A so-called "black widow" star with a tightly orbiting stellar partner has been caught in act of consuming its companion by a NASA space telescope, scientists say.
The fast-spinning pulsar, known as PSRJ1311-3430 (J1311 for short), is part of a unique class of pulsars named for dangerous redback and black widow spiders that devour their cosmic mates. In time, the pulsar is expected to completely absorb its smaller companion star, a celestial partner that may have caused its characteristic quick spin. You can see a video animation of the pulsar's deadly embrace here.
"The essential feature of black widow and redback binaries are that they place a normal but very low-mass star in close proximity to a millisecond pulsar, which has disastrous consequences for the star," Roger Romani, a member of the Kavli Institute for Particle Astrophysics and Cosmology in California, said in a statement. [The Star Quiz: Test Your Stellar Smarts]
A fast-spinning gamma ray emitter
When a massive star explodes in a supernova, its leftover core can survive as a neutron star, an incredibly dense body that can pack the mass of the sun into a city-sized ball. Neutron stars that rotate a few thousand times per minute, sweeping a beam of radio, visible light, x-rays, and gamma rays like a light house are known as pulsars. Astronomers can detect the stream of emission when it points towards Earth in a brief pulse.
But some pulsars rotate at a dazzling speeds, turning on their axis at least once every ten milliseconds, or a few thousand times a minute. Known as millisecond pulsars, more than half of these fast-spinning stars have companions, while their slower cousins tend to appear in isolation. The high companion rates suggest to scientists that interactions with a second star can accelerate the spin of a normal pulsar.
In 2012, Romani was part of a team that used NASA's Fermi Gamma-ray Space Telescope to characterize J1311 using only its gamma-ray emission. While Fermi frequently identifies gamma-ray sources, radio telescope follow-ups have been the key source of detection of the rapid pulsation that identifies the source as a millisecond pulsar, though slower pulsars are frequently identifiable by the telescope.
The gamma-ray detection is key because many of the sedate pulsars are quiet in the radio spectrum, where the millisecond pulsars are frequently identified, potentially allowing numerous radio-quiet millisecond pulsars to pass by unnoticed.
Initially imaging the system in visible light, Romani noticed that the faint companion star changed colors from intense blue to dull red, indicating a shift from hot to cold every half an hour. His finding suggested that the star was being dramatically heated by a compact object such as a pulsar, leading him to suggest that the system was a new black widow.
"This was the first time a millisecond pulsar has ever been detected solely by pulsed gamma rays," Holger Pletsch at the Albert Einstein Institute in Germany said in the same statement. Pletsch combed through four years of Fermi Large Area Telescope (LAT) data with an international effort to hunt for millisecond pulsars. Orbital information provided by Romani's work helped to narrow the search, allowing Pletsch to confirm J1311's black widow status.
A dangerous pairings
J1311 spins 390 times a second, rotating about a million times between each detection made by Fermi. A companion star, weighing in at 12 to 17 times the mass of Jupiter, orbits the dense neutron star, which is twice the mass of the sun. The stars orbit once every 93 minutes in a setup that is the tightest of its kind ever seen.
As J1311 sweeps its beam past its partner, it heats the side of the star facing the pulsar to more than 21,000 degrees Fahrenheit (12,000 degrees Celsius), more than twice as hot as the sun's surface. The opposite side of the star reaches far lower temperatures of 5,000 F (2,700 C). Flares from the companion revealed with additional observations indicated variable heating, which allowed scientists to narrow down the masses of the two.
As the companions in such systems intercept energy from the pulsar, they act as vanity mirrors, revealing the beam in great detail. But these pairings are named for the black widow spider and its cousin, the Australian redback, for a reason, with black widow systems containing smaller, less massive stars than redbacks. Just like the two deadly spiders consume their partners after mating, the companion star won't find a happy ending. As the pulsar flashes toward the companion star, it strips away the outer layers of its partner, ultimately destroying it.
"The high-energy emission and wind from the pulsar basically heats and blows off the normal star's material and, over millions to billions of years, can eat away the entire star," said Alice Harding, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md.
"These systems can completely consume their companion stars, and that's how we think solitary millisecond pulsars form."
Over 300 millisecond pulsars have been cataloged. Of them, 18 black widows and nine redbacks have been found in the Milky Way galaxy, with additional pairs located in the dense globular clusters orbiting the galaxy. Almost all of these deadly pairings have been detected by Fermi.
Although J1311 was first detected by gamma rays rather than by radio, it does emit the occasional radio signal. A team led by Paul Ray at the Naval Research Laboratory in Washington used the Green Bank Telescope in West Virginia and other radio telescopes to further study the system. They found that the system emits radio pulses at brief, irregular moments.
"The pulsar heating is ablating its companion, literally blowing it away, so ionized gas fills the system," Ray said. "This scatters or absorbs the radio emission most of the time."
Higher energy gamma rays pass through the gas more easily, allowing Fermi to make detailed observations and potentially register other companion-consuming millisecond pulsars.