ブラックホール衝突後のキックを初めて測定（First measurement of a black hole’s ‘kick’ after cosmic collision）

2025-09-23 ペンシルベニア州立大学（PennState）

国際研究チームが、二つのブラックホールの衝突・合体後に生じる「キック」（反動運動）を初めて完全に測定しました。質量が不均衡なブラックホール同士の合体GW190412を対象に、LIGOとVirgoが2019年に検出した重力波データを解析。重力波の方向依存性と一般相対性理論の予測を組み合わせることで、合体後のブラックホールが秒速50km超（約31マイル/秒）で弾き出され、元の星団から離脱可能な速度に達していたことが判明しました。さらに、地球や元の連星系に対するキックの方向も特定。これにより、重力波のみで数十億光年彼方の天体の三次元運動を再構築できる新技術が実証されました。本成果はNature Astronomyに掲載され、今後は非対称合体の観測や電磁波とのマルチメッセンジャー天文学にも応用が期待されます。

When two black holes collide and merge, they each emit gravitational waves that “kick” the resulting newborn black hole, causing it to rapidly move and sometimes escape its own galaxy. If the two original black holes are of unequal masses, the gravitational waves appear significantly different when detected on Earth. This imbalance allowed an international team of researchers to fully characterize motion of this kick for the first time. Credit: Galician Institute for High Energy Physics. All Rights Reserved.

＜関連情報＞

高次重力波モードによるブラックホール反動の完全測定 A complete measurement of a black-hole recoil through higher-order gravitational-wave modes

Juan Calderón Bustillo,Samson H. W. Leong & Koustav Chandra
Nature Astronomy Published:09 September 2025
DOI:https://doi.org/10.1038/s41550-025-02632-5

Abstract

General relativity predicts that gravitational waves (GWs) carry linear momentum. Consequently, the remnant black hole of a black-hole merger can inherit a recoil velocity or ‘kick’ of crucial implications in, for example, black-hole formation scenarios. While the kick magnitude is determined by the mass ratio and spins of the source, estimating its direction requires a measurement of the two orientation angles of the source. While the orbital inclination angle is commonly reported in GW observations, the scientific potential of the azimuthal one has not been exploited so far. Here we show how the presence of more than one GW emission mode allows one to constrain this angle and, consequently, the kick direction of a real GW event. We analyse the GW190412 signal, which contains higher-order modes, with a numerical relativity surrogate waveform model for black-hole mergers. We rule out kick magnitudes below the typical escape velocity of dense globular clusters v_esc ≈ 50 km s⁻¹ with a Bayes factor of ~21 (or ~95% probability). The kick forms angles θ^-100M_KL=32⁺³⁵_-14with the orbital angular momentum defined at a reference time t_ref = −100 M before merger (with M denoting the system mass in geometric units), Φθ_KN=44⁺¹⁹_-17deg with the line of sight and ^-100M_KN =69⁺³³_-38with the projection of the latter onto the former, all quoted at a 90% credible level. We anticipate that complete characterization of black-hole recoils will aid in evaluating candidate multi-messenger observations of black-hole mergers in active galactic nuclei, by testing the consistency of observed signals with proposed electromagnetic emission mechanisms.