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  • richardmitnick 1:17 pm on October 27, 2017 Permalink | Reply
    Tags: , , , , , , , Pulsars   

    From astrobites: “Observing a Strange Pulsar in X-ray and Radio” 

    Astrobites bloc

    astrobites

    27 October 2017
    Joshua Kerrigan

    Title: Simultaneous Chandra and VLA Observations of the Transitional Millisecond Pulsar PSR J1023+0038: Anti-correlated X-ray and Radio Variability
    Authors: Slavko Bogdanov, Adam T. Deller, James C. A. Miller-Jones, et al.
    First Author’s Institution: Columbia University

    Status: Submitted to ApJ, open access

    What’s more interesting than a rapidly spinning neutron star that emits electromagnetic radiation parallel to its magnetic poles? One that doesn’t exactly behave as expected, of course. One such weirdly acting pulsar, PSR J1023+0038, is a transitional millisecond pulsar (tMSP) — which is fancy speak for a pulsar with a millisecond rotational period that switches between radio and X-ray emission on a several-year timescale. The fact that this pulsar emits in both X-ray and radio on these longer timescales isn’t what piques the interest of astronomers, however, in the case of the study in this astrobite.

    Weird Pulsar Behavior

    Pulsars can typically fall into one of the following categories: radio pulsars are powered by exchanging rotational energy from the spinning neutron star into emitting radiation. This means that their rotation slows and their pulse length increases. Meanwhile, X-ray pulsars are accretion powered, meaning they turn heated infalling matter into X-ray emission. What distinguishes PSR J1023+0038 from the background of pulsars that switch between accretion-powered X-ray and rotation-powered radio pulsars is that it has a simultaneous anti-correlated X-ray and radio emission. The authors looked at about 5 hours of overlapping and concurrent observations from the Chandra X-ray Observatory and the Very Large Array (VLA) to try and understand this weird relationship between the X-ray and radio emissions.

    NASA/Chandra Telescope

    NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

    This is very clearly shown in Fig. 1 where we can see a tiny sample of time of overlapping X-ray and radio flux measurements. The anti-correlation is quite strong, meaning that when the X-ray emissions are weakest, the radio emission is strongest.

    1
    Figure 1: Radio emissions (black) and x-ray emissions (blue) recorded by the VLA and Chandra respectively over time. This shows that when radio emissions drop off, X-ray emissions pick up.

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

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  • richardmitnick 5:13 pm on August 1, 2017 Permalink | Reply
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    From Goddard: “NASA Continues to Study Pulsars, 50 Years After Their Chance Discovery” 

    NASA Goddard Banner
    NASA Goddard Space Flight Center

    Aug. 1, 2017
    Clare Skelly
    clare.a.skelly@nasa.gov
    NASA’s Goddard Space Flight Center in Greenbelt, Md.

    A little bit of “scruff” in scientific data 50 years ago led to the discovery of pulsars – rapidly spinning dense stellar corpses that appear to pulse at Earth.

    Astronomer Jocelyn Bell made the chance discovery using a vast radio telescope in Cambridge, England. Although it was built to measure the random brightness flickers of a different category of celestial objects called quasars, the 4.5-acre telescope produced unexpected markings on Bell’s paper data recorder every 1.33730 seconds.

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    The 4.5 Acre Array. Reproduced with permission from 40 Years of Pulsars—Millisecond Pulsars, Magnetars, and More, edited by C. G. Bassa, Z. Wang, A. Gumming, and V. M. Kaspi. Copyright 2008, AIP Publishing LLC

    “The pulses were so regular, so much like a ticking clock, that Bell and her supervisor Anthony Hewish couldn’t believe it was a natural phenomenon,” said Zaven Arzoumanian of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Once they found a second, third and fourth they started to think differently.”

    The unusual stellar objects had been previously predicted but never observed. Today, scientists know of over 2,000 pulsars. These rotating “lighthouse” neutron stars begin their lives as stars between about seven and 20 times the mass of our sun. Some are found to spin hundreds of times per second, faster than the blades of a household blender, and they possess enormously strong magnetic fields.

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    Most known neutron stars are observed as pulsars, emitting narrow, sweeping beams of radiation. They squeeze up to two solar masses into a city-size volume, crushing matter to the highest possible stable densities. To explore these exotic states of matter, NICER measures X-ray emissions across the surfaces of neutron stars as they spin, ultimately confronting the predictions of nuclear physics theory.
    Credits: NASA’s Goddard Space Flight Center

    Technology advances in the past half-century allowed scientists to study these compact stellar objects from space using different wavelengths of light, especially those much more energetic than the radio waves received by the Cambridge telescope. Several current NASA missions continue to study these natural beacons.

    The Neutron star Interior Composition Explorer, or NICER, is the first NASA mission dedicated to studying pulsars.

    NASA NICER

    In a nod to the anniversary of Bell’s discovery, NICER observed the famous first pulsar, known today as PSR B1919+21.

    NICER launched to the International Space Station in early June and started science operations last month. Its X-ray observations – the part of the electromagnetic spectrum in which these stars radiate both from their million-degree solid surfaces and from their strong magnetic fields – will reveal how nature’s fundamental forces behave within the cores of these objects, an environment that doesn’t exist and can’t be reproduced anywhere else. “What’s inside a pulsar?” is one of many long-standing astrophysics questions about these ultra-dense, fast-spinning, powerfully magnetic objects.

    The “stuff” of pulsars is a collection of particles familiar to scientists from over a century of laboratory studies on Earth – neutrons, protons, electrons, and perhaps even their own constituents, called quarks. However, under such extreme conditions of pressure and density, their behavior and interactions aren’t well understood. New, precise measurements, especially of the sizes and masses of pulsars are needed to pin down theories.

    “Many nuclear-physics models have been developed to explain how the make-up of neutron stars, based on available data and the constraints they provide,” said Goddard’s Keith Gendreau, the principal investigator for NICER. “NICER’s sensitivity, X-ray energy resolution and time resolution will improve these by more precisely measuring their radii, to an order of magnitude improvement over the state of the art today.”

    3
    NICER is currently installed on the International Space Station. This turntable animation of the payload calls out the locations of NICER’s star tracker camera, electronics, space station attachment mechanism, 56 sunshields, pointing actuators and stow/deploy actuator.
    Credits: NASA’s Goddard Space Flight Center

    The mission will also pave the way for future space exploration by helping to develop a Global Positioning System-like capability for the galaxy. The embedded Station Explorer for X-ray Timing and Navigation Technology, or SEXTANT, demonstration will use NICER’s X-ray observations of pulsar signals to determine NICER’s exact position in orbit.

    “You can time the pulsations of pulsars distributed in many directions around a spacecraft to figure out where the vehicle is and navigate it anywhere,” said Arzoumanian, who is also the NICER science lead. “That’s exactly how the GPS system on Earth works, with precise clocks flown on satellites in orbit.”

    Scientists have tested this method using computer and lab simulations. SEXTANT will demonstrate pulsar-based navigation for the first time in space.

    NICER-SEXTANT is the first astrophysics mission dedicated to studying pulsars, 50 years after their discovery. “I think it is going to yield many more scientific discoveries than we can anticipate now,” said Gendreau.

    NICER-SEXTANT is a two-in-one mission. NICER is an Astrophysics Mission of Opportunity within NASA’s Explorer program, which provides frequent flight opportunities for world-class scientific investigations from space utilizing innovative, streamlined, and efficient management approaches within the heliophysics and astrophysics science areas. NASA’s Space Technology Mission Directorate supports the SEXTANT component of the mission, demonstrating pulsar-based spacecraft navigation.

    More about NICER: https://www.nasa.gov/nicer/

    Read about five famous pulsars from the past 50 years: https://nasa.tumblr.com/post/163637443034/five-famous-pulsars-from-the-past-50-years ­­­­

    See the full article here.

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    NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

    Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.


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  • richardmitnick 8:32 pm on May 2, 2017 Permalink | Reply
    Tags: , , , , , , Mystery glow of Milky Way likely not dark matter, Pulsars,   

    From Symmetry: “Mystery glow of Milky Way likely not dark matter” 


    Symmetry

    05/02/17
    Manuel Gnida

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    NASA/CXC/University of Massachusetts/D. Wang et al.; Greg Stewart, SLAC National Accelerator Laboratory

    According to the Fermi LAT collaboration, the galaxy’s excessive gamma-ray glow likely comes from pulsars, the remains of collapsed ancient stars.

    NASA/Fermi LAT

    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 on the Fermi LAT collaboration. 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,” says Mattia Di Mauro from the Kavli Institute for Particle Astrophysics and Cosmology, a joint institute of Stanford University and the US Department of Energy’s SLAC National Accelerator Laboratory. “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.”

    Di Mauro led the analysis, which looked at the glow with the Large Area Telescope on NASA’s Fermi Gamma-ray Space Telescope, which has been orbiting Earth since 2008. The LAT, a sensitive “eye” for gamma rays, the most energetic form of light, was conceived of and assembled at SLAC, which also hosts its operations center.

    The collaboration’s findings, submitted to The Astrophysical Journal for publication, are available as a preprint.

    A mysterious glow

    Dark matter is one of the biggest mysteries of modern physics. Researchers know that dark matter exists because it bends light from distant galaxies and affects how galaxies rotate. But they don’t know what the substance is made of. Most scientists believe it’s composed of yet-to-be-discovered particles that almost never interact with regular matter other than through gravity, making it very hard to detect them.

    One way scientific instruments might catch a glimpse of dark matter particles is when the particles either decay or collide and destroy each other. “Widely studied theories predict that these processes would produce gamma rays,” says Seth Digel, head of KIPAC’s Fermi group. “We search for this radiation with the LAT in regions of the universe that are rich in dark matter, such as the center of our galaxy.”

    Previous studies have indeed shown that there are more gamma rays coming from the galactic center than expected, fueling some scientific papers and media reports that suggest the signal might hint at long-sought dark matter particles. However, gamma rays are produced in a number of other cosmic processes, which must be ruled out before any conclusion about dark matter can be drawn. This is particularly challenging because the galactic center is extremely complex, and astrophysicists don’t know all the details of what’s going on in that region.

    Most of the Milky Way’s gamma rays originate in gas between the stars that is lit up by cosmic rays, charged particles produced in powerful star explosions called supernovae. This creates a diffuse gamma-ray glow that extends throughout the galaxy. Gamma rays are also produced by supernova remnants, pulsars—collapsed stars that emit “beams” of gamma rays like cosmic lighthouses—and more exotic objects that appear as points of light.

    “Two recent studies by teams in the US and the Netherlands have shown that the gamma-ray excess at the galactic center is speckled, not smooth as we would expect for a dark matter signal,” says KIPAC’s Eric Charles, who contributed to the new analysis. “Those results suggest the speckles may be due to point sources that we can’t see as individual sources with the LAT because the density of gamma-ray sources is very high and the diffuse glow is brightest at the galactic center.”

    Remains of ancient stars

    The new study takes the earlier analyses to the next level, demonstrating that the speckled gamma-ray signal is consistent with pulsars.

    “Considering that about 70 percent of all point sources in the Milky Way are pulsars, they were the most likely candidates,” Di Mauro says. “But we used one of their physical properties to come to our conclusion. Pulsars have very distinct spectra—that is, their emissions vary in a specific way with the energy of the gamma rays they emit. Using the shape of these spectra, we were able to model the glow of the galactic center correctly with a population of about 1,000 pulsars and without introducing processes that involve dark matter particles.”

    The team is now planning follow-up studies with radio telescopes to determine whether the identified sources are emitting their light as a series of brief light pulses—the trademark that gives pulsars their name.

    Discoveries in the halo of stars around the center of the galaxy, the oldest part of the Milky Way, also reveal details about the evolution of our galactic home, just as ancient remains teach archaeologists about human history.

    “Isolated pulsars have a typical lifetime of 10 million years, which is much shorter than the age of the oldest stars near the galactic center,” Charles says. “The fact that we can still see gamma rays from the identified pulsar population today suggests that the pulsars are in binary systems with companion stars, from which they leach energy. This extends the life of the pulsars tremendously.”

    Dark matter remains elusive

    The new results add to other data that are challenging the interpretation of the gamma-ray excess as a dark matter signal.

    “If the signal were due to dark matter, we would expect to see it also at the centers of other galaxies,” Digel says. “The signal should be particularly clear in dwarf galaxies orbiting the Milky Way. These galaxies have very few stars, typically don’t have pulsars and are held together because they have a lot of dark matter. However, we don’t see any significant gamma-ray emissions from them.”

    The researchers believe that a recently discovered strong gamma-ray glow at the center of the Andromeda galaxy, the major galaxy closest to the Milky Way, may also be caused by pulsars rather than dark matter.

    But the last word may not have been spoken. Although the Fermi-LAT team studied a large area of 40 degrees by 40 degrees around the Milky Way’s galactic center (the diameter of the full moon is about half a degree), the extremely high density of sources in the innermost four degrees makes it very difficult to see individual ones and rule out a smooth, dark matter-like gamma-ray distribution, leaving limited room for dark matter signals to hide.

    This work was funded by NASA and the DOE Office of Science, as well as agencies and institutes in France, Italy, Japan and Sweden.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 7:33 am on February 22, 2017 Permalink | Reply
    Tags: , , , , , furthest pulsar in the Universe, Pulsars, The brightest   

    From ESA: “The brightest, furthest pulsar in the Universe” 

    ESA Space For Europe Banner

    European Space Agency

    21 February 2017
    Markus Bauer








    ESA Science and Robotic Exploration Communication Officer









    Tel: +31 71 565 6799









    Mob: +31 61 594 3 954









    Email: markus.bauer@esa.int

    Gian Luca Israel
    INAF, Osservatorio Astronomico di Roma, Italy
    Email: gianluca@oa-roma.inaf.it

    Norbert Schartel
    XMM-Newton project scientist
    Email: Norbert.Schartel@esa.int

    1
    NGC 5907 X-1: record-breaking pulsar

    ESA’s XMM-Newton has found a pulsar – the spinning remains of a once-massive star – that is a thousand times brighter than previously thought possible.

    ESA/XMM Newton
    ESA/XMM Newton

    The pulsar is also the most distant of its kind ever detected, with its light travelling 50 million light-years before being detected by XMM-Newton.

    Pulsars are spinning, magnetised neutron stars that sweep regular pulses of radiation in two symmetrical beams across the cosmos. If suitably aligned with Earth these beams are like a lighthouse beacon appearing to flash on and off as it rotates. They were once massive stars that exploded as a powerful supernova at the end of their natural life, before becoming small and extraordinarily dense stellar corpses.

    This X-ray source is the most luminous of its type detected to date: it is 10 times brighter than the previous record holder. In one second it emits the same amount of energy released by our Sun in 3.5 years.

    XMM-Newton observed the object several times in the last 13 years, with the discovery a result of a systematic search for pulsars in the data archive – its 1.13 s periodic pulses giving it away.

    The signal was also identified in NASA’s Nustar archive data, providing additional information.

    NASA NuSTAR
    NASA/NuSTAR

    “Before, it was believed that only black holes at least 10 times more massive than our Sun feeding off their stellar companions could achieve such extraordinary luminosities, but the rapid and regular pulsations of this source are the fingerprints of neutron stars and clearly distinguish them from black holes,” says Gian Luca Israel, from INAF-Osservatorio Astronomica di Roma, Italy, lead author of the paper describing the result published in Science this week.

    The archival data also revealed that the pulsar’s spin rate has changed over time, from 1.43 s per rotation in 2003 to 1.13 s in 2014. The same relative acceleration in Earth’s rotation would shorten a day by five hours in the same time span

    “Only a neutron star is compact enough to keep itself together while rotating so fast,” adds Gian Luca.

    Although it is not unusual for the rotation rate of a neutron star to change, the high rate of change in this case is likely linked to the object rapidly consuming mass from a companion.

    “This object is really challenging our current understanding of the ‘accretion’ process for high-luminosity stars,” says Gian Luca. “It is 1000 times more luminous than the maximum thought possible for an accreting neutron star, so something else is needed in our models in order to account for the enormous amount of energy released by the object.”

    The scientists think there must be a strong, complex magnetic field close to its surface, such that accretion onto the neutron star surface is still possible while still generating the high luminosity.

    “The discovery of this very unusual object, by far the most extreme ever discovered in terms of distance, luminosity and rate of increase of its rotation frequency, sets a new record for XMM-Newton, and is changing our ideas of how such objects really ‘work’,” says Norbert Schartel, ESA’s XMM-Newton project scientist.

    An accreting pulsar with extreme properties drives an ultraluminous X-ray source in NGC 5907 by G.L. Israel is published in Science.

    See the full article here .

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    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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  • richardmitnick 12:36 pm on January 19, 2017 Permalink | Reply
    Tags: , , , , Geminga and B0355+54, , Pulsars   

    From Chandra: “Geminga and B0355+54: Chandra Images Show That Geometry Solves a Pulsar Puzzle” 

    NASA Chandra Banner

    NASA Chandra Telescope

    NASA Chandra
    [I know a guy, JLT, who just might love to see these images.]
    January 18, 2017

    1
    Credit X-ray: NASA/CXC/PSU/B.Posselt et al; Infrared: NASA/JPL-Caltech; Illustration: Nahks TrEhnl
    Release Date January 18, 2017

    X-ray images from Chandra have shown distinctly different shapes for the structures around two pulsars.

    Pulsars are rapidly rotating, highly magnetized, neutron stars born in supernova explosions triggered by the collapse of massive stars.

    In certain cases, pulsars generate extensive clouds of high-energy particles called pulsar wind nebulas.

    By studying the shape and orientation of these structures, astronomers may be able to explain the presence or absence of radio and gamma-ray pulses from these systems.

    NASA’s Chandra X-ray Observatory has taken deep exposures of two nearby energetic pulsars flying through the Milky Way galaxy. The shape of their X-ray emission suggests there is a geometrical explanation for puzzling differences in behavior shown by some pulsars.

    Pulsars – rapidly rotating, highly magnetized, neutron stars born in supernova explosions triggered by the collapse of massive stars- were discovered 50 years ago via their pulsed, highly regular, radio emission. Pulsars produce a lighthouse-like beam of radiation that astronomers detect as pulses as the pulsar’s rotation sweeps the beam across the sky.

    Since their discovery, thousands of pulsars have been discovered, many of which produce beams of radio waves and gamma rays. Some pulsars show only radio pulses and others show only gamma-ray pulses. Chandra observations have revealed steady X-ray emission from extensive clouds of high-energy particles, called pulsar wind nebulas, associated with both types of pulsars. New Chandra data on pulsar wind nebulas may explain the presence or absence of radio and gamma-ray pulses.

    This four-panel graphic shows the two pulsars observed by Chandra. Geminga is in the upper left and B0355+54 is in the upper right. In both of these images, Chandra’s X-rays, colored blue and purple, are combined with infrared data from NASA’s Spitzer Space Telescope that shows stars in the field of view.

    NASA/Spitzer Telescope
    NASA/Spitzer Telescope

    Below each data image, an artist’s illustration depicts more details of what astronomers think the structure of each pulsar wind nebula looks like.

    For Geminga, a deep Chandra observation totaling nearly eight days over several years was analyzed to show sweeping, arced trails spanning half a light year and a narrow structure directly behind the pulsar. A five-day Chandra observation of the second pulsar, B0355+54, showed a cap of emission followed by a narrow double trail extending almost five light years.

    The underlying pulsars are quite similar, both rotating about five times per second and both aged about half a million years. However, Geminga shows gamma-ray pulses with no bright radio emission, while B0355+54 is one of the brightest radio pulsars known yet not seen in gamma rays.

    A likely interpretation of the Chandra images is that the long narrow trails to the side of Geminga and the double tail of B0355+54 represent narrow jets emanating from the pulsar’s spin poles. Both pulsars also contain a torus, a disk-shaped region of emission spreading from the pulsar’s spin equator. These donut-shaped structures and jets are crushed and swept back as the pulsars fly through the Galaxy at supersonic speeds.

    In the case of Geminga, the view of the torus is close to edge-on, while the jets point out to the sides. B0355+54 has a similar structure, but with the torus viewed nearly face-on and the jets pointing nearly directly towards and away from Earth. In B0355+54, the swept-back jets appear to lie almost on top of each other, giving a doubled tail.

    Both pulsars have magnetic poles quite close to their spin poles, as is the case for the Earth’s magnetic field. These magnetic poles are the site of pulsar radio emission so astronomers expect the radio beams to point in a similar direction as the jets. By contrast the gamma-ray emission is mainly produced along the spin equator and so aligns with the torus.

    For Geminga, astronomers view the bright gamma-ray pulses along the edge of the torus, but the radio beams near the jets point off to the sides and remain unseen. For B0355+54, a jet points almost along our line of sight towards the pulsar. This means astronomers see the bright radio pulses, while the torus and its associated gamma-ray emission are directed in a perpendicular direction to our line of sight, missing the Earth.

    These two deep Chandra images have, therefore, exposed the spin orientation of these pulsars, helping to explain the presence, and absence, of the radio and gamma-ray pulses.

    The Chandra observations of Geminga and B0355+54 are part of a large campaign, led by Roger Romani of Stanford University, to study six pulsars that have been seen to emit gamma-rays. The survey sample covers a range of ages, spin-down properties and expected inclinations, making it a powerful test of pulsar emission models.

    A paper on Geminga led by Bettina Posselt of Penn State University was accepted for publication in The Astrophysical Journal and is available online. A paper on B0355+54 led by Noel Klingler of the George Washington University was published in the December 20th, 2016 issue of The Astrophysical Journal and is available online.

    See the full article here .

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    NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

     
  • richardmitnick 3:41 pm on January 10, 2017 Permalink | Reply
    Tags: , , , , , Pulsars   

    From astrobites: “A Too-Hot Pulsar Speeding Through the Galaxy” 

    Astrobites bloc

    Astrobites

    Jan 10, 2017
    Thankful Cromartie

    Title: Hubble Space Telescope detection of the millisecond pulsar J2124-3358 and its far-ultraviolet bow shock nebula
    Authors: B. Rangelov, G. G. Pavlov, O. Kargaltsev, A. Reisenegger, S. Guillot, M. van Kerkwijk & C. Reyes
    First Author’s Institution: Department of Physics, The George Washington University, Washington, DC
    1
    Status: Accepted to ApJ [open access]

    1
    Pulsars Are Spinning Neutron Stars
    CREDIT: Bill Saxton, NRAO/AUI/NSF

    Pulsars – the rapidly rotating, highly magnetized neutron stars that beam radiation from their magnetic axes — are as mysterious as they are exotic. They’re most often observed at radio frequencies using single-dish telescopes, and are sometimes glimpsed in X-ray and gamma-ray bands. Far rarer are pulsar observations at “in-between” frequencies, such as ultraviolet (UV), optical, and infrared (IR) (collectively, UVOIR); in fact, only about a dozen pulsars have been detected this way. However, their study in this frequency range has proved enlightening, as we will see in today’s post.
    A pulsar too hot to handle

    While one would expect a neutron star to cool with age if an internal heating mechanism does not operate throughout its lifetime, observations of the millisecond pulsar J0437–4715 (an interesting object in its own right) yielded surprising results. In a 2016 study, far-UV observations revealed the 7-billion-year-old pulsar to have a surface temperature of about 2 × 105 K — about 35 times the temperature of the Sun’s photosphere. This finding inspired Rangelov et al. to observe another millisecond pulsar, J2124-3358 (a 3.8-billion-year-old pulsar with a spin period of 4.93 ms), in the far-UV and optical bands using the Hubble Space Telescope (HST).

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    Because so few pulsars have been studied in these frequency ranges, their spectral energy distributions (SEDs) in this regime are poorly understood. Generally speaking, the spectra of normal, rotation-powered pulsars reveal a nonthermal (not dependent on temperature) component in optical and X-rays caused by electrons and positrons in the pulsar magnetosphere. In the far-UV, some pulsars show a thermal (blackbody) component in their spectra, thought to come from the surface of the cooling object. Analysis of the team’s HST images revealed an SED that is best modeled by a combined nonthermal and thermal spectral fit, with nonthermal emission dominating at optical wavelengths and thermal emission appearing in the far-UV (see Figure 1). If their interpretation is correct, this implies a surface temperature for J2124-3358 that is between 0.5 × 105 and 2.1 × 105 K, which is very much in line with the temperature of J0437-4715. If this proves to be the case, these two measurements will strongly suggest the presence of a heating mechanism in millisecond pulsars. However, various fits using only nonthermal components in the far-UV are still valid, so it is impossible to make an absolute determination of the correct fit.

    There are quite a few heating mechanisms that could be invoked to explain these objects’ high temperatures, ranging from the release of stored strain energy from the pulsar’s crust to dark matter annihilation in the pulsar’s interior. More spectral coverage of J2124-3358 is necessary to both check the validity of the nonthermal and thermal combined fit and to get closer to determining more specifically the heating mechanism in play.

    2
    Figure 1: Thermal (red dashed) and nonthermal (blue dashed) combined spectral fit to HST far-UV/optical data for J2124-3358. The black line signifies the sum of both components. Because there is uncertainty about the nature of the nonthermal component, two possible spectral slopes are shown. Figure 7 in the paper.

    A (bow) shocking find in the far-UV

    Images of J2124-3358 also show the presence of a bow shock, which is an arc-shaped shock that occurs when an object is moving faster than the interstellar medium (ISM) sound speed. J2124-3358 was known before this study to be accompanied by such a shock in H-alpha (Hydrogen transition from n=3 to n=2) filters, for which plenty of neutral hydrogen is required. As a result of the HST observations, J2124-3358 was found to have an (albeit fainter) far-UV shock coincident with the H-alpha shock (see Figure 2). This is only the second such object (after J0437-4715) to show a far-UV bow shock. It is absolutely possible that many pulsars cause bow shocks that don’t emit in H-alpha, but do in other wavelength regimes. Studying these more carefully will yield information about the nature of the ISM.

    In order to learn more about the heating mechanisms operating in these objects as well as the bow shocks that sometimes accompany them, many more pulsars will need to be studied using various optical, UV, and IR filters. Studies in the far-UV are only possible with Hubble, so it will be a long time before a sufficient number of objects will be studied at these frequencies in order to make solid conclusions about the nature of such interesting phenomena.

    3
    Figure 2: New observations of J2124-3358 from this study using the HST at three different wavelengths are shown in the top (left and right) and bottom left images. The shock is clearly visible in the far-UV using the F125LP filter. The bottom right image shows a previous H-alpha observation of the same pulsar. Figure 1 in the paper.

    See the full article here .

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    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
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  • richardmitnick 10:08 pm on December 23, 2016 Permalink | Reply
    Tags: AR Scorpii, , , Pulsars   

    From astrobites: “AR Sco — The First White Dwarf Pulsar” 

    Astrobites bloc

    Astrobites

    Dec 23, 2016
    Matthew Green

    Title: Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii
    Authors: D.A.H. Buckley, P.J. Meintjes, S.B. Potter, T.R. Marsh, & B.T Gänsicke
    First Author’s Institution: South African Astronomical Observatory, PO Box 9, Observatory, 7935, Cape Town, South Africa
    Status: Published on arXiv, open access

    In 1967, Jocelyn Bell Burnell and Anthony Hewish saw a signal that they didn’t understand: a regular flash of radio emission coming from the same point on the sky, once every 1.3 seconds. It was named CP 1919, although privately they nicknamed the star LGM-1 (standing for Little Green Men) after the suggestion that the radio pulses were signals from an alien civilisation. While their signal was — spoiler alert — not aliens, it was the discovery of two things: the first known pulsar, and the first known neutron star.

    Until now, every known pulsar has contained a neutron star with a strong magnetic field. The magnetic field accelerates charged particles in its atmosphere and causes them to emit synchrotron radiation in two beams, pointing away from the north and south magnetic poles of the star. If the magnetic pole is not lined up with the rotation poles, these two beams sweep through space like rays of light from a lighthouse, appearing to observers on Earth as regular flashes of radio waves. Neutron stars have long been thought to be the only stars dense and magnetic enough to cause these beams. Today we see that this is no longer true, as we take a look at the first ever known white dwarf pulsar.

    2
    Figure 1: Artist’s impression of AR Sco. Image by Mark Garlick, taken from the discovery’s press release.

    AR Scorpii

    AR Scorpii, or AR Sco, is a binary system containing a white dwarf and a main sequence star. Earlier this year, it was discovered to pulsate incredibly strongly — its brightness can increase or decrease by as much as a factor of four in as little as thirty seconds! Some of these pulsations are shown in Figure 2. These pulsations are seen across the electromagnetic spectrum, from radio all the way up to ultraviolet. There are three time periods we see in the pulsations. Two are the orbital period of the system (3.5 hours) and the rotation period of the white dwarf (2 minutes, which is much faster than a white dwarf normally spins). The third period we see, which is also around 2 minutes long, is a so-called ‘beat’ period that comes from interference between the orbital and rotation periods. The beat period implies some interaction between the white dwarf itself and the main sequence star, such as pulses from the white dwarf reflecting from the other star’s surface. Strangely, this beat period is the most pronounced period in the data. If it is indeed a reflection effect, we see more of the reflected light than we see light from the white dwarf itself, a state of affairs which is hard to explain.

    3
    Figure 2: The pulsations of AR Sco. These data cover approximately 30 minutes, which is 15% of a full orbital cycle. This is Figure 1 from today’s paper.

    Polarisation

    4
    Figure 3: Percentage of photons which were polarised. The spikes in polarisation line up well with the pulsations in the previous figure. This is Figure 3 from today’s paper.

    Today’s paper presents a new set of data on this system. For the first time, the polarisation of radiation from the system has been measured. Polarisation is the amount by which light is aligned; if you think of light as a collection of waves, polarised light would would have the peaks of each set of waves pointed in the same direction, while unpolarised light would have them pointed in random directions.

    The team behind today’s paper measured how polarised the light was from AR Sco, and found interesting results. Between pulses, the light is only around 5% polarised (meaning that around 5% of photons are polarised). During each pulse, however, they saw the polarisation rise to more like 30%. Take a look at Figure 3 to see for yourself. Clearly, the process causing the pulsations must be able to produce polarised light. The most likely candidate is synchrotron radiation, the process that powers pulsars.

    The Nature of AR Sco

    So where does that leave us? The white dwarf in AR Sco must have formed with an unusually strong magnetic field, up to 500 mega-Gauss (this is around 10 times as strong as an MRI machine, or 10,000 times as strong as a fridge magnet). Its rotation was then sped up to the short rotation period we now see. In neutron star pulsars this ‘spin-up’ occurs during the formation of the neutron star: as the star collapses from a puffy giant star core to a dense neutron star, conservation of angular momentum forces it to spin faster. In AR Sco, because the white dwarf is not as dense as a neutron star, the same explanation won’t cover the extremely fast rotation that we see. It is suggested that the spin-up may instead have involved mass transfer between the two stars in AR Sco. However it happened, we were left with a dense, fast-spinning, highly-magnetic object emitting two beams of synchrotron radiation. There are still questions to be answered, but for now it seems likely that AR Sco is the first white dwarf pulsar!

    Merry Christmas!

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 9:48 am on November 6, 2016 Permalink | Reply
    Tags: , , , , , Pulsars,   

    From CfA: “Pulsar Wind Nebulae” 

    Harvard Smithsonian Center for Astrophysics


    Center For Astrophysics

    November 4, 2016

    Neutron stars are the detritus of supernova explosions, with masses between one and several suns and diameters only tens of kilometers across. A pulsar is a spinning neutron star with a strong magnetic field; charged particles in the field radiate in a lighthouse-like beam that can sweep past the Earth with extreme regularity every few seconds or less. A pulsar also has a wind, and charged particles, sometimes accelerated to near the speed of light, form a nebula around the pulsar: a pulsar wind nebula. The particles’ high energies make them strong X-ray emitters, and the nebulae can be seen and studied with X-ray observatories. The most famous example of a pulsar wind nebula is the beautiful and dramatic Crab Nebula.

    Supernova remnant Crab nebula. NASA/ESA Hubble
    Supernova remnant Crab nebula. NASA/ESA Hubble

    When a pulsar moves through the interstellar medium, the nebula can develop a bow-shaped shock. Most of the wind particles are confined to a direction opposite to that of the pulsar’s motion and form a tail of nebulosity. Recent X-ray and radio observations of fast-moving pulsars confirm the existence of the bright, extended tails as well as compact nebulosity near the pulsars. The length of an X-ray tail can significantly exceed the size of the compact nebula, extending several light-years or more behind the pulsar.

    CfA astronomer Patrick Slane was a member of a team that used the Chandra X-ray Observatory to study the nebula around the pulsar PSR B0355+54, located about 3400 light-years away.

    NASA/Chandra Telescope
    NASA/Chandra Telescope

    The pulsar’s observed movement over the sky (its proper motion) is measured to be about sixty kilometer per second. Earlier observations by Chandra had determined that the pulsar’s nebula had a long tail, extending over at least seven light-years (it might be somewhat longer, but the field of the detector was limited to this size); it also has a bright compact core. The scientists used deep Chandra observations to examine the nebula’s faint emission structures, and found that the shape of the nebula, when compared to the direction of the pulsar’s motion through the medium, suggests that the spin axis of the pulsar is pointed nearly directly towards us. They also estimate many of the basic parameters of the nebula including the strength of its magnetic field, which is lower than expected (or else turbulence is re-accelerating the particles and modifying the field). Other conclusions include properties of the compact core and details of the physical mechanisms powering the X-ray and radio radiation.
    Reference(s):

    Deep Chandra Observations of the Pulsar Wind Nebula Created by PSR B0355+54</emKlingler, Noel; Rangelov, Blagoy; Kargaltsev, Oleg; Pavlov, George G.; Romani, Roger W.; Posselt, Bettina; Slane, Patrick; Temim, Tea; Ng, C.-Y.; Bucciantini, Niccolò; Bykov, Andrei; Swartz, Douglas A.; Buehler, Rolf, ApJ 2016 (in press).

    See the full article here .

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    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

     
  • richardmitnick 10:15 am on June 22, 2016 Permalink | Reply
    Tags: , , , , , , Pulsars   

    From Goddard: “Astronomers Find the First ‘Wind Nebula’ Around a Magnetar” 

    NASA Goddard Banner
    NASA Goddard Space Flight Center

    June 21, 2016
    Francis Reddy
    francis.j.reddy@nasa.gov
    NASA’s Goddard Space Flight Center, Greenbelt, Maryland

    Astronomers have discovered a vast cloud of high-energy particles called a wind nebula around a rare ultra-magnetic neutron star, or magnetar, for the first time. The find offers a unique window into the properties, environment and outburst history of magnetars, which are the strongest magnets in the universe.

    1
    This X-ray image shows extended emission around a source known as Swift J1834.9-0846, a rare ultra-magnetic neutron star called a magnetar. The glow arises from a cloud of fast-moving particles produced by the neutron star and corralled around it. Color indicates X-ray energies, with 2,000-3,000 electron volts (eV) in red, 3,000-4,500 eV in green, and 5,000 to 10,000 eV in blue. The image combines observations by the European Space Agency’s XMM-Newton spacecraft taken on March 16 and Oct. 16, 2014. Credits: ESA/XMM-Newton/Younes et al. 2016

    ESA/XMM Newton
    ESA/XMM Newton

    A neutron star is the crushed core of a massive star that ran out of fuel, collapsed under its own weight, and exploded as a supernova. Each one compresses the equivalent mass of half a million Earths into a ball just 12 miles (20 kilometers) across, or about the length of New York’s Manhattan Island. Neutron stars are most commonly found as pulsars, which produce radio, visible light, X-rays and gamma rays at various locations in their surrounding magnetic fields. When a pulsar spins these regions in our direction, astronomers detect pulses of emission, hence the name.

    2
    This illustration compares the size of a neutron star to Manhattan Island in New York, which is about 13 miles long. A neutron star is the crushed core left behind when a massive star explodes as a supernova and is the densest object astronomers can directly observe. Credits: NASA’s Goddard Space Flight Center

    Typical pulsar magnetic fields can be 100 billion to 10 trillion times stronger than Earth’s. Magnetar fields reach strengths a thousand times stronger still, and scientists don’t know the details of how they are created. Of about 2,600 neutron stars known, to date only 29 are classified as magnetars.

    The newfound nebula surrounds a magnetar known as Swift J1834.9-0846 — J1834.9 for short — which was discovered by NASA’s Swift satellite on Aug. 7, 2011, during a brief X-ray outburst.

    NASA/SWIFT Telescope
    NASA/SWIFT Telescope

    Astronomers suspect the object is associated with the W41 supernova remnant, located about 13,000 light-years away in the constellation Scutum toward the central part of our galaxy.

    “Right now, we don’t know how J1834.9 developed and continues to maintain a wind nebula, which until now was a structure only seen around young pulsars,” said lead researcher George Younes, a postdoctoral researcher at George Washington University in Washington. “If the process here is similar, then about 10 percent of the magnetar’s rotational energy loss is powering the nebula’s glow, which would be the highest efficiency ever measured in such a system.”

    A month after the Swift discovery, a team led by Younes took another look at J1834.9 using the European Space Agency’s (ESA) XMM-Newton X-ray observatory, which revealed an unusual lopsided glow about 15 light-years across centered on the magnetar. New XMM-Newton observations in March and October 2014, coupled with archival data from XMM-Newton and Swift, confirm this extended glow as the first wind nebula ever identified around a magnetar. A paper describing the analysis will be published by The Astrophysical Journal.

    “For me the most interesting question is, why is this the only magnetar with a nebula? Once we know the answer, we might be able to understand what makes a magnetar and what makes an ordinary pulsar,” said co-author Chryssa Kouveliotou, a professor in the Department of Physics at George Washington University’s Columbian College of Arts and Sciences.

    The most famous wind nebula, powered by a pulsar less than a thousand years old, lies at the heart of the Crab Nebula supernova remnant in the constellation Taurus. Young pulsars like this one rotate rapidly, often dozens of times a second. The pulsar’s fast rotation and strong magnetic field work together to accelerate electrons and other particles to very high energies. This creates an outflow astronomers call a pulsar wind that serves as the source of particles making up in a wind nebula.

    Supernova remnant Crab nebula. NASA/ESA Hubble
    The best-known wind nebula is the Crab Nebula, located about 6,500 light-years away in the constellation Taurus. At the center is a rapidly spinning neutron star that accelerates charged particles like electrons to nearly the speed of light. As they whirl around magnetic field lines, the particles emit a bluish glow. This image is a composite of Hubble observations taken in late 1999 and early 2000. The Crab Nebula spans about 11 light-years. Credits: NASA, ESA, J. Hester and A. Loll (Arizona State University)

    “Making a wind nebula requires large particle fluxes, as well as some way to bottle up the outflow so it doesn’t just stream into space,” said co-author Alice Harding, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We think the expanding shell of the supernova remnant serves as the bottle, confining the outflow for a few thousand years. When the shell has expanded enough, it becomes too weak to hold back the particles, which then leak out and the nebula fades away.” This naturally explains why wind nebulae are not found among older pulsars, even those driving strong outflows.

    A pulsar taps into its rotational energy to produce light and accelerate its pulsar wind. By contrast, a magnetar outburst is powered by energy stored in the super-strong magnetic field. When the field suddenly reconfigures to a lower-energy state, this energy is suddenly released in an outburst of X-rays and gamma rays. So while magnetars may not produce the steady breeze of a typical pulsar wind, during outbursts they are capable of generating brief gales of accelerated particles.

    “The nebula around J1834.9 stores the magnetar’s energetic outflows over its whole active history, starting many thousands of years ago,” said team member Jonathan Granot, an associate professor in the Department of Natural Sciences at the Open University in Ra’anana, Israel. “It represents a unique opportunity to study the magnetar’s historical activity, opening a whole new playground for theorists like me.”

    ESA’s XMM-Newton satellite was launched on Dec. 10, 1999, from Kourou, French Guiana, and continues to make observations. NASA funded elements of the XMM-Newton instrument package and provides the NASA Guest Observer Facility at Goddard, which supports use of the observatory by U.S. astronomers.

    See the full article here.

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    NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

    Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.

    NASA Goddard campus

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  • richardmitnick 5:13 pm on September 30, 2015 Permalink | Reply
    Tags: , , , Pulsars   

    From AAS NOVA: ” A Pulsar Eases Off the Brakes” 

    AASNOVA

    Amercan Astronomical Society

    30 September 2015
    Susanna Kohler

    1
    This still from an animation shows an artist’s impression of a pulsar, surrounded by strong magnetic field lines (blue) and emitting a beam of radiation (purple). [NASA]

    In 2006, pulsar PSR 1846–0258 unexpectedly launched into a series of energetic X-ray outbursts. Now a study has determined that this event may have permanently changed the behavior of this pulsar, raising questions about our understanding of how pulsars evolve.

    Between Categories

    A pulsar — a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation — can be powered by one of three mechanisms:

    Rotation-powered pulsars transform rotational energy into radiation, gradually slowing down in a predictable way.
    Accretion-powered pulsars convert the gravitational energy of accreting matter into radiation.
    Magnetars are powered by the decay of their extremely strong magnetic fields.

    Astronomical classification often results in one pesky object that doesn’t follow the rules. In this case, that object is PSR 1846–0258, a young pulsar categorized as rotation-powered. But in 2006, PSR 1846–0258 suddenly emitted a series of short, hard X-ray bursts and underwent a flux increase — behavior that is usually only exhibited by magnetars. After this outburst, it returned to normal, rotation-powered-pulsar behavior.

    Since the discovery of this event, scientists have been attempting to learn more about this strange pulsar that seems to straddle the line between rotation-powered pulsars and magnetars.

    Unprecedented Drop

    One way to examine what’s going on with PSR 1846–0258 is to evaluate what’s known as its “braking index,” a measure of how quickly the pulsar’s rotation slows down. For a rotation-powered pulsar, the braking index should be roughly constant. The pulsar then slows down according to a fixed power law, where the slower it rotates, the slower it slows down.

    In a recent study, Robert Archibald (McGill University) and collaborators report on 7 years’ worth of timing observations of PSR 1846–0258 after its odd magnetar-like outburst. They then compare these observations to 6.5 years of data from before the outburst. The team finds that the braking index for this bizarre pulsar dropped suddenly by 14.5σ after the outburst — a change that’s unprecedented both in how large and how long-lived it’s been.

    Why is this a problem? Many of the quoted properties of pulsars (like ages, magnetic fields, and luminosities) are determined based on models that envision pulsars as magnetic dipoles in a vacuum. But if this is the case, a pulsar’s braking index should be constant — or, in more realistic scenarios, we might expect it to change slightly over the span of thousands of years. The fact that PSR 1846–0258 underwent such a drastic change during its outburst poses a significant challenge to these models of pulsar behavior and evolution.

    Citation

    R. F. Archibald et al 2015 ApJ 810 67. doi:10.1088/0004-637X/810/1/67

    See the full article here .

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