Kavli Affiliate: Qingjuan Yu

| First 5 Authors: M. Ghasemi-Nodehi, Chandrachur Chakraborty, Qingjuan Yu, Youjun Lu,

| Summary:

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*.

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