Decoding signatures of extra dimensions and estimating spin of quasars from the continuum spectrum

Abstract

The continuum spectrum emitted by the accretion disk around quasars holds a wealth of information regarding the strong gravitational field produced by the massive central object. Such a strong gravity regime is often expected to be the ideal place to look for deviations from general relativistic predictions. One possible avenue, which may lead to deviations from general relativity, corresponds to the presence of extra dimensions. Since extra dimensions are well motivated from the perspective of high energy physics, it is instructive to investigate the effect of more than four spacetime dimensions on the black hole continuum spectrum, an effective astrophysical probe to the strong gravity regime. To explore such a scenario, we compute the optical luminosity emitted by a thin accretion disk around a rotating supermassive black hole albeit in the presence of extra dimensions. The background metric resembles the Kerr-Newman spacetime of general relativity where the tidal charge parameter, inherited from extra dimensions, can also assume negative values. The theoretical luminosity computed in such a background is contrasted with optical observations of eighty quasars. The difference between the theoretical and observed luminosity for these quasars is used to infer the most favored choice of the rotation parameter for each quasar and the tidal charge parameter. This has been achieved by minimizing/maximizing several error estimators, e.g., ${\ensuremath{\chi}}^{2}$, Nash-Sutcliffe efficiency, index of agreement etc. Intriguingly, all of them favor a negative value for the tidal charge parameter, a characteristic signature of extra dimensions.