Category: Astronomy

Herbig-Haro 211 consists of two jets of material, visible at lower right. Image credit: A.A. Muench-Nasrallah, CfA. Click to enlarge.
Astronomers find jets everywhere when they look into space. Small jets spout from newborn stars, while huge jets blast out of the centers of galaxies. Yet despite their commonness, the processes that drive them remain shrouded in mystery. Even relatively nearby stellar jets hide their origins behind almost impenetrable clouds of dust. All stars, including our sun, pass through a jet phase during their “childhood,” so astronomers are eager to understand how jets form and how they may influence star and planet formation.

At this week’s meeting on submillimeter astronomy in Cambridge, Mass., astronomers described the latest results from an international collaboration using the Submillimeter Array (SMA) atop Mauna Kea, Hawaii. The SMA has begun to peer through the dust and home in on the sources of nearby stellar jets.

“Using the SMA, we can stare into the throat of the jet,” said SMA project scientist Paul Ho of the Harvard-Smithsonian Center for Astrophysics (CfA). “We’re getting close to seeing its launching point.”

Astronomer Hsien Shang of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) and her colleagues have created a model of jet formation that calculates temperatures, densities and brightnesses within stellar jets. SMA observations of a young star system prosaically named Herbig-Haro (HH) 211 have confirmed the validity of the model.

“Our model predicts what we will see about 100 astronomical units from the star,” Shang said. (One astronomical unit is the average Earth-Sun distance of 93 million miles.) “With the SMA, we can begin to look at the HH 211 system at the scale of the model and test those predictions. So far, everything checks out.”

HH 211 is located about 1,000 light-years away in the constellation Perseus. Astronomers estimate that the small protostar hidden within HH 211 is less than 1,000 years old-a mere baby by astronomical standards, so young that it is still growing by accumulating matter from a surrounding disk of gas and dust. The protostar eventually will become a low-mass star similar to the sun.

Although most of the matter in the disk will flow onto the star, some must be ejected outward to carry away excess angular momentum. Complex physical processes funnel that ejected matter into dual jets that shoot outward in opposite directions.

“Jets form very close to a protostar, within about 5 million miles of its surface according to the model we applied” said researcher Naomi Hirano (ASIAA). “The SMA can help test the jet model on the youngest protostars using molecular tracers from within that innermost region.”

SMA’s successor, the planned ALMA project, should finally reveal the nature of the engine powering these jets by peering into the core where they form.

“The SMA has brought us tantalizingly close to our goal-the answer to the question of how jets form,” said Ho. “ALMA will take us those final few steps.”

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint 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.

Original Source: Harvard CfA News Release

A cosmic view of neutrino ripples. Image credit: Oxford. Click to enlarge.
Astrophysicists from the Universities of Oxford and Rome have for the first time found evidence of ripples in the Universe?s primordial sea of neutrinos, confirming the predictions of both Big Bang theory and the Standard Model of particle physics.

Neutrinos are elementary particles with no charge and very little mass, which are extremely difficult to study due to their very weak interaction with matter. Yet pinning down the physical properties of neutrinos is of paramount importance to scientists attempting to understand the fundamental building blocks of Nature. According to the standard Big Bang model, neutrinos permeate the Universe at a density of about 150 per cubic centimetre. The Earth is therefore immersed in an ocean of neutrinos, without us ever noticing.

Although it is impossible to measure this ?Cosmic Neutrino Background? directly with present-day technology, physicists predict that ripples or waves in it have an impact on the growth of structures in the Universe.

In research to be published in the journal Physical Review Letters, Dr. Roberto Trotta, Lockyer Fellow of the Royal Astronomical Society at Oxford?s Department of Physics, and Dr. Alessandro Melchiorri of La Sapienza University in Rome were able to demonstrate for the first time the existence of ripples of primordial origin in the Cosmic Neutrino Background.

The discovery, made by combining data produced by the NASA WMAP (Wilkinson Microwave Anisotropy Probe) satellite and the Sloan Digital Sky Survey, confirms the predictions of both the Big Bang theory and the Standard Model of particle physics. The research has important implications for the study of neutrinos, showing that theories of the infinitely large (cosmology) and the infinitely small (particle physics) are in agreement.

Dr. Trotta said: ?This research provides important new evidence in favour of the current cosmological model, unifying it with fundamental physics theories. Cosmology is becoming a more and more powerful laboratory where physics not easily accessible on Earth can be tested and verified. The high quality of recent cosmological data allows us to investigate neutrinos in the cosmological framework, obtaining measurements which are competitive with ? if not superior to ? particle accelerator findings.?

Original Source: Oxford News Release

Artist interpretation of protoplanetary systems forming inside a nebula. Image credit: CfA. Click to enlarge.
Meeting this week in Cambridge, Mass., astronomers using the Submillimeter Array (SMA) on Mauna Kea, Hawaii, confirmed, for the first time, that many of the objects termed “proplyds” found in the Orion Nebula do have sufficient material to form new planetary systems like our own.

“The SMA is the only telescope that can measure the dust within the Orion proplyds, and thereby assess their true potential for forming planets. This is critical in our understanding of how solar systems form in hostile regions of space,” said Jonathan Williams of the University of Hawaii Institute for Astronomy, lead author on a paper submitted to The Astrophysical Journal.

Surviving in the chaotic regions within the Orion Nebula where stellar winds can reach a staggering two million miles per hour and temperatures exceed a searing 18,000 degrees Fahrenheit, the question remained – would enough material endure to form a new solar system or would it be eroded away into space like wind and sand eroding away desert cliffs? It now appears that these protoplanetary disks are quite tenacious, bringing new grounds for optimism in the search for planetary systems.

Imaged by the Hubble Space Telescope back in the early 1990s as misshapen silhouettes against the nebular background, the most spectacular proplyds appear bright. Their surrounding ionized cocoons glow due to their close proximity to a nearby hot star formation called the Trapezium. The Trapezium is a star cluster consisting of more than 1,000 young, hot stars that are only 1 million years old. They condensed out of the original cold, dark cloud of gas that now glows from their ionizing light. They are crowded into a space about 4 light-years in diameter, the same as the distance between the Sun and Proxima Centauri, the next closest star in space.

Blasted by the solar winds of the Trapezium, the proplyds are the next generation of smaller stars to arise in Orion, this time with visible discs that may be forming planets. It has remained unclear, however, whether they contained enough material to form stable planetary systems. Using the SMA, astronomers now have been able to probe deep inside these disks to measure their mass and to unravel the formation process presented by these potential infant solar systems.

“While the Hubble pictures were spectacular, they revealed only disk-like shapes that did not tell us the amount of material present,” said David Wilner, of the Harvard-Smithsonian Center for Astrophysics (CfA). Since some of the discs appear to be comparable in size and mass to our own solar system, this strengthens the connection between the Orion proplyds and our origins.

Since most Sun-like stars in the Galaxy eventually form in environments like the Orion Nebula, the SMA results suggest that the formation of solar systems like our own is common and a continuing event in the Galaxy.

“The same cycle of birth, life and death we experience here on Earth is repeated in the stars overhead. Now, the SMA provides us with a front-row seat in unraveling the wonder of these cosmic events,” reflected Wilner.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint 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.

Original Source: CfA News Release

Supernova remnant Cassiopeia A. Image credit: NASA/JPL. Click to enlarge.
An enormous light echo etched in the sky by a fitful dead star was spotted by the infrared eyes of NASA’s Spitzer Space Telescope.

The surprising finding indicates Cassiopeia A, the remnant of a star that died in a supernova explosion 325 years ago, is not resting peacefully. Instead, this dead star likely shot out at least one burst of energy as recently as 50 years ago.

“We had thought the stellar remains inside Cassiopeia A were just fading away,” said Dr. Oliver Krause, University of Arizona, Tucson. “Spitzer came along and showed us this exploded star, one of the most intensively studied objects in the sky, is still undergoing death throes before heading to its final grave.”

Infrared echoes trace the dusty journeys of light waves blasted away from supernova or erupting stars. As the light waves move outward, they heat up clumps of surrounding dust, causing them to glow in infrared light. The echo from Cassiopeia A is the first witnessed around a long-dead star and the largest ever seen. It was discovered by accident during a Spitzer instrument test.

“We had no idea that Spitzer would ever see light echoes,” said Dr. George Rieke of the University of Arizona. “Sometimes you just trip over the biggest discoveries.”

To view the echoes dancing through clouds of dust surrounding Cassiopeia A, visit:
http://www.spitzer.caltech.edu/Media/releases/ssc2005-14/visuals.shtml.

A supernova remnant like Cassiopeia A typically consists of an outer, shimmering shell of expelled material and a core skeleton of a once-massive star, called a neutron star. Neutron stars come in several varieties, ranging from intensely active to silent. Typically, a star that has recently died will continue to act up. Consequently, astronomers were puzzled that the star responsible for Cassiopeia A appeared to be silent so soon after its death.

The new infrared echo indicates the Cassiopeia A neutron star is active and may even be an exotic, spastic type of object called a magnetar. Magnetars are like screaming dead stars, with eruptive surfaces that rupture and quake, pouring out tremendous amounts of high-energy gamma rays. Spitzer may have captured the “shriek” of such a star in the form of light zipping away through space and heating up its surroundings.

“Magnetars are very rare and hard to study, especially if they are no longer associated with their place of origin. If we have indeed uncovered one, then it will be just about the only one for which we know what kind of star it came from and when,” Rieke said.

Astronomers first saw hints of the infrared echo in strange, tangled dust features that showed up in the Spitzer test image. When they looked at the same dust features again a few months later using ground-based telescopes, the dust appeared to be moving outward at the speed of light. Follow-up Spitzer observations taken one year later revealed the dust was not moving, but was being lit up by passing light.

A close inspection of the Spitzer pictures revealed a blend of at least two light echoes around Cassiopeia A, one from its supernova explosion, and one from the hiccup of activity that occurred around 1953. Additional Spitzer observations of these light echoes may help pin down their enigmatic source.

Krause was lead author with Rieke of a study about the discovery appearing this week in the journal Science.

JPL manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate. Science operations are conducted at the Spitzer Science Center, California Institute of Technology, Pasadena, Calif. JPL is a division of Caltech. Spitzer’s multiband imaging photometer, which made the new observations, was built by Ball Aerospace Corporation, Boulder, Colo.; the University of Arizona; and Boeing North America, Canoga Park, Calif. Its development was led by Rieke.

For additional images and information about Spitzer on the Web, visit: http://www.spitzer.caltech.edu/Media. For information about NASA and agency programs on the Web, visit: http://www.nasa.gov/home/index.html.

Original Source: NASA/JPL News Release

All-sky map of the best fitting ‘halo+disk’ model of 511 keV gamma-ray line emission. Image credit: INTEGRAL. Click to enlarge.
The positron, the anti-matter counterpart to the electron, was predicted by Paul Dirac’s – at the time revolutionary – quantum wave equation for the electron. A few years later, in 1932, Carl Anderson discovered the positron in cosmic rays, and Dirac got the Nobel Prize in 1933 and Anderson in 1936.

When a positron meets an electron, they annihilate, producing two gamma rays. Sometimes however, the annihilation is preceded by the formation of positronium, which is like a hydrogen atom with the proton replaced by a positron (positronium has its own symbol, Ps). Positronium comes in two forms, is unstable, and decays into either two gammas (within about 0.1 nanoseconds) or three (within about 100 nanoseconds).

Astronomers have known since the 1970s that there must be a lot of positrons in the universe. Why? Because when a positron and electron annihilate to give two gammas, both have the same wavelength, about 0.024 Å, or 0.0024 nm (astronomers, like particle physicists, don’t talk about the wavelengths of gamma rays, they talk about their energy; 511 keV in this case). So, if you look at the sky with gamma-ray vision – from above the atmosphere of course! – you know there was lots of positrons because you can see lots of gammas of a single ‘colour’, 511 keV (it’s similar to concluding there’s lots of hydrogen in the universe by noticing lots of the red (1.9 eV) H alpha in the night sky).

From the spectrum of the three-gamma decay of positronium, compared with the 511 keV line intensity, astronomers four years ago worked out that about 93% of positrons whose annihilation we see form positronium before they decay.

How much positronium? In the Milky Way bulge, about 15 billion (thousand million) tons of positrons are annihilated every second. That’s as much mass as the electrons in tens of trillions of tons of stuff we’re used to, like rocks or water; about as much as in a mid-sized asteroid, 40 km across.

By analyzing the publicly released INTEGRAL data (about one year’s worth), J?rgen Kn?dlseder and his colleagues found that:

• the positrons which are being annihilated in the Milky Way disk most likely come from the beta+ (i.e. positron) decay of the isotopes Aluminium-26 and Titanium-44, which themselves were produced in recent supernovae (remember, astronomers call even 10 million years ago ‘recent’)
• however, there are more positrons being annihilated in the Milky Way bulge than in the disk, by a factor of five
• there don’t seem to be any ‘point’ sources.

Of course, to an INTEGRAL scientist, a ‘point’ source doesn’t have quite the same meaning as it does to an amateur astronomer! Gamma-ray vision in the positronium line is incredibly blurry, an object six Moons across (3?) would look like a ‘point’! Nonetheless, Kn?dlseder and his team of astrophysics sleuths are able to say that “none of the sources we searched for showed a significant 511 keV flux”; these 40 ‘usual suspects’ include pulsars, quasars, black holes, supernovae remnants, star-forming regions, rich galaxy clusters, satellite galaxies, and blazars. But, they’re still looking, “We have indeed [planned,] dedicated INTEGRAL observations of the usual suspects, such as Type Ia supernovae (SN1006, Tycho), and LMXB (Cen X-4) which might help to solve this problem.”

So, where do the 15 billion tons of positrons being annihilated every second in the bulge come from? “For me the most important thing about the positron annihilation is that the principal source is still a mystery,” says Kn?dlseder. “We can explain the faint emission from the disk by Aluminium-26 decay, but the bulk of positrons are situated in the bulge region of the Galaxy, and we have no source that can easily explain all observational characteristics. In particular, if you compare the 511 keV sky to the sky observed at other wavelengths you recognise that the 511 keV sky is unique! There is no other sky that resembles to what we observe.”

The INTEGRAL team feel they can rule out massive stars, collapsars, pulsars, or cosmic ray interactions, for if these were the source of the bulge positrons, then the disk would be much brighter in 511 keV light.

The bulge positrons may come from low-mass X-ray binaries, classical novae, or Type 1a supernovae, through a variety of processes. The challenge in each case is to understand how sufficient positrons created by these could survive long enough afterwards and diffuse far enough from their birthplaces.

What about cosmic strings? While the recent Tanmay Vachaspati paper proposing these as a possible source of the bulge positrons came out too recently for Kn?dlseder et al. to consider for their paper, “Yet for me it is not obvious that we have enough observational constraints to state that cosmic strings make the 511 keV; we don’t even know if cosmic strings exist. One would need a unique characteristic of cosmic strings that exclude all other sources, and today I think we are far from this.”

Perhaps most excitingly, the positrons may come from the annihilation of a low-mass dark matter particle and its anti-particle, or as Kn?dlseder et al. put it “Light dark matter (1-100 MeV) annihilation, as suggested recently by Boehm et al. (2004), is probably the most exotic but also the most exciting candidate source of galactic positrons.” Dark matter is even more exotic than positronium; dark matter is not anti-matter, and no one has been able to capture it, let alone study it in a lab. Astronomers accept that it is ubiquitous and tracking down its nature is one of the hottest topics in both astrophysics and particle physics. If the billions of tons per second of positrons that are annihilated in the Milky Way bulge cannot have come from classical novae or thermonuclear supernovae, then perhaps good old dark matter is to blame.

Chandra image of SN1970G. Image credit: NASA. Click to enlarge.
As astronomers look out over the Universe, one principle stands out in bas relief above the vast welter of data and information captured by their instruments – the Universe is a work in progress. From hydrogen atom to galaxy cluster, things undergo change in surprisingly similar ways. A principle of growth, maturation, death, and rebirth is at play in the Universe. Nowhere is that principle more fully embodied than in the primary sources of light we see through our instruments – the stars.

On June 1 2005, a pair of investigators (Stefan Immler of NASA’s Goddard Space Flight Center and K.D. Kuntz of John Hopkins University) published X-ray data collected from a variety of space-borne instruments. The data reveals how one massive star passing within a nearby galaxy (M101) can help us understand the relatively short period between a star’s death and the transformation of its luminous wreath of gas into a supernova remnant. That star – supernova SN 1970G – has now experienced some 35 years of a visible “afterlife” in the form of a rapidly spinning neutronic core within an expansive circumstellar aura of gas and dust (the CSM or circumstellar matter). Even now (from our perception) heavy metals race outward at a speed of thousands of kilometers per second – potentially planting seeds of organic matter within the Interstellar Medium (ISM) of a 27 million light year distant galaxy – one easily visible in the smallest of instruments within the spring constellation of Ursa Majoris. Only when the energy within that matter reaches the ISM, will 1970G have completed its cycle of birth and potential rebirth to take form in new stars and planets.

The destiny of a star is primarily determined by its mass. Surviving for as little as 50,000 years, the most massive stars (as great as 150 suns) condense out of vast concentrations of cold gas and dust to eventually live very fast lives. In youth, such stars exult as brilliant blue giants radiating near-ultraviolet light from a photosphere whose temperature may be five times greater than that of our own Sun. Within such stars nuclear furnaces rapidly accumulate giving off prodigious amounts of extremely intense radiation. Pressure from this radiation propels the star’s outer shroud outward many times over even as a howling gale of highly charged particles boils off its surface to become the stars CSM. Due to pressure exerted by its rapidly expanding core, such a star’s nuclear engine eventually becomes starved for fuel. The subsequent collapse is marked by a brilliant light show – one that can potentially outshine an entire galaxy. At magnitude 12.1, type II supernova 1970G never became bright enough to overcome its 8th magnitude host. But for some 30,000 years prior to its efflorescence, 1970G boiled off copious quantities of hydrogen and helium gas in the form of a powerful solar wind. Later, that same diaphanous aura of matter took the brunt of 1970G’s outburst shocking it into X-ray excitation. And it is that period of expanding shockwaves that has dominated the energy signature or “flux” of 1970G over the past 35 years of observation.

According to a paper entitled “Discovery of X-Ray Emission from Supernova 1970G with Chandra” Immler and Kuntz report that, “As the oldest SN detected in X-rays, SN 1970G allows, for the first time, direct observation of the transition from a SN to its supernova remnant (SNR) phase.”

Although the report cites X-ray data from a variety of X-ray satellites, the bulk of the information comes out of a series of five sessions using the NASA’s Chandra X-Ray Observatory during the period July 5-11, 2004. During those sessions a total of almost 40 hours of soft X-rays were collected. Chandra’s superior spatial resolution and the sensitivity gained from long-term observation allowed astronomers to fully resolve the supernova’s X-ray lightcurve from that of a nearby HII region within the galaxy – a region bright enough in visible light to have been included in J.L.E Dreyer’s New General Catalog compiled during the late 19th century – NGC 5455.

Results from this – and a handful of other observations of supernova afterglow using NASA’s Chandra and ESA’s XMM-Newton – have confirmed one of the leading theories of post-supernova X-ray lightcurves. From the paper: “high-quality X-ray spectra have confirmed the validity of the circumstellar interaction models which predict a hard spectral component for the forward shock emission during the early epoch (less than 100 days) and a soft thermal component for the reverse shock emission after the expanding shell has become optically thin.”

For tens of thousands of years before going supernova, the star that became SN 1970G quietly boiled away matter into space. This created an expansive extrastellar aura of hydrogen and helium in the form of a CSM. When it went supernova, a massive flux of hot matter shot into space as SN 1970G’s mantle rebounded after collapse onto its superheated core. For roughly 100 days, the density of this matter remained exceedingly high and – as it smacked into the CSM – hard X-rays dominated the output of the noval flux. These hard X-rays contain ten to twenty times as much energy as those to follow.

Later as this highly energized matter expanded enough to become optically transparent, a new period supervened – X-ray flux from the CSM itself caused a reverse flood of lower-energy “soft” X-rays. That period is expected to continue until the CSM expands to the point of fusion with Interstellar Matter (the ISM). At that time the supernova remnant will form and thermal energy within the CSM will ionize the ISM itself. Out of this will come the characteristically “blue-green” glow visible in such supernovae remnants as the Cygnus Loop when seen through even modest amateur instruments and appropriate filters.

Has SN 1970G evolved into a supernova remnant yet?

One important clue to solving this question is seen in the mass-loss rate of the supernova before eruption. According to Immler and Kuntz: “The measured mass-loss rate for SN 1970G is similar to those inferred for other Type II SNe, which typically range from 10^-5 to 10^-4 solar masses per year. This is indicative that the X-ray emission arises from shock-heated CSM deposited by the progenitor rather than shock-heated ISM, even at this late epoch after the outburst.”

According to Stefan Immler, “Supernovae usually fade away quickly in the near aftermath of their explosion as the shock wave reaches the outer boundaries of the stellar wind, which becomes thinner and thinner. A few hundred years later, however, the shock runs into the interstellar medium, and produces copious X-ray emission due to the high densities of the ISM. Measurements of the densities at the shock front of 1970G showed that they are characteristic of stellar winds, which are more than an order of magnitude smaller than the densities of the ISM.”

Because of the low levels of X-ray output, the authors have concluded that 1970G has yet to reach the supernova remnant phase – even at an age of 35 years after the explosion. Based on studies associated with supernova remnants such as the Cygnus Loop we know that once remnants are formed, they can persist for tens of thousands of years as superheated matter fuses with the ISM. Later, after the shock-heated ISM has finally cooled off, new stars and planets may form enriched by heavy atoms such as carbon, oxygen, and nitrogen along with even heavier elements (such as iron) produced during the brief moment of the actual supernova explosion – the stuff of life.

Clearly SN 1970G has a great deal more to teach us about the afterlife of massive stars and its march toward supernova remnant status will continue to be carefully monitored well into the future.

Written by Jeff Barbour

The 1987A supernova remnant doesn’t seem to have a neutron star. Image credit: Hubble. Click to enlarge.
In 1987, earthbound observers saw a star explode in the nearby dwarf galaxy called the Large Magellanic Cloud. Astronomers eagerly studied this supernova-the closest seen in the past 300 years-and have continued to examine its remains. Although its blast wave has lit up surrounding clouds of gas and dust, the supernova appears to have left no core behind. Astronomers now report that even the sharp eyes of the Hubble Space Telescope failed to locate the black hole or ultracompact neutron star they believe was created by the star’s death 18 years ago.

“We think a neutron star was formed. The question is: Why don’t we see it?” said astronomer Genevieve Graves of UC Santa Cruz, first author on the paper announcing these results.

“Therein lies the mystery-where is that missing neutron star?” mused co-author Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics (CfA).

When a massive star explodes, it leaves behind some sort of compact object, either a city-sized ball of subatomic particles called a neutron star, or a black hole. The outcome depends on the mass of the progenitor star. Smaller stars form neutron stars while larger stars form black holes.

The progenitor of supernova (SN) 1987A weighed 20 times as much as the sun, placing it right on the dividing line and leaving astronomers uncertain about what type of compact object it produced. All observations to date have failed to detect a light source in the center of the supernova remnant, leaving the question of the outcome unanswered.

Detecting a black hole or neutron star is challenging. A black hole can be detected only when it swallows matter, because the matter heats up and emits light as it falls into the black hole. A neutron star at the distance of the Large Magellanic Cloud can be detected only when it emits beams of radiation as a pulsar, or when it accretes hot matter like a black hole.

“A neutron star could just be sitting there inside SN 1987A, not accreting matter and not emitting enough light for us to see,” said astronomer Peter Challis (CfA), second author on the study.

Observations have ruled out the possibility of a pulsar within SN 1987A. Even if the pulsar’s beams were not aimed at the earth, they would light the surrounding gas clouds. However, theories predict that it can take anywhere from 100 to 100,000 years for a pulsar to form following a supernova, because the neutron star must gain a sufficiently strong magnetic field to power the pulsar beam. SN 1987A may be too young to hold a pulsar.

As a result, the only way astronomers might detect the central object is to search for evidence of matter accreting onto either a neutron star or a black hole. That accretion could happen in one of two ways: spherical accretion in which matter falls in from all directions, or disk accretion in which matter spirals inward from a disk onto the compact object.

The Hubble data rule out spherical accretion because light from that process would be bright enough to detect. If disk accretion is taking place, the light it generates is very faint, meaning that the disk itself must be quite small in both mass and radial extent. Also, the lack of detectable radiation indicates that the disk accretion rate must be extremely low-less than about one-fifth the mass of the Moon per year.

In the absence of a definitive detection, astronomers hope to learn more about the central object by studying the dust clouds surrounding it. That dust absorbs visible and ultraviolet light and re-radiates the energy at infrared wavelengths.

“By studying that reprocessed light, we hope to find out what’s powering the supernova remnant and lighting the dust,” said Graves. Future observations by NASA’s Spitzer Space Telescope should provide new clues to the nature of the hidden object.

Additional observations by Hubble also could help solve the mystery. “Hubble is the only existing facility with the resolution and sensitivity needed to study this problem,” said Kirshner.

The paper describing these findings is online at http://arxiv.org/abs/astro-ph?0505066

Original Source: CfA News Release

Swift’s X-Ray telescope captured this image of GRB050509b embedded in the diffuse X-ray emission associated with the galaxy cluster. Image credit: NASA. Click to enlarge.
Two billion years and 25 days ago, an event destined to be a watershed in the astronomical community took place in a distant galaxy ? a blast of gamma rays lasting a mere a thirtieth of a second. The aptly-named Swift observatory ‘saw’ the gammas with its Burst Alert Telescope (BAT) instrument, worked out roughly where they were coming from, and turned its X-ray and UV telescopes. The international GCN (GRB Coordinates Network) lit up with notices from observatories all over the world (and out in space), reporting what they found when they looked there. Data came in from Namibia, the Canaries, continental US, Chile, India, the Netherlands, and above all Hawaii. The world?s leading optical telescopes, the VLT, the Kecks, Gemini, Subaru, all swung into action; the electromagnetic spectrum was covered from extremely high energy gammas to the radio.

And all for what? A few dozen gamma rays plus about a dozen X-rays? Astronomers have known for over a decade that gamma ray bursts (GRBs) come in two different kinds: ?long-soft? and ?short-hard?. GRB050509b was a short-hard one. It lasted about 30 ms, its gamma spectrum had more ?hard? gammas than ?soft? ones, and it was the first time an X-ray afterglow was ever detected.

Astronomers have been “desperately seeking afterglows” for years. These are the X-ray, UV, optical, IR, and radio waves streaming from the site of the GRB, after the gamma radiation tails off. Because we can pinpoint the source of these more accurately than the GRBs themselves, finding afterglows is the first step to working out what they are.

Before GRB050509b, astronomers were leaning towards the theory that long-soft GRBs are core-collapse supernovae (collapsars). While there have been dozens of theoretical papers published on what short-hard GRBs might be, only three scenarios seemed to fit the gamma ray data ? the merger (or collision) of a neutron star with another (or a black hole), a giant flare from a magnetar (a ?starquake? in an intensely magnetic neutron star), or some variation on the collapsar theme.

Now the first of what will likely be hundreds of papers on GRB050509b has been submitted for publication. The 28 authors conclude that “there is now observational support for the hypothesis that short-hard bursts arise during the merger of a compact binary (two neutron stars, or a neutron star and a black hole).”

The key to the researchers? conclusion is the ‘localization’ of the X-ray afterglow.

Swift?s X-ray telescope detected X-rays coming from the same region of the sky as the gammas; after some sleuthing to tie the apparent X-ray position to the astronomers? coordinate system (RA and Dec), the Swift XRT team determined that the afterglow came from a circle about 15″ (arc seconds) across, whose centre is about 10″ from the heart of an elliptical galaxy (which now has the memorable name G1), itself a member of a rich cluster of galaxies bathed in X-rays. How did they know it was an afterglow? Because it faded; the diffuse X-ray glow from clusters doesn?t do that.

And despite looking very carefully, no other electromagnetic afterglow was detected.

So now our 28 astronomers had to work out whether G1?s suburbs is where the stardeath happened, or somewhere else; what is the ?host?, in astronomer-speak.

Modern astronomy makes heavy use of statistics; to be sure they don?t have a fluke, researchers normally want lots and lots of examples. In this case, the only stats the paper?s authors could do is a calculation ? how likely is it that a short-hard GRB (assuming that such are stardeath events) would occur ?near? an elliptical galaxy, in a rich cluster, just by chance? Many different ?how likely? questions were asked; the answers in all cases are, ?not very likely?. However, no one is ruling out bad luck.

Our researchers could now turn to the various theoretical models of short-hard GRBs, and of GRB afterglows, to see how well the observational data fit the theoretical expectations, assuming the GRB went off in G1.

Good news (#1) is that the afterglow data matches well: short-hard GRBs release a lot less (gamma) energy than do long-soft ones (so afterglows from short-hard GRBs should be fainter; the gamma energy is an indicator of the energy used to power the afterglow). Better yet, since what the burst debris smashes into determines how bright the afterglow will be, the faint GRB050509b afterglow is just what you?d expect if it happened in the rarified gas of the interstellar medium of an elliptical (collapsar afterglows are bright in part because they happen in the messy remnants of the gas-dust clouds from which they were born a mere few million years earlier).

The second piece of good news is that, no trace of recent star formation could be found in G1, thus pretty much ruling out a collapsar as the progenitor. Why? Because collapsars are very young stars, and so cannot have moved far from their birthplace before their death. Further, the debris of even the wimpiest collapsar supernova would have been visible, several days afterwards.

What about a giant flare from a magnetar? This cannot be strongly ruled out for GRB050509b, but a magnetar in a galaxy like G1 is not very likely, and GRB050509b was a thousand times brighter than the strongest magnetar flare we?ve seen, to date.

That leaves the merger of a neutron star binary (or NS-BH binary). Where would we find such a binary, just ready to merge? They certainly could be found in the suburbs of spiral galaxies, or in globular clusters, but giant elliptical galaxies like G1 is mostly where.

So it?s ?case closed?? Not quite. ?Other progenitor models are still viable, and additional rapidly localized bursts from the Swift mission will undoubtedly help to further clarify the progenitor picture.?

Could GRB050509b be a stardeath in a much more distant galaxy? Maybe one of the dozen or so fuzzy blobs (a much more distant galaxy cluster? such chance alignments are very common) in or near the X-ray afterglow? Perhaps this will be discussed in future papers on GRB050509b.

Original Source: http://arxiv.org/abs/astro-ph/0505480

Simulated image that shows the distribution of matter in the Universe. Image credit: MPG. Click to enlarge.
The Virgo consortium, an international group of astrophysicists from the UK, Germany, Japan, Canada and the USA has today (June 2nd) released first results from the largest and most realistic simulation ever of the growth of cosmic structure and the formation of galaxies and quasars. In a paper published in Nature, the Virgo Consortium shows how comparing such simulated data to large observational surveys can reveal the physical processes underlying the build-up of real galaxies and black holes.

The “Millennium Simulation” employed more than 10 billion particles of matter to trace the evolution of the matter distribution in a cubic region of the Universe over 2 billion light-years on a side. It kept the principal supercomputer at the Max Planck Society’s Supercomputing Centre in Garching, Germany occupied for more than a month. By applying sophisticated modelling techniques to the 25 Terabytes (25 million Megabytes) of stored output, Virgo scientists are able to recreate evolutionary histories for the approximately 20 million galaxies which populate this enormous volume and for the supermassive black holes occasionally seen as quasars at their hearts.

Telescopes sensitive to microwaves have been able to image the Universe directly when it was only 400,000 years old. The only structure at that time was weak ripples in an otherwise uniform sea of matter and radiation. Gravitationally driven evolution later turned these ripples into the enormously rich structure we see today. It is this growth which the Millennium Simulation is designed to follow, with the twin goals of checking that this new paradigm for cosmic evolution is indeed consistent with what we see, and of exploring the complex physics which gave rise to galaxies and their central black holes.

Recent advances in cosmology demonstrate that about 70 percent of our Universe currently consists of Dark Energy, a mysterious force field which is causing it to expand ever more rapidly. About one quarter apparently consists of Cold Dark Matter, a new kind of elementary particle not yet directly detected on Earth. Only about 5 percent is made out of the ordinary atomic matter with which we are familiar, most of that consisting of hydrogen and helium. All these components are treated in the Millennium Simulation.

In their Nature article, the Virgo scientists use the Millennium Simulation to study the early growth of black holes. The Sloan Digital Sky Survey (SDSS) has discovered a number of very distant and very bright quasars which appear to host black holes at least a billion times more massive than the Sun at a time when the Universe was less than a tenth its present age.

“Many astronomers thought this impossible to reconcile with the gradual growth of structure predicted by the standard picture”, says Dr Volker Springel (Max Planck Institute for Astrophysics, Garching) the leader of the Millennium project and the first author of the article, “Yet, when we tried out our galaxy and quasar formation modelling we found that a few massive black holes do form early enough to account for these very rare SDSS quasars. Their galaxy hosts first appear in the Millennium data when the Universe is only a few hundred million years old, and by the present day they have become the most massive galaxies at the centres of the biggest galaxy clusters.”

For Prof Carlos Frenk (Institute for Computational Cosmology, University of Durham) the head of Virgo in the UK, the most interesting aspect of the preliminary results is the fact that the Millennium Simulation demonstrates for the first time that the characteristic patterns imprinted on the matter distribution at early epochs and visible directly in the microwave maps, should still be present and should be detectable in the observed distribution of galaxies. “If we can measure the baryon wiggles sufficiently well”, says Prof Frenk, “then they will provide us with a standard measuring rod to characterise the geometry and expansion history of the universe and so to learn about the nature of the Dark Energy.”

“These simulations produce staggering images and represent a significant milestone in our understanding of how the early Universe took shape.” said PPARC’s Chief Executive, Prof Richard Wade. “The Millennium Simulation is a brilliant example of the interaction between theory and experiment in astronomy as the latest observations of astronomical objects can be used to test the predictions of theoretical models of the Universe’s history.”

The most interesting and far-reaching applications of the Millennium Simulation are still to come according to Prof Simon White (Max Planck Institute for Astrophysics) who heads Virgo efforts in Germany. “New observational campaigns are providing us with information of unprecedented precision about the properties of galaxies, black holes and the large-scale structure of our Universe,” he notes. “Our ability to predict the consequences of our theories must reach a matching level of precision if we are to use these surveys effectively to learn about the origin and nature of our world. The Millennium Simulation is a unique tool for this. Our biggest challenge now is to make its power available to astronomers everywhere so that they can insert their own galaxy and quasar formation modelling in order to interpret their own observational surveys.”

Original Source: PPARC News Release

VLBA image of quasar 3C 273, with its long jet blasting out. Image credit: NRAO. Click to enlarge.
When a pair of researchers aimed the National Science Foundation’s Very Long Baseline Array (VLBA) radio telescope toward a famous quasar, they sought evidence to support a popular theory for why the superfast jets of particles streaming from quasars are confined to narrow streams. Instead, they got a surprise that “may send the theorists back to the drawing boards,” according to one of the astronomers.

“We did find the evidence we were looking for, but we also found an additional piece of evidence that seems to contradict it,” said Robert Zavala, an astronomer at the U.S. Naval Observatory’s Flagstaff, Arizona, station. Zavala and Greg Taylor, of the National Radio Astronomy Observatory and the Kavli Institute of Particle Astrophysics and Cosmology, presented their findings to the American Astronomical Society’s meeting in Minneapolis, Minnesota.

Quasars are generally thought to be supermassive black holes at the cores of galaxies, the black hole surrounded by a spinning disk of material being drawn inexorably into the black hole’s gravitational maw. Through processes still not well understood, powerful jets of particles are propelled outward at speeds nearly that of light. A popular theoretical model says that magnetic-field lines in the spinning disk are twisted tightly together and confine the fast-moving particles into narrow “jets” streaming from the poles of the disk.

In 1993, Stanford University and Kavli Institute astrophysicist Roger Blandford suggested that such a twisted magnetic field would produce a distinct pattern in the alignment, or polarization, of radio waves coming from the jets. Zavala and Taylor used the VLBA, capable of producing the most detailed images of any telescope in astronomy, to seek evidence of Blandford’s predicted pattern in a well-known quasar called 3C 273.

“We saw exactly what Blandford predicted, supporting the idea of a twisted magnetic field. However, we also saw another pattern that is not explained by such a field,” Zavala said.

In technical terms, the twisted magnetic field should cause a steady change, or gradient, in the amount by which the alignment (polarization) of the radio waves is rotated as one looks across the width of the jet. That gradient showed up in the VLBA observations. However, with a twisted magnetic field, the percentage of the waves that are similarly aligned, or polarized, should be at its greatest at the center of the jet and decrease steadily toward the edges. Instead, the observations showed the percentage of polarization increasing toward the edges.

That means, the astronomers say, there either is something wrong with the twisted-magnetic-field model or its effects are washed out by interactions between the jet and the interstellar medium that it is drilling through. “Either way, the theorists have to get to work to figure out how this can happen,” Zavala said.

When notified of the new results, Blandford said, “these observations are good enough to warrant further development of the theory.”

3C 273 is one of the most famous quasars in astronomy, and was the first to be recognized as a very distant object in 1963. Caltech astronomer Maarten Schmidt was working on a brief scientific article about 3C273 on the afternoon of February 5 that year when he suddenly recognized a pattern in the object’s visible-light spectrum that allowed an immediate calculation of its distance. He later wrote that “I was stunned by this development…” Just minutes later, he said, he met his colleague Jesse Greenstein, who was studying another quasar, in a hallway. In a matter of another few minutes, they found that the second one also was quite distant. 3C 273 is about two billion light-years from Earth in the constellation Virgo, and is visible in moderate-sized amateur telescopes.

The VLBA is a system of ten radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in space. Dedicated in 1993, the VLBA has an ability to see fine detail equivalent to being able to stand in New York and read a newspaper in Los Angeles.

“The extremely sharp radio ‘vision’ of the VLBA was absolutely necessary to do this work,” Zavala explained. “We used the highest radio frequencies at which we could detect 3C273’s jet to maximize the detail we could get, and this effort paid off with great science,” he added.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release