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In late June 2013, an exceptional binary system containing a rapidly spinning neutron star underwent a dramatic change in behavior never before observed. The pulsar's radio beacon vanished, while at the same time the system brightened fivefold in gamma rays, the most powerful form of light, according to measurements by NASA's Fermi Gamma-ray Space Telescope. The system, known as AY Sextantis, is located about 4,400 light-years away in the constellation Sextans. It pairs a 1.7-millisecond pulsar named PSR J1023+0038 -- J1023 for short -- with a star containing about one-fifth the mass of the sun. The stars complete an orbit in only 4.8 hours, which places them so close together that the pulsar will gradually evaporate its companion. To better understand J1023's spin and orbital evolution, the system was routinely monitored in radio. These observations revealed that the pulsar's radio signal had turned off and prompted the search for an associated change in its gamma-ray properties. What's happening, astronomers say, are the last sputtering throes of the pulsar spin-up process. Researchers regard the system as a unique laboratory for understanding how millisecond pulsars form and for studying details of how accretion takes place on neutron stars. In J1023, the stars are close enough that a stream of gas flows from the sun-like star toward the pulsar. The pulsar's rapid rotation and intense magnetic field are responsible for both the radio beam and its powerful pulsar wind. When the radio beam is detectable, the pulsar wind holds back the companion's gas stream, preventing it from approaching too closely. But now and then the stream surges, pushing its way closer to the pulsar and establishing an accretion disk. When gas from the disk falls to an altitude of about 50 miles (80 km), processes involved in creating the radio beam are either shut down or, more likely, obscured. Some of the gas may be accelerated outward at nearly the speed of light, forming dual particle jets firing in opposite directions. Shock waves within and along the periphery of these jets are a likely source of the bright gamma-ray emission detected by Fermi.

Astronomers, using NASA’s Hubble Space Telescope have come up with what they say is some of their best evidence yet for the presence of a rare class of “intermediate-sized” black hole that may be lurking in the heart of the closest globular star cluster to Earth, located 6,000 light-years away. Like intense gravitational potholes in the fabric of space, virtually all black holes seem to come in two sizes: small and humongous. It’s estimated that our galaxy is littered with 100 million small black holes (several times the mass of our Sun) created from exploded stars. The universe at large is flooded with supermassive black holes, weighing millions or billions of times our Sun’s mass and found in the centers of galaxies. A long-sought missing link is an intermediate-mass black hole, weighing in somewhere between 199 and 10,000 solar masses. How would they form, where would they hang out, and why do they seem to be so rare? For more information, visit https://nasa.gov/hubble.