Imaging ultra-compact objects with radiatively inefficient accretion flows

TL;DR

This study compares the images of a naked singularity model and a black hole using relativistic simulations, finding their observational signatures are remarkably similar, which challenges the uniqueness of black hole identification.

Contribution

It introduces the JMN-1 naked singularity model as an alternative to black holes and demonstrates its observational indistinguishability through detailed simulations.

Findings

JMN-1 images closely resemble Schwarzschild black hole images

Differences between the models are minimal in simulated images

JMN-1 shows slightly higher intensity than Schwarzschild black hole

Abstract

Recent Event Horizon Telescope observations of M87* and Sgr A* strongly suggests the presence of supermassive black hole at their respective cores. In this work, we use the semi-analytic Radiatively Inefficient Accretion Flows (RIAF) model to investigate the resulting images of Joshi-Malafarina-Narayan (JMN-1) naked singularity and the Schwarzschild BH. We aim at choosing the JMN-1 naked singularity model and compare the synchrotron images with the Schwarzschild solution to search any distinct features which can distinguish the two objects and find alternative to the black hole solution. We perform general relativistic ray-tracing and radiative transfer simulations using Brahma code to generate synchrotron emission images utilising thermal distribution function for emissivity and absorptivity. We investigate effects in the images by varying inclination angle, disk width and frequency. The shadow images simulated by the JMN-1 model closely resemble those generated by the Schwarzschild black hole. When we compare these images, we find that the disparities between them are minimal. We conduct simulations using various plasma parameters, but the resulting images remain largely consistent for both scenarios. This similarity is evident in the horizontal cross-sectional brightness profiles of the two instances. Notably, the JMN-1 model exhibits slightly higher intensity in comparison to the Schwarzschild black hole. We conclude that JMN-1 presents itself as a viable substitute for the black hole scenario. This conclusion is not solely grounded in the fact that they are indistinguishable from their respective shadow observations, but also in the consideration that JMN-1 emerges as an end state of a continual gravitational collapse. This paradigm not only allows for constraints on spacetime but also provides a good probe for the nature of the central compact object.