Maximum Observable Blueshift from Circular Equatorial Kerr Orbiters

Abstract

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 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 momentum of supermassive black holes from the broadening of their surrounding line emission.

arXiv comment22 pages, 8 figures

References in corpus (25)

• Observation of Gravitational Waves from a Binary Black Hole Merger
• First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
• Tests of general relativity with GW150914
• First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
• Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits
• The Kerr/CFT Correspondence
• Black Hole Shadows, Photon Rings, and Lensing Rings
• Constraining Black Hole Spin Via X-ray Spectroscopy
• Black Hole Spin in X-Ray Binaries: Observational Consequences
• The Extreme Kerr Throat Geometry: A Vacuum Analog of AdS_2 x S^2
• The Black-Hole Mass in M87 from Gemini/NIFS Adaptive Optics Observations
• Universal Interferometric Signatures of a Black Hole's Photon Ring
• Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets
• Broad emission lines for negatively spinning black holes
• The M87 Black Hole Mass from Gas-dynamical Models of Space Telescope Imaging Spectrograph Observations
• The Shape of the Black Hole Photon Ring: A Precise Test of Strong-Field General Relativity
• Lensing by Kerr Black Holes
• The Null Geodesics of the Kerr Exterior
• Universal Polarimetric Signatures of the Black Hole Photon Ring
• Observational Signature of High Spin at the Event Horizon Telescope
• Particle motion near high-spin black holes
• Photon Emission Near Extreme Kerr Black Holes
• Venturing beyond the ISCO: Detecting X-ray emission from the plunging regions around black holes
• Critical Emission from a High-Spin Black Hole
• Blueshifted absorption lines from X-ray reflection in IRAS 13224-3809

Cited by in corpus (29)

• Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon
• Can the EHT M87 results be used to test general relativity?
• Photon rings of spherically symmetric black holes and robust tests of non-Kerr metrics
• Polarimetry and Astrometry of NIR Flares as Event Horizon Scale, Dynamical Probes for the Mass of Sgr A*
• The Polarized Image of a Synchrotron Emitting Ring of Gas Orbiting a Black Hole
• Photon Ring Autocorrelations
• Polarized Image of Equatorial Emission in the Kerr Geometry
• Holography of the Photon Ring
• Adaptive Analytical Ray Tracing of Black Hole Photon Rings
• Kerr Geodesics in Terms of Weierstrass Elliptic Functions
• Image of the Schwarzschild black hole pierced by a cosmic string with a thin accretion disk
• Photon Emission from Circular Equatorial Kerr Orbiters
• Complete classification of photon escape in the Kerr black hole spacetime
• Extreme lensing induces spectro-temporal correlations in black-hole signals
• Gravity versus astrophysics in black hole images and photon rings: Equatorial emissions and spherically symmetric space-times
• Photon emission from inside the innermost stable circular orbit
• Photon emissions from near-horizon extremal and near-extremal Kerr equatorial emitters
• High spin expansion for null geodesics
• Observability of Zero-angular-momentum Sources Near Kerr Black Holes
• Shadow and thin accretion disk around Ayón-Beato-García black hole coupled with cloud of strings
• Photon escape in the extremal Kerr black hole spacetime
• Observational properties of a Schwarzschild black hole surrounded by a Dehnen-type dark matter halo
• Morphology of Relativistically Broadened Line Emission from Axisymmetric Equatorial Accretion Disks
• Near-Horizon Collisions around Near-Extremal Black Holes
• Classification of radial Kerr geodesic motion
• Thin accretion disk around Schwarzschild-like black hole in bumblebee gravity
• Redshift Spectroscopy as a Probe of Regular Black Holes, Black Bounces, and Scalar-Hair Compact Objects
• Polarization Signatures of Inspiraling Hotspots around Kerr Black Holes
• How narrow is the M87* ring? I. The choice of closure likelihood function