Excavating black hole continuum spectrum: Possible signatures of scalar hairs and of higher dimensions

TL;DR

This paper investigates how alternative gravity theories, including higher dimensions and scalar fields, imprint signatures on black hole continuum spectra, with observational data suggesting potential evidence for negative tidal charge parameters.

Contribution

It explores the effects of three distinct alternative gravity models on black hole spectra and identifies observational signatures indicating negative tidal charges, hinting at higher dimensions or scalar fields.

Findings

Negative tidal charge parameter minimizes differences between theory and observations.

Observational data favor negative tidal charge values in black hole models.

Signatures consistent with higher dimensions or scalar charges are suggested by spectral analysis.

Abstract

Continuum spectrum from black hole accretion disc holds enormous information regarding the strong gravity regime around the black hole and hence about the nature of gravitational interaction in extreme situations. Since in such strong gravity regime the dynamics of gravity should be modified from the \EH one, its effect should be imprinted on the continuum spectrum originating from the black hole accretion. To explore the effects of these alternative theories on the black hole continuum spectrum in an explicit manner, we have discussed three alternative gravitational models having their origin in three distinct paradigms --- (a) higher dimensions, (b) higher curvature gravity and (c) Horndeski theories. All of them can have signatures sculptured on the black hole continuum spectrum, distinct from the standard general relativistic scenario. Interestingly all these models exhibit black hole solutions with tidal charge parameter which in these alternative gravity scenarios can become negative, in sharp contrast with the Reissner-Nordstr\"om black hole. Using the observational data of optical luminosity for eighty Palomer Green quasars we have illustrated that the difference between the theoretical estimates and the observational results gets minimized for negative values of the tidal charge parameter. As a quantitative estimate of this result we concentrate on several error estimators, including reduced $\chi^{2}$, Nash-Sutcliffe efficiency, index of agreement etc. Remarkably, all of them indicates a negative value of the tidal charge parameter, signaling the possibility of higher dimensions as well as scalar charge at play in those high gravity regimes.