A Window to the Stars

ESA’s orbiting gamma-ray observatory, Integral, has made a pioneering unequivocal discovery of radioactive iron-60 in our galaxy that provides powerful insight into the workings of massive stars that pervade and shape it.

Found drifting in space, the radioactive isotope has been sought for long. All past reported sightings of iron-60 have been subject to controversy. Now Integral has provided unequivocal evidence.
Since late 2002, Integral has been collecting data from across the galaxy. It shows an enhancement in gamma rays at two characteristic energies, 1173 and 1333 kilo electron Volts. These are produced by radioactive decay of iron-60 into cobalt-60.
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Roland Diehl of the Max-Planck-Institut für extraterrestrische Physik, headed the work and believes it is a major step forward. “These gamma-ray lines have been detected before with some dispute. Integral, the only instrument capable of doing this, shows that iron-60 does exist in interstellar space in our Galaxy,” he says.

More than a curiosity, its presence opens a door into the very heart of the most massive stars in the cosmos. The majority of chemical elements are built inside stars from raw ingredients present during star formation from an interstellar gas cloud. In addition to hydrogen and helium produced during the Big Bang, the gas contained enrichments, known to astronomers as ‘metals’, from previous generations of stars and their nuclear reactions.

Until this detection, astronomers had only one radioactive isotope to probe into the current build-up of chemical elements in stars and their distribution with respect to future star formation. That was the radioactive isotope aluminium-26, first discovered in 1978. “The study of aluminium-26 has developed into its own branch of astronomy,” says Diehl.

Iron-60 gives astronomers valuable new insight - although produced in the same stars as aluminium-26, its production differs markedly. Iron-60 is synthesised both later in a star’s life and deeper inside.

As massive stars age, they develop a layered structure in which different chemical elements are fused together. While aluminium-26 is one rung on the ladder of nuclear reactions, iron-60 is produced from pre-existing stable iron isotopes by a process called ‘neutron capture’ in the respective layers where helium and carbon atoms are undergoing fusion.

“Iron-60 provides the entry into studying neutron capture in stars through contemporaneous radioactivity,” says Diehl. It has also prompted a number of particle accelerators to begin more detailed studies of how easily iron captures neutrons.

Unlike aluminium-26, iron-60 is only expelled into space when the star explodes at the end of its life. It then decays with a half-life of 1.5 million years, producing the gamma rays that Integral detected.

The new data pins down the ratio of iron-60 to aluminium-26, which has a half-life of 740 000 years. Previous predictions have fallen anywhere between 10 and 100 percent. Integral shows it to be 15 percent, which agrees well with current theoretical estimates. But theoreticians and nuclear physicists have been stimulated by Integral’s results to strive for more precise predictions.

Radioactive iron, a window to the stars from ESA and Max Planck Institut

Although Integral clearly sees the telltale gamma rays, they are too faint for it to map out enhancements and paucities across the Galaxy. Mapping the distribution of iron-60 is a job for the next generation of gamma-ray instruments.

Nevertheless, the team will continue observing with Integral for as long as they can, in the hope of gaining some coarse ideas about the isotope’s spread across the Galaxy.
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GZK cut-off Cosmic Rays & Cosmic Showers from Bee @ Backreaction
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Integral View of Sky

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Credits: IBIS survey team

Integral expands our view of the gamma-ray sky
The upper image shows the sky distribution of four of the main soft gamma-ray source populations observed in the third Integral/IBIS survey catalogue. This newly-released catalogue contains 421 sources. Of the known systems, the low-mass X-ray binaries (LMXB) are old systems mainly populating the galactic bulge, the high-mass X-ray binaries (HMXB) are younger systems seen along the galactic plane, and the active galactic nuclei (AGN) are extragalactic sources seen over the whole sky. Around one out of four of the sources seen by Integral are unidentified, and their distribution is also shown.

The lower picture is a false colour image of the central region of our galaxy. This is a composite image based on all-sky IBIS/ISGRI maps in three energy windows (between 17 and 100 keV) and represents the true 'X-ray colours' of the sources. Red sources are dominated by emission below 30 keV, while blue sources have harder spectra, emitting strongly above 40 keV.

One of the most remarkable "relic" gamma-ray clouds of the new catalog, the source HESS J1825-137 which is 100 light-years across. The zoomed box shows the much smaller X-ray nebula (data from the XMM-Newton satellite), surrounding the middle-aged (21000 years old) pulsar PSR B1823-13. (Credit: Astroparticule et Cosmologie (APC), CNRS)

High-Energy 'Relic' Wind Reveals Past Behavior Of Dead Stars
Winds from pulsars have been known for many years. The most famous example is that from the pulsar in the center of the Crab Nebula, a bright cloud of expanding gas from a star that exploded in the year 1054. In that case, the wind generates X-rays (which have less energy than gamma rays) through synchrotron radiation and gamma rays through the inverse Compton scattering. These X-rays and gamma rays are seen coming from gas a few light years across at most.

The objects detected by the H.E.S.S. team are far more extended. The glow of gamma rays seen from the pulsar PSR B1823-13, for example, is approximately 100 light years across. A light year is the distance a particle of light, traveling 186,000 miles per second travels in one year.

The larger size of this gamma-ray emitting region means the electrons producing the gamma rays have traveled further and so come from a period earlier in the pulsar’s history. This in turn means that studying the gamma rays from pulsar winds can give astronomers insight into the history of the pulsar itself and how its magnetic field has changed over the past tens of thousands of years.

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Powerful Solar Winds colliding head-on from Universe Today
NASA Scientists Find High Energy Systems Hidden in 'Gas Cocoon'
First X-Ray detection of a Colliding Wind Binary from Science Daily
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Blue electron light

The blue streak in this photograph shows the dramatic gain in energy made by some of the electrons in a bunch after passing through plasma (ionized gas). The white spot shows the electrons in the bunch that generated the plasma to propel the other electrons to double their energy, to 85 billion electron volts (GeV). The electrons can be photographed because they emit blue light as they pass through air.
Click on image for larger version

Electrons already travel at near light's speed in an accelerator, but physicists from the Stanford Linear Accelerator (SLAC) actually doubled the energy of the electrons, not their speed.

This achievement demonstrates a technology that may drive the future of accelerator design. To reach the high energies required to answer the new set of mysteries confronting particle physics—such as dark energy and the origin of mass—the newest accelerators are vastly larger, and consequently more expensive, than their predecessors. Very high-energy particle beams will be needed to detect the very heavy and very short-lived particles that have eluded scientists so far.

While still in early development stages, the research shows that acceleration using plasma, or ionized gas, can dramatically boost the energy of particles in a short distance.

The electrons first traveled two miles through the linear accelerator at SLAC, gaining 42 billion electron volts (or GeV) of energy. Then they passed through a 33-inch long (84-centimeter) plasma chamber and picked up another 42 GeV of energy. Like an afterburner on a jet engine, the plasma provides extra thrust. The plasma chamber is filled with lithium gas. As the electron bunch passes through the lithium, the front of the bunch creates plasma. This plasma leaves a wake that flows to the back of the bunch and shoves it forward, giving electrons in the back more energy.

The experiment created one of the largest acceleration gradients ever achieved. The gradient is a measure of how quickly particles amass energy. In this case, the electrons hurtling through the plasma chamber gained 3,000 times more energy per meter than usual in the accelerator.

A current experimental limitation is that most of the electrons in a bunch lose their energy to the plasma. Energy out of one part of the beam is put into another part.

During the last two years, the team has improved the plasma acceleration gradient by a factor of 200. One of the next steps is to attempt a two-bunch system, where the first bunch provides all the energy to the trailing bunch. In a full-scale plasma accelerator, physicists would use those second bunches to create high-energy particle collisions in their detectors.

New Accelerator Technique Doubles Particle Energy in Just One Meter
SLAC press release 14/02/07

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Supergiant fast X-Ray transients
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Credits: ESA This artist's impression shows a high-mass binary system, composed of a supergiant luminous star (in blue) and a compact stellar object, such as a neutron star.

As discovered by ESA's Integral observatory, many of these supergiant systems produce strong and exceptionally fast-rising X-ray outbursts lasting a few hours only, hence their name 'supergiant fast X-ray transients'.

The outbursts may depend on the way stellar material is exchanged between the supergiant star and the compact object.

The light curve at the bottom-right was retrieved by Integral from the supergiant fast X-ray transient source IGR J17544-2619 on 17 September 2003.

The curve shows a very fast X-ray outburst from the compact object, lasting about two hours only, with very fast rise and slow decay. The counterpart of this source is a luminous supergiant, unambiguously identified by ESA's XMM-Newton and NASA's Chandra X-ray observatories.


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Credits: JM Blondin, North Carolina State University

This simulated sequence shows the interaction between the stellar material carried by the wind of a supergiant star and its 'receiving' companion - a compact stellar object such as a neutron star. In the vicinity of the compact object it is possible to see the development of a turbulent shocked flow.

Integral reveals new class of ‘supergiant’ X-ray binary stars
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LHC - The worlds largest microscope
by Sabine Hossenfelder @ Backreaction
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Gamma Ray Astronomy

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PPARC simulation of the Microquasar LS5039 ENLARGE Image
This image was created using software developed by Dr. Rob Hynes of LSU.

Gamma-rays are produced in extreme cosmic particle accelerators such as supernova explosions and provide a unique view of the high energy processes at work in the Milky Way.

VHE (very high energy) gamma-ray astronomy is still a young field and the High Energy Stereoscopic System (H.E.S.S.) is conducting the first sensitive survey at this energy range, finding previously unknown sources.

The object that is producing the high energy radiation is thought to be a 'microquasar'. These objects consist of two stars in orbit around each other. One star is an ordinary star, but the other has used up all its nuclear fuel, leaving behind a compact corpse. Depending on the mass of the star that produced it, this compact object is either a neutron star or a black hole, but either way its strong gravitational pull draws in matter from its companion star. This matter spirals down towards the neutron star or the black hole, in a similar way to water spiraling down a plughole.

However, sometimes the compact object receives more matter than it can cope with. The material is then squirted away from the system in a jet of matter moving at speeds close to that of light, resulting in a microquasar. Only a few such objects are known to exist in our galaxy and one of them, an object called LS5039, has now been detected by the H.E.S.S. team.
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The companion star to the compact object is a massive star that is losing material from its surface. This matter is then captured by the compact object's strong gravitational field and spirals down towards the surface. Some of this material is then ejected in two jets travelling at 20% of the speed of light. In fact, the real nature of LS5039 is something of a mystery. It is not clear what the compact object is. Some of the characteristics suggest it is a neutron star, some that it is a black hole. Not only that, but the jet isn't much of a jet; although it is moving at about 20% of the speed of light, which might seem a lot, in the context of these objects it's actually quite slow.

Nor is it clear how the gamma rays are being produced. Very high energy gamma rays emitted close to the companion star are more likely to be absorbed, creating a matter/antimatter cascade, than escape from the system.

The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of four 13 m diameter telescopes is currently the most sensitive detector of VHE gamma-rays - radiation that is a million, million times more energetic than the visible light.

These high energy gamma rays are quite rare even for relatively strong sources; only about one gamma ray per month hits a square metre at the top of the Earth's atmosphere. Also, since they are absorbed in the atmosphere, a direct detection of a significant number of the rare gamma rays would require a satellite of huge size.

The H.E.S.S. telescopes employ a trick - they use the atmosphere as detector medium. When gamma rays are absorbed in the air, they emit short flashes of blue light, named Cherenkov light, lasting a few billionths of a second. This light is collected by the H.E.S.S. telescopes with large mirrors and extremely sensitive cameras and can be used to create images of astronomical objects as they appear in gamma-rays.

More from PPARC PressRelease:
Mystery compact object producing high energy radiation
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Integral Catches a new erupting blackhole
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ESA's gamma-ray observatory, Integral, has spotted a rare kind of gamma-ray outburst.

The vast explosion of energy allowed astronomers to pinpoint a possible black hole in our Galaxy.

The outburst was discovered on 17 September 2006 by staff at the Integral Science Data Centre (ISDC), Versoix, Switzerland.

"The galactic centre is one of the most exciting regions for gamma ray astronomy because there are so many potential gamma-ray sources," says Roland Walter, an astronomer at the ISDC, and lead author of these results.

In this case, the outburst continued to rise in brightness for a few days before beginning a gradual decline that lasted for weeks. The way the brightness of an outburst rises and falls is known to astronomers as a light curve. "It was only after a week that we could see the shape of the light curve and realised what a rare event we had observed," says Walter.

Comparing the shape of the light curve to others on file revealed that this was an eruption thought to come from a binary star system in which one component is a star like our Sun whereas the other is a black hole.

In these systems, the gravity of the black hole is ripping the Sun-like star to pieces. As the doomed star orbits the black hole, it lays down its gas in a disc, know as an accretion disc, surrounding the black hole.

Occasionally, this accretion disc becomes unstable and collapses onto the black hole, causing the kind of outburst that Integral witnessed. Astronomers are still not sure why the accretion disc should collapse like this but one thing is certain: when it does collapse, it releases thousands of times the energy than at other times.

Because such active star–black hole binaries are thought to be rare in the Galaxy, astronomers expect Integral to see such an outbursts only once every few years. That makes each and every one a precious resource for astronomers to study.

INTEGRAL catches a new erupting black hole
Additional Material to ESA's Press Release of 27 Nov. 2006
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