What would a gamma-ray burst sound like? No one really knows, but members of the team that work with the Fermi Large Area Telescope (LAT) have translated gamma-ray measurements into musical notes and have created a “song” from the photons from one of the most energetic of these powerful explosions, GRB 080916C which occurred in September of 2008.
“In translating the gamma-ray measurements into musical notes we assigned the photons to be “played” by different instruments (harp, cello, or piano) based on the probabilities that they came from the burst,” the team wrote in the Fermi blog. “By converting gamma rays into musical notes, we have a new way of representing the data and listening to the universe.” Continue reading “A Gamma-Ray Burst as Music”
A pulsar with its magnetic field lines illustrated. The beams emitting from the poles are what washes over our detectors as the dead star spins.
Researchers using the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope have developed a new method to detect a special class of stellar remnant, known as pulsars. A pulsar is a special type of neutron star, which spin hundreds of times per second. When the intense spin is combined with beams of energy caused by intense magnetic fields, a “lighthouse” pulse is generated. When the “lighthouse” beam sweeps across Earth’s field of view, the object is referred to as a pulsar.
Led by Matthew Kerr (Kavli Institute for Particle Astrophysics and Cosmology), and Fernando Camilo (Columbia University), a research team recently announced a new method for detecting pulsars. How will Kerr’s research help astronomers better understand (and locate) these small, elusive stellar remnants?
Every three hours, the LAT surveys the entire sky, searching for the high energy signatures associated with gamma-ray outbursts. In general the energy levels of the photos detected by the LAT are 20 million to over 300 billion times as energetic as the photons associated with visible light.
By combining observations from the LAT and data obtained from the Parkes radio telescope in Australia, the team is able to detect pulsar candidates. The team’s approach combines a “wide area” approach of an all-sky telescope like the LAT with the sensitivity of a radio telescope. So far, the team’s discovery of five “millisecond” class pulsars, including one unusual pulsar has proven their technique to be successful.
The unusual pulsar, officially named PSR J0101–6422, had an additional 35 days of study devoted to better understanding its properties. Once the radio pulsation period and phase were determined, an incredible amount of data, including data on its gamma-ray pulsations was obtained. Using the data, the team was able to determine PSR J0101–6422 is roughly 1750 light-years away from Earth, and has an unusual light curve which features a “sandwich” of two gamma-ray peaks with an intense radio peak in the center, like a cosmic ham sandwich.
The team was unable to explain the phenomenon with standard pulsar emission models.which the team could not explain with standard geometric pulsar emission models, and have proposed that J0101–6422 is a new hybrid class of pulsar that features radio emissions that originate from low and high altitudes above the neutron star.
The results of Kerr’s research have been published in the Astrophysical Journal.
Image #1 Caption:Clouds of charged particles move along the pulsar’s magnetic field lines (blue) and create a lighthouse-like beam of gamma rays (purple) in this illustration. Image Credit: NASA
IceCube team poses for a picture in front of deployment tower after the completion of the IceCube Neutrino Detector in December of 2010. Photo by: Chad Carpenter/NSF
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The origin of cosmic rays has been one of the most enduring mysteries in physics, and it looks like it’s going to stay that way for a while longer. One of the leading candidates for where cosmic rays come from is gamma ray bursts, and physicists were hoping a huge Antarctic detector called the IceCube Neutrino Observatory would confirm that theory. But observations of over 300 GRB’s turned up no evidence of cosmic rays. In short, cosmic rays aren’t what we thought they were.
But, just like Thomas Edison who said that “every wrong attempt discarded is another step forward,” physicists view this latest finding as progress.
“Although we have not discovered where cosmic rays come from, we have taken a major step towards ruling out one of the leading predictions,” said IceCube principal investigator and University of Wisconsin–Madison physics professor Francis Halzen.
Cosmic rays are electrically charged particles, such as protons, that strike Earth from all directions, with energies up to one hundred million times higher than those created in man-made accelerators. The intense conditions needed to generate such energetic particles have focused physicists’ interest on two potential sources: the massive black holes at the centers of active galaxies and gamma ray bursts (GRBs), flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies.
IceCube is using neutrinos, which are believed to accompany cosmic ray production, to explore these two theories. In a paper published in the April 19 issue of the journal Nature, IceCube scientists describe a search for neutrinos emitted from 300 gamma ray bursts observed, most recently in coincidence with the SWIFT and Fermi satellites, between May 2008 and April 2010. Surprisingly, they found none – a result that contradicts 15 years of predictions and challenges one of the two leading theories for the origin of the highest energy cosmic rays.
Aurora seen behind the IceCube Lab. Photo by: Sven Lidstrom/NSF
The detector searches for high-energy (teraelectronvolt; 1012-electronvolt) neutrinos, and in their paper the team said they found an upper limit on the flux of energetic neutrinos associated with GRBs that is at least a factor of 3.7 below the predictions. This implies that either GRBs are not the only sources of cosmic rays with energies greater than 1018More info on IceCube.
Today's Journal Club is about a new addition to the Standard Model of fundamental particles.
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According to Wikipedia, a journal club is a group of individuals who meet regularly to critically evaluate recent articles in the scientific literature. And of course, the first rule of Journal Club is… don’t talk about Journal Club.
So, without further ado – today’s journal article is about the latest findings in neutrino astronomy.
This paper presents some recent observations from the IceCube neutrino telescope at the South Pole – which acually observes neutrinos from the northern sky – using the Earth to filter out some of the background noise. Cool huh?
Firstly, a quick recap of neutrino physics. Neutrinos are sub-atomic particles of the lepton variety and are essentially neutrally charged versions of the other leptons – electrons, muons and taus – which all have a negative charge. So, we say that neutrinos come in three flavours – electron neutrinos, muon neutrinos and tau neutrinos.
Neutrinos were initially proposed by Pauli (a proposal later refined by Fermi) to explain how energy could be transported away from a system undergoing beta decay. When solar fusion began to be understood in the 1930s – the role of neutrinos was problematic since only a third or more of the neutrinos that were predicted to be produced by fusion were being detected – an issue which became known as the solar neutrino problem in the 1960’s.
The solar neutrino problem was only resolved in the late 1990s when the three neutrino flavours idea gained wide acceptance and each were finally detected in 2001 – confirming that solar neutrinos in transit actually oscillate between the three flavours (electron, muon and tau) – which means that if your detector is set up to detect only one flavour you will detect only about one third of all the neutrinos coming from the Sun.
Ten years later, the Ice Cube the neutrino observatory is using our improved understanding of neutrinos to try and detect high energy neutrinos of extragalactic origin. The first challenge is to distinguish atmospheric neutrinos (produced in abundance as cosmic rays strike the atmosphere) from astrophysical neutrinos.
Using what we have learnt from solving the solar neutrino problem, we can be confident that any neutrinos from distant sources have had time to oscillate – and hence should arrive at Earth in approximately equal ratios. Atmospheric neutrinos produced from close sources (also known as ‘prompt’ neutrinos) don’t have time to oscillate before being detected.
When looking for point sources of high energy astrophysical neutrinos, IceCube is most sensitive to muon neutrinos – which are detected when the neutrino weakly interacts with an ice molecule – emitting a muon. A high energy muon will then generate Cherenkov radiation – which is what IceCube actually detects. Unfortunately muon neutrinos are also the most common source of cosmic ray induced atmospheric neutrinos, but we are steadily getting better at determining what energy levels represent astrophysical rather than atmospheric neutrinos.
So, it’s still early days with this technology – with much of the effort going in to learning how to observe, rather than just observing. But maybe one day we will be observing the cosmic neutrino background – and hence the first second of the Big Bang. One day…
So… comments? Are neutrinos the fundamentally weirdest fundamental particle out there? Could IceCube be used to test the faster-than-light neutrino hypothesis? Want to suggest an article for the next edition of Journal Club?
This all-sky Fermi view includes only sources with energies greater than 10 GeV. From some of these sources, Fermi's LAT detects only one gamma-ray photon every four months. Brighter colors indicate brighter gamma-ray sources. Credit: NASA/DOE/Fermi LAT Collaboration
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It scans the entire visible sky every three hours. Its job is to gather light – but not just any light. What’s visible to our eyes averages about 2 and 3 electron volts, but NASA’s Fermi Gamma-Ray Space Telescope is taking a deep look into a higher realm… the electromagnetic range. Here the energy doesn’t need a boost. It slams out gamma-rays with energies ranging from 20 million to more than 300 billion electron volts (GeV). After three years of space time, the Fermi Large Area Telescope (LAT) has produced its first census of these extreme energy sources.
Over its current operating time, Fermi has continued to paint an ever-deepening portrait of the gamma-ray sky. Even with the huge amount of data which pours in over its 180 minute window, high energy events are not common. When it comes to sources above 10 GeV, even Fermi’s LAT detects only one source about three times a year.
“Before Fermi, we knew of only four discrete sources above 10 GeV, all of them pulsars,” said David Thompson, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “With the LAT, we’ve found hundreds, and we’re showing for the first time just how diverse the sky is at these high energies.”
Just what exactly is out there which can produce such a powerful process? When it comes to gamma-rays, more than half of Fermi’s nearly 500 findings are active galaxies where matter falling into their central supermassive black holes produces intense jets spewing out at close to light speed. A small portion – around 10% – of the census belongs to sources within the Milky Way. These are pulsars, supernova debris and a handful of binary systems which house massive stars. What’s really interesting is the portion of totally unidentifiable sources that constitute about a third of the findings. They simply don’t have any spectroscopic counterparts and astronomers are hoping that these higher energy sources will give them new material to compare their findings against.
New sources emerge and old sources fade as the LAT's view extends into higher energies. Credit: NASA/DOE/Fermi LAT Collaboration and A. Neronov et al.
When it comes to light – obey the rules. Just as we understand that sources of infra-red light fade away when viewed in the ultra-violet, gamma-ray sources above 1 GeV can disappear without a trace when viewed at higher, or “harder,” energies. “One example is the well-known radio galaxy NGC 1275, which is a bright, isolated source below 10 GeV.” says the Fermi team. ” At higher energies it fades appreciably and another nearby source begins to appear. Above 100 GeV, NGC 1275 becomes undetectable by Fermi, while the new source, the radio galaxy IC 310, shines brightly.” The Fermi hard-source list is the product of an international team led by Pascal Fortin at the Ecole Polytechnique’s Laboratoire Leprince-Ringuet in Palaiseau, France, and David Paneque at the Max Planck Institute for Physics in Munich.
More than half of the sources above 10 GeV are black-hole-powered active galaxies. More than a third of the sources are completely unknown, having no identified counterpart detected in other parts of the spectrum. Credit: NASA's Goddard Space Flight Center
The new Fermi census will be a unique source of comparative information to assist ground-based facilities called Atmospheric Cherenkov Telescopes. These sources have confirmed 130 gamma-ray sources with energies above 100 GeV. They include the Major Atmospheric Gamma Imaging Cherenkov telescope (MAGIC) on La Palma in the Canary Islands, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona, and the High Energy Stereoscopic System (H.E.S.S.) in Namibia.
“Our catalog will have a significant impact on ground-based facilities’ work by pointing them to the most likely places to find gamma-ray sources emitting above 100 GeV,” Paneque said.
But big ground-based telescopes have big limitations. In this case, their field of view is very constricted and they can’t operate during daylight hours, full Moon or bad weather. But don’t count them out.
“As Fermi’s exposure constantly improves our view of hard sources, ground-based telescopes are becoming more sensitive to lower-energy gamma rays, allowing us to bridge these two energy regimes,” Fortin added.
Gamma-rays detected by Fermi's LAT show that the remnant of Tycho's supernova shines in the highest-energy form of light. This portrait of the shattered star includes gamma rays (magenta), X-rays (yellow, green, and blue), infrared (red) and optical data. Image Credit: Gamma ray, NASA/DOE/Fermi LAT Collaboration; X-ray, NASA/CXC/SAO; Infrared, NASA/JPL-Caltech; Optical, MPIA, Calar Alto, O. Krause et al. and DSS)
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By no means are we suggesting that NASA’s Fermi Gamma-Ray Space Telescope can induce altered states of awareness, but this ‘far-out’ image is akin to 1960’s era psychedelic art. However, the data depicted here provides a new and enlightened way of looking at an object that’s been observed for over 400 years. After years of study, data collected by Fermi has revealed Tycho’s Supernova Remnant shines brightly in high-energy gamma rays.
The discovery provides researchers with additional information on the origin of cosmic rays (subatomic particles that are on speed). The exact process that gives cosmic rays their energy isn’t well understood since charged particles are easily deflected by interstellar magnetic fields. The deflection by interstellar magnetic fields makes it impossible for researchers to track cosmic rays to their original sources.
“Fortunately, high-energy gamma rays are produced when cosmic rays strike interstellar gas and starlight. These gamma rays come to Fermi straight from their sources,” said Francesco Giordano at the University of Bari in Italy.
But here’s some not-so-psychedelic facts about supernova remnants in general and Tycho’s in particular:
When a massive star reaches the end of its lifetime, it can explode, leaving behind a supernova remnant consisting of an expanding shell of hot gas propelled by the blast shockwave. In many cases, a supernova explosion can be visible on Earth – even in broad daylight. In November of 1572, a new “star” was discovered in the constellation Cassiopeia. The discovery is now known to be the most visible supernova in the past 400 years. Often called “Tycho’s supernova”, the remnant shown above is named after Danish astronomer Tycho Brahe, who spent a great deal of time studying the supernova.
Tycho's map shows the supernova's position (largest symbol, at top) relative to the stars that form Cassiopeia. Image credit: University of TorontoThe 1572 supernova event occurred when the night sky was considered to be a fixed and unchanging part of the universe. Tycho’s account of the discovery gives a sense of just how profound his discovery was. Regarding his discovery, Tycho stated, “When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes, and so, turning to the servants who were accompanying me, I asked them whether they too could see a certain extremely bright star…. They immediately replied with one voice that they saw it completely and that it was extremely bright”
In 1949, physicist Enrico Fermi (the namesake for the Fermi Gamma-ray Space Telescope) theorized that high-energy cosmic rays were accelerated in the magnetic fields of interstellar gas clouds. Following up on Fermi’s work, astronomers learned that supernova remnants might be the best candidate sites for magnetic fields of such magnitude.
One of the main goals of the Fermi Gamma-ray Space Telescope is to better understand the origins of cosmic rays. Fermi’s Large Area Telescope (LAT) can survey the entire sky every three hours, which allows the instrument to build a deeper view of the gamma-ray sky. Since gamma rays are the most energetic form of light, studying gamma ray concentrations can help researchers detect the particle acceleration responsible for cosmic rays.
Co-author Stefan Funk (Kavli Institute for Particle Astrophysics and Cosmology) adds, “This detection gives us another piece of evidence supporting the notion that supernova remnants can accelerate cosmic rays.”
After scanning the sky for nearly three years, Fermi’s LAT data showed a region of gamma-ray emissions associated with the remnant of Tycho’s supernova. Keith Bechtol, (KIPAC graduate student) commented on the discovery, saying, “We knew that Tycho’s supernova remnant could be an important find for Fermi because this object has been so extensively studied in other parts of the electromagnetic spectrum. We thought it might be one of our best opportunities to identify a spectral signature indicating the presence of cosmic-ray protons”
The team’s model is based on LAT data, gamma-rays mapped by ground-based observatories and X-ray data. The conclusion the team has come to regarding their model is that a process called pion production is the best explanation for the emissions. The animation below depicts a proton moving at nearly the speed of light and striking a slower-moving proton. The protons survive the collision, but their interaction creates an unstable particle — a pion — with only 14 percent of the proton’s mass. In 10 millionths of a billionth of a second, the pion decays into a pair of gamma-ray photons.
If the team’s interpretation of the data is accurate, then within the remnant, protons are being accelerated to near the speed of light. After being accelerated to such tremendous speeds, the protons interact with slower particles and produce gamma rays. With all the amazing processes at work in the remnant of Tycho’s supernova, one could easily imagine how impressed Brahe would be.
A neutron star's outer atmosphere engulfs another star in this concept rendering. (NASA/GSFC)
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On December 25, 2010, at 1:38 p.m. EST, NASA’s Swift Burst Alert Telescope detected a particularly long-lived gamma-ray burst in the constellation Andromeda. Lasting nearly half an hour, the burst (known as GRB 101225A) originated from an unknown distance, leaving astronomers to puzzle over exactly what may have created such a dazzling holiday display.
Now there’s not just one but two theories as to what caused this burst, both reported in papers by a research team from the Institute of Astrophysics in Granada, Spain. The papers will appear in the Dec. 1 issue of Nature.
Gamma-ray bursts are the Universe’s most luminous explosions. Most occur when a massive star runs out of nuclear fuel. As the star’s core collapses, it creates a black hole or neutron star that sends intense jets of gas and radiation outwards. As the jets shoot into space they strike gas previously shed by the star and heat it, generating bright afterglows.
NASA's Swift observatory is a satellite in low-Earth orbit, scanning the sky for the presence of gamma-ray bursts and gravitational wave forces. (NASA)
If a GRB jet happens to be aimed towards Earth it can be detected by instruments like those aboard the Swift spacecraft.
Luckily GRBs usually come from vast distances, as they are extremely powerful and could potentially pose a danger to life on Earth should one strike directly from close enough range. Fortunately for us the odds of that happening are extremely slim… but not nonexistent. That is one reason why GRBs are of such interest to astronomers… gazing out into the Universe is, in one way, like looking down the barrels of an unknown number of distant guns.
The 2010 “Christmas burst”, as the event also called, is suspected to feature a neutron star as a key player. The incredibly dense cores that are left over after a massive star’s death, neutron stars rotate extremely rapidly and have intense magnetic fields.
One of the new theories envisions a neutron star as part of a binary system that also includes an expanding red giant. The neutron star may have potentially been engulfed by the outer atmosphere of its partner. The gravity of the neutron star would have caused it to acquire more mass and thus more momentum, making it spin faster while energizing its magnetic field. The stronger field would have then fired off some of the stellar material into space as polar jets… jets that then interacted with previously-expelled gases, creating the GRB detected by Swift.
This scenario puts the source of the Christmas burst at around 5.5 billion light-years away, which coincides with the observed location of a faint galaxy.
An alternate theory, also accepted by the research team, involves the collision of a comet-like object and a neutron star located within our own galaxy, about 10,000 light-years away. The comet-like body could have been something akin to a Kuiper Belt Object which, if in a distant orbit around a neutron star, may have survived the initial supernova blast only to end up on a spiraling path inwards.
The object, estimated to be about half the size of the asteroid Ceres, would have broken up due to tidal forces as it neared the neutron star. Debris that impacted the star would have created gamma-ray emission detectable by Swift, with later-arriving material extending the duration of the GRB into the X-ray spectrum… also coinciding with Swift’s measurements.
Both of these scenarios are in line with processes now accepted by researchers as plausible explanations for GRBs thanks to the wealth of data provided by the Swift telescope, launched in 2004.
“The beauty of the Christmas burst is that we must invoke two exotic scenarios to explain it, but such rare oddballs will help us advance the field,” said Chryssa Kouveliotou, a co-author of the study at NASA’s Marshall Space Flight Center in Huntsville, Alabama.
More observations using other instruments, such as the Hubble Space Telescope, will be needed to discern which of the two theories is most likely the case… or perhaps rule out both, which would mean something else entirely is the source of the 2010 Christmas burst!
Cygnus X hosts many young stellar groupings, including the OB2 and OB9 associations and the cluster NGC 6910. The combined outflows and ultraviolet radiation from the region's numerous massive stars have heated and pushed gas away from the clusters, producing cavities of hot, lower-density gas. In this 8-micron infrared image, ridges of denser gas mark the boundaries of the cavities. Bright spots within these ridges show where stars are forming today. Credit: NASA/IPAC/MSX
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Situated about 4,500 light-years away in the constellation of Cygnus is a veritable star factory called Cygnus X… one estimated to have enough “raw materials” to create as many as two million suns. Caught in the womb are stellar clusters and OB associations. Of particular interest is one labeled Cygnus OB2 which is home to 65 of the hottest, largest and meanest O-type stars known – and close to 500 B members. The O boys blast out holes in the dust clouds in intense outflows, disrupting cosmic rays. Now, a study using data from NASA’s Fermi Gamma-ray Space Telescope is showing us this disturbance can be traced back to its source.
Discovered some 60 years ago in radio frequencies, the Cygnus X region has long been of interest, but dust-veiled at optical wavelengths. By employing NASA’s Fermi Gamma-ray Space Telescope, scientists are now able to peer behind the obscuration and take a look at the heart through gamma ray observations. In regions of star formation like Cygnus X, subatomic particles are produced and these cosmic rays shoot across our galaxy at light speed. When they collide with interstellar gas, they scatter – making it impossible to trace them to their point of origin. However, this same collision produces a gamma ray source… one that can be detected and pinpointed.
“The galaxy’s best candidate sites for cosmic-ray acceleration are the rapidly expanding shells of ionized gas and magnetic field associated with supernova explosions.” says the FERMI team. “For stars, mass is destiny, and the most massive ones — known as types O and B — live fast and die young.”
Because these star types aren’t very common, regions like Cygnus X become important star laboratories. Its intense outflows and huge amount of mass fills the prescription for study. Within its hollowed-out walls, stars reside in layers of thin, hot gas enveloped in ribbons of cool, dense gas. It is this specific area in which Fermi’s LAT instrumentation excels – detecting an incredible amount of gamma rays.
“We are seeing young cosmic rays, with energies comparable to those produced by the most powerful particle accelerators on Earth. They have just started their galactic voyage, zig-zagging away from their accelerator and producing gamma rays when striking gas or starlight in the cavities,” said co-author Luigi Tibaldo, a physicist at Padova University and the Italian National Institute of Nuclear Physics.
Clocked at up to 100 billion electron volts by the LAT, these highly accelerated particles are revealing the extreme origin of gamma-ray emission. For example, visible light is only two to three electron volts! But why is Cygnus X so special? It entangles its sources in complex magnetic fields and keeps the majority of them from escaping. All thanks to those high mass stars…
“These shockwaves stir the gas and twist and tangle the magnetic field in a cosmic-scale jacuzzi so the young cosmic rays, freshly ejected from their accelerators, remain trapped in this turmoil until they can leak into quieter interstellar regions, where they can stream more freely,” said co-author Isabelle Grenier, an astrophysicist at Paris Diderot University and the Atomic Energy Commission in Saclay, France.
However, there’s more to the story. The Gamma Cygni supernova remnant is also nearby and may impact the findings as well. At this point, the Fermi team considers it may have created the initial “cocoon” which holds the cosmic rays in place, but they also concede the accelerated particles may have originated through multiple interactions with stellar winds.
“Whether the particles further gain or lose energy inside this cocoon needs to be investigated, but its existence shows that cosmic-ray history is much more eventful than a random walk away from their sources,” Tibaldo added.
This cluster is 27,000 light-years away and lies farther than the center of our galaxy in the constellation Sagittarius. Credit: NASA/ESA/I. King, Univ. of Calif., Berkeley/Wikisky.org
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Astronomers using the Fermi Gamma-ray Space Telescope have found a surprisingly young, powerful and luminous millisecond pulsar. Over the past three years, Fermi has detected more than 100 gamma-ray pulsars and typically the ages of these objects are at least a billion year old. But this new object is just a youngster, born only about 25 million years ago.
“It is a bit like finding Justin Bieber when you thought you were at a Rolling Stones concert,” said Victoria Kaspi, physics professor, McGill University in Montreal, during a teleconference about two new discoveries made with the Fermi telescope. “Fermi has represented a huge leap forward in finding things that couldn’t have been imagined 25 years ago.”
In addition to the very young and bright pulsar, researchers announced they have also discovered a set of nine previously unknown gamma-ray pulsars, a new type that have extremely low luminosity. These were uncovered with a new technique to more efficiently sift through Fermi data.
The young millisecond pulsar, named PSR J1823?3021A was found within the globular cluster NGC 6624, not far from the center of our galaxy. Fermi has detected pulsars in globular clusters before, but usually what it finds are the combined gamma rays from many ancient pulsars within the clusters. But this time, surprisingly, the gamma rays originated from just one very powerful millisecond pulsar.
“At first we thought it was perhaps one hundred millisecond pulsars, but now we see it is just one,” said Paulo Freire, from the Max Planck Institute for Radio Astronomy in Bonn, Germany, also speaking to reporters during the teleconference. Freire is the lead author on a new paper published in the Astrophysical Journal. “It must have formed recently based on how rapidly it’s emitting energy. It’s a bit like finding a screaming baby in a quiet retirement home. This was a rather surprising discovery for everyone involved.”
A pulsar is a type of neutron star that emits electromagnetic energy at periodic intervals, sending out signals almost like a lighthouse. Pulsars that combine incredible density with extreme rotation are called millisecond pulsars. These millisecond pulsars are especially fascinating, as they are city-sized spheres about half millions times Earth’s mass, spinning at up to 43,000 revolutions per minute.
Millisecond pulsars are thought to achieve such speeds because they are gravitationally bound in binary systems with normal stars. During part of their stellar lives, gas flows from the normal star to the pulsar. Over time, the impact of this falling gas gradually spins up the pulsar’s rotation.
This plot shows the positions of nine new pulsars (magenta) discovered by Fermi and of an unusual millisecond pulsar (green) that Fermi data reveal to be the youngest such object known. With this new batch of discoveries, Fermi has detected more than 100 pulsars in gamma rays. Credit: AEI and NASA/DOE/Fermi LAT Collaboration
The nine new low luminosity pulsars found with Fermi emit less gamma radiation than those previously known and rotate only between three and twelve times per second. Only one of these pulsars was later also found to emit radio waves. Without the new technique, astronomers wouldn’t have found this faint pulsars.
““We used a new kind of hierarchical algorithm which we had originally developed for the search for gravitational waves, and we were quickly rewarded,” said Bruce Allen, director of the Max Planck Institute for Gravitational Physics, a co-author on the recent discoveries.
Using what is called a blind search, computers check many different combinations of position and rotational behavior, to see if they match the arrival times of photons hitting the Fermi Large Area Telescope (LAT) coming from the same direction. The search used the 8,000 photons deemed most probable to come from a pulsar at the recognized position, which Fermi’s LAT had collected during its three years in orbit. When the photon arrival times match up with the putative pulsar position and rotation model, a regular pattern of peaks appears in the gamma-ray photon counts, as a function of the rotational position of the pulsar, and a new gamma-ray pulsar has been discovered.
“It is a little like sifting through a pile of sand looking for diamonds,” Allen said, adding that the search is ongoing and they hope to find more.
Additionally, Allen said, users of the Einstein@Home project can now be part of this search, to help specifically to search for the first pure gamma-ray millisecond-pulsar. Allen is the director of this project and said this discovery would be a significant contribution to our understanding of pulsars.
Artist’s impression of a gamma-ray burst shining through two young galaxies in the early Universe. Credit: ESO
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“Shot through the heart and you’re to blame…” There’s nothing more powerful than a gamma-ray burst. These abrupt, mega-bright events are captured by orbiting telescopes where the information is immediately relayed to the ground for observation in visible light and infra-red. Some events are so powerful that they linger for hours or even days. But just how quick can we spot them? A burst cataloged as GRB 090323 was picked up by the NASA Fermi Gamma-ray Space Telescope, then confirmed by the X-ray detector on NASA’s Swift satellite and with the GROND system at the MPG/ESO 2.2-metre telescope in Chile. Within a day it was being studied by ESO’s Very Large Telescope. It was so intense it penetrated its host galaxy and another… heading out on a 12 billion light year journey just to get here.
“When we studied the light from this gamma-ray burst we didn’t know what we might find. It was a surprise that the cool gas in these two galaxies in the early Universe proved to have such an unexpected chemical make-up,” explains Sandra Savaglio (Max-Planck Institute for Extraterrestrial Physics, Garching, Germany), lead author of the paper describing the new results. “These galaxies have more heavy elements than have ever been seen in a galaxy so early in the evolution of the Universe. We didn’t expect the Universe to be so mature, so chemically evolved, so early on.”
As the brilliant beacon passed through the galaxies, the gases performed as a filter, absorbing some wavelengths of light. But the real kicker here is we wouldn’t have even known these galaxies existed if it weren’t for the gamma-ray burst! Because the light was affected, astronomers were able to detect the “composition of the cool gas in these very distant galaxies, and in particular how rich they were in heavy elements.” It had been surmised that early galaxies would have less heavy elements since their stellar populations weren’t old enough to have produced them… But the findings pointed otherwise. These new galaxies were rich in heavy elements and going against what we thought we knew about galactic evolution.
So exactly what does that mean? It would appear these new, young galaxies are forming stars at an incredible rate. To enrich their gases so quickly, it’s possible they are in a merger process. While this isn’t a new concept, it just may support the theory that gamma-ray bursts can be associated with “vigorous massive star formation”. Furthermore, it’s surmised that rapid stellar growth may have simply stopped in the primordial Universe. What’s left that we can observe some 12 billion years later are mere shadows of what once was… like cool dwarf stars and black holes. These two newly discovered galaxies are like finding a hidden stain on the outskirts of the distant Cosmos.
“We were very lucky to observe GRB 090323 when it was still sufficiently bright, so that it was possible to obtain spectacularly detailed observations with the VLT. Gamma-ray bursts only stay bright for a very short time and getting good quality data is very hard. We hope to observe these galaxies again in the future when we have much more sensitive instruments, they would make perfect targets for the E-ELT,” concludes Savaglio.
