BY DR EMILY BALDWIN
Posted: 07 January, 2009
NASA's Fermi Gamma-ray Space Telescope has discovered 12 new gamma-ray-only pulsars and has detected gamma-ray pulses from 18 others, adding vital new information to our understanding of how these stellar powerhouses operate.
"We know of 1,800 pulsars, but until Fermi we saw only little wisps of energy from all but a handful of them," says Roger Romani of Stanford University. "Now, for dozens of pulsars, we're seeing the actual power of these machines."
Fermi has found 12 previously unknown pulsars (orange) and detected gamma-ray emissions from known radio pulsars (magenta, cyan) and from known or suspected gamma-ray pulsars identified by NASA's now-defunct Compton Gamma-Ray Observatory (green). Image: NASA/Fermi/LAT Collaboration.
A pulsar is a highly magnetized rapidly rotating neutron star, the crushed core left behind when a massive star explodes. Many pulsars are detected by their signature radio wavelength pulses, emitted in narrow, lighthouse-like beams emanating from the star's magnetic poles. If the magnetic poles and the star's spin axis don't align exactly, the spinning pulsar sweeps the beams across the sky, and should one swing our way, Earth-based telescopes will hear their cry. Unfortunately, any census of pulsars is automatically biased because we only see those beams that happen to bathe the Earth with their radiation.
"That has coloured our understanding of neutron stars for 40 years," says Romani, who comments that the radio beams are easy to detect, but they represent only a few parts per million of a pulsar's total power. A pulsar’s gamma rays on the other hand, account for 10 percent or more. "For the first time, Fermi is giving us an independent look at what heavy stars do."
Pulsars are like giant particle accelerators, and through processes not fully understood, their intense electric and magnetic fields combined with a rapid spin, accelerates particles to speeds near that of light. By studying the gamma rays, astronomers can take a glimpse of the pulsar’s beating heart. The latest survey has revealed 12 new gamma-ray-only pulsars, and gamma-ray pulses from 18 others.
"We used to think the gamma rays emerged near the neutron star's surface from the polar cap, where the radio beams form," says Alice Harding of NASA's Goddard Space Flight Center. "The new gamma-ray-only pulsars put that idea to rest."
solated pulsars gradually slow their spins, but the opposite happens if the pulsar is joined by a companion star as part of a binary system. Gas accreted from the star can force the pulsar to spin faster, resulting in rotation periods of just a few milliseconds. Credit: NASA/Dana Berry.
Astronomers now believe the pulsating gamma rays originate far above the neutron star as particles accelerate along arcs of open magnetic field. For one particular pulsar known as the Vela pulsar, which is also the brightest persistent gamma-ray source in the sky, the emission region is thought to lie nearly 500 kilometres from the star, which itself is only 30 kilometres across. Many existing models place the gamma-ray emission along the boundary between open and closed magnetic field lines. One model suggests it starts at high altitudes; another implies emission from the star's surface all the way out. "So far, Fermi observations to date cannot distinguish which of these models is correct,"says Harding.
Fermi also picked up pulsating gamma rays from seven ‘millisecond pulsars’, so called because they spin between 100 and 1,000 times a second. Far older than pulsars like Vela or the 10,000 year old CTA 1 pulsar, these unusual objects reside in binary systems containing a normal star. Stellar matter accreted from the companion can spin up the pulsar until its surface moves at an appreciable fraction of light speed. Isolated pulsars slow down over time, like CTA 1, which slows down by about a second every 87,000 years. In the binary systems the opposite happens, with gas accreted from the star forcing the pulsar to spin up its rotation to the order of milliseconds.