“Shadow of Kerr Black Hole Surrounded by Cloud of Strings in Rastall Gravity: Observations

Motivated by the first image of a black hole captured by the EHT, there has
been a surge of research using observations of black hole shadows to test
gravity theories. In this paper, we carry out the related study about shadow of
Kerr black hole surrounded by a cloud of strings in Rastall gravity, which
deviates from the Kerr black hole due to the presence of the string parameter
$a_0$ and the parameter $beta$. The horizons, ergospheres, and photon region
of the black hole are shown. Moreover, we explore the shadow and observations
of the black hole, which are closely linked to the parameters $a_0$ and
$beta$. Treating M87* as Kerr black hole surrounded by a cloud of strings
under Rastall gravity, we constrain the black hole parameters by the EHT
observations. For a given $beta$, the circularity deviation of the black hole
obeys $Delta Clesssim0.1$ in all regions. The angular diameter
$theta_{d}=42pm3mu as$ can give the upper bound of parameters $a$ and $a_0$
for fixed $beta$. The shadow axis ratio satisfies the observation data of EHT
($1<D_xlesssim4/3$) in the whole space for a given $beta$. These results are
consistent with the public information of EHT. In other words, candidates for
real astrophysical black holes can be Kerr black holes surrounded by a cloud of
strings in Rastall gravity.

Conclusions

The research presented in this paper focuses on the shadow of a Kerr black hole surrounded by a cloud of strings in Rastall gravity. The study examines the horizons, ergospheres, and photon region of the black hole, as well as the parameters $a_0$ and $beta$ that affect its properties. The observations of the black hole shadow are closely linked to these parameters.

Using the EHT observations of M87*, the paper also discusses how the black hole parameters can be constrained. The circularity deviation of the black hole is found to be $Delta Clesssim0.1$ in all regions for a given $beta$. The angular diameter $theta_{d}=42pm3mu as$ provides an upper bound for the parameters $a$ and $a_0$ with fixed $beta$. The shadow axis ratio is also found to be consistent with the EHT observation data (

Based on these findings, it is concluded that Kerr black holes surrounded by a cloud of strings in Rastall gravity can serve as candidates for real astrophysical black holes.

Future Roadmap

Building on this research, there are several potential challenges and opportunities on the horizon:

1. Further observational verification: It is important to continue gathering observational data of black holes to further verify the consistency of the parameters and properties discussed in this study. This could involve utilizing data from future EHT observations or other telescopes and instruments.
2. Refining parameter constraints: The current constraints on the black hole parameters are based on a fixed $beta$ value. Future research could explore how the constraints vary with different values of $beta$ to provide a more comprehensive understanding of the underlying physics.
3. Exploring other gravity theories: While this study focuses on Rastall gravity, there are several alternative theories of gravity that could also be investigated. Comparing the results across different theories can help shed light on the fundamental properties of black holes.
4. Investigating the nature of the string cloud: The presence of a cloud of strings around black holes is an intriguing concept. Further research could delve into the nature and behavior of these strings, potentially revealing new insights into the interactions between gravity and quantum physics.

By addressing these challenges and opportunities, future research in this field can contribute towards a more comprehensive understanding of black holes and their role in the universe.

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