MSc Defense: Tianyi Zhou
On Monday June 19 2023 at 12.30 in meeting room Hb1 at Blegdamsvej, Tianyi Zhou will defend his master thesis supervised by Vitor Cardoso. Censor will be Marta Orselli, University of Perugia. The thesis defense will be in English.
Title: Quantifying the nature of compact objects: Geodesic synchrotron radiation from light rings
Abstract: Black holes (BHs) are the simplest macroscopic objects predicted by Einstein’s theory of General Relativity (GR). Nowadays, the concept of BH is widely used in astrophysics to refer to those densest astrophysical objects. However, the curvature singularity in the interior of BHs leads to breakdown of known physics. Exotic compact objects (ECOs) are proposed as BH mimickers and tightly connected with the dark matter puzzle. Therefore, using observations to quantify the nature of compact objects is of paramount importance. Predicted by GR, geodesic synchrotron radiation (GSR) has attracted attention again in recent years, providing approaches to probe spacetimes and the nature of ECOs. Uniform-density stars are studied in this thesis as a proxy for ECOs due to its simplicity. A sufficiently compact uniform-density star can host two light rings, an unstable light ring in the exterior and a stable light ring in the interior, corresponding to circular null geodesics. Circular time-like geodesics are unstable when close to the unstable light ring, while circular time-like geodesics inside the stable light ring are stable. Charged particles can emit synchrotron radiation when orbiting on these circular time-like geodesics.
Scalar radiation in the vicinity of uniform-density stars is computed in both numerical and analytical approaches. Outside the star, scalar radiation has similar features to that in BH case: near the unstable light ring, the radiation is suppressed in each mode and dominated by high-frequency modes. It has also distinctive excitations due to those trapped modes. Inside the star, scalar radiation is suppressed when particles are close to the stable light ring, as well as when it is compared with the radiation emitted by particles orbiting the unstable light ring. Analytical result reveals that the suppression is due to the potential barrier between the stable light ring and turning point as the barrier corresponds to an exponentially decreasing factor. Less compact stars are expected to have lower potential barriers thus the suppression is less significant. Electromagnetic and axial gravitational radiation are also the same as those in BH case except for excitations. Radiations from the stable light ring are similarly suppressed. Future work shall first investigate polar gravitational radiation for a complete understanding of GSR. Orbital evolution and gravitational waveform are also of interest. Other ECO models such as boson stars should also be considered and studied.