Khagol Bulletin # 131

Debarati Chatterjee is a Theoretical Astrophysics researcher specialising in Compact Stars and Gravitational Waves. She is an Associate Professor at the Inter-University Centre for Astronomy and Astrophysics, Pune, India, and a member of several international collaborations, including LIGO Scientific Collaboration. She is the current Chair of Education and Public Outreach for the LIGO-Indiamega-science project. Beyond academics, Debarati is passionate about creative writing, learning new languages, dancing, and sports like high-altitude trekking, rock climbing andmixedmartial arts. The breakthrough in this field came with the discovery of gravitational waves (GW) - ripples in space-time produced when massive cosmic objects collide or are perturbed. Although predicted by Einstein back in 1915, these elusive waves could not be detected given their extremely weak nature: their amplitude is one part in a septillion! Catching a gravitational wave required a century of technological advancement to build one of the most precise instruments in the world, a gravitational wave detector. Huge "L" shaped detectors with several kilometre arm lengths were suspended in ultra-high vacuum to isolate them from surrounding vibrations. At the same time, powerful lasers were made to reflect in the arms multiple times and then interfere so that any passingwavewould change its pattern. These detectors, such as the LIGO detector in the US and Virgo detector in Europe, were then successively upgraded over a decade to reach their design sensitivity when they ultimately discovered the first GW signal from a pair of binary black holes in 2015 and soon after, from a pair of neutron stars in 2017. regimes do not allow us to constrain the models precisely. The NSGW group at IUCAA, led by Debarati Chatterjee, studies fundamental physics using gravitational waves. Over the past few years, the group made several remarkable contributions to research, with implications for multidisciplinary domains such as Nuclear Physics, Particle Physics andGWastronomy. Research In particular, several studies led by PhD s t ud e n t B i k r am Ke s ha r i P r a dhan (published in Nuclear Physics A, Physical Review C, Physical Review D and the Astrophysical Journal in 2022-2023) searched for signatures of the neutron star's internal composition in unstable oscillation modes that lead to continuous GW emission. The results showed that dense matter theories in nuclear physics could be better constrained using signals from such oscillations, in both isolated NSs or binary, by planned future-generation GW detectors. They also investigated how such detections may allow us to distinguish between neutron stars and other stable families of compact stars, such as strange stars, or probe the nature of a possible phase transition in their interior. In a series of works led by PhD student Suprovo Ghosh, constraints were imposed on dense matter theories using multidisciplinary Physics (nuclear theory, heavy-ion physics, Multi-messenger astrophysical data) at different densities. PhD student Swarnim Shirke extended this scheme to hybrid stars (with phase transition in the interior) by additionally imposing constraints from perturbative theories of QCD (Quantum Chromodynamics, the theory of strong interaction) at very high densities. These works were published in the European Physical Journal, Frontiers in Astronomy and Space Sciences and the Astrophysical Journal in 2022-2023. Suprovo Ghosh also proposed tidal heating in the initial inspiral phase of NSmergers as a novel probe of strangeness. Further, Swarnim Shirke is investigating the possibility of detecting exotic matter in the interior of neutron stars. In his recent work published in the Journal of Cosmology and Astroparticle Physics, NS oscillationmodes were suggested as a new probe of the presence of dark matter in NSs. Post- d o c t o r a l s t u d e n t D h r u v P a t h a k collaborated in several of these efforts and led studies to improve limits for continuous GWsearches by LIGO. As the current generation (LIGO-Virgo- KAGRA) of GW detectors near the end of their fourth observation run, many more are on the horizon. In May 2023, the Indian government gave the final nod to constructing a GW detector on Indian soil. Joining the global array of GW detectors, the LIGO-India project will revolutionise the landscape of GW research with its cutting- edge technology and global position, leading to a highly accurate localisation of GW sources. Future-generation detectors such as the Cosmic Explorer in the US, the Einstein Telescope in Europe, and the space-based Laser Interferometer Space Antenna (LISA) are planned over the next decade. A dedicated high-frequency GW interferometer called NEMO (Neutron Star Extreme Matter Observatory), designed to measure the fundamental properties of nuclear matter at extreme densities with GWs, has also been proposed. Joining hands with space and ground-based t e l e s c o p e s , m u l t i - m e s s e n g e r collaborative research on neutron stars is expected to crack the code in mysteries of fundamental physics, from the infinite to the infinitesimal. The best is yet to come inGWresearch. | 04 | KHAG L | No. 131 - JANUARY 2024