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String inspired models can serve as potential candidates to replace GR in thehigh energy regime where quantum gravity is expected to play a vital role. Suchmodels not only subsume the ultraviolet nature of gravity but also exhibitpromising prospects in resolving issues like dark matter and dark energy, whichcannot be adequately addressed within the framework of GR. The Einstein-Maxwelldilaton-axion (EMDA) theory, which is central to this work is one such stringinspired model arising in the low energy effective action of the heteroticstring theory with interesting implications in inflationary cosmology and inthe late time acceleration of the universe. It is therefore important to surveythe role of such a theory in explaining astrophysical observations, e.g. thecontinuum spectrum of black holes which is expected to hold a wealth ofinformation regarding the background metric. The Kerr-Sen spacetime correspondsto the exact, stationary, and axisymmetric black hole solution in EMDA gravity,possessing dilatonic charge and angular momentum originating from the axionicfield. In this work, we compute the theoretical spectrum from the accretiondisk around quasars in the Kerr-Sen background assuming the thin accretion diskmodel due to Novikov \& Thorne. This is then used to evaluate the theoreticalestimates of optical luminosity for a sample of eighty Palomar-Green quasarswhich are subsequently compared with the available observations. Our resultsbased on $\chi^2$ analysis indicate that the dilaton parameter $r_2\sim 0.2$ isfavored by optical observations of quasars which is further corroborated byother error estimators e.g., the Nash-Sutcliffe efficiency, the index ofagreement, and their modified versions. We further report that strong dilatoncharges ($r_2>1.6$) are disfavored by quasar optical data and the spinsassociated with the quasars are also estimated.

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