23-Jun-12 3:00 AM EST

Gamma-ray outbursts shed new light on pulsars

A team of researchers has developed a new method of detecting pulsars by using the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope with the sensitivity of a radio telescope. By combining a “wide area” approach of an all-sky telescope like the LAT with radio observations, the team has discovered five “millisecond” class pulsars. Their discoveries include one very unusual pulsar that is a new hybrid class of pulsar that features radio emissions that originate from low and high altitudes above the neutron star.

Matthew Kerr, the lead author on the current paper, told Waves and Packets, “The first pulsar detected in gamma rays was the Crab Nebula back in 1974 with the SAS-2 satellite. Before the launch of the Fermi-LAT, additional missions (primarily COS-B and CGRO on EGRET) brought this up to 6 pulsars known in high energy gamma rays. Fermi-LAT has let us find well over 100!”

The unusual pulsar that Kerr’s team found, officially named PSR J0101–6422, has an unusual light curve (a plot of brightness vs time) that features a “sandwich” of two gamma-ray peaks with an intense radio peak in the center. “The pulses are locked in the sense that they always maintain the same separation in a light curve,” says Kerr.

This work touches upon a complicated question for pulsars. That is, at what altitude above the neutron star’s surface are the sources of the radio and gamma emissions? This is an area of active research, and it is especially complicated for millisecond pulsars. Kerr explains that by studying many hundreds of pulsars in the radio, it has been generally concluded that, for young pulsars (not the millisecond variety reported in the current paper, which are older pulsars) radio emission originates at relatively low altitudes.

For young pulsars, one can infer that gamma rays originate at high altitudes, (1) because they lag behind the radio pulses by an appropriate amount, governed essentially by light travel time, and (2) because astronomers do not see attenuation of gamma rays due to the very strong magnetic field present at the neutron star’s surface.

It is not clear how well these arguments apply to millisecond pulsars, which – according to the leading theory - have accreted mass and thus have sped up. Where some millisecond pulsars seem to be very similar to young pulsars, others have quite different properties. For example, some pulsars are observed to have radio and gamma pulses arriving at the same time, indicating both come from high altitude.

Scott Ransom, an astronomer at NRAO and a co-author in the current paper, explains that the altitude of the emission sources are estimated based on the beam size observation and models based of the light cone geometry. Moreover the wavelengths of the emissions can be a proxy for the magnetic field strength, which is of course position dependent. “The magnetic fields around pulsars are like giant synchrotrons, and are powerful enough to create new particles that lead to other cascade events. The field can even strip particles from the surface of the star.”

Kerr adds, "The spinning magnetic field (typical surface values are 10⁸ Tesla for a young neutron star, 10⁴ T for a recycled millisecond pulsar) combined with the rapid spin (~10 Hertz for a young pulsar, 100s of Hz for an millisecond pulsar) induce a huge electric field. "Huge" meaning a total potential drop of order 10¹⁴ volts, sufficient to accelerate particles to ultra-relativistic energies, and it's these particles that power the radio and gamma pulses.

Because the induced electric field is so strong, it can rip particles off of the neutron star surface, or even create positron/electron pairs from vacuum. These charged particles will tend to arrange themselves in such a way to cancel out the electric field (analogously to what happens in a conductor). Thus, the cartoon picture of the exterior of a pulsar is of a big dipolar magnetic field with a tenuous collection of charged particles that shorts out the electric field and brings the system to equilibrium.

However, there are certain places in this magnetosphere where this shorting of the electric field might fail, called "gaps" in our jargon. In these gaps, the induced electric field can reach appreciable fractions of the "huge" value mentioned above, accelerate particles, and power radio and gamma pulses. Where these gaps occur, what causes them, how they relate to the global structure of the magnetosphere, and how the particles convert their energy to the radiation we see, these are all active topics of research."

Pulsars indeed are perhaps the most extraordinary physics laboratories in the Universe. Their behavior covers fundamental questions in nuclear and condensed matter physics, as well as astrophysics.