In the 4 June Physical Review Letters, a team offers an explanation for this puzzling deceleration. When two black holes merge, the resulting larger black hole usually shoots away from its birthplace, but it immediately slows down in some cases, according to computer simulations. The new object will shoot off to the left but will quickly slow down as the highly curved region smoothes out and emits gravitational waves. A pair of black holes in the process of merging generates high curvature (red) in the merged horizon on the side of the smaller black hole. "As these clumps fall down to the black hole, they also modulate the X-ray emission there."įuture observations of other tidal disruption events will be needed to further clarify the origin of optical and ultraviolet light.M. "Returning clumps of debris strike the incoming stream, which results in shock waves that emit visible and ultraviolet light," said Goddard's Bradley Cenko, the acting Swift principal investigator and a member of the science team. Tidal debris initially falls toward the black hole but overshoots, arcing back out along elliptical orbits and eventually colliding with the incoming stream. In a paper describing the results published March 15 in The Astrophysical Journal Letters, Pasham, Cenko and their colleagues show how interactions among the infalling debris could create the observed optical and UV emission. The researchers supplemented later Swift observations with optical data from the Las Cumbres Observatory headquartered in Goleta, California. Follow-up observations with Swift's X-ray and Ultraviolet/Optical telescopes began eight days later and continued every few days for the next nine months. 22, 2014, in images obtained by the All Sky Automated Survey for SuperNovae (ASASSN), which includes robotic telescopes in Hawaii and Chile. Additionally, the gas emitting the light seemed to remain at steady temperatures for much longer than expected.ĪSASSN-14li was discovered Nov. In some of the best-studied events, this emission seems to be located much farther than where the black hole's tides could shatter the star. But the location of optical and UV light was unclear, even puzzling. Tidal disruption flares carry important information about how this debris initially settles into an accretion disk.Īstronomers know the X-ray emission in these flares arises very close to the black hole. Matter falling toward a black hole collects into a spinning accretion disk, where it becomes compressed and heated before eventually spilling over the black hole's event horizon, the point beyond which nothing can escape and astronomers cannot observe. Astronomers call this a tidal disruption event. When a star passes too close to a black hole with 10,000 or more times the sun's mass, tidal forces outstrip the star's own gravity, converting the star into a stream of debris. For comparison, the event horizon of a black hole like this is about 13 times bigger than the sun, and the accretion disk formed by the disrupted star could extend to more than twice Earth's distance from the sun. "We think this means the optical and UV emission arose far from the black hole, where elliptical streams of orbiting matter crashed into each other."Īstronomers think ASASSN-14li was produced when a sun-like star wandered too close to a 3-million-solar-mass black hole similar to the one at the center of our own galaxy. "We discovered brightness changes in X-rays that occurred about a month after similar changes were observed in visible and UV light," said Dheeraj Pasham, an astrophysicist at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, and the lead researcher of the study.
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