天文学者は、ガンマ線バーストがどのように形成されるかを再考する必要があるかもしれません

中性子星によるガンマ線バーストの想像図。 クレジット: Nuria Jordana-Mitjans

英国のバース大学の最近の研究によると、ガンマ線バーストの原因は、ブラック ホールではなく、生まれたばかりの超大質量星である可能性があります。

地球を周回する衛星は、数ミリ秒から数百秒続く非常にエネルギーの高いガンマ線放射の閃光として、ガンマ線バースト (GRB) を検出しました。 これらの壊滅的な爆発は、地球から数十億光年離れた遠方の銀河で発生します。

2 つの中性子星が衝突すると、短時間 GRB と呼ばれるタイプの GRB が生成されます。 私たちの太陽の質量を都市より小さいサイズに圧縮したこれらの超高密度の星は、時空の波紋と呼ばれる波紋を生成します。[{” attribute=””>gravitational waves just before triggering a GRB in their final moments.

Until now, space scientists have largely agreed that the ‘engine’ powering such energetic and short-lived bursts must always come from a newly formed

Dr. Jordana-Mitjans said: “Such findings are important as they confirm that newborn neutron stars can power some short-duration GRBs and the bright emissions across the electromagnetic spectrum that have been detected accompanying them. This discovery may offer a new way to locate neutron star mergers, and thus gravitational waves emitters when we’re searching the skies for signals.”

Competing theories

Much is known about short-duration GRBs. They start life when two neutron stars, which have been spiraling ever closer, constantly accelerating, finally crash. And from the crash site, a jetted explosion releases the gamma-ray radiation that makes a GRB, followed by a longer-lived afterglow. A day later, the radioactive material that was expelled in all directions during the explosion produced what researchers call a kilonova.

However, precisely what remains after two neutron stars collide – the ‘product’ of the crash – and consequently the power source that gives a GRB its extraordinary energy, has long been a matter of debate. Scientists may now be closer to resolving this debate, thanks to the findings of the Bath-led study.

Space scientists are split between two theories. The first theory has it that neutron stars merge to briefly form an extremely massive neutron star, only for this star to then collapse into a black hole in a fraction of a second. The second argues that the two neutron stars would result in a less heavy neutron star with a higher life expectancy.

So the question that has been needling astrophysicists for decades is this: are short-duration GRBs powered by a black hole or by the birth of a long-lived neutron star?

To date, most astrophysicists have supported the black hole theory, agreeing that to produce a GRB, it is necessary for the massive neutron star to collapse almost instantly.

Electromagnetic signals

Astrophysicists learn about neutron star collisions by measuring the electromagnetic signals of the resultant GRBs. The signal originating from a black hole would be expected to differ from that coming from a neutron star remnant.

The electromagnetic signal from the GRB explored for this study (named GRB 180618A) made it clear to Dr. Jordana-Mitjans and her collaborators that a neutron star remnant rather than a black hole must have given rise to this burst.

Elaborating, Dr. Jordana-Mitjans said: “For the first time, our observations highlight multiple signals from a surviving neutron star that lived for at least one day after the death of the original neutron star binary.”

Professor Carole Mundell, study co-author and professor of Extragalactic Astronomy at Bath, where she holds the Hiroko Sherwin Chair in Extragalactic Astronomy, said: “We were excited to catch the very early optical light from this short gamma-ray burst – something that is still largely impossible to do without using a robotic telescope. But when we analyzed our exquisite data, we were surprised to find we couldn’t explain it with the standard fast-collapse black hole model of GRBs.

“Our discovery opens new hope for upcoming sky surveys with telescopes such as the Rubin Observatory LSST with which we may find signals from hundreds of thousands of such long-lived neutron stars before they collapse to become black holes.”

Disappearing afterglow

What initially puzzled the researchers was that the optical light from the afterglow that followed GRB 180618A disappeared after just 35 minutes. Further analysis showed that the material responsible for such a brief emission was expanding close to the speed of light due to some source of continuous energy that was pushing it from behind.

What was more surprising was that this emission had the imprint of a newborn, rapidly spinning and highly magnetized neutron star called a millisecond magnetar. The team found that the magnetar after GRB 180618A was reheating the leftover material of the crash as it was slowing down.

In GRB 180618A, the magnetar-powered optical emission was one-thousand times brighter than what was expected from a classical kilonova.

Reference: “A Short Gamma-Ray Burst from a Protomagnetar Remnant” by N. Jordana-Mitjans, C. G. Mundell, C. Guidorzi, R. J. Smith, E. Ramírez-Ruiz, B. D. Metzger, S. Kobayashi, A. Gomboc, I. A. Steele, M. Shrestha, M. Marongiu, A. Rossi and B. Rothberg, 10 November 2022, The Astronomical Journal.
DOI: 10.3847/1538-4357/ac972b

The study was funded by the Hiroko and Jim Sherwin Postgraduate Studentship. 

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