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

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

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  • richardmitnick 8:17 pm on August 10, 2015 Permalink | Reply
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    From BBC: “The Most Accurate Clocks in Space” 

    BBC
    BBC

    Aug 10, 2015
    Marcus Woo

    1
    Fast-spinning pulsars can act as the universe’s timekeepers

    At first, Shri Kulkarni didn’t think it was a big deal. It was the middle of the night in September 1982, and he was at Arecibo Observatory in Puerto Rico, using the enormous radio dish to hunt for pulsars: the ultra-dense, rapidly spinning corpses of massive stars.

    Arecibo Observatory
    Arecibo Observatory

    He had just detected his first pulsar, and it was rotating really fast – once every 1.5 milliseconds – which was more than 20 times faster than any known at the time.

    Temp 1
    Credit: Detlev van Ravenswaay/SPL

    For Kulkarni, who was still a graduate student then, the rapid rotation didn’t mean much. It was just a fast pulsar, he thought. He called his project advisor, the late Don Backer, an astronomer at the University of California, Berkeley, US and delivered the news.

    “There was a long silence,” Kulkarni recalls. Probably because Backer knew this was big.

    He reminded Kulkarni what such a fast pulsar meant: this was an object spinning 641 times per second. “Many people thought that pulsars going at that speed would break apart,” says Kulkarni, now an astronomer at the California Institute of Technology in the US. Pulsars are as big as a city – about 20 km in diameter – and the assumption was that if it were rotating that fast, the centrifugal force would rip it to smithereens.

    Temp 2
    Neutron stars become extraordinarily dense (Credit: Jupe/Alamy)

    But now Kulkarni’s discovery upended that assumption. It changed not only his burgeoning career, but also an entire field. The pulsar, known as PSR B1937+21, became the first of a new class of remarkable objects called millisecond pulsars.

    Not only are they fast, but they also spin with such amazing regularity that they’re among the most accurate clocks in the universe. Using these celestial timekeepers, astronomers are answering questions about stars, matter – and even space and time itself – that would otherwise be impossible.

    Extreme objects

    Even ordinary pulsars are extraordinary. They’re some of the universe’s most extreme objects, the remains of stars between about eight and 20 times as massive as the sun. When such a star burns up its fuel and dies, it explodes in a supernova, blowing off its outer layers of gas.

    Temp 3
    Credit: Julian Baum/SPL

    What’s left is a core so dense that its electrons have fused with protons, forming a solid sphere of mostly neutrons. It’s become a neutron star. These objects squeeze between about 1.2 and 2 suns worth of mass into a ball no more than 20 km in diameter. Just a teaspoonful weighs a trillion kilograms – comparable to the mass of every person on Earth.

    Such density means the gravity on a neutron star’s surface is extremely strong – 100 billion times greater than Earth’s. If you tried standing on a neutron star (ignoring the million-degree temperatures, of course), you’d be squished, your atoms smeared across the surface. In fact, this overwhelming gravitational pull prevents the formation of any bumps greater than a few centimetres high, giving neutron stars some of the smoothest surfaces in space.

    And then there are the magnetic fields, the most powerful in the universe. Even the weakest is a hundred million times stronger than Earth’s – strong enough to warp the structure of an atom. At the poles, a neutron star’s magnetic field accelerates charged particles – positrons and electrons stripped off the surface by powerful electric fields – and blasts them into space in the form of jets. Those particles produce beams of radiation at radio frequencies, which eventually reach radio telescopes on Earth.

    It’s these beams that give pulsars their namesake. When a neutron star rotates rapidly, it swings these beams around like a lighthouse. From Earth, it appears as a steady, pulsating signal, sometimes as slow as once every 10 seconds.

    But they start out faster. They were cranking up the speed before they were pulsars, when they were stellar cores. As a star runs out of nuclear fuel, it can’t maintain the pressure needed to hold itself up, and the core contracts due to its own gravity.

    Like the way ice skaters spin faster when tucking their arms in, the core of a dying star rotates faster as it collapses. By the time the star dies and you’re left with a neutron star, it can be spinning as fast as 100 times a second. Over time, its rotating magnetic field loses energy, which slows the pulsar down.

    Which is why Kulkarni’s discovery of a pulsar going so much faster was so astounding. To whip it up to such speeds, astronomers realised, a pulsar must receive help from a companion star in orbit. As the companion exhausts its fuel, it swells – as all stars do eventually – and its outer layers start to spill onto the pulsar, forming a disk of hot gas spiraling inward like water circling a drain. The swirling disk spins up the pulsar.

    Temp 5
    Credit: Julian Baum/SPL

    The discovery of millisecond pulsars revitalized a moribund field, which started in 1967 when Jocelyn Bell discovered the first pulsar. The field’s landmark discovery came in 1974, when Russell Hulse and Joseph Taylor found two pulsars spiraling in toward each other. For that to happen, the energy of the pulsar’s orbits must be dissipating in the form of gravitational waves, ripples in the fabric of space-time.

    Their measurements were the clearest evidence yet that these waves exist, confirming a prediction of [Albert] Einstein’s theory of general relativity; they would later win the Nobel Prize in 1993. “That was the one highlight of the field,” Kulkarni says. It seemed all that was left to do was find more pulsars. “By 1982,” he says, “there was a sense that everything about pulsars had been discovered.”

    Cosmic laboratories

    That changed when Kulkarni found the first millisecond pulsar. Since then, astronomers have identified about 300 more. They estimate that the Milky Way Galaxy is home to 20,000 millisecond pulsars, and about an equal number of regular pulsars – a meagre number compared to the galaxy’s hundreds of billions of stars. PSR B1937+21 held the speed record until 2006, when Jason Hessels – who, like Kulkarni, was a graduate student at the time – discovered Terzan 5ad, a faint pulsar that spins 716 times per second.

    Temp 6
    Black holes may produce gravitational waves (Credit: Gl0ck/Alamy)

    With such high speeds and masses – lots of angular momentum, in physics-speak -millisecond pulsars are hard to slow down. That makes them incredibly consistent over a long period of time. When millisecond pulsars were first discovered, they rivaled the stability of atomic clocks. Today, atomic clocks have surpassed pulsars in accuracy. But if you were to compare them over a longer period of time – say, decades – pulsars can be just as good, says Hessels, who’s now at the University of Amsterdam in the Netherlands. Even after billions of years, a millisecond pulsar may slow down by only a few milliseconds. But because astronomers can precisely pin down its rate of deceleration, they can compensate and still use them as clocks.

    Millisecond pulsars are so stable that astronomers have measured their spin periods to an accuracy of one part in a million trillion (that’s 18 decimal places). They know when a pulse arrives on Earth to a precision of 100 nanoseconds. Because the pulses are so reliable, the tiniest deviations can reveal with great detail what’s going on in and around the pulsar – and in the space between the stars.

    In this space is dust and gas, called the interstellar medium, which obstructs and scatters a pulsar’s signals. By measuring the pulses’ delay, their intensity, and how sharp they are, astronomers can probe the properties of the interstellar medium, which plays a key role in how stars and galaxies form and evolve.

    Temp 7
    Credit: Claus Lunau/SPL

    Around the pulsar is the companion star that helped speed it up. The size of the star and how it evolves over time – for example, how changing magnetic activity can alter its shape – influences its orbit. Delays, modulations, or other variations in the pulses reveal what the companion star is like and how it interacts with the pulsar.

    Thanks to the precision of these pulses, astronomers can detect even the most subtle gravitational tugs. In 1992, astronomers discovered a planetary system orbiting a millisecond pulsar – the first planets found outside the solar system. The gravity of the planets were causing the pulsar to wobble ever so slightly, changing the arrival times of the pulses. In the case of Kulkarni’s pulsar, PSR B1937+21, these kinds of timing variations have recently suggested the presence of objects as small as asteroids.

    Detecting those pulses of radio waves – and, in some cases, X rays and gamma rays – is crucial because it’s often the only way for astronomers to observe and study these exotic pulsar systems. It’s also one of the only ways to study the weird structure and composition of the pulsar itself.

    Pulsars are essentially giant atomic nuclei. They can have a thin atmosphere not much more than 10 cm thick made of helium, hydrogen, and carbon, and an outer crust that’s mostly iron. As you go deeper, the matter becomes denser, full of neutrons (and some protons and electrons) in increasingly exotic forms, merging together to form strands and even sheets. But no one really knows what it’s like inside.

    Millisecond pulsars offer clues. The pulses allow scientists to precisely determine the pulsars’ orbits and thus their masses – crucial data that theorists need to constrain and devise new hypotheses. Nowhere in the universe can you find matter at such high densities and pressure. For physicists, pulsars are like laboratories for exploring such extremes – and maybe discovering entirely new types of matter.

    “It’s almost miraculous that there’s this type of star that’s so useful for testing areas of physics that would otherwise be inaccessible,” Hessels says.

    Testing Einstein

    Those areas include gravity itself. Einstein’s theory of general relativity describes gravity as bends and curves in the fabric of space-time, and so far, its predictions have been proven true again and again. But the theory may work differently in the enormous densities and strong gravity of pulsars—as strong as you can get without becoming a black hole. To find out whether that’s the case, researchers can look for discrepancies in the pulses.

    Temp 8
    Credit: Mark Garlick/SPL

    Recently, Hessels was part of a team that discovered a millisecond pulsar in a triple system with two white dwarfs—the remnants of stars not massive enough to form neutron stars. This rare configuration gives scientists a way to test one of the hallmarks of relativity: the equivalence principle.

    The principle says that gravity is the same for everyone and everything. Perhaps the most dramatic example is when astronaut Dave Scott dropped a hammer and a feather on the moon in 1971. Both hit the lunar surface at the same time, showing that the moon’s gravity pulled on both equally. Likewise, researchers want to see if the gravity of one of the white dwarfs pulls on the pulsar in the same way as the other white dwarf. They haven’t done the experiment yet, but the researchers say it could be the most accurate test ever of the equivalence principle.

    Of course, no one has found Einstein to be wrong just yet. One of the most successful confirmations of relativity was the Hulse-Taylor binary pulsar system, the big pre-millisecond-pulsar discovery that proved gravitational waves were real. Still, the evidence was indirect, based on measurements of orbits that allowed Hulse and Taylor to infer the existence of gravitational waves. To this day, a direct detection remains elusive.

    Temp 9
    A pulsar radiating light (Credit: Stocktrek Images Inc/Alamy)

    That’s despite the efforts of ground-based experiments such as LIGO, the Laser Interferometer Gravitational-Wave Observatory, which is designed to detect gravitational waves from colliding neutron stars or black holes. Its first observing run between 2002 and 2010 turned up nothing. After significant upgrades, it’s set to start up again in the fall of 2015.

    Caltech LIGO
    Caltech LIGO

    Meanwhile, an international effort has been racing to beat LIGO using – you guessed it – millisecond pulsars. “The idea is to use them as a galactic GPS,” says Hessels, who is part of the European contingent. When gravitational waves pass through Earth, the planet bobs like a buoy on the water. Those tiny motions alter the arrival times of the pulses.

    Over the last few years, astronomers have continued to refine their techniques, meticulously timing a few dozen of the best cosmic clocks known. And they hope to see something soon. “There’s a reasonable prospect of detecting gravitational waves in this way in the next five years or so,” says Ingrid Stairs, an astronomer at the University of British Columbia in Canada and member of the North American team.

    Temp 10
    The ultimate cosmic clock (Credit: Stocktrek Images Inc/Alamy)

    Still, Stairs thinks LIGO probably will beat them to it. But while LIGO is designed to detect waves from merging neutron stars and black holes several times as massive as the sun, the pulsar method is sensitive to collisions between supermassive black holes, which are millions to billions of times heftier than the sun. “It’s looking at a totally different source of gravitational waves,” she says. “Even if we’re later than LIGO, it doesn’t mean they’ve totally scooped us.”

    Regardless of who wins the race, the millisecond pulsar has been vital for understanding a range of cosmic phenomena. “It’s nature’s gift to us,” Kulkarni says. “It’s a precise, physical laboratory – but in the heavens.” It was a gift received more than three decades ago, and if it didn’t seem like a big deal then, it certainly does now.

    See the full article here.

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  • richardmitnick 8:07 pm on May 19, 2015 Permalink | Reply
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    From CAASTRO: “Where are all the pulsars at the Galactic Centre?” 

    CAASTRO bloc

    CAASTRO ARC Centre of Excellence for All Sky Astrophysics

    19 May 2015
    No Writer Credit

    As stable, regularly pulsing rotators, pulsars are superb instruments for testing theories of gravity. An important test of General Relativity is the behaviour of bodies in extremely strong gravitational fields. The detection of a pulsar in the gravitational field of a black hole therefore potentially constitutes an exceptional test of theories of gravity in the strong field regime.

    Astronomers have long sought to find pulsar-black hole binary systems by conducting large surveys of the Galactic pulsar population. But there is another way to find such systems: find a supermassive black hole and then search for orbiting pulsars. For this reason, the region immediately surrounding the four million solar mass black hole at the Galactic Centre, Sgr A* has been the site for a number of deep pulsar surveys over the past several decades.

    1
    Sagittarius A*. This image was taken with NASA’s Chandra X-Ray Observatory. Ellipses indicate light echoes.

    NASA Chandra Telescope
    NASA/Chandra

    Yet, despite these searches, none of these searches has uncovered a single regular pulsar.

    1

    Where are all the pulsars at the Galactic Centre? In their current paper, CAASTRO Associate Investigator Jean-Pierre Macquart (Curtin University) and colleague Nissim Kanekar (National Centre for Radio Astrophysics, Pune / India) argue that the high stellar density in the central parsec around the Galactic Centre is likely to result in a pulsar population dominated by millisecond pulsars (MSPs). Earlier pulsar searches have been largely insensitive to such an MSP population, accounting for the lack of pulsar detections. We estimate the best search frequency for such an MSP population, taking into account new information on the scattering environment towards Sgr A* provided by the recently-detected magnetar near the Galactic Centre. The optimal search frequency is near 8 GHz for pulsars with periods 1-20ms, assuming that the pulsars have a luminosity distribution similar to those in the rest of the Milky Way. We find that 10-30 hour integrations with the Green Bank Telescope or the Very Large Array would be sufficient to detect MSPs at the Galactic Centre.

    Interestingly, observations of the Galactic Centre at X-ray and GeV energies have, just within the last few months, independently suggested evidence for such an MSP population (Perez et al. 2015, O’Leary et al. 2015).

    Publication details:
    J.-P. Macquart and N. Kanekar in the Astrophysical Journal (2015) On Detecting Millisecond Pulsars at the Galactic Center

    See the full article here.

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    Astronomy is entering a golden age, in which we seek to understand the complete evolution of the Universe and its constituents. But the key unsolved questions in astronomy demand entirely new approaches that require enormous data sets covering the entire sky.

    In the last few years, Australia has invested more than $400 million both in innovative wide-field telescopes and in the powerful computers needed to process the resulting torrents of data. Using these new tools, Australia now has the chance to establish itself at the vanguard of the upcoming information revolution centred on all-sky astrophysics.

    CAASTRO has assembled the world-class team who will now lead the flagship scientific experiments on these new wide-field facilities. We will deliver transformational new science by bringing together unique expertise in radio astronomy, optical astronomy, theoretical astrophysics and computation and by coupling all these capabilities to the powerful technology in which Australia has recently invested.

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  • richardmitnick 10:14 am on April 10, 2015 Permalink | Reply
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    From ASTRON: “LOFAR Discovers its First RRAT” 

    ASTRON bloc

    Netherlands Institute for Radio Astronomy

    10-04-2015
    Daniele Michilli

    1

    Rotating RAdio Transients (RRATs; McLaughlin et al. 2006) are sporadically emitting pulsars. Finding more RRATs is important in order to have a complete picture of the radio-emitting neutron star population. Also, understanding why their behavior is in some cases quite different compared to “steady” pulsars is important for understanding the pulsar radio-emission mechanism.

    The bottom panel of the plot shows all the significant pulses detected in beam 56 of sub-array pointing 2 of observation L202425. These are plotted as a function of dispersion measure and time. The discovered RRAT has a dispersion measure close to 78 pc/cm3 and a period of 2.23 s (or some integer fraction of this). About ten bright single pulses have been detected from the source in the one-hour discovery observation.

    LOFAR’s first RRAT discovery is shown in this plot, where the pulses from the neutron star are highlighted in blue.

    ASTRON LOFAR Map
    LOFAR map

    ASTRON LOFAR Radio Antenna Bank
    ASTRON LOFAR Radio Antenna Bank

    The discovery has been made as part of the LOFAR Tied-Array All-Sky Survey (LOTAAS), an ongoing survey for pulsars and fast radio transients, which has previously discovered another 13 new pulsars (www.astron.nl/lotaas). The irregular emission of RRATs makes them difficult to detect in periodicity searches. For my PhD I am developing new techniques to sift through the LOTAAS data in order to find more individual dispersed pulses. This is a very challenging task because each LOTAAS pointing contains 222 beams, each with thousands of frequency channels, and millions of time samples!

    See the full article here.

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    ASTRON-Westerbork Synthesis Radio Telescope
    Westerbork Synthesis Radio Telescope (WSRT)

    ASTRON was founded in 1949, as the Foundation for Radio radiation from the Sun and Milky Way (SRZM). Its original charge was to develop and operate radio telescopes, the first being systems using surplus wartime radar dishes. The organisation has grown from twenty employees in the early 1960’s to about 180 staff members today.

     
  • richardmitnick 4:38 pm on December 5, 2014 Permalink | Reply
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    From Daily Galaxy: “Discovery of a Pulsar and Supermassive Black Hole Pairing Could Help Unlock the Enigma of Gravity” 

    Daily Galaxy
    The Daily Galaxy

    Last year, the very rare presence of a pulsar (named SGR J1745-2900) was also detected in the proximity of a supermassive black hole (Sgr A**, made up of millions of solar masses), but there is a combination that is still yet to be discovered: that of a pulsar orbiting a ‘normal’ black hole; that is, one with a similar mass to that of stars.

    b

    sgr

    Supermassive Black Hole Sagittarius A*

    The center of the Milky Way galaxy, with the supermassive black hole Sagittarius A* (Sgr A*), located in the middle, is revealed in these images. As described in our press release, astronomers have used NASA’s Chandra X-ray Observatory to take a major step in understanding why material around Sgr A* is extraordinarily faint in X-rays.

    NASA Chandra Telescope
    NASA/Chandra

    The large image contains X-rays from Chandra in blue and infrared emission from the Hubble Space Telescope in red and yellow. The inset shows a close-up view of Sgr A* in X-rays only, covering a region half a light year wide. The diffuse X-ray emission is from hot gas captured by the black hole and being pulled inwards. This hot gas originates from winds produced by a disk-shaped distribution of young massive stars observed in infrared observations.

    NASA Hubble Telescope
    NASA/ESA Hubble

    These new findings are the result of one of the biggest observing campaigns ever performed by Chandra. During 2012, Chandra collected about five weeks worth of observations to capture unprecedented X-ray images and energy signatures of multi-million degree gas swirling around Sgr A*, a black hole with about 4 million times the mass of the Sun. At just 26,000 light years from Earth, Sgr A* is one of very few black holes in the universe where we can actually witness the flow of matter nearby.

    The authors infer that less than 1% of the material initially within the black hole’s gravitational influence reaches the event horizon, or point of no return, because much of it is ejected. Consequently, the X-ray emission from material near Sgr A* is remarkably faint, like that of most of the giant black holes in galaxies in the nearby Universe.

    The captured material needs to lose heat and angular momentum before being able to plunge into the black hole. The ejection of matter allows this loss to occur.

    This work should impact efforts using radio telescopes to observe and understand the “shadow” cast by the event horizon of Sgr A* against the background of surrounding, glowing matter. It will also be useful for understanding the impact that orbiting stars and gas clouds might make with the matter flowing towards and away from the black hole.

    The paper is available online and is published in the journal Science. The first author is Q.Daniel Wang from University of Massachusetts at Amherst, MA; and the co-authors are Michael Nowak from Massachusetts Institute of Technology (MIT) in Cambridge, MA; Sera Markoff from University of Amsterdam in The Netherlands, Fred Baganoff from MIT; Sergei Nayakshin from University of Leicester in the UK; Feng Yuan from Shanghai Astronomical Observatory in China; Jorge Cuadra from Pontificia Universidad de Catolica de Chile in Chile; John Davis from MIT; Jason Dexter from University of California, Berkeley, CA; Andrew Fabian from University of Cambridge in the UK; Nicolas Grosso from Universite de Strasbourg in France; Daryl Haggard from Northwestern University in Evanston, IL; John Houck from MIT; Li Ji from Purple Mountain Observatory in Nanjing, China; Zhiyuan Li from Nanjing University in China; Joseph Neilsen from Boston University in Boston, MA; Delphine Porquet from Universite de Strasbourg in France; Frank Ripple from University of Massachusetts at Amherst, MA and Roman Shcherbakov from University of Maryland, in College Park, MD. Image credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

    k.
    This image was taken with NASA’s Chandra X-Ray Observatory.

    The intermittent light emitted by pulsars, the most precise timekeepers in the universe, allows scientists to verify Einstein’s theory of relativity, especially when these objects are paired up with another neutron star or white dwarf that interferes with their gravity. However, this theory could be analysed much more effectively if a pulsar with a black hole were found, except in two particular cases, according to researchers from Spain and India.

    Pulsars are very dense neutron stars that are the size of a city (their radius approaches ten kilometres), which, like lighthouses for the universe, emit gamma radiation beams or X-rays when they rotate up to hundreds of times per second. These characteristics make them ideal for testing the validity of the theory of general relativity, published by Einstein between 1915 and 1916.

    “Pulsars act as very precise timekeepers, such that any deviation in their pulses can be detected,” Diego F. Torres, ICREA researcher from the Institute of Space Sciences (IEEC-CSIC), explains to SINC. “If we compare the actual measurements with the corrections to the model that we have to use in order for the predictions to be correct, we can set limits or directly detect the deviation from the base theory.”

    These deviations can occur if there is a massive object close to the pulsar, such as another neutron star or a white dwarf. A white dwarf can be defined as the stellar remnant left when stars such as our Sun use up all of their nuclear fuel. The binary systems, comprised of a pulsar and a neutron star (including double pulsar systems) or a white dwarf, have been very successfully used to verify the theory of gravity.

    Until now scientists had considered the strange pulsar/black hole pairing to be an authentic ‘holy grail’ for examining gravity, but there exist at least two cases where other pairings can be more effective. This is what is stated in the study that Torres and the physicist Manjari Bagchi, from the International Centre of Theoretical Sciences (India) and now postdoc at the IEEC-CSIC, have published in the Journal of Cosmology and Astroparticle Physics. The work also received an Honourable Mention in the 2014 Essays of Gravitation prize.

    The first case occurs when the so-called principle of strong equivalence is violated. This principle of the theory of relativity indicates that the gravitational movement of a body that we test only depends on its position in space-time and not on what it is made up of, which means that the result of any experiment in a free fall laboratory is independent of the speed of the laboratory and where it is found in space and time.

    The other possibility is if one considers a potential variation in the gravitational constant that determines the intensity of the gravitational pull between bodies. Its value is G = 6.67384(80) x 10-11 N m2/kg2. Despite it being a constant, it is one of those that is known with the least accuracy, with a precision of only one in 10,000.

    In these two specific cases, the pulsar-black hole combination would not be the perfect ‘holy grail’, but in any case scientists are anxious to find this pair, because it could be used to analyse the majority of deviations. In fact, it is one of the desired objectives of X-ray and gamma ray space telescopes (such as Chandra, NuStar or Swift), as well as that of large radio telescopes that are currently being built, such as the enormous ‘Square Kilometre Array’ (SKA) in Australia and South Africa.

    NASA NuSTAR
    NASA/Nu-STAR

    NASA SWIFT Telescope
    NASA/Swift

    SKA Square Kilometer Array

    The image at the top of the page shows dynamic rings, wisps and jets of matter and antimatter around the pulsar in the Crab Nebula as observed in X-ray light by Chandra Space Observatory in 2001.

    Manjari Bagchi y Diego F. Torres. “In what sense a neutron star−black hole binary is the holy grail for testing gravity?”. Journal of Cosmology and Astroparticle Physics, 2014. Doi:10.1088/1475-7516/2014/08/055.

    See the full article here.

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  • richardmitnick 12:07 pm on November 18, 2014 Permalink | Reply
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    From SPACE.com: ” Dark Matter Murder Mystery: Is Weird Substance Destroying Neutron Stars?” 

    space-dot-com logo

    SPACE.com

    November 18, 2014
    Charles Q. Choi

    dm
    This illustration shows a dark matter annihilation map. Credit: Illustris Collaboration

    The mysterious substance that makes up most of the matter in the universe may be destroying neutron stars by turning them into black holes in the center of the Milky Way, new research suggests.

    ns
    When an image from NASA’s Chandra X-ray Observatory of PSR B1509-58 — a spinning neutron star surrounded by a cloud of energetic particles –was released in 2009, it quickly gained attention because many saw a hand-like structure in the X-ray emission. In a new image of the system, X-rays from Chandra in gold are seen along with infrared data from NASA’s Wide-field Infrared Survey Explorer (WISE) telescope in red, green and blue. NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, also took a picture of the neutron star nebula in 2014, using higher-energy X-rays than Chandra. PSR B1509-58 is about 17,000 light-years from Earth.
    JPL, a division of the California Institute of Technology in Pasadena, manages the WISE mission for NASA. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for the NASA Science Mission Directorate. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.

    If astronomers successfully detect a neutron star dying at the metaphorical hands of dark matter, such a finding could yield critical insights on the elusive properties of material, scientists added.

    Dark matter — an invisible substance thought to make up five-sixths of all matter in the universe — is currently one of the greatest mysteries in science. The consensus among researchers suggests that dark matter is composed of a new type of particle, one that interacts very weakly at best with all the known forces of the universe. As such, dark matter is invisible and nearly completely intangible, mostly detectable only via the gravitational pull it exerts.

    A number of ongoing experiments based on massive sensor arrays buried underground are attempting to identify the weak signals dark matter is expected to give off when it makes a rare encounter with other particles. In addition, the most powerful particle accelerator on Earth, the Large Hadron Collider (LHC), is attempting to create particles that might be dark matter. So far, none of these studies have confirmed any signs of dark matter, leaving much uncertain about its properties.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    Now, physicists suggest answers to the mystery of dark matter might lie in another puzzle, known as the missing pulsar problem.

    A pulsar is a kind of neutron star, which is a super-dense remnant of a massive star left behind after dying in a gigantic explosion known as a supernova. Neutron stars can devour matter from companion stars, acts of cannibalization that make neutron stars give off pulses of radiation, earning such neutron stars the name pulsar.

    According to current astrophysical and cosmological models, several hundred pulsars should be orbiting the supermassive black hole at the heart of the Milky Way. However, searches for these pulsars by looking for the radio waves they emit have so far come up empty-handed.

    Now researchers suggest dark matter could destroy these neutron stars, transforming them into black holes.

    Dark matter, like ordinary matter, is drawn to the gravity of other matter. The greatest concentration of normal matter in the Milky Way is at its center, so the greatest concentration of dark matter is there as well.

    In a region of high dark matter density such as the heart of the Milky Way, an enormous amount of dark matter particles could accumulate in a pulsar, causing it to grow massive enough to collapse and form a black hole.

    “It is possible that pulsars imploding into black holes may provide the first concrete signal of particulate dark matter,” said study co-author Joseph Bramante, a physicist at the University of Notre Dame.

    The models of dark matter that are most consistent with this idea, and with observations of pulsars seen outside the galactic center, are ones that suggest dark matter is asymmetric, meaning there is more of one kind of dark matter particle than its antiparticle counterpart. Normal matter is asymmetric as well — there are far more protons in the universe than anti-protons. (When a particle and its antimatter counterpart meet, they annihilate each other, releasing a burst of energy — a proof of Einstein’s famous equation, E=mc2, which revealed mass can be converted to energy and vice versa.)

    “For me, the most surprising result is that already existing models of dark matter could cause pulsars at the galactic center to collapse into black holes,” Bramante told Space.com.

    If dark matter is asymmetric, this would be consistent with “why there is more matter than antimatter in the universe, and why there is five times more dark matter than visible matter,” Bramante added.

    The mass of the dark matter particle responsible for imploding pulsars in the galactic core might be 100 times lighter than an electron or heavier than 100 million protons. If dark matter is as massive as 100 million protons, it would take more than 1,000 times the energies capable at the LHC to create them, Bramante noted. This suggests that looking for an imploding pulsar in the centers of galaxies might be a more feasible way to learn about dark matter.

    There might be other explanations for the missing pulsar problem. For instance, massive stars may form short-lived, highly magnetic pulsars known as magnetars in the galactic center rather than ordinary long-lived pulsars, perhaps because stars in the galactic core might be highly magnetized. The researchers are exploring how astronomers might identify whether a pulsar in the galactic core died because of dark matter, supporting their idea.

    Bramante and his colleague Tim Linden detailed their findings Oct. 10 in the journal Physical Review Letters.

    See the full article here.

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