At arbejde sammen som et virtuelt teleskop, observatorier rundt om i verden producerer de første direkte billeder af et sort hul

Et internationalt hold på over 200 astronomer, inklusive videnskabsmænd fra MIT's Haystack Observatory, har taget de første direkte billeder af et sort hul. De opnåede denne bemærkelsesværdige bedrift ved at koordinere kraften fra otte store radioobservatorier på fire kontinenter, at arbejde sammen som en virtuel, Teleskop i jordstørrelse.

I en række artikler offentliggjort i dag i et særnummer af Astrofysiske tidsskriftsbreve , holdet har afsløret fire billeder af det supermassive sorte hul i hjertet af Messier 87, eller M87, en galakse i Virgo-galaksehoben, 55 millioner lysår fra Jorden.

Alle fire billeder viser et centralt mørkt område omgivet af en lysring, der virker skæv - lysere på den ene side end på den anden.

Albert Einstein, i hans almene relativitetsteori, forudsagde eksistensen af ​​sorte huller, i form af uendeligt tæt, kompakte områder i rummet, hvor tyngdekraften er så ekstrem, at intet, ikke engang lys, kan flygte indefra. Per definition, sorte huller er usynlige. Men hvis et sort hul er omgivet af lysemitterende materiale såsom plasma, Einsteins ligninger forudsiger, at noget af dette materiale skulle skabe en "skygge, "eller et omrids af det sorte hul og dets grænse, også kendt som dens begivenhedshorisont.

Baseret på de nye billeder af M87, forskerne mener, at de ser skyggen af ​​et sort hul for første gang, i form af det mørke område i midten af ​​hvert billede.

Relativitet forudsiger, at det enorme gravitationsfelt vil få lys til at bøje sig rundt om det sorte hul, danner en lys ring omkring dens silhuet, og vil også få det omgivende materiale til at kredse omkring objektet tæt på lysets hastighed. De lyse, en skæv ring i de nye billeder giver visuel bekræftelse af disse effekter:Materialet, der er på vej mod vores udsigtspunkt, mens det roterer rundt, virker lysere end den anden side.

Fra disse billeder, teoretikere og modelbyggere på holdet har fastslået, at det sorte hul er omkring 6,5 milliarder gange så massivt som vores sol. Små forskelle mellem hvert af de fire billeder tyder på, at materialet lyner rundt om det sorte hul med lynets hast.

"Dette sorte hul er meget større end Neptuns bane, og Neptun tager 200 år at gå rundt om solen, " siger Geoffrey Crew, en forsker ved Haystack Observatory. "Når det sorte hul M87 er så massivt, en planet i kredsløb ville gå rundt om den inden for en uge og rejse tæt på lysets hastighed."

"Folk har en tendens til at se himlen som noget statisk, at tingene ikke ændrer sig i himlen, eller hvis de gør, det er på tidsskalaer, der er længere end et menneskes liv, " siger Vincent Fish, en forsker ved Haystack Observatory. "Men hvad vi finder for M87 er, ved de meget fine detaljer, vi har, objekter ændres på tidsskalaen af ​​dage. I fremtiden, vi kan måske producere film af disse kilder. I dag ser vi startrammerne."

"Disse bemærkelsesværdige nye billeder af det sorte hul M87 beviser, at Einstein havde ret igen, " siger Maria Zuber, MIT's vicepræsident for forskning og E.A. Griswold professor i geofysik i Department of Earth, Atmosfæriske og planetariske videnskaber. "Opdagelsen blev muliggjort af fremskridt inden for digitale systemer, hvor Haystack-ingeniører længe har udmærket sig."

"Naturen var venlig"

Billederne er taget af Event Horizon Telescope, eller EHT, et array i planetskala bestående af otte radioteleskoper, hver i en fjernbetjening, højtliggende miljø, inklusive Hawaiis bjergtoppe, Spaniens Sierra Nevada, den chilenske ørken, og den antarktiske iskappe.

På en given dag, hvert teleskop fungerer uafhængigt, observere astrofysiske objekter, der udsender svage radiobølger. Imidlertid, et sort hul er uendeligt meget mindre og mørkere end nogen anden radiokilde på himlen. For at se det klart, astronomer skal bruge meget korte bølgelængder - i dette tilfælde, 1,3 millimeter - der kan skære gennem materialeskyerne mellem et sort hul og Jorden.

At lave et billede af et sort hul kræver også en forstørrelse, eller "vinkelopløsning, " svarende til at læse en tekst på en telefon i New York fra en fortovscafé i Paris. Et teleskops vinkelopløsning øges med størrelsen på dets modtageskål. selv de største radioteleskoper på Jorden er ikke nær store nok til at se et sort hul.

Men når flere radioteleskoper, adskilt af meget store afstande, er synkroniseret og fokuseret på en enkelt kilde på himlen, de kan fungere som en meget stor radioskål, gennem en teknik kendt som meget lang baseline interferometri, eller VLBI. Deres kombinerede vinkelopløsning som et resultat kan forbedres væsentligt.

For EHT, de otte deltagende teleskoper opsummerede til en virtuel radioskål så stor som Jorden, med evnen til at løse et objekt ned til 20 mikrobuesekunder - omkring 3 millioner gange skarpere end 20/20 syn. Ved et lykkeligt tilfælde, det er omtrent den præcision, der kræves for at se et sort hul, ifølge Einsteins ligninger.

"Naturen var venlig mod os, og gav os noget lige stort nok til at se ved at bruge state-of-the-art udstyr og teknikker, " siger Crew, medleder af EHT-korrelationsarbejdsgruppen og ALMA Observatory VLBI-teamet.

"Drammer af data"

Den 5. april 2017, EHT begyndte at observere M87. Efter at have konsulteret adskillige vejrudsigter, astronomer identificerede fire nætter, der ville skabe klare forhold for alle otte observatorier - en sjælden mulighed, hvor de kunne arbejde som én fælles ret for at observere det sorte hul.

I radioastronomi, teleskoper registrerer radiobølger, ved frekvenser, der registrerer indkommende fotoner som en bølge, med en amplitude og fase, der måles som en spænding. Som de observerede M87, hvert teleskop indtog datastrømme i form af spændinger, repræsenteret som digitale tal.

"We're recording gobs of data—petabytes of data for each station, " Crew says.

I alt, each telescope took in about one petabyte of data, equal to 1 million gigabytes. Each station recorded this enormous influx that onto several Mark6 units—ultrafast data recorders that were originally developed at Haystack Observatory.

After the observing run ended, researchers at each station packed up the stack of hard drives and flew them via FedEx to Haystack Observatory, in Massachusetts, and Max Planck Institute for Radio Astronomy, i Tyskland. (Air transport was much faster than transmitting the data electronically.) At both locations, the data were played back into a highly specialized supercomputer called a correlator, which processed the data two streams at a time.

As each telescope occupies a different location on the EHT's virtual radio dish, it has a slightly different view of the object of interest—in this case, M87. The data received by two separate telescopes may encode a similar signal of the black hole but also contain noise that's specific to the respective telescopes.

The correlator lines up data from every possible pair of the EHT's eight telescopes. From these comparisons, it mathematically weeds out the noise and picks out the black hole's signal. High-precision atomic clocks installed at every telescope time-stamp incoming data, enabling analysts to match up data streams after the fact.

"Precisely lining up the data streams and accounting for all kinds of subtle perturbations to the timing is one of the things that Haystack specializes in, " says Colin Lonsdale, Haystack director and vice chair of the EHT directing board.

Teams at both Haystack and Max Planck then began the painstaking process of "correlating" the data, identifying a range of problems at the different telescopes, fixing them, and rerunning the correlation, until the data could be rigorously verified. Only then were the data released to four separate teams around the world, each tasked with generating an image from the data using independent techniques.

"It was the second week of June, and I remember I didn't sleep the night before the data was released, to be sure I was prepared, " says Kazunori Akiyama, co-leader of the EHT imaging group and a postdoc working at Haystack.

All four imaging teams previously tested their algorithms on other astrophysical objects, making sure that their techniques would produce an accurate visual representation of the radio data. When the files were released, Akiyama and his colleagues immediately ran the data through their respective algorithms. Vigtigt, each team did so independently of the others, to avoid any group bias in the results.

"The first image our group produced was slightly messy, but we saw this ring-like emission, and I was so excited at that moment, " Akiyama remembers. "But simultaneously I was worried that maybe I was the only person getting that black hole image."

His concern was short-lived. Soon afterward all four teams met at the Black Hole Initiative at Harvard University to compare images, og fandt, with some relief, and much cheering and applause, that they all produced the same, lopsided, ring-like structure—the first direct images of a black hole.

"There have been ways to find signatures of black holes in astronomy, but this is the first time anyone's ever taken a picture of one, " Crew says. "This is a watershed moment."

"A new era"

The idea for the EHT was conceived in the early 2000s by Sheperd Doeleman Ph.D. '95, who was leading a pioneering VLBI program at Haystack Observatory and now directs the EHT project as an astronomer at the Harvard-Smithsonian Center for Astrophysics. På det tidspunkt, Haystack engineers were developing the digital back-ends, recorders, and correlator that could process the enormous datastreams that an array of disparate telescopes would receive.

"The concept of imaging a black hole has been around for decades, " Lonsdale says. "But it was really the development of modern digital systems that got people thinking about radio astronomy as a way of actually doing it. More telescopes on mountaintops were being built, and the realization gradually came along that, Hej, [imaging a black hole] isn't absolutely crazy."

In 2007, Doeleman's team put the EHT concept to the test, installing Haystack's recorders on three widely scattered radio telescopes and aiming them together at Sagittarius A*, the black hole at the center of our own galaxy.

"We didn't have enough dishes to make an image, " recalls Fish, co-leader of the EHT science operations working group. "But we could see there was something there that's about the right size."

I dag, the EHT has grown to an array of 11 observatories:ALMA, APEX, the Greenland Telescope, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the Kitt Peak Telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope.

Coordinating observations and analysis has involved over 200 scientists from around the world who make up the EHT collaboration, with 13 main institutions, including Haystack Observatory. Key funding was provided by the National Science Foundation, the European Research Council, and funding agencies in East Asia, including the Japan Society for the Promotion of Science. The telescopes contributing to this result were ALMA, APEX, the IRAM 30-meter telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope.

More observatories are scheduled to join the EHT array, to sharpen the image of M87 as well as attempt to see through the dense material that lies between Earth and the center of our own galaxy, to the heart of Sagittarius A*.

"We've demonstrated that the EHT is the observatory to see a black hole on an event horizon scale, " Akiyama says. "This is the dawn of a new era of black hole astrophysics."

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