Investigating the existence of gravitomagnetic monopole in M87*

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 ^∘. 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_∗≲ 1.1 for the prograde rotation (a_∗> 0), and the lower limit of n_∗ cannot be less than - 1.1 , i.e., n_∗≳ - 1.1 for the retrograde rotation (a_∗< 0). Moreover, if the circularity of the shadow can be measured on a precision of ≲ 1 % , the Kerr and KTN naked singularities can be falsified for M87*.