Eur. Phys. J. C (2021) 81: 939
https://doi.org/10.1140/epjc/s10052-021-09696-3

Regular Article - Theoretical Physics

Investigating the existence of gravitomagnetic monopole in M87*

¹ National Astronomical Observatories, Chinese Academy of Sciences, 100101, Beijing, China
² School of Astronomy, Institute for Research in Fundamental Sciences, 19395-5531, Tehran, Iran
³ Kavli Institute for Astronomy and Astrophysics, Peking University, 100871, Beijing, China
⁴ Depatment of Physics, Indian Institute of Science, 560012, Bengaluru, India
⁵ School of Astronomy and Space Science, University of Chinese Academy of Sciences, 100049, Beijing, China

^a mghasemin@ipm.ir

Received: 10 August 2021
Accepted: 28 September 2021
Published online: 23 October 2021

Abstract

We examine the possibility for the existence of gravitomagnetic monopole ($n_{\ast }$) 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 $|n_{\ast }|$ for a fixed Kerr parameter $|a_{\ast }|$ in case of the KTN black hole, whereas for a KTN naked singularity it increases with increasing $n_{\ast }$ for a fixed $a_{\ast }>0$ if $n_{\ast }>-cot{17}^{\circ }$ . 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 $n_{\ast }$ cannot be greater than 1.1, i.e., $n_{\ast }\lesssim 1.1$ for the prograde rotation ($a_{\ast }>0$), and the lower limit of $n_{\ast }$ cannot be less than $-1.1$, i.e., $n_{\ast }\gtrsim -1.1$ for the retrograde rotation ($a_{\ast }<0$). 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*.