Posted on by Fraser Cain

Dwarf Galaxies Have Been Through a Lot

Astronomers have shown for the first time that even the smallest galaxies in the Universe have complex structures that indicate a complex history. Using the Subaru Telescope, a team of astronomers from the National Astronomical Observatory of Japan, the Institute of Physics in Lithuania, the University of Durham, Paris Observatory, Kyoto University, Gunma Astronomical Observatory, and the University of Tokyo have discovered an extended halo of stars with a sharp cutoff in the dwarf irregular galaxy Leo A, a member of the Local Group of galaxies that includes the Milky Way. The discovery challenges current scenarios of galaxy formation by showing that instead of being the preservers of pristine building blocks that combined to form larger galaxies, dwarf irregular galaxies have their own history of build-up.

Understanding galaxy formation and evolution on time scales comparable to the age of the Universe is one of astronomy’s greatest challenges. In the scenarios of standard cosmology (Note 1), galaxies are built up via hierarchical merging: small primordial density fluctuations in the smooth distribution of matter in the early Universe grow and combine to form larger structures like the Milky Way. The most numerous type of galaxies in the universe — dwarf irregular galaxies (Note 2) — are supposed to preserve their properties unchanged over billions of years and represent pristine primeval building blocks. This is one reason why astronomers have recently been studying dwarf irregular galaxies with great interest.

The team led by Professors Nobuo Arimoto (National Astronomical Observatory of Japan) and Vladas Vansevicius (Institute of Physics, Lithuania) has studied Leo A — an isolated and extremely gas rich dwarf irregular galaxy with only 0.01% of the mass of the Milky Way and a low fraction chemical elements produced by earlier generations of stars. These characteristics suggest that this galaxy has been evolving without significant interaction with other galaxies. This galaxy has been believed to have quite a simple structure, in contrast to large disk galaxies like the Milky Way. However, this view needs to be changed due to deep imaging of the outer regions of this galaxy with the Subaru Telescope.

Prior to these observations, Leo A was already known to have a large angular size (7′ x 5′; Note 3) and Subaru Telescope equipped with its Prime Focus Camera (Suprime-Cam) was an ideal instrument to study the stars at the galaxy’s outer limits (Fig. 1). A single exposure with Suprime-Cam covers a field of view of 34′ x 27′ (pixel size 0”.2 x 0”.2) with high sensitivity. The team acquired optical images of the dwarf irregular galaxy Leo A with three broad band filters in November 2001. In order to trace the entire extent of the old stars in Leo A, the team employed red giant branch (RGB) stars which are evolved low-mass stars with very high luminosity and are expected to represent well the extended structures of galaxies. They investigated inside an ellipse of semi-major axis a = 12′ which fully covers the galaxy, and detected 1394 RGB stars distributed symmetrically and smoothly within this field.

Fig. 2 shows the radial profile of the surface number density of the red giant stars. The team found significantly larger disk structure (with a semi-major axis of 5.5′) than previously known (3.5′). Moreover, the deep observations permitted the discovery of a new stellar component in dwarf irregular galaxies, which the team calls a ?halo? (5.5′-7.5′), which has a less steep slope in the number density of RGB stars. The halo component ends at 8′ from the center of the galaxy with a sharp cutoff in the RGB star distribution. The existence of such a halo structure in dwarf irregular galaxies had been unconfirmed before these observations.

The size of Leo A revealed by these new observations is twice as large as its previously accepted size, suggesting that even in the nearby universe we see galaxies only as ?tips of icebergs” that are actually a few times more extended.

The newly discovered halo with a sharp stellar cutoff and the disk of the dwarf irregular galaxy Leo A closely resembles the structure as well as stellar and gaseous content found in large full-fledged disk galaxies like the Milky Way. The complicated structure of large massive galaxies has been believed to be a result of the merging of less massive galaxies like dwarf irregular ones. However, this study clearly reveals that the dwarf irregular galaxy Leo A already has disk and halo components, and suggests complex build-up histories for even very low mass galaxies like Leo A, which are supposed to form directly from the primordial density fluctuations in the early universe (Note 1), and challenges contemporary understanding of galaxy evolution. Professors N. Arimoto and V. Vansevicius believe Leo A is a ?Rosetta stone? (Note 4) for understanding the process of galaxy formation and evolution.

The scientific paper on this research has been accepted for publication in the August 20, 2004, Astrophysical Journal Letters (Volume 611, Number 2, L93).

Original Source: Subaru News Release

Posted on by Fraser Cain

Perseids Will Peak on August 11

The Perseid meteor shower, an annual celestial event beloved by millions of skywatchers around the world, returns to the night sky this coming week.

Sky & Telescope magazine predicts that the Perseid shower will reach its peak late Wednesday night and early Thursday morning, August 11?12. The rate of activity should pick up steam after midnight until the first light of dawn. North America, especially the West and Hawaii, is optimally positioned to catch the best of the shower.

An observer under a dark sky might typically see more than 60 Perseids per hour between midnight and dawn. Since the waning crescent Moon will be only three days from new at the time of shower maximum, posing minimal interference with the view, this is an opportune year for watching them.

You’ll need no equipment but your eyes. The darker your sky, the better ? any artificial light pollution in your sky will reduce the number of meteors that are visible. But even if you live in an urban or suburban area, you have a good chance of seeing at least some meteors. Find a dark spot with a wide-open view of the sky. Bring a reclining lawn chair, insect repellent, and blankets or a sleeping bag; clear August nights can get surprisingly chilly.

“Go out after about 11 p.m. or so, lie back, and watch the stars,” says Sky & Telescope senior editor Alan MacRobert. “Relax, be patient, and let your eyes adapt to the dark. With a little luck you’ll see a ‘shooting star’ every couple of minutes on average.”

Perseids can appear anywhere and everywhere in the sky. So the best direction to watch is wherever your sky is darkest, probably straight up. Faint Perseids appear as tiny, quick streaks. Occasional brighter ones may sail across the heavens for several seconds and leave a brief train of glowing smoke.

If you trace each meteor’s direction of flight backward far enough across the sky, you’ll find that your imaginary line crosses a spot in the constellation Perseus, near Cassiopeia. This is the shower’s radiant, the perspective point from which all the Perseids would appear to come if you could see them approaching from interplanetary space. The radiant is low in the north-northeast before midnight and rises higher in the northeast during the early-morning hours.

Don’t give up if it’s cloudy Wednesday night. The Perseid shower lasts for about two weeks, with good rates in the predawn hours of August 10th through 15th. This year the ever-thinning Moon becomes less of a problem with each passing night. Far fewer meteors will appear before midnight, even on the night of the shower’s maximum, because the radiant is then quite low in the sky. The radiant is always low or below the horizon for Southern Hemisphere countries like Australia, New Zealand, and South Africa, where few, if any, Perseids can be seen.

The Perseid meteoroids are tiny, sand- to pea-size bits of rocky debris that were shed long ago by Comet Swift-Tuttle. This comet, like others, is slowly disintegrating as it orbits the Sun. Over the centuries, its crumbly remains have spread all along its 130-year orbit to form a sparse “river of rubble” hundreds of millions of miles long.

Earth’s own path around the Sun carries us through this stream of particles every mid-August. The particles, or meteoroids, are traveling 37 miles per second with respect to Earth at the place where we encounter them. So when one of them strikes the upper atmosphere (about 50 to 80 miles up), it creates a quick, white-hot streak of superheated air.

For several years in the early 1990s the Perseids performed spectacularly, flaring with outbursts of up to hundreds of meteors per hour. The particles responsible for these outbursts were probably shed during Comet Swift-Tuttle’s swing by the Sun in 1862.

Astronomers Esko Lyytinen of Finland and Tom Van Flandern of Washington, DC, have alerted skygazers to the possibility that this “extra” Perseid peak could make a comeback in 2004. They predict that this year, the rubble trail released in 1862 will pass just 200,000 kilometers (125,000 miles)) inside Earth’s orbit on August 11th, just as observing conditions become optimal for meteor watchers in Eastern Europe and eastern North Africa eastward to central Russia, India, and western China.

Will the Perseids “storm” in 2004? There’s only one way to find out: Get outside and watch the show!

More about the Perseid meteors ? and how to watch and photograph them ? appears in the August 2004 issue of Sky & Telescope magazine and online in the articles listed at the end of this press release.

Original Source: Sky and Telescope News Release

Posted on by Fraser Cain

NASA Extends TRMM Mission through 2004

NASA will extend operation of the Tropical Rainfall Measuring Mission (TRMM) through the end of 2004, in light of a recent request from the National Oceanic and Atmospheric Administration (NOAA). The extension, to be undertaken jointly with NASA’s TRMM partner, the Japan Aerospace Exploration Agency (JAXA), will provide data during another storm season in the U.S. and Asia.

TRMM has yielded significant scientific research data over the last seven years to users around the globe. In addition, TRMM data has aided NOAA, other government agencies, and other users in their operational work of monitoring and predicting rainfall and storms, as well as in storm research. Launched in 1997, TRMM was originally designed as a three-year research mission. Following four years of extending TRMM, NASA and JAXA recently announced a decision to decommission TRMM, and proceed with a safe, controlled deorbit. Options for safe re-entry become increasingly limited the longer TRMM is operated, as it is already more than 3 years beyond design life.

“NASA is committed to working with our partner agencies to help them carry out their mission. We have decided to extend TRMM through this year’s hurricane season in our effort to aid NOAA in capturing another full season of storm data,” said Dr. Ghassem Asrar, Deputy Associate Administrator of NASA’s Science Mission Directorate. “The United States is a leader in Earth remote sensing, and NASA is proud of our role in building that leadership. Our work in partnership with NOAA and international partners such as JAXA is an important part of the world’s scientific research on global precipitation and weather. TRMM has been a valuable part of that legacy and we look to our follow-on missions to continue to reap great public benefit,” he added.

TRMM is the first satellite to measure rainfall over the global tropics, allowing scientists to study the transfer of water and energy among the global atmosphere and ocean surface that form the faster portions of the Earth’s climate system. Because TRMM’s radar enables it to “see through” clouds, it allows weather researchers to make the equivalent of a CAT-scan of hurricanes and helps weather forecasters to use TRMM data to improve prediction of severe storms.

“TRMM has proven helpful in complementing the other satellite data used by NOAA’s National Weather Service in its operations,” said Retired Air Force Brig. Gen. David L. Johnson, Director of NOAA’s National Weather Service.

JAXA welcomes and supports the decision to extend TRMM. The extension will be of benefit to the worldwide science and research communities. NASA and JAXA look forward to continuing their close collaboration beyond TRMM through establishment of a new advanced capability for the measurement of precipitation globally with the Global Precipitation Measurement Mission (GPM). GPM will use an extensive ground validation network to further improve the accuracy of its measurements compared to those made by TRMM.

NASA and NOAA have asked the National Academy of Sciences to convene a workshop next month to advise NASA and NOAA on the best use of TRMM’s remaining spacecraft life; the overall risks and benefits of the TRMM mission extension options; the advisability of transfer of operational responsibility for TRMM to NOAA; any requirement for a follow-on operational satellite to provide comparable TRMM data; and optimal use of GPM, a follow-on research spacecraft to TRMM, planned for launch by the end of the decade.

“It’s important to note that we are able to extend TRMM for this brief period and are vigilant in maintaining our requirement for a safe, controlled re-entry and deorbit of the spacecraft,” said Asrar. “We also welcome the opportunity to receive advice from the National Academy of Sciences next month on the best use of TRMM’s remaining spacecraft life, TRMM re-entry risk, and the best use of our upcoming next-generation research spacecraft, GPM,” he added

NASA and NOAA will work with the National Academy of Sciences to share with the public outcomes from next month’s workshop.

For more information about TRMM on the Internet, visit:

http://trmm.gsfc.nasa.gov/

Original Source: NASA News Release

Posted on by Fraser Cain

Outbound View of Saturn After Initial Orbit

A frigid ball of gas in the blackness of space, Cassini?s new home appears cool and serene in this natural color image.

The spacecraft obtained this view as it sped outward from the planet on its initial orbit. At left, Saturn?s shadow stretches almost completely across the rings, while at right the planet?s illuminated face appears to gaze down at the far-off Sun.

Images taken through blue, green and red filters with the wide angle camera were combined to create this natural color view. The images were taken on July 17, 2004, from a distance of about 5.8 million kilometers (3.6 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 93 degrees. The image scale is 346 kilometers (215 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colorado.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Posted on by Fraser Cain

Cassini Sees Lightning on Saturn

The Cassini spacecraft, which began its tour of the Saturn system just over a month ago, has detected lightning and a new radiation belt at Saturn, and a glow around the planet’s largest moon, Titan.

The spacecraft’s radio and plasma wave science instrument detected radio waves generated by lightning. “We are detecting the same crackle and pop one hears when listening to an AM radio broadcast during a thunderstorm,” said Dr. Bill Kurth, deputy principal investigator on the radio and plasma wave instrument, University of Iowa, Iowa City. “These storms are dramatically different than those observed 20 years ago.”

Cassini finds radio bursts from this lightning are highly episodic. There are large variations in the occurrence of lightning from day to day, sometimes with little or no lightning, suggesting a number of different, possibly short-lived storms at middle to high latitudes. Voyager observed lightning from an extended storm system at low latitudes, which lasted for months and appeared highly regular from one day to the next.

The difference in storm characteristics may be related to very different shadowing conditions in the 1980s than are found now. During the Voyager time period when lightning was first observed, the rings cast a very deep shadow near Saturn’s equator. As a result, the atmosphere in a narrow band was permanently in shadow — making it cold — and located right next to the hottest area in Saturn’s atmosphere. Turbulence between the hot and cold regions could have led to long-lived storms. However, during Cassini’s approach and entry into Saturn’s orbit, it is summer in the southern hemisphere and the ring shadow is distributed widely over a large portion of the northern hemisphere, so the hottest and coldest regions are far apart.

A major finding of the magnetospheric imaging instrument is the discovery of a new radiation belt just above Saturn’s cloud tops, up to the inner edge of the D-ring. This is the first time that a new Saturnian radiation belt has been discovered with remote sensing.

This new radiation belt extends around the planet. It was detected by the emission of fast neutral atoms created as its magnetically trapped ions interact with gas clouds located planetward of the D-ring, the innermost of Saturn’s rings. With this discovery, the radiation belts are shown to extend far closer to the planet than previously known.

“This new radiation belt had eluded detection by any of the spacecraft that previously visited Saturn. With its discovery we have seen something that we did not expect, that radiation belt particles can ‘hop’ over obstructions like Saturn’s rings, without being absorbed by the rings in the process,” said Dr. Donald G. Mitchell, instrument scientist for the magnetospheric imaging instrument at the Johns Hopkins University Applied Physics Laboratory, Laurel, Md.

Saturn’s largest moon, Titan, is also shining for attention. Cassini’s visual and infrared mapping spectrometer captured Titan glowing both day and night, powered by emissions from methane and carbon monoxide gases in the moon’s extensive, thick atmosphere.

“Not only is Titan putting on a great light show but it is also teaching us more about its dense atmosphere,” said Dr. Kevin Baines, science team member for the visual and infrared mapping spectrometer at JPL. “What is amazing is that the size of this glow or emission of gases is a sixth the diameter of the planet.”

The Sun-illuminated fluorescent glow of methane throughout Titan’s upper atmosphere ? revealing the atmosphere’s immense thickness and extending more than 700 kilometers (435 miles) above the surface, was expected. However, the nighttime glow, persistently shining over the night side of Titan, initially surprised scientists.

“These images are as if you were seeing Titan through alien eyes. Titan glows throughout the near-infrared spectrum. If you were an alien it would be hard to get a good night’s sleep on Titan because the light would always be on,” said Baines.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter.

For the latest images and more information about the Cassini- Huygens mission, visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Proton Launches Amazonas Satellite

A Proton rocket launched by International Launch Services (ILS) successfully carried the Amazonas satellite into orbit today.

The 191-foot-tall (58.2-meters) vehicle lifted off at 4:32 a.m. from Baikonur (6:32 p.m. EDT Wednesday, 22:32 GMT Wednesday). The rocket?s Breeze M upper stage placed the satellite into a transfer orbit 9 hours and 11 minutes later.

The launch vehicle was built by Khrunichev State Research and Production Space Center of Moscow, a partner in the ILS joint venture along with Lockheed Martin Corp. [NYSE:LMT]. This was the third Proton launch of the year for ILS, and the seventh mission overall for the company in 2004.

The satellite was built for Hispasat of Spain, and will be used by its subsidiaries Hispamar of Brazil and Hispasat Canarias to provide a multitude of communications services at both C- and Ku-band on both sides of the Atlantic Ocean. When it reaches its operating position of 61 degrees West longitude, the Amazonas satellite will be the largest in the Hispasat fleet. The high-power satellite is a Eurostar E3000 model built by EADS Astrium of Europe, the third E3000 spacecraft launched by ILS this year.

?We thank Hispasat for again selecting an ILS vehicle,? said ILS President Mark Albrecht. ?We have demonstrated that Hispasat can rely on us for a good launch, whether they choose Proton, or whether they choose Atlas as they have twice before.?

Albrecht added that the Proton vehicle has now completed five missions this year, three for ILS and two for the Russian government. ?We call this the workhorse of the Russian fleet, because it flies so often and so reliably. Everyone from start-up businesses to telecommunications giants to participants in the International Space Station have put their trust in Proton,? he said. ?ILS is proud to market this vehicle for commercial satellite launches.?

“This is a major milestone for EADS Astrium, with Amazonas being the third Eurostar E3000 version satellite to be launched this year by a Proton Breeze M launcher from ILS,” said Antoine Bouvier, CEO of EADS Astrium. ?Amazonas is the most powerful satellite ordered by Hispasat. The satellite was delivered in time, which was very important. This satellite is equipped with a new lithium-ion battery offering higher efficiency. All our teams in Europe were mobilized to make this event a success.?

ILS has established itself as the indisputable launch services leader by offering the industry’s two best launch systems: Proton and the Lockheed Martin-built Atlas. With a remarkable launch rate of 65 missions since 2000, the Atlas and Proton launch vehicles have consistently demonstrated the reliability and flexibility that have made them the vehicles of choice. Since the beginning of 2003, ILS has signed more new commercial contracts than all its competitors combined. ILS was formed in 1995, and is based in McLean, Va., a suburb of Washington, D.C.

Original Source: ILS News Release

Posted on by Fraser Cain

da Vinci Project Announces X Prize Attempt

The Canadian da Vinci Project Team has notified the ANSARI X PRIZE of its intention to launch its rocket on October 2nd, 2004, marking its official entry in the international, commercially-funded space race competition.

The da Vinci Project, which unveiled its rocket Wild Fire today, joins one American team — in a field of 26 — to announce its launch date. “With two ANSARI X PRIZE teams launching within days of each other for the $10 million prize (U.S.), we truly have a remarkable race for space,” said Dr. Peter H. Diamandis, Chairman and Founder of the X PRIZE Foundation. The recipient of the $10 million prize will be the first team to travel safely to space twice within a two-week period on a privately funded, re-useable spacecraft.

Brian Feeney, who plans to pilot Wild Fire approximately 85 kilometers into suborbital space, said the team is finalizing construction of the rocket as well as logistical details related to the event, which will be held in Kindersley, Saskatchewan.

“We’re very close to achieving our mission, thanks to the organizations and individuals that understand the significance of this race,” said Feeney. “The da Vinci Project is on the cusp of a new era of space travel for humankind. Our team is proof positive that ingenuity and innovation can overcome the impossible.”

In addition, Feeney announced a new sponsor to finance the project. Golden Palace.Com, the world’s largest online casino, has signed on as the title sponsor. The Golden Palace.Com Space Program — Powered By The da Vinci Project — is poised to make history.

According to sources at the casino, GoldenPalace.com is excited and very proud to be a part of the historic flight of the Wild Fire. In the continuing pursuit for innovative ideas for exposure, Golden Palace sources believes the ANSARI X Prize competition provides the advertising exposure as well as the added incentive of a history-making event that will eventually serve to benefit all humankind.

The da Vinci Project, which Feeney describes as the largest volunteer technology project in Canada, has achieved several critical milestones since officially entering the competition in 1996. These milestones have included the filing of necessary papers to Canadian government for launch approval; securing the site for launch; the testing of engine components; testing the design and securing a wide range of sponsors including Sun Microsystems of Canada, Blake Cassels Graydon, ANSYS, Hinz Automation and Kindersley Transport.

“The da Vinci Project is emblematic of the spirit of the ANSARI X PRIZE,” said Diamandis. “It brings together many of the best and brightest to break down preconceived notions on space travel and prove that we are no longer bound to one planet.”

The X PRIZE Foundation is working with the Federation Aeronautique Internationale (FAI), the international body responsible for all aviation and space world records, to ensure that the ANSARI X PRIZE winner will also receive official international recognition for his achievement. In addition to duration, altitude and distance, the FAI has introduced new world record categories for ‘minimum time between two consecutive sub-orbital flights in a reusable vehicle’ and ‘number of persons carried in sub-orbital flight’.

Original Source: da Vinci Project News Release

Posted on by Fraser Cain

Wallpaper: NGC 3949

Our Sun and solar system are embedded in a broad pancake of stars deep within the disk of the Milky Way galaxy. Even from a distance, it is impossible to see our galaxy’s large-scale features other than the disk.

The next best thing is to look farther out into the universe at galaxies that are similar in shape and structure to our home galaxy. Other spiral galaxies like NGC 3949, pictured in the Hubble image, fit the bill. Like our Milky Way, this galaxy has a blue disk of young stars peppered with bright pink star-birth regions. In contrast to the blue disk, the bright central bulge is made up of mostly older, redder stars.

NGC 3949 lies about 50 million light-years from Earth. It is a member of a loose cluster of some six or seven dozens of galaxies located in the direction of the Big Dipper, in the constellation Ursa Major (the Great Bear). It is one of the larger galaxies of this cluster.

This image was created from Hubble data taken with the Wide Field Planetary Camera 2 in October 2001. Separate exposures through blue, visible, and near-infrared filters have been combined to make the natural color picture. This image was produced by the Hubble Heritage Team (STScI).

Original Source: Hubble News Release

Posted on by Fraser Cain

Slides on Olympus Mons

This image from ESA’s Mars Express show the western flank of the shield volcano Olympus Mons in the Tharsis region of the western Martian hemisphere.

The image was taken by the High Resolution Stereo Camera (HRSC) during orbit 143 from an altitude of 266 kilometres. It were taken with a resolution of about 25 metres per pixel and is centred at 222? East and 22? North. North is to the left.

The image shows the western part of the escarpment, rising from the surface level to over 7000 metres. In the foreground, part of the extensive plains west of the escarpment are shown, known as an ‘aureole’ (from the Latin for ‘circle of light’).

To the north and west of the volcano, these ‘aureole’ deposits are regions of gigantic ridges and blocks extending some 1000 kilometres from the summit like petals of a flower. An explanation for the origin of the deposits has challenged planetary scientists for decades.

The most persistent explanation, however, has been landslides. Large masses of shield material can be found in the aureole area. Several indications also suggest a development and resurfacing connected to glacial activity.

Original Source: ESA News Release

Posted on by Fraser Cain

Closer, Dimmer Gamma Ray Burst Spotted

A gamma-ray burst detected by ESA’s Integral gamma-ray observatory on 3 December 2003 has been thoroughly studied for months by an armada of space and ground-based observatories. Astronomers have now concluded that this event, called GRB 031203, is the closest cosmic gamma-ray burst on record, but also the faintest. This also suggests that an entire population of sub-energetic gamma-ray bursts has so far gone unnoticed…

Cosmic gamma-ray bursts (GRBs) are flashes of gamma rays that can last from less than a second to a few minutes and occur at random positions in the sky. A large fraction of them is thought to result when a black hole is created from a dying star in a distant galaxy. Astronomers believe that a hot disc surrounding the black hole, made of gas and matter falling onto it, somehow emits an energetic beam parallel to the axis of rotation.

According to the simplest picture, all GRBs should emit similar amounts of gamma-ray energy. The fraction of it detected at Earth should then depend on the ‘width’ (opening angle) and orientation of the beam as well as on the distance. The energy received should be larger when the beam is narrow or points towards us and smaller when the beam is broad or points away from us. New data collected with ESA’s high energy observatories, Integral and XMM-Newton, now show that this picture is not so clear-cut and that the amount of energy emitted by GRBs can vary significantly. “The idea that all GRBs spit out the same amount of gamma rays, or that they are ‘standard candles’ as we call them, is simply ruled out by the new data,” said Dr Sergey Sazonov, from the Space Research Institute of the Russian Academy of Sciences, Moscow (Russia) and the Max-Planck Institute for Astrophysics, Garching near Munich (Germany).

Sazonov and an international team of researchers studied the GRB detected by Integral on 3 December 2003 and given the code-name of GRB 031203. Within a record 18 seconds of the burst, the Integral Burst Alert System had pinpointed the approximate position of GRB 031203 in the sky and sent the information to a network of observatories around the world. A few hours later one of them, ESA’s XMM-Newton, determined a much more precise position for GRB 031203 and detected a rapidly fading X-ray source, which was subsequently seen by radio and optical telescopes on the ground.

This wealth of data allowed astronomers to determine that GRB 031203 went off in a galaxy less than 1300 million light years away, making it the closest GRB ever observed. Even so, the way in which GRB 031203 dimmed with time and the distribution of its energy were not different from those of distant GRBs. Then, scientists started to realise that the concept of the ‘standard candle’ may not hold. “Being so close should make GRB 031203 appear very bright, but the amount of gamma-rays measured by Integral is about one thousand times less than what we would normally expect from a GRB,” Sazonov said.

A burst of gamma rays observed in 1998 in a closer galaxy appeared even fainter, about one hundred times less bright than GRB 031203. Astronomers, however, could not conclusively tell whether that was a genuine GRB because the bulk of its energy was emitted mostly as X-rays instead of gamma-rays. The work of Sazonov’s team on GRB 031203 now suggests that intrinsically fainter GRBs can indeed exist.

A team of US astronomers, coordinated by Alicia Soderberg from the California Institute of Technology, Pasadena (USA), studied the ‘afterglow’ of GRB 031203 and gave further support to this conclusion. The afterglow, emitted when a GRB’s blastwave shocks the diffuse medium around it, can last weeks or months and progressively fades away. Using NASA’s Chandra X-ray Observatory, Soderberg and her team saw that the X-ray brightness of the afterglow was about one thousand times fainter than that of typical distant GRBs. The team’s observations with the Very Large Array telescope of the National Radio Astronomy Observatory in Socorro (USA) also revealed a source dimmer than usual.

Sazonov and Soderberg explain that their teams looked carefully for signs that GRB 031203 could be tilted in such a way that most of its energy would escape Integral’s detection. However, as Sazonov said, “the fact that most of the energy that we see is emitted in the gamma-ray domain, rather than in the X-rays, means that we are seeing the beam nearly on axis.” It is, therefore, unlikely that much of its energy output can go unnoticed.

This discovery suggests the existence of a new population of GRBs much closer but also dimmer than the majority of those known so far, which are very energetic but distant. Objects of this type may also be very numerous and thus produce more frequent bursts.

The bulk of this population has so far escaped our attention because it lies at the limit of detection with past and present instruments. Integral, however, may be just sensitive enough to reveal a few more of them in the years to come. These could be just the tip of the iceberg and future gamma-ray observatories, such as the planned NASA’s Swift mission, should be able to extend this search to a much larger volume of the Universe and find many more sub-energetic GRBs.

Original Source: ESA News Release