From University of Arizona: “Pulses from a Dead Star, Little Green Men and a Historic Discovery”

John Cocke (Photo courtesy of Nathaniel Johnston/njohnstonphotography.com)

Feb. 8, 2019
Daniel Stolte

In January 1969, only months before Neil Armstrong would step onto the moon, three UA scientists were the first to detect the optical flash from a pulsar — a stellar corpse thought to pack at least one-and-a-half times the mass of our sun into a city-sized, fast-spinning neutron star.

Fifty years ago, a team of three undeterred University of Arizona astrophysicists huddled around a 36-inch telescope inside the dome of the UA’s observatory on top of Kitt Peak.

U Arizona Steward Observatory at Kitt Peak, AZ, USA, altitude 2,096 m (6,877 ft)

With cobbled-together electronic equipment, W. John Cocke, Mike Disney and Don Taylor and made a historic discovery: the first detection of light flashes coming from a pulsar, a fast-spinning neutron star.

On Jan. 15, a public lecture at the UA’s Steward Observatory recounted the discovery. In the audience was none other than Cocke, a member of the original team and now professor emeritus. Cocke spoke to UANews about those few days in January 1969 that spawned a new field in astrophysics: the science of pulsars, ultra-dense corpses of formerly massive stars whose bizarre nature is only surpassed by black holes.

Located about 6,500 light-years from Earth in the constellation Taurus, the Crab Nebula is still expanding at a rate of more than 600 miles per second. (Image: Adam Block/UA Mt. Lemmon Sky Center)

U Arizona Catalina Sky Survey, on Mount Lemmon, AR, USA, 9,171 ft (2,795 m)

What are pulsars, and what can they tell us about the universe?
Cocke: Pulsars are rotating neutron stars, which are the cores of exploded stars. A pulsar essentially is a rotating magnet, which generates an electric field, and these things are spinning so rapidly that the electric field is sucking material out from the surface of the star. That generates the high-intensity emission in radio, optical and ultraviolet wavelengths, even gamma rays. Out of each magnetic pole comes a continuous beam of electromagnetic emission as the thing rotates around. In order for us to see pulsars, their magnetic field axis has to be offset from the rotation axis, so their beam sweeps around like a lighthouse. As the beam sweeps past the Earth, we can see right down the pole just briefly as it flashes past, creating the sensation of seeing a pulse.

In a way, pulsars teach us only about the very violent fates of the few stars that are massive enough to blow themselves completely to smithereens or collapse into a neutron star and finally a black hole. Most stars are going to die very, very slowly, and as the universe continues to expand, everything gets cooler and cooler, and the universe then dies, as T.S. Eliot would say, “not with a bang, but a whimper.”

When your boss, Steward Observatory Director Bart Bok, learned about your observations and what you found, he was “horrified.” Why?
Cocke: Because only one of us (Taylor) had experience with telescopes and instrumentation. Mike Disney and I were theorists, and the whole thing was so improbable, you see, that nobody, including us, thought that we would actually find something. A month before, I had asked a number of pundits at the American Astronomical Society Meeting whether it was a good idea or just a waste of time to look for this thing in the middle of the Crab Nebula, and they said, “Don’t bother, it won’t pan out.”

John Cocke next to the 21-inch telescope at Steward Observatory on the UA campus. (Photo courtesy of Nathaniel Johnston/njohnstonphotography.com)

How did the project come about?
Cocke: The first radio signals from what we now know are pulsars were detected by by Jocelyn Bell and Antony Hewish in the autumn of 1967.

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.

Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

At that point, radio astronomers were really concerned how something as massive as a star could emit pulses that were only a second or a second and a half long. The first joke that came out was that these were radio signals from advanced civilizations, which became known as the LGM, the “Little Green Men” idea. Of course, nobody really believed that except people wearing tin foil helmets.

At first the signals were believed to come from white dwarfs (burned-out stars similar to our sun) as they expanded and contracted. But then a very fast pulsating star was discovered in the Vela constellation in the Southern Hemisphere emitting pulses lasting one-tenth of a second, and that blew the white-dwarf theory right out of the water.

In early November 1968, radio astronomers discovered this thing associated with the Crab Nebula that emitted about 30 pulses per second. At that point, everybody understood that they had to be neutron stars, and I had wondered about looking for optical counterparts of these things for a few months before. This pulsar, then, that was located rather near the Crab Nebula made me think of a very peculiar star in the middle of the Crab Nebula named for its discoverer, Swiss astronomer Walter Baade. He recognized that star was very peculiar and may be the collapsed remnant of the supernova explosion that had created the nebula itself. It is emitting a lot of light and even shows up on old photographic plates taken of the Crab Nebula. Mike Disney and I then teamed together, once we realized we were both theoreticians interested in gaining some experience doing observing. He suggested we cobble together some instrumentation that would allow us to do this.

How did you go about making the first optical observations of a neutron star?
Cocke: We were looking into a pretty broad spectrum in the visible light spectrum, and we knew that any optical signal coming through the 36-inch telescope from Baade’s star would be pretty faint. We weren’t really sure what was needed, except that we needed something that allowed us to build up signals in a computer synced to the pulsar itself, so we could gather up enough signal with overlapping pulses coming in that we could build up a detection out of the noise.

Interestingly, there was a report in the 1950s about an experienced pilot who looked at Baade’s star during a public telescope viewing and remarked that it appeared to flash, but her observation was dismissed. However, we did not know this at the time. We knew there were other groups of astronomers looking at pulsars with slower signal frequency, and they were not having success. We attached a photometer to the telescope and connected that to an off-the-shelf device called CAT – computer of average transients – which had a total memory of 400 bytes and could build up a signal above the noise so you could actually see something interesting. All of this instrumentation was put together properly by Don Taylor, and he became the third member of the team.

Can you tell us about the night of the discovery?
Cocke: The first two nights were clear but wasted because I had made a mistake in calculating the Doppler shift due to Earth’s motion through space. A few cloudy nights followed, and we ran out of our allocated observing time. But it turned out that our colleague Bill Tifft was able to give us some of his observing nights because he had to take care of a family emergency. On January 15, within a few minutes of observing, we could see the pulse as it built up on the screen. We moved the telescope off Baade’s star to a nearby star or just a blank spot to see whether or not the signal would still come through like that, and it didn’t. Then we’d move it back on the pulsar but change the frequency setting so it was off, and we didn’t see that signal, so that was a good check. We then rechecked everything and did another round on the proper position and at the proper period, and the pulse would come up again. These were all checks we had to run to make sure this thing was real. On our screen, we saw a big main pulse and a smaller, secondary pulse – the exact pattern we expected from what the radio pulses look like. That was the final clincher.

Are there any practical applications for pulsar science?
Cocke: No. (pauses) Sorry about that.

See the full article here .

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From Washington University in St.Louis: “Reaching for neutron stars”

Washington University in St.Louis

November 29, 2017.
Chuck Finder
chuck.finder@wustl.edu

Crab Nebula in the constellation Taurus contains a pulsar at its core that is a younger neutron star, the very type brought into clearer focus by a Physics Review Letters study by researchers at Washington University in St. Louis. Elements of this image are furnished by NASA. (Photo: Shutterstock)

For more than a decade, a cross-disciplinary team of chemists and physicists in Arts & Sciences at Washington University in St. Louis has been chasing the atomic nucleus. With progressive studies, they moved up the element chain to Calcium-48, an extremely rare solid commodity that has more neutrons than protons and, as such, carries a hefty price tag of $100,000 per gram.

It is a quirky material, with this particular study taking Washington University chemists Robert J. Charity and Lee G. Sobotka from Duke’s Triangle Universities Nuclear Laboratory to the Department of Energy’s Los Alamos (N.M.) National Laboratory.

“If you leave it on a table, it turns to powder,” said co-author Charity, a research professor of chemistry in Arts & Sciences. “Calcium oxidizes very quickly in air. It was a worry.”

Ultimately, three grams of Ca-48 helped to produce a double-edged finding for Charity and co-author Willem H. Dickhoff, professor of physics. Their team discovered both a framework to predict where neutrons will inhabit a nucleus and a way to predict the skin thickness of a nucleus.

In their research published Nov. 29 in Physics Review Letters, they predicted how the neutrons would create a thick skin, and that this skin of Ca-48 — 3.5 femtometers (fm) in radius — measured 0.249 + 0.023 fm.

To convert that into centimeters, it would measure 2.49×10^-14 cm. The researchers say the key finding is that the skin is thicker and more neutron-rich than previously believed.

“That links us to astrophysics and, in particular, neutron-star physics,” Dickhoff said of the research results. “The Los Alamos experiment was critical for the analysis we pursued. In the end — because it has this additional set of neutrons — it gets us to information that helps us to further clarify the physics of neutron stars, where there are many more neutrons relative to protons.

“And it gives us the opportunity to predict where the neutrons are in Ca-48,” Dickhoff said. “That is the critical information, which leads to the prediction of the neutron skin.”

For Charity, Dickhoff and co-authors Hossein Mahzoon, PhD ’15, a lecturer in physics at Truman State University in Kirksville, Mo., and Mack Atkinson, a PhD candidate in physics at Washington University, the chase continues.

They watch with interest as Ca-48 is scheduled to undergo the cleanest skin-thickness test available via the electron accelerator at the Thomas Jefferson National Accelerator Facility in Newport News, Va.

Moreover, they proceed to move up the element chain of neutron-rich nuclei to what Charity called the “famous nucleus” of Lead-208. Michael Keim, a senior in physics, is spearheading a study of Lead-208.

This graph basically shows where the protons are (more solid lines exp and ch ) and where the neutrons are (dotted lines n and w) in the nucleus. The neutrons are located in the thick skin, where the dotted lines separate from the solids. To be precise, the experimental (maroon staggered line) and fitted (black) charge distribution are the solids and the neutron matter distribution (blue) and the weak charge distribution (red hashes) are the dotted lines. (Graphic: the authors)

“It will give us an experimental handle on whether our analysis is really predictive,” Dickhoff said. “We think we have a good argument why we think it has a thick skin. There is a large group of people … who predict a smaller skin. This is directly relevant for the understanding of the size of neutron stars. It is not yet crystal clear how big a neutron star is — its radius.”

How they made their analysis and reached this predictive framework is part of their decade-long pursuit as well. Their chemistry-physics group subscribes to “dispersion relations,” which Sobotka, who is a professor of chemistry and of physics, explained simply: “It’s what tells you not to laugh before you are tickled. That means causality is properly taken into account.”

In short, they analyze all energies simultaneously rather than focusing on one single energy.

Since first publishing together in 2006, they have used the dispersive optical model (DOM) developed a quarter-century ago by Claude Mahauxa, a nuclear theorist from Belgium. They expanded upon it — across energy domains and isotopes — so they could attempt to predict where the nuclear particles are.

“When you put extra neutrons in, it doesn’t like that, right?” Charity said of the atomic nucleus. “It has to figure out how to accommodate these extra neutrons. It can put them evenly throughout the nucleus. Or it could put them on the surface. So the question is: Is this force stronger in the low density region of the nucleus or weaker?”

“We know where the protons are,” Dickhoff added. “That is well established experimentally. But you can’t do that easily with neutrons. I simply want to know what a nucleon, a proton or a neutron, is doing. How is it spending its time? Nucleons are more interactive — they do other things than sit quietly in their orbits. That’s what this method can sort of tell us.”

Their nonlocal DOM framework — a decade-plus in the making — uses computer modeling and computations as well as the lab experimentation. It allows them to “make a prediction that is well founded and taken seriously,” Dickhoff said. “Next, we will have a measurement for Lead-208.”

This study was funded by the U.S. Department of Energy, Division of Nuclear Physics grant DE-FG02-87ER-40316, and National Science Foundation grants PHY-1304242, PHY-1613362 and PHY-1520971.

See the full article here .

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From H.E.S.S.: “The Crab Nebula is Extended”

H.E.S.S.

October 2017

The Crab Nebula is one of the best-studied astrophysical objects of all time and shines across the whole accessible electromagnetic spectrum, from radio wavelengths up to very high energy (VHE, E > 100 GeV*) gamma-rays ([1], [2]).

Supernova remnant Crab nebula. NASA/ESA Hubble

X-ray picture of Crab pulsar, taken by Chandra

It is powered by the Crab Pulsar, a fast-rotating neutron star (period P = 33 ms) possessing an ultra-strong magnetic field. It was created during a supernova explosion seen in the constellation Taurus in 1054 A.D. A part of the rotational energy of the Crab Pulsar is converted into electron-positron pairs, forming a pulsar wind. The electrons and positrons are shocked and accelerated to ultra-relativistic energies at the wind termination shock. These high-energy particles propagate outwards and lose energy via the emission of synchrotron and Inverse Compton (IC) radiation resulting from interactions with magnetic and photon fields, respectively. The synchrotron radiation ranges from the radio regime up to around 1 GeV, and its emissivity depends on the density of high-energy particles and the strength of the magnetic field. Thus, the spatial extent of the synchrotron emission is determined by a convolution of the local electron distributions with the magnetic field, where the latter is expected to vary significantly through the nebula. The second important emission mechanism is IC scattering: Energetic electrons and positrons can transfer a fraction of their energy onto photons thereby transforming these to high-energy and VHE gamma-rays. In the case of the Crab Nebula, the dominant target photon field is the synchrotron emission of the nebula itself, generated by the same particle population. This photon field is expected to be more homogeneous than the magnetic field. Thus the IC emission can be taken as a much more reliable tracer of the distribution of relativistic electrons, revealing more about the underlying physics of one of the Milky Way’s most prominent and interesting particle accelerators.

Fig 1: Top: Distribution of gamma-ray candidates from the Crab Nebula (Data ON) together with background events (Data OFF) as a function of the squared distance to the source position. The simulated PSF and the PSF convolved with the best-fit Gaussian are shown for comparison as well. Bottom: Significance of the bin-wise deviation (MC – Data) of the measured events when compared to the PSF (black) and the convolved one (orange).

Until now, the morphology of the Crab Nebula has only been resolved with radio, optical, and X-ray telescopes, up to photon energies of around 80 keV [1,3]; at higher energies, no extension could be measured mainly due to the worse angular resolution of the corresponding instruments. For telescopes like H.E.S.S., the expected size of the Crab Nebula is several times smaller than the point spread function (PSF**). In such a case, the intrinsic extension of the source only leads to a slight broadening of the signal as compared to the PSF, which itself however strongly depends on the actual observation and instrument conditions. For the first time, we employ simulations that take into account all these conditions and thus lead to a considerably improved PSF description [4]. This allows to probe source extensions well below one arcminute***, which corresponds to a new level of resolving source sizes in VHE gamma-ray astronomy.

Here we used 25.7 hours of high-quality observations of the Crab Nebula, taken with all four of the small telescopes of H.E.S.S. The analysis settings were chosen to achieve a good PSF; for this source and observation conditions, 68% of the gamma-rays from a point source are reconstructed within 0.05° (3 arcminutes) of the source direction.

The distribution of events from the Crab Nebula as a function of the (squared) distance to the source is shown in Figure 1 (blue crosses). For comparison, the PSF is shown as well (black) and is obviously highly inconsistent with the data, where the probability for consistency of the two distributions amounts to merely around 10-14. The broadening of the data distribution with respect to the PSF can only be explained when assuming an intrinsic source extension. The PSF was iteratively convolved with a Gaussian source model of different width. The extension is obtained by comparing the compatibility of the data and the convolved PSF each time, and the best-fit extension is found to be σCrab = 52.2” ± 2.9” ± 7.8”sys. This Gaussian width σCrab corresponds to 39% source containment. The size of the Crab Nebula in gamma-rays has been measured for the first time and can now be put in context to its morphology seen at other wavelengths.

Fig 2: Extension of the Crab Nebula as seen with H.E.S.S. (solid white circles, corresponding to Gaussian width), overplotted on the UV (top) and X-ray (bottom) image. The bright dot in the middle corresponds to emission from the Crab Pulsar, whereas the inner ring around the pulsar that is visible in the X-ray image is supposed to be related to the wind termination shock ([5], [6]). For illustration purposes, the VHE extension circle is centered on the pulsar position.

Our resulting Gaussian width is overplotted on images of the Crab Nebula at UV wavelengths (λ = 291 nm) and X-ray energies (0.1 – 10 keV) on the top and bottom of Figure 2, respectively. While the gamma-ray extension is obviously small compared to the optical/UV size, it is the other way around when comparing the gamma-ray to X-ray extension. This can be understood when considering the energetics of the electron population responsible for the respective emission.

The X-ray emission of the Crab Nebula is confined to a smaller region than the UV emission, because the latter is mainly from electrons with E ~ 1 TeV, whereas the former, the X-ray emission, is mainly from electrons with larger energies (~ 10 TeV). Since electrons lose energy more efficiently at higher energies, a shrinking of the pulsar wind nebula with increasing energy is indeed expected. Until now, the window between UV and X-ray energies and the corresponding morphology has never been constrained observationally. Now with our new H.E.S.S. measurement, we are closing that gap by measuring the extension of the IC emission of the Crab Nebula in the range where electrons with energies of several TeV dominate. This naturally explains why the size we obtain lies in between the UV and X-ray extension.

Resolving the extension of the Crab Nebula with the H.E.S.S. array is a milestone in the study of the gamma-ray sky with Cherenkov telescopes, and demonstrates the power of simulations to characterize the instrument performance, with potential applications for the upcoming CTA era.

(*) 1 GeV = 109 eV and one eV (abbreviation of electron-volt) is a unit of energy which, by definition, represents the amount of energy gained by an electron when accelerated by an electric potential difference of 1 volt.

(**) The PSF corresponds to the distribution of reconstructed event directions from a point source. In other words, it describes how a gamma-ray point source appears widened up the instrument.

(***) One arcminute corresponds to 1/60th of 1°. An arcminute (or 1′) can be further subdivided into 60 arcseconds (60”). 1° thus equals 3600”, making it the angular equivalent of an hour.

References [sorry, no links]:

[1] Hester, J. J. The Crab Nebula: An Astrophysical Chimera. ARA&A 46, 127-155 (Sept. 2008)
[2] Bühler, R. & Blandford, R.: The surprising Crab pulsar and its nebula: a review. Reports on Progress in Physics 77, 066901 (June 2014)
[3] Madsen et al.: Broadband X-ray Imaging and Spectroscopy of the Crab Nebula and Pulsar with NuStar. ApJ 801, 66 (March 2015)
[4] Holler et al.: Run-Wise Simulations for Imaging Atmospheric Cherenkov Telescope Arrays. Proceedings of the 35th ICRC, contribution 755 (July 2017)
[5] Weisskopf et al.: Discovery of Spatial and Spectral Structure in the X-ray Emission from the Crab Nebula. ApJ Letter 536, 81-84 (June 2000)
[6] Gaensler, B. M. & Slane, P. O.: The Evolution and Structure of Pulsar Wind Nebulae. ARA&A 44, 17-47 (Sept. 2006)

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The High Energy Stereoscopic System

H.E.S.S. is a system of Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma rays in the energy range from 10s of GeV to 10s of TeV. The name H.E.S.S. stands for High Energy Stereoscopic System, and is also intended to pay homage to Victor Hess , who received the Nobel Prize in Physics in 1936 for his discovery of cosmic radiation. The instrument allows scientists to explore gamma-ray sources with intensities at a level of a few thousandths of the flux of the Crab nebula (the brightest steady source of gamma rays in the sky). H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality. The first of the four telescopes of Phase I of the H.E.S.S. project went into operation in Summer 2002; all four were operational in December 2003, and were officially inaugurated on September 28, 2004. A much larger fifth telescope – H.E.S.S. II – is operational since July 2012, extending the energy coverage towards lower energies and further improving sensitivity.

Crab nebula

From U Arizona: “Stellar Corpse Sheds Light on Cosmic Rays”

University of Arizona

This composite image of the Crab Nebula was assembled with arbitrary color scaling by combining data from five telescopes spanning nearly the entire electromagnetic spectrum. (Image credits: NASA, ESA, NRAO/AUI/NSF and G. Dubner/University of Buenos Aires)

Sept. 4, 2017
Daniel Stolte

New research revealed that the entire zoo of electromagnetic radiation streaming from the Crab Nebula has its origin in one population of electrons and must be produced in a different way than scientists have traditionally thought.

The origin of cosmic rays, high-energy particles from outer space unceasingly impinging on Earth, is among the most challenging open questions in astrophysics.

Discovered more than 100 years ago and considered a potential health risk to airplane crews and astronauts, cosmic rays are believed to be produced by shock waves — for example, those resulting from supernovae explosions. The most energetic cosmic rays streaking across the universe carry 10 to 100 million times the energy generated by particle colliders such as the Large Hadron Collider at CERN. New research published in the Monthly Notices of the Royal Astronomical Society sheds new light on the origin of those energetic particles.

“The new result represents a significant advance in our understanding of particle acceleration at shock waves, traditionally regarded as the main sources of energetic particles in the universe,” said the study’s lead author, Federico Fraschetti, a staff scientist at the University of Arizona’s Departments of Planetary Sciences and Astronomy.

The Crab Nebula, remnant of a supernova explosion that was observed almost 1,000 years ago, is one of the best studied objects in the history of astronomy and a known source of cosmic rays. It emits radiation across the entire electromagnetic spectrum, from gamma rays, ultraviolet and visible light, to infrared and radio waves.

“Most of what we observe comes from very energetic particles such as electrons that did not yet leave the source,” said Fraschetti. “Since we can only observe the electromagnetic radiation that they emit from the source itself, we rely on models to reproduce the radiation spectrum we see from the nebula.”

The new study, co-authored by Martin Pohl at the University of Potsdam, Germany, revealed that the entire zoo of electromagnetic radiation streaming from the Crab Nebula can arise from a single population of electrons, previously deemed impossible, and that they originate in a different way than scientists have traditionally thought.

According to the generally accepted model, once the particles reach the shock, they bounce back and forth many times due to the magnetic turbulence. During this process they gain energy — in a similar way to a tennis ball being bounced between two rackets that are steadily moving nearer to each other — and are pushed closer and closer to the speed of light. Such a model follows an idea introduced by Italian physicist Enrico Fermi in 1949.

“The current models do not include what happens when the particles reach their highest energy,” said Federico Fraschetti. “Only if we include a different process of acceleration can we explain the entire electromagnetic spectrum we see, and that tells us that while the shock wave still is the source of the acceleration of the particles, the mechanisms must be different.”

At the heart of the Crab Nebula lies a pulsar, a rapidly rotating neutron star originating from the explosion of a star a few times more massive than the sun. When it exploded, the star shredded its outer layers, creating the stunning colorscape that makes the Crab Nebula so popular with professional and amateur astronomers. The pulsar emits a wind of electrons and positrons traveling at what astrophysicists call relativistic speed — close to the speed of light.

“Those particles are the fastest things in the universe,” Fraschetti said. “Anything we experience in our everyday lives is very far from relativistic effects. But these highly energetic particles still need to be accelerated even more to produce the electromagnetic radiation that we see coming from the Crab Nebula.”

That acceleration, scientists believe, happens at a boundary called the termination shock, where the particle wind slams into the cloud of gas and dust that the star blew off into space when it went supernova.

Except that just when the particles become energetic enough to leave the system and become cosmic radiation, they go beyond the limits of the models traditionally used to account for the origin of cosmic radiation, Fraschetti and Pohl found. The authors conclude that a better understanding is needed of how particles are accelerated in cosmic sources, and how the acceleration works when the energy of the particles become very large.

Several NASA missions, including ACE, STEREO and WIND, are dedicated to studying the effects of shocks caused by plasma explosions on the surface of the sun as they travel to Earth.

Scientists hope that results from those experiments may shed light on the mechanisms of acceleration in objects such as the Crab Nebula.

See the full article here .

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Where else in the world can you find an astronomical observatory mirror lab under a football stadium? An entire ecosystem under a glass dome? Visit our campus, just once, and you’ll quickly understand why the UA is a university unlike any other.

From astrobites: “The birth of a new field”

Astrobites

Apr 14, 2017
Kelly Malone

Title: Observation of TeV gamma rays from the Crab Nebula using the Atmospheric Cerenkov Imaging technique
Authors: Weekes et. al
First Author’s Institution: Harvard-Smithsonian Center of Astrophysics

Status: Published in The Astrophysical Journal (1989), [open access]

Today’s paper is historical in nature rather than a current summary – it describes the 1989 paper that essentially birthed the field of ground-based gamma-ray astrophysics by making the first > 5 sigma detection of a TeV gamma-ray source!

A brief history of TeV gamma-ray astronomy

Gamma rays lie on the highest-frequency end of the electromagnetic spectrum and have been observed spanning a few orders of magnitude in energy, starting from a few hundred keV, going through the MeV range, the GeV range, and beyond. The most energetic gamma rays observed to date have been in the TeV range, which is roughly the same energy the proton collisions at the Large Hadron Collider take place at. TeVCat currently lists 198 known TeV gamma-ray sources.

http://tevcat.uchicago.edu/

They are associated with some of the most energetic and violent things in our universe, including supernova explosions and active galactic nuclei.

TeV gamma-ray sources are of particular interest because of their ability to probe phenomena associated with some of the big unsolved problems in astroparticle physics – they are associated with the acceleration sites of charged cosmic rays, but are somewhat easier to study since gamma rays are electrically neutral and don’t curve in magnetic fields on their way to us. This means that they point directly back to their sources. The origins and acceleration sites of charged cosmic rays are still open questions – we know a large portion of the galactic cosmic rays originate in supernova explosions, but don’t know a whole lot else. They can also be used for other science; for example, many current gamma-ray observatories are involved in finding electromagnetic counterparts to gravitational waves.

The Whipple 10 m telescope used to make the observations described in this Astrobite. (Source: Michael Richmond, used under a Creative Commons license)

CfA Whipple Observatory, near Amado, Arizona on the slopes of Mount Hopkins

When a gamma ray hits the Earth’s atmosphere, it interacts with the air molecules and creates what is known as an extensive air shower. This means that it is not possible to directly observe the gamma ray from the Earth and its indirect products must be studied instead. The extensive air shower consists of many electrons and positrons, some of which are traveling faster than the phase velocity of light in air. This leads to the emission of a type of radiation known as Cherenkov radiation. Detecting this radiation is one of the ways that we can indirectly detect gamma rays on the Earth, and many currently running experiments (such as VERITAS, MAGIC, and HESS) have used this technique to great success.

MAGIC Cherenkov gamma ray telescope on the Canary island of La Palma, Spain

HESS Cherenko Array, located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg

A few decades ago, the prospects for detecting gamma rays from the Earth was not so rosy. Techniques to separate the showers caused by gamma rays from those caused by the very large background caused by hadrons were still in their infancy. Experiments were publishing only weak detections (~3 sigma) and contradictory results. Statistical tests that we use today to check the validity of results were not widely used yet. An overview of the field from 1988 stresses that it is likely that some of the “sources” in their list will likely be removed as techniques are refined. (For more information about this time period, see section 2.2 of this history of gamma-ray astronomy, written in 2012.)

A pioneering observation

The spectrum of the Crab Nebula. The “W” is the measurement from this paper, the others are results from earlier experiments, which were less significant detections. The lines are the predicted spectrum for two different values of the magnetic field.

In 1989, everything changed for the field of ground-based gamma-ray astrophysics. That was the year that scientists published a 9 sigma detection of TeV gamma rays from the Crab Nebula, the first unambiguous detection of gamma rays (from any source) at TeV energies.

The data was collected at the Whipple Observatory in Arizona, which had a 10 m reflector outfitted with a 37 pixel camera to detect the Cherenkov radiation described in the preceding section. The 37 phototubes were arranged in a hexagonal pattern and were capable of tracking sources across the sky.

It was the improved gamma/background discrimination that led to the unambiguous detection. After each observation, the data was calibrated, the observed showers parameterized, and then candidate gamma rays were selected. Monte Carlo simulations were used to predict how the camera would respond to gamma-ray initiated showers and hadron-initiated background showers. When the analysis was finished, the Crab Nebula was seen with a significance of 9 sigma above an energy threshold of 0.7 TeV. No variability was observed over the months or years the data was taken over, and it was established that the emission was likely coming from the hard Compton synchrotron spectrum in the Nebula.

The authors close the paper by noting that observing a steady source such as the Crab Nebula is important for the field of TeV gamma ray astronomy, since such a source can be used as a standard candle in calibrating new detectors. In fact, this is still true today. Nearly every gamma-ray experiment starts off their life by publishing a paper with their observations of the Crab Nebula, as it is still the most significant source in the gamma ray sky!

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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.
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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.

From Hubble: “Hubble Captures the Beating Heart of the Crab Nebula”

Hubble

July 7, 2016
No writer credit

Data Description: Data were provided by the HST proposals 9787 and 10526: J. Hester and J. Foy (ASU), and J. Morse (RPI); and proposal 13109: M. Weisskopf, A. Tennant, and C. Wilson-Hodge (NASA/MSFC), J. Arons (UC Berkeley), R. Blandford, R. Buehler, S. Funk, and Y. Uchiyama (Stanford University), P. Caraveo, A. De Luca, and M. Tavani (INAF), E. Costa (CNR), C. Ferrigno (Integral Science Data Center, Switzerland), D. Horns (DESY, Germany), A. Lobanov (Max Planck Institute for Radio Astronomy, Germany), E. Max (Lawrence Livermore National Laboratory), and R. Mignani (University College London).
Instrument: ACS/WFC
Filters: F550M(V) and F606W(V)+POLV60
Exposure Date(s): F550M(V): September 15, 2005, and January 10, 2013
F606W(V)+POLV60: August 8, 2003, and September 6, 2005
About the Release
Image Credit: NASA and ESA
Release Date: July 7, 2016
Color: This image is a composite of separate exposures acquired by the ACS/WFC instrument. Several filters were used to sample various wavelengths. The color results from assigning different hues (colors) to each monochromatic (grayscale) image associated with an individual filter. In this case, the assigned colors represent not only changes in different filters, but also the same filters taken on different exposure dates to highlight features that change over time.

At the center of the Crab Nebula, located in the constellation Taurus, lies a celestial “beating heart” that is an example of extreme physics in space. The tiny object blasts out blistering pulses of radiation 30 times a second with unbelievable clock-like precision. Astronomers soon figured out that it was the crushed core of an exploded star, called a neutron star, which wildly spins like a blender on puree. The burned-out stellar core can do this without flying apart because it is 10 billion times stronger than steel. This incredible density means that the mass of 1.4 suns has been crushed into a solid ball of neutrons no bigger than the width of a large city. This Hubble image captures the region around the neutron star. It is unleashing copious amounts of energy that are pushing on the expanding cloud of debris from the supernova explosion — like an animal rattling its cage. This includes wave-like tsunamis of charged particles embedded in deadly magnetic fields.

On July 4, 1054, Chinese astronomers recorded the supernova that formed the Crab Nebula. The ultimate celestial firework, this “guest star” was visible during the daytime for 23 days, shining six times brighter than the planet Venus. The supernova was also recorded by Japanese, Arabic, and Native American stargazers. While searching for a comet that was predicted to return in 1758, French astronomer Charles Messier discovered a hazy nebula in the direction of the long-vanished supernova. He would later add it to his celestial catalog as “Messier 1.” Because M1 didn’t move across the sky like a comet, Messier simply ignored it other than just marking it as a “fake comet.” Nearly a century later the British astronomer William Parsons sketched the nebula. Its resemblance to a crustacean led to M1’s other name, the Crab Nebula. In 1928 Edwin Hubble first proposed associating the Crab Nebula to the Chinese “guest star” of 1054.

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The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI), is a free-standing science center, located on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy (AURA) for NASA, conducts Hubble science operations.

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

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.

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

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.

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

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.

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

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

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

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

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

See the full article here.

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

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

NASA/Goddard Campus

From CosmosUp: “[Info and Images] The Crab Pulsar And Its Nebula” New Take on an Old Subject, Worth Your Time

CosmosUp

The Crab Pulsar was born with supernova explosion which was widely observed on Earth in the year 1054. The Crab Nebula is located 6,500-light-years away from us in the direction of Constellation Taurus. Here, in this article, we will present this amazing object, once thought to be the most energetic light in universe.

A brief history of stars

When a massive star, with a mass several times that of the Sun, reach the end of its live, it compresses and explodes as supernovae, leaving behind a good-looking corpse, a neutron star. Neutron stars are the smallest and densest stars known to exist in the Universe.

They are only a few miles across, with a large fraction of the star’s original mass, composed almost entirely of neutrons — subatomic particles with no net electrical charge. Neutron stars are very hot and spins spectacularly fast on its axis emitting beams of electromagnetic radiation that are detected as pulsars.

The Crab Nebulae

After a massive explosion powerful enough to turn a huge star into cloud of dust, the crab nebula took shape- the eye of the storm, a speeding pulsating star, a pulsar. The gravity squeezed the giant star’s core into an object with 10km diameter, rotating 30 times per second.

Scientists estimate the crab pulsar’s mass to that of 1.5 solar masses. It’s so dense that one pint of this will weigh thousands maybe millions of tons. Two beams of light, energy and radiation, spinning 30 time per second power the huge cloud of dust, the crab nebula.

There’s so much radiations there, more even on the Sun, that’s could easily be considered one of the deadliest things in the universe.

Chandra image of the Crab Nebula

Crab Nebula in Multiple Wavelengths
Based on File:Crab Nebula in multiwavelength.png by Torres997

The Crab Pulsar

In Jan 2016 MAGIC, a ground-based gamma-ray instrument located on the Canary island of La Palma, Spain, discovered unexpected very energetic photons, the most energetic pulsed emission radiation ever detected to date coming from the center of the supernova of 1054 A.D., the Crab pulsar.

MAGIC Cherenkov gamma ray telescope on the Canary island of La Palma, Spain

“We performed deep observation of the Crab pulsar with MAGIC to understand this phenomenon, expecting to measure the maximum energy of the pulsating photons,”

Roberta Zanin from (ICCUB-IEEC, Barcelona) continues:

The new observations extend this tail to much higher, above TeV energies, that is, several times more energetic than the previous measurement, violating all the theory models believed to be at work in neutron stars.”

Crab pulsar, astro.uu.nl/ jleeuwen

This is a mosaic image, one of the largest ever taken by NASA’s Hubble Space Telescope of the Crab Nebula, a six-light-year-wide expanding remnant of a star’s supernova explosion. Japanese and Chinese astronomers recorded this violent event nearly 1,000 years ago in 1054, as did, almost certainly, Native Americans.

The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula’s eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second due to the neutron star’s rotation. A neutron star is the crushed ultra-dense core of the exploded star.

The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844, using a 36-inch telescope. When viewed by Hubble, as well as by large ground-based telescopes such as the European Southern Observatory’s Very Large Telescope, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of a star, 6,500 light-years away.
The newly composed image was assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.

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