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Topic Name: New Research have Made the Best Determination of the Power of a Supernova Explosion Using X-ray and Optical Observations

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Kem Cook and Sergei Nikolaev

Location: Lawrence Livermore National Laboratory, United States

Details

Astronomers have made the best determination of the power of a supernova explosion long after it was visible from Earth. This technique, using X-ray and optical observations, may help reveal the details of how some stars come to a cataclysmic death.

Using data from NASA’s Chandra X-ray Observatory, the Gemini Observatory and ESA’s XMM-Newton Observatory, two teams of international researchers, including Lawrence Livermore National Laboratory scientists Kem Cook and Sergei Nikolaev, determined that a supernova that occurred about 400 years ago was unusually bright and energetic.

By observing the remnant of a supernova and a light echo from the initial explosion, the teams have established the validity of a new method for studying a type of supernova that produces most of the iron in the universe. The two teams of researchers studied the supernova remnant and the supernova light echo that are located in the Large Magellanic Cloud (LMC), a small galaxy about 160,000 light years from Earth

This is the first time two methods – X-ray observations of the supernova remnant and optical observations of the expanding light echoes – have been combined to study a supernova. Until now, scientists could only estimate the power of explosions from the light seen soon after a star exploded, or from remnants that are several hundred years old, but not from both.

And the results could have implications in identifying similar incidents in the Milky Way.

“Classifying outbursts associated with centuries-old remnants is likely to be successful in providing new constraints on additional LMC supernovae as well as their historical counterparts in our own galaxy,” Cook said.

In 2004, scientists used Chandra to determine that a supernova remnant, known as SNR 0509-67.5 in the LMC, was a Type Ia supernova, which is caused by a white dwarf star in a binary system that reaches a critical mass and explodes.

In the new optical study, an estimate of the explosion’s power came from studying the original light of the explosion as it travels through space. Just as sound bounces off walls of a canyon, light waves create an echo by bouncing off dust clouds in space. The light from these echoes travels a longer path than the light that travels straight toward us, and so can be seen hundreds of years after the original explosion.

"People didn’t have advanced telescopes to study supernovas when they went off hundreds of years ago," said Armin Rest of Harvard University, who led the light echo observations. "But we’ve done the next best thing by looking around the site of the explosion and constructing an action replay of it."

First seen by the Cerro-Tololo Inter-American Observatory in Chile, the light echoes were observed in greater detail by Gemini Observatory in Chile. The optical spectra of the light echo were used to confirm the supernova was a Type Ia and to unambiguously determine the particular class of explosion and its energy.

The Chandra and XMM data were then independently used to calculate the amount of energy involved in the original explosion, using analysis of the supernova remnant and state-of-the-art explosion models. The conclusion was that the explosion was an especially energetic and bright variety of Type Ia supernova, providing strong evidence that the detailed explosion models are accurate.

Cook and Nikolaev are active members of the SuperMACHO project, a five-year microlensing survey of the LMC. The light echo research evolved out of the serendipitous discovery of light echos in SuperMACHO.

Both methods estimated a similar time since the explosion of about 400 years. An extra constraint on the age comes from the lack of recorded historical evidence for a recent supernova in the LMC. Because this star appears in the Southern Hemisphere, it likely would have been seen by navigators who noted similarly bright celestial events, if it had occurred less than about 400 years ago.

Because a Type Ia supernova brightness can be determined from its spectrum or the way its apparent brightness fades, Type Ia supernovae are important tools to study the expansion of the universe and the nature of dark energy.

“This is the first time that spectra were obtained of an ancient supernova, and they were good enough to allow us to identify the supernova as belonging to a particularly bright class of type Ia supernovae,” Cook said.

This work also is being extended to other supernova remnants and light echoes.

These results appear in two recently accepted papers in The Astrophysical Journal. The first discusses the spectrum obtained by Gemini, led by Rest. The second, with Carlos Badenes of Princeton as first author, details the Chandra observations of SNR 0509-67.5.

Note for Supernova
A supernova is a stellar explosion that creates an extremely luminous object. A supernova causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

Several types of supernovae exist that may be triggered in one of two ways, involving either turning off or suddenly turning on the production of energy through nuclear fusion. After the core of an aging massive star ceases to generate energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing gravitational potential energy that heats and expels the star's outer layers. Alternatively, a white dwarf star may accumulate sufficient material from a stellar companion (usually through accretion, rarely via a merger) to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it. Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the Chandrasekhar limit, while accreting white dwarfs ignite as they approach this limit (roughly 1.38 times the mass of the Sun). White dwarfs are also subject to a different, much smaller type of thermonuclear explosion fueled by hydrogen on their surfaces called a nova. Solitary stars with a mass below approximately nine solar masses, such as the Sun itself, evolve into white dwarfs without ever becoming supernovae.

On average, supernovae occur about once every 50 years in a galaxy the size of the Milky Way and play a significant role in enriching the interstellar medium with heavy elements. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.

Because supernovae are relatively rare events, occurring about once every 50 years in a galaxy like the Milky Way, many galaxies must be monitored regularly in order to obtain a good sample of supernovae to study.

Supernovae in other galaxies cannot be predicted with any meaningful accuracy. When they are discovered, they are already in progress. Most scientific interest in supernovae—as standard candles for measuring distance, for example—require an observation of their peak luminosity. It is therefore important to discover them well before they reach their maximum. Amateur astronomers, who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an optical telescope and comparing them to earlier photographs.

Towards the end of the 20th century, astronomers increasingly turned to computer-controlled telescopes and CCDs for hunting supernovae. While such systems are popular with amateurs, there are also larger installations like the Katzman Automatic Imaging Telescope. Recently, the Supernova Early Warning System (SNEWS) project has also begun using a network of neutrino detectors to give early warning of a supernova in the Milky Way galaxy. A neutrino is a particle that is produced in great quantities by a supernova explosion, and it is not obscured by the interstellar gas and dust of the galactic disk.

Supernova searches fall into two classes: those focused on relatively nearby events and those looking for explosions farther away. Because of the expansion of the universe, the distance to a remote object with a known emission spectrum can be estimated by measuring its Doppler shift (or redshift); on average, more distant objects recede with greater velocity than those nearby, and so have a higher redshift. Thus the search is split between high redshift and low redshift, with the boundary falling around a redshift range of z = 0.1–0.3—where z is a dimensionless measure of the spectrum's frequency shift.

High redshift searches for supernovae usually involve the observation of supernova light curves. These are useful for standard or calibrated candles to generate Hubble diagrams and make cosmological predictions. At low redshift, supernova spectroscopy is more practical than at high redshift, and this is used to study the physics and environments of supernovae. Low redshift observations also anchor the low distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies.

Note for Large Magellanic Cloud
The Large Magellanic Cloud (LMC) is a nearby satellite galaxy of our own galaxy, the Milky Way. At a distance of slightly less than 50 kiloparsecs (≈160,000 light-years), the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal (~ 16 kiloparsecs) and Canis Major Dwarf Galaxy (~ 12.9 kiloparsecs) lying closer to the center of the Milky Way. It has a mass equivalent to approximately 10 billion times the mass of our Sun (1010 solar masses), making it roughly 1/10 as massive as the Milky Way. The LMC is the fourth largest galaxy in the local group, with the Andromeda Galaxy (M31) and Triangulum Galaxy (M33) also having more mass.

While the LMC is often considered an irregular type galaxy, (the NASA Extragalactic Database lists the Hubble sequence type as Irr/SB(s)m), the LMC contains a very prominent bar in its center, suggesting that it may have previously been a barred spiral galaxy. The LMC's irregular appearance is possibly the result of tidal interactions with both the Milky Way, and the Small Magellanic Cloud (SMC).

It is visible as a faint 'cloud' in the night sky of the southern hemisphere, straddling the border between the constellations of Dorado and Mensa.

The LMC was long considered to be a planar galaxy that could be assumed to lie at a single distance from us. However, in 1986, Caldwell and Coulson found that field Cepheid variables in the northeast portion of the LMC lie closer to the Milky Way than Cepheids in the southwest portion. More recently, this inclined geometry for fields stars in the LMC has been confirmed via observations of Cepheids, core helium burning red clump stars and the tip of the red giant branch. All three of these papers find an inclination of ~ 35o, where a face on galaxy has an inclination of 0o. Further work on the structure of the LMC using the kinematics of carbon stars showed that the LMC's disk is both thick and flared. Regarding the distribution of star clusters in the LMC, Schommer et al.measured velocities for ~80 clusters and found that the LMC's cluster system has kinematics consistent with the clusters moving in a disk-like distribution. These results were confirmed by Grocholski et al., who calculated distances to a number of clusters and showed that the LMC's cluster system is in fact distributed in the same plane as the field stars.

Determining a precise distance to the LMC, as with any other galaxy, is challenging due to the use of standard candles for calculating distances, with the primary problem being that many of the standard candles are not as 'standard' as one would like; in many cases, the age and/or metallicity of the standard candle plays a role in determining the intrinsic luminosity of the object. The distance to the LMC has been calculated using a variety of standard candles, with Cepheid variables being one of the most popular. Cepheids have been shown to have a relationship between their absolute luminosity and the period over which their brightness varies. However, Cepheids appear to suffer from a metallicity effect, where Cepheids of different metallicities have different period-luminosity relations. Unfortunately, the Cepheids in the Milky Way typically used to calibrate the period-luminosity relation are more metal rich than those found in the LMC. Recently, the Cepheid absolute luminosity has been re-calibrated using Cepheid variables in the galaxy NGC 4258 that cover a range of metallicities. Using this improved calibration, they find an absolute distance modulus of (m − M)0 = 18.41, or 48 kpc (~157000 lightyears).

Note for Milky Way
The Milky Way is a barred spiral galaxy that is part of the Local Group of galaxies. Although the Milky Way is one of billions of galaxies in the observable universe, the Galaxy has special significance to humanity as it is the home galaxy of the planet Earth. The plane of the Milky Way galaxy is visible from Earth as a band of light in the night sky, and it is the appearance of this band of light which has inspired the name for our galaxy.

Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the observation of the band of light, while the full name Milky Way Galaxy, or alternatively the Galaxy should be used to describe our galaxy as an astrophysical whole. It is unclear how widespread the usage of this convention is, however, and the term Milky Way is routinely used in either context.

Visible from Earth as a hazy band of white light that is seen in the night sky, arching across the entire celestial sphere, the visual phenomenon of the Milky Way (as seen in the night sky) originates from stars and other material which lies within the galactic plane.

The Milky Way looks brightest in the direction of the constellation of Sagittarius, toward the galactic center. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that our Solar System lies close to the galactic plane. The Milky Way has a relatively low surface brightness, making it difficult to see from any urban or suburban location suffering from light pollution.

The stellar disk of the Milky Way galaxy is approximately 100,000 light years in diameter, and is believed to be, on average, about 1,000 light years thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass stars, which is highly uncertain. Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12,000 light years - twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if it were reduced to 130 km (80 mi) in diameter, the Solar System would be a mere 2 mm (0.08 inches) in width. The Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 light-years.

About Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the mass limit for white dwarf stars to become neutron stars. "Chandra" also means "moon" or "luminous" in Sanskrit.

Chandra Observatory is the third of NASA's four Great Observatories. The first was Hubble Space Telescope; second the Compton Gamma Ray Observatory, launched in 1991; and last is the Spitzer Space Telescope. Prior to successful launch, the Chandra Observatory was known as AXAF, the Advanced X-ray Astrophysics Facility. AXAF was assembled and tested by TRW (now Northrop Grumman Space Technology) in Redondo Beach, California.

Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes, requiring a space-based telescope to make these observations.

Unlike optical telescopes which possess simple aluminized parabolic surfaces (mirrors), X-ray telescopes generally use a Wolter telescope consisting of nested cylindrical paraboloid and hyperboloid surfaces coated with iridium or gold. X-ray photons would be absorbed by normal mirror surfaces, so mirrors with a low grazing angle are necessary to reflect them. Chandra uses four pairs of nested iridium mirrors, together with their support structure, called the High Resolution Mirror Assembly (HRMA).

Chandra's highly elliptical orbit allows it to observe continuously for up to 55 hours of its 65 hour orbital period. At its furthest orbital point from earth, Chandra is one of the furthest from earth earth-orbiting satellites. This orbit takes it beyond the geostationary satellites and beyond the outer Van Allen belt

With an angular resolution of 0.5 arcsecond (2.4 µrad), Chandra possesses a resolution over one thousand times better than that of the first orbiting X-ray telescope. The Science Instrument Module (SIM) holds the two focal plane instruments, the Advanced CCD Imaging Spectrometer (ACIS) and the High Resolution Camera (HRC), moving whichever is called for into position during an observation.

ACIS consists of 10 CCD chips and provides images as well as spectral information of the object observed. It operates in the range of 0.2 - 10 keV. HRC has two micro-channel plate components and images over the range of 0.1 - 10 keV. It also has a time resolution of 16 microseconds. Both of these instruments can be used on their own or in conjunction with one of the observatory's two transmission gratings.

The transmission gratings, which swing into the optical path behind the mirrors, provide Chandra with high resolution spectroscopy. The High Energy Transmission Grating Spectrometer (HETGS) works over 0.4 - 10 keV and has a spectral resolution of 60-1000. The Low Energy Transmission Grating Spectrometer (LETGS) has a range of 0.09 - 3 keV and a resolution of 40-2000.

About Gemini Observatory
The Gemini Observatory is an astronomical observatory consisting of two 8-metre telescopes at different sites. The Northern Operations Center is located in Hilo, Hawaii, and the Southern Operations Center is in La Serena, Chile. The Gemini telescopes were built and are operated by a consortium consisting of the United States, United Kingdom, Canada, Chile, Brazil, Argentina, and Australia. This partnership is managed by the Association of Universities for Research in Astronomy (AURA). The United Kingdom had dropped out of the partnership in late 2007, only to be re-instated again two and a half months later.

One telescope (Gemini North, also called the Frederick C. Gillett telescope) is located on Hawaii's Mauna Kea. Its location makes for excellent viewing conditions due to the superb atmospheric conditions on top of the over 4200 m (13,800 ft) high dormant volcano. It saw first light in 1999 and began scientific operations in 2000.

The other (Gemini South) is located at over 2700 m (9000 ft) elevation on a mountain in the Chilean Andes called Cerro Pachón. Very dry air and negligible cloud cover make this another prime telescope location (shared by several other observatories, including the SOAR Telescope and Cerro Tololo Inter-American Observatory). Gemini South saw first light in 2000.

The Gemini Observatory's international headquarters is located in Hilo, Hawaii at the University of Hawaii at Hilo University Park. The Gemini South base-facility is located on the Cerro Tololo Inter-American Observatory (CTIO) campus in La Serena Chile.

Together, the twin Gemini telescopes provide complete unobstructed coverage of both the northern and southern skies. They are currently among the largest and most advanced optical/infrared telescopes available to astronomers. Both employ a range of advanced technologies to deliver the highest quality images, including laser guide stars, adaptive optics and multi-object spectroscopy. In addition, the two telescopes allow very high-quality infrared observations due to the advanced protected silver coating of their mirrors and advanced ventilation systems. Thanks to a high degree of networking, the Gemini telescopes can be operated remotely, and observations can be run when atmospheric conditions suit them best, reducing unnecessary travel by astronomers.

Other institutions involved in the research include Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Harvard-Smithsonian Center for Astrophysics, Gemini Observatory, McMaster University, Texas A&M University, Ohio State University, Washington University, University of Washington, Las Campanas Observatory, Pontificia Universidad Católica de Chile, Universidad de Chile and UC Berkeley.

Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by Lawrence Livermore National Security, LLC for the U.S. Department of Energy's National Nuclear Security Administration.

In figure 1, This combination of X-ray and optical images shows the aftermath of a powerful supernova explosion in the Large Magellanic Cloud (LMC), a small galaxy about 160,000 light years from Earth. The debris from this explosion (upper inset) shows the lowest energy X-rays are shown in red, the intermediate energies are green and the highest energies are blue. The light echo image (lower inset) shows light from the original supernova explosion that has bounced off dust clouds in the neighboring regions of the LMC (the light echoes are shown in blue and stars in orange). The large optical image shows emission lines of hydrogen (H-alpha) in red, singly-ionized sulfur in green and doubly-ionized oxygen in blue. The image highlights regions of star formation in the LMC, including supernova remnants and giant structures carved out by multiple supernovas.

In figure 2, A series of five images taken between 2001-2006 of the light echo in a time-lapse movie, showing the light from the original supernova explosion that has bounced off dust clouds in the neighboring regions of the Large Magellanic Cloud (LMC).

In figure 3, sequence of artist's illustrations begins with a view of a bright, massive star in the center, surrounded by several dust clouds. A bright flash of light marks the destruction of the star in a supernova explosion. As time passes the supernova dims and its light spreads outwards, eventually reflecting off the dust clouds to create light echoes. A supernova remnant eventually appears at the site of the supernova explosion, and increases in size with time.