When a star reaches the end of its life cycle, it will blow off its outer layers in a fiery explosion known as a supernova. Where less massive stars are concerned, a white dwarf is what will be left behind. Similarly, any planets that once orbited the star will also have their outer layers blown off by the violent burst, leaving behind the cores behind.
For decades, scientists have been able to detect these planetary remnants by looking for the radio waves that are generated through their interactions with the white dwarf’s magnetic field. According to new research by a pair of researchers, these “radio-loud” planetary cores will continue to broadcast radio signals for up to a billion years after their stars have died, making them detectable from Earth.
A nova star is like a vampire that siphons gas from its binary partner. As it does so, the gas is compressed and heated, and eventually it explodes. The remnant gas shell from that explosion expands outward and is lit up by the stars at the center of it all. Most of these novae explode about once every 10 years.
But now astrophysicists have discovered one remnant so large that the star that created it must have been erupting yearly for millions of years.
This image is a software-calibrated image with high signal-to-noise ratio at a frequency of 120 MHz, of the radio sky above Effelsberg, Germany, on November10, 2009. It has North at the top and East at the left, just as a person would have seen the entire sky when lying on their back on a flat field near Effelsberg late in the afternoon on November 10, if their eyes were sensitive to radio waves.
The two bright (yellow) spots are Cygnus A – a giant radio galaxy powered by a supermassive black hole – near the center of the image, and Cassiopeia A – a bright radio source created by a supernova explosion about 300 years ago – at the upper-left in the image. The plane of our Milky Way galaxy can also be seen passing by both Cassiopeia A and Cygnus A, and extending down to the bottom of the image. The North Polar Spur, a large cloud of radio emission within our own galaxy, can also be seen extending from the direction of the Galactic center in the South, toward the western horizon in this image. “We made this image with a single 60 second “exposure” at 120 MHz using our high-band LOFAR field in Effelsberg”, says James Anderson, project manager of the Effelsberg LOFAR station.
“The ability to make all-sky images in just seconds is a tremendous advancement compared to existing radio telescopes which often require weeks or months to scan the entire sky,” Anderson went on. This opens up exciting possibilities to detect and study rapid transient phenomena in the universe.
LOFAR, the LOw Frequency ARray, was designed and developed by ASTRON (Netherlands Institute for Radio Astronomy) with 36 stations centered on Exloo in the northeast of The Netherlands. It is now an international project with stations being built in Germany, France, the UK and Sweden connected to the central data processing facilities in Groningen (NL) and the ASTRON operations center in Dwingeloo (NL). The first international LOFAR station (IS-DE1) was completed on the area of the Effelsberg radio observatory next to the 100-m radio telescope of the Max-Planck-Institut für Radioastronomie (MPIfR).
Operating at relatively low radio frequencies from 10 to 240 MHz, LOFAR has essentially no moving parts to track objects in the sky; instead digital electronics are used to combine signals from many small antennas to electronically steer observations on the sky. In certain electronic modes, the signals from all of the individual antennas can be combined to make images of the entire radio sky visible above the horizon.
LOFAR uses two different antenna designs, to observe in two different radio bands, the so-called low-band from 10 to 80 MHz, and the high-band from 110 to 240 MHz. All-sky images using the low-band antennas at Effelsberg were made in 2007.
Following the observation for the first high-band, all-sky image, scientists at MPIfR made a series of all-sky images covering a wide frequency range using both the low-band and high-band antennas at Effelsberg.
The movie of these all-sky images has been compiled and is shown above. The movie starts at a frequency of 35 MHz, and each subsequent frame is about 4 MHz higher in frequency, through 190 MHz. The resolution of the Effelsberg LOFAR telescope changes with frequency. At 35 MHz the resolution is about 10 degrees, at 110 MHz it is about 3.4 degrees, and at 190 MHz it is about 1.9 degrees. This change in resolution can be seen by the apparent size of the two bright sources Cygnus A and Cassiopeia A as the frequency changes.
Scientists at MPIfR and other institutions around Europe will use measurements such as these to study the large-sky structure of the interstellar matter of our Milky Way galaxy. The low frequencies observed by LOFAR are ideal for studying the low energy cosmic ray electrons in the Milky Way, which trace out magnetic field structures through synchrotron emission. Other large-scale features such as supernova remnants, star-formation regions, and even some other nearby galaxies will need similar measurements from individual LOFAR telescopes to provide accurate information on the large-scale emission in these objects. “We plan to search for radio transients using the all-sky imaging capabilities of the LOFAR telescopes”, says Michael Kramer, director at MPIfR, in Bonn. “The detection of rapidly variable sources using LOFAR could lead to exciting discoveries of new types of astronomical objects, similar to the discoveries of pulsars and gamma-ray bursts in the past decades.”
“The low-frequency sky is now truly open in Effelsberg and we have the capability at the observatory to observe in a wide frequency range from 10 MHz to 100 GHz”, says Anton Zensus, also director at MPIfR. “Thus we can cover four orders of magnitude in the electromagnetic spectrum.”
The origin of cosmic rays has been a mystery since their discovery nearly a century ago. But new images from the Fermi Gamma-ray Space Telescope may bring astronomers a step closer to understanding the source of the Universe’s most energetic particles. The images show where supernova remnants emit radiation a billion times more energetic than visible light. “Fermi now allows us to compare emission from remnants of different ages and in different environments,” said Stefan Funk, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC).
Cosmic rays are made up of electrons, positrons and atomic nuclei and they constantly bombard the Earth. In their near-speed-of-light journey across the galaxy, the particles are deflected by magnetic fields, which scramble their paths and mask their origins. When cosmic rays collide with interstellar gas, they produce gamma rays. While instruments can infer the presence of cosmic rays by looking for the glow of gamma ray emissions, so far, no specific sources have been located. .
“Understanding the sources of cosmic rays is one of Fermi’s key goals,” said said Funk, who presented the new images and findings at the American Physical Society meeting in Washington, D.C. on Monday.
Fermi’s Large Area Telescope (LAT) mapped billion-electron-volt (GeV) gamma rays from three middle-aged supernova remnants — known as W51C, W44, and IC 443 — that were never before resolved at these energies. (The energy of visible light is between 2 and 3 electron volts.) Each remnant is the expanding debris of a massive star that blew up between 4,000 and 30,000 years ago.
In addition, Fermi’s LAT also spied GeV gamma rays from Cassiopeia A (Cas A), a supernova remnant only 330 years old. Ground-based observatories, which detect gamma rays thousands of times more
energetic than the LAT was designed to see, have previously detected Cas A.
“Older remnants are extremely bright in GeV gamma rays, but relatively faint at higher energies. Younger remnants show a different behavior,” explained Yasunobu Uchiyama, a Panofsky Fellow at SLAC. “Perhaps the highest-energy cosmic rays have left older remnants, and Fermi sees emission from trapped particles at lower energies.”
In 1949, physicist Enrico Fermi — for whom the Fermi telescope was named –suggested that the highest-energy cosmic rays were accelerated in the magnetic fields of gas clouds. In the decades that followed,
astronomers showed that supernova remnants are the galaxy’s best candidate sites for this process.
Young supernova remnants seem to possess both stronger magnetic fields and the highest-energy cosmic rays. Stronger fields can keep the highest-energy particles in the remnant’s shock wave long enough to speed them to the energies observed.
The Fermi observations show GeV gamma rays coming from places where the remnants are known to be interacting with cold, dense gas clouds.
“We think that protons accelerated in the remnant are colliding with gas atoms, causing the gamma-ray emission,” Funk said. An alternative explanation is that fast-moving electrons emit gamma rays as they fly past the nuclei of gas atoms. “For now, we can’t distinguish between these possibilities, but we expect that further observations with Fermi will help us to do so,” he added.
Either way, these observations validate the notion that supernova remnants act as enormous accelerators for cosmic particles.
“How fitting it is that Fermi seems to be confirming the bold idea advanced over 60 years ago by the scientist after whom it was named,” noted Roger Blandford, director of KIPAC.