How To Differentiate Between Black Holes and Their Mimickers? (Planetary Science)

Tidal forces can tear apart a massive object due to the gravitational influence of another. This fact assumes importance in the context of supermassive black holes (BHs) with masses M ∼ 106– 1010M that are believed to exist at the center of most galaxies. Stars that come in the vicinity of such large masses are often disrupted by extreme gravity, when the tidal forces become comparable to, or larger than, the self gravity of the star. Such tidal disruption events (TDEs) of stars in the background of a massive BH often produce an observable luminous flare, that can reveal important properties of the stellar structure, as well as that of the BH. Indeed, TDEs are known to be one of the main physical processes responsible for the formation of accretion disks around BHs, a topic that has received considerable attention of late, after the advent of the event horizon telescope. Although several such TDEs have been observed, and seminal works have appeared in the literature over the last several decades, it is perhaps fair to say that a complete theoretical understanding of these processes is still lacking. This assumes importance in the light of the Large Synoptic Survey Telescope (LSST) which is expected to provide more data on TDEs in the near future.

In the context of a central mass M, the tidal radius Rt ∼ R∗ (M/M∗)^1/3 , defined as the closest radial distance a stellar object of mass M∗ and radius R∗ can exist without getting tidally disrupted, is ubiquitous. This “1/3 law” is a standard textbook result where one uses the fact that at the tidal disruption limit, the tidal force equals the force due to self gravity at the surface of a star. This Newtonian result is true for any central mass M, regardless of the geometry that it produces, and it is perfectly legitimate to apply this to solutions of general relativity (GR) other than black holes (BHs), which might not have any obvious interpretation as a singularity covered by an event horizon. Indeed, while tidal disruption events (TDEs) are believed to be common near galactic centers, relatively less attention has been paid in the literature to the fact that such objects that do not have an event horizon (i.e are not black holes) can also tidally disrupt stars. One such example is provided by the wormhole (WH) geometry.

As, wormholes have various observational features that are similar to black holes, these can act as black hole mimickers. Considering this, Sarkar and colleagues studied, to what extent these behave differently as far as tidal disruptions are considered.

They showed that although at large distances both behave like Newtonian objects, close to the event horizon or to the throat, black holes and wormholes have different tidal effects on stars, due to their respective geometries.

They quantified this difference by a numerical procedure in the Schwarzschild black hole and the exponential wormhole backgrounds, and compared the peak fallback rates of tidal debris in these geometries. They also computed tidal disruption rates in these backgrounds.

Finally, they found that the peak fallback rate for wormholes might be about 4 times higher than that of a black hole with similar mass. Secondly, the tidal disruption rates for the wormhole examples they considered are greater than those for the corresponding black holes.

“Our analysis here is numerical, and specialized to a static scenario. A time-dependent analysis of tidal disruptions will be a natural extension of this work.”

— concluded authors of the study

Reference: Pritam Banerjee, Suvankar Paul, Rajibul Shaikh and Tapobrata Sarkar, “Tidal disruption near black holes and their mimickers”, Journal of Cosmology and Astroparticle Physics, Volume 2021, March 2021. Link to paper

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