cosmic rays Archives

August 21, 2010December 24, 2015 by Steve Nerlich

Astronomy Without A Telescope – A Universe Free Of Charge?

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If there were equal amounts of matter and anti-matter in the universe, it would be easy to deduce that the universe has a net charge of zero, since a defining ‘opposite’ of matter and anti-matter is charge. So if a particle has charge, its anti-particle will have an equal but opposite charge. For example, protons have a positive charge – while anti-protons have a negative charge.

But it’s not apparent that there is a lot of anti-matter around as neither the cosmic microwave background, nor the more contemporary universe contain evidence of annihilation borders – where contact between regions of large scale matter and large scale anti-matter should produce bright outbursts of gamma rays.

So, since we do apparently live in a matter-dominated universe – the question of whether the universe has a net charge of zero is an open question.

It’s reasonable to assume that dark matter has either a net zero charge – or just no charge at all – simply because it is dark. Charged particles and larger objects like stars with dynamic mixtures of positive and negative charges, produce electromagnetic fields and electromagnetic radiation.

So, perhaps we can constrain the question of whether the universe has a net charge of zero to just asking whether the total sum of all non-dark matter has. We know that most cold, static matter – that is in an atomic, rather than a plasma, form – should have a net charge of zero, since atoms have equal numbers of positively charged protons and negatively charged electrons.

Stars composed of hot plasma might also be assumed to have a net charge of zero, since they are the product of accreted cold, atomic material which has been compressed and heated to create a plasma of dissociated nuclei (+ve) and electrons (-ve).

The principle of charge conservation (which is accredited to Benjamin Franklin) has it that the amount of charge in a system is always conserved, so that the amount flowing in will equal the amount flowing out.

Apollo 15's Lunar Surface Experiments Package (ALSEP). The Moon represents a good vantage point to measure the balance of incoming cosmic rays versus outgoing solar wind.

An experiment which has been suggested to enable measurement of the net charge of the universe, involves looking at the solar system as a charge-conserving system, where the amount flowing in is carried by charged particles in cosmic rays – while the amount flowing out is carried by charged particles in the Sun’s solar wind.

If we then look at a cool, solid object like the Moon, which has no magnetic field or atmosphere to deflect charged particles, it should be possible to estimate the net contribution of charge delivered by cosmic rays and by solar wind. And when the Moon is shadowed by the tail of the Earth’s magnetosphere, it should be possible to detect the flux attributable to just cosmic rays – which should represent the charge status of the wider universe.

Drawing on data collected from sources including Apollo surface experiments, the Solar and Heliospheric Observatory (SOHO), the WIND spacecraft and the Alpha Magnetic Spectrometer flown on a space shuttle (STS 91), the surprising finding is a net overbalance of positive charges arriving from deep space, implying that there is an overall charge imbalance in the cosmos.

Either that or a negative charge flux occurs at energy levels lower than the threshold of measurement that was achievable in this study. So perhaps this study is a bit inconclusive, but the question of whether the universe has a net charge of zero still remains an open question.

Further reading: Simon, M.J. and Ulbricht, J. (2010) Generating an electrical potential on the Moon by cosmic rays and solar wind?

February 15, 2010December 24, 2015 by Nancy Atkinson

Sources of Cosmic Rays Found? Fermi Telescope Closes In

A multiwavelength look at Cas A. Credit: NASA/DOE/Fermi LAT Collaboration

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.

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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.”

Fermi mapped GeV-gamma-ray emission regions (magenta) in the W44 supernova remnant. The features clearly align with filaments detectable in other wavelengths. This composite merges X-ray data (blue) from the Germany/U.S./UK ROSAT mission, infrared (red) from NASA’s Spitzer Space Telescope, and radio (orange) from the Very Large Array near Socorro, N.M. Credit: NASA/DOE/Fermi LAT Collaboration, NASA/ROSAT, NASA/JPL-Caltech, and NRAO/AUI

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.

Source: Fermi/Sonoma State University

December 28, 2009December 24, 2015 by Nicholos Wethington

An Exotic Source for Cosmic Rays: ‘Baby’ Black Holes

Cosmic rays – particles that have been accelerated to near the speed of light – stream out from our Sun all of the time, though they are positively sluggish compared to what are called Ultra-High-Energy Cosmic Rays (UHECRs). These types of cosmic rays originate from sources outside of the Solar System, and are much more energetic than those from our Sun, though also much rarer. The merger between a white dwarf and neutron star or black hole may be one source of these rays, and such mergers may occur often enough to be the most significant source of these energetic particles.

The Sloan White dwArf Radial velocity data Mining Survey (SWARMS) – which is part of the Sloan Digital Sky Survey – recently uncovered a binary system of exotic objects only 50 parsecs away from the Solar System. This system, named SDSS 1257+5428, appears to be a white dwarf star that is orbiting a neutron star or low-mass black hole. Details about the system and its initial discovery can be found in a paper by Carles Badenes, et al. here.

Co-author Todd Thompson, assistant professor in the Department of Astronomy at Ohio State University, argues in a recent letter to The Astrophysical Journal Letters that this type of system, and subsequent merger of these exotic remnants of stars, may be commonplace, and could account for the amount of UHECRs that are currently observed. The merger between the white dwarf and neutron star or black hole may also create a black hole of low mass, a so-called “baby” black hole.

Thompson wrote in an email interview:

“White dwarf/neutron star or black hole binaries are thought to be quite rare, although there is a huge range in the number per Milky Way-like galaxy in the literature. SWARMS was the first to detect such a system using the “radial velocity” technique, and the first to find such an object so nearby, only 50 parsecs away (about 170 light years). For this reason, it was very surprising, and its relative proximity is what allowed us to make the argument that these systems must be quite common compared to most previous expectations. SWARMS would have had to be very lucky to see something so rare so near by.”

Thompson, et al. argue that this type of merger may be the most significant source of UHECRs in the Milky Way galaxy, and that one should merge in the galaxy about every 2,000 years. These types of mergers may be slightly less common than Type Ia supernovae, which originate in binary systems of white dwarfs.

A white dwarf merging with a neutron star would also create a low-mass black hole of about 3 times the mass of the Sun. Thompson said, “In fact, this scenario is likely since we think that neutron stars cannot exist above 2-3 times the mass of the Sun. The idea is that the WD would be disrupted and accrete onto the neutron star and then the neutron star would collapse to a black hole. In this case, we might see the signal of BH formation in gravity waves.”

The gravity waves produced in such a merger would be above the detectable range by the Laser Interferometer Gravitational-Wave Observatory (LIGO), an instrument that uses lasers to detect gravity waves (of which none have been detected…yet), and even possibly a spaced base gravitational wave observatory, NASA’s Laser Interferometer Space Antenna, LISA.

Common cosmic rays that come from our Sun have an energy on the scale of 10^7 to 10^10 electron-volts. Ultra-high-energy cosmic rays are a rare phenomenon, but they exceed 10^20 electron-volts. How do systems like SDSS 1257+5428 produce cosmic rays of such high energy? Thompson explained that there are two equally fascinating possibilities.

In the first, the formation of a black hole and subsequent accretion disk from the merger would generate a jet somewhat like those seen at the center of galaxies, the telltale sign of a quasar. Though these jets would be much, much smaller, the shockwaves at the front of the jet would accelerate particles to the necessary energies to create UHECRs, Thompson said.

In the second scenario, the neutron star steals matter off of the white dwarf companion, and this accretion starts it rotating rapidly. The magnetic stresses that build at the surface of the neutron star, or “magnetar”, would be able to accelerate any particles that interact with the intense magnetic field to ultra-high energies.

The creation of these ultra-high-energy cosmic rays by such systems is highly theoretical, and just how common they may be in our galaxy is only an estimate. It remains unclear so soon after the discovery of SDSS 1257+5428 whether the companion object of the white dwarf is a black hole or neutron star. But the fact that SWARMS made such a discovery so early in the survey is encouraging for the discovery of further exotic binary systems.

“It is not likely that SWARMS will see 10 or 100 more such systems. If it did, the rate of such mergers would be very (implausibly) high. That said, we’ve been surprised many times before. However, given the total area of the sky surveyed, if our estimate of the rate of such mergers is correct, SWARMS should see only about 1 more such system, and they may see none. A similar survey in the southern sky (there is nothing at present comparable to the Sloan Digital Sky Survey, on which SWARMS is based) should turn up approximately 1 such system,” Thompson said.

Observations of SDSS 1257+5428 have already been made using the Swift X-ray observatory, and some measurements have been taken in the radio spectrum. No source of gamma-rays was to be found in the location of the system using the Fermi telescope.

Thompson said, “Probably the most important forthcoming observation of the system is to get a true distance via parallax. Right now, the distance is based on the properties of the observed white dwarf. In principle,
it should be relatively easy to watch the system over the next year and get a parallax distance, which will alleviate many of the uncertainties surrounding the physical properties of the white dwarf.”

Source: Arxiv, email interview with Todd Thompson

November 2, 2009April 26, 2016 by Nancy Atkinson

Solving the Mystery of Cosmic Rays’ Origins

What accelerates cosmic rays to nearly the speed of light? Astronomer have pondered that question for nearly 100 years, and now new evidence supports a theory held for two decades that cosmic rays likely are powered by exploding stars and stellar winds. “This discovery has been predicted for almost 20 years, but until now no instrument was sensitive enough to see it,” said Wystan Benbow, an astrophysicist at the Smithsonian Astrophysical Observatory who coordinated this project for the Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaboration.

Nearly 100 years ago, scientists detected the first signs of cosmic rays, which are actually not rays or beams but subatomic particles (mostly protons) that zip through space at nearly the speed of light. The most energetic cosmic rays hit with the punch of a 98-mph fastball, even though they are smaller than an atom. Astronomers questioned what natural force could accelerate particles to such a speed.

The rarest cosmic rays carry over 100 billion times as much energy as generated by any particle accelerator on Earth. Astronomers have devised ingenious methods for detecting cosmic rays that hit Earth’s atmosphere. However, detecting cosmic rays from a distance requires much more effort.

This representative-color figure shows the very-high-energy gamma-ray emission observed by VERITAS coming from the Cigar Galaxy, also known as Messier 82. The black star is the location of the active starburst region. The emission from M82 is effectively point-like for VERITAS, and the white circle indicates the size of a simulated point source. The entire galaxy would be contained within the circle. Credit: CfA/V.A. Acciari

VERITAS has found new evidence for cosmic rays in the “Cigar Galaxy,” also known as Messier 82 (M82), which is located 12 million light-years from Earth in the direction of the constellation Ursa Major, which strongly support the long-held theory that supernovae and stellar winds from massive stars are the dominant accelerators of cosmic-ray particles.

Galaxies with high levels of star formation like M82, also known as “starburst” galaxies, have large numbers of supernovae and massive stars. If the theory holds, then starburst galaxies should contain more cosmic rays than normal galaxies. The VERITAS discovery confirms that expectation, indicating that the cosmic-ray density in M82 is approximately 500 times the average density in our Galaxy, the Milky Way.

“This discovery provides fundamental insight into the origin of cosmic rays,” said Rene Ong, a professor of physics at the University of California, Los Angeles, and the spokesperson for the VERITAS collaboration.

Using gamma rays to infer cosmic rays

VERITAS could not detect M82’s cosmic rays directly because they are trapped within the Cigar Galaxy. Instead, VERITAS looked for clues to the presence of cosmic rays: gamma rays. Gamma rays are the most energetic form of light, far more powerful than ultraviolet light or even X-rays. When cosmic rays interact with interstellar gas and radiation within M82, they produce gamma rays, which can then escape their home galaxy and reach Earthbound detectors.

It took two years of dedicated data collection to tease out the faint signal coming from M82.

“We knew that the detection of M82 would have important scientific implications. As a result, we scheduled an exceptionally long exposure immediately after the experiment became fully operational” said Benbow. “The data needed to be meticulously analyzed to extract the gamma-ray signal, which is over a million times smaller than the background noise. Although the signal is only a tiny fraction of the data, we made many checks for possible bias and we are confident that the signal is genuine.”

“The detection of M82 indicates that the universe is full of natural particle accelerators, and as ground-based gamma-ray observatories continue to improve, further discoveries are inevitable.” said Martin Pohl, a professor of physics at Iowa State University who helped lead the study. A next-generation VHE gamma-ray observatory, the Advanced Gamma-ray Imaging System (AGIS), is already under development.

VERITAS is operated by a collaboration of more than 100 scientists from 22 different institutions in the United States, Ireland, England and Canada. Click here for more information on VERITAS.

Lead image caption: A composite of multi-wavelength images of the active galaxy M82 from Hubble, Chandra, and Spitzer. Credit: NASA, ESA, CXC, and JPL-Caltech

Source: Harvard Smithsonian Center for Astrophysics