Universe Today

Image credit: NASA

A team of astronomers were lucky enough to observe the rare event of a neutron star turning into a magnetic object called a magnetar. Ten magnetars have been seen to date, but this object, a transient magnetar, is brand new. A normal neutron star is the rapidly spinning remnant of a star that went supernova; they typically possess a very strong magnetic field. A magnetar is similar, but it has a magnetic field up to 1,000 times as strong as a neutron star. This new discovery could indicate that magnetars are more common in the Universe than previously thought.

In a lucky observation, scientists say they have discovered a neutron star in the act of changing into a rare class of extremely magnetic objects called magnetars. No such event has been witnessed definitively until now. This discovery marks only the tenth confirmed magnetar ever found and the first transient magnetar.

The transient nature of this object, discovered in July 2003 with NASA’s Rossi X-ray Timing Explorer, may ultimately fill in important gaps in neutron star evolution. Dr. Alaa Ibrahim of George Washington University and NASA Goddard Space Flight Center in Greenbelt, Md., presents this result today at the meeting of the American Astronomical Society in Atlanta.

A neutron star is the core remains of a star at least eight times more massive than the Sun that exploded in a supernova event. Neutron stars are highly compact, highly magnetic, fast-spinning objects with about a Sun’s worth of mass compressed into a sphere roughly ten miles in diameter.

A magnetar is up to a thousand times more magnetic than ordinary neutron stars. At a hundred trillion (10^14) Gauss, they are so magnetic that they could strip a credit card clean at a distance of 100,000 miles. The Earth’s magnetic field, in comparison, is about 0.5 Gauss, and a strong refrigerator magnet is about 100 Gauss. Magnetars are brighter in X rays than they are in visible light, and they are the only stars known that shine predominantly by magnetic power.

The observation presented today supports the theory that some neutron stars are born with these ultrahigh magnetic fields, but they may be at first too dim to see and measure. In time, however, these magnetic fields act to slow the neutron star’s spin. This act of slowing releases energy, making the star brighter. Additional disturbances in the star’s magnetic field and crust can make it brighter yet, leading to the measurement of its magnetic field. The newly discovered star, dim as recent as a year ago, is named XTE J1810-197.

“The discovery of this source came courtesy of another magnetar that we were monitoring, named SGR 1806-20,” said Ibrahim. He and his colleagues detected XTE J1810-197 with the Rossi Explorer about a degree to the northeast of SGR 1806-20, within the Milky Way galaxy about 15,000 light years away in the constellation Sagittarius.

Scientists pinpointed the location of the source with NASA’s Chandra X-ray Observatory, which provides more accurate positioning than Rossi. Checking archive data from the Rossi Explorer, Dr. Craig Markwardt of NASA Goddard estimated that XTE J1810-197 became active (that is, 100 times brighter than before) around January 2003. Looking back even further with archived data from ASCA and ROSAT, two decommissioned international satellites, the team could spot XTE J1810-197 as a very dim, isolated neutron star as early as 1990. Thus, the history of XTE J1810-197 emerged.

The inactive state of XTE J1810-197, Ibrahim said, was similar to that of other puzzling objects called Compact Central Objects (CCOs) and Dim Isolated Neutron Stars (DINSs). These objects are thought to be neutron stars created in the hearts of star explosions, and some still reside there, but they are too dim to study in detail.

One mark of a neutron star is its magnetic field. But to measure this, scientists need to know the neutron star’s spin period and the rate that it is slowing down, called the “spin down”. When XTE J1810-197 lit up, the team could measure its spin (1 revolution per 5 seconds, typical of magnetars), its spin down, and thus its magnetic field strength (300 trillion Gauss).

In the alphabet soup of neutron stars, there are also Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs). Both of these are now considered to be the same kind of objects, magnetars; and another presentation at today’s meeting by Dr. Peter Woods et al. supports this connection. These objects periodically but unpredictably erupt with X-ray and gamma-ray light. CCOs and DINSs appear not to have a similar active state.

Although the concept is still speculative, an evolutionary pattern may be emerging, Ibrahim said. The same neutron star, endowed with an ultrahigh magnetic field, may pass through each of these four phases during its lifetime. The proper order, however, remains unclear. “Discussion of such a pattern has surfaced in the scientific community in recent years, and XTE J1810-197’s transient nature provides the first tangible evidence in favor of such a kinship,” Ibrahim said. “With a few more examples of stars showing a similar trend, a magnetar family tree may emerge.”

“The observation implies that magnetars could be more common than what is seen but exist in a prolonged dim state,” said team member Dr. Jean Swank of NASA Goddard.

“Magnetars seem now to be in a perpetual carnival mode; SGRs are turning into AXPs and AXPs can start behaving like SGRs anytime and without warning,” said team member Dr. Chryssa Kouveliotou of NASA Marshall, who is receiving the Rossi Award at the AAS meeting for her work on magnetars. “What started with a few odd sources, may soon be proven to encompass a huge number of objects in our Galaxy.”

Additional supporting data came from the Interplanetary Network and the Russian-Turkish Optical Telescope. Ibrahim’s colleagues on this observation also include Dr. William Parke of George Washington University; Drs. Scott Ransom, Mallory Roberts and Vicky Kaspi of McGill University; Dr. Peter Woods of NASA Marshall; Dr. Samar Safi-Harb of the University of Manitoba; Dr. Solen Balman of the Middle East Technical University in Ankara; and Dr. Kevin Hurley of University of California at Berkeley. Drs. Eric Gotthelf and Jules Halpern of Columbia University provided important data from Chandra.

Original Source: NASA News Release

Image credit: ESA

When the second brightest supernova seen in modern times, SN 1993J, blew up several years ago, it did leave a survivor. Using the Hubble Space Telescope, and several ground-based observatories, an international team of astronomers discovered a massive companion star that must have been orbiting the supernova at the time it exploded. This discovery is very important because it will allow astronomers watch what the remnant of SN 1993J does to its companion star. They might even be able to detect a neutron star or black hole forming in real time.

A joint European/University of Hawaii team of astronomers has for the first time observed a stellar ?survivor? to emerge from a double star system involving an exploded supernova.

Supernovae are some of the most significant sources of chemical elements in the Universe, and they are at the heart of our understanding of the evolution of galaxies.

Supernovae are some of the most violent events in the Universe. For many years astronomers have thought that they occur in either solitary massive stars (Type II supernovae) or in a binary system where the companion star plays an important role (Type I supernovae). However no one has been able to observe any such companion star. It has even been speculated that the companion stars might not survive the actual explosion…

The second brightest supernova discovered in modern times, SN 1993J, was found in the beautiful spiral galaxy M81 on 28 March 1993. From archival images of this galaxy taken before the explosion, a red supergiant was identified as the mother star in 1993 – only the second time astronomers have actually seen the progenitor of a supernova explosion (the first was SN 1987A, the supernova that exploded in 1987 in our neighbouring galaxy, the Large Magellanic Cloud).

Initially rather ordinary, SN 1993J began to puzzle astronomers as its ejecta seemed too rich in the chemical element helium and instead of fading normally it showed a bizarre sharp increase in brightness. The astronomers realised that a normal red supergiant alone could not have given rise to such a weird supernova. It was suggested that the red supergiant orbited a companion star that had shredded its outer layers just before the explosion.

Ten years after this cataclysmic event, a European/University of Hawaii team of astronomers has now peered deep into the glowing remnants of SN 1993J using the NASA/ESA Hubble Space Telescope?s Advanced Camera for Surveys (ACS) and the giant Keck telescope on Mauna Kea in Hawaii. They have discovered a massive star exactly at the position of the supernova that is the long sought companion to the supernova progenitor.

This is the first supernova companion star ever to be detected and it represents a triumph for the theoretical models. In addition, this observation allows a detailed investigation of the stellar physics leading to supernova explosions. It is now clear that during the last 250 years before the explosion 10 solar masses of gas were torn violently from the red supergiant by its partner. By observing the companion closely in the coming years it may even be possible to detect a neutron star or black hole emerge from the remnants of the explosion ?in real time?.

Given the paucity of observations of supernova progenitor systems this result, published in Nature on 8 January 2004, is likely to ‘be crucial to understanding how very massive stars explode and why we see such peculiar supernovae’ according to first author Justyn R. Maund from the University of Cambridge, UK.

The team is composed of Stephen J. Smartt and Justyn R. Maund (University of Cambridge, UK), Rolf. P. Kudritzki (University of Hawaii, USA), Philipp Podsiadlowski (University of Oxford, UK) and Gerry F. Gilmore (University of Cambridge, UK).

Stephen Smartt, also from the University of Cambridge, says, ?Supernova explosions are at the heart of our understanding of the evolution of galaxies and the formation of chemical elements in the Universe. It is essential that we know what type of stars produce them.?

For the last ten years astronomers have believed that they could understand the very peculiar behaviour of 1993J by invoking the existence of a binary companion star and now this picture has proved correct.

According to Rolf Kudritzki, from the University of Hawaii, ?The combination of the outstanding spatial resolution of Hubble and the huge light gathering power of the Keck 10- metre telescope in Hawaii has made this fantastic discovery possible.?

Supernovae occur when a star of more than about eight times the mass of the Sun reaches the end of its nuclear fuel reserves and can no longer produce enough energy to keep the star from collapsing under its own immense weight. The core of the star collapses, and the outer layers are ejected in a fast-moving shock wave.

This huge energy release causes the visible supernova we see. While astronomers are convinced that observations will match this theoretical model, they are in the embarrassing position that they have confidently identified only two stars that later exploded as supernovae ? the precursors of supernovae 1987A and 1993J.

There have been more than 2000 supernovae discovered in galaxies beyond the Milky Way and there appear to be about eight distinct sub-classes. However identifying which stars produce which flavours has proved incredibly difficult. This team has now embarked on a parallel project with the Hubble Space Telescope to image a large number of galaxies and then wait patiently for a supernova to explode.

Supernovae appear in spiral galaxies like M81 on average once every 100 years or so. The team, led by Stephen Smartt, hope to increase the numbers of supernova progenitors known from 2 to 20 over the next five years.

Original Source: ESA News Release

Image credit: Harvard CfA

New calculations by a pair of Harvard astronomers predict that the first “Sun-like” stars in the Universe were alone; devoid of planets or life. The very first generation of stars was hot and massive; they lived hard and died young. After they exploded as supernovae and seeded the Universe with heavier materials, other stars formed in stellar nurseries. The next generation of stars was probably similar in mass and composition to our own Sun, but there weren’t enough minerals to create rocky planets like the Earth. It took a succession of supernovae before there was enough heavy material that planets could form – probably 500 million to 2 billion years after the Big Bang.

To most people, the phrase “Sun-like star” calls to mind images of a friendly, warm yellow star accompanied by a retinue of planets possibly capable of nurturing life. But new calculations by Harvard astronomers Volker Bromm and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics), which were announced today at the 203rd meeting of the American Astronomical Society in Atlanta, show that the first Sun-like stars were lonely orbs moving through a universe devoid of planets or life.

“The window for life opened sometime between 500 million and 2 billion years after the Big Bang” says Loeb. “Billions of years ago, the first low-mass stars were lonely places. The reason for that youthful solitude is embedded in the history of our universe.”

In The Beginning
The very first generation of stars were not at all like our Sun. They were white-hot, massive stars that were very short-lived. Burning for only a few million years, they collapsed and exploded as brilliant supernovae. Those very first stars began the seeding process in the universe, spreading vital elements like carbon and oxygen, which served as planetary building blocks.

“Previously, with Lars Hernquist and Naoki Yoshida (also at the CfA), I have simulated those first supernova explosions to calculate their evolution and how much heavy elements (elements heavier than hydrogen or helium) they produced,” says Bromm. “Now, in this work, Avi Loeb and I have determined that a single first-generation supernova could produce enough heavy elements to enable the first Sun-like stars to form.”

Bromm and Loeb showed that many second-generation stars had sizes, masses, and hence temperatures similar to our Sun. Those properties resulted from the cooling influence of carbon and oxygen when the stars formed. Even elemental abundances as low as one-ten thousandth those found in the Sun proved sufficient to allow smaller, low-mass stars like our Sun to be born.

Yet those same low abundances prohibited rocky planets from forming around those first Sun-like stars due to a lack of raw materials. Only as further generations of stars lived, died, and enriched the interstellar medium with heavy elements did the birth of planets, and life itself, become possible.

“Life is a recent phenomenon,” Loeb states unequivocally. “We know that it took many supernova explosions to make all the heavy elements we find here on Earth and in our Sun and our bodies.”

Recent observational evidence corroborates their finding. Studies of known extrasolar planets have found a strong correlation between the presence of planets and the abundance of heavy elements (“metals”) in their stars. That is, a star with higher metallicity and more heavy elements is more likely to possess planets. Conversely, the lower a star’s metallicity, the less likely it is to have planets.

“We’re now just beginning to investigate the metallicity threshold for planet formation, so it’s hard to say when exactly the window for life opened. But clearly, we’re fortunate that the metallicity of the matter that birthed our solar system was high enough for the Earth to form,” says Bromm. “We owe our existence in a very direct way to all the stars whose life and death preceded the formation of our Sun. And this process began right after the Big Bang with the very first stars. As the universe evolved, it progressively seeded itself with all the heavy elements necessary for planets and life to form. Thus, the evolution of the universe was a step-by-step process that resulted in a stable G-2 star capable of sustaining life. A star we call the Sun.”

Original Source: Harvard CfA News Release