Investigating the existence of gravitomagnetic monopole in M87*

Abstract

Abstract We examine the possibility for the existence of gravitomagnetic monopole ( $$n_*$$ 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 $$|n_*|$$ | n ∗ | for a fixed Kerr parameter $$|a_*|$$ | a ∗ | in case of the KTN black hole, whereas for a KTN naked singularity it increases with increasing $$n_*$$ n ∗ for a fixed $$a_* > 0$$ a ∗ > 0 if $$n_* > -\cot 17^{\circ }$$ 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 $$n_*$$ n ∗ cannot be greater than 1.1, i.e., $$n_* \lesssim 1.1$$ n ∗ ≲ 1.1 for the prograde rotation ( $$a_* > 0$$ a ∗ > 0 ), and the lower limit of $$n_*$$ n ∗ cannot be less than $$-1.1$$ - 1.1 , i.e., $$ n_* \gtrsim -1.1$$ n ∗ ≳ - 1.1 for the retrograde rotation ( $$a_* < 0$$ a ∗ < 0 ). Moreover, if the circularity of the shadow can be measured on a precision of $$\lesssim 1\%$$ ≲ 1 % , the Kerr and KTN naked singularities can be falsified for M87*.