Investigating the existence of gravitomagnetic monopole in M87*

TL;DR

This study investigates the potential existence of a gravitomagnetic monopole in M87* using EHT data, showing that such a monopole could be compatible with observations and could be constrained or falsified with more precise measurements.

Contribution

It introduces a method to constrain the gravitomagnetic monopole parameter in M87* using shadow size and shape analysis from EHT data, considering Kerr-Taub-NUT spacetime.

Findings

Non-zero gravitomagnetic monopole is compatible with current EHT data.

Upper limit of monopole parameter n_* is approximately 1.1 for prograde rotation.

More precise shadow shape measurements could falsify Kerr and KTN naked singularity models.

Abstract

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*.