Neutron Star
New insights into neutron star matter
Near-impossibly massive neutron star detected.
‘Alien’ radio signals are coming from a neutron star in a galaxy far, far away
Cosmic radio waves which have puzzled astronomers for more than a decade, and even led to speculation that they were made by aliens, are probably produced by a huge neutron star, scientists have said.
Telescopes first picked up Fast Radio Bursts in 2001 and they have been recorded hitting Earth in a regular pattern ever since.
They last just a few milliseconds but seem to carry as much energy as the Sun releases in a month, suggesting they must come from a huge source of power.
The bursts were probably made by huge neutron stars called magnetars
Although pulsars are known to emit bursts of radio waves, they do not do so regularly or with anything like the power of FRBs. The mystery signals led to speculation that they could be the first hint of an advanced alien civilisation.
Now scientists have traced the origin of the waves to a dwarf galaxy more than three billion light years from Earth, where a powerful neutron star called a magnetar could have formed.
They found the signal’s home by focussing on one recurring burst which had been detected at the Arecibo Observatory in Puerto Rico and managed to pick up the mystery bust nine more times over a six month period in 2016.
<img src=”/content/dam/science/2017/01/04/capture51-small_trans_NvBQzQNjv4BqRf-JwjkUI04RQsPVbslsYMmbaZFXdNrb1l4HX9Zj3vs.png” alt=” Arecibo Observatory in Puerto Rico” width=”320″ height=”199″ class=”responsive-image–fallback”/> 
By tracing the orientation of the bursts their origin was narrowed down to a region about 100 light years in diameter.
Deep imaging of that region by the Gemini North Telescope in Hawaii turned up an faint dwarf galaxy which was discovered to also emit low-level radio waves.
Such galaxies are thought to hold massive, highly magnetic and rapidly rotating neutron stars called magnetars, which could hold enough energy to emit huge solar flares.
It means that the bursts picked up by telescopes erupted from the star before the first complex lifeforms appeared, and have been travelling ever since.
“We are the first to show that this is a cosmological phenomenon. It’s not something in our backyard. And we are the first to see where this thing is happening, in this little galaxy, which I think is a surprise,” Dr Casey Law said.
“Now our objective is to figure out why that happens.”
<img src=”/content/dam/science/2017/01/04/130104_web-small_trans_NvBQzQNjv4BqBSHGRGHg_oZk_Ec3dgP_Qor4OrsmXGYYl75LDT78nrM.jpg” alt=”Fast Radio Bursts have been picked up by telescopes across the world since 2001 ” width=”320″ height=”199″ class=”responsive-image–fallback”/> 
Dr Law, team leader Dr Shami Chatterjee of Cornell University and other astronomers on the team will present their findings today at the American Astronomical Society meeting in Grapevine, Texas. The work is also published in the scientific journal Nature and Astrophysical Journal Letters.
Other theories suggest the signals are coming from material which is being jettisoned from the region surrounding a supermassive black hole.
“Finding the host galaxy of this FRB, and its distance, is a big step forward, but we still have much more to do before we fully understand what these things are,” Dr Chatterjee said.
Space-Time Warp Measured With Aid Of Vanishing Neutron Star
It’s not easy to weigh a star, but an international team of astronomers has done just that.
In fact, they’ve measured the masses of both stars in an odd binary star system some 25,000 light-years from Earth–and gauged the space-time warp resulting from the system’s intense gravitation.
“Our result is important because weighing stars while they freely float through spaceis exceedingly difficult,” Dr. Joeri van Leeuwen, a University of Amsterdam astrophysicist and the leader of the team, said in a written statement. “That is a problem because such mass measurements are required for precisely understanding gravity, the force that is intimately linked to the behavior of space and time on all scales in our universe.”
The binary system under study is known to astronomers as J1906. It features a fast-spinning neutron star, or pulsar, in orbit around another star that is believed to be either another neutron star or a white dwarf. Neutron stars are the smallest, densest stars known to exist. Each of the stars in the system is more massive than our sun, and they are 100 times nearer to each other than the Earth is to the sun.
To gauge the pulsar’s mass and measure the warping of space within the system, the team tracked the pulsar’s rotations using observations from the Arecibo Observatoryin Puerto Rico (where the original observations were made) and four other radio telescopes around the world.
The measurements showed that the pulsar’s mass is about 1.29 times the mass of the sun, Dr. Ingrid Stairs, a professor of physics and astronomy at The University of British Columbia in Vancouver, told The Huffington Post in an email. Its companion star is about 1.32 times as massive as the sun.
The extreme gravity within the system causes a wobble in the axis of the pulsar’s spin (see video above), meaning the portion of the pulsar’s emission that we are able to see changes over time.
“We have observed this, and in fact it turns out that we are starting to get close to the edge of the emission region, so that the pulsar is getting fainter and fainter,” Stairs told The Huffington Post in an email. “We were lucky to catch it before it disappeared.”
But the pulsar isn’t gone forever.
“This cosmic spinning top is expected to wobble back into view,” van Leeuwen said in the statement, “but it might take as long as 160 years.”
Watch the video. URL: https://www.youtube.com/watch?feature=player_embedded&v=IgihQG8t0kI
New neutrino cooling theory changes understanding of neutron stars’ surfaces.
Massive X-ray superbursts near the surface of neutron stars are providing a unique window into the operation of fundamental forces of nature under extreme conditions.
A small, dense object only 12 miles in diameter is responsible for this beautiful X-ray nebula that spans 150 light years. At the center of this image made by NASA’s Chandra X-ray Observatory is a very young and powerful pulsar, known as PSR B1509-58 (B1509). The pulsar is a rapidly spinning neutron star which is spewing energy out into the space around it to create complex and intriguing structures, including one that resembles a large cosmic hand.
NASA
Massive X-ray superbursts near the surface of neutron stars are providing a unique window into the operation of fundamental forces of nature under extreme conditions.“Scientists are intrigued by what exactly powers these massive explosions, and understanding this would yield important insights about the fundamental forces in nature, especially on the astronomical–cosmological scale,” said Peter Moller of Los Alamos National Laboratory’s Theoretical Division.A neutron star is created during the death of a giant star more massive than the Sun, compressed to a tiny size but with gravitational fields exceeded only by those of black holes. And in the intense neutron-rich environment, nuclear reactions cause strong explosions that manifest themselves as X-ray bursts and the X-ray superbursts that are more rare and 1,000 times more powerful.The importance of discovering an unknown energy source of titanic magnitude in the outermost layers of accreting neutron star surfaces is heightened by the unresolved issue of neutrino masses, the recent discovery of the Higgs boson, and the fact that highly neutron-rich nuclei with low-lying states enable “weak interactions,” prominent in stellar explosions. The weak nuclear force is one of four fundamental sources, such as gravity, that interact with the neutrinos; it is responsible for some types of radioactive decay.These hitherto celestially operative nuclei are expected to be within the experimental reach of the Facility for Rare Isotope Beams (FRIB), a proposed user facility at Michigan State University (MSU) funded by the U.S. Department of Energy Office of Science.
Previously, a common assumption was that that the energy released in these radioactive decays would power the X-ray superburst explosions. This was based on simple models of nuclear beta decay, sometimes postulating the same decay properties for all nuclei. It turns out, however, that it is of crucial importance to develop computer models that realistically describe the shape of each individual nuclide since they are not all spherical.
At Los Alamos, scientists have carried out detailed calculations of the specific individual beta-decay properties of thousands of nuclides, all with different decay properties, and created databases with these calculated properties.
The databases are then used at MSU as input into models that trace the decay pathways with the passage of time in accreting neutron stars and compute the total energy that is released in these reactions.
The new unexpected result is that so much energy escapes by neutrino emission that the remaining energy released in the beta decays is not sufficient to ignite the X-ray superbursts that are observed. Thus the superbursts’ origin has now become a puzzle.
Solving the puzzle will require that scientists calculate in detail the consequences of shapes of neutron-rich nuclei, the authors said, and it requires that they simultaneously analyze the role played by neutrinos in neutron star X-ray bursts whose energetic magnitudes are exceeded only by explosions in the nova–supernova class.
The strong nuclear deformations that formed the basis for the neutrino cooling in neutron star crusts also play a role in a number of astrophysical settings and have been taken into account in studies of supernova explosions and subsequent collapses.
Gamma Ray Bursts and The Fireball Model.
Gamma-Ray Bursts (GRBs) are some of the most energetic events in the universe. The energy that is released during a GRB is impressively high (the most powerful bursts can eject energy equal to over 9000 supernovae). These energy levels are so extreme that they cannot be created by thermal processes. So, what causes these high energy levels?
The Fireball Model is one of the few models that has been put forth to explain why GRBs tend to have such high energy levels. It also attempts to explain the time scales that govern them and why they generate an afterglow. More importantly, the model helps answer pressing questions about GRBs, like why they are so variable (liable to change) over short time scales. Ultimately, it seems that this variability is directly related to the high energy levels, as the variability indicates that it occurs over a very small area (with the emission of a GRB being on the order of 10^52 ergs, coming from a very small area, it was then theorized that a Lorentz Factor of ~100 much be associated with the GRB).
The fireball model uses two different shock wave models to explain both the initial burst of gamma-rays and the extended afterglow that is detected after the GRB. To understand the fireball model, the data must be considered in its separate parts. First, there is the energy output. It can have a range of several orders of magnitude — from 10^49 all the way through to 10^54 ergs. Second, there is the burst duration, which can be as short as a few milliseconds and as long as several hours. It took many years before physicists were able to get close to determining exactly how GRBs operate, as many different theories were proposed, but they all struggled to explain all of the different characteristics that are observed between the different types of GRBs. In short, the fireball model must be able to encompass all of these variables in order to apply to all GRBs (and thus be a plausible model). Fortunately, this is something the model has excelled at throughout the years.
The name of the fireball model suggests the mechanism to which a GRB occurs — in a fireball of ultra-relativistic energy consisting of optically thin material with very few baryons. In essence, during the GRB event, the inner engine remains undetectable due to the optical thickness and the lack of a thermal profile due to the compactness of the inner engine. The internal shocks cause the detectable GRB, and the external shocks form the gradual afterglow.
Mechanisms of the Inner Engine:
The inner engine is of great importance, as it needs to be able to push material out very near the speed of light. The inner engine of a GRB is a highly compact source, and it is the highly compact nature of this object that leads to the idea that the core of the inner engine of a GRB is either a neutron star or a black hole (as they’re the two most compact sources that we’re currently aware of).
Internal Shocks:
The internal shocks are the mechanism for the production of the observed highly energetic gamma-rays. Moments after the initial GRB event, shock waves emanate from the inner engine at relativistic speeds [99.995% of the speed of light at a Lorentz factor of ~100]. The fireball is dynamic; it isn’t just one shock wave emanating from the compact source. Instead, different shock waves will be traveling at different relativistic speeds, and it is the interaction between these different shock fronts that cause the energetic gamma-ray emissions.
The internal shocks traveling at relativistic speeds convert kinetic energy into gamma-ray photons, this is the only way to get the high energy gamma-rays that are observed (as previously mentioned, they cannot be emitted through a thermal process). When the internal shocks interact with each other as they are moving at different relativistic speeds, the interactions produce Inverse Compton and Synchrotron emission.
Initially, the fireball is optically thick but as it expands and cools it becomes optically thin, allowing the gamma-ray photons to escape. Early models had the fireball and the internal shock waves as being purely radiative, but this didn’t follow what was being observed (it would have made a profile too smooth). To solve this problem, some baryonic mass was added. This allowed for the internal shocks to become effectively contaminated. The added baryonic mass also aids in the conversion of some radiation energy into kinetic energy, which helps with an added kick to the relativistic kinetic energy of the shock waves, this in turn increases the gamma-ray energy more.
Even if all of the shock waves emanate from the core at the same speed they will eventually cross over multiple times. As the shells are emitting through inverse Compton, it is slowing the shock front, thus increasing the times that many shock waves interact with one another. The earlier shock waves are likely to be emitted slower than the later emitted shock waves, this would also increase the amount of interactivity between the different shock waves.
External Shocks:
The external shock waves are used to explain the afterglow that was first detected by BeppoSAX in 1997, as the internal shock waves are not able to explain the duration of the afterglow nor the wavelengths that are detected (which range from soft x-ray through to radio). The name can be a little misleading at first; the external waves actually refer to the internal waves at a later stage –once they’ve cooled down and continue emanating from the source. As the shock waves continue out they will eventually interact with the Interstellar Medium [ISM] (such as a molecular cloud or some other impedance), and it is the shock waves’ interaction with the dust/gas that cause the afterglow. Unlike the internal shocks, the external shocks are primarily a thermal emission. The energy transferred from the shock waves is deposited into the ISM; this material can then be caught up in the shock front and emit radiation. As the shock waves began with a lot of energy, there is a lot that can be deposited into the ISM, this is what can cause such long afterglow and why it covers all parts of the energy spectrum.
Although it would be correct to assume that all GRBs have an external shock, about half of detected GRBs don’t have a detectable afterglow. The reason that no afterglow is being detected is not thought to be because the exposures aren’t long enough, or because we’re observing too early or too late. Rather, GRBs occur in high mass systems, whether it be through a supernova or NS-NS and NS-BH merges, this means that they’ve had very short stellar lives and may still be inside of a molecular cloud. Molecular clouds are very optically thick environments so the reason we’re not able to detect the afterglow in about 50% of the time could just be due to reddening, absorption, or scattering.
Over the decades, many theoretical models have been created to explain these deeply mysterious events. The Fireball model has been around for a long time, has been revised over and over again in an attempt to explain GRBs. It is a neat model, for a more detailed explanation I would recommend reading an article published in 1999 by one of the men who has been at the forefront of GRB astronomy for decades.
NASA’S Swift Reveals New Phenomenon in a Neutron Star.
Astronomers using NASA’s Swift X-ray Telescope have observed a spinning neutron star suddenly slowing down, yielding clues they can use to understand these extremely dense objects.
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. A neutron star can spin as fast as 43,000 times per minute and boast a magnetic field a trillion times stronger than Earth’s. Matter within a neutron star is so dense a teaspoonful would weigh about a billion tons on Earth.
This neutron star, 1E 2259+586, is located about 10,000 light-years away toward the constellation Cassiopeia. It is one of about two dozen neutron stars called magnetars, which have very powerful magnetic fields and occasionally produce high-energy explosions or pulses.
Observations of X-ray pulses from 1E 2259+586 from July 2011 through mid-April 2012 indicated the magnetar’s rotation was gradually slowing from once every seven seconds, or about eight revolutions per minute. On April 28, 2012, data showed the spin rate had decreased abruptly, by 2.2 millionths of a second, and the magnetar was spinning down at a faster rate.
“Astronomers have witnessed hundreds of events, called glitches, associated with sudden increases in the spin of neutron stars, but this sudden spin-down caught us off guard,” said Victoria Kaspi, a professor of physics at McGill University in Montreal. She leads a team that uses Swift to monitor magnetars routinely.
Astronomers dubbed the event an “anti-glitch,” said co-author Neil Gehrels, principal investigator of the Swift mission at NASA’s Goddard Space Flight Center in Greenbelt, Md. “It affected the magnetar in exactly the opposite manner of every other clearly identified glitch seen in neutron stars.”
The discovery has important implications for understanding the extreme physical conditions present within neutron stars, where matter becomes squeezed to densities several times greater than an atomic nucleus. No laboratory on Earth can duplicate these conditions.
A report on the findings appears in the May 30 edition of the journal Nature.
The internal structure of neutron stars is a long-standing puzzle. Current theory maintains a neutron star has a crust made up of electrons and ions; an interior containing oddities that include a neutron superfluid, which is a bizarre state of matter without friction; and a surface that accelerates streams of high-energy particles through the star’s intense magnetic field.
The streaming particles drain energy from the crust. The crust spins down, but the fluid interior resists being slowed. The crust fractures under the strain. When this happens, a glitch occurs. There is an X-ray outburst and the star gets a speedup kick from the faster-spinning interior.
Processes that lead to a sudden rotational slowdown constitute a new theoretical challenge.
On April 21, 2012, just a week before Swift observed the anti-glitch, 1E 2259+586 produced a brief, but intense X-ray burst detected by the Gamma-ray Burst Monitor aboard NASA’s Fermi Gamma-ray Space Telescope. The scientists think this 36-millisecond eruption of high-energy light likely signaled the changes that drove the magnetar’s slowdown.
“What is really remarkable about this event is the combination of the magnetar’s abrupt slowdown, the X-ray outburst, and the fact we now observe the star spinning down at a faster rate than before,” said lead author Robert Archibald, a graduate student at McGill.
Goddard manages Swift, which was launched in November 2004. The telescope is operated in collaboration with Pennsylvania State University in University Park, Pa., the Los Alamos National Laboratory in New Mexico and Orbital Sciences Corp. in Dulles, Va. International collaborators are in the United Kingdom and Italy, and the mission includes contributions from Germany and Japan.
Source: NASA
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