Discovery of an Outbursting 12.8 Minute Ultracompact X-Ray Binary^∗

Abstract

We report the discovery of OGLE-UCXB-01, a 12.8 minute variable object located in the central field of Galactic bulge globular cluster Djorg 2. The presence of frequent, short-duration brightenings at such an ultrashort period in long-term OGLE photometry together with the blue color of the object in Hubble Space Telescope images and the detection of moderately hard X-rays by Chandra observatory point to an ultracompact X-ray binary system. The observed fast period decrease makes the system a particularly interesting target for gravitational-wave detectors such as the planned Laser Interferometer Space Antenna.

Metallicity estimation of RR Lyrae stars from their I-band light curves

“…We note that the halo MDF shown in Fig. 10 is based on less than a thousand RRab stars toward the Magellanic cloud footprint of the OGLE survey, which were separated from endemic LMC and SMC objects based on their apparent magnitudes following the criteria of Pietrukowicz et al (2020). The mode of the halo MDF is in good agreement with the approximately −1.6 dex median metallicity of the halo population derived by Crestani et al (2021) from a sample of 211 RR Lyrae stars with HR spectroscopy.…”

“…Perhaps the most important one is their effect on the photometric MDFs derived for various stellar populations in and around the Milky Way, while capitalizing on the vast amount of accurate I-band time-series data from the OGLE surveys of the bulge, southern disk, and the Magellanic Clouds. Most recently, Pietrukowicz et al (2020) exploited the entire OGLE RR Lyrae catalog to study the MDFs of the old stellar populations of the Galaxy, the LMC, SMC, and the Sagittarius dwarf galaxy, and drew inferences on their formation histories therefrom. To assess the effect of the systematic difference between the various earlier formulae and our new regression model on our view of the MDFs of these populations, we deployed all of them on the I-band light curves of RR Lyrae stars in the OCVS and computed the MDFs of the same cosmic environments.…”

“…To assess the effect of the systematic difference between the various earlier formulae and our new regression model on our view of the MDFs of these populations, we deployed all of them on the I-band light curves of RR Lyrae stars in the OCVS and computed the MDFs of the same cosmic environments. We used the same simple positional and brightness criteria in selecting the pertinent stellar populations as the ones that were applied by Pietrukowicz et al (2020), which are sufficient to keep sample contamination on the level of a few percent, and aid direct comparability and reproducibil- ity. With the additional quality criteria of {C > 0.9 and S/N > 100 and N ep.…”

“…What makes OGLE unique among the large number of recent and ongoing photometric surveys is that it provides homogeneous, high-quality light curves for billions of objects over large areas of the observationally most challenging regions of the Local Group, namely the Galactic bulge and mid-plane, and the Small and Large Magellanic Clouds. These data were utilized by a plethora of studies, employing the RR Lyrae stars not only as distance indicators (e.g., Jacyszyn-Dobrzeniecka et al 2017), but also as metallicity tracers (e.g., Skowron et al 2016;Pietrukowicz et al 2020).…”

Error Analysis of the Light Curve Solution of Contact Binaries Based on the W-D Code

“…Accurate basic physical parameters (masses, radii and luminosities of each component) are the key information for studying these two physical processes. Furthermore, contact binaries may be progenitors of luminous red novae (LRNs; e.g., V1309 Sco, Tylenda et al 2011;Stȩpień 2011;Zhu et al 2016;Pietrukowicz et al 2017) and blue stragglers (BSs; e.g., Eggleton 2012;Ferreira et al 2019). The basic physical parameters of contact binaries are also necessary to understand the formation of these peculiar objects.…”