Gamma-ray pulsars in the Milky Way's plane, found by volunteers using Einstein@Home. The sky map is from Fermi's Large Area Telescope. The brighter the color you see, the more intense the radiation in that spot. The small flags show the nationality of the volunteers whose computers spotted the pulsars. Credit: Knispel/Pletsch/AEI/NASA/DOE/Fermi LAT Collaboration
Imagine that you’re innocently running your computer in pursuit of helping data crunch a huge science project. Then, out of the thousands of machines running the project, yours happens to stumble across a discovery. That’s what happened to several volunteers with Einstein@Home, which seeks pulsars in data from the Fermi Gamma-Ray Space Telescope, among other projects.
“At first I was a bit dumbfounded and thought someone was playing a hoax on me. But after I did some research,” everything checked out. That someone as insignificant as myself could make a difference was amazing,” stated Kentucky resident Thomas M. Jackson, who contributed to the project.
Pulsars, a type of neutron star, are the leftovers of stars that exploded as supernovae. They rotate rapidly, with such precision in their rotation periods that they have sometimes been likened to celestial clocks. Although the discovery is exciting to the eight volunteers because they are the first to find these gamma-ray pulsars as part of a volunteer computing project, the pulsars also have some interesting scientific features.
Artist’s illustration of a neutron star, a tiny remnant that remains after its predecessor star explodes. Here, the 12-mile (20-kilometer) sphere is compared with the size of Hannover, Germany. Credit: NASA’s Goddard Space Flight Center
The four pulsars were discovered in the plane of the Milky Way in an area that radio telescopes had looked at previously, but weren’t able to find themselves. This means that the pulsars are likely only visible in gamma rays, at least from the vantage point of Earth; the objects emit their radiation in a narrow direction with radio, but a wider stripe with gamma rays. (After the discoveries, astronomers used the Max Planck Institute for Radio Astronomy’s 100-meter Effelsberg radio telescope and the Australian Parkes Observatory to peer at those spots in the sky, and still saw no radio signals.)
Two of the pulsars also “hiccup” or exhibit a pulsar glitch, when the rotation sped up and then fell back to the usual rotation period a few weeks later. Astronomers are still learning more about these glitches, but they do know that most of them happen in young pulsars. All four pulsars are likely between 30,000 and 60,000 years old.
Artist’s conception of a gamma-ray pulsar. Gamma rays are shown in purple, and radio radiation in green. Credit: NASA/Fermi/Cruz de Wilde
“The first-time discovery of gamma-ray pulsars by Einstein@Home is a milestone – not only for us but also for our project volunteers. It shows that everyone with a computer can contribute to cutting-edge science and make astronomical discoveries,” stated co-author Bruce Allen, principal investigator of Einstein@Home. “I’m hoping that our enthusiasm will inspire more people to help us with making further discoveries.”
Einstein@Home is run jointly by the Center for Gravitation and Cosmology at the University of Wisconsin–Milwaukee and the Albert Einstein Institute in Hannover, Germany. It is funded by the National Science Foundation and the Max Planck Society. As for the volunteers, their names were mentioned in the scientific literature and they also received certificates of discovery for their work.
They are what is known as the “lighthouses” of the universe – rotating neutron stars that emit a focused beam of electromagnetic radiation that is only visible if you’re standing in it’s path. Known as pulsars, these stellar relics get their name because of the way their emissions appear to be “pulsating” out into space.
Not only are these ancient stellar objects very fascinating and awesome to behold, they are very useful to astronomers as well. This is due to the fact that they have regular rotational periods, which produces a very precise internal in its pulses – ranging from milliseconds to seconds.
Description:
Pulsars are types of neutron stars; the dead relics of massive stars. What sets pulsars apart from regular neutron stars is that they’re highly magnetized, and rotating at enormous speeds. Astronomers detect them by the radio pulses they emit at regular intervals.
An artist’s impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry
Formation:
The formation of a pulsar is very similar to the creation of a neutron star. When a massive star with 4 to 8 times the mass of our Sun dies, it detonates as a supernova. The outer layers are blasted off into space, and the inner core contracts down with its gravity. The gravitational pressure is so strong that it overcomes the bonds that keep atoms apart.
Electrons and protons are crushed together by gravity to form neutrons. The gravity on the surface of a neutron star is about 2 x 1011 the force of gravity on Earth. So, the most massive stars detonate as supernovae, and can explode or collapse into black holes. If they’re less massive, like our Sun, they blast away their outer layers and then slowly cool down as white dwarfs.
But for stars between 1.4 and 3.2 times the mass of the Sun, they may still become supernovae, but they just don’t have enough mass to make a black hole. These medium mass objects end their lives as neutron stars, and some of these can become pulsars or magnetars. When these stars collapse, they maintain their angular momentum.
But with a much smaller size, their rotational speed increases dramatically, spinning many times a second. This relatively tiny, super dense object, emits a powerful blast of radiation along its magnetic field lines, although this beam of radiation doesn’t necessarily line up with it’s axis of rotation. So, pulsars are simply rotating neutron stars.
And so, from here on Earth, when astronomers detect an intense beam of radio emissions several times a second, as it rotates around like a lighthouse beam – this is a pulsar.
History:
The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewis, and it surprised the scientific community by the regular radio emissions it transmitted. They detected a mysterious radio emission coming from a fixed point in the sky that peaked every 1.33 seconds. These emissions were so regular that some astronomers thought it might be evidence of communications from an intelligent civilization.
Although Burnell and Hewis were certain it had a natural origin, they named it LGM-1, which stands for “little green men”, and subsequent discoveries have helped astronomers discover the true nature of these strange objects.
Astronomers theorized that they were rapidly rotating neutron stars, and this was further supported by the discovery of a pulsar with a very short period (33-millisecond) in the Crab nebula. There have been a total of 1600 found so far, and the fastest discovered emits 716 pulses a second.
Later on, pulsars were found in binary systems, which helped to confirm Einstein’s theory of general relativity. And in 1982, a pulsar was found with a rotation period of just 1.6 microseconds. In fact, the first extrasolar planets ever discovered were found orbiting a pulsar – of course, it wouldn’t be a very habitable place.
Interesting Facts:
When a pulsar first forms, it has the most energy and fastest rotational speed. As it releases electromagnetic power through its beams, it gradually slows down. Within 10 to 100 million years, it slows to the point that its beams shut off and the pulsar becomes quiet.
When they are active, they spin with such uncanny regularity that they’re used as timers by astronomers. In fact, it is said that certain types of pulsars rival atomic clocks in their accuracy in keeping time.
Pulsars also help us search for gravitational waves, probe the interstellar medium, and even find extrasolar planets in orbit. In fact, the first extrasolar planets were discovered around a pulsar in 1992, when astronomers Aleksander Wolszczan and Dale Frail announced the discovery of a multi-planet planetary system around PSR B1257+12 – a millisecond pulsar now known to have two extrasolar planets.
Artist’s impression of the planets orbiting PSR B1257+12. Credit: NASA/JPL-Caltech/R. Hurt (SSC)
It has even been proposed that spacecraft could use them as beacons to help navigate around the Solar System. On NASA’s Voyager spacecraft, there are maps that show the direction of the Sun to 14 pulsars in our region. If aliens wanted to find our home planet, they couldn’t ask for a more accurate map.
Artist's rendition of TESS in space. (Credit: MIT Kavli Institute for Astrophysics Research).
Move over Kepler. NASA has recently green-lighted two new missions as part of its Astrophysics Explorer Program.
These come as the result of four proposals submitted in 2012. The most anticipated and high profile mission is TESS, the Transiting Exoplanet Survey Satellite.
Slated for launch in 2017, TESS will search for exoplanets via the transit method, looking for faint tell-tale dips in brightness as the unseen planet passes in front of its host star. This is the same method currently employed by Kepler, launched in 2009. Unlike Kepler, which stares continuously at a single segment of the sky along the galactic plane in the direction of the constellations Cygnus, Hercules, and Lyra, TESS will be the first dedicated all-sky exoplanet hunting satellite.
The mission will be a partnership of the Space Telescope Science Institute, the MIT Lincoln Laboratory, the NASA Goddard Spaceflight Center, Orbital Sciences Corporation, the Harvard-Smithsonian Center for Astrophysics and the MIT Kavli Institute for Astrophysics and Space Research (MKI).
TESS will launch onboard an Orbital Sciences Pegasus XL rocket released from the fuselage of a Lockheed L-1011 aircraft, the same system that deployed IBEX in 2008 & NuSTAR in 2012. NASA’s Interface Region Imaging Spectrograph (IRIS) will also launch using a Pegasus XL rocket this summer in June.
An Orbital Sciences Pegasus XL rocket attached to the fuselage of an L1011 for the launch of IBEX. (Credit: NASA).
“TESS will carry out the first space-borne all-sky transit survey, covering 400 times as much sky as any previous mission. It will identify thousands of new planets in the solar neighborhood, with a special focus on planets comparable in size to the Earth,” said George Riker, a senior researcher from MKI.
TESS will utilize four wide angle telescopes to get the job done. The effective size of the detectors onboard is 192 megapixels. TESS is slated for a two year mission. Unlike Kepler, which sits in an Earth-trailing heliocentric orbit, TESS will be in an elliptical path in Low Earth Orbit (LEO).
TESS will examine approximately 2 million stars brighter than 12th magnitude including 1,000 of the nearest red dwarfs. Not only will TESS expand the growing catalog of exoplanets, but it is also expected to find planets with longer orbital periods.
One dilemma with the transit method is that it favors the discovery of planets with short orbital periods, which are much more likely to be seen transiting their host star from a given vantage point in space.
TESS will also serve as a logical progression from Kepler to later proposed exoplanet search platforms. TESS will also discover candidates for further scrutiny by as the James Webb Space Telescope to be launched in 2018 and the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrometer based at La Silla Observatory in Chile.
Artist’s conception of NICER on the exterior of the International Space Station. (Credit: NASA).
Also on the board for launch in 2017 is NICER, the Neutron Star Interior Composition Explorer to be placed on the exterior of the International Space Station. NICER will employ an array 56 telescopes which will collect and study X-rays from neutron stars. NICER will specialize in the study of a particular sub-class of neutron star known as millisecond pulsars. The X-ray telescopes are in a configuration utilizing a set of nested glass shells looking like the layers of an onion.
Observing pulsars in the X-ray range of the spectrum will offer scientists tremendous insight into their inner workings and structure. The International Space Station offers a unique vantage point to do this sort of science. Like the Alpha Magnetic Spectrometer (AMS-02), the power requirements of NICER dictate that it cannot be a free-flying satellite. X-Ray astronomy must also be done above the hindering effects of the Earth’s atmosphere.
NICER will be deployed as an exterior payload aboard an ISS ExPRESS Logistics Carrier. These are unpressurized platforms used for experiments that must be directly exposed to space.
Another fascinating project working in tandem with NICER is SEXTANT, the Station Explorer for X-ray Timing And Navigation Technology. This project seeks to test the precision of millisecond pulsars for interplanetary navigation.
“They (pulsars) are extremely reliable celestial clocks and can provide high-precision timing just like the atomic signals supplied through the 26-satellite military operated Global Positioning System (GPS),” said NASA Goddard scientist Zaven Arzoumanian. The chief difficulty with relying on this system for interplanetary journeys is that the signal gets progressively weaker the farther you travel from the Earth.
“Pulsars, on the other hand, are accessible in virtually every conceivable flight regime, from LEO to interplanetary and deepest space,” said NICER/SEXTANT principle investigator Keith Gendreau.
Both NICER and TESS follow the long legacy of NASA’s Astrophysics Explorer Program, which can be traced all the way back to the launch Explorer 1. This was the very first U.S. satellite launched in 1958. Explorer 1 discovered the Van Allen radiation belts surrounding the Earth.
(From left) William Pickering, James Van Allen, and Wernher von Braun hold aloft a mock up of Explorer 1 shortly after launch. (Credit NASA/JPL-Caltech).
“The Explorer Program has a long and stellar history of deploying truly innovative missions to study some of the most exciting questions in space science,” stated NASA associate administrator for science John Grunsfeld. “With these missions, we will learn about the most extreme states of matter by studying neutron stars and we will identify many nearby star systems with rocky planets in the habitable zones for further study by telescopes such as the James Webb Space Telescope.”
Of course, Grunsfeld is referring to planets orbiting red dwarf stars, which will be targeted by TESS. These are expected have a habitable zone much closer to their primary star than our own Sun. It has even been suggested by MIT scientists that the first exoplanets visited by humans on some far off date might be initially discovered by TESS. The spacecraft may also discover future targets for follow up spectroscopic analysis, the best chance of discovering alien life on an exoplanet in the next 50 years. One can imagine the excitement that a positive detection of a chemical exclusive to life as we know it such as chlorophyll in the spectra of a far of world would generate. More ominously, detection of such synthetic elements as plutonium in the atmosphere of an exoplanet might suggest we found them… but alas, too late.
But on a happier note, it’ll be exciting times for space exploration to see both projects get underway. Perhaps human explorers will indeed one day visit the worlds discovered by TESS… and use navigation techniques pioneered by SEXTANT to do it!
Space Shuttle Atlantis passes behind the Parkes radio telescope after final undocking from the International Space Station in July 2011. (Image Copyright: John Sarkissian; used with permission).
Chalk another one up for Citizen Science. Earlier this month, researchers announced the discovery of 24 new pulsars. To date, thousands of pulsars have been discovered, but what’s truly fascinating about this month’s discovery is that came from culling through old data using a new method.
A pulsar is a dense, highly magnetized, swiftly rotating remnant of a supernova explosion. Pulsars where first discovered by Jocelyn Bell Burnell and Antony Hewish in 1967. The discovery of a precisely timed radio beacon initially suggested to some that they were the product of an artificial intelligence. In fact, for a very brief time, pulsars were known as LGM’s, for “Little Green Men.” Today, we know that pulsars are the product of the natural death of massive stars.
The data set used for the discovery comes from the Parkes 64-metre radio observatory based out of New South Wales, Australia. The installation was the first to receive telemetry from the Apollo 11 astronauts on the Moon and was made famous in the movie The Dish. The Parkes Multi-Beam Pulsar Survey (PMPS) was conducted in the late 1990’s, making thousands of 35-minute recordings across the plane of the Milky Way galaxy. This survey turned up over 800 pulsars and generated 4 terabytes of data. (Just think of how large 4 terabytes was in the 90’s!)
Artist’s conception of a pulsar. (Credit: NASA/GSFC).
The nature of these discoveries presented theoretical astrophysicists with a dilemma. Namely, the number of short period and binary pulsars was lower than expected. Clearly, there were more pulsars in the data waiting to be found.
Enter Citizen Science. Using a program known as Einstein@Home, researchers were able to sift though the recordings using innovative modeling techniques to tease out 24 new pulsars from the data.
“The method… is only possible with the computing resources provided by Einstein@Home” Benjamin Knispel of the Max Planck Institute for Gravitational Physics told the MIT Technology Review in a recent interview. The study utilized over 17,000 CPU core years to complete.
Einstein@Home is a program uniquely adapted to accomplish this feat. Begun in 2005, Einstein@Home is a distributed computing project which utilizes computing power while machines are idling to search through downloaded data packets. Similar to the original distributed computing program SETI@Home which searches for extraterrestrial signals, Einstein@Home culls through data from the LIGO (Laser Interferometer Gravitational Wave Observatory) looking for gravity waves. In 2009, the Einstein@Home survey was expanded to include radio astronomy data from the Arecibo radio telescope and later the Parkes observatory.
Among the discoveries were some rare finds. For example, PSR J1748-3009 Has the highest known dispersion measure of any millisecond pulsar (The dispersion measure is the density of free electrons observed moving towards the viewer). Another find, J1750-2531 is thought to belong to a class of intermediate-mass binary pulsars. 6 of the 24 pulsars discovered were part of binary systems.
These discoveries also have implications for the ongoing hunt for gravity waves by such projects as LIGO. Specifically, a through census of binary pulsars in the galaxy will give scientists a model for the predicted rate of binary pulsar mergers. Unlike radio surveys, LIGO seeks to detect these events via the copious amount of gravity waves such mergers should generate. Begun in 2002, LIGO consists of two gravity wave observatories, one in Hanford Washington and one in Livingston Louisiana just outside of Baton Rouge. Each LIGO detector consists of two 2 kilometre Fabry-Pérot arms in an “L” configuration which allow for ultra-precise measurements of a 200 watt laser beam shot through them. Two detectors are required to pin-point the direction of an incoming gravity wave on the celestial sphere. You can see the orientation of the “L’s” on the display on the Einstein@Home screensaver. Two geographically separate detectors are also required to rule out local interference. A gravity wave from a galactic source would ripple straight through the Earth.
Such a movement would be tiny, on the order of 1/1,000th the diameter of a proton, unnoticed by all except the LIGO detectors. To date, LIGO has yet to detect gravity waves, although there have been some false alarms. Scientists regularly interject test signals into the data to see if system catches them. The lack of detection of gravity waves by LIGO has put some constraints on certain events. For example, LIGO reported a non-detection of gravity waves during the February 2007 short gamma-ray burst event GRB 070201. The event arrived from the direction of the Andromeda Galaxy, and thus was thought to have been relatively nearby in the universe. Such bursts are thought to be caused by neutron star and/or black holes mergers. The lack of detection by LIGO suggests a more distant event. LIGO should be able to detect a gravitational wave event out to 70 million light years, and Advanced LIGO (AdLIGO) is set to go online in 2014 and will increase its sensitivity tenfold.
The control room at LIGO Livingston. (Photo by Author).
Knowledge of where these potential pulsar mergers are by such discoveries as the Parkes radio survey will also give LIGO researchers clues of targets to focus on. “The search for pulsars isn’t easy, especially for these “quiet” ones that aren’t doing the equivalent of “screaming” for our attention,” Says LIGO Livingston Data Analysis and EPO Scientist Amber Stuver. The LIGO consortium developed the data analysis technique used by Einstein@Home. The direct detection of gravitational waves by LIGO or AdLIGO would be an announcement perhaps on par with CERN’s discovery of the Higgs Boson last year. This would also open up a whole new field of gravitational wave astronomy and perhaps give new stimulus to the European Space Agencies’ proposed Laser Interferometer Space Antenna (LISA) space-based gravity wave detector. Congrats to the team at Parkes on their discovery… perhaps we’ll have the first gravity wave detection announcement out of LIGO as well in years to come!
-Read the original paper on the discovery of 24 new pulsars here.
This image compresses the Vela movie sequence into a single snapshot by merging pie-slice sections from eight individual frames.
Credit: NASA/DOE/Fermi LAT Collaboration
I loved my Spirograph when I was young, and obviously Eric Charles, a physicist with the Fermi Gamma-ray Space Telescope team did too. Charles has taken data from Fermi’s Large Area Telescope and turned it into a mesmerizing movie of the Vela Pulsar. It actually is a reflection of the complex motion of the spacecraft as it stared at the pulsar.
The video shows the intricate pattern traced by the Fermi Gamma-ray Space Telescope’s view of the Vela Pulsar over the spacecraft’s 51 months in orbit.
Fermi orbits our planet every 95 minutes, building up increasingly deeper views of the universe with every circuit. Its wide-eyed Large Area Telescope (LAT) sweeps across the entire sky every three hours, capturing the highest-energy form of light — gamma rays — from sources across the universe. The Fermi telescope has given us our best view yet of the bizarre world of the high energy Universe, which include supermassive black holes billions of light-years away to intriguing objects in our own galaxy, such as X-ray binaries, supernova remnants and pulsars.
Francis Reddy from the Goddard Spaceflight Center describes the movie:
The Vela pulsar outlines a fascinating pattern in this movie showing 51 months of position and exposure data from Fermi’s Large Area Telescope (LAT). The pattern reflects numerous motions of the spacecraft, including its orbit around Earth, the precession of its orbital plane, the manner in which the LAT nods north and south on alternate orbits, and more. The movie renders Vela’s position in a fisheye perspective, where the middle of the pattern corresponds to the central and most sensitive portion of the LAT’s field of view. The edge of the pattern is 90 degrees away from the center and well beyond what scientists regard as the effective limit of the LAT’s vision. Better knowledge of how the LAT’s sensitivity changes across its field of view helps Fermi scientists better understand both the instrument and the data it returns.
The pulsar traces out a loopy, hypnotic pattern reminiscent of art produced by the colored pens and spinning gears of a Spirograph, a children’s toy that produces geometric patterns.
The Vela pulsar spins 11 times a second and is the brightest persistent source of gamma rays the LAT sees. While gamma-ray bursts and flares from distant black holes occasionally outshine the pulsar, the Vela pulsar is like a persistant beacon, much like the light from a lighthouse.
Find out more about this movie and the Fermi Telescope here.
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
An artist's impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry
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It’s a millisecond pulsar… a rapidly rotating neutron star and it’s about to reach the end of its mass gathering phase. For ages the vampire of this binary system has been sucking matter from a donor star. It has been busy, spinning at incredibly high rotational speeds of about 1 to 10 milliseconds and shooting off X-rays. Now, something is about to happen. It is going to lose a whole lot of energy and age very quickly.
Astrophysicist Thomas Tauris of Argelander-Institut für Astronomie and Max-Planck-Institut für Radioastronomie has published a paper in the February 3 issue of Science where he has shown through numerical equations the root of stellar evolution and accretion torques. In this model, millisecond pulsars are shown to dissipate approximately half of their rotational energy during the last phase of the mass-transfer process and just before it turns into a radio source. Dr. Tauris’ findings are consistent with current observations and his conclusions also explain why a radio millisecond pulsar appears age-advanced over their companion stars. This may be the answer as to why sub-millisecond pulsars don’t exist at all!
“Millisecond pulsars are old neutron stars that have been spun up to high rotational frequencies via accretion of mass from a binary companion star.” says Dr. Tauris. “An important issue for understanding the physics of the early spin evolution of millisecond pulsars is the impact of the expanding magnetosphere during the terminal stages of the mass-transfer process.”
By drawing mass and angular momentum from a host star in a binary system, a millisecond pulsar lives its life as a highly magnetized, old neutron star with an extreme rotational frequency. While we might assume they are common, there are only about 200 of these pulsar types known to exist in galactic disk and globular clusters. The first of these millisecond pulsars was discovered in 1982. What counts are those that have spin rates between 1.4 to 10 milliseconds, but the mystery lay in why they have such rapid spin rates, their strong magnetic fields and their strangely appearing ages. For example, when do they switch off? What happens to the spin rate when the donor star quits donating?
“We have now, for the first time, combined detailed numerical stellar evolution models with calculations of the braking torque acting on the spinning pulsar”, says Thomas Tauris, the author of the present study. “The result is that the millisecond pulsars lose about half of their rotational energy in the so-called Roche-lobe decoupling phase. This phase is describing the termination of the mass transfer in the binary system. Hence, radio-emitting millisecond pulsars should spin slightly slower than their progenitors, X-ray emitting millisecond pulsars which are still accreting material from their donor star. This is exactly what the observational data seem to suggest. Furthermore, these new findings can help explain why some millisecond pulsars appear to have characteristic ages exceeding the age of the Universe and perhaps why no sub-millisecond radio pulsars exist.”
Thanks to this new study we’re now able to see how a spinning pulsar could possibly brake out of an equilibrium spin. At this age, the mass-transfer rate slows down and affects the magnetospheric radius of the pulsar. This in turn expands and forces the incoming matter to act as a propeller. The action then causes the pulsar to slow down its rotation and – in turn – slow its spin rate.
“Actually, without a solution to the “turn-off” problem we would expect the pulsars to even slow down to spin periods of 50-100 milliseconds during the Roche-lobe decoupling phase”, concludes Thomas Tauris. “That would be in clear contradiction with observational evidence for the existence of millisecond pulsars.”
Using an array of millisecond pulsars, astronomers can detect tiny changes in the pulse arrival times in order to detect the influence of gravitational waves. Credit: NRAO
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A special project to search for pulsars has bagged the first student discovery of a millisecond pulsar – a super-fast spinning star, and this one rotates about 324 times per second. The Pulsar Search Collaboratory (PSC) has students analyzing real data from the National Radio Astronomy Observatory’s (NRAO) Robert C. Byrd Green Bank Telescope (GBT) to find pulsars. Astronomers involved with the project said the discovery could help detect elusive ripples in spacetime known as gravitational waves.
“Gravitational waves are ripples in the fabric of spacetime predicted by Einstein’s theory of General Relativity,” said Dr. Maura McLaughlin, from West Virginia University. “We have very good proof for their existence but, despite Einstein’s prediction back in the early 1900s, they have never been detected.”
Four other pulsars have been discovered by high school students participating in this project.
Pulsar hunters Sydney Dydiw of Trinity High School, Emily Phan of George C. Marshall High School, Anne Agee of Roanoke Valley Governor's School, and Jessica Pal of Rowan County High School. Not pictured: Max Sterling of Langley High School. Credit: NRAO
“When you discover a pulsar, you feel like you’re walking on air! It is the best experience you can ever have,” said student co-discoverer Jessica Pal of Rowan County High School in Kentucky. “You get to meet astronomers and talk to them about your experience. I still can’t believe I found a pulsar. It is wonderful to know that there is something out there in space that you discovered.”
The other student involved in the discovery was Emily Phan of George C. Marshall High School in Virginia, who along with Pal found the millisecond pulsar on January 17, 2012. It was later confirmed by Max Sterling of Langley High School, Sydney Dydiw of Trinity High School, and Anne Agee of Roanoke Valley Governor’s School, all in Virginia.
“I am considering pursuing astronomy as a career choice,” said Agee. “The Pulsar Search Collaboratory has opened my eyes to how fun astronomy can be!”
Once the pulsar candidate was reported to NRAO, a followup observing session was scheduled on the giant, 17-million-pound telescope. On January 24, 2012, observations confirmed that the pulsar was real.
Pulsars are spinning neutron stars that sling “lighthouse beams” of radio waves around as they rotate. A neutron star is what is left after a massive star explodes at the end of its “normal” life. With no nuclear fuel left to produce energy to offset the stellar remnant’s weight, its material is compressed to extreme densities. The pressure squeezes together most of its protons and electrons to form neutrons; hence, the name “neutron star.” One tablespoon of material from a pulsar would weigh 10 million tons.
On January 24, 2012, observations with the Green Bank Telescope at 800 MHz confirmed that the signal was astronomical and zeroed in on its position. Pulsars are brighter at lower frequencies (like 350 MHz, above) than at higher frequencies, and so the confirmation plot is noisier than the original data. Since this pulsar spins so fast, it may be used as part of the pulsar timing array used to detect gravitational waves. Courtesy NRAO.
The object that the students discovered is a special class of pulsars called millisecond pulsars, which are the fastest-spinning neutron stars. They are highly stable and keep time more accurately than atomic clocks.
Astronomers don’t know much about them, however. But because of their stability, these pulsars may someday allow astronomers to detect gravitational waves.
Millisecond pulsars, however, could hold the key to that discovery. Like buoys bobbing on the ocean, pulsars can be perturbed by gravitational waves.
“Gravitational waves are invisible,” said McLaughlin. “But by timing pulsars distributed across the sky, we may be able to detect very small changes in pulse arrival times due to the influence of these waves.”
Millisecond pulsars are generally older pulsars that have been “spun up” by stealing mass from companion stars, but much is left to discover about their formation.
“This latest discovery will help us understand the genesis of millisecond pulsars,” said Dr. Duncan Lorimer, who is also part of the project. “It’s a very exciting time to be finding pulsars!”
Robert C. Byrd Green Bank Telescope CREDIT: NRAO/AUI/NSF
The PSC is a joint project of the National Radio Astronomy Observatory and West Virginia University, funded by a grant from the National Science Foundation. The PSC includes training for teachers and student leaders, and provides parcels of data from the GBT to student teams. The project involves teachers and students in helping astronomers analyze data from the GBT.
Approximately 300 hours of the observing data were reserved for analysis by student teams. These students have been working with about 500 other students across the country. The responsibility for the work, and for the discoveries, is theirs. They are trained by astronomers and by their teachers to distinguish between pulsars and noise.
The PSC will continue through the 2012-2013 school year. Teachers interested in participating in the program can learn more at this link. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Today's journal article involves the mysterious case of PSR J1841-500, the pulsar that doesn't pulse
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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. Since this is Universe Today if we occasionally stray into critically evaluating each other’s critical evaluations, that’s OK too.
And of course, the first rule of Journal Club is… don’t talk about Journal Club. So, without further ado – today’s journal article involves the mysterious case of PSR J1841-500, the pulsar that didn’t pulse.
Pulsars are neutron stars – with polar jets. They rotate quite fast and when one of those polar jets line up with Earth we detect a pulse of radio light. This is a very regular and very predictable pulse – although when measured over long time periods, the pulses slowly decline in frequency as magnetic forces create drag that slows the neutron star’s spin.
But PSR J1841-500 is an oddball – when first discovered, it pulsed every 0.9 seconds. Then for reasons unknown, it stopped pulsing for a period of over 500 days – after which it just picked up where it left off. There are at least two other pulsars known to have demonstrated such ‘switch on – switch off’ behaviour, although neither had anything like the switch-off duration of PSR J1841-500.
It is known that now and again neutron stars experience a ‘glitch’, a kind of a starquake, as the intense gravity of the star crunches its internal structure down to an even more compact state. This can change its spin and hence its pulse rate.
However, it’s not clear if glitch phenomena would help explain the extended ‘switch-off’ phenomenon observed in PSR J1841-500. Looking for other anomalies, the authors point to the unusual proximity of the magnetar IE 1841-045, which might be having some (unknown) effect on its neighbor.
The authors then conclude with the interesting suggestion that if this behavior is common, then we may be missing lots of pulsars that were in a ‘switched-off’ state when their particular region of the sky was last surveyed. This means we actually need to undertake repeated surveys – which should hopefully keep more astronomers in a job.
So – comments? Are the authors just building an overblown story out of a measurement error? Is there some weird magnetic dance going on between two proximal neutron stars? Want to suggest an article for the next edition of Journal Club?
This is an artist's concept of a pulsar (blue-white disk in center) pulling in matter from a nearby star (red disk at upper right). The stellar material forms a disk around the pulsar (multicolored ring) before falling on to the surface at the magnetic poles. The pulsar's intense magnetic field is represented by faint blue outlines surrounding the pulsar. Credit: NASA
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Neutron stars have been classed as “undead”… real zombie stars. Even though technically defunct, the neutron star continues to shine – and occasionally feed on a neighbor if it gets too close. They are born when a massive star collapses under its gravity and its outer layers are blown far and wide, outshining a billion suns, in a supernova event. What’s left is a stellar corpse… a core of inconceivable density… where one teaspoon would weigh about a billion tons on Earth. How would we study such a curiosity? NASA has proposed a mission called the Neutron Star Interior Composition Explorer (NICER) that would detect the zombie and allow us to see into the dark heart of a neutron star.
The core of a neutron star is pretty incredible. Despite the fact that it has blown away most of its exterior and stopped nuclear fusion, it still radiates heat from the explosion and exudes a magnetic field which tips the scales. This intense form of radiation caused by core collapse measures out at over a trillion times stronger than Earth’s magnetic field. If you don’t think that impressive, then think of the size. Originally the star could have been a trillion miles or more in diameter, yet now is compressed to the size of an average city. That makes a neutron star a tiny dynamo – capable of condensing matter into itself at more than 1.4 times the content of the Sun, or at least 460,000 Earths.
“A neutron star is right at the threshold of matter as it can exist – if it gets any denser, it becomes a black hole,” says Dr. Zaven Arzoumanian of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We have no way of creating neutron star interiors on Earth, so what happens to matter under such incredible pressure is a mystery – there are many theories about how it behaves. The closest we come to simulating these conditions is in particle accelerators that smash atoms together at almost the speed of light. However, these collisions are not an exact substitute – they only last a split second, and they generate temperatures that are much higher than what’s inside neutron stars.”
If approved, the NICER mission will be launched by the summer of 2016 and attached robotically to the International Space Station. In September 2011, NASA selected NICER for study as a potential Explorer Mission of Opportunity. The mission will receive $250,000 to conduct an 11-month implementation concept study. Five Mission of Opportunity proposals were selected from 20 submissions. Following the detailed studies, NASA plans to select for development one or more of the five Mission of Opportunity proposals in February 2013.
This is an artist's concept of the NICER instrument on board the International Space Station. NICER is the cube in the foreground on the left. The circular objects protruding from the cube are telescopes that focus X-rays from the pulsar on to the detector. Credit: NASA
What will NICER do? First off, an array of 56 telescopes will gather X-ray information from a neutron stars magnetic poles and hotspots. It is from these areas that our zombie stars release X-rays, and as they rotate create a pulse of light – thereby the term “pulsar”. As the neutron star shrinks, it spins faster and the resultant intense gravity can pull in material from a closely orbiting star. Some of these pulsars spin so fast they can reach speeds of several hundred of rotations per second! What scientists are itching to understand is how matter behaves inside a neutron star and “pinning down the correct Equation Of State (EOS) that most accurately describes how matter responds to increasing pressure. Currently, there are many suggested EOSs, each proposing that matter can be compressed by different amounts inside neutron stars. Suppose you held two balls of the same size, but one was made of foam and the other was made of wood. You could squeeze the foam ball down to a smaller size than the wooden one. In the same way, an EOS that says matter is highly compressible will predict a smaller neutron star for a given mass than an EOS that says matter is less compressible.”
Now all NICER will need to do is help us to measure a pulsar’s mass. Once it is determined, we can get a correct EOS and unlock the mystery of how matter behaves under intense gravity. “The problem is that neutron stars are small, and much too far away to allow their sizes to be measured directly,” says NICER Principal Investigator Dr. Keith Gendreau of NASA Goddard. “However, NICER will be the first mission that has enough sensitivity and time-resolution to figure out a neutron star’s size indirectly. The key is to precisely measure how much the brightness of the X-rays changes as the neutron star rotates.”
So what else does our zombie star do that’s impressive? Because of their extreme gravity in such small volume, they distort space/time in accordance with Einstein’s theory of General Relativity. It is this space “warp” that allows astronomers to reveal the presence of a companion star. It also produces effects like an orbital shift called precession, allowing the pair to orbit around each other causing gravitational waves and producing measurable orbital energy. One of the goals of NICER is to detect these effects. The warp itself will allow the team to determine the neutron star’s size. How? Imagine pushing your finger into a stretchy material – then imagine pushing your whole hand against it. The smaller the neutron star, the more it will warp space and light.
Here light curves become very important. When a neutron star’s hotspots are aligned with our observations, the brightness increases as one rotates into view and dims as it rotates away. This results in a light curve with large waves. But, when space is distorted we’re allowed to view around the curve and see the second hotspot – resulting in a light curve with smoother, smaller waves. The team has models that produce “unique light curves for the various sizes predicted by different EOSs. By choosing the light curve that best matches the observed one, they will get the correct EOS and solve the riddle of matter on the edge of oblivion.”
