From NASA Astrophysics. Hidden deep within a group of ancient stars, there lurks a young and powerful enigma. This is NGC 6624, a globular cluster near our galaxy’s center thought to be about 10 billion years old. NASA’s Fermi Gamma-ray Space Telescope detects high-energy radiation from many globular clusters.
Usually what Fermi is seeing is the cumulative gamma rays from all of the old pulsars in these clusters. A pulsar is a rapidly spinning neutron star, which is the small, incredibly dense remnant of a much more massive star.
A teaspoon of matter from a neutron star weighs as much Mount Everest, and a neutron star is so compact that a ball about 15 miles across contains more matter than our sun. Neutron stars spin between 7 and 40,000 times a minute and form with incredibly strong magnetic fields. Rapid spin and intense magnetic fields drive powerful beams of electromagnetic radiation, including gamma rays. As the pulsar rotates, these beams sweep the sky like a lighthouse.
To a distant observer, the pulsar appears to blink on and off. Pulsars slow down as they age but some of the oldest pulsars spin hundreds of times a second. Each of these millisecond pulsars orbits a normal star. Over time, the impact of gas pulled from the normal star, has spun the pulsar up to incredible speeds. This accretion may be the cause of their weaker magnetic fields.
Despite this, these pulsars also emit gamma rays. But the millisecond pulsar in NGC 6624 doesn’t fit neatly into this picture. It’s so bright that Fermi directly detects its gamma rays, and so far it’s the only one seen in a globular cluster with such power. It’s losing energy so fast that it must be only around 25 million years old–the youngest millisecond pulsar ever found. It also possesses the strongest magnetic field yet observed in a millisecond pulsar. It’s high energy output dooms it to fade out quickly on astronomical time scales and scientists wonder if this object represents a new way to make millisecond pulsars.
In three years, Fermi has detected more than 100 gamma-ray pulsars, shown here using animated pulses fifty slower than actual speed. Recent advances in data analysis helped Fermi reach this milestone, and these techniques promise to find many more gamma-ray pulsars. Some of these are historical –the first gamma-ray pulsars ever discovered. Others, like the pulsar in NGC 6624, were first seen by radio telescopes and then observed by Fermi. Some were first spotted in radio after investigating unknown sources detected by Fermi.
And about a third of gamma-ray pulsars were discovered by Fermi on the basis of their gamma-ray pulsations alone. Fermi’s gamma-ray observations are literally showing us these incredible stellar lighthouses in a new light.