A long-standing mystery is the mechanism by which cosmic rays are accelerated in our galaxy. While the most powerful accelerators on Earth, such as the Large Hadron Collider, can accelerate particles to teraelectronvolt, or TeV, energy scales, fluxes of cosmic rays reaching the Earth have been detected with energies up to one million times higher. Fermi-LAT’s high-energy gamma-ray emission data can be used to indirectly study the spatial distribution and energies of cosmic rays in distant astrophysical environments -- regions where cosmic-ray acceleration is believed to be taking place.
Likely sites of cosmic-ray acceleration are supernova remnants (SNRs), which are also strong sources of gamma-ray emission. Supernovae, the catastrophic deaths of stars, are the most violent phenomena known in our galaxy, producing a brilliant flash of light at least ten billion times brighter than the Sun. These gigantic explosions drive shock waves into interstellar medium, a process similar to the atmospheric shock wave produced by a supersonic aircraft.
It is widely believed that the shock waves in these SNRs are the sources of cosmic rays arriving at the Earth, at least for those whose energies are below 1 PeV. Recent observations with the Fermi-LAT have identified a new class of middle-aged SNR in which low-energy (several gigaelectronvolts) cosmic rays are preferentially generated. The results from the Fermi-LAT suggest that high-energy cosmic rays cannot be efficiently accelerated in evolved remnants. Fermi-LAT observations provide a new probe to study how cosmic-ray acceleration is affected by environmental effects such as shock propagation into dense clouds of interstellar gas, and how cosmic rays are released into the interstellar medium. The discovery made with the Fermi-LAT sheds new light on the long-standing problem of the origin of cosmic rays.
One of the most studied of all SNR is the Crab Nebula, a relatively young SNR left behind from a historical supernova explosion recorded in 1054 AD. The explosion left behind a rapidly rotating neutron star, or pulsar, spinning on its axis thirty times a second, as well as a surrounding nebula of material ejected from the star. The pulsar creates a wind of high-energy particles and strong electromagnetic fields that interact with the surrounding material and emits electromagnetic radiation across the spectrum, from radio waves to gamma rays. Its bright and apparently steady emission has been used as a reference source to calibrate telescopes in several wavebands, in particular at high energies. It has also become one of the main astronomical laboratories to test theories of particle acceleration and plasma physics under extreme conditions. The Fermi-LAT has been used to make the surprising discovery that the emission from the Crab Nebula is highly variable at “soft” gamma-ray energies (photons ranging from 0.1-10 GeV). In particular, two extreme outbursts were detected in February 2009 and December 2010, with flux increases by factors of about 6 and with durations lasting only a few days.
The brevity and strength of the flares imply that the soft gamma-ray emission must come from a relatively small region, likely located less than a light year from the neutron star. Even more interestingly, it shows that the emission is radiated by electrons of energies greater than 1 PeV. These are the most energetic particles associated with any discrete astronomical source.
The central engines of pulsar wind nebulae (PWN) like the Crab Nebula are pulsars which are basically electrical generators on a cosmic scale. Nearly 200 years ago, Michael Faraday discovered the principle of induction: moving a conductor (say, a loop of wire) within a magnetic field will produce an electric field inside the conductor, driving a current. Today, nearly all electrical power on Earth is generated via induction. Pulsars generate fields in effectively the same way with the rotating wire loop replaced by a spinning, superconducting neutron star. They are more massive than the sun, but could fit within the city limits of New York with room to spare! And these neutron stars possess the strongest known magnetic fields in the universe, some up to 10 billion times stronger than the most powerful electromagnets on earth, the kind used, for example, in MRI machines or supercolliders.
In the case of the Crab Nebula, the electric field results in an electrical potential of up to 100 quadrillion volts. By comparison, the effective potential difference of the Crab Nebula is more than 1000 times greater than that of the most powerful man-made particle accelerator, the Large Hadron Collider (LHC). And just as the field induced in a rotating wire accelerates the electrons in the metal and causes them to flow, the field produced by the neutron star accelerates any charged particle near it. In the wire, the speed of the electrons is limited by interactions with the atoms of the metal. In a pulsar, however, the electric field is so intense that electrons are almost instantly accelerated to near the speed of light. Accelerated charges emit radiation with an intensity that grows with their speed and acceleration, so their speed is eventually limited by the copious emission of light at a variety of wavelengths. Since the pulsar is spinning, we get a glimpse of this intense radiation once each revolution, giving the characteristic on-off lighthouse signal for which pulsars are known.
It's been more than 40 years since the discovery of the first radio pulsar. And despite the simple description of pulsars as cosmic inductors, a detailed and consistent theory of how these fascinating "stars" work remains elusive. Indeed, even fundamental questions about, say, whether the radio waves generated by neutron stars come from X-rays or gamma rays remain unsettled. To answer these questions, we must study many pulsars at many different wavelengths. And although pulsars have been most historically (and thoroughly) studied in radio, it is through an examination of their gamma-ray emissions that we may finally come to understand them.
Almost all of the energy emitted by young pulsars comes out in the form of gamma-rays. Moreover, nearly anywhere there is particle acceleration, there will be gamma-rays. This makes gamma-rays a much more effective tool for studying pulsars than radio waves, which, are produced coherently, meaning that tiny patches above the star dominate what we see. Trying to infer global properties of the pulsar from the small regions contributing to the radio flux is all but hopeless.
The Fermi-LAT is opening up a new era for gamma-ray studies of pulsars. Prior to the LAT, gamma-rays from only half a dozen pulsars had been detected. Nearly three years into Fermi's mission, that number has increased to about 100. Not only do we see more sources but we also see them more sharply. One of the first important findings, generated with only a few months’ worth of data, was the precise measurement of the spectrum of the Vela pulsar, the brightest steady source of gamma-rays in the sky. Because the Vela pulsar’s spectrum does not die off quickly at energies above 1 GeV, the production of gamma-rays near its surface has been ruled out.
Ongoing studies are focusing on light curves -- the rise and fall of gamma-ray emission we see as the pulsar rotates -- to help pin down the location of the site of acceleration within the pulsar from the outer magnetosphere (often referred to as the "outer gap" picture) and emission arising from both low and high altitudes (known as the "two-pole caustic" or "slot gap" picture) predict significantly different shapes for these light curves, and with the wealth of new pulsars detected by the LAT, we are on the way to a basic understanding of these amazing objects.
- Fermi-LAT count maps (2-10 GeV) of the middle-aged SNRs interacting with molecular clouds:(a) W51C; (b) W44; (c) IC443; and (d) W28. Superposed are the contours of the VLA radio images.251658240
- Exposure corrected gamma-ray count map of the Crab Nebula for the first 25 months of Fermi observations (left), the February 2009 flare (middle), and the September 2010 flare (right). The nebula phase (in which there is no pulsar emission) is shown in the upper panels, the pulsar-phase in the lower panels. 251658240
- Image of the Crab Nebula as seen in X-ray images taken by the Chandra X-ray Observatory. The filamentary wisps trace the locations of energetic particles traveling along magnetic field lines within the nebula and emitting synchrotron radiation. The bright central nucleus corresponds to the pulsar that powers the nebular emission by injecting high-energy particles. Image Credit: NASA/CXC/SAO /F.Seward et al.
- Illustration of the geometry of a Pulsar. The pulsar spins rapidly about its rotation axis (blue line) at a rate of many times a second. Radiation is observed when the magnetic dipole axis (red line) sweeps across our line of sight giving rise to the pulsed gamma-ray emission observed by the Fermi-LAT. Figure taken from “Handbook of Pulsar Astronomy” by Lorimer and Kramer.251658240