Observatory reveals key evidence of cosmic ray acceleration limit in W51 for first time

(a)The UHE gamma-ray emission is clearly observed from the W51 complex, which hosts the supernova remnant W51C and star forming region W51B. (b) The "bending" feature around tens TeV indicates the cosmic-ray acceleration limit in the W51 complex at around 400TeV. Credit: Science China Press

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The Large High Altitude Air Shower Observatory (LHAASO) officially released the precise measurements of high-energy gamma radiation from the W51 complex, confirming it as a cosmic-ray accelerator boosting particles up to so-called ultra-high energies (UHE, above 1014 electronvolts). The results also provide key evidence about the cosmic-ray acceleration limit in this complex.

The findings, titled "Evidence for particle acceleration approaching PeV energies in the W51 complex," were recently published online in Science Bulletin. The research was conducted by the LHAASO International Collaboration, led by the Institute of High Energy Physics, Chinese Academy of Sciences.

The W51 complex is one of the largest and the most active stellar factories in the Milky Way and one of the few regions confirmed to host GeV energy cosmic-ray accelerators. It plays a crucial role in unraveling the century-old mystery of the origin of cosmic rays.

Researchers utilized data from the LHAASO experiment to, for the first time, extend the measurements of the energy spectrum of gamma-rays from this region to the UHE range. They clearly observed a bending structure in the gamma-ray spectrum at tens of TeV, indicating the acceleration limit of cosmic rays in this region.

The energy spectrum measured by LHAASO can be smoothly connected with that which was measured by the Fermi-LAT collaboration at lower energies. Spanning six orders of magnitude of gamma-ray energy, the spectrum provides important evidence that the radiation originates from collisions between cosmic rays and molecular clouds. It also indicates that the W51 complex has a cosmic-ray acceleration limit of around 400 TeV.

"The supernova remnant W51C, located in the W51 complex, is the most plausible cosmic-ray accelerator responsible for the wideband gamma-ray emission," Prof. Li Zhe said, one of the co-corresponding authors.

LHAASO is a national major science and technology infrastructure located on Haizi Mountain at an altitude of 4,410 meters in Daocheng, Sichuan province, China. It consists of an array of 5,216 electromagnetic particle detectors and 1,188 muon detectors distributed over 1 km2, a water Cherenkov detector array covering 78,000 m2 and an array of 18 wide-field-of-view Cherenkov telescopes.

LHAASO was completed and began high-quality stable operation in July 2021. It is the most sensitive UHE gamma-ray detection device in the world, characterized by the large field of view and all-weather capability.

by Science China Press

Source: Phys.org (Provided by Science China Press)

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More information: Zhen Cao et al, Evidence for particle acceleration approaching PeV energies in the W51 complex, Science Bulletin (2024). DOI: 10.1016/j.scib.2024.07.017

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Explore further

• Physicists discover elbow-like feature in mean logarithmic mass spectrum of ultra-high-energy cosmic rays

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Multi-wavelength Observations Reveal Impact of Black Hole on M87 Galaxy

A Black Hole and its Far-Reaching Effects
Credit: Sophia Dagnello; NRAO/AUI/NSF; CXC; EHT Collaboration

To better understand the black hole at the core of galaxy M87, the EHT Collaboration mounted a multi-wavelength observing campaign. Observations across the electromagnetic spectrum in radio, visible-light, ultraviolet, X-ray, and gamma-ray revealed the far-reaching impact of the supermassive black hole on its surroundings. Credit: EHT Collaboration; NASA/Swift; NASA/Fermi; Caltech-NuSTAR; CXC; CfA-VERITAS; MAGIC; HESS. Hi-res image

In 2019, a worldwide collaboration of scientists used a global collection of radio telescopes called the Event Horizon Telescope (EHT) to make the first-ever image of a black hole — the supermassive black hole at the core of the galaxy M87, some 55 million light-years from Earth. This long-sought achievement was an important scientific landmark. However, any image at a single wavelength can give only a partial picture of the entire phenomenon.

“We knew that the first direct image of a black hole would be groundbreaking,” said Kazuhiro Hada of the National Astronomical Observatory of Japan, a co-author on the new study. “But to get the most out of this remarkable image, we need to know everything we can about the black hole’s behavior at that time by observing over the entire electromagnetic spectrum.”

The tremendous gravitational pull of a supermassive black hole can power jets of particles that travel at nearly the speed of light across vast distances. The result produces electromagnetic radiation spanning the entire range from radio waves to visible light, to gamma rays.

In this video, results from each telescope across the observing campaign reveal previously unseen structures and the impact of the black hole on its surroundings in regions spanning one to 100,000 light-years across.

“Understanding the particle acceleration is really central to our understanding of both the EHT image as well as the jets, in all their ‘colors’,” said co-author Sera Markoff, from the University of Amsterdam. “These jets manage to transport energy released by the black hole out to scales larger than the host galaxy, like a huge power cord. Our results will help us calculate the amount of power carried, and the effect the black hole’s jets have on its environment.”

To expand their view of the region around the 6.5-million-solar-mass black hole, scientists mounted a multi-wavelength observing campaign, including 19 ground-and space-based observatories working at gamma-ray, X-ray, visible-light, and radio wavelengths. The study used the Atacama Large Millimeter/submillimeter Array (ALMA) and the National Science Foundation’s Very Long Baseline Array (VLBA).

“There are multiple groups eager to see if their models are a match for these rich observations, and we’re excited to see the whole community use this public data set to help us better understand the deep links between black holes and their jets,” said co-author Daryl Haggard of McGill University.

This new study, reported in The Astrophysical Journal Letters, provides a valuable resource for helping scientists understand the physics of how such monster black holes operate and strongly affect their surroundings.

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Dave Finley
Public Information Officer
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Amy C. Oliver
NRAO News and Public Information Manager
Public Information Officer, ALMA
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Scientific Paper: “Broadband Multi-wavelength Properties of M 87 During the 2017 Event Horizon Telescope Campaign,” Algaba, J.C. et al, The Astrophysical Journal Letters, 911, L11, April 14, 2021, doi: 3847/2041-8213/abef71

EHT Official Press Release

 

Source:  National Radio Astronomy Observatory (NRAO)/News

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Gamma-ray Scientists "Dust Off" Intensity Interferometry, Upgrade Technology with Digital Electronics, Larger Telescopes, and Improved Sensitivity

Artist's conception illustrating improved angular resolution, as was achieved using a scalable version of the intensity interferometry technique developed at VERITAS.  Credit: M. Weiss. High Resolution (jpg) - Low Resolution (jpg)

Cambridge, MA - Scientists in the VERITAS Collaboration have measured the angular diameter of stars using Stellar Intensity Interferometry for the first time in nearly 50 years, and demonstrated both improvements to the sensitivity of the technique and its scalability using digital electronics.

Led by astronomers from the Center for Astrophysics | Harvard & Smithsonian and the University of Utah, VERITAS (Very Energetic Radiation Imaging Telescope Array System) scientists measured the angular diameters of Beta Canis Majoris—a blue giant star located 500 light-years from the sun—and Epsilon Orionis—a blue supergiant star located 2,000 light-years from the sun.

"A proper understanding of stellar physics is important for a massive range of astronomical fields, from exoplanet studies to cosmology, and yet they are often seen as point sources of light due to their great distances from Earth," said Nolan Matthews, University of Utah. "Interferometry has been widely successful in achieving the angular resolution needed to spatially resolve stars and we've demonstrated the capability to perform optical intensity interferometry measurements with an array of many telescopes that in turn will help to improve our understanding of stellar systems." Michael Daniel, Operations Manager, VERITAS, added, "Resolving something the size of a coin on the moon is a marvelous thing. Knowing if that coin is a dime or a nickel is something even more special still. If you want that level of detail, then you want intensity interferometry to work on this scale."

VERITAS used all four of its gamma-ray telescopes, located at the Fred Lawrence Whipple Observatory in Amado, Arizona, to increase its coverage and provide greater resolution for observation.

"This is the first demonstration of the original Hanbury Brown and Twiss technique using an array of optical telescopes," said David Kieda, astronomer, University of Utah, and Principal Investigator. "Modern electronics allow us to computationally combine light signals from each telescope. The resulting instrument has the optical resolution of a football-field-sized reflector."

Typically observing dark, moonless skies for Cherenkov light—blue flashes indicative of the presence of gamma-rays—VERITAS scientists made use of the nights surrounding the full moon to conduct the study. "The moon doesn’t disrupt observations for intensity interferometry,” said Daniel. "This opens up new scientific horizons for the VERITAS telescopes and similar facilities."

The first telescopes to perform stellar measurements using intensity interferometry were the Narrabri telescopes in the 1970s. "Narrabri measured 32 stars in the southern hemisphere, and to significantly improve upon that result required a large leap in technology," said Wystan Benbow, Director, VERITAS. "Right now we are pathfinding for the future Cherenkov Telescope Array (CTA); we have proven that we can add 100 telescopes to this design, enabling astronomers to image features on stellar surfaces with unparalleled optical resolution."

The future for intensity interferometry is bright, and VERITAS scientists have a few ideas about where it could go, from creating a larger catalog of stars, to measuring space objects and phenomena, like the properties of interacting binary star systems, rapidly rotating stars, and potentially the pulsation of Cepheid variables, among others.

Having previously measured the apparent diameter of some very small stars in the sky using the asteroid occultation method, the study is one more indicator that gamma-ray telescopes, and their scientists, are more than meets the eye.

"New technology is a science multiplier," said Peter Kurczynski, Program Director for Advanced Technologies and Instrumentation at the National Science Foundation, which contributed funding for the project. "It enables discoveries that would be otherwise impossible." Benbow added, "There's great potential for intensity interferometry to make leaps forward now that we know it can work on gamma-ray telescopes. We're excited to see, and create, what comes next."

The VERITAS SII project was supported with AST and PHYS grants from the National Science Foundation, and by the University of Utah. The results of the study are published in Nature Astronomy.

About VERITAS

VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a ground-based array of four, 12-m optical reflectors for gamma-ray astronomy located at the Center for Astrophysics | Harvard & Smithsonian, Fred Lawrence Whipple Observatory in Amado, Arizona. VERITAS is the world's most sensitive very-high-energy gamma-ray observatory, and it detects gamma rays via the extremely brief flashes of blue "Cherenkov" light they create when they are absorbed in Earth's atmosphere.

VERITAS is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, and the Smithsonian Institution, NSERC in Canada, and the Helmholtz Association in Germany.

The VERITAS Collaboration consists of about 80 scientists from 20 institutions in the United States, Canada, Germany and Ireland.

For more information about VERITAS visit http://veritas.sao.arizona.edu

About Center for Astrophysics | Harvard & Smithsonian

Headquartered in Cambridge, Mass., the Center for Astrophysics | Harvard & Smithsonian (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Amy Oliver
Public Affairs
Center for Astrophysics | Harvard & Smithsonian
Fred Lawrence Whipple Observatory
520-879-4406
amy.oliver@cfa.harvard.edu

Source: Harvard-Smithsonian Center for Astrophysics (CfA)/News

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Binary star as a cosmic particle accelerator

In the shock region where the supersonic stellar winds of the two stars collide, subatomic particles are accelerated to such an extent that they produce very high-energy gamma radiation. Illustration: DESY, Science Communication Lab.  Download [6.2 MB, 3840 x 2160]

Very high-energy (VHE) gamma radiation from Eta Carinae could be detected with H.E.S.S. around the time of the next encounter of the two giant stars. Illustration: DESY, Science Communication Lab.  Download [6.5 MB, 3840 x 2160]

Specialised telescope provides evidence of very high-energy gamma radiation from Eta Carinae

With a specialised telescope in Namibia a DESY-led team of researchers has proven a certain type of binary star as a new kind of source for very high-energy cosmic gamma-radiation. Eta Carinae is located 7500 lightyears away in the constellation Carina (the ship’s keel) in the Southern Sky and, based on the data collected, emits gamma rays with energies all the way up to 400 gigaelectronvolts (GeV), some 100 billion times more than the energy of visible light. The team headed by DESY’s Stefan Ohm, Eva Leser and Matthias Füßling is presenting its findings, made at the gamma-ray observatory High Energy Stereoscopic System (H.E.S.S.), in the journal Astronomy & Astrophysics. A specially created multimedia animation explains the phenomenon. “With such visualizations we want to make the fascination of research tangible,” emphasises DESY's Director of Astroparticle Physics, Christian Stegmann.

Eta Carinae is a binary system of superlatives, consisting of two blue giants, one about 100 times, the other about 30 times the mass of our sun. The two stars orbit each other every 5.5 years in very eccentric elliptical orbits, their separation varying approximately between the distance from our Sun to Mars and from the Sun to Uranus. Both these gigantic stars fling dense, supersonic stellar winds of charged particles out into space. In the process, the larger of the two loses a mass equivalent to our entire Sun in just 5000 years or so. The smaller one produces a fast stellar wind travelling at speeds around eleven million kilometres per hour (about one percent of the speed of light).

A huge shock front is formed in the region where these two stellar winds collide, heating up the material in the wind to extremely high temperatures. At around 50 million degrees Celsius, this matter radiates brightly in the X-ray range. The particles in the stellar wind are not hot enough to emit gamma radiation, though. “However, shock regions like this are typically sites where subatomic particles are accelerated by strong prevailing electromagnetic fields,” explains Ohm, who is the head of the H.E.S.S. group at DESY. When particles are accelerated this rapidly, they can also emit gamma radiation. In fact, the satellites “Fermi”, operated by the US space agency NASA, and AGILE, belonging to the Italian space agency ASI, already detected high-energy gamma rays of up to about 10 GeV coming from Eta Carinae in 2009.

Subatomic hailstorm

“Different models have been proposed to explain how this gamma radiation is produced,” Füßling reports. “It could be generated by accelerated electrons or by high-energy atomic nuclei.” Determining which of these two scenarios is correct is crucial: very energetic atomic nuclei account for the bulk of the so-called Cosmic Rays, a subatomic cosmic hailstorm striking Earth constantly from all directions. Despite intense research for more than 100 years, the sources of the Cosmic Rays are still not exhaustively known. Since the electrically charged atomic nuclei are deflected by cosmic magnetic fields as they travel through the universe, the direction from which they arrive at Earth no longer points back to their origin. Cosmic gamma rays, on the other hand, are not deflected. So, if the gamma rays emitted by a specific source can be shown to originate from high-energy atomic nuclei, one of the long-sought accelerators of cosmic particle radiation will have been identified.

“In the case of Eta Carinae, electrons have a particularly hard time getting accelerated to high energies, because they are constantly being deflected by magnetic fields during their acceleration, which makes them lose energy again,” says Leser. “Very high-energy gamma radiation begins above the 100 GeV range, which is rather difficult to explain in Eta Carinae to stem from electron acceleration.” The satellite data already indicated that Eta Carinae also emits gamma radiation beyond 100 GeV, and H.E.S.S. has now succeeded in detecting such radiation up to energies of 400 GeV around the time of the close encounter of the two blue giants in 2014 and 2015. This makes the binary star the first known example of a source in which very high-energy gamma radiation is generated by colliding stellar winds.

“The analysis of the gamma radiation measurements taken by H.E.S.S. and the satellites shows that the radiation can best be interpreted as the product of rapidly accelerated atomic nuclei,” says DESY’s PhD student Ruslan Konno, who has published a companion study, together with scientists from the Max Planck Institute for Nuclear Physics in Heidelberg. “This would make the shock regions of colliding stellar winds a new type of natural particle accelerator for cosmic rays.” With H.E.S.S., which is named after the discoverer of Cosmic Rays, Victor Franz Hess, and the upcoming Cherenkov Telescope Array (CTA), the next-generation gamma-ray observatory currently being built in the Chilean highlands, the scientists hope to investigate this phenomenon in greater detail and discover more sources of this kind.

Cosmic roadtrip

Thanks to detailed observations of Eta Carinae at all wavelengths, the properties of the stars, their orbits and stellar winds have been determined relatively accurately. This has given astrophysicists a better picture of the binary star system and its history. To illustrate the new observations of Eta Carinae, the DESY astrophysicists have produced a video animation together with the animation specialists of the award-winning Science Communication Lab. The computer-generated images are close to reality because the measured orbital, stellar and wind parameters were used for this purpose. The internationally acclaimed multimedia artist Carsten Nicolai, who uses the pseudonym Alva Noto for his musical works, created the sound for the animation.

“I find science and scientific research extremely important,” says Nicolai, who sees close parallels in the creative work of artists and scientists. For him, the appeal of this work also lay in the artistic mediation of scientific research results: “particularly the fact that it is not a film soundtrack, but has a genuine reference to reality,” emphasizes the musician and artist. Together with the exclusively composed sound, this unique collaboration of scientists, animation artists and musician has resulted in a multimedia work that takes viewers on an extraordinary journey to a superlative double star some 7500 light years away.

Eta Carinae - a new source of very high-energy cosmic gamma radiation

Animation: DESY, Science Communication Lab; Sound by Alva Noto.. The animation is available in UHD and without annotations to media. Please contact the DESY press office at presse@desy.de

Source:  Deutsches Elektronen-Synchrotron DESY/News

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Reference:

Detection of very-high-energy γ-ray emission from the colliding wind binary η Car with H.E.S.S.; H.E.S.S. Collaboration (for DESY: Matthias Füßling, Eva Leser, Stefan Ohm); Astronomy & Astrophysics, 2020; DOI: 10.1051/0004-6361/201936761

Gamma-ray and X-ray constraints on non-thermal processes in η Carinae; R. White, M.Breuhaus, R. Konno, S. Ohm, B. Reville, and J.A. Hinton; Astronomy & Astrophysics, 2020; DOI:   10.1051/0004-6361/201937031

Interview

Carsten Nicolai aka Alva Noto talks about the sound of astroparticle physics

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Signs of Collisions to Come

Artist’s impression of a neutron star threaded with a dipole magnetic field.
Credit:[ESO/L.Calçada]. Hi-res Image

Artist’s impression of the collision and merger of two neutron stars.
Credit:[NSF/LIGO/Sonoma State University/A. Simonnet]. Hi-res Image

We know that when two neutron stars — the dense, compact cores of evolved stars — collide, they produce signals that span the electromagnetic spectrum. But could these binaries also flare before they merge, as well?

A Broad Range of Signals

The discovery and follow-up of the gravitational-wave event GW170817, a collision of two neutron stars, provided the first direct evidence of the many forms of light that are emitted in these mergers. Between the instant of collision and the months that followed, observatories around the world recorded everything from high-energy gamma rays to late-time radio emission.

But emission might not be restricted to during and after the merger! A new study conducted by two researchers from the Flatiron Institute, Elias R. Most (also of Goethe University Frankfurt, Germany) and Alexander Philippov, explores the possibility that neutron star binaries may also produce flares of emission in the time leading up to their final impact.

This plot of the out-of-plane magnetic field density indicates the twist in flux tubes connecting the two neutron stars seen at the center of the plot. Here, an electromagnetic flare is launched from the binary after a significant twist has built up due to relative rotation of the right star. [Most & Philippov 2020]. Hi-res Image

What About Magnetic Fields?

In particular, Most and Philippov focus on how the magnetospheres of the two neutron stars — the magnetized environment surrounding each body — interact shortly before the objects collide.

The authors conduct special-relativistic force-free simulations of orbiting pairs of neutron stars in which each star is threaded with the strong dipole magnetic field expected for these bodies. The simulations then track how the stars’ magnetic fields evolve, twist, and interact as the bodies orbit each other.

A Twisted Fate

Most and Philippov find that dramatic releases of magnetic energy are a common outcome if the neutron stars orbit close enough to one another that their magnetospheres interact.

The authors show that the brightness of the flare luminosity depends only on how far apart the neutron stars are in the simulation: the smaller the separation, the brighter the flare. This dependence demonstrates that the flaring events are driven primarily by the energy stored in the twisted tube of magnetic flux that forms connecting the two neutron stars.

When the two neutron stars spin at different speeds, the magnetic field loop that forms between the stars becomes progressively more twisted — until this stored rotational energy is abruptly ejected. And even if neither neutron star is spinning, the authors show that magnetic flux twist still builds up and releases as a result of the binary’s orbital motion, assuming that the magnetic fields of the two stars are not aligned.

Here, the twisted flux tube and resultant flaring is caused by orbital motion of 45° misaligned magnetic fields, rather than by one star spinning. The bottom panel shows a 3D visualization of the field line configuration at the time of flaring. [Most & Philippov 2020]. Hi-res Image

Look for Radio Clues

So can we observe these sudden releases of energy? Most and Philippov argue that we should be able to spot the drama in radio emission: a radio afterglow will be produced behind the magnetized bubble that’s ejected from the twisted loop, and additional radio emission can be produced when the bubble collides with surrounding plasma.

Future work on this topic will explore the impacts of the neutron stars’ inspiral, and how the interactions of the magnetospheres change when the neutron stars carry unequal charge. The current study, however, indicates it’s worth keeping a radio eye out to see if we can spot signs of collisions to come!

Citation

“Electromagnetic Precursors to Gravitational-wave Events: Numerical Simulations of Flaring in Pre-merger Binary Neutron Star Magnetospheres,” Elias R. Most and Alexander A. Philippov 2020 ApJL 893 L6. doi:10.3847/2041-8213/ab8196

Source: American Astronomical Society (AAS)

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The Fastest-Spinning Known Millisecond Pulsar in the Galactic Field

Timing residuals as a function of time and orbital phase are shown in panels (a) and (b) respectively. Folded pulse profiles of PSR J0952-0607 are shown as a function of orbital phase for the orbital phases covered by our observations (panel (c)). Eclipses of the radio signal in black widow systems occur around orbital phase φ=0.25 but are not obvious in PSR J0952-0607. Sloan r´-band light curve of the binary companion of PSR J0952-0607 (panel (d)). The Icarus model fit to the light curve is shown with the solid line. Large format: PNG.

Using observations across the entire electro-magnetic spectrum, astronomers have discovered a radio pulsar spinning 707 times every second, making it the fastest known spinning pulsar in the Galactic field and the second fastest known overall. 

The pulsar emits pulsed electromagnetic radiation at very low radio frequencies and very high gamma-rays, while the low-mass binary companion, heated by the energetic radiation from the pulsar, is detected at optical wavelengths. The discovery of this system at very low radio frequencies suggests there may be an as yet unseen population of fast spinning radio pulsars.

The pulsar, PSR J0952-0607, was discovered with LOFAR, the Low-Frequency Array, a radio telescope consisting of a dense core of antenna stations in the Netherlands, and international stations in Germany, France, Sweden, the United Kingdom, Sweden, Poland and Ireland. Operating at very low radio frequencies of 110 to 150 MHz, the LOFAR telescope targeted unassociated high-energy gamma-ray sources discovered with the space-based Fermi gamma-ray telescope, searching for pulsed radio emission of radio and gamma-ray bright millisecond pulsars.

Radio observations revealed that PSR J0952-0607 is part of a binary system, where the pulsar orbits a very low mass (2% the mass of the Sun) binary companion every 6.42 hours. In these so-called 'black widow' systems, referencing the spider which consumes its mate, the proximity of the companion to the pulsar meant the hemisphere facing the pulsar is heated by the energetic pulsar emission, leading to the evaporation of matter from the companion. This heating leads to large variations in brightness of the companion over the course of an orbit.

"Optical observations with the Isaac Newton Telescope were crucial in accurately pinpointing the location of the pulsar, since both LOFAR and Fermi only provide localizations of a few arcminute accuracy.", says Cees Bassa, lead author of the paper presenting the discovery of PSR J0952-0607. Due to the large variation in optical brightness, modulated at the orbital period of the binary, Bassa was able to quickly identify the companion to PSR J0952-0607 in the time series photometry, obtained with the Isaac Newton Telescope last January in service mode.

The subarcsecond optical localization of the counterpart allowed the astronomers to constrain the spindown rate of the millisecond pulsar, which shows that it has a low magnetic field. Furthermore, modelling the optical light curve reveals that the companion does not fill its Roche lobe. The absence of occasional eclipses of the radio emission is consistent with this, but is contrary to what's found in the majority of black widow systems.

The fast spin period of PSR J0952-0607 makes it a prime candidate for further optical studies, as further modelling of the light curve in multiple filters, combined with optical spectroscopy, may allow for the mass of the pulsar to be determined. Knowledge of the mass of such a rapidly spinning pulsar may provide constraints on the composition of pulsars.

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More Information

C. G. Bassa, Z. Pleunis, J. W. T. Hessels, E. C. Ferrara, R. P. Breton, N. V. Gusinskaia, V. I. Kondratiev, S. Sanidas, L. Nieder, C. J. Clark, T. Li, A. S. van Amesfoort, T. H. Burnett, F. Camilo, P. F. Michelson, S. M. Ransom, P. S. Ray, and K. Wood, 2017, "LOFAR Discovery of the Fastest-spinning Millisecond Pulsar in the Galactic Field", ApJL, 846, 20 [ ADS ].

"LOFAR Radio Telescope Discovers Record-Breaking Pulsar", ASTRON press release, 5th September 2017.

Source:  Isaac Newton Group of Telescopes

Crab Nebula: Observatories Combine to Crack Open the Crab Nebula

NGC 1952/Crab Nebula

Credit  X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL/Caltech; 

Radio: NSF/NRAO/VLA; Ultraviolet: ESA/XMM-Newton

JPEG (472.7 kb) - Large JPEG (10.6 MB) - Tiff (31.1 MB) - More Images

A Quick Look at the Crab Nebula

 

More Animations

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Astronomers have produced a highly detailed image of the Crab Nebula, by combining data from telescopes spanning nearly the entire breadth of the electromagnetic spectrum, from radio waves seen by the Karl G. Jansky Very Large Array (VLA) to the powerful X-ray glow as seen by the orbiting Chandra X-ray Observatory. And, in between, the Hubble Space Telescope's crisp visible-light view and the infrared perspective of the Spitzer Space Telescope.

The Crab Nebula, the result of a bright supernova explosion seen by Chinese and other astronomers in the year 1054, is 6,500 light-years from Earth. At its center is a super-dense neutron star, rotating once every 33 milliseconds, shooting out rotating lighthouse-like beams from radio waves to gamma-ray wavelengths — a pulsar. The nebula's intricate shape is caused by a complex interplay of the pulsar, a fast-moving wind of particles coming from the pulsar, and material originally ejected by the supernova explosion and by the star itself before the explosion.

This image combines data from five different telescopes: The VLA (radio) in red; Spitzer Space Telescope (infrared) in yellow; Hubble Space Telescope (visible) in green; XMM-Newton (ultraviolet) in blue; and Chandra X-ray Observatory (X-ray) in purple.

The new VLA, Hubble, and Chandra observations were largely made at about the same time in November 2012. Chandra has been observing the Crab Nebula since shortly after the telescope was launched into space in 1999 and has repeatedly done so in the years since. X-ray data reveal the distribution and behavior of the high-energy particles being spewed from the pulsar at the center of the Crab, which provides important clues to the workings of this mighty cosmic generator producing energy at the rate of 1,000 suns.

A paper describing the latest multi-wavelength work on the Crab, led by Gloria Dubner (IAFE), appears in The Astrophysical Journal and is available online. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

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Fast Facts for Crab Nebula:

Scale: Image is about 5 arcmin across (10 light years)
Category: Supernovas & Supernova Remnants, Neutron Stars/X-ray Binaries
Coordinates (J2000): RA 05h 34m 32s | Dec +22° 0.0' 52.00"
Constellation: Taurus
Observation Date: 48 pointings between March 2000 and Nov 2013
Observation Time: 25 hours 28 min. (1 day 1 hour 28 min)

Obs. ID: 769-773, 1994-2001, 4607, 13139, 13146, 13147, 13150-13154, 13204-13210, 13750-13752, 13754-13757, 14416, 14458, 14678-14682, 14685, 16245, 16257, 16357, 16358

Instrument: ACIS
Also Known As: NGC 1952
References: Dubner, G. et al., 2017, ApJ [in print]; arXiv: 1704.02968
Color Code: X-ray (Purple), Ultraviolet (Blue), Optical (Green), Infrared (Yellow-Green), Radio (Red)
Distance Estimate: About 6,500 light years

Source: NASA’s Chandra X-ray Observatory

Origin of Milky Way’s Hypothetical Dark Matter Signal May Not Be So Dark

Galaxy’s Excessive Gamma-Ray Glow Likely Comes from Pulsars, the Remains of Collapsed Ancient Stars

Menlo Park, Calif. — A mysterious gamma-ray glow at the center of the Milky Way is most likely caused by pulsars – the incredibly dense, rapidly spinning cores of collapsed ancient stars that were up to 30 times more massive than the sun. That’s the conclusion of a new analysis by an international team of astrophysicists, including researchers from the Department of Energy’s SLAC National Accelerator Laboratory. The findings cast doubt on previous interpretations of the signal as a potential sign of dark matter – a form of matter that accounts for 85 percent of all matter in the universe but that so far has evaded detection.

An excess of gamma-rays coming from the center of the Milky Way is likely due to a population of pulsars – rapidly spinning, very dense and highly magnetized neutron stars that emit “beams” of gamma rays like cosmic lighthouses. The pulsars’ location in the oldest region of the galaxy suggests that they leach energy from companion stars, which prolongs the pulsars’ lifetime. The background image shows the galactic center as seen by NASA’s Chandra X-ray Observatory. (NASA/CXC/University of Massachusetts/D. Wang et al.; Greg Stewart/SLAC National Accelerator Laboratory)

“Our study shows that we don’t need dark matter to understand the gamma-ray emissions of our galaxy,” said Mattia Di Mauro from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint institute of Stanford University and SLAC. “Instead, we have identified a population of pulsars in the region around the galactic center, which sheds new light on the formation history of the Milky Way.”

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

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

A Mysterious Glow

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

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

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

When astrophysicists model the Milky Way’s gamma-ray sources to the best of their knowledge, they are left with an excess glow at the galactic center. Some researchers have argued that the signal might hint at hypothetical dark matter particles. However, it could also have other cosmic origins. (NASA; A. Mellinger/Central Michigan University; T. Linden/University of Chicago)

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

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

An excess of gamma rays coming from the center of the Milky Way has fueled hopes the signal might stem from hypothetical dark matter particles that collide and destroy each other (left). The radiation could also be produced by pulsars – rapidly rotating neutron stars with strong magnetic fields (right). (Greg Stewart/SLAC National Accelerator Laboratory)

Remains of Ancient Stars

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

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

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

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

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

Simulated distribution of gamma-ray sources in the inner 40 degree by 40 degree region of the Milky Way with the galactic center in the middle. The map shows pulsars in the galactic disk (red stars) and in the galaxy’s central region (black circles). (NASA/DOE/Fermi LAT Collaboration)

Dark Matter Remains Elusive

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

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

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

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

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

-Written by Manuel Gnida

Source:  SLAC National Accelerator Laboratory

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Citation: 

The Fermi-LAT Collaboration, arXiv:1705.00009, 02 May 2017.

Press Office Contact: 

Andrew Gordon,
agordon@slac.stanford.edu,
(650) 926-2282

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SLAC National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Wide View of the Crab Nebula

Crab Nebula, NGC 1952, Messier 1

Credit: ESO / Manu Mejias

The Crab Nebula, which also goes by the names Messier 1, NGC 1952 and Taurus A, is one of the best studied astronomical objects in the sky. It is the remnant of a supernova explosion which was observed by Chinese astronomers in 1054. The tangled filaments visible in this image are the remains of the exploded star, which are still expanding outwards at about 1500 kilometres per second.

Although not visible to the naked eye due to foreground filaments of helium and hydrogen the heart of the nebula hosts two faint stars. It is one of these that is responsible for the nebula that we see today — a star that is known as the Crab Pulsar, or CM Tau. This is the small, dense, corpse of the original star that caused the supernova. It is now only about 20 kilometres in diameter and rotates around its axis 30 times every second!

The star emits pulses of radiation in all wavelengths, ranging from gamma rays — for which it is one of the brightest sources in the sky — to radio waves. The radiation from the star is so strong that it is creating a wave of material that is deforming the inner parts of the nebula. The appearance of these structures changes so fast that astronomers can actually observe how they reshape. This provides a rare opportunity as cosmic timescales are usually much too long for change to be observed to this extent.

The data from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile used to make this image were selected from the ESO archive by Manu Mejias as part of the Hidden Treasures competition.

Source: ESO/Images

Giant, Magnetized Outflows from our Galactic Center

 A false-color image of our Milky Way as seen in a projection that shows the galactic center at the center of the image, the plane of the galaxy stretching across the central band, and the two arc-shaped radio lobes of emission seen extending north and south of the plane. Several of the newly discovered magnetic structures are labeled.  Credit: Carretti et al., and Nature.  Low Resolution Image (jpg)

Two years ago, CfA astronomers reported the discovery of giant, twin lobes of gamma-ray emission protruding about 50,000 light-years above and below the plane of our Milky Way galaxy, and centered on the supermassive black hole at our galaxy's core. The scientists argued then that the bubbles were produced either by an eruption from the black hole sometime in the past, or else by a burst of star formation in that vicinity. 

It now appears that these giant bubbles of hot gas can be seen at radio wavelengths as well. Writing in the new issue of the journal Nature, CfA astronomer Gianni Bernardi and eight of his colleagues describe finding humongous lobes of radio emission emanating from the Galactic Center. Moreover, the emission is polarized, a general property that electromagnetic radiation can have; some sunglasses take advantage of the fact that reflected sunlight becomes polarized. In the case of radio wavelengths, the explanation for polarization is the presence of strong magnetic fields. 

The scientists calculate that the radio lobes, which closely match the gamma-ray lobes in overall dimensions but which contain three ridge-like substructures, are probably polarized by the presence of strong magnetic fields that extend out of the galactic plane in both directions for tens of thousands of light-years, and which contain an energy roughly equivalent to the total current output of the Sun for a time equal to the lifetime of the universe. They argue that the activity is driven by star-formation activity, rather than black-hole activity, and that it originates in a region around the Galactic Center about 650 light-years in size. Not least, the scientists argue that the ridges seen in the magnetically-shaped outflow are the result of several episodes of star-formation that constitute a phonograph-like record of star formation in this region over at least the past ten million years. 

Source: Smithsonian Astrophysical Observatory

NASA'S Fermi Measures Cosmic 'Fog' Produced by Ancient Starlight

This animation tracks several gamma rays through space and time, from their emission in the jet of a distant blazar to their arrival in Fermi's Large Area Telescope (LAT). During their journey, the number of randomly moving ultraviolet and optical photons (blue) increases as more and more stars are born in the universe. Eventually, one of the gamma rays encounters a photon of starlight and the gamma ray transforms into an electron and a positron. The remaining gamma-ray photons arrive at Fermi, interact with tungsten plates in the LAT, and produce the electrons and positrons whose paths through the detector allows astronomers to backtrack the gamma rays to their source.  (Credit: NASA's Goddard Space Flight Center/Cruz deWilde) .  Download video clip in high-resolution

This plot shows the locations of 150 blazars (green dots) used in the EBL study. The background map shows the entire sky and was constructed from four years of gamma rays with energies above 10 billion electron volts (GeV) detected by Fermi. The plane of our Milky Way galaxy runs along the middle of the plot. The Fermi LAT instrument is the first to detect more than 500 sources in this energy range. (Credit: NASA/DOE/Fermi LAT Collaboration) .  Larger image  -  Image without blazar positions

Fermi measured the amount of gamma-ray absorption in blazar spectra produced by ultraviolet and visible starlight at three different epochs in the history of the universe. (Credit: NASA's Goddard Space Flight Center) .   Larger image

This illustration places the Fermi measurements in perspective with other well-known features of cosmic history. Star formation reached a peak when the universe was about 3 billion years old and has been declining ever since. (Credit: NASA's Goddard Space Flight Center) .   Larger image

Click here for press briefing multimedia associated with this story.

Astronomers using data from NASA's Fermi Gamma-ray Space Telescope have made the most accurate measurement of starlight in the universe and used it to establish the total amount of light from all of the stars that have ever shone, accomplishing a primary mission goal.

"The optical and ultraviolet light from stars continues to travel throughout the universe even after the stars cease to shine, and this creates a fossil radiation field we can explore using gamma rays from distant sources," said lead scientist Marco Ajello, a postdoctoral researcher at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University in California and the Space Sciences Laboratory at the University of California at Berkeley.

 

Gamma rays are the most energetic form of light. Since Fermi's launch in 2008, its Large Area Telescope (LAT) observes the entire sky in high-energy gamma rays every three hours, creating the most detailed map of the universe ever known at these energies.

The total sum of starlight in the cosmos is known to astronomers as the extragalactic background light (EBL). To gamma rays, the EBL functions as a kind of cosmic fog. Ajello and his team investigated the EBL by studying gamma rays from 150 blazars, or galaxies powered by black holes, that were strongly detected at energies greater than 3 billion electron volts (GeV), or more than a billion times the energy of visible light.

"With more than a thousand detected so far, blazars are the most common sources detected by Fermi, but gamma rays at these energies are few and far between, which is why it took four years of data to make this analysis," said team member Justin Finke, an astrophysicist at the Naval Research Laboratory in Washington.

As matter falls toward a galaxy's supermassive black hole, some of it is accelerated outward at almost the speed of light in jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, the galaxy appears especially bright and is classified as a blazar.

Gamma rays produced in blazar jets travel across billions of light-years to Earth. During their journey, the gamma rays pass through an increasing fog of visible and ultraviolet light emitted by stars that formed throughout the history of the universe.

Occasionally, a gamma ray collides with starlight and transforms into a pair of particles -- an electron and its antimatter counterpart, a positron. Once this occurs, the gamma ray light is lost. In effect, the process dampens the gamma ray signal in much the same way as fog dims a distant lighthouse.

From studies of nearby blazars, scientists have determined how many gamma rays should be emitted at different energies. More distant blazars show fewer gamma rays at higher energies -- especially above 25 GeV -- thanks to absorption by the cosmic fog.

The farthest blazars are missing most of their higher-energy gamma rays.

The researchers then determined the average gamma-ray attenuation across three distance ranges between 9.6 billion years ago and today.

From this measurement, the scientists were able to estimate the fog's thickness. To account for the observations, the average stellar density in the cosmos is about 1.4 stars per 100 billion cubic light-years, which means the average distance between stars in the universe is about 4,150 light-years.

A paper describing the findings was published Thursday on Science Express.

"The Fermi result opens up the exciting possibility of constraining the earliest period of cosmic star formation, thus setting the stage for NASA's James Webb Space Telescope," said Volker Bromm, an astronomer at the University of Texas, Austin, who commented on the findings. "In simple terms, Fermi is providing us with a shadow image of the first stars, whereas Webb will directly detect them."

Measuring the extragalactic background light was one of the primary mission goals for Fermi.

"We're very excited about the prospect of extending this measurement even farther," said Julie McEnery, the mission's project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.

Goddard manages the Fermi astrophysics and particle physics research partnership. Fermi was developed in collaboration with the U.S. Department of Energy with contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.

NASA's Fermi Telescope Finds Giant Structure in our Galaxy

Using data from NASA's Fermi Gamma-ray Space Telescope, scientists have recently discovered a gigantic, mysterious structure in our galaxy. This feature looks like a pair of bubbles extending above and below our galaxy's center. Each lobe is 25,000 light-years tall and the whole structure may be only a few million years old. (Video credit: NASA's Goddard Space Flight Center).

From end to end, the newly discovered gamma-ray bubbles extend 50,000 light-years, or roughly half of the Milky Way's diameter, as shown in this illustration. Hints of the bubbles' edges were first observed in X-rays (blue) by ROSAT, a Germany-led mission operating in the 1990s. The gamma rays mapped by Fermi (magenta) extend much farther from the galaxy's plane. Credit: NASA's Goddard Space Flight Center. Larger image. Unlabeled image

A giant gamma-ray structure was discovered by processing Fermi all-sky data at energies from 1 to 10 billion electron volts, shown here. The dumbbell-shaped feature (center) emerges from the galactic center and extends 50 degrees north and south from the plane of the Milky Way, spanning the sky from the constellation Virgo to the constellation Grus. Credit: NASA/DOE/Fermi LAT/D. Finkbeiner et al. Larger image. Video showing processing steps

When an electron moving near the speed of light strikes a low-energy photon, the collision slightly slows the electron and boosts the photon's energy to the gamma-ray regime. Credit: NASA's Goddard Space Flight Center. View video

The bubbles display a spectrum with higher peak energies than the diffuse gamma-ray glow seen throughout the sky. In addition, the bubbles display sharp edges in Fermi LAT data. Both of these qualities suggest that the structure arose in a sudden, impulsive event. Credit: NASA/DOE/Fermi LAT/D. Finkbeiner et al. Larger image

WASHINGTON -- NASA's Fermi Gamma-ray Space Telescope has unveiled a previously unseen structure centered in the Milky Way. The feature spans 50,000 light-years and may be the remnant of an eruption from a supersized black hole at the center of our galaxy.

"What we see are two gamma-ray-emitting bubbles that extend 25,000 light-years north and south of the galactic center," said Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who first recognized the feature. "We don't fully understand their nature or origin."

The structure spans more than half of the visible sky, from the constellation Virgo to the constellation Grus, and it may be millions of years old. A paper about the findings has been accepted for publication in The Astrophysical Journal.

Finkbeiner and his team discovered the bubbles by processing publicly available data from Fermi's Large Area Telescope (LAT). The LAT is the most sensitive and highest-resolution gamma-ray detector ever launched. Gamma rays are the highest-energy form of light.

Other astronomers studying gamma rays hadn't detected the bubbles partly because of a fog of gamma rays that appears throughout the sky. The fog happens when particles moving near the speed of light interact with light and interstellar gas in the Milky Way. The LAT team constantly refines models to uncover new gamma-ray sources obscured by this so-called diffuse emission. By using various estimates of the fog, Finkbeiner and his colleagues were able to isolate it from the LAT data and unveil the giant bubbles.

Scientists now are conducting more analyses to better understand how the never-before-seen structure was formed. The bubble emissions are much more energetic than the gamma-ray fog seen elsewhere in the Milky Way. The bubbles also appear to have well-defined edges. The structure's shape and emissions suggest it was formed as a result of a large and relatively rapid energy release - the source of which remains a mystery.

One possibility includes a particle jet from the supermassive black hole at the galactic center. In many other galaxies, astronomers see fast particle jets powered by matter falling toward a central black hole. While there is no evidence the Milky Way's black hole has such a jet today, it may have in the past. The bubbles also may have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way's center several million years ago.

"In other galaxies, we see that starbursts can drive enormous gas outflows," said David Spergel, a scientist at Princeton University in New Jersey. "Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics."

Hints of the bubbles appear in earlier spacecraft data. X-ray observations from the German-led Roentgen Satellite suggested subtle evidence for bubble edges close to the galactic center, or in the same orientation as the Milky Way. NASA's Wilkinson Microwave Anisotropy Probe detected an excess of radio signals at the position of the gamma-ray bubbles.

The Fermi LAT team also revealed Tuesday the instrument's best picture of the gamma-ray sky, the result of two years of data collection.

"Fermi scans the entire sky every three hours, and as the mission continues and our exposure deepens, we see the extreme universe in progressively greater detail," said Julie McEnery, Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.

NASA's Fermi is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

"Since its launch in June 2008, Fermi repeatedly has proven itself to be a frontier facility, giving us new insights ranging from the nature of space-time to the first observations of a gamma-ray nova," said Jon Morse, Astrophysics Division director at NASA Headquarters in Washington. “These latest discoveries continue to demonstrate Fermi's outstanding performance.”

For more information about Fermi, visit:

http://www.nasa.gov/fermi


Trent Perrotto
Headquarters, Washington
202-358-0321
trent.j.perrotto@nasa.gov

Lynn Chandler
Goddard Space Flight Center, Greenbelt, Md.
301-286-2806
lynn.chandler-1@nasa.gov

INTEGRAL completes the deepest all-sky survey in hard X-rays

A newly developed image analysis technique has significantly improved the sensitivity limits reached by the IBIS imager on board INTEGRAL, resulting in the deepest survey ever compiled of the entire sky in the energy range between 17 and 60 keV. Pushing the instrument towards its very limits, the novel method discloses a vast number of previously undetected faint sources, galactic and extragalactic alike.

For more than seven years, the INTEGRAL observatory has been surveying the entire X-ray and gamma-ray sky and has accumulated a copious amount of exposure time, targeting both the crowded regions along the Galactic Plane and the high-latitude portions of the sky, this latter region being dominated by extragalactic sources. Theoretically, a longer exposure time translates into an improvement in sensitivity, but this connection is not always straightforward: a number of systematic effects plague the observations and limit the sensitivity of the instruments despite the increased exposure time. Hence, new techniques are sought, and implemented, in order to overcome these systematic effects and to fully exploit the instrument performance.

A successful example of this synergy is a novel image analysis algorithm, recently developed to improve the sensitivity achieved by IBIS, the Imager on Board the INTEGRAL Satellite.

Region near the Galactic Plane in the INTEGRAL/IBIS 7-year All-Sky Hard X-ray Survey. This alternating image shows the difference in results achieved by using a) the previous sky reconstruction method and b) the improved, newly developed image analysis algorithm. Credit: Krivonos, et al. 2010, A&A, in press

Since high-energy photons, such as hard X-rays and gamma rays, cannot be focussed using traditional lenses and mirrors, IBIS is a coded-mask telescope, consisting of a metal plate (or mask) with a pattern of holes, which is placed on top of a detector. Photons coming from an astronomical source pass through the holes and, depending on their incoming direction, cast a series of shadows on the detector. "The principle is very similar to that of a pinhole camera," explains INTEGRAL Project Scientist Chris Winkler. "From the pattern of dark and bright pixels, or shadowgram, recorded by the detector, it is possible to reconstruct the position in the sky and the intensity of the sources that produced the shadowgram. The reconstruction relies on complex image analysis techniques, which are unfortunately prone to a variety of systematic effects, especially along the Galactic Plane," adds Winkler. The so-called Galactic Ridge emission, a strong, diffuse X-ray radiation coming from the Galactic Plane, represents a serious problem in this process, which is further complicated by the vast number of sources in the crowded field of the Galactic Centre.

"In order to isolate individual sources on the sky, images have to be cleaned by removing the background signal, which in turn has to be properly modelled," says Roman Krivonos, a researcher at the Max Planck Institute for Astrophysics and at the Space Research Institute of the Russian Academy of Science, who led the study. In this case, the background includes emission from the Cosmic X-Ray Background, instrumental noise and, depending on the galactic latitude of the observed fields, additional Ridge emission from the Galactic Plane.

Galactic Bulge region in the INTEGRAL/IBIS 7-year map. The overlaid COBE/DIRBE 4.9 μm brightness contours trace the Galaxy's disk/bulge structure. Credit: R. Krivonos

"The main source of systematic effects is the mismatch between the model of the background, used in the image analysis, and the true background, actually present in the images," Krivonos explains. "Our new method contains an improved model of the Ridge emission based on near-infrared observations, a good tracer of the galactic X-ray emission; this enables us to remove this particular source of systematic effects," he adds. The algorithm also contains a further cleaning step, through which all large-scale artefacts, due to residual systematic effects and mimicking extended structures on the sky, are removed.

The newly developed method suppresses systematic effects almost completely in extragalactic, high-latitude fields, and yields a significant, albeit not total, removal also in the portion of the sky dominated by the Galaxy. Observations have now a more-or-less uniform sky background, enabling the detection of previously unnoticeable faint sources.

The improved sky reconstruction method at work in the sky region around the Seyfert-1 galaxy NGC 4151 in the INTEGRAL/IBIS 7-year All-Sky Hard X-ray Survey. Left: image created using the previous sky reconstruction method. Right: image created using the improved, newly developed algorithm, which allowed the detection of a new hard X-ray source, IGR J11203+4531 (highlighted by the green circle). Credit: Krivonos, et al. 2010, A&A, in press

The result is the deepest all-sky survey compiled to date in hard X-rays, covering the energy range between 17 and 60 keV. The sensitivity has reached instrumental limits on extragalactic observations, where the IBIS imager aboard INTEGRAL is working at its maximum efficiency; on galactic fields, observations do not reach, but significantly approach, the instrumental limits, delivering a survey of the Galaxy with the best currently available sensitivity in this energy band. "After 7 years of operations, IBIS has collected data over very long exposure times, and we can thus finally profit from the instrument's full capabilities. At this point in the mission's lifetime, such a technique, pushing the instrument towards its limits, is especially valuable," comments Winkler.

The catalogue of extragalactic sources detected in the study, mostly Active Galactic Nuclei (AGN), benefits enormously from the newly developed method, resulting in a much deeper survey than previously achieved. The higher sensitivity of the new observations enables a significant number of sources up to a redshift of z~0.1 to be detected, thus probing how the properties of hard X-ray emitting AGN evolved over the last thousand million years of cosmic history. The sample of galactic sources, comprising compact sources of X-ray radiation such as accreting black holes and neutron stars, is also considerably larger than previous comparable catalogues. "It represents a prototype of the compact source samples that will be detected in nearby galaxies by future hard X-ray missions," says Krivonos.

Notes for editors:

INTEGRAL is an ESA project with instruments and science data centre funded by ESA Member States (especially the PI countries: Denmark, France, Germany, Italy, Spain, Switzerland) and Poland, and with the participation of Russia and the USA.

The INTEGRAL/IBIS 7-year All-Sky Hard X-Ray Survey is based on observations performed with the IBIS coded mask telescope in the energy band 17-60 keV.

The survey covers 90% of the sky down to the flux limit of 6.2 × 10-11 erg/s/cm2, and 10% of the sky down to the flux limit of 8.6 × 10-12 erg/s/cm2. The faintest galactic source detected is the type-I X-ray burster AX J1754.2-2754, with a flux of 4.6 × 10-12 erg/s/cm2. For comparison, the brightest source in the hard X-ray band is the Crab Nebula, with a flux of 1.43 × 10-8 erg/s/cm2.

Most of the exposures were collected in the region of the Galactic Plane, where the maximum available exposure is an approximately 20 million second deep field of the Galactic Centre. The newly implemented algorithm decrease the systematic noise by about 44% in this field, and practically removes it from high-latitude sky images.

The data used in this study were obtained from the European and Russian INTEGRAL Science Data Centres.

Related publications:

Krivonos, R., et al. [2010], INTEGRAL/IBIS 7-year All-Sky Hard X-Ray Survey – Part I: Image Reconstruction, A&A accepted – http://dx.doi.org/10.1051/0004-6361/200913814

Krivonos, R., et al. [2010], INTEGRAL/IBIS 7-year All-Sky Hard X-Ray Survey – Part II: Catalog of Sources, A&A accepted - http://arxiv.org/abs/1006.4437

Contacts:

Chris Winkler, INTEGRAL Project Scientist
Research and Scientific Support Department
Directorate of Science and Robotic Exploration, ESA, The Netherlands
Email: cwinkler@rssd.esa.int
Phone: +31 71 565 3591

Roman Krivonos
Max Planck Institute for Astrophysics, Germany &
Space Research Institute, Russian Academy of Science, Moscow, Russia
Email: krivonos@mpa-garching.mpg.de, krivonos@iki.rssi.ru
Phone: +49 (89) 30000-2275