FIG. The NUT parameter (n/M ) versus the spin parameter (a/M ) space, which is divided into a black hole region and a naked singularity region by the black dashed line. A zoomed version is shown in the inset for clarity. This figure shows that there is a range of n/M and a/M values for GRO J1655–40 allowed by all the three methods. Credit : American Physical Society
North and south magnetic poles always go hand in hand in nature. Cutting a bar magnet in half just creates two magnets, each of which still has two poles, rather than creating separate north and south poles. However, their electrostatic cousins, positive and negative charges, can exist independently. In 1931, British physicist P. A. M. Dirac taught us that the existence of magnetic monopoles implies that electric charge is quantized. Although some condensed matter systems contain phenomena that are mathematically analogous to magnetic monopoles, there is no experimental or observational evidence that magnetic monopoles exist indeed. When the analogy of Einstein’s general relativity is drawn with electromagnetism,‘gravitoelectric charge’ exists and is referred to as ‘mass’ which is an important constituent of the physical world. The next evident question that arises is ‘does gravitomagnetic charge or the so-called gravitomagnetic monopole exist in nature?’
Historically, Newman, Tamburino and Unti discovered a stationary and spherically symmetric vacuum solution (which is now known as the NUT solution) of the Einstein’s equation, that contains the gravitomagnetic monopole or the so-called NUT parameter. Though this gravitomagnetic monopole is physically interpreted as “a linear source of pure angular momentum”, i.e., “a massless rotating rod”, and is the proposed gravitational analogue of Dirac’s magnetic monopole, an observational evidence of this aspect of fundamental physics was elusive. However,the first significant observational indication of the gravitomagnetic monopole has recently been reported by Dr. Chandrachur Chakraborty from Kavli Institute for Astronomy and Astrophysics, Beijing, China and Prof. Sudip Bhattacharyya from Tata Institute of Fundamental Research, Mumbai, India, based on the X-ray observations of an astrophysical collapsed object : GRO J1655-40. They have inferred that this object not only contains the non-zero gravitomagnetic monopole but their result tentatively suggests that the above mentioned astrophysical collapsed object could also be a naked singularity.
Here, we should note that a rotating astrophysical collapsed object is generally described by the Kerr spacetime, and is characterized by only two parameters : mass and spin. Depending on the spin value of the rotating astrophysical collapsed object, it could be a black hole, or a naked singularity (also sometimes called a “superspinar”) which is visible in principle to external observers. Now, if the Kerr spacetime contains the gravitomagnetic monopole or vice-versa, it is regarded as the Kerr-Taub-NUT (KTN) spacetime.
The earlier three independent primary X-ray observational methods provided significantly different spin values for GRO J1655-40, which were 0.286 ± 0.003, 0.65 − 0.75 and 0.90 − 0.99. Employing a new technique, the two scientists have demonstrated that the inclusion of one extra parameter (i.e., gravitomagnetic monopole) not only makes the spin and other parameter values inferred from the three methods consistent with each other (see figure), but also makes the inferred black hole mass consistent with an independently measured value. Therefore, their inference of the first significant observational indication of the gravitomagnetic monopole, which, even though is not a direct detection, can have an exciting impact on fundamental physics and astrophysics. Moreover, as the accreting collapsed object GRO J1655-40 could be better described with the more general KTN spacetime, instead of the Kerr spacetime, this makes the KTN spacetime astrophysically relevant.
Paper link : https://link.aps.org/doi/10.1103/PhysRevD.98.043021