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Stimulated radiation from superradiant scalar cloud in scalar-tensor theory

Wenyi Wang, Sousuke Noda, Taishi Katsuragawa

TL;DR

The paper addresses how fundamental scalars in scalar–tensor theories can form superradiant clouds around Kerr black holes and emit stimulated photons, with the process governed by environmental mass variation via the chameleon mechanism. It combines Kerr perturbation theory with a Boltzmann treatment of $\varphi \to \gamma\gamma$ decay, highlighting how uniform and non-uniform matter backgrounds yield qualitatively different growth and emission profiles; the non-uniform case, in particular, can enhance the instability and alter emission timescales. For uniform matter, the bound-state spectrum is hydrogenic for $\mu_0 M \ll 1$, and the fastest-growing mode $l=m=1$ can reach photon luminosities up to $\sim 10^{37}$ erg s$^{-1}$, with emission strongest near the equator; the $l=2,m=1$ mode peaks at $\theta=\pi/4$ with $\sim 10^{35}$ erg s$^{-1}$. The environmental dependence driven by the chameleon mechanism provides a distinctive observational handle to differentiate scalar–tensor scalars from axions or other light bosons, though future work should incorporate more realistic disk-like matter distributions and connect with terrestrial constraints to robustly map the viable parameter space.

Abstract

Scalar-tensor theories predict fundamental scalar fields of considerable interest in astrophysics and cosmology. We investigate the superradiant instability of scalar clouds around Kerr black holes, showing that stimulated decay generates detectable electromagnetic signals. The growth of the superradiant scalar cloud differs from that of other bosonic fields and depends sensitively on the matter distribution surrounding the black hole, which originates from the scalar-matter coupling realized by the chameleon mechanism in modified gravity theories. In non-uniform matter distributions, stimulated emission from scalar clouds offers an observational signature that distinguishes fundamental scalars from other light bosonic fields.

Stimulated radiation from superradiant scalar cloud in scalar-tensor theory

TL;DR

The paper addresses how fundamental scalars in scalar–tensor theories can form superradiant clouds around Kerr black holes and emit stimulated photons, with the process governed by environmental mass variation via the chameleon mechanism. It combines Kerr perturbation theory with a Boltzmann treatment of decay, highlighting how uniform and non-uniform matter backgrounds yield qualitatively different growth and emission profiles; the non-uniform case, in particular, can enhance the instability and alter emission timescales. For uniform matter, the bound-state spectrum is hydrogenic for , and the fastest-growing mode can reach photon luminosities up to erg s, with emission strongest near the equator; the mode peaks at with erg s. The environmental dependence driven by the chameleon mechanism provides a distinctive observational handle to differentiate scalar–tensor scalars from axions or other light bosons, though future work should incorporate more realistic disk-like matter distributions and connect with terrestrial constraints to robustly map the viable parameter space.

Abstract

Scalar-tensor theories predict fundamental scalar fields of considerable interest in astrophysics and cosmology. We investigate the superradiant instability of scalar clouds around Kerr black holes, showing that stimulated decay generates detectable electromagnetic signals. The growth of the superradiant scalar cloud differs from that of other bosonic fields and depends sensitively on the matter distribution surrounding the black hole, which originates from the scalar-matter coupling realized by the chameleon mechanism in modified gravity theories. In non-uniform matter distributions, stimulated emission from scalar clouds offers an observational signature that distinguishes fundamental scalars from other light bosonic fields.
Paper Structure (10 sections, 39 equations, 18 figures)

This paper contains 10 sections, 39 equations, 18 figures.

Table of Contents

  1. Introduction
  2. Scalar-tensor theory and field equations
  3. The uniform distribution of matter
  4. Superradiant instability and stimulated decay of scalar cloud
  5. $l=m=1$
  6. $l=2, m=1$
  7. The non-uniform distribution of matter
  8. Superradiant instability
  9. Stimulated radiation
  10. conclusions and discussion

Figures (18)

  • Figure 1: Model of the superradiant “2p” scalar cloud around a Kerr black hole.
  • Figure 2: The evolution of scalar (blue line) and photon (red line) number in the non-spherical for $l=m=1$. The figure is generated based on the parameter values given in Eq. \ref{['uniform-1']}. The left panel shows the entire time scale considered here, while the right panel presents an enlarged view around the time at which the photon number begins to increase.
  • Figure 3: scalar (top panels) and photon (bottom panels) number in the non-spherical for different angles (left panel:$\theta = \pi/2$, right panel:$\theta = \pi/4$).
  • Figure 4: The evolution of the average photon luminosity after the stimulated decay takes over.
  • Figure 5: Model of the superradiant “$l=2,m=1$” scalar cloud around a Kerr black hole.
  • ...and 13 more figures