地球の気象や人体に与える影響は？
「群発」の高速電波バーストを観測、研究
2016年03月03日 09:16 発信地：パリ/フランス
【3月3日 AFP】
宇宙のはるかかなたで瞬間的に強力な電波が爆発的に放射される「高速電波バースト（FRB）」で、電波が複数回にわたって発せられている謎の現象を発見したとの研究結果が2日、発表された。
FRBは、その存在が初めて発見されてからまだ10年足らずしかたっていない。
英科学誌ネイチャー（Nature）に掲載された研究論文によると、この繰り返し続くパルス状の電波は、天の川銀河（Milky Way Galaxy）のはるか遠方から発せられているという。
1000分の1秒間に放たれるエネルギー量が、太陽放射の約1万年分に匹敵するFRBは、電波が単発的に放射される現象とこれまで考えられていた。
2007年に発見されたFRBは、これまでに20回足らずしか観測されていないが、日々1万回以上は発生していると考えられている。
発生源はいまだに不明だ。天文学者らはこれまで、恒星が爆発して超新星になったり、中性子星が崩壊してブラックホールに変化したりなどの激変的な事象によってFRBが発生するとの仮説を立てていた。
だが、今回の最新観測データが示している複数回のパルスは、これらのどの仮説シナリオにも合致しないため、このデータをどのように解釈すべきかについて、科学者らは困惑している。
論文の共同執筆者で、カナダ・マギル大学（McGill University）宇宙研究所の大学院生であるピーター・ショルツ（Peter Scholz）氏は、米自治領プエルトリコ（Puerto Rico）にあるアレシボ天文台（Arecibo Observatory）の電波望遠鏡で収集された観測データを調べていて、2015年11月に複数回の高速電波バーストが発生した証拠を発見した。
ショルツ氏は、声明で「この発見が、FRBの研究で極めて重要なものになるということがすぐに分かった」と述べている。
同天文台の世界最大級の電波望遠鏡は、合計10個の電波パルスをとらえていた。これらは全て、1分以内にまとまって観測されていた。
論文の主執筆者で、ドイツ・ボン（Bonn）にあるマックス・プランク電波天文学研究所（Max Planck Institute for Radio Astronomy）の研究者であるローラ・スピットラー（Laura Spitler）氏は「バーストが繰り返されることだけでなく、光度とスペクトルも、他のFRBとは異なっている」と話す。
今回の最新研究は、ネイチャー誌に先週発表された研究と食い違う結果になっているように思われる。先週発表された研究では、FRBが1回限りの激変現象で発生すると結論付けられていた。
だが目下のところ、FRBの発生源は2種類以上存在する可能性があると考えられている。
複数回のパルスは、極めて強力なエネルギーを放つ回転する中性子星などの「異種天体」に由来するものである可能性があると研究チームは推測している。
(c)AFP/Marlowe Hood
Mystery Cosmic Radio Pulse Leaves Magnetic Clue
Dec 3, 2015 02:21 PM ET // by Ian O'Neill
news.discovery.com/space/astronomy/mystery-cosmic-radio-pulse-leaves-magnetic-clue-151203
Somewhere out there in the darkest depths of our universe, something is generating mysterious eruptions of energy that are being detected as fast radio bursts — otherwise known as FRBs.
Since these weird phenomena were first discovered nearly a decade ago, astronomers have been at a loss to explain what they could be. Everything from the exotic (aliens?!) to the “mundane” (supernovas, boo) have been cited as possible culprits. Now, by painstakingly analyzing the signal of one of these pulses, astronomers may be on the trail to finally working out what the heck is rumbling the cosmos.
FRBs last only a fraction of a second but pack a hell of an energetic punch.
Because they are so transient, however, they’re not easy to record.
They flash, and they’re gone. Also, as they have often traveled vast distances, their signals are stretched and blurred out. According to a Carnegie Mellon University press release, though only a few have been detected, astronomers think the universe sees tens of thousands of these pulses every single day, but to see one you need to be looking at the right place at the right time.
After analyzing hundreds of hours of data from the National Science Foundation’s (NSF) Green Bank Telescope (GBT) in West Virginia, researchers stumbled across the most detailed signature of a FRB recorded to date. From this signal, they were able to gain a measure on its magnetic “fingerprint” to reveal the environment it must have been spawned.
“We now know that the energy from this FRB passed through a dense, magnetized region shortly after it formed,”
said astronomer Kiyoshi Masui, of the University of British Columbia and the Canadian Institute for Advanced Research.
“This significantly narrows down the source’s environment and type of event that triggered the burst.”
What’s dense and extremely magnetized?
Supernovas and star forming regions.
But exactly where did this FRB originate?
So far, the detected FRBs have originated in random parts of the sky; there appears to be no pattern, and this particular FRB was no different. But the one thing that set this event apart — dubbed “FRB 110523″ — is the detail held in its signal. Now the detective work could begin.
Finding FRB 110523 was no easy task; the researchers had to scan through 40 terabytes of radio data looking for the split-second pulses. They found 6,000 candidate FRBs in the data, but the vast majority were of too low a quality for analysis or confirmation. Another problem is that, after travelling potentially billions of light-years to reach Earth, these pulses become smeared or blurred, an effect known as “dispersion delay.” This has the effect of blending FRBs into the radio data, hiding it from view. The researchers therefore had to create a computer algorithm to track down these smeared-out pulses.
“Hidden within an incredibly massive dataset, we found a very peculiar signal, one that matched all the known characteristics of a Fast Radio Burst, but with a tantalizing extra polarization element that we simply have never seen before,”
said Jeffrey Peterson, a faculty member in Carnegie Mellon’s McWilliams Center for Cosmology.
“I feel extremely lucky to have identified the 16th (FRB),”
said Hsiu-Hsien Lin, also from Carnegie Mellon, who made the identification of FRB 110523.
“Not only is this the first FRB in this frequency range, our FRB has provided us with a great deal of information that help us to better understand this astrophysical phenomenon.”
This particular signal appeared like a diamond in the rough. Not only was its signal clear, it also held a polarization fingerprint (called Faraday rotation, a corkscrew-like twist in electromagnetic radiation) from where it was born.
“This tells us something about the magnetic field that the burst traveled through on its way to us, giving a hint about the burst’s environment,”
said Masui.
“It also gives the theorists a bit more to work with when they come up with explanations for these bursts.”
Measurements of the dispersion delay of FRB 110523 revealed the source of the signal was 6 billion light-years away (it was therefore triggered 6 billion years ago, when the universe was a little over half the age it is now), but additional analysis revealed the size of the source region — it came from another galaxy. But that’s not all.
Even more information was gleaned from the pulse’s dispersion delay; it revealed the signal passed through two distinct regions of ionized gas. One of these regions was very close to the source (within the source’s galaxy) and most likely
“a nebula surrounding the source or the environment near the center of a galaxy.”
“Taken together, these remarkable data reveal more about an FRB than we have ever seen before and give us important constraints on these mysterious events,”
said Masui.
“We also have an exciting new tool to search through otherwise overwhelming archival data to uncover more examples and get closer to truly understanding their nature.”
This voyage of discovery reminds me of the story behind the first detection of a pulsar in 1967. At the time, the periodic signal coming from the pulsar PSR B1919+21 was mysterious, so astronomers nicknamed it “LGM-1″ — a.k.a. “Little Green Men-1.
” The signal, however, was quickly identified as a rapidly-spinning neutron star generating powerful radio emissions as it rotated (creating pulses 1.34 seconds apart) and not evidence of an intelligent alien civilization.
Although the source of FRBs is not clear quite yet, it seems their mystery is slowly unraveling and the mechanism may lie deep inside the magnetic hearts of galaxies billions of light-years away.