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 is expected to hold a wealth of information regarding the background metric. The Kerr-Sen spacetime corresponds to the exact, stationary, and axisymmetric 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 results based on χ2 analysis indicate that the dilaton parameter r2 ∼ 0.2 is favored by optical observations of quasars which is further corroborated by other error estimators e.g., the Nash-Sutcliffe efficiency, the index of agreement and their modified versions. We further report that strong dilaton charges (r2 > 1.6) are disfavored by quasar optical data and the spins associated with the quasars are also estimated.

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Extra dimensions, which led to the foundation and inception of string theory, provide an elegant approach to force-unification. With bulk curvature as high as the Planck scale, higher curvature terms, namely f (R) gravity seems to be a natural addendum in the bulk action. These can not only pass the classic tests of general relativity but also serve as potential alternatives to dark matter and dark energy. With interesting implications in inflationary cosmology, gravitational waves and particle phenomenology it is worth exploring the impact of extra dimensions and higher curvature in black hole accretion. Various classes of black hole solutions have been derived which bear nontrivial imprints of these ultraviolet corrections to general relativity. This in turn gets engraved in the continuum spectrum emitted by the accretion disk around black holes. Since the near horizon regime of supermassive black holes manifest maximum curvature effects, we compare the theoretical estimates of disk luminosity with quasar optical data to discern the effect of the modified background on the spectrum. In particular, we explore a certain class of black hole solution bearing a striking resemblance with the well-known Reissner-Nordström de Sitter/anti-de Sitter/flat spacetime which unlike general relativity can also accommodate a negative charge parameter. By computing error estimators like chi-square, Nash-Sutcliffe efficiency, index of agreement, etc. we infer that optical observations of quasars favor a negative charge parameter which can be a possible indicator of extra dimensions. The analysis also supports an asymptotically de Sitter spacetime with an estimate of the magnitude of the cosmological constant whose origin is solely attributed to higher curvature terms in higher dimensions. * tpib@iacs.res.in † tpbm3@iacs.res.in ‡ tpssg@iacs.res.in arXiv:1905.12820v2 [gr-qc] 1 Aug 2019 is still elusive and ill-understood [15][16][17]. All this makes the quest for a more complete theory of gravity increasingly compelling such that it yields GR in the low energy limit. Consequently a surfeit of alternate gravity models are proposed which can potentially fulfill the deficiencies in GR. A viable alternate gravity theory must be free from ghost modes, be consistent with solar system based tests, should not engender a fifth force in local physics and should successfully explain observations that GR fails to address. The alternate gravity models which fulfill these benchmark can be broadly classified into three categories: (i) Modified gravity models where the gravity action is supplemented with higher curvature terms, e.g., f(R) gravity [18][19][20][21], Lanczos-Lovelock models etc. [22][23][24][25][26] (ii) Extra-dimensional models that alter the effective 4-dimensional gravitational field equations due to the bulk Weyl stresses and higher order corrections to the stress-tensor [27][28][29][30][31][32][33] and (iii) Scalar-tensor theories of gravity which include the Brans-Dicke theory and the more general Horndeski models [34][35][36][...

The presence of axionic field can provide plausible explanation to several long standing problems in physics such as dark matter and dark energy. The pseudo-scalar axion whose derivative corresponds to the Hodge dual of the Kalb-Ramond field strength in four dimensions plays crucial roles in explaining several astrophysical and cosmological observations. Therefore, the detection of axionic hairs/Kalb-Ramond field which appears as closed string excitations in the heterotic string spectrum may provide a profound insight to our understanding of the current universe. The current level of precision achieved in solar-system based tests employed to test general relativity, is not sufficient to detect the presence of axion. However, the near horizon regime of quasars where the curvature effects are maximum seems to be a natural laboratory to probe such additions to the matter sector. The continuum spectrum emitted from the accretion disk around quasars encapsulates the imprints of the background spacetime and hence acts as a storehouse of information regarding the nature of gravitational interaction in extreme situations. The surfeit of data available in the electromagnetic domain provides a further motivation to explore such systems. Using the optical data for eighty Palomar Green quasars we demonstrate that the theoretical estimates of optical luminosity explain the observations best when the axionic field is assumed to be absent. However, axion which violates the energy condition seems to be favored by observations which has several interesting consequences. Error estimators, including reduced χ 2 , Nash-Sutcliffe efficiency, index of agreement and modified versions of the last two are used to solidify our conclusion and the implications of our result are discussed.

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