An international team of scientists studying what amounts to a
computer-simulated “pulsar in a box” are gaining a more detailed
understanding of the complex, high-energy environment around spinning
neutron stars, also called pulsars. The model traces the paths of
charged particles in magnetic and electric fields near the neutron star,
revealing behaviors that may help explain how pulsars emit gamma-ray
and radio pulses with ultraprecise timing.
“Efforts to understand how pulsars do what they do began as soon as
they were discovered in 1967, and we’re still working on it,” said
Gabriele Brambilla, an astrophysicist at NASA’s Goddard Space Flight
Center in Greenbelt, Maryland, and the University of Milan who led a
study of the recent simulation. “Even with the computational power
available today, tracking the physics of particles in the extreme
environment of a pulsar is a considerable challenge
.”
A pulsar is the crushed core of a massive star that ran out of fuel,
collapsed under its own weight and exploded as a supernova. Gravity
forces more mass than the Sun’s into a ball no wider than Manhattan
Island in New York City while also revving up its rotation and
strengthening its magnetic field. Pulsars can spin thousands of times a
second and wield the strongest magnetic fields known.
These characteristics also make pulsars powerful dynamos, with superstrong electric fields that can rip particles out of the surface and accelerate them into space.
NASA’s Fermi Gamma-ray Space Telescope has detected gamma rays from
216 pulsars. Observations show that the high-energy emission occurs
farther away from the neutron star than the radio pulses. But exactly
where and how these signals are produced remains poorly known.
Various physical processes ensure that most of the particles around a
pulsar are either electrons or their antimatter counterparts,
positrons.
“Just a few hundred yards above a pulsar’s magnetic pole, electrons
pulled from the surface may have energies comparable to those reached by
the most powerful particle accelerators on Earth,” said Goddard’s Alice
Harding. “In 2009, Fermi discovered powerful gamma-ray flares from the Crab Nebula pulsar that indicate the presence of electrons with energies a thousand times greater.”
Speedy electrons emit gamma rays, the highest-energy form of light, through a process called curvature radiation. A gamma-ray photon can, in turn, interact with the pulsar’s magnetic field in a way that transforms it into a pair of particles, an electron and a positron.
To trace the behavior and energies of these particles, Brambilla,
Harding and their colleagues used a comparatively new type of pulsar
model called a “particle in cell” (PIC) simulation. Goddard’s
Constantinos Kalapotharakos led the development of the project’s
computer code. In the last five years, the PIC method has been applied
to similar astrophysical settings by teams at Princeton University in
New Jersey and Columbia University in New York.
“The PIC technique lets us explore the pulsar from first principles.
We start with a spinning, magnetized pulsar, inject electrons and
positrons at the surface, and track how they interact with the fields
and where they go,” Kalapotharakos said. “The process is computationally
intensive because the particle motions affect the electric and magnetic
fields and the fields affect the particles, and everything is moving
near the speed of light.”
The simulation shows that most of the electrons tend to race outward
from the magnetic poles. The positrons, on the other hand, mostly flow
out at lower latitudes, forming a relatively thin structure called the
current sheet. In fact, the highest-energy positrons here — less than
0.1 percent of the total — are capable of producing gamma rays similar
to those Fermi detects, confirming the results of earlier studies.
Some of these particles likely become boosted to tremendous energies
at points within the current sheet where the magnetic field undergoes reconnection, a process that converts stored magnetic energy into heat and particle acceleration.
One population of medium-energy electrons showed truly odd behavior, scattering every which way — even back toward the pulsar.
The particles move with the magnetic field, which sweeps back and
extends outward as the pulsar spins. Their rotational speed rises with
increasing distance, but this can only go on so long because matter
can’t travel at the speed of light.
The distance where the plasma’s rotational velocity would reach light
speed is a feature astronomers call the light cylinder, and it marks a
region of abrupt change. As the electrons approach it, they suddenly
slow down and many scatter wildly. Others can slip past the light
cylinder and out into space.
The simulation ran on the Discover supercomputer at NASA’s Center for Climate Simulation at Goddard and the Pleiades supercomputer
at NASA’s Ames Research Center in Silicon Valley, California. The model
actually tracks “macroparticles,” each of which represents many
trillions of electrons or positrons. A paper describing the findings was published May 9 in The Astrophysical Journal.
“So far, we lack a comprehensive theory to explain all the
observations we have from neutron stars. That tells us we don’t yet
completely understand the origin, acceleration and other properties of
the plasma environment around the pulsar,” Brambilla said. “As PIC
simulations grow in complexity, we can expect a clearer picture.”
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and
particle physics partnership, developed in collaboration with the U.S.
Department of Energy and with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden and
the United States.
By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Editor: Rob Garner
Source: NASA/Pulsar
