2 Static spherically symmetric black hole solutions in a
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/KalbRamond 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 kh, 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.