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.
Explore a new "pulsar in a box" computer simulation that tracks the fate of electrons (blue) and their antimatter kin, positrons (red), as they interact with powerful magnetic and electric fields around a neutron star. Lighter tracks indicate higher particle energies. Each particle seen in this visualization actually represents trillions of electrons or positrons. Better knowledge of the particle environment around neutron stars will help astronomers understand how they produce precisely timed radio and gamma-ray pulses.
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.
Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.
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.
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 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 high-energy light is more plentiful and weirder than anyone expected.
The sleepy sun turns out to be a factory of extremely energetic light.
Scientists have discovered that the sun puts out more of this light, called high-energy gamma rays, overall than predicted. But what's really weird is that the rays with the highest energies appear when the star is supposed to be at its most sluggish, researchers report in an upcoming study in Physical Review Letters. The research is the first to examine these gamma rays over most of the solar cycle, a roughly 11-year period of waxing and waning solar activity.
That newfound oddity is probably connected to the activity of the sun's magnetic fields, the researchers say, and could lead to new insights about the mysterious environment.
"The almost certain thing that's going on here is the magnetic fields are much more powerful, much more variable, and much more weirdly shaped than we expect," says astrophysicist John Beacom of the Ohio State University in Columbus.
The sun's high-energy gamma rays aren't produced directly by the star. Instead, the light is triggered by cosmic rays — protons that zip through space with some of the highest energies known in nature — that smack into solar protons and produce high-energy gamma rays in the process (SN: 10/14/27, p. 7).
All of those gamma rays would get lost inside the sun, if not for magnetic fields. Magnetic fields are known to take charged particles like cosmic rays and spin them around like a house in a tornado. Theorists have predicted that cosmic rays whose paths have been scrambled by the tangled mass of magnetic fields at the solar surface should send high-energy gamma rays shooting back out of the sun, where astronomers can see them.
Beacom and colleagues, led by astrophysicist Tim Linden of Ohio State, sifted through data from NASA's Fermi Gamma-ray Space Telescope from August 2008 to November 2017.
The observations spanned a period of low solar activity in 2008 and 2009, a period of higher activity in 2013 and a decline in activity to the minimum of the next cycle, which started in 2018 (SN: 11/2/13, p. 22). The team tracked the number of solar gamma rays emitted per second, as well as their energies and where on the sun they came from.
There were more high-energy gamma rays, above 50 billion electron volts, or GeV, than anyone predicted, the team reports. Weirder still, rays with energies above 100 GeV appeared only during the solar minimum, when the sun's activity level was low. One photon emitted during the solar minimum had an energy as high as 467.7 GeV.
Strangest of all, the sun seems to emit gamma rays from different parts of its surface at different times in its cycle. Because cosmic rays that hit the sun come in from all directions, you would expect the entire sun to light up in gamma rays uniformly. But Beacom's team found that during the solar minimum, gamma rays came mainly from near the equator, and during the solar maximum, when the sun's activity level was high, they clustered near the poles.
These plots show that the sun shot light called high-energy gamma rays from its middle during a period of low solar activity (from about August 2008 to the end of 2009, left), but not during a period of high activity (from 2010 until 2017, right). The gamma rays seem to migrate from the equator to the poles after 2010. Rays with less than 100 billion electron volts, or GeV, of energy are depicted as circles; those with 100 GeV or more are triangles. The bar graphs represent the number of gamma rays that came from different latitudes.
Beacom and colleagues tried to connect the excess gamma rays to other solar behaviors that change with magnetic activity, like solar flares or sunspots (SN: 9/30/17, p. 6). "So far nothing has really held up to any sort of scrutiny," says astrophysicist Annika Peter, also at Ohio State.
More observations are coming soon. NASA's Parker Solar Probe, which launched on August 12, will take the first direct measurements of the magnetic field in the sun's outer atmosphere, or corona (SN: 7/21/18, p. 12).
And as the sun enters the next solar minimum, the highest-energy gamma rays are starting to return. In February, Fermi caught its first gamma ray with an energy above 100 GeV since 2009.