Investigating the existence of gravitomagnetic monopole in M87*

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We examine the possibility for the existence of gravitomagnetic monopole (n_*) in M87* by using the results obtained from its first Event Horizon Telescope image. By numerically deducing the shadow sizes in Kerr-Taub-NUT (KTN) spacetime, we show that the shadow size increases with increasing |n_*| for a fixed Kerr parameter |a_*| in case of the KTN black hole, whereas for a KTN naked singularity it increases with increasing n_* for a fixed a_* > 0 if n_* > -cot 17^{circ } . In general, the asymmetry of shadow shape increases if the central dark object in M87 is a KTN/Kerr naked singularity instead of a KTN/Kerr black hole. We find that a non-zero gravitomagnetic monopole is still compatible with the current EHT observations, in which case the upper limit of n_* cannot be greater than 1.1, i.e., n_* lesssim 1.1 for the prograde rotation (a_* > 0), and the lower limit of n_* cannot be less than -1.1, i.e., n_* gtrsim -1.1 for the retrograde rotation (a_* < 0). Moreover, if the circularity of the shadow can be measured on a precision of lesssim 1%, the Kerr and KTN naked singularities can be falsified for M87*.