English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

The Kerr-Sen black hole is a rotating charged black hole solution arisingfrom heterotic string theory. In 4-dimensions effective theory the bosonicfields are: a $U(1)$ gauge boson, a Kalb-Ramond 3-form which is equivalent to apseudoscalar axion in 4-dimensions, the dilaton and the graviton. The couplingconstants in the theory are $\alpha^{\prime}$ (inverse string tension) and$\kappa$ (inverse reduced Planck mass in 4-dimensions) and the charge of the$U(1)$ field and the axion-photon coupling are related to these two. Sen founda black hole solution (the Kerr-Sen black hole) with these fields as theexternal hair of the black hole. In this paper we investigate the possibilityof determining the Sen solution from observations. The observations which cantest the Kerr-Sen black hole are: (a) determination of the shape of the photonshadow, and (b) the rotation of polarization of photon due to axion hair. Thedeviation from circularity gives the $U(1)$ charge of the black hole andidentification of this charge in terms of the photon coupling leads to aprediction of frequency independent "Faraday rotation" in terms of black holeparameters already determined from the shadow. Similar measurements ofKerr-Newman black hole with axion hair have no correlation between the shape ofthe image and the amount of "Faraday rotation". This correlation can be adistinctive test of the Sen metric. In the recent observation from EHT of M87*shadow, the deviation from circularity has an upper bound of 10%. If thisobservation is refined to 1% accuracy then a definitive prediction of thecharge of the Kerr-Sen black hole and the "Faraday rotation" can be made.Interestingly observations of "Faraday rotation" have shown that the effect isindependent of frequency pointing to an axionic hair interpretation for theeffect.

Test of Kerr-Sen metric with black hole observations