Maximum observable blueshift from circular equatorial Kerr orbiters

Delilah E.A. Gates, Shahar Hadar, Alexandru Lupsasca

Research output: Contribution to journalArticlepeer-review


The region of spacetime near the event horizon of a black hole can be viewed as a deep potential well at large gravitational redshift relative to distant observers. However, matter orbiting in this region travels at relativistic speeds and can impart a significant Doppler shift to its electromagnetic emission, sometimes resulting in a net observed blueshift at infinity. Thus, a black hole broadens the line emission from monochromatic sources in its vicinity into a smoothly decaying "red wing"- whose flux vanishes at large redshift - together with a "blue blade"that retains finite flux up to a sharp edge corresponding to the maximum observable blueshift. In this paper, we study the blue blade produced by isotropic monochromatic emitters on circular equatorial orbits around a Kerr black hole, and we obtain simple relations describing how the maximum blueshift encodes black hole spin and inclination. We find that small values of the maximum blueshift yield an excellent probe of inclination, while larger values provide strong constraints on spin or inclination in terms of the other. These results bear direct relevance to ongoing and future observations aiming to infer the angular momenta of supermassive black holes from the broadening of their surrounding line emission.

Original languageEnglish
Article number104041
JournalPhysical Review D
Issue number10
StatePublished - 16 Nov 2020
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2020 American Physical Society.

ASJC Scopus subject areas

  • Physics and Astronomy (miscellaneous)


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