlier suggestions that hot $10^7$ K plasma in active coronae arises from flaring activity, and that this flaring activity increases markedly once the stellar surface becomes covered with active regions.Comment: 53 pages, 19 figures, accepted for publication in Astrophysical Journal. A version of the paper with higher quality figures is available from http://www.astropa.unipa.it/Library/preprint.htm

Continuous heating of a giant X-ray flare on Algol

Giant flares can release large amounts of energy within a few days: X-ray emission alone can be up to ten percent of the star's bolometric luminosity. These flares exceed the luminosities of the largest solar flares by many orders of magnitude, which suggests that the underlying physical mechanisms supplying the energy are different from those on the Sun. Magnetic coupling between the components in a binary system or between a young star and an accretion disk has been proposed as a prerequisite for giant flares. Here we report X-ray observations of a giant flare on Algol B, a giant star in an eclipsing binary system. We observed a total X-ray eclipse of the flare, which demonstrates that the plasma was confined to Algol B, and reached a maximum height of 0.6 stellar radii above its surface. The flare occurred around the south pole of Algol B, and energy must have been released continously throughout its life. We conclude that a specific extrastellar environment is not required for the presence of a flare, and that the processes at work are therefore similar to those on the Sun.Comment: Nature, Sept. 2 199

The binary properties of the pulsating subdwarf B eclipsing binary PG 1336-018 (NY Virginis)

Aims. We present an unbiased orbit solution and mass determination of the components of the eclipsing binary PG1336−018 as a critical test for the formation scenarios of subdwarf B stars. Methods. We obtained high-resolution time series VLT/UVES spectra and high-speed multicolour VLT/ULTRACAM photometric observations of PG1336−018, a rapidly pulsating subdwarf B star in a short period eclipsing binary. Results. Combining the radial velocity curve obtained from the VLT/UVES spectra with the VLT/ULTRACAM multicolour lightcurves, we determined numerical orbital solutions for this eclipsing binary. Due to the large number of free parameters and their strong correlations, no unique solution could be found, only families of solutions. We present three solutions of equal statistical significance, two of which are compatible with the primary having gone through a core He-flash and a common-envelope phase described by the α-formalism. These two models have an sdB primary of 0.466 M and 0.389 M, respectively. Finally, we report the detection of the Rossiter-McLaughlin effect for PG1336−018

Attitude determination and control system of Sharjah-sat-1

Sharjah-Sat-1 is the first CubeSat mission of the Sharjah Academy for Astronomy, Space Sciences, and Technology (SAASST), in collaboration with Istanbul Technical University Space Systems Design and Test Laboratory (ITU-SSDTL) and Sabanci University (SU). The 3U+ CubeSat's primary payload is an improved X-Ray detector (iXRD), with the objectives of detecting hard X-rays from very bright X-ray sources, as well as studying the solar coronal holes. The secondary payload is a dual-camera system for Earth imaging. A dual-camera system will be used to image the SAASST building with a size of about 100m. The iXRD requires an attitude accuracy of 1 degree or better. Its full width at half maximum is 4.26 degrees with a linear response for the square collimator used. Every 1-degree pointing error will lead to 23% information loss. The ADCS subsystem of Sharjah-Sat-1 has been carefully determined and selected to ensure the success of its sophisticated mission, considering the constraints on the CubeSat standards with regards to size, mass, and power, the operational requirements of the mission, and space environmental disturbances expected throughout the mission's lifetime. Those disturbances are mostly encountered at low altitudes, and Sharjah-Sat-1 is planned to have a Sun-synchronous orbit with an altitude of about 500 km, not exceeding 600km. The planned launch date of the mission is June 2022. The paper presents a discussion and evaluation of the ADCS system in detail and its importance in terms of the achievement of the Sharjah-Sat-1 scientific mission (both star/Sun imaging and Earth-imaging), the hardware and the software implemented for active control, and the various attitude determination and control modes for different sensors and actuators' configurations

First light of Sharjah-Sat-1: potential targets and early science

Sharjah-Sat-1 is a 3U+ CubeSat developed as a collaborative research project between the Sharjah Academy for Astronomy, Space Sciences, and Technology (SAASST), University of Sharjah (UoS), Istanbul Technical University (ITU), and Sabanci University (SU). Sharjah-Sat-1 was launched on January 3, 2023, equipped with a dual payload onboard: (i) an improved X-ray Detector (iXRD) and (ii) a system of two optical cameras. Its primary scientific mission is to study bright, hard X-ray sources in our Galaxy and solar coronal holes. The primary science payload onboard is the iXRD (improved X-ray Detector developed by Sabanci University) with CdZnTe-based crystal as the active material and a Tungsten collimator with a field of view of 4 degrees. The energy coverage ranges from 20 keV to 200 keV, with a spectral resolution of 6 keV @ 60 keV. Sharjah-Sat-1 is currently in the commissioning phase, and we anticipate early observation in the next few weeks. Its main science goal is to observe the brightest galactic hard X-ray sources, transient and persistent. Black hole candidates and pulsars emit radiation up to a few 100 keVs, making them potential targets. Long observations of bright Black Hole Candidates will allow observing spectral transition (hard/soft) and studying their variability. Besides, solar observations will be conducted to study hard X-ray spectra of flares and coronal holes. Other targets of opportunity are transient bright events, such as gamma-ray bursts (GRB). While the detector's effective area is only 6.5 cm2, it is expected to reach a sensitivity of around 60 mCrab in a single day in the 20 keV to 100 keV band, assuming 600s exposure for each orbit. However, further in-flight calibrations are needed. By the time of the conference, we anticipate having and presenting the first high-level products (i.e., spectra and lightcurves) of the brightest X-ray galactic sources (e.g., Cyg X-1, Sco X-1, etc.) observed by Sharjah-Sat-1

An evolved donor star in the long-period cataclysmic variable HS 0218+3229

Context. We present time-resolved spectroscopy and photometry of HS 0218+3229, a new long-period cataclysmic variable discovered within the Hamburg Quasar Survey. It is one of the few systems that allow a dynamical measurement of the masses of the stellar components. Aims. We combine the analysis of time-resolved optical spectroscopy and R-band photometry with the aim of measuring the mass of the white dwarf and the donor star and the orbital inclination. Methods. Cross-correlation of the spectra with K-type dwarf templates is used to derive the radial velocity curve of the donor star. An optimal subtraction of the broadened templates is performed to measure the rotational broadening and constrain the spectral type of the donor. Finally, an ellipsoidal model is fitted to the R-band light curve to obtain constraints upon the orbital inclination of the binary system. Results. The orbital period of HS 0218+3229 is found to be 0.297229661 ± 0.000000001 d (7.13351186 ± 0.00000002 h), and the amplitude of the donor’s radial velocity curve is K2 = 162.4 ± 1.4 kms−1. Modelling the ellipsoidal light curves gives an orbital inclination in the range i = 59◦ ±3◦. A rotational broadening between 82.4±1.2 km s−1 and 89.4±1.3 km s−1 is found when assuming zero and continuum limb darkening, respectively. The secondary star has most likely a spectral type K5 and contributes ∼80–85% to the R-band light. Our analysis yields a mass ratio of 0.52 < q < 0.65, a white dwarf mass of 0.44 < M1(M) < 0.65, and a donor star mass of 0.23 < M2(M) < 0.44. Conclusions. We find that the donor star in HS 0218+3229 is significantly undermassive for its spectral type. It is therefore very likely that it has undergone nuclear evolution prior to the onset of mass transfer