arXiv subjects

A. Kosakowski

Publications and source records attributed to A. Kosakowski.

3 recordsLinked to original sources

arXiv2026

Discovery of the first outbursting hot subdwarf binary: ZTF J0007+4804

Hot subdwarf binaries with white dwarf companions with orbital periods of less than two hours are progenitor candidates for massive single white dwarfs as well as a variety of thermonuclear explosions. Our aim is to determine the binary properties of the hot subdwarf -- white dwarf system ZTF J000742.62+480414.51, model its future evolution, and characterize the brightening events seen in TESS photometry. Using data from ZTF and TESS, we performed a Lomb Scargle analysis to find the orbital period and the period of the brightening events. Analysis of time-resolved spectroscopy was combined with light curve modeling to determine the effective temperature, surface gravity, and radius of the primary star, the masses of both stars, and to confirm the presence of an accretion disk. X-ray observations were performed with Swift, and MESA modeling was used to find the future evolution of the system. The kinematics of the system were also calculated. ZTF J000742.62+480414.51 consists of an accreting $0.48\pm0.01\,M_\odot$ white dwarf with a $0.42\pm0.01\,M_\odot$ B-type hot subdwarf acting as a donor. The system exhibits SU UMa type dwarf nova outbursts with a recurrence time of $P_{\mathrm{out}} \approx 9$ days. No X-rays were detected, with an upper limit on the X-ray luminosity of about $3\times10^{31}$ erg/sec. The system lies in the Galactic thin disk, and has an orbital period of $P_{\mathrm{orb}} = 108.72\pm0.01$ minutes. The system has likely formed from a main sequence binary with component masses $\gtrsim2\,\mathrm{M_{\odot}}$ and will likely merge into a single white dwarf, but a thermonuclear explosion cannot be ruled out. ZTF J000742.62+480414.51 consists of a low mass white dwarf actively accreting hydrogen rich material from a B-type hot subdwarf, and is the first hot subdwarf -- white dwarf system discovered that produces dwarf nova outbursts.

astro-ph.SR↗

arXiv2026

BlackGEM observations of compact pulsating stars: Mode identification for the DOV PG 1159-035 using multi-colour photometry

While space-based telescopes offer unparalleled precision for asteroseismology, ground-based observations remain crucial for identifying compact pulsator candidates and enabling their pulsational study through multi-colour photometry. The BlackGEM telescope array significantly enhances the detection and characterisation possibilities for compact pulsators that tend to be much fainter than dwarf or giant pulsators. Using BlackGEM multi-colour photometry of the hot pre-white dwarf PG 1159-035, we demonstrate its capability to detect short, multi-periodic pulsations with amplitudes down to a few milli-magnitudes. The primary aim of this study is to establish the feasibility of pulsation mode identification in hot subdwarfs and white dwarfs via mode amplitude-ratio analysis derived from BlackGEM multi-colour observations. Pulsation frequencies were extracted from our target using iterative pre-whitening analysis. To validate our data-driven mode identification concept using multi-colour photometry, we used the well-studied hot pre-white dwarf PG 1159-035, with previously identified pulsation modes, as a prototypical object that served for validation. The pre-whitening analysis using BlackGEM's standard q-, u-, and i-band light curves of PG 1159-035 revealed pulsation frequencies of l=1 and l=2 modes, consistent with values obtained from the literature. Subsequent amplitude ratio calculations resulted in discernible distributions for the l=1 and l=2 modes. The future assembly of more BlackGEM amplitude ratios for well-known white dwarfs with already identified modes will lead to density estimators suitable for identifying newly detected modes in known or as-yet-undiscovered pulsators. Our proof-of-concept study paves the way for large-scale asteroseismic analyses of optically faint compact pulsating stars using ground-based facilities, such as BlackGEM.

astro-ph.SR↗

arXiv2024

The BlackGEM telescope array I: Overview

The main science aim of the BlackGEM array is to detect optical counterparts to gravitational wave mergers. Additionally, the array will perform a set of synoptic surveys to detect Local Universe transients and short time-scale variability in stars and binaries, as well as a six-filter all-sky survey down to ~22nd mag. The BlackGEM Phase-I array consists of three optical wide-field unit telescopes. Each unit uses an f/5.5 modified Dall-Kirkham (Harmer-Wynne) design with a triplet corrector lens, and a 65cm primary mirror, coupled with a 110Mpix CCD detector, that provides an instantaneous field-of-view of 2.7~square degrees, sampled at 0.564\arcsec/pixel. The total field-of-view for the array is 8.2 square degrees. Each telescope is equipped with a six-slot filter wheel containing an optimised Sloan set (BG-u, BG-g, BG-r, BG-i, BG-z) and a wider-band 440-720 nm (BG-q) filter. Each unit telescope is independent from the others. Cloud-based data processing is done in real time, and includes a transient-detection routine as well as a full-source optimal-photometry module. BlackGEM has been installed at the ESO La Silla observatory as of October 2019. After a prolonged COVID-19 hiatus, science operations started on April 1, 2023 and will run for five years. Aside from its core scientific program, BlackGEM will give rise to a multitude of additional science cases in multi-colour time-domain astronomy, to the benefit of a variety of topics in astrophysics, such as infant supernovae, luminous red novae, asteroseismology of post-main-sequence objects, (ultracompact) binary stars, and the relation between gravitational wave counterparts and other classes of transients

astro-ph.IM↗