Speaker: Dr. Priyanka Sarmah (NTHU)
Talk title: Effects of Superradiance in  Active Galactic Nuclei
Time: 2025/05/19 (Mon.) 12:30
Place: NCTS Physics 4F Lecture Hall, Cosmology Hall, NTU

Abstract: 
A spinning supermassive black hole (SMBH) at the core of an active galactic nucleus (AGN) provides room for the elusive ultra-light scalar particles (ULSP) to be produced through a phenomenon called superradiance. As a result of this phenomenon, a cloud of scalar particles forms around the black hole by draining the spin angular momentum of the SMBH. In this work, we present a study of the superradiant instability due to a scalar field in the vicinity of the central SMBH in an AGN. We begin by showing that the time-evolution of the gravitational coupling 𝛼 in a realistic ambiance created by the accretion disk around the SMBH in AGN leads to interesting consequences such as the amplified growth of the scalar cloud, enhancement of the gravitational wave emission rate, and appearance of higher modes of superradiance within the age of the Universe (∼ 10^10 years). We then explore the consequence of superradiance on the characteristics of the AGN. Using the Novikov-Thorne model for an accretion disk, we divide the full spectrum into three distinct wavelength bands- X-ray (10^4−10^2 𝜇m), UV (0.010-0.4 𝜇m), and Vis-IR (0.4 𝜇m-100 𝜇m) and observe sudden drops in the time-variations of the luminosities across these bands and Eddington ratio ( 𝑓Edd) with a characteristic timescale of superradiance. Using a uniform distribution of spin and mass of the SMBHs in AGNs, we demonstrate the appearance of depleted regions and accumulations along the boundaries of these regions in the planes of different band-luminosities and 𝑓Edd. Finally, we discuss some possible signatures of superradiance that can be drawn from the observed time-variation of the AGN luminosities.