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Iron Line from Neutron Star Accretion Disks in Scalar Tensor Theories

The Fe K_αfluorescent line at 6.4 keV is a powerful probe of the space-time metric in the vicinity of accreting compact objects. We investigated here how some alternative theories of gravity, namely Scalar tensor Theories, that invoke the presence of a non-minimally coupled scalar field and predict the existence of strongly scalarized neutron stars, change the expected line shape with respect to General Relativity. By taking into account both deviations from the general relativistic orbital dynamics of the accreting disk, where the Fe line originates, and the changes in the light propagation around the neutron star, we computed line shapes for various inclinations of the disk with respect to the observer. We found that both the intensity of the low energy tails and the position of the high energy edge of the line change. Moreover we verified that even if those changes are in general of the order of a few percent, they are potentially observable with the next generation of X-ray satellites.