Universe Today

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

Galaxy Cluster Abell 2218 distorting the light from several more distant galaxies. Image credit: ESO. Click to enlarge.
Fifty years after his death, Albert Einstein’s work still provides new tools for understanding our universe. An international team of astronomers has now used a phenomenon first predicted by Einstein in 1936, called gravitational lensing, to determine the shape of stars. This phenomenon, due to the effect of gravity on light rays, led to the development of gravitational optics techniques, among them gravitational microlensing. It is the first time that this well-known technique has been used to determine the shape of a star.

Most of the stars in the sky are point-like, making it very difficult to evaluate their shape. Recent progress in optical interferometry has made it possible to measure the shape of a few stars. In June 2003, for instance, the star Achernar (Alpha Eridani) was found to be the flattest star ever seen, using observations from the Very Large Telescope Interferometer (see ESO Press Release for details about this discovery). Until now, only a few measurements of stellar shape have been reported, partly due to the difficulty of carrying such measurements. It is important, however, to obtain further accurate determinations of stellar shape, as such measurements help to test theoretical stellar models.

For the first time, an international team of astronomers [1], led by N.J. Rattenbury (from Jodrell Bank Observatory, UK), applied gravitational lensing techniques to determine the shape of a star. These techniques rely on the gravitational bending of light rays. If light coming from a bright source passes close to a foreground massive object, the light rays will be bent, and the image of the bright source will be altered. If the foreground massive object (the ‘lens’) is point-like and perfectly aligned with the Earth and the bright source, the altered image as seen from the Earth will be a ring shape, the so-called ‘Einstein ring’. However, most real cases differ from this ideal situation, and the observed image is altered in a more complicated way. The image below shows an example of gravitational lensing by a massive galaxy cluster.

Gravitational microlensing, as used by Rattenbury and his colleagues, also relies on the deflection of light rays by gravity. Gravitational microlensing is the term used to describe gravitational lensing events where the lens is not massive enough to produce resolvable images of the background source. The effect can still be detected as the distorted images of the source are brighter than the unlensed source. The observable effect of gravitational microlensing is therefore a temporary apparent magnification of the background source. In some cases, the microlensing effect may increase the brightness of the background source by a factor of up to 1000. As already pointed out by Einstein, the alignments required for the microlensing effect to be observed are rare. Moreover, as all stars are in motion, the effect is transitory and non-repeating. Microlensing events occur over timescales from weeks to months, and require long-term surveys to be detected. Such survey programs have existed since the 1990s. Today, two survey teams are operating: a Japan/New Zealand collaboration known as MOA (Microlensing Observations in Astrophysics) and a Polish/Princeton collaboration known as OGLE (Optical Gravitational Lens Experiment). The MOA team observes from New Zealand and the OGLE team from Chile. They are supported by two follow-up networks, MicroFUN and PLANET/RoboNET, that operate about a dozen telescopes around the globe.

The microlensing technique has been applied to search for dark matter around our Milky Way and other galaxies. This technique has also been used to detect planets orbiting around other stars. For the first time, Rattenbury and his colleagues were able to determine the shape of a star using this technique. The microlensing event that was used was detected in July 2002 by the MOA group. The event is named MOA 2002-BLG-33 (hereafter MOA-33). Combining the observations of this event by five ground-based telescopes together with HST images, Rattenbury and his colleagues performed a new analysis of this event.

The lens of event MOA-33 was a binary star, and such binary lens systems produce microlensing lightcurves that can provide much information about both the source and lens systems. The particular geometry of the observer, lens and source systems during the MOA-33 microlensing event meant that the observed time-dependent magnification of the source star was very sensitive to the actual shape of the source itself. The shape of the source star in microlensing events is usually assumed to be spherical. Introducing parameters describing the shape of the source star into the analysis allowed the shape of the source star to be determined.

Rattenbury and his colleagues estimated the MOA-33 background star to be slightly elongated, with a ratio between the polar and equatorial radius of 1.02 -0.02/+0.04. However, given the uncertainties of the measurement, a circular shape of the star cannot be completely excluded. The figure below compares the shape of the MOA-33 background star with those recently measured for Altair and Achernar. While both Altair and Achernar are only a few parsecs from the Earth, the MOA-33 background star is a more distant star (about 5000 parsecs from the Earth). Indeed, interferometric techniques can only be applied to bright (thus nearby) stars. On the contrary, the microlensing technique makes it possible to determine the shape of much more distant stars. Indeed, there is currently no alternative technique to measure the shape of distant stars.

This technique, however, requires very specific (and rare) geometrical configurations. From statistical considerations, the team estimated that about 0.1% of all detected microlensing events will have the required configurations. About 1000 microlensing events are observed every year. They should become even more numerous in the near future. The MOA group is presently commissioning a new Japan-supplied 1.8m wide-field telescope that will detect events at an increased rate. Also, a US led group is considering plans for a space-based mission called Microlensing Planet Finder. This is being designed to provide a census of all types of planets within the Galaxy. As a by-product, it would also detect events like MOA-33 and provide information on the shapes of stars.

Original Source: Jodrell Bank Observatory