Search for ultralight bosons in Cygnus X-1 with Advanced LIGO

TL;DR

This study searches for gravitational waves from ultralight boson clouds around Cygnus X-1 using Advanced LIGO data, setting new limits on boson masses and informing dark matter models.

Contribution

It provides the first directed search for such signals from Cygnus X-1 and constrains boson masses in specific models, including the string axiverse scenario.

Findings

No gravitational wave evidence found in 250-750 Hz band.

Constraints on boson masses in the range 6.4 to 8.0 x 10^{-13} eV.

Additional constraints for high-spin scenarios in the string axiverse.

Abstract

Ultralight scalars, if they exist as theorized, could form clouds around rapidly rotating black holes. Such clouds are expected to emit continuous, quasimonochromatic gravitational waves that could be detected by LIGO and Virgo. Here we present results of a directed search for such signals from the Cygnus X-1 binary, using data from Advanced LIGO's second observing run. We find no evidence of gravitational waves in the 250-750 Hz band. Without incorporating existing measurements of the Cygnus X-1 black hole spin, our results disfavor boson masses in $6.4 \leq \mu/(10^{-13} {\rm eV}) \leq 8.0$, assuming that the black hole was born $5 \times 10^6$ years ago with a nearly-extremal spin. We then focus on a string axiverse scenario, in which self-interactions enable a cloud for high black-hole spins consistent with measurements for Cygnus X-1. In that model, we constrain the boson masses in $9.6 \leq \mu/(10^{-13} {\rm eV}) \leq 15.5$ for a decay constant $f_a\sim 10^{15}$ GeV. Future application of our methods to other sources will yield improved constraints.