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天文学研究3项新成果
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天文学研究新成果（1）
美国爱荷华大学（University of Iowa）的研究人员提供了一种理论来解释宇宙是如何成为充满光亮的，他们的研究结果已经在《皇家天文学会月纪》（Monthly Notices of the Royal Astronomical Society）杂志网站发表——
Abstract
Chandra observations of the nearby, Lyman-continuum (LyC) emitting galaxy Tol 1247-232 resolve the X-ray emission and show that it is dominated by a point-like source with a hard spectrum (Γ = 1.6 ± 0.5) and a high luminosity [(9 ± 2) × 1040 erg s− 1]. Comparison with an earlier XMM–Newton observation shows flux variation of a factor of 2. Hence, the X-ray emission likely arises from an accreting X-ray source: a low-luminosity active galactic nucleus or one or a few X-ray binaries. The Chandra X-ray source is similar to the point-like, hard spectrum (Γ = 1.2 ± 0.2), high-luminosity (1041 erg s− 1) source seen in Haro 11, which is the only other confirmed LyC-emitting galaxy that has been resolved in X-rays. We discuss the possibility that accreting X-ray sources contribute to LyC escape.
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Researchers propose how the universe became filled with light
August 30, 2017
Soon after the Big Bang, the universe went completely dark. The intense, seminal event that created the cosmos churned up so much hot, thick gas that light was completely trapped. Much later—perhaps as many as one billion years after the Big Bang—the universe expanded, became more transparent, and eventually filled up with galaxies, planets, stars, and other objects that give off visible light. That's the universe we know today.
How it emerged from the cosmic dark ages to a clearer, light-filled state remains a mystery.
In a new study, researchers at the University of Iowa offer a theory of how that happened. They think black holes that dwell in the center of galaxies fling out matter so violently that the ejected material pierces its cloudy surroundings, allowing light to escape. The researchers arrived at their theory after observing a nearby galaxy from which ultraviolet light is escaping.
"The observations show the presence of very bright X-ray sources that are likely accreting black holes," says Philip Kaaret, professor in the UI Department of Physics and Astronomy and corresponding author on the study. "It's possible the black hole is creating winds that help the ionizing radiation from the stars escape. Thus, black holes may have helped make the universe transparent."
Kaaret and his team focused on a galaxy called Tol 1247-232, located some 600 million light years from Earth, one of only three nearby galaxies from which ultraviolet light has been found to escape. In May 2016, using an Earth-orbiting telescope called Chandra, the researchers saw a single X-ray source whose brightness waxed and waned and was located within a vigorous star-forming region of Tol 1247-232.
The team determined it was something other than a star.
"Stars don't have changes in brightness," Kaaret says. "Our sun is a good example of that.
"To change in brightness, you have to be a small object, and that really narrows it down to a black hole," he says.
But how would a black hole, whose intense gravitational pull sucks in everything around it, also eject matter?
The quick answer is no one knows for sure. Black holes, after all, are hard to study, in part because their immense gravitational pull allows no light to escape and because they're embedded deep within galaxies. Recently, however, astronomers have offered an explanation: The jets of escaping matter are tapping into the accelerated rotational energy of the black hole itself.
Imagine a figure skater twirling with outstretched arms. As the skater folds her arms closer to her body, she spins faster. Black holes operate much the same way: As gravity pulls matter inward toward a black hole, the black hole likewise spins faster. As the black hole's gravitational pull increases, the speed also creates energy.
"As matter falls into a black hole, it starts to spin and the rapid rotation pushes some fraction of the matter out," Kaaret says. "They're producing these strong winds that could be opening an escape route for ultraviolet light. That could be what happened with the early galaxies."
Kaaret plans to study Tol 1247-232 more closely and find other nearby galaxies that are leaking ultraviolet light, which would help corroborate his theory.
Explore further:Image: Computer simulation of a supermassive black hole
天文学研究新成果（2）
遥远的星系发出15种高能无线电脉冲
Distant galaxy sends out 15 high-energy radio bursts
August 30, 2017 by Robert Sanders
Breakthrough Listen, an initiative to find signs of intelligent life in the universe, has detected 15 brief but powerful radio pulses emanating from a mysterious and repeating source – FRB 121102 – far across the universe.
Fast radio bursts are brief, bright pulses of radio emission from distant but largely unknown sources, and FRB 121102 is the only one known to repeat: more than 150 high-energy bursts have been observed coming from the object, which was identified last year as a dwarf galaxy about 3 billion light years from Earth.
Possible explanations for the repeating bursts range from outbursts from rotating neutron stars with extremely strong magnetic fields – so-called magnetars – to a more speculative idea: They are directed energy sources, powerful laser bursts used by extraterrestrial civilizations to power spacecraft, akin to Breakthrough Starshot's plan to use powerful laser pulses to propel nano-spacecraft to Earth's nearest star, Proxima Centauri.
"Bursts from this source have never been seen at this high a frequency," said Andrew Siemion, director of the Berkeley SETI Research Center and of the Breakthrough Listen program.
As astronomers around the globe try to understand the mechanism generating fast radio bursts, they have repeatedly turned their radio telescopes on FRB 121102. Siemion and his team alerted the astronomical community to the high-frequency activity via an Astronomer's Telegram on Monday evening, Aug. 28.
"As well as confirming that the source is in a newly active state, the high resolution of the data obtained by the Listen instrument will allow measurement of the properties of these mysterious bursts at a higher precision than ever possible before," said Breakthrough Listen postdoctoral researcher Vishal Gajjar, who discovered the increased activity.
First detected with the Parkes Telescope in Australia, fast radio bursts have now been seen by several radio telescopes around the world. FRB 121102 was discovered on Nov. 2, 2012, (hence its name) and in 2015 it was the first fast radio burst seen to repeat, ruling out theories of bursts' origins that involved the catastrophic destruction of the progenitor, at least in this instance.
Regardless of FRB 121102's ultimate source, when the recently detected pulses left their host galaxy, our solar system was less than 2 billion years old, noted Steve Croft, a Breakthrough Listen astronomer at UC Berkeley. Life on Earth consisted only of single-celled organisms; it would be another billion years before even the simplest multi-cellular life began to evolve.
As part of Breakthrough Listen's program to observe nearby stars and galaxies for signatures of extraterrestrial technology, the project science team at UC Berkeley added FRB 121102 to its list of targets. In the early hours of Saturday, Aug. 26, Gajjar observed that area of the sky using the Breakthrough Listen backend instrument at the Green Bank Telescope in West Virginia.
The instrument accumulated 400 terabytes (a million million bytes) of data over a five-hour period, observing across the entire 4 to 8 GHz frequency band. This large dataset was searched for signatures of short pulses from the source over a broad range of frequencies, with a characteristic dispersion, or delay as a function of frequency, caused by the presence of gas in space between Earth and the source. The distinctive shape that the dispersion imposes on the initial pulse is an indicator of the amount of material between us and the source, and hence an indicator of the distance to the host galaxy.
Analysis by Gajjar and the Breakthrough Listen team revealed 15 new pulses from FRB 121102. The observations show for the first time that fast radio bursts emit at higher frequencies than previously observed, with the brightest emission occurring at around 7 GHz.
"The extraordinary capabilities of the backend receiver, which is able to record several gigahertz of bandwidth at a time, split into billions of individual channels, enable a new view of the frequency spectrum of FRBs, and should shed additional light on the processes giving rise to FRB emission." Gajjar said.
"Whether or not fast radio bursts turn out to be signatures of extraterrestrial technology, Breakthrough Listen is helping to push the frontiers of a new and rapidly growing area of our understanding of the universe around us," Siemion said.
Explore further:Mysterious bursts of energy do come from outer space
天文学研究新成果（3）
天体物理学家将土星的卫星和土星环转变为音乐
Astrophysicists convert moons and rings of Saturn into music
August 30, 2017
After centuries of looking with awe and wonder at the beauty of Saturn and its rings, we can now listen to them, thanks to the efforts of astrophysicists at the University of Toronto (U of T).
"To celebrate the Grand Finale of NASA's Cassini mission next month, we converted Saturn's moons and rings into two pieces of music," says astrophysicist Matt Russo, a postdoctoral researcher at the Canadian Institute for Theoretical Astrophysics (CITA) in the Faculty of Arts & Science at U of T.
The conversion to music is made possible by orbital resonances, which occur when two objects execute different numbers of complete orbits in the same time, so that they keep returning to their initial configuration. The rhythmic gravitational tugs between them keep them locked in a tight repeating pattern which can also be converted directly into musical harmony.
"Wherever there is resonance there is music, and no other place in the solar system is more packed with resonances than Saturn," says Russo.
The Cassini spacecraft has been collecting data while orbiting Saturn since its arrival in 2004 and is now in the throes of a final death spiral. It will plunge into the planet itself on September 15 to avoid contaminating any of its moons.
The orbital periods of the six 1st order resonances of Janus that affect the ring system. The 1:1 resonance is with Janus' co-orbital moon Epimetheus. The corresponding frequencies of these resonances were scaled up by 23 octaves by astrophysicists at the University of Toronto, producing a musical scale. Credit: SYSTEM Sounds/NASA/JPL/Space Science Institute
Russo was joined by astrophysicist Dan Tamayo, a postdoctoral researcher at CITA and the Centre for Planetary Sciences at U of T Scarborough, and together they were able to play music with an instrument measuring over a million kilometers long. The musical notes and rhythms both come from the orbital motion of Saturn's moons along with the orbits of the trillions of small particles that make up the ring system.
"Saturn's magnificent rings act like a sounding board that launches waves at locations that harmonize with the planet's many moons, and some pairs of moons are themselves locked in resonances," says Tamayo.
Music of the moons and rings
For the first piece which follows Cassini's final plunge, the researchers increased the natural orbital frequencies of Saturn's six large inner moons by 27 octaves to arrive at musical notes. "What you hear are the actual frequencies of the moons, shifted into the human hearing range" says Russo. The team then used a state of the art numerical simulation of the moon system developed by Tamayo to play the resulting notes every time a moon completes an orbit.
The moon system has two orbital resonances which give rhythmic and harmonic structure to the otherwise unsteady lullaby-style melody. The first and third moons Mimas and Tethys are locked in a 2:1 resonance so that Mimas orbits twice for every orbit of Tethys. The same relationship links the orbits of the second and fourth moons Enceledus and Dione, and the combination of the two simple rhythms creates interesting musical patterns as they fall in and out of synchronicity.
A wood carving of Saturn's main ring system designed for the visually impaired, commissioned by astrophysicists at the University of Toronto. One will be able to feel many complex structures within the rings while also listening to their audio form. Credit: SYSTEM Sounds
"Since doubling the frequency of a note produces the same note an octave higher, the four inner moons produce only two different notes close to a perfect fifth apart," says Russo, who is also a graduate of U of T's Jazz performance program. "The fifth moon Rhea completes a major chord that is disturbed by the ominous entrance of Saturn's largest moon, Titan."
Russo and Tamayo are joined in the project by Toronto musician, and Matt's long-time bandmate, Andrew Santaguida. "Dan understands orbital resonances as deeply as anyone and Andrew is a music production wizard. My job is to connect these two worlds."
Titan actually gives the Cassini probe the final push which sends it hurtling towards its death in the heart of Saturn. The music follows Cassini's final flight over the ring system by converting the constantly increasing orbital frequencies of the rings into a dramatic rising pitch; the volume of the tone increases and decreases along with the observed bright and dark bands of the rings. The death of Cassini as it crashes into Saturn is heard as a final crash of a final piano chord, which was inspired by The Beatles' "A Day in the Life", in which a rich major chord follows a similarly tense crescendo.
In addition to the soundtrack, Russo has had a large wood carving made of Saturn's rings so people can follow along with their fingertips while listening. The carving will be part of a tactile-audio astronomy exhibit at the Canadian National Institute for the Blind's Night Steps fundraising event for the visually impaired in Toronto on September 15, the same day the Cassini mission is scheduled to end.
Resonances of Janus translated into music
The second piece demonstrates the scales played by Janus and Epimetheus, two small irregular moons that share an orbit just outside Saturn's main ring system. Together they are an example of 1:1 resonance, the only one in the solar system. The pair orbit at slightly different distances from Saturn but with a difference that is so negligible they swap places every four years. The composition simulates the final few months of Cassini's mission, while Janus is inching closer to Epimetheus before stealing its place in 2018. Together, the two moons play a unison drone but with a constantly shifting rhythm that repeats every eight years.
Russo played a C# note on his guitar once for every orbit while a cello sustains a note for each resonance within the rings.
"Each ring is like a circular string, being continuously bowed by Janus and Epimetheus as they chase each other around their shared orbit," says Russo. Cassini recently captured an image of one of the ripples this creates within the rings. To turn this into music, Russo and Santaguida used the brightness variations in this image to control the intensity of the cello.
"Saturn's dancing moons now have a soundtrack," says Russo.
Russo, Tamayo and Santaguida are the same group who converted the recently discovered TRAPPIST-1 planetary system into music a few months ago. They've dubbed their astro-sonic side-project SYSTEM Sounds) and hope to continue exploring the universe for other evidence of naturally occurring harmonic resonance.
Explore further:Image: Saturn and rings, 7 June 2017
https://blog.sciencenet.cn/blog-212210-1073647.html
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