Friday, August 22, 2014

A silver needle in the sky

Credit: NASA & ESA
Acknowledgement: Roelof de Jong

This stunning new image from the NASA/ESA Hubble Space Telescope shows part of the sky in the constellation of Canes Venatici (The Hunting Dogs).

Although this region of the sky is not home to any stellar heavyweights, being mostly filled with stars of average brightness, it does contain five Messier objects and numerous intriguing galaxies — including NGC 5195, a small barred spiral galaxy considered to be one of the most beautiful galaxies visible, and its nearby interacting partner the Whirlpool Galaxy (heic0506a). The quirky Sunflower Galaxy is another notable galaxy in this constellation, and is one of the largest and brightest edge-on galaxies in our skies.

Joining this host of characters is spiral galaxy NGC 4244, nicknamed the Silver Needle Galaxy, shown here in a new image from Hubble. This galaxy spans some 65 000 light-years and lies around 13.5 million light-years away. It appears as a wafer-thin streak across the sky, with its loosely wound spiral arms hidden from view as we observe the galaxy side on. It is part of a group of galaxies known as the M94 Group [1].

Numerous bright clumps of gas can be seen scattered across its length, along with dark dust lanes surrounding the galaxy’s core. NGC 4244 also has a bright star cluster at its centre. Although we can make out the galaxy’s bright central region and star-spattered arms, we cannot see any more intricate structure due to the galaxy’s position; from Earth, we see it stretched out as a flattened streak across the sky.

A number of different observations were pieced together to form this mosaic, and gaps in Hubble’s coverage have been filled in using ground-based data. The Hubble observations were taken as part of the GHOSTS survey, which is scanning nearby galaxies to explore how they and their stars formed to get a more complete view of the history of the Universe.

Notes


[1] Our home group, containing the Milky Way and many others, is known as the Local Group. M94 is relatively close to the Local Group.

Source:  NASA & ESA - Space Telescope

Thursday, August 21, 2014

Swirling Electrons in the Whirlpool Galaxy

LOFAR radio map of the whirlpool galaxy M51 and its neighbourhood at a frequency of 150 MHz. The field covers 4 by 2.6 degrees. The observations were performed with the Dutch LOFAR high-band antennas. The map shows the distribution of hot electrons in M51 and also a large number of background galaxies.The inlay shows an enlarged view of M51 at 150 MHz (white contour lines) overlayed onto an optical image of M51 from the Digital Sky Survey (DSS). © David Mulcahy et al., Astronomy & Astrophysics 

The whirlpool galaxy Messier 51 (M51) is seen from a distance of approximately 30 million light years. This galaxy appears almost face-on and displays a beautiful system of spiral arms.

A European team of astronomers was able to observe M51 with the International LOFAR Telescope in the frequency range 115-175 MHz, just above the normal commercial FM radio frequency band of 88-108 MHz. The team obtained the most sensitive image of any galaxy at frequencies below 1 GHz so far.

With LOFAR's high sensitivity, the disk of M51 in the radio regime could be traced much further out than before. The astronomers detected cosmic electrons and magnetic fields 40,000 light years away from the center of M51. With LOFAR's high angular resolution, the spiral arms are clearly visible. Magnetic fields and cosmic rays are densest in spiral arms. Compared to higher radio frequencies, spiral arms appear broader due to the diffusion of cosmic electrons away from the spiral arms where they have been formed. 

The view of galaxies in the radio regime is different to their optical appearance. Whereas optical images show predominantly the visible light from stars, the radio waves unravel two constituents of galaxies that are invisible to optical telescopes: electrons, almost as fast as light, and magnetic fields. Their role for the stability and evolution of galaxies is increasingly under discussion. The electrons are "cosmic ray" particles produced in the shock fronts of giant supernova explosions. Magnetic fields are generated by dynamo processes driven by gas motions. When the electrons spiral around the magnetic field lines, radio waves are emitted, a process called synchrotron emission. Its intensity increases with the number and energy of the electrons and with magnetic field strength. 

For many decades, radio astronomy has been unable to explore low frequencies below 300 MHz because the ionosphere acts as a barrier of low-frequency radio waves (which are completely blocked below about 10 MHz). Sophisticated methods of data processing and superfast computers are needed to recover the emission. Due to these technical challenges, spiral galaxies have hardly been studied before at these very low radio frequencies. The only observations were of poor resolution and no details could be made out.

The target of investigation in David Mulcahy's PhD project was the beautiful spiral galaxy Messier 51 at a distance of about 30 million light years which is visible already in a small telescope in the constellation "Canes Venatici", not far away from the famous Big Dipper (in German: "Großer Wagen") in the sky.  

"Low-frequency radio waves are important as they carry information about electrons of relatively low energies that are able to propagate further away from their places of origin in the star-forming spiral arms and are able to illuminate the magnetic fields in the outer parts of galaxies", says David Mulcahy. "We need to know whether magnetic fields are expelled from galaxies and what their strength is out there." 

"This beautiful image, coupled with the important scientific result it represents, illustrates the fantastic advances that can be made at low radio frequencies with the LOFAR telescope", continues Anna Scaife from Southampton University, co-author of the paper. "Unravelling the mysteries of magnetic fields is crucial to understanding how our Universe works. For too long, many of the big questions about magnetic fields have simply been untestable and this new era of radio astronomy is very exciting." 

The Low Frequency Array (LOFAR), designed and constructed by ASTRON in the Netherlands, is a brand new radio telescope giving access to very low radio frequencies. 

© ASTRON, The Netherlands 

LOFAR explores the relatively unexplored frequency range below 240 MHz and consists of a multitude of small and simple antennas without moving parts. LOFAR consists of 38 stations in the Netherlands, 6 stations in Germany and one station each in the UK, France and Sweden. The novelty is the online combination of the signals from all stations in a powerful computing cluster located at the University of Groningen (Netherlands). 

Observations of M51 with LOFAR below FM radio frequencies (at 30-80 MHz) have already taken place. „This opens a new window to the Universe where we do not know how galaxies will look like", concludes Rainer Beck, who supervised David Mulcahy's PhD project. „Maybe we will see how galaxies are magnetically connected to intergalactic space. This is a key experiment in preparation for the planned Square Kilometre Array (SKA) that should tell us how cosmic magnetic fields are generated." 

Original paper:
The nature of the low-frequency emission of M51: First observations of a nearby galaxy with LOFAR, by D.D. Mulcahy, A. Horneffer, R. Beck et al., 2014, Astronomy & Astrophysics  (DOI: 10.1051/0004-6361/201424187).



Why NASA Studies the Ultraviolet Sun

Spacecraft record solar activity as a binary code, 1s and 0s, which computer programs can translate into black and white. Scientists colorize the images for realism, and then zoom in on areas of interest. Image Credit: NASA/Karen Fox. Hi-Res Image

Four of the telescopes on the Solar Dynamics Observatory observe extreme ultraviolet light activity on the sun that is invisible to the naked eye. Image Credit: NASA/SDO. Hi-Res Image

The Solar Dynamics Observatory observed a solar flare (upper left) and a coronal mass ejection (right) erupting from the sun’s limb in extreme ultraviolet light on August 6, 2010. Image Credit: NASA/SDO. Hi-Res Image

You cannot look at the sun without special filters, and the naked eye cannot perceive certain wavelengths of sunlight. Solar physicists must consequently rely on spacecraft that can observe this invisible light before the atmosphere absorbs it.
“Certain wavelengths either do not make it through Earth’s atmosphere or cannot be seen by our eyes, so we cannot use normal optical telescopes to look at the spectrum,” said Dean Pesnell, the project scientist for the Solar Dynamics Observatory, or SDO, at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Several spacecraft can observe these invisible light wavelengths. SDO for example has four telescopes that image the sun in the ultraviolet spectrum. As beams of ultraviolet light pass into the telescope, a mirror with special coatings filters and amplifies the ultraviolet light’s otherwise poor reflection. The incoming photons are then recorded as pixels and converted into electrical signals, similar to how your cell phone camera sees visible light.

“It’s exactly the same process, whether it’s ultraviolet light, infrared light, visible light, or radio,” said Joseph Gurman, project scientist for both the Solar and Heliospheric Observatory and the Solar Terrestrial Relations Observatory at Goddard. “In this case we’re trying to understand how the sun changes and how those changes affect life here on Earth.”

Ultraviolet light causes molecular radiation damage to our skin, seen as sunburns that can lead to cancer. Its cousin, extreme ultraviolet radiation, and the associated solar storms have the potential to disrupt communications and spacecraft navigation. “These are very damaging, energetic photons, and we want to understand what chain of events produces these photons,” Pesnell said.

Thankfully our planet’s atmosphere absorbs much of this solar radiation, making life on Earth possible. 

However, this means that to study extreme ultraviolet light, instruments must do it from the vacuum of space.

“Ultraviolet light from the sun can show us the origins of solar storms that can lead to power outages, cell phone disruptions, and delays in shipping packages due to the rerouting of planes from over the pole,” Gurman said.

By understanding what occurs in the sun’s atmosphere, scientists hope to predict when powerful solar events such as coronal mass ejections and solar flares may occur.

“You really want to know what’s happening on the sun as soon as you can,” said Jack Ireland, a solar visualization specialist at Goddard. “We can then use computer models to estimate how solar events will affect Earth’s space environment.”

The information can then be used by NOAA’s Space Weather Prediction Center, in Boulder, Co. to alert power companies and airlines to take the necessary precautions, thus avoiding power outages and keeping airplane passengers safe.

Max Gleber
NASA's Goddard Space Flight Center in Greenbelt, Maryland

Wednesday, August 20, 2014

A Spectacular Landscape of Star Formation

PR Image eso1425a
Star formation in the southern Milky Way

Star formation regions in the constellation of Carina (The Keel) 

Star formation in the constellation of Carina 

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Videos

Zooming in on star formation in the southern Milky Way
Zooming in on star formation in the southern Milky Way

A close-up look at star formation in the southern Milky Way
A close-up look at star formation in the southern Milky Way


This image, captured by the Wide Field Imager at ESO’s La Silla Observatory in Chile, shows two dramatic star formation regions in the southern Milky Way. The first is of these, on the left, is dominated by the star cluster NGC 3603, located 20 000 light-years away, in the Carina–Sagittarius spiral arm of the Milky Way galaxy. The second object, on the right, is a collection of glowing gas clouds known as NGC 3576 that lies only about half as far from Earth.

NGC 3603 is a very bright star cluster and is famed for having the highest concentration of massive stars that have been discovered in our galaxy so far. At the centre lies a Wolf–Rayet multiple star system, known as HD 97950. Wolf–Rayet stars are at an advanced stage of stellar evolution, and start off with around 20 times the mass of the Sun. But, despite this large mass, Wolf–Rayet stars shed a considerable amount of their matter due to intense stellar winds, which blast the star’s surface material off into space at several million kilometres per hour, a crash diet of cosmic proportions.

NGC 3603 is in an area of very active star formation. Stars are born in dark and dusty regions of space, largely hidden from view. But as the very young stars gradually start to shine and clear away their surrounding cocoons of material they become visible and create glowing clouds in the surrounding material, known as HII regions. HII regions shine because of the interaction of ultraviolet radiation given off by the brilliant hot young stars with the hydrogen gas clouds. HII regions can measure several hundred light-years in diameter, and the one surrounding NGC 3603 has the distinction of being the most massive in our galaxy.

The cluster was first observed by John Herschel on 14 March 1834 during his three-year expedition to systematically survey the southern skies from near Cape Town. He described it as a remarkable object and thought that it might be a globular star cluster. Future studies showed that it is not an old globular, but a young open cluster, one of the richest known.

NGC 3576, on the right of the image, also lies in the Carina–Sagittarius spiral arm of the Milky Way. But it is located only about 9000 light-years from Earth — much closer than NGC 3603, but appearing next to it in the sky.

NGC 3576 is notable for two huge curved objects resembling the curled horns of a ram. These odd filaments are the result of stellar winds from the hot, young stars within the central regions of the nebula, which have blown the dust and gas outwards across a hundred light-years. Two dark silhouetted areas known as Bok globules are also visible in this vast complex of nebulae. These black clouds near the top of the nebula also offer potential sites for the future formation of new stars.

NGC 3576 was also discovered by John Herschel in 1834, making it a particularly productive and visually rewarding year for the English astronomer.

More information


ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links


Contacts

Richard Hook
ESO Public Information Officer
Garching bei München, Germany

Tel: +49 89 3200 6655
Email:
rhook@eso.org

Source: ESO


Type Ia supernovae stem from the explosion of white dwarfs coupled with twin stars

Study discards possibility that type Ia supernovae might stem from explosions of white dwarfs nourished by normal stars. Were these conclusions to become generalized, type Ia supernovae might no longer serve as “standard candles” to measure astronomical distances

Type Ia supernovae happen when a white dwarf, the “corpse” of a star similar to the Sun, absorbs material from a twin star until it reaches a critical mass--1.4 times that of the Sun—and explodes. Because of their origin, all these explosions share a very similar luminosity. This uniformity made type Ia supernovae ideal objects to measure distances in the universe, but the study of supernova 2014J suggests a scenario that would invalidate them as “standard candles".

"Type Ia supernovae are considered standard candles because their constitution is very homogeneous and practically all of them reach the same maximum luminosity. They even allowed us to discover that the universe was expanding at an accelerating rate. However, we still don’t know what stellar systems give rise to this type of supernovae,” says Miguel Ángel Pérez Torres, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) in charge of the study.

A new model postulating the fusion of two white dwarfs is now challenging the predominant one, consisting of a white dwarf and a normal star. The new scenario does not imply the existence of a maximum mass limit and will not, therefore, necessarily produce explosions of similar luminosity. Credit: NASA/CXC/M Weiss

Credit: NASA/CXC/M Weiss

SN 2014J, A SUPERNOVA VERY NEAR BY

The results mentioned above were obtained from the study of supernova 2014J, situated 11.4 million light years away from our planet, using the EVN and eMERLIN networks of radio telescopes. “It is a phenomenon that very seldom occurs in our immediate universe. 2014J has been the Ia type supernova closest to us since 1986, when the telescopes were much less sensitive, and it may well be the only one we’ll be able to observe in such vicinity in the next one hundred and fifty years,” says Pérez Torres (IAA-CSIC).

Radio observation makes it possible to reveal what stellar systems lie behind type Ia supernovae. If the explosion proceeds from a white dwarf being nourished by a twin star, for example, a great amount of gas should be present in the environment; after the explosion, the material ejected by the supernova will collide with this gas and produce an intense emission of X rays and radio waves. By contrast, a couple of white dwarfs will not generate this gaseous envelope and, therefore, there will be no emission of either X rays or radio waves.

"We have not detected radio emissions on SN 2014J, which favours the second scenario", says Pérez Torres. "If these results were to gain general acceptance, the cosmological consequences would be weighty, because the use of type Ia supernovae to measure distances would come into question,” the researcher concludes.

Reference
M. A. Perez-Torres, P. Lundqvist, R. J. Beswick, C. I. Bjornsson, T. W. B. Muxlow, Z. Paragi, S. Ryder, A. Alberdi, C. Fransson, J. M. Marcaide, I. Marti-Vidal, E. Ros, M. K. Argo, J. C. Guirado. Constraints on the progenitor system and the environs of SN 2014J from deep radio observations. The Astrophysical Journal. ApJ, vol. 792, pág. 38.
Contact:
Instituto de Astrofísica de Andalucía (IAA-CSIC)
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll@iaa.es - 958230532
http://www.iaa.es
http://www-divulgacion.iaa.es


Tuesday, August 19, 2014

NASA's RXTE Satellite Decodes the Rhythm of an Unusual Black Hole

Explore M82 X-1 and learn more about how astronomers used X-ray fluctuations to determine its status as an intermediate-mass black hole. Image Credit: NASA Goddard Space Flight Center

Astronomers have uncovered rhythmic pulsations from a rare type of black hole 12 million light-years away by sifting through archival data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite.

The signals have helped astronomers identify an unusual midsize black hole called M82 X-1, which is the brightest X-ray source in a galaxy known as Messier 82. Most black holes formed by dying stars are modestly-sized, measuring up to around 25 times the mass of our sun. And most large galaxies harbor monster, or supermassive, black holes that contain tens of thousands of times more mass.

“Between the two extremes of stellar and supermassive black holes, it's a real desert, with only about half a dozen objects whose inferred masses place them in the middle ground," said Tod Strohmayer, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"For reasons that are very hard to understand, these objects have resisted standard measurement techniques," said Richard Mushotzky, a professor of astronomy at UMCP.

By going over past RXTE observations, the astronomers found specific changes in brightness that helped them determine M82 X-1 measures around 400 solar masses.

As gas falls toward a black hole, it heats up and emits X-rays.  Variations in X-ray brightness reflect changes occurring in the gas. The most rapid fluctuations happen  near the brink of the black hole’s event horizon, the point beyond which nothing, not even light, can escape.

Astronomers call these rhythmic pulses quasi-periodic oscillations, or QPOs.  For stellar black holes, astronomers have established that the larger the mass, the slower the QPOs, but they could not be sure what they were seeing from M82 X-1 was an extension of this pattern.

"When we study fluctuations in X-rays from many stellar-mass black holes, we see both slow and fast QPOs, but the fast ones often come in pairs with a specific 3:2 rhythmic relationship," explained Dheeraj Pasham, UMCP graduate student. For every three pulses from one member of a QPO pair, its partner pulses twice.

By analyzing six years of RXTE data, the team located X-ray variations that reliably repeat about 5.1 and 3.3 times a second, a 3:2 relationship. The combined presence of slow QPOs and a faster pair in a 3:2 rhythm sets a standard scale allowing astronomers to extend proven relationships used to determine the masses of stellar-mass black holes.

The results of the study were published online in the Aug. 17 issue of the journal Nature.

Launched in late 1995 and decommissioned in 2012, RXTE is one of NASA's longest-serving astrophysics missions. Its legacy of unique measurements continues to provide researchers with valuable insights into the extreme environments of neutron stars and black holes.

A new NASA X-ray mission called the Neutron Star Interior Composition Explorer (NICER) is slated for launch to the International Space Station in late 2016. Pasham has identified six potential middle-mass black holes that NICER may be able to explore for similar signals.

For more information, visit: http://www.nasa.gov/topics/universe/index.html  

Related Links: 

Felicia Chou
Headquarters, Washington
202-358-0257

felicia.chou@nasa.gov

Lynn Chandler
Goddard Space Flight Center, Greenbelt, Md.
301-286-2806

lynn.chandler-1@nasa.gov



Monday, August 18, 2014

Protostars in Orion

OMC-2 FIR 4
Credit: Herschel image: ESA/Herschel/Ph. André, D. Polychroni, A. Roy, V. Könyves, N. Schneider for the Gould Belt survey Key Programme; inset and layout: ESA/ATG medialab.

Results from Herschel may have helped to solve a puzzle surrounding our own Sun's past. By studying very young stars in the Orion Nebula, astronomers have been able to gain an insight into how the Sun may have been behaving in its youth.

By using the HIFI spectrometer to detect the far-infrared light at very specific frequencies, the team obtained the chemical fingerprint of a range of atoms and molecules within the clump of gas and dust in which the stars are forming. This also allows a study of the temperature, and showed that the gas and dust are relatively warm, though still at around -200 Celsius. It is thought that the Sun formed in a similar environment, and so studies of such regions allow astronomers to examine one possible history of the Sun's life.

The mystery about our own Sun relates to evidence for a particular form, or isotope, of the element beryllium in meteorites found on Earth. This particular isotope, called beryllium-10, is relatively unusual in that it is not formed in the fusion reactions within stars, or in the supernova explosions which occur when massive stars die. Rather, it is formed when very energetic particles slam into heavier, and more common, elements such as oxygen.

Beryllium-10 doesn't normally hang around, though, because it decays into a specific set of lighter elements. The fact that breyllium-10 is present means that it was being created as the rocks formed, which suggests the that material in the early Solar System was being bombarded by very energetic particles. The big question has been whether these particles were produced by the young Sun, or whether they originated further afield in the deaths of massive stars.

One way of gaining a handle on that question is to study stars at a similar stage of their life - hence the interest in the young stars in Orion. An interesting result from the latest study was the relative amounts of molecules containing hydrogen combined with other atoms such as carbon, oxygen and nitrogen. In most star-forming environments the nitrogen-bearing molecules such as diazenylium (N2H+) are destroyed relatively easily, leaving an over-abundance of molecules such as hydrogen carbonate (HCO).

However, in the case of this particular clump, known as "OMC-2 FIR4", it appears that the HCO has been destroyed almost as quickly as the N2H+. The only explanation for the destruction of both types of molecule is a surprisingly high rate of energetic particles - and at a rate easily high enough to explain the presence of beryllium-10 in the meteorites. While it doesn't rule out other possibilities, it is strong evidence that the Sun may have been much more active in its youth.  

DETAILED INFORMATION 

Object Name: OMC-2 FIR 4
Type of Object: Starforming region
Science category: Star formation
Image Scale: The image is around 2.5 degrees across
Coordinates: Right Ascension: 5h 35m 27s ; Declination: −5° 9′ 55
Constellation: Orion the Hunter
Instrument: Image: SPIRE and PACS; Study: HIFI
Wavelengths: Image: 70 (blue), 160 (green), 250 (red) microns; Study: 480-1244 GHz
Distance of Object: 1500 light years
Date of Release: 01/07/2014

Key Programme:
Image: Gould Belt Survey
Study: CHESS (Chemical Herschel Surveys of Star forming regions)

Publication:
Ceccarelli et al. (2014) ApJ 790 L1, "Herschel finds evidence for stellar wind particles in a protostellar envelope: is this what happened to the young Sun? http://iopscience.iop.org/2041-8205/790/1/L1/

Further information: 

Friday, August 15, 2014

Galactic soup

2MASX J16133219+5103436, SDSS J161330.18+510335 and Zw I 136
Credit:  Image credit: ESA/Hubble & NASA 
Acknowledgement: Judy Schmidt

This new NASA/ESA Hubble Space Telescope image shows a whole host of colourful and differently shaped galaxies; some bright and nearby, some fuzzy, and some so far from us they appear as small specks in the background sky.

The most prominent characters are the two galaxies on the left — 2MASX J16133219+5103436 at the bottom, and its blue-tinted companion SDSS J161330.18+510335 at the top. The latter is slightly closer to us than its partner, but the two are still near enough to one another to interact. Together, the two make up a galactic pair named Zw I 136.

Both galaxies in this pair have disturbed shapes and extended soft halos. They don’t seem to conform to our view of a “typical” galaxy — unlike the third bright object in this frame, a side-on spiral seen towards the right of the image.

Astronomers classify galaxies according to their appearance and their shape. The most famous classification scheme is known as the Hubble sequence, devised by its namesake Edwin Hubble. One of the great questions in galaxy evolution is how interactions between galaxies trigger waves of star formation, and why these stars then abruptly stop forming. Interacting pairs like this one present astronomers with perfect opportunities to investigate this.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Judy Schmidt.

Source:  ESA/Hubble  - Space Telescope


Thursday, August 14, 2014

NASA's Chandra Observatory Searches for Trigger of Nearby Supernova

The data gathered on the Jan. 21 explosion, a Type Ia supernova, allowed scientists to rule out one possible cause. These supernovas may be triggered when a white dwarf takes on too much mass from its companion star, immersing it in a cloud of gas that produces a significant source of X-rays after the explosion.

Astronomers used NASA's Swift and Chandra telescopes to search the nearby Messier 82 galaxy, the location of the explosion, for such an X-ray source. However, no source was found, revealing the region around the site of the supernova is relatively devoid of material.

"While it may sound a bit odd, we actually learned a great deal about this supernova by detecting absolutely nothing," said Raffaella Margutti of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, who led the study. "Now we can essentially rule out that the explosion was caused by a white dwarf continuously pulling material from a companion star."

This supernova, SN 2014J, could instead have been caused by the merger of two white dwarf stars, an event that should result in little or no X-rays after the explosion. Further observations could rule out or confirm other possible triggers.

"Being able to eliminate one of the main possible explanations for what caused SN 2014J to explode is a big step," said CfA's Atish Kamble, a co-author of the study. "The next step is to narrow things down even further."

Type Ia supernovas are used as cosmic distance-markers, and have played a key role in the discovery of the universe's accelerated expansion. At about 12 million light-years from Earth, SN 2014J and its host galaxy are close -- from a cosmic perspective. This offers scientists a chance to observe details that would be too hard to detect in more distant supernovas.

"It's crucial that we understand exactly how these stars explode because so much is riding on our observations of them for cosmology," said co-author Jerod Parrent also from CfA. "SN 2014J might be a chance of a lifetime to study one of these supernovas in detail as it happens."

The study of SN 2014J is similar to a study led by Margutti about another supernova, SN 2011fe, in the nearby galaxy M101.

This study was conducted by CfA's Supernova Forensics Team, led by Alicia Soderberg. The results were published online and in the July 20 print issue of The Astrophysical Journal.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

For an additional interactive image, podcast, and video on the findings, visit: http://chandra.si.edu

For a preprint of the study results in The Astrophysical Journal, visit: http://arxiv.org/abs/1405.1488

For Chandra images, multimedia and related materials, visit: http://www.nasa.gov/chandra

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.

For more information, contact:

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998

mwatzke@cfa.harvard.edu

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462

daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463

cpulliam@cfa.harvard.edu



Wednesday, August 13, 2014

NASA's NuSTAR sees rare blurring of black hole light

Scientists have used NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), an orbiting X-ray telescope, to capture an extreme and rare event in the regions immediately surrounding a supermassive black hole. A compact source of X-rays that sits near the black hole, called the corona, has moved closer to the black hole over a period of just days. The researchers publish their results in Monthly Notices of the Royal Astronomical Society.

An artist’s impression of a supermassive black hole and its surroundings. The regions around supermassive black holes shine brightly in X-rays. Some of this radiation comes from a surrounding disk, and most comes from the corona, pictured here as the white light at the base of a jet. This is one possible configuration for the Mrk 335 corona, as its actual shape is unclear. Credit: NASA-JPL / Caltech. Click here a full resolution image

"The corona recently collapsed in towards the black hole, with the result that the black hole's intense gravity pulled all the light down onto its surrounding disk, where material is spiralling inward," said Michael Parker of the Institute of Astronomy in Cambridge, lead author of the new paper.

As the corona shifted closer to the black hole, the black hole's gravitational field exerted a stronger tug on the x-rays emitted by the corona. The result was an extreme blurring and stretching of the X-ray light. Such events had been observed previously, but never to this degree and in such detail.

Supermassive black holes are thought to reside in the centres of all galaxies. Some are more massive and rotate faster than others. The black hole in this new study, referred to as Markarian 335, or Mrk 335, is about 324 million light-years from Earth in the direction of the Pegasus constellation. It is one of the most extreme systems of which the mass and spin rate have ever been measured. The black hole squeezes about 10 million times the mass of our Sun into a region only 30 times as wide as the Sun's diameter, and it spins so rapidly that space and time are dragged around with it.

Even though some light falls into a supermassive black hole never to be seen again, other high-energy light emanates from both the corona and the surrounding accretion disk of superheated material. Though astronomers are uncertain of the shape and temperature of coronas, they know that they contain particles that move close to the speed of light.

NASA's Swift satellite has monitored Mrk 335 for years, and recently noted a dramatic change in its X-ray brightness. In what is called a 'target-of-opportunity' observation, NuSTAR was redirected to take a look at high-energy X-rays from this source in the range of 3 to 79 kiloelectron volts. This particular energy range offers astronomers a detailed look at what is happening near the event horizon, the region around a black hole from which light can no longer escape gravity's grasp.

Follow-up observations indicate that the corona still is in this close configuration, months after it moved. Researchers don't know whether and when the corona will shift back. What is more, the NuSTAR observations reveal that the grip of the black hole's gravity pulled the corona's light onto the inner portion of its superheated disk, better illuminating it. The shifting corona lit up the precise region they wanted to study, almost as if somebody had shone a flashlight for the astronomers.

The new data could ultimately help determine more about the mysterious nature of black hole coronas. In addition, the observations have provided better measurements of Mrk 335's furious relativistic spin rate. Relativistic speeds are those approaching the speed of light, as described by Albert Einstein's theory of relativity.

"We still don't understand exactly how the corona is produced or why it changes its shape, but we see it lighting up material around the black hole, enabling us to study the regions so close in that effects described by Einstein's theory of general relativity become prominent," said NuSTAR Principal Investigator Fiona Harrison of the California Institute of Technology (Caltech) in Pasadena. "NuSTAR's unprecedented capability for observing this and similar events allows us to study the most extreme light-bending effects of general relativity."

Media contacts

J.D. Harrington
Headquarters, Washington
202-358-5241

j.d.harrington@nasa.gov

Whitney Clavin
Jet Propulsion Laboratory
Pasadena
California
United States
Tel: +1 818 354 4673

whitney.clavin@jpl.nasa.gov

Science contact

Prof Michael Parker
Institute of Astronomy
Cambridge
United Kingdom
Tel: +44 (0)1223 337 511

mlparker@ast.cam.ac.uk


Further information

The new work appears in B. Agís-González et al., 2014, "Black hole spin and size of the X-ray-emitting region(s) in the Seyfert 1.5 galaxy ESO 362−G18", Monthly Notices of the Royal Astronomical Society, vol. 443, pp. 2862-2873, published by Oxford University Press. The paper is available free of charge via the link.

Notes for editors

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA's Jet Propulsion Laboratory (JPL) in Pasadena for NASA's Science Mission Directorate in Washington. The spacecraft was built by Orbital Sciences Corporation in Dulles, Virginia. Its instrument was built by a consortium including Caltech, JPL, the University of California, Berkeley, Columbia University, New York, NASA's Goddard Space Flight Center, Greenbelt, Maryland, the Danish Technical University in Denmark, Lawrence Livermore National Laboratory in Livermore, California, ATK Aerospace Systems in Goleta, California, and with support from the Italian Space Agency (ASI) Science Data Center.

NuSTAR's mission operations centre is at UC Berkeley, with the ASI providing its equatorial ground station located in Malindi, Kenya. The mission's outreach program is based at Sonoma State University, Rohnert Park, California. NASA's Explorer Program is managed by Goddard. JPL is managed by Caltech for NASA.

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter via @royalastrosoc



Tuesday, August 12, 2014

A Possible Signal from Dark Matter?

An X-ray image of the hot gas in the central region of the Perseus Cluster of galaxies, taken by the Chandra X-ray Observatory. The Perseus Cluster is one of the most massive objects in the Universe with thousands of galaxies immersed in an enormous cloud of superheated gas. The image shows enormous bright loops, ripples, and jet-like streaks throughout the cluster. Astronomers may have detected an emission line from a form of dark matter, the sterile neutrino, in the spectrum of galaxy clusters like Perseus. Credit: Chandra/NASA/ESA

Galaxies are often found in groups or clusters, the largest known aggregations of matter and dark matter. The Milky Way, for example, is a member of the "Local Group" of about three dozen galaxies, including the Andromeda Galaxy located about 2 million light-years away. Very large clusters can contain thousands of galaxies, all bound together by gravity. The closest large cluster of galaxies to us, the Virgo Cluster with about 2000 members, is about 50 million light-years away.

The space between galaxies is not empty. It is filled with hot intergalactic gas whose temperature is of order ten million kelvin, or even higher. The gas is enriched with heavy elements that escape from the galaxies and accumulate in the intracluster medium over billions of years of galactic and stellar evolution. These intracluster gas elements can be detected from their emission lines in X-ray, and include oxygen, neon, magnesium, silicon, sulfur, argon, calcium, iron, nickel, and even chromium and manganese.

The relative abundances of these elements contain valuable information on the rate of supernovae in the different types of galaxies in the clusters since supernovae make and/or disburse them into the gas. Therefore it came as something of a surprise when CfA astronomers and their colleagues discovered a faint line corresponding to no known element. Esra Bulbul, Adam Foster, Randall Smith, Scott Randall and their team were studying the averaged X-ray spectrum of a set of seventy-three clusters (including Virgo) looking for emission lines too faint to be seen in any single one when they uncovered a line with no known match in a particular spectral interval not expected to have any features.

The scientists propose a tantalizing suggestion: the line is the result of the decay of a putative, long-sought-after dark matter particle, the so-called sterile neutrino. It had been suggested that the hot X-ray emitting gas in a galaxy cluster might be a good place to look for dark matter signatures, and if the sterile neutrino result is confirmed it would mark a breakthrough in dark matter research (it is of course possible that it is a statistical or other error). Recent unpublished results from another group tend to support the detection of this feature; the team suggests that observations with the planned Japanese Astro-H X-ray mission in 2015 will be critical to confirm and resolve the nature of this line.

Reference(s): 


"Detection of an Unidentified Emission Line in the Stacked X-Ray Spectrum of Galaxy Clusters,” Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, and Scott W. Randall, ApJ 789, 13, 2014.


X-ray diagnostics of the donor star in ultra-compact X-ray binaries

Fig. 1: An artist’s impression of an accreting Low Mass X-ray Binary. The donor star fills its Roche lobe and its material overflows the inner Lagrangian points and accretes on the relativistic star (in this case a black hole). Due to the large angular momentum of the infalling material an accretion disk is formed around the compact object. Credit: ESA 2002/medialab

Fig. 2: A sketch of the innermost part (~1000 gravitational radii) in a low mass X-ray binary in the so called hard state. The inner part of the accretion flow is filled with hot and tenuous, optically thin plasma. Comptonization of the low frequency radiation in the plasma cloud is the main mechanism of the spectral formation in this state. Some fraction of this radiation illuminates the surface of the accretion disk and of the donor star. It is reprocessed by the material of the accretion disk and of the donor star giving rise to the so called ‘reflected component’, depicted in Fig.3. Credit: Gilfanov M., 2010, in Belloni T., ed., Lecture Notes in Physics, Vol. 794, The Jet Paradigm. Springer-Verlag, Berlin, p. 17

Fig. 3: The spectrum of the reflected component for an accretion disk of solar abundance. Superposed on top of the reflected continuum produced by Compton scatterings on electrons, are absorption edges and fluorescence lines of various elements. Also shown is the Comptonized continuum produced by the hot plasma cloud in the vicinity of the compact object (see Fig.2). An observer near the Earth will observe the sum of the two components

Ultra-compact X-ray binaries are a small but fascinating subclass of low-mass X-ray binaries, in which the donor is a white dwarf - a remnant of a moderately massive normal star. In order to understand the formation and evolution of these systems, it is critically important to identify the nature of the donor, which can be made of either helium or carbon and oxygen. MPA scientists have recently proposed and tested using - XMM-Newton observations - a principally new method to answer this question by the means of X-ray spectroscopy. 

Low mass X-ray binaries (LMXBs) are stellar systems consisting of two stars, one of which is a relativistic object - a neutron star or a black hole - and the other is a normal low-mass star, like our Sun, for example (Fig.1). If the separation between the two objects is comparable to the size of the normal star (which is hundreds thousands to millions of times larger than its relativistic companion), it may overfill it’s Roche lobe - the region of space where dynamics of matter are dominated by the gravitational attraction of the star. Consequently, it will start losing its outer layers under the gravitational pull of the second star. Material is predominantly lost through the so called inner Lagrangian point - the point on the line connecting the two stars where the forces of gravity and the centrifugal force balance each other out. The material of the donor star will flow through this point and will fall into the gravitation potential well of the relativistic star, initiating the process which is called accretion. Due to its large angular momentum, the infalling matter will form an accretion disk around the relativistic object (Fig.1). The classical theory of accretion disks around black holes and neutron stars was developed by Nikolai Shakura and Rashid Sunyaev in 1972. Due to the small size of the relativistic object (~15 km for a neutron star) the gravitational energy released during accretion constitutes a significant fraction of the rest mass energy of the accreting material, typically about 5-20%. This makes these systems very luminous sources of X-ray emission. 

There is a small but fascinating subclass of low-mass X-ray binaries, called Ultra-compact X-ray binaries (UCXBs) in which the donor star is a white dwarf - a remnant of a moderately massive normal star. These systems are extremely compact (hence their name) and have orbital periods shorter than 40 minutes, the fastest one having a period as short as 11 minutes. 

An interesting feature of these systems is that the chemical composition of the donor star is dramatically different from the composition of the donor star in ‘normal’ low-mass X-ray binaries. While donor stars in normal LMXBs have chemical composition similar to our Sun, i.e. are made of mostly hydrogen and helium with small admixture of metals, UCXBs feature donors that are depleted of hydrogen. They can be made of the ashes of nuclear burning of hydrogen (mostly helium and nitrogen), of helium (mostly carbon and oxygen) or carbon (mostly oxygen and neon). 

Depending on the particular evolutionary path through which UCXBs form, they may have a variety of donors ranging from non-degenerate helium stars to white dwarfs. It is critically important to distinguish between these possibilities, in order to understand the processes that lead to UCXB formation and control their evolution. So far this task has been performed using methods of optical astronomy, with various degrees of success. 

MPA scientists have recently proposed and tested a principally new method of diagnostics of the nature of the donor star in UCXBs by the means of X-ray spectroscopy. 

The method is using the phenomenon called X-ray reflection. A fraction of the emission produced near the compact objects illuminates the surface of the accretion disk and the donor star (Fig.2) and is reprocessed by this material. In the jargon of high energy astrophysics this reprocessed emission is called “reflected component”. An example of its spectrum is shown in Fig.3

On top of the continuum produced by the Compton scatterings off electrons in the accretion disk, the reflected component also contains a number of characteristic lines. These lines (called emission lines) are due to the different chemical elements present in the accreting material. They are produced by the process called fluorescence and have well known energies, unique for each chemical element. Their shape and relative strength carry information about the geometry of the accretion flow and chemical composition of the accreting material. 

The reflected component is heavily diluted by the primary emission, therefore the fluorescent lines of most of the elements are very weak and difficult to detect. Except for the fluorescent line of iron, which in the case of neutral iron is located at 6.4 keV. Thanks to the high fluorescent yield and abundance of iron, this is the brightest spectral feature in an otherwise relatively smooth continuum. All normal LMXBs have this line easily observable in their X-ray spectra. 

While the reprocessing of X-ray radiation by the accretion disc and particularly the shape and strength of the iron line has been thoroughly investigated since 1970s, all prior work concentrated on accretion disks of nearly solar abundance of elements, with only moderate variations of the element abundances considered in a few papers. MPA scientists have now taken the first step in modeling X-ray reflection off hydrogen poor material with anomalous abundances, as expected in the accretion disks in Ultra-compact X-ray binaries. The model developed using the Monte Carlo technique is the first simulation of reflection spectra of C/O, O/Ne/Mg or helium rich disks. 

Using these simulations, MPA scientists came to a paradoxical conclusion: The strongest and most easily observable effect of the hydrogen poor, C/O rich material is not an appearance of strong fluorescent lines of carbon and oxygen - as one might expect - but nearly complete disappearance of the fluorescent line of iron! This is caused by the screening of iron by the much more abundant carbon and oxygen. 

In a neutral material of solar abundance, the most likely process for a photon with energy exceeding 7.1 keV - the photoionisation threshold of K-shell electrons in iron (so called K-edge) - is absorption by iron due to the photoionisation of its atoms. Photoionisation of iron is followed in about one-third of the cases by the emission of a 6.4 keV fluorescent photon. Consequently, the majority of photons with energies above this threshold will be absorbed by iron and will, therefore, contribute to its fluorescent line. 

In the case of a C/O (or O/Ne) white dwarf though, the overwhelming overabundance of oxygen makes it the dominant absorbing agent even at energies far beyond its own K-edge, leaving only a few photons to fuel the iron fluorescent line. Although the fluorescent line of oxygen produced in the process is significantly boosted, it is still strongly diluted by the primary continuum and therefore is difficult to detect. A much more visible effect is the significant attenuation or complete disappearance of the iron line. 

Helium, on the other hand, is not capable of screening iron, due to its smaller charge and, correspondingly smaller absorption cross-section at the iron K-edge. Therefore in the case of a helium-rich donor reflection proceeds ‘as usual’ and the iron line has its nominal strength. 

This opens an exciting possibility to discriminate between helium and oxygen rich donors by means of X-ray spectroscopy. MPA scientists calibrated the method using extensive Monte-Carlo simulations, investigated its luminosity dependence and proposed observational tests of the picture. They used the data of XMM-Newton satellite to verify results of theoretical calculations using observations of UCXB systems with a donor star of known composition. Furthermore, they provided tentative identifications of the donor star in several ultra-compact binaries, where its nature remained so far unknown.

Filippos Koliopanos and Marat Gilfanov


References:

1. Koliopanos F., Gilfanov M., Bildsten L., 2013, MNRAS, 432, 1264

2. Koliopanos F., Gilfanov M., Bildsten L., M.Diaz Trigo, 2014 MNRAS
 
 


Monday, August 11, 2014

Astronomers find stream of gas – 2.6 million light years long

The bridge of gas (shown in green) stretches from the large galaxy at the bottom left to the group of galaxies at the top. A third nearby galaxy to the right also has a shorter stream of gas attached to it. The three insets show expanded views of the different galaxies and the green circle indicates the Arecibo telescope beam. Credit: Rhys Taylor/Arecibo Galaxy Environment Survey/The Sloan Digital Sky Survey Collaboration.

Astronomers and students have found a bridge of atomic hydrogen gas 2.6 million light years long between galaxies 500 million light years away. They detected the gas using the William E. Gordon Telescope at the Arecibo Observatory, a radio astronomy facility of the US National Science Foundation sited in Puerto Rico. The team publish their results today in a paper in Monthly Notices of the Royal Astronomical Society.

The stream of atomic hydrogen gas is the largest known, a million light years longer than a gas tail found in the Virgo Cluster by another Arecibo project a few years ago. Dr Rhys Taylor, a researcher at the Czech Academy of Sciences and lead author of the paper, said "This was totally unexpected. We frequently see gas streams in galaxy clusters, where there are lots of galaxies close together, but to find something this long and not in a cluster is unprecedented." 

It is not just the length of the stream that is surprising but also the amount of gas found in it. Roberto Rodriguez, a 2014 graduate from the University of Puerto Rico in Humacao who worked on the project as an undergraduate, explained "We normally find gas inside galaxies, but here half of the gas – 15 billion times the mass of the Sun – is in the bridge. That’s far more than in the Milky Way and Andromeda galaxies combined!"

The team is still investigating the origin of the stream. One notion surmises that the large galaxy at one end of the stream passed close to the group of smaller galaxies at the other end in the past, and that the gas bridge was drawn out as they moved apart. A second notion suggests that the large galaxy plowed straight through the middle of the group, pushing gas out of it. The team plan to use computer simulations to find out which of these ideas can best match the shape of the bridge that is seen with the Arecibo Telescope.

The project involved three undergraduate researchers: Roberto Rodriguez and Clarissa Vazquez from UPR Humacao, and Hanna Herbst, now a graduate student at the University of Florida. Dr Robert Minchin, a staff astronomer at Arecibo Observatory and the principal investigator on the project, said "Student involvement is very important to us. We are proud to be inspiring the next generation of astronomers, and particularly proud of the involvement of Puerto Rican students."

The bridge was found in data taken between 2008 and 2011 for the Arecibo Galaxy Environment Survey (AGES), which is using the power of the Arecibo Telescope to survey a large area of sky with a high level of sensitivity.

Media contacts

Ruth E. Torres Hernández
Public Relations Officer
Arecibo Observatory
Puerto Rico
Tel: +1 787 878 2612 x615

rutorres@suagm.edu

Yvonne Guadalupe Negrón
Director- Public Relations Office
Universidad Metropolitana
Puerto Rico
Tel: +1 787 766 1717 x6405, +1 787 242 0806


Erin Carver
Media and Communications Manager
Universities Space Research Association
United States
Tel: +1 410 227 7078

ecarver@usra.edu

Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 / 4582
Mob: +44 (0)794 124 8035

rm@ras.org.uk

Science contacts

Dr Rhys Taylor
Czech Academy of Sciences

rhysyt@gmail.com

Dr Robert Minchin
Arecibo Observatory
Puerto Rico

rminchin@naic.edu

Image and caption

An image is available to accompany this release.
Caption: The bridge of gas (shown in green) stretches from the large galaxy at the bottom left to the group of galaxies at the top. A third nearby galaxy to the right also has a shorter stream of gas attached to it. Credit: Rhys Taylor / Arecibo Galaxy Environment Survey / The Sloan Digital Sky Survey Collaboration


Further information

The new work appears in R. Taylor et al., 2014, "The Arecibo Galaxy Environment Survey VII : A Dense Filament With Extremely Long HI Streams", Monthly Notices of the Royal Astronomical Society, vol. 443, pp. 2634-2649, published by Oxford University Press. A pre-print of the paper can be found on the arXiv.

Notes for editors

The Arecibo Observatory is operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968), and in alliance with Ana G. Méndez-Universidad Metropolitana, and the Universities Space Research Association. The Arecibo Planetary Radar program is supported by NASA's Near Earth Object Observation program. For more information see the Observatory's Facebook page and follow its Twitter feed.

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter.



Saturday, August 09, 2014

White dwarfs crashing into Neutron Stars explain loneliest supernovae

An artist’s illustration of a white dwarf star (the stretched object right of centre) being dragged on to a neutron star (bottom centre). At the top left is the galaxy where the pair originated and other more distant galaxies can be seen elsewhere in the image. Credit: (c) Mark A. Garlick / space-art.co.uk / University of Warwick. Click here a full resolution image.

A research team led by astronomers and astrophysicists at the University of Warwick have found that some of the Universe’s loneliest supernovae are likely created by the collisions of white dwarf stars into neutron stars. Dr Joseph Lyman from the University of Warwick is the lead researcher on the paper, which will appear in the journal Monthly Notices of the Royal Astronomical Society.

"Our paper examines so-called 'calcium-rich' transients" says Dr Lyman. "These are luminous explosions that last on the timescales of weeks, however, they're not as bright and don't last as long as traditional supernovae, which makes them difficult to discover and study in detail".

Previous studies had shown that calcium comprised up to half of the material thrown off in such explosions compared to only a tiny fraction in normal supernovae. This means that these curious events may actually be the dominant producers of calcium in our universe.

"One of the weirdest aspects is that they seem to explode in unusual places. For example, if you look at a galaxy, you expect any explosions to roughly be in line with the underlying light you see from that galaxy, since that is where the stars are" comments Dr Lyman. "However, a large fraction of these are exploding at huge distances from their galaxies, where the number of stellar systems is miniscule.

"What we address in the paper is whether there are any systems underneath where these transients have exploded, for example there could be very faint dwarf galaxies there, explaining the weird locations. We present observations, going just about as faint as you can go, to show there is in fact nothing at the location of these transients - so the question becomes, how did they get there?"

Calcium-rich transients observed to date can be seen tens of thousands of parsecs away from any potential host galaxy, with a third of these events at least 65 thousand light years from a potential host galaxy.

The researchers used the Very Large Telescope in Chile and Hubble Space Telescope observations of the nearest examples of these calcium rich transients to attempt to detect anything left behind or in the surrounding area of the explosion.

The deep observations taken allowed them to rule out the presence of faint dwarf galaxies or globular star clusters at the locations of these nearest examples. Furthermore, an explanation for core-collapse supernovae, which calcium-rich transients resemble, although fainter, is the collapse of a massive star in a binary system where material is stripped from the massive star undergoing collapse. The researchers found no evidence for a surviving binary companion or other massive stars in the vicinity, allowing them to reject massive stars as the progenitors of calcium rich transients.

Professor Andrew Levan from the University of Warwick’s Department of Physics and a researcher on the paper said: "It was increasingly looking like hypervelocity massive stars could not explain the locations of these supernovae.  They must be lower mass longer lived stars, but still in some sort of binary systems as there is no  known way that a single low mass star can go supernova by itself, or create an event that would look like a supernova."

The researchers then compared their data to what is known about short-duration gamma ray bursts (SGRBs). These are also often seen to explode in remote locations with no coincident galaxy detected. SGRBs are understood to occur when two neutron stars collide, or when a neutron star merges with a black hole – this has been backed up by the detection of a 'kilonova' accompanying a SGRB thanks to work led by Professor Nial Tanvir, a collaborator on this study.

Although neutron star and black hole mergers would not explain these brighter calcium rich transients, the research team considered that if the collision was instead between a white dwarf star and neutron star, it would fit their observations and analysis as it:
•    Would provide enough energy to generate the luminosity of calcium rich transients.
•    The presence of a white dwarf would provide a mechanism to produce calcium rich material.
•    The presence of the Neutron star could explain why this binary star system was found so far from a host galaxy.

Dr Lyman said: "What we therefore propose is these are systems that have been ejected from their galaxy. A good candidate in this scenario is a white dwarf and a neutron star in a binary system. The neutron star is formed when a massive star goes supernova. The mechanism of the supernova explosion causes the neutron star to be 'kicked' to very high velocities (100s of km/s). This high velocity system can then escape its galaxy, and if the binary system survives the kick, the white dwarf and neutron star will merge causing the explosive transient."

The researchers note that such merging systems of white dwarfs and neutron stars are postulated to produce high energy gamma-ray bursts, motivating further observations of any new examples of calcium rich transients to confirm this. Additionally, such merging systems will contribute significant sources of gravitational waves, potentially detectable by upcoming experiments that will shed further light on the nature of these exotic systems.

Media contacts

Tom Frew
International Press Officer
University of Warwick
Tel: +44 (0)24 765 75910
a.t.frew@warwick.ac.uk

Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 / 4582
Mob: +44 (0)794 124 8035
rm@ras.org.uk

Science contact

Dr Joseph Lyman
Department of Physics
University of Warwick
J.D.Lyman@warwick.ac.uk


Further information

The work used observations made with the ESO Telescopes at the Paranal Observatory under programme ID 092.D-0420 and the NASA/ESA Hubble Space Telescope, with obtained from the data archive at the Space Telescope Science Institute.

The University of Warwick acknowledges the support from the UK Science and Technology Facilities Council (grant ID ST/I001719/1).

R.P. Church and M.B. Davis of the Lund University Observatory, Department of Astronomy and Theoretical Physics and N.R.Tanvir of the Department of Physics and Astronomy, University of Leicester made significant contributions to the work in addition to the University of Warwick researchers.

Notes for editors

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter


The research will appear in “The progenitors of calcium-rich transients are not formed in situ”, J. D. Lyman, A. J. Levan, R. P. Church, M. B. Davies, N. R. Tanvir, Monthly Notices of the Royal Astronomical Society, Oxford University Press, in press. A preprint of the paper is available on the arXiv