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  • richardmitnick 11:39 am on February 9, 2019 Permalink | Reply
    Tags: , , , , First observed in 2008 a binary system known as IGR J18245–2452 from its x-ray outbursts and PSR J1824–2452I for its radio emissions, , Millisecond pulsars, , , The fastest millisecond pulsar PSR J1748–2446ad   

    From Medium: “IGR J18245–2452: The most important neutron star you’ve never heard of” 

    From Medium

    Jan 21, 2019
    Graham Doskoch

    Astronomers have spent thirty years on the theory behind how millisecond pulsars form. Now we know they got it right.

    Neutron stars are known for their astonishing rotational speeds, with most spinning around their axes many times each second. The mechanism behind this is simple: When a fairly massive star several times the radius of the Sun collapses into a dense ball about ten kilometers in diameter, conservation of angular momentum dictates that it must spin quicker.

    However, one class of neutron stars can’t be explained this way: millisecond pulsars. These exotic objects spin hundreds of times each second, with the fastest, PSR J1748–2446ad, rotating at over 700 Hertz! Since their discovery in the 1980s, a slightly different evolutionary path has been proposed. After studying dozens of systems, astronomers theorized that millisecond pulsars are very old — old enough that they’ve lost much of their original angular momentum to radiation. However, they’re also in binary systems, and under certain conditions, a companion star can transfer matter — and thus angular momentum — to the pulsar, spinning it back up again.

    1
    A plot of the periods and magnetic fields of pulsars. Millisecond pulsars have extremely short periods, and comparatively weak magnetic fields. Image credit: Swinburne University of Technology

    During this period of accretion, the system should become an x-ray binary, featuring strong emission from the hot plasma in the neutron star’s accretion disk. There should also be periods where the neutron star behaves like an ordinary radio pulsar, emitting radio waves we can detect on Earth. If we could detect both types of radiation from a single system, it might be the clinching bit of evidence for the spin-up model of millisecond pulsar formation.

    In 2013, astronomers discovered just that: a binary system known as IGR J18245–2452 from its x-ray outbursts, and PSR J1824–2452I for its radio emissions. First observed in 2008, it had exhibited both radio pulsations and x-ray outbursts within a short period of time, clear evidence of the sort of transitional stage everyone had been looking for. This was it: a confirmation of the ideas behind thirty years of work on how these strange systems form.

    2
    INTEGRAL observations of IGR J18245–2452 from February 2013 (top) and March/April 2013 (bottom). The system is only visible in x-rays in the second period. Image credit: ESA/INTEGRAL/IBIS/Jörn Wilms.

    ESA/Integral

    The 2013 outburst

    Towards the end of March of 2013, the INTEGRAL and Swift space telescopes detected x-rays from an energetic event coming from the core of the globular cluster M28 (Papitto et al. 2013).

    NASA Neil Gehrels Swift Observatory

    It appeared to be an outburst of some kind — judging by the Swift observations, likely a thermonuclear explosion. A number of scenarios can lead to x-ray transients, including novae and certain types of supernovae. Binary systems are often the culprits, where mass can be transferred from one star or compact object to another.

    3
    Fig. 7, Papitto et al. Swift data from observations of an outburst show its characteristic exponentially decreasing cooling.

    One thermonuclear burst observed by Swift followed a time evolution profile expected for such a detonation: An increase in luminosity for 10 seconds, followed by an exponential decrease with a time constant of 38.9 seconds. This decrease represents the start of post-burst cooling. The other outbursts from the system should have had similar profiles characteristic of x-ray-producing thermonuclear explosions, and indeed later observations of the system have confirmed that this is indeed the case (De Falco et al. 2017 [Astronomy and Astrophysics]), albeit with slightly different rise times and decay constants.

    To determine the identity of the transient, now designated IGR J18245–2452, astronomers made follow-up observations using the XMM-Newton telescope.

    ESA/XMM Newton

    The nature of the outburst would determine how it evolved over time. For instance, supernovae (usually) decrease in brightness over the course of weeks or months. In this case, however, the x-rays were still detected — albeit a bit weaker. More surprisingly, the strength of the emission appeared to be modulated, varying with a period of 3.93 milliseconds.

    Such a short period seemed to indicate that a pulsar might be responsible. The team checked databases of known radio pulsars and found one that matched the x-ray source: PSR J1824–2452I, a millisecond pulsar in a binary system. Even after this radio counterpart had been found, however, two questions remained: Were these x-ray pulses new or a long-term process, and how did they relate to the radio emission?

    Diving into the archives

    A handy tool for observational astronomers is archival images. By looking at observations taken months, years or decades before an event, scientists can — if they’re lucky — peek into the past to see what an object of interest looked like long before it became interesting. Archival data is often of use for teams studying supernovae, as even a previously uninteresting or unnoticed star can tell the story of a supernova’s progenitor.

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    Fig. 3, Papitto et al. Chandra images from 2008, showing the system in quiescent (top) and active (bottom) states.

    NASA/Chandra X-ray Telescope

    In this case, Papitto et al. looked at Chandra observations from 2008, comparing them with new data from April 2013. They found x-ray variability occurring shortly after a period of radio activity by the pulsar, indicating that the system had switched off its radio emissions and started emitting x-rays. This was extremely interesting, because new observations with three sensitive radio telescopes — Green Bank, Parkes, and Westerbork — indicated that the pulsar was no longer active in radio waves.

    Green Bank Radio Telescope, West Virginia, USA, now the center piece of the GBO, Green Bank Observatory, being cut loose by the NSF

    CSIRO/Parkes Observatory, located 20 kilometres north of the town of Parkes, New South Wales, Australia

    Westerbork Synthesis Radio Telescope, an aperture synthesis interferometer near World War II Nazi detention and transit camp Westerbork, north of the village of Westerbork, Midden-Drenthe, in the northeastern Netherlands

    It was possible that the pulsar had been eclipsed and emission was ongoing, and this may indeed have happened at some points, but was not likely to be the main factor behind the apparent quiescence.

    A few weeks later, however, the exact opposite happened: the pulsar exited its quiescent radio state and was again picked up by the three radio telescopes. In short, over a period of months, it had oscillated between behaving like an x-ray binary and a normal millisecond pulsar. Finally, x-ray observations had conclusively shown that this sort of bizarre transitional state was possible!

    The mechanism

    IGR J18245–2452 spends the vast majority of its time in what is known as a “quiescent” state, during which there is comparatively little x-ray activity. The pulsar’s magnetosphere exerts a pressure on the infalling gas, forming a disk at a suitable distance from the surface. Eventually, however, there is enough buildup that an x-ray outburst occurs, lasting for a few months. The outburst decreases the mass accretion rate, and the magnetosphere pushes away much of the transferred gas, allowing radio pulsations to take place once more.

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    Fig. 2, De Falco et al. Over a period of a few weeks, IGR J18245–2452 underwent a number of individual x-ray outbursts, themselves indicative of a brief period of x-ray activity and radio silence.

    It’s expected that the pulsar will eventually be spun-up until its rotational period is on the order of a millisecond or so. It will cease x-ray emissions, and be visible mainly through radio pulses. All of this, however, is far in the future, and during our lifetimes, IGR J18245–2452 will stay in its current transitional state, halfway between an x-ray binary and a millisecond pulsar.

    Women in STEM – Dame Susan Jocelyn Bell Burnell

    Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

    See the full article here .

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    About Medium

    Medium is an online publishing platform developed by Evan Williams, and launched in August 2012. It is owned by A Medium Corporation. The platform is an example of social journalism, having a hybrid collection of amateur and professional people and publications, or exclusive blogs or publishers on Medium, and is regularly regarded as a blog host.

    Williams developed Medium as a way to publish writings and documents longer than Twitter’s 140-character (now 280-character) maximum.

     
  • richardmitnick 9:06 pm on June 7, 2018 Permalink | Reply
    Tags: , , , , , Millisecond pulsars,   

    From Stanford University: Stanford-led international collaboration discovered an elusive neutron star 

    Stanford University Name
    From Stanford University

    June 1, 2018
    Kimberly Hickok

    Media Contact

    Amy Adams, Stanford News Service:
    (650) 497-5908,
    amyadams@stanford.edu

    Two of the most powerful telescopes in the world worked together to find the faintest millisecond pulsar ever discovered. The collaboration between the Fermi Large Area Telescope and China’s FAST radio telescope was spearheaded by Stanford physicist Peter Michelson.

    1
    The Gamma-ray sky map and integrated pulse profiles of the new MSP: Upper panel shows the region of the gamma-ray sky where the new MSP is located. Lower panel a) shows the observed radio pulses in a one-hour tracking observation of FAST. Lower panel b) shows the folded pulses from more than nine years of Fermi-LAT gamma-ray data. Credit: Pei Wang/NAOC

    China’s 500-meter Aperture Spherical radio Telescope (FAST) discovered a radio millisecond pulsar (MSP) coincident with the unassociated gamma-ray source 3FGL J0318.1+0252 in the Fermi Large Area Telescope (LAT) point-source list. This is another milestone of FAST.phys.org

    __________________________________________________
    CSIRO is a world leader in receiver design. CSIRO and engineers from the Chinese Academy of Sciences recently worked together to develop a receiver for China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST). In addition, the Parkes telescope is following up radio sources detected with FAST.

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    Receiver in the anechoic chamber.©CSIRO

    __________________________________________________

    NASA/Fermi LAT


    NASA/Fermi Gamma Ray Space Telescope

    FAST radio telescope, now operating, located in the Dawodang depression in Pingtang county Guizhou Province, South China, https://astronomynow.com

    During the early morning hours of Feb. 17, 2018, Chinese scientists emailed data showing evidence of a rapidly spinning pulsar detected with China’s Five-hundred-meter Aperture Spherical Telescope (FAST) to the Fermi Gamma-ray Space Telescope–Large Area Telescope (LAT) team.

    “One of our collaborators in Germany, who was up at the time, used the FAST data to search in 10 years of Fermi data – and boom! There was the pulsar,” said Stanford physicist Peter Michelson.

    FAST had detected a faint pulsar with a spin period of just 5.19 milliseconds, and estimated to be 4,000 light-years away – likely the faintest millisecond pulsar ever detected. The discovery was the first of its kind from the collaboration between Fermi LAT and FAST, a partnership spearheaded by Michelson.

    Searching the sky

    With Michelson as the principal investigator, the Fermi LAT team, an international collaboration, has discovered hundreds of pulsars since its launch 10 years ago this June. Pulsars are rapidly spinning neutron stars that release beams of electromagnetic waves as they rotate. Similar to the rotating beam of light from a lighthouse, the pulses of energy from pulsars occur at regular intervals ranging from milliseconds to seconds. Large radio telescopes detect pulses in the radio wave range of the electromagnetic spectrum while the Fermi LAT detects pulses in the gamma-ray range.

    The partnership between Fermi LAT and China’s FAST significantly improves the ability of scientists to detect the faintest pulsars, called millisecond pulsars. The Fermi LAT can detect gamma-rays from suspected pulsars, but can’t determine the rotation period of a rapidly spinning pulsar. That’s where radio telescopes such as FAST come in. When directed to search for radio pulses from the regions of the sky where Fermi detected gamma-rays, FAST can determine the rotation period.

    But that’s only if the radio telescope is sensitive enough to detect the radio pulses. FAST’s enormous 500-meter diameter dish makes it the most sensitive radio telescope on the planet for this purpose, which means FAST can detect pulsars that other radio telescopes overlook, such as the extremely faint millisecond pulsar detected in February.

    A universal effort

    The Fermi LAT collaboration has been international from the start, involving hundreds of scientists from institutions in the United States, Japan, France, Italy and Sweden. Since its launch, scientists from China, Germany, Spain, South Africa and Thailand have joined the team.

    In the spring of 2017, Michelson, who is also the Luke Blossom Professor in the School of Humanities and Sciences, spoke with Chinese physicist Xian Hou about initiating a collaboration with FAST. Hou is a collaborator on the Fermi LAT team and also a scientist at the Chinese Academy of Science’s Yunnan Observatory.

    To kick off the collaboration, Fermi LAT scientists gave the Chinese team a list of locations in the sky where they had detected possible pulsars. The FAST team looked at a source that had previously been examined by Arecibo, a radio telescope in Puerto Rico operated by the University of Central Florida, but that failed to detect radio pulsations from the source.

    NAIC Arecibo Observatory operated by University of Central Florida, Yang Enterprises and UMET, Altitude 497 m (1,631 ft)

    FAST’s more sensitive equipment succeeded, revealing one of the faintest pulsars detected to date.

    The discovery demonstrated the capability of FAST to detect pulsars that are too faint to be detected by less-sensitive radio telescopes like Arecibo. “That was pretty exciting,” said Michelson.

    From a scientific standpoint, the finding is significant because it suggests future discoveries of many more pulsars, which together, Michelson explained, may help detect low-frequency gravitational waves traveling through the galaxy that can modulate the arrival times of pulsations from these sources.

    Valuable global partnerships

    Michelson is proud of the team’s discovery but is most proud of the collaborative effort. “It’s not just the science. The part I think is important to me is that it’s truly an international collaboration,” he said. One of the reasons he thinks collaborations are so important: “Particularly with countries we sometimes have strained relations with, it’s important to work on things where you share a common purpose and there is a benefit to all involved. That’s important in the long run.”

    Michelson also sees cost benefits from international collaborations, especially in the field of astronomy, due to the expensive facilities required for experiments. “No one nation can afford to invest in all the experiments,” he said. “In astrophysics in particular, state-of-the-art facilities cost a lot. It’s important for scientists around the globe to share access to data from these facilities that will enable important science. Everyone can benefit from this.”

    As a mentor to graduate students in Stanford’s Department of Physics, Michelson strives to teach his students the importance of international collaborations through working with Fermi. “It’s what science does beyond just doing science,” he said. “It connects cultures.”

    See the full article here .


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    Stanford University campus. No image credit

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  • richardmitnick 12:48 pm on September 5, 2017 Permalink | Reply
    Tags: , , , , LOFAR radio telescope discovers record-breaking pulsar, Millisecond pulsars, PSR J0952-0607   

    From ASTRON: “LOFAR radio telescope discovers record-breaking pulsar” 

    ASTRON bloc

    Netherlands Institute for Radio Astronomy

    05/09/2017

    ASTRON LOFAR Radio Antenna Bank, Nethrlands

    Astronomers have discovered two rapidly rotating radio pulsars with the Low-Frequency Array (LOFAR) radio telescope in the Netherlands by investigating unknown gamma-ray sources uncovered by NASA’s Fermi Gamma-ray Space Telescope.

    NASA/Fermi LAT


    NASA/Fermi Telescope

    The first pulsar (PSR J1552+5437) rotates 412 times per second. The second pulsar (PSR J0952-0607) rotates 707 times per second, making it the fastest-spinning pulsar in the disk of our Galaxy and the second-fastest known spinning-pulsar overall.

    Lighthouse

    Pulsars are neutron stars, the remnants of massive stars that exploded as a supernova, which emit radio waves from their magnetic poles that sweep past Earth as they rotate. As a result, they act like lighthouses where we see pulses of radio emission for each rotation. Neutron stars are the size of a city packed in more mass than the Sun. That’s why they are used to study the behaviour of matter under extreme densities. By studying the fastest-spinning pulsars, astronomers hope to discover more about the internal structure of neutron stars and the extremes of the Universe.

    New technique

    Pulsars shine the brightest at low frequency radio waves and this makes LOFAR an ideal telescope for studying them. “However, finding pulsars with LOFAR is extra hard work because gas and dust between stars disrupts low frequency radio waves,” says Cees Bassa from ASTRON, the Netherlands Institute for Radio Astronomy. That’s why astronomers usually look for pulsars at higher radio frequencies.

    Bassa and his colleagues have now found a way to overcome this problem. “We have developed a new processing technique, which uses graphics cards (originally designed for gaming) in the large DRAGNET computer cluster in Groningen to process the LOFAR data.”
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    DRAGNET computer cluster
    This cluster is funded through an ERC starting grant to Jason Hessels from ASTRON and the University of Amsterdam.

    Millisecond pulsar

    Ziggy Pleunis, working together with Bassa and Hessels, was the first to test this technique in a pilot survey with LOFAR in 2016. He struck gold finding PSR J1552+5437, a pulsar rotating once every 2.43 milliseconds or 412 times per second. This is the first pulsar spinning at millisecond spin periods found with LOFAR.

    “As millisecond pulsars are known to emit both high-energy gamma radiation as well as radio waves, we specifically looked at gamma-ray sources of unknown origin,” says Pleunis, now a PhD student at McGill University in Montreal, Canada.

    He was able to show that the gamma-rays from the millisecond pulsar arrive at the same rotational phases as the radio pulses, suggesting a common mechanism for producing both types of radiation.

    Record-breaking pulsar

    Spurred by the success of the pilot survey, Bassa, Hessels and Pleunis continued searching for millisecond pulsars with LOFAR and quickly found an even faster-spinning pulsar. Rotating 707 times per second, the so called PSR J0952-0607 is the fastest-spinning pulsar known in the disk of our Galaxy. Of the known pulsars, PSR J0952-0607 is surpassed in rotation speed only by a pulsar in a dense star cluster outside of the Galactic disk, which rotates 716 times per second.

    “Because PSR J0952-0607 is much closer to us than the pulsar in the star cluster, it allows us to study it in much more detail,” says Bassa. Using the Isaac Newton Telescope on the island of La Palma, Spain, the astronomers identified a low-mass star orbiting the pulsar, which provided additional measurements of the distance and energetics of PSR J0952-0607. Future optical observations of the binary companion star will help to determine the mass of the rapidly spinning pulsar, allowing astronomers to discern its composition.

    Unseen population

    Both pulsars (J1552+5437 and J0952-0607) are unexpectedly bright at the low radio frequencies, and quickly become dimmer at higher radio frequencies. This means that they would probably not have been found at higher radio frequencies where most previous radio telescopes searched for pulsars. Hence, there may be an as-yet unseen population of fast-spinning millisecond pulsars in our Galaxy.

    “We are finding growing evidence that the fastest-spinning pulsars are the brightest at low radio frequencies, and that there may be a link with the production of high energy gamma-rays,” says Hessels. If this is indeed the case, then LOFAR is expected to find more, possibly even faster-spinning, millisecond pulsars whose rotation rate can give astronomers a better understanding of the internal structure of neutron stars.

    Two papers detailing Pleunis’ and Bassa’s pulsar discoveries will appear in The Astrophysical Journal Letters on 5 September 2017.

    A Millisecond Pulsar Discovery in a Survey of Unidentified Fermi γ-Ray Sources with LOFAR

    LOFAR Discovery of the Fastest-spinning Millisecond Pulsar in the Galactic Field

    See the full article here .

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    LOFAR is a radio telescope composed of an international network of antenna stations and is designed to observe the universe at frequencies between 10 and 250 MHz. Operated by ASTRON, the network includes stations in the Netherlands, Germany, Sweden, the U.K., France, Poland and Ireland.

    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 4:31 pm on September 1, 2017 Permalink | Reply
    Tags: , , , , How a Black Widow Consumes Its Companion, Intrabinary shock, Millisecond pulsars   

    From AAS NOVA: “How a Black Widow Consumes Its Companion” 

    AASNOVA

    American Astronomical Society

    1 September 2017
    Susanna Kohler

    1
    Artist’s impression of the optical and X-ray emission surrounding the original “Black Widow” pulsar B1957+20. [NASA/CXC/M.Weiss]

    Hanging out in a binary system with a hot millisecond pulsar can be hazardous to your health! A new study has examined how these perilous objects can heat and evaporate away their companions.

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    Panel (a) shows the intrabinary shock and the companion star (the pulsar would lie to the right). Panel (b) shows the companion star and the magnetic field lines funneling into its front pole. [Adapted from Sanchez & Romani 2017]

    Predatory Stars

    Millisecond pulsars — highly magnetized neutron stars that we detect through their beamed pulses of radiation — lose energy rapidly as they spin slower and slower. When such an object is locked in a binary system with a star or a planetary-mass object, the energy lost by the pulsar can blast its companion, causing it to evaporate.

    Such systems, termed “black widows” in an acknowledgement of how the pulsar effectively consumes its partner, show optical emission revealing their strong heating. We hope that by studying these systems, we can learn more about the properties of the energetic winds emitted by pulsars, and by measuring the companion dynamics we can determine the masses of the pulsars and companions in these systems.

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    Different geometries for the companion’s magnetic field lines can alter the resulting light curve for the system. [Sanchez & Romani 2017]

    How Heating Happens

    Past models of black widows — necessary to interpret the observations — have generally assumed that the companion’s evaporation was due only to direct heating by the energetic gamma-ray photons emitted by the pulsar. This scenario, however, doesn’t successfully reproduce some of the quirks we’ve observed for these systems, such as very large temperatures and asymmetric light curves.

    This picture also ignores the fact that much of the pulsar’s spin-down energy — the energy lost as it gradually spins slower and slower — is carried away by not just the gamma-ray photons, but also a magnetized wind of electrons and positrons. Two scientists at Stanford University, Nicolas Sanchez and Roger Romani, asked the following: how could the particles in the pulsar wind contribute to the heating of a black widow’s companion?

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    The authors’ heating model, as fit to PSR J1301+0833. [Sanchez & Romani 2017]

    A Shock Assists

    Sanchez and Romani’s alternative model relies on the fact that somewhere between the pulsar and its companion lies an intrabinary shock — the collision point between the pulsar’s relativistic wind and the companion’s ordinary, baryonic wind. The shock is anchored to the companion via magnetic fields, which provides an entry point for shock particles to be funneled along the magnetic field lines onto the companion’s surface. These energetic particles, in addition to the direct irradiation by the pulsar’s photons, cause the heating of the companion that results in its evaporation.

    Sanchez and Romani show via simulations that this model can reproduce the observed light curves of several known black widow systems — including the strange features that the direct-heating model didn’t account for. They then use their model to make estimates for the masses of the pulsars and companions in these systems.

    The authors caution that this model is still incomplete, but it illustrates that other sources of heating are important to consider in addition to heating by photons. Applying this and similar models to more black-widow systems will surely help us to better understand how these predatory compact stars cause their companions’ ultimate demise.

    Citation

    Nicolas Sanchez and Roger W. Romani 2017 ApJ 845 42. doi:10.3847/1538-4357/aa7a02

    See the full article here .

    Related Journal Articles
    See the full article for further references comlete with links.

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    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
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  • richardmitnick 9:55 pm on October 12, 2016 Permalink | Reply
    Tags: , , Millisecond pulsars,   

    From SAO: “Millisecond Pulsars” 

    Smithsonian Astrophysical Observatory
    Smithsonian Astrophysical Observatory

    September 30, 2016 [This never came to social media.]

    1
    An artist’s impression of a millisecond pulsar and its companion. The pulsar (seen in blue with two radiation beams) is accreting material from its bloated red companion star and increasing its rotation rate. Astronomers have measured the orbital parameters of four millisecond pulsars in the globular cluster 47 Tuc and modeled their possible formation and evolution paths. European Space Agency & Francesco Ferraro (Bologna Astronomical Observatory)

    When a star with a mass of roughly ten solar masses finishes its life, it explodes as a supernova, leaving behind a neutron star as remnant “ash.” Neutron stars have masses of one-to-several suns but they are tiny in diameter, only tens of kilometers. They spin rapidly, and when they have associated magnetic fields, charged particles caught in them emit electromagnetic radiation in a lighthouse-like beam that can sweep past the Earth with great regularity every few seconds or less. These kinds of neutron stars are called pulsars, and they are dramatic, powerful probes of supernovae, their progenitor stars, and the properties of nuclear matter under the extreme conditions that exist in these stars.

    Millisecond pulsars are ones that spin particularly rapidly, hundreds of times per second. Astronomers have concluded that these objects must be increasing their rotation rates through the accretion of material from a nearby companion star. There are nearly 3000 known millisecond pulsars. About five percent of them are found in globular clusters — gravitationally bound, roughly spherical ensembles of stars containing as many as a million stars, with sizes as small as only tens of light-years in diameter. Their crowded environments provide ideal conditions for forming binary stars, and nearly eighty percent of the pulsars in globular clusters are millisecond pulsars. The globular cluster 47 Tucanae (47 Tuc) has twenty-five of them.

    CfA astronomer Maureen van den Berg was part of a team of astronomers that studied four unusual millisecond binary pulsars in 47 Tuc whose orbital parameters were unknown. Orbits are key to understanding the origin and evolution of pulsars, their mass transfer and speed-up rates, and even the precise masses of the stars. The scientists analyzed data from 519 radio observations of 47 Tuc assembled over sixteen years. The most shortest period pulsar in the set has a period of only 0.15 days. The longest one is 10.9 days (by the way, both are known to nine decimal places) and has an orbit that is even more circular than the Earth’s — in fact, it is the most circular system ever found in a globular cluster. The astronomers estimate that this binary pulsar probably formed when a neutron star encountered a binary star, captured its companion from the binary, and then began accreting material from it to become a pulsar. (A second, less likely scenario is also possible in which the binary pair formed and also evolved together.) The scientists completed similar analyses for the other three objects. The results, the first in a series of papers on the millisecond pulsars in 47 Tuc, characterize for the first time four of its pulsars including one of its most unusual ones, and provide new insights into how these objects formed and the environmental conditions within a globular cluster.

    Reference(s):

    Long-term Observations of the Pulsars in 47 Tucanae – I. A Study of Four Elusive Binary Systems, A. Ridolfi, P. C. C. Freire, P. Torne, C. O. Heinke, M. van den Berg, C. Jordan, M. Kramer, C. G. Bassa, J. Sarkissian, N. D’Amico, D. Lorimer, F. Camilo, R. N. Manchester and A. Lyne, MNRAS 462, 2918, 2016.

    See the full article here .

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    About CfA

    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. The long relationship between the two organizations, which began when the SAO moved its headquarters to Cambridge in 1955, was formalized by the establishment of a joint center in 1973. The CfA’s history of accomplishments in astronomy and astrophysics is reflected in a wide range of awards and prizes received by individual CfA scientists.

    Today, some 300 Smithsonian and Harvard scientists cooperate in broad programs of astrophysical research supported by Federal appropriations and University funds as well as contracts and grants from government agencies. These scientific investigations, touching on almost all major topics in astronomy, are organized into the following divisions, scientific departments and service groups.

     
  • richardmitnick 3:22 pm on January 11, 2016 Permalink | Reply
    Tags: , , Millisecond pulsars   

    From Astrobites: “Are millisecond pulsars causing excess gamma-rays?” 

    Astrobites bloc

    Astrobites

    Jan 11, 2016
    Kelly Malone

    Paper Title: Title: The Gamma-Ray Luminosity Function of Millisecond Pulsars and Implications for the GeV Excess

    Authors: Dan Hooper – FNAL
    Gopolang Mohlabeng

    1
    Illustration of a millisecond pulsar emitting beams of radiation. As the pulsar rotates, this beam is periodically pointed toward the Earth. Credit: NASA

    The Galactic Center is an exciting area. In addition to the well-known central black hole and the thousands of stars, there is a curious excess of GeV gamma rays. The origin of these gamma rays is currently unknown. This excess gets a fair amount of attention because it has roughly the same characteristics that we would expect from annihilating dark matter (see this past astrobite for more details). One of the other competing explanations is a population of unresolved millisecond pulsars (MSPs), which would be expected to have roughly the same spectral shape as the GeV excess. Millisecond pulsars are rotating neutron stars that emit a beam of radiation with a periods on a millisecond time scale. They probably form when an old neutron star is spun up via accretion of matter onto it from a companion star.

    Previous studies have looked at the millisecond pulsar explanation and concluded that if it were true, the inner portion of the Milky Way should have many more bright MSPs than the Fermi Gamma-Ray Space Telescope has already detected.

    NASA Fermi Telescope
    NASA/Fermi

    However, this conclusion may suffer from a systematic problem with the measurement of distances to many millisecond pulsars. This is because the distances are often estimated using radio dispersion measurements, which in turn rely on models of how electrons are distributed in interstellar space. If you propagate all the resulting uncertainties through, they can end up being fairly significant! If this potential mismeasurement turns out to be a problem, Fermi might not be sensitive to all of the MSPs responsible for the excess at the Galactic Center.

    Hooper and Mohlabeng used a different method to describe the characteristics of the Milky Way MSP population which does not use distances based on the potentially problematic radio dispersion measurements. Instead, they determined the luminosity function (or number of stars as a function of brightness) of the MSPs using the best fit to a model describing the MSP population in the Milky Way that they constructed and observations about gamma-ray emission that come from groups of stars orbiting the Milky Way’s center. The parameters in the model were constrained using the few MSPs that have distances obtained via the motion of the stars over a long period of time, which is more accurate. They also looked at three variables that effect the luminosity of a given MSP: the magnetic field, the period of rotation, and the gamma-ray efficiency.

    Temp 2
    This figure shows how the number of millisecond pulsars changes as a function of luminosity. The black line is the luminosity function from this paper. The red line is from a previous study that uses the uncertain distance calculation. They disagree greatly about the number of very bright MSPs, but mostly agree at lower luminosities. (Source: the paper)

    The authors used their resulting luminosity function and the probability that a given MSP would be detected by Fermi to estimate how many MSP candidates Fermi should have already detected, and came to the conclusion that Fermi still should have detected significantly more MSPs than it has. However, they are not ruling out the millisecond pulsar theory completely. It is quite possible that MSPs near the Galactic Center are less luminous than those found elsewhere, such as in the Galactic Plane or in globular clusters. One theory has MSPs accumulating in the central stellar cluster/Galactic bulge as the result of the groups of stars getting too close to the central black hole. This would lead to older MSPs in the Galactic center than elsewhere. Since pulsars lose rotational kinetic energy with age, this would lead to lower luminosity MSPs. More studies are needed to get to the bottom of this mystery. Further constraints on the MSP population would give weight to the annihilating dark matter explanation for the GeV excess.

    See the full article here .

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