Sometimes, astronomers get lucky and catch an event they can watch to see how the properties of some of the most massive objects in the universe evolve. That happened in February 2020, when a team of international astronomers led by Dheeraj (DJ) Pasham at MIT found one particular kind of exciting event that helped them track the speed at which a supermassive black hole was spinning for the first time. Dr. Pasham found AT2020ocn, a bright flash captured by the Zwicky Transient Facility at Palomar Observatory. He thought it might signify a tidal disruption event (TDE). In these extreme events, a black hole rips apart a star. Part of the star’s remnants are flung from the black hole, but part falls into the accretion disk. And how they fall could hold the key to understanding how a black hole is spinning. How that disk accretes is attributable to a cosmological theory called Lense-Thirring precession, which shows how space-time is warped by powerful gravitational fields—like those around black holes. Lense-Thirring theory predicts that an accretion disk formed after a TDE would “wobble” soon after the event before settling down into a more standard pattern of matter orbiting a black hole. The key would be to catch a TDE event very early after it happened and then watch the resulting “wobbling” over as long of a time span as possible. Fraser discusses measuring the spin of a black hole. So catching AT2020ocn was just the first step—then the authors had to monitor it—preferably for months. To do so, they recruited the Neutron Star Interior Composition ExploreR (NICER), an X-ray telescope attached to the ISS. NICER watched the galaxy containing AT2020ocn for 200 days immediately following the bright flash caught by Zwicky.  They began to notice a pattern. Every 15 days, the amount of X-rays emitted around the black hole peaked sharply, indicating the potential “wobble” they were looking for. Plugging that frequency into equations for the Lense-Thirring theory, along with estimates of the star’s mass and the black hole’s mass, they determined the black hole was spinning at 25% of the speed of light—which is actually relatively slow for a black hole. A black hole’s rotational speed can increase or decrease depending on its local environment. As it absorbs more material, typically in the form of matter from its accretion disk falling into it, its rotational speed increases. On the other hand, if it collides with another black hole, the overall rotational speed could decrease, as the two black holes’ spins could be opposite. That appears to be what has happened with the black hole that caused the AT2020ocn TDE, given its relatively slow speed compared to other black holes. Black holes typically spin exceptionally fast, as Fraser discusses in this video. The findings of this work were recently published in a paper in Nature. They also potentially lay the groundwork for calculating the spin of other supermassive black holes in the galaxy. Dr Pasham believes astronomers could calculate the spins of hundreds of black holes, opening up insights into their formation and life cycle. But to do that, they will still need a lot of luck. TDEs are relatively rare events, and even when they do happen, there are obvious resource constraints on telescope time. The Vera Rubin Observatory might help, as it will monitor large chunks of the sky, but it’s not scheduled to come online until mid-next year. Until then, those interested in tracking black hole spins might have to rely on serendipity to find a rare event and have the telescope time to monitor it. Learn More:MIT – Using wobbling stellar material, astronomers measure the spin of a supermassive black hole for the first timePasham et al. – Lense–Thirring precession after a supermassive black hole disrupts a starUT – Black Holes are Firing Beams of Particles, Changing Targets Over TimeUT – The Milky Way’s Black Hole is Spinning as Fast as it Can Lead Image:Artist’s depiction of how the accretion disk around a black hole could wobble in frequency with its spin, and how that wobble might be captured by a sensor near Earth.Credits: Michal Zajacek & Dheeraj Pasham The post A New Way to Measure the Rotation of Black Holes appeared first on Universe Today.

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Approximately 60 Rock songs have generated over a billion streams on Sportify. Often the songs are not the act’s best but they are the ones that were streamed the most. Here are the 10 best. 10. Creedence Clearwater Revival: Fortunate Son The song became a ’60s anti-war anthem and a symbol of the counterculture’s opposition to the U.S. involvement in the Vietnam war while expressing solidarity with the soldiers fighting it. 9. Black Sabbath: Paranoid With Tony Iommi’s machine-gun guitar and Ozzy Osbourne’s menacingly troubled vocals the song is widely regarded as one of the greatest Heavy Metal songs of all time. 8. Oasis: Wonderwall According to the band’s guitarist Noel Gallagher, “Wonderwall” described “an imaginary friend who’s gonna come and save you from yourself.” The song reached the Top 10 in 13 other countries, including the U.S. and Canada. 7. Metallica: Enter Sandman “Enter Sandman” was the first song Metallica had written for their ’91 eponymous album. The single achieved platinum certification for more than 1,000,000 copies shipped. 6. Red Hot Chili Peppers: Californication The song is notable for its sparse combination of guitar and bass notes in the main riff; Frusciante drew inspiration from “Carnage Visors” by The Cure. It was named the Red Hot Chili Peppers’ best song by both Kerrang and Rolling Stone.  5. Guns N’ Roses: Sweet Child O’ Mine The band’s first single topped the Billboard 100 chart becoming the band’s only U.S. #1 single. Lead singer Axl Rose based the lyrics on his girlfriend Erin Everly. 4. Nirvana: Smells Like Teen Spirit “I was trying to write the ultimate pop song.” said Nirvana frontman Kurt Cobain. “I was basically trying to rip off the Pixies. We used their sense of dynamics, being soft and quiet and then loud and hard.” 3. U2: With or Without You As the group’s most successful single at the time, it became their first #1 hit in both the United States and Canada topping the Billboard 100 for three weeks. The track’s lyrics were inspired by Bono’s conflicting feelings about the lives he led as a musician and domestic man. 2. The White Stripes: Seven Nation Army “Seven Nation Army” received widespread critical acclaim and won the Best Rock Song Award at the 46th Grammy Awards. It has become a sports anthem and has also served as a theme song for sports teams, personalities, and events. 1. Queen: Bohemian Rapsody Written by lead singer Freddie Mercury, the song is a six-minute suite that is notable for not having a refraining chorus but rather several sections. It is one of the few Prog Rock songs of the ’70s to achieve both widespread acclaim and commercial success.  ### The post 10 Best Rock Songs With Over A Billion Streams On Spotify appeared first on RockinTown.