My first Magazine Sky & Telescope - 02.2019

IN BRIEF BLACK HOLES Gas Seen Moving near the Event Horizon AN INTERNATIONAL TEAM of astron- omers has caught the motion of hot, magnetized gas right near the event horizon of our galaxy’s central super- massive black hole. This gas is likely part of a puffy disk that the black hole feeds from. The disk provides a ﬂickering light source, its steady glow sometimes spiking in ﬂares that observers see in wavelengths from X-ray to radio. While watching the star S2 whiz by the black hole last summer (S&T: Sept. 2018, p. 22), astronomers with the Gravity Collaboration at the Very Large Telescope Interferometer in Chile saw three bright infrared ﬂares from near the black hole. Each ﬂare lasted between 30 and 90 minutes and didn’t stay in place. Instead, each appeared to race around the black hole at 30% of the speed of light, tracing out about two-thirds of a clockwise loop that has the black hole at its cen- ter. Changing patterns in the light’s polarization conﬁrmed the twirling. As the team reports in the October Astronomy & Astrophysics, the data point to hotspots orbiting very close to the black hole’s event horizon, com- pleting a pass every 45 minutes or so. “This is an incredible measure- ment,” says Avery Broderick (Uni- versity of Waterloo, Canada). If astronomers see more ﬂ ares behaving the same way, then “this presents an extraordinary opportunity to make precise tests of gravity in its most extreme environments.” Attendees at a galactic center workshop in Germany were excited but cautious. “Everyone believes there is some kind of motion in a ﬂ are near the black hole,” says Sera Markoff (University of Amsterdam, The Neth- erlands). Hotspots are the simplest solution, which is why the team went with them. But it’s not yet clear if an alternative solution might be better. “Most of us believe that more com- plicated things are possible, such as motions associated with magnetic ﬂ ares or jets.” ■ CAMILLE M. CARLISLE Weird Stripes on Dione Saturn’s moons are a hodgepodge of misfi ts. Iapetus looks like the Death Star from Star Wars. Tiny Pan resembles a cosmic empa- nada. And now something appears to be drawing straight lines across the surface of Dione. The bright lines stretch for up to hun- dreds of kilometers, are less than 5 kilometers wide, run parallel to the equator, and reside only at lower latitudes. They appear to lie atop extant features such as ridges and craters, which means they’ve been emplaced fairly recently. The most likely culprit is something in the same plane as Dione’s orbit that gently rained down on the surface, researchers argue October 15th in Geophysical Research Letters. A passing comet, Saturn’s rings, or the moons Helene and Polydeuces — which share Dione’s orbit — are all suspects. ■ CHRISTOPHER CROCKETT Hubble Takes a Nap The Hubble Space Telescope has fully recov- ered from a three-week hiatus. The nearly 29-year-old observatory put itself into safe mode on October 6th after noting anomalous behavior from one of its gyroscopes, which are vital for ensuring that the telescope stays on target. Support crews suspected that the backup gyro might have developed an air bubble or stiff power wire after sitting inactive for years. To clear out the blockage, the team commanded Hubble to go through a series of turns, hoping to reset the gyro — and it worked. The venerable observatory got back to work on October 27th by turning its gaze toward a remote star-forming galaxy. ■ CHRISTOPHER CROCKETT Colossal Supercluster Lurks in Early Universe This visualization of gas orbiting at about 30% of the speed of light around the Milky Way’s supermassive black hole relies on data from simulations. Researchers have identifi ed what may be a gargantuan predecessor to modern super- clusters, complex webs of galaxies spanning hundreds of millions of light-years. The light from this proto-supercluster has taken about 11 billion years to reach Earth. Seven galaxy clusters appear linked together by fi laments of galaxies across roughly 20 million billion billion (2 × 10 25 ) cubic light-years of space. The entire ensemble is about as massive as 4.6 quadrillion Suns. While not the fi rst galaxy cluster seen in the early universe, none are as massive or as sprawling as this one. Olga Cucciati (National Institute for Astrophysics, Italy) and colleagues reported the discovery in the November Astronomy & Astrophysics. ■ CHRISTOPHER CROCKETT sk yandtelescope.com • FE B RUA RY 2 019 11

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