fermi gamma ray space telescope

Five Famous Pulsars from the Past 50 Years

Early astronomers faced an obstacle: their technology. These great minds only had access to telescopes that revealed celestial bodies shining in visible light. Later, with the development of new detectors, scientists opened their eyes to other types of light like radio waves and X-rays. They realized cosmic objects look very different when viewed in these additional wavelengths. Pulsars — rapidly spinning stellar corpses that appear to pulse at us — are a perfect example.

The first pulsar was observed 50 years ago on August 6, 1967, using radio waves, but since then we have studied them in nearly all wavelengths of light, including X-rays and gamma rays.

Typical Pulsar

Most pulsars form when a star — between 8 and 20 times the mass of our sun — runs out of fuel and its core collapses into a super dense and compact object: a neutron star

These neutron stars are about the size of a city and can rotate slowly or quite quickly, spinning anywhere from once every few hours to hundreds of times per second. As they whirl, they emit beams of light that appear to blink at us from space.

First Pulsar

One day five decades ago, a graduate student at the University of Cambridge, England, named Jocelyn Bell was poring over the data from her radio telescope - 120 meters of paper recordings.

Image Credit: Sumit Sijher

She noticed some unusual markings, which she called “scruff,” indicating a mysterious object (simulated above) that flashed without fail every 1.33730 seconds. This was the very first pulsar discovered, known today as PSR B1919+21.

Best Known Pulsar

Before long, we realized pulsars were far more complicated than first meets the eye — they produce many kinds of light, not only radio waves. Take our galaxy’s Crab Nebula, just 6,500 light years away and somewhat of a local celebrity. It formed after a supernova explosion, which crushed the parent star’s core into a neutron star. 

The resulting pulsar, nestled inside the nebula that resulted from the supernova explosion, is among the most well-studied objects in our cosmos. It’s pictured above in X-ray light, but it shines across almost the entire electromagnetic spectrum, from radio waves to gamma rays.

Brightest Gamma-ray Pulsar

Speaking of gamma rays, in 2015 our Fermi Gamma-ray Space Telescope discovered the first pulsar beyond our own galaxy capable of producing such high-energy emissions. 

Located in the Tarantula Nebula 163,000 light-years away, PSR J0540-6919 gleams nearly 20 times brighter in gamma-rays than the pulsar embedded in the Crab Nebula.

Dual Personality Pulsar

No two pulsars are exactly alike, and in 2013 an especially fast-spinning one had an identity crisis. A fleet of orbiting X-ray telescopes, including our Swift and Chandra observatories, caught IGR J18245-2452 as it alternated between generating X-rays and radio waves. 

Scientists suspect these radical changes could be due to the rise and fall of gas streaming onto the pulsar from its companion star.

Transformer Pulsar

This just goes to show that pulsars are easily influenced by their surroundings. That same year, our Fermi Gamma Ray Space Telescope uncovered another pulsar, PSR J1023+0038, in the act of a major transformation — also under the influence of its nearby companion star. 

The radio beacon disappeared and the pulsar brightened fivefold in gamma rays, as if someone had flipped a switch to increase the energy of the system. 

NICER Mission

Our Neutron star Interior Composition Explorer (NICER) mission, launched this past June, will study pulsars like those above using X-ray measurements.

With NICER’s help, scientists will be able to gaze even deeper into the cores of these dense and mysterious entities.

For more information about NICER, visit https://www.nasa.gov/nicer

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MASSIVE STAR’S DYING BLAST CAUGHT BY RAPID-RESPONSE TELESCOPES

** Synopsis: A blast of gamma rays from space detected in June 2016 is helping astronomers resolve long-standing questions about the universe’s most powerful explosions. **

In June 2016, an international team of 31 astronomers, led by the University of Maryland’s Eleanora Troja and including Arizona State University’s Nathaniel Butler, caught a massive star as it died in a titanic explosion deep in space.

The blast of the dying star released in about 40 seconds as much energy as the Sun releases over its entire lifetime, all focused into a tight beam of gamma rays aimed by chance toward Earth.

The team’s findings, reported in the scientific journal Nature, provide strong evidence for one of two competing models for how gamma-ray bursters (GRBs) produce their energy.

“These are the brightest explosions in the universe,” says Butler, an associate professor in ASU’s School of Earth and Space Exploration. “And we were able to measure this one’s development and decay almost from the initial blast.”

Quick reflexes

The gamma-ray blast on June 25, 2016, was detected by two NASA satellites that monitor the sky for such events, the Fermi Gamma-ray Space Telescope and the Swift Gamma-Ray Burst Mission.

The satellite observatories detected the burst of gamma rays, identified where in the sky it came from, and sent its celestial position within seconds to automated telescopes on the ground.

The MASTER-IRC telescope at the Teide Observatory in the Canary Islands observed it first, within a minute of the satellite notification. The telescope is part of Russia’s MASTER network of robotic telescopes at the Teide Observatory. It made optical light observations while the initial phase was still active, gathering data on the amount of polarized optical light relative to the total light produced.

After the Sun set over this facility eight and a half hours later, the RATIR camera in which ASU is involved began observing. RATIR stands for Reionization And Transients InfraRed camera; it is mounted on a 1.5-meter (60-inch) robotically controlled telescope located on San Pedro Mártir Peak, at Mexico’s National Astronomical Observatory in Baja California. Butler is the principal investigator for the fully-automated camera.

Butler explains, “At best, it takes a minute or two for our telescope to slew to the burst’s position. In this case, we had to wait for it to rise over the horizon. This means the gamma-ray burst itself had ended, and we were observing what’s called the afterglow. This is the fading explosion as the radiation shocks up the interstellar medium around the star that exploded.”

He says, “The RATIR camera lets us take simultaneous images in six colors, two optical and four near-infrared. Over the past five years, RATIR has imaged 155 gamma-ray bursts.”

Mystery beams of energy

While gamma-ray bursters have been known for about fifty years, astronomers are still mostly in the dark about how they erupt.

“Despite a long history of observations,” Butler says, “the emission mechanism driving gamma-ray bursters remains largely mysterious.”

Gamma-ray bursts are detected approximately once per day and are brief, but intense, flashes of gamma radiation. They come from all different directions in the sky, and they last from tens of milliseconds to about a minute, making it hard to observe them in detail.

Astronomers believe most of these explosions are associated with supernovas. These occur when a massive star reaches the end of its normal existence and blows up in a colossal explosion. A supernova throws off some of the star’s outer layers, while its core and remaining layers collapse in a few seconds into a neutron star or, in the case of highly massive stars, a black hole.

Continued RATIR observations over weeks following the June 2016 outburst showed that the gamma rays were shot out in a beam about two degrees wide, or roughly four times the apparent size of the Moon. It was sheer chance that Earth happened to lie within the beam.

Beaming effects, Butler says, may result from the spin of the black hole produced after the supernova explosion, as it releases material along its poles.

Magnetic focus

“We think the gamma-ray emission is due to highly energetic electrons, propelled outward like a fireball,” Butler says. Magnetic fields must also be present, he adds, and theories differ as to how the fields are produced and to what extent the flow of magnetic energy outward is important.

A key diagnostic is measuring the radiation’s polarization, he explains. This, astronomers think, is largely controlled by the strength of the magnetic fields that focus the radiation. Butler says, "Measuring the strength of magnetic fields by their polarization effects can tell us about the mechanisms that accelerate particles such as electrons up to very high energies and cause them to radiate at gamma-ray energies.”

In the case of the June 2016 blast, the scientists were able to measure polarization using MASTER within minutes, an unprecedented early discovery. The large amount of polarization the team observed indicates that powerful magnetic fields were confining and directing it. This lends support for the magnetic origin model for gamma-ray bursters.

While gamma-ray bursters have many more mysteries to be unfolded, Butler says, “this is the first strong evidence that the early shocks generated by these bursts are magnetically driven.”

Hubble dates black hole's last big meal

For the supermassive black hole at the center of our Milky Way galaxy, it’s been a long time between dinners. NASA’s Hubble Space Telescope has found that the black hole ate its last big meal about 6 million years ago, when it consumed a large clump of infalling gas. After the meal, the engorged black hole burped out a colossal bubble of gas weighing the equivalent of millions of suns, which now billows above and below our galaxy’s center.

The immense structures, dubbed the Fermi Bubbles, were first discovered in 2010 by NASA’s Fermi Gamma-ray Space Telescope. But recent Hubble observations of the northern bubble have helped astronomers determine a more accurate age for the bubbles and how they came to be.

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Txch This Week: Lab-On-A-Chip Gets Big Boost And The Lemon-Shaped Moon

by Annie Epstein

This week on Txchnologist, we felt a tug on our heartstrings when we read about Lyman Connor’s story. After a fall, Connor spent some time in the hospital and met a small boy who had lost his hand. Inspired by the boy’s plight, he set out to make a low-cost, app-controlled bionic hand. The engineering part has been a success. Now he just needs to find the boy again.

A Swiss team is hard at work creating a new way to filter water at the speedy rate of almost a liter per minute. Their personal water treatment technology uses an advanced polymer membrane with nanoscopic pores that block bacteria, viruses and other microbes from passing through. The device screws on to any plastic bottle and can filter 300 liters of contaminated water, one person’s hydration requirements over the course of a year. 

Manufacturing has begun on parts for the world’s largest experimental nuclear fusion reactor, expected to begin operating in 2020. The international project, now estimated to cost around $20 billion to construct, involves cooperation between Europe, the U.S., Russia, Japan, China, South Korea and India. If it works, the reactor is expected to generate 500 megawatts of power, and one gram of hydrogen fuel will generate as much power as eight tons of oil.

Finally, agricultural scientists are making potatoes for Millenials. Using the age-old method of selective plant breeding, researchers are coming out with tubers colored like raspberries and others splashed with purple accents. Move over bland russets, potatoes with flair will be making their way to markets soon.

Now we’re bringing you the news we’ve been following this week in the world of science, technology and innovation.

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Origin of Milky Way's hypothetical dark matter signal may not be so dark

A mysterious gamma-ray glow at the center of the Milky Way is most likely caused by pulsars – the incredibly dense, rapidly spinning cores of collapsed ancient stars that were up to 30 times more massive than the sun. That’s the conclusion of a new analysis by an international team of astrophysicists, including researchers from the Department of Energy’s SLAC National Accelerator Laboratory. The findings cast doubt on previous interpretations of the signal as a potential sign of dark matter – a form of matter that accounts for 85 percent of all matter in the universe but that so far has evaded detection.

“Our study shows that we don’t need dark matter to understand the gamma-ray emissions of our galaxy,” said Mattia Di Mauro from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint institute of Stanford University and SLAC. “Instead, we have identified a population of pulsars in the region around the galactic center, which sheds new light on the formation history of the Milky Way.”

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NASA's Fermi mission expands its search for dark matter

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA’s Fermi Gamma-ray Space Telescope, have broadened the mission’s dark matter hunt using some novel approaches.

“We’ve looked for the usual suspects in the usual places and found no solid signals, so we’ve started searching in some creative new ways,” said Julie McEnery, Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it.”

Dark matter neither emits nor absorbs light, primarily interacts with the rest of the universe through gravity, yet accounts for about 80 percent of the matter in the universe. Astronomers see its effects throughout the cosmos – in the rotation of galaxies, in the distortion of light passing through galaxy clusters, and in simulations of the early universe, which require the presence of dark matter to form galaxies at all.

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NASA’s Fermi mission expands its search for dark matter

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA’s Fermi Gamma-ray Space Telescope, have broadened the mission’s dark matter hunt using some novel approaches.
“We’ve looked for the usual suspects in the usual places and found no solid signals, so we’ve started searching in some creative new ways,” said Julie McEnery, Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it.”

Dark matter neither emits nor absorbs light, primarily interacts with the rest of the universe through gravity, yet accounts for about 80 percent of the matter in the universe. Astronomers see its effects throughout the cosmos – in the rotation of galaxies, in the distortion of light passing through galaxy clusters, and in simulations of the early universe, which require the presence of dark matter to form galaxies at all.

The leading candidates for dark matter are different classes of hypothetical particles. Scientists think gamma rays, the highest-energy form of light, can help reveal the presence of some of types of proposed dark matter particles. Previously, Fermi has searched for tell-tale gamma-ray signals associated with dark matter in the center of our galaxy and in small dwarf galaxies orbiting our own. Although no convincing signals were found, these results eliminated candidates within a specific range of masses and interaction rates, further limiting the possible characteristics of dark matter particles.

Among the new studies, the most exotic scenario investigated was the possibility that dark matter might consist of hypothetical particles called axions or other particles with similar properties. An intriguing aspect of axion-like particles is their ability to convert into gamma rays and back again when they interact with strong magnetic fields.

These conversions would leave behind characteristic traces, like gaps or steps, in the spectrum of a bright gamma-ray source.

Manuel Meyer at Stockholm University led a study to search for these effects in the gamma rays from NGC 1275, the central galaxy of the Perseus galaxy cluster, located about 240 million light-years away.

High-energy emissions from NGC 1275 are thought to be associated with a supermassive black hole at its center. Like all galaxy clusters, the Perseus cluster is filled with hot gas threaded with magnetic fields, which would enable the switch between gamma rays and axion-like particles. This means some of the gamma rays coming from NGC 1275 could convert into axions – and potentially back again – as they make their way to us.

Meyer’s team collected observations from Fermi’s Large Area Telescope (LAT) and searched for predicted distortions in the gamma-ray signal. The findings, published April 20 in Physical Review Letters, exclude a small range of axion-like particles that could have comprised about 4 percent of dark matter.

“While we don’t yet know what dark matter is, our results show we can probe axion-like models and provide the strongest constraints to date for certain masses,” Meyer said. “Remarkably, we reached a sensitivity we thought would only be possible in a dedicated laboratory experiment, which is quite a testament to Fermi.”

Another broad class of dark matter candidates are called Weakly Interacting Massive Particles (WIMPs). In some versions, colliding WIMPs either mutually annihilate or produce an intermediate, quickly decaying particle. Both scenarios result in gamma rays that can be detected by the LAT.

Regina Caputo at the University of California, Santa Cruz, sought these signals from the Small Magellanic Cloud (SMC), which is located about 200,000 light-years away and is the second-largest of the small satellite galaxies orbiting the Milky Way. Part of the SMC’s appeal for a dark matter search is that it lies comparatively close to us and its gamma-ray emission from conventional sources, like star formation and pulsars, is well understood. Most importantly, astronomers have high-precision measurements of the SMC’s rotation curve, which shows how its rotational speed changes with distance from its center and indicates how much dark matter is present. In a paper published in Physical Review D on March 22, Caputo and her colleagues modeled the dark matter content of the SMC, showing it possessed enough to produce detectable signals for two WIMP types.

“The LAT definitely sees gamma rays from the SMC, but we can explain them all through conventional sources,” Caputo said. “No signal from dark matter annihilation was found to be statistically significant.”

In the third study, researchers led by Marco Ajello at Clemson University in South Carolina and Mattia Di Mauro at SLAC National Accelerator Laboratory in California took the search in a different direction. Instead of looking at specific astronomical targets, the team used more than 6.5 years of LAT data to analyze the background glow of gamma rays seen all over the sky.

The nature of this light, called the extragalactic gamma-ray background (EGB) has been debated since it was first measured by NASA’s Small Astronomy Satellite 2 in the early 1970s. Fermi has shown that much of this light arises from unresolved gamma-ray sources, particularly galaxies called blazars, which are powered by material falling toward gigantic black holes. Blazars constitute more than half of the total gamma-ray sources seen by Fermi, and they make up an even greater share in a new LAT catalog of the highest-energy gamma rays.

Some models predict that EGB gamma rays could arise from distant interactions of dark matter particles, such as the annihilation or decay of WIMPs. In a detailed analysis of high-energy EGB gamma rays, published April 14 in Physical Review Letters, Ajello and his team show that blazars and other discrete sources can account for nearly all of this emission.

“There is very little room left for signals from exotic sources in the extragalactic gamma-ray background, which in turn means that any contribution from these sources must be quite small,” Ajello said. “This information may help us place limits on how often WIMP particles collide or decay.”

Although these latest studies have come up empty-handed, the quest to find dark matter continues both in space and in ground-based experiments. Fermi is joined in its search by NASA’s Alpha Magnetic Spectrometer, a particle detector on the International Space Station.