Bogumił Pilecki (CAMK, Warsaw)
Since 2020, we have been systematically identifying and characterizing double-lined (SB2) binary Cepheids in the Large Magellanic Cloud, the Small Magellanic Cloud, and the Milky Way. Our primary detection method exploits the apparent overbrightness of Cepheids as a signature of a luminous companion, complemented by searches for systems composed of two Cepheid components. This effort has increased the known sample of confirmed SB2 Cepheids to 62 objects —an order-of-magnitude improvement over previous numbers— and expanded the number of double-Cepheid binaries from one to ten. For 37 systems, we detected anticorrelated orbital motion of both components, providing definitive proof of binarity. Preliminary orbital solutions have been derived for 24 systems with periods up to seven years, and full spectroscopic orbits have been determined for 15 systems with periods up to 3 years. I will present the orbital and physical properties of these binaries and discuss their implications for Cepheid multiplicity, evolution, and origin. Remarkably, at least 10% of them exhibit evidence of past mergers, suggesting that a significant fraction of Cepheids may form through binary interaction. I will also address the possible consequences of these findings for calibrating the Cepheid distance scale.
Samik Mitra (Astrophysics and Relativity Group International Centre for Theoretical Sciences, Bengaluru, India)
Magneto-rotational instability (MRI) is the primary mechanism for driving turbulence in accretion disks. Such instability disrupts the axial symmetry of the system and can stochastically excite the quasi-normal ringing of the black holes (BHs), which could emit gravitational waves (GWs). Motivated with this, we investigate stochastic GW signals from turbulent, magnetized accretion disks around spinning BHs. In doing so, we conduct three global general relativistic magnetohydrodynamic (GRMHD) simulations of (i) standard and normal evolution (SANE), (ii) sub-SANE, and (iii) magnetically arrested disk (MAD) models. The results from these simulations are used as a source in the Teukolsky equation to compute the emitted GW energy. We compare the GW signals from these models and identify what aspects of the gas physics, including turbulence and magnetic fields, effectively excite BH quasi-normal modes (QNMs). We assess whether the stochastic GW backgrounds (SGWBs) generated in each scenario approach the sensitivity curves of future gravitational wave detectors, offering new insights into multimessenger astrophysics and accretion physics.
Andrzej Kruszewicz (Warsaw Zoological Garden)
Rameshan Thimmappa
