「休眠状態」の恒星質量ブラックホール、銀河系外で初検出　研究

【パリＡＦＰ＝時事】天の川銀河（銀河系）の近傍銀河で、恒星と連星系を形成している「休眠状態」の恒星質量ブラックホールを初めて発見したと、天体物理学者チームが先週発表した。（写真は太陽の２５倍の質量を持つ青色巨星と「休眠状態」の恒星質量ブラックホールで構成される連星系「ＶＦＴＳ ２４３」の想像図）
　英科学誌ネイチャー・アストロノミーに掲載された最新論文の筆頭執筆者で、オランダ・アムステルダム大学の天体物理学者トメル・シェナー氏は、「太陽の２５倍の質量を持つ大質量星が、見えない何かの周りを回っているのを発見した」とＡＦＰの取材に語った。
　天の川銀河の伴銀河、大マゼラン雲にあるこの青色巨星は、太陽の９倍の質量を持つブラックホールとの「死のダンス」に巻き込まれていると研究チームは考えている。
　この種のブラックホールは通常、連星系の伴星から物質を取り込む際に発するＸ線放射によって検出される。
　だが、この連星系「ＶＦＴＳ ２４３」は、Ｘ線を発していないため、休眠状態にあるとみられる。伴星の物質を取り込めるほど近い距離にはないのだ。
　論文の共同執筆者で、ベルギー・ルーベンカトリック大学の天体物理学者ユーグ・サナ氏は、超大質量ブラックホールに比べてはるかに小さい恒星質量ブラックホールは、天の川銀河内だけでも約１億個存在すると考えられていると説明する。
　ただその一方で、これまでに見つかっている恒星質量ブラックホールは１０個しかないとも指摘した。これは、その多くが伴星をのみ込むチャンスが訪れるのを待ちながら、休眠状態にあるからなのかもしれない。
　休眠ブラックホールの観測は、白い服を着た人と黒い服を着た人が暗室の中でダンスをするのを見るようなものだと、サナ氏は表現する。白い服を着た人が踊るのが見えるだけで、もう一人の存在が分かる仕掛けだ。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2022/07/27-14:01）

A team of astrophysicists known for debunking previous supposed black holes announced a discovery of their own on Monday: the first dormant stellar-mass black hole spotted orbiting a star in a nearby galaxy.
While these black holes are thought to be common throughout the universe, they have proved difficult to find, and they have themselves rejected several possible candidates in recent years.
Now the international team has found a needle in a haystack, said Tomer Shenar, an astrophysicist at the University of Amsterdam and lead author of a new study in the Nature Astronomy journal.
The team was searching the skies for something that could eventually become a binary black hole, in which two black holes orbit each other after swallowing their stars in a supernovae explosion.
We found a quite massive star, that weighs 25 times the mass of our Sun, that is orbiting around something that we do not see, Shenar told AFP.
They believe the blue star, which is in the Large Magellanic Cloud galaxy that neighbours our Milky Way, is locked in a death dance with a black hole that has nine times the mass of our Sun.
These kinds of black holes are normally detected by the X-ray radiation they emit as they collect material from their companion star.
But this binary system, known as VFTS 243, is called dormant because it does not emit X-rays -- it is not close enough to suck matter from its star.
- 'Black hole destroyer' -
Hugues Sana, astrophysicist at the KU Leuven University in Belgium, said the Milky Way alone is thought to have around 100 million stellar-mass black holes, which are far smaller than their supermassive big brothers.
However only 10 have been found, said Sana, a co-author of the study.
This could be because many are laying dormant, biding their time to eventually swallow their companion star.
Sana said observing them was like watching two people dance in a dark room, one dressed in white and the other in black. You might only see one dancer, but you know the other one is there.
We've never really detected such systems before, Shenar told AFP. There have been a few claims in the last years, but they have all more or less been refuted, Shenar told AFP.
Indeed, members of his team were among those rejecting previous discoveries, by laying out alternatives for what the data could indicate.
Because of this, Shenar said they expected extra scrutiny.
So they went about meticulously eliminating all the other possibilities, Shenar said, until they were satisfied that it's either a fat, invisible alien -- or a black hole.
Then they called the most famous black hole debunker they knew.
Kareem El-Badry of the Harvard-Smithsonian Center for Astrophysics has been debunking black holes one after another over the last couple of years, Tomer said, dubbing him the black hole destroyer.
I sent him the data and I told him, listen, we found this object -- prove me wrong, Tomer said.
I had my doubts, said El-Badry, who joined the team and ran his own simulations.
But I could not find a plausible explanation for the data that did not involve a black hole.
- Not with a bang, but a whimper -
The discovery could also give an insight into how black holes are formed.
Stellar-mass black holes are believed to be born during the death of a large star, in a massive supernovae explosion.
The force of the blast knocks black holes in a binary system into an elliptical, rather than circular orbit.
However, VFTS 243 has an orbit that is also perfectly circular.
That means that the star immediately vanished into the black hole, Shenar said.
This has a lot of implications as to how these black hole pairs form, he said, adding that VFTS 243's star could eventually collapse in a similar way.
Andrew Norton, an astrophysicist at Britain's Open University who was not involved in the study, said this is important evidence that all such stars may not end their lives in supernovae explosions.
Shenar said he welcomed other scientists trying to debunk the debunkers.
If someone comes and debunks this as well, I'm sure they will have a pretty fantastic explanation -- like the fat alien.