Implications of Einstein-Maxwell dilaton-axion gravity from the black hole continuum spectrum

Abstract

String inspired models can serve as potential candidates to replace general relativity (GR) in the high energy/high curvature regime where quantum gravity is expected to play a vital role. Such models not only subsume the ultraviolet nature of gravity but also exhibit promising prospects in resolving issues like dark matter and dark energy, which cannot be adequately addressed within the framework of GR. The Einstein-Maxwell dilaton-axion (EMDA) theory, which is central to this work is one such string inspired model arising in the low energy effective action of the heterotic string theory with interesting implications in inflationary cosmology and in the late time acceleration of the universe. It is therefore important to survey the role of such a theory in explaining astrophysical observations, e.g. the continuum spectrum of black holes which are expected to hold a wealth of information regarding the background metric. The Kerr-Sen spacetime corresponds to the exact, stationary and axi-symmetric black hole solution in EMDA gravity, possessing dilatonic charge and angular momentum originating from the axionic field. In this work, we compute the theoretical spectrum from the accretion disk around quasars in the Kerr-Sen background assuming the thin accretion disk model due to Novikov & Thorne. This is then used to evaluate the theoretical estimates of optical luminosity for a sample of eighty Palomar-Green quasars which are subsequently compared with the available observations. Our analysis based on error estimators like the χ 2 , the Nash-Sutcliffe efficiency, the index of agreement etc., indicates that black holes carrying non-existent or weak dilaton charges (viz, 0 r2 0.1) are observationally more favored. The spins associated with the quasars are also estimated. Interestingly, a similar conclusion has been independently achieved by studying the observed jet power and the radiative efficiencies of microquasars. The implications are discussed.

Einstein-Maxwell-Dilaton-Axion mass formulas for black holes with struts and strings

“…Rotating electrically charged black hole solution of EMDA theory was obtained by Sen 28 within the first approach and independently, with the inclusion of NUT and magnetic charge, by Gal'tsov and Kechkin 31 in the second context. Recently, these solutions have attracted attention as an alternative to the Kerr metric in an effort to find astrophysical evidence for new physics [32][33][34] .…”