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Repeating outflows of hot wind found close to a galactic black hole

An international team of astrophysicists from Southampton, Oxford and South Africa have detected a very hot, dense outflowing wind close to a black hole at least 25,000 light-years from Earth.

Lead researcher Professor Phil Charles from the University of Southampton explained that the gas (ionised helium and hydrogen) was emitted in bursts which repeated every 8 mins, the first time this behaviour has been seen around a black hole. The findings have been published in the journal Monthly Notices of the Royal Astronomical Society.

The object Professor Charles’ team studied was Swift J1357.2-0933 which was first discovered as an X-ray transient – a system that exhibits violent outbursts – in 2011. These transients all consist of a low-mass star, similar to our Sun and a compact object, which can be a white dwarf, neutron star or black hole. In this case, Swift J1357.2-0933 has a black hole compact object which is at least 6 times the mass of our Sun.

Material from the normal star is pulled by the compact object into a disc in between the two. Massive outbursts occur when the material in the disc becomes hot and unstable and it releases copious amounts of energy.

Professor Charles said: “What was particularly unusual about this system was that ground-based telescopes had revealed that its optical brightness displayed periodic dips in its output and that the period of these dips slowly changed from around 2 mins to about 10 mins as the outburst evolved. Such strange behaviour has never been seen in any other object.

“The cause of these remarkable, fast dips has been a hot topic of scientific debate ever since their discovery. So it was with great excitement that astronomers greeted the second outburst of this object in mid-2017, presenting an opportunity to study this strange behaviour in greater detail.”

Professor Charles and his team recognised that key to getting the answer was to obtain optical spectra a number of times during each dip cycle, essentially studying how their colour changed with time. But with the object about 10,000 times fainter than the faintest star visible to the naked eye and the dip period of only around 8 minutes, a very big telescope had to be used.

So, they used SALT, the Southern African Large Telescope, the largest optical telescope in the southern hemisphere.

The University of Southampton is one of the founding UK partners in SALT, and together with their South African collaborators, are part of a multi-partner Large Science Programme to study transients of all types. Not only does SALT have the necessary huge collecting area (it has a 10m diameter mirror), but it is operated in a 100% queue-scheduled way by resident staff astronomers, meaning that it can readily respond to unpredictable transient events. This was perfect for Swift J1357.2-0933, and SALT obtained more than an hour of spectra, with one taken every 100 secs.

“Our timely observations of this fascinating system demonstrates how the quick response of SALT, through its flexible queue-scheduled operation, makes it an ideal facility for follow-up studies of transient objects”, said Dr David Buckley, the Principal Investigator of the SALT transient programme, based at the South African Astronomical Observatory, who also added, “With the instantaneous availability of a number of different instruments on SALT, we can also dynamically modify our observing plans to suit the science goals and react to results, almost in real-time”

Professor Charles added: “The results from these spectra were stunning. They showed ionised helium in absorption, which had never been seen in such systems before. This indicated that it must be both dense and hot – around 40,000 degrees. More remarkably, the spectral features were blue-shifted (due to the Doppler effect), indicating that they were blowing towards us at about 600km/s. But what really astonished us was the discovery that these spectral features were visible only during the optical dips in the light-curve. We have interpreted this quite unique property as due to a warp or ripple in the inner accretion disc that orbits the black hole on the dipping timescale. This warp is very close to the black hole at just 1/10 the radius of the disc.”

What is driving this matter away from the black hole? It is almost certainly the radiation pressure of the intense X-rays generated close to the black hole. But it has to be much brighter than we see directly, suggesting that the material falling on to the black hole obscures it from direct view, like clouds obscuring the Sun. This occurs because we happen to be viewing the binary system from a vantage point where the disc appears edge-on, as depicted in the schematic illustration, and rotating blobs in this disc obscure our view of the central black hole.

Interestingly there are no eclipses by the companion star seen in either the optical or X-ray as might be expected. This is explained by it being very small, and constantly in the shadow of the disc. This inference comes from detailed theoretical modelling of winds being blown off accretion discs that was undertaken by one of the team, James Matthews at the University of Oxford, using supercomputer calculations.

This object has remarkable properties amongst an already interesting group of objects that have much to teach us about the end-points of stellar evolution and the formation of compact objects. We already know of a couple of dozen black hole binary systems in our Galaxy, which all have masses in the 5-15 solar mass range, and the single black hole at our Galactic Centre is around 4 million solar masses. They all grow by the accretion of matter that we have witnessed so spectacularly in this object. We also know that a substantial fraction of the accreting material is being blown away. When that happens from the supermassive black holes at the centres of galaxies, those powerful winds and jets can have a huge impact on the rest of the galaxy.

These short-period binary versions are a perfect way to study this physics in action.

Notes to editors

1. For further information or interview requests please contact (SAAO: David Buckley:; Daniel Cunnama: (Southampton: Steve Bates, Media Relations Officer, University of Southampton.; 02380 593212)

2. The attached image produced by John Paice a graduate student at the University of Southampton and the Inter-University Centre for Astronomy & Astrophysics in Pune, India, with support from a UKIERI (UK-India) grant.

3. This study is to be published in Monthly Notices of the Royal Astronomical Society as a Letter to the Editor, authored by Phil Charles (University of Southampton), James H. Matthews (University of Oxford), David A.H. Buckley (South African Astronomical Observatory; SAAO), Poshak Gandhi (Southampton), Enrico Kotze (SAAO and South African Large Telescope), and John A. Paice (Southampton and Inter-University Centre for Astronomy and Astrophysics). This work was supported by the South African National Research Foundation, the Leverhulme Trust, STFC, and a UGC-UKIERI Thematic Partnership.

4. Founded in 1820, the South African Astronomical Observatory (SAAO; is the national centre for optical and infrared astronomy in South Africa. Its primary role is to conduct fundamental research in astronomy and astrophysics by providing a world-class facility to scientists. The SAAO also promotes astronomy and astrophysics in Southern Africa, by sharing research findings and discoveries, and participating in outreach activities to enthuse citizens about physics and astronomy. The SAAO is a facility of the National Research Foundation, which operates under the South African Department of Science and Technology. The SAAO encompasses headquarters in the eponymous suburb of Observatory in Cape Town, and a dedicated research and observation station with several working telescopes (including SALT; outside the Karoo town of Sutherland in the Northern Cape.

5. The University of Southampton drives original thinking, turns knowledge into action and impact, and creates solutions to the world’s challenges. We are among the top 100 institutions globally (QS World University Rankings 2019). Our academics are leaders in their fields, forging links with high-profile international businesses and organisations, and inspiring a 24,000-strong community of exceptional students, from over 135 countries worldwide. Through our high-quality education, the University helps students on a journey of discovery to realise their potential and join our global network of over 200,000 alumni.

6. 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, recognises 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 4,000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. The RAS accepts papers for its journals based on the principle of peer review, in which fellow experts on the editorial boards accept the paper as worth considering. The Society issues press releases based on a similar principle, but the organisations and scientists concerned have overall responsibility for their content.


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SAAO 1.0m Telescope pivotal in the discovery of a ‘Forbidden’ Planet in the ‘Neptunian Desert’

The South African Astronomical Observatory’s 1.0m telescope provided crucial follow-up observations in new research published today by members of the Astronomy and Astrophysics Group at the University of Warwick, UK, detailing the discovery of an exoplanet in the so-called “Neptunian Desert”.

The planet NGTS-4b, also nick-named ‘The Forbidden Planet’ by researchers is about 20 times the mass of the Earth and about 3 times the size. The planet orbits its host star in just 1.3 days with temperatures exceeding 1000 degrees Celsius.

NGTS-4b was first noticed using the state-of-the-art Next-Generation Transit Survey (NGTS) observing facility, designed to search for transiting planets on bright stars. NGTS is situated at the European Southern Observatory’s Paranal Observatory in the heart of the Atacama Desert, Chile.

The discovery relied heavily on follow-up observations made by Dr Matt Burleigh (University of Leicester) using the Sutherland High-speed Optical Cameras (SHOC) on the SAAO 1.0m Telescope in November 2017. This triggered an international effort to obtain further observations and a few weeks later it was confirmed that the transit was indeed a sub-Neptune exoplanet.

When looking for new planets astronomers look for a dip in the light of a star – this is the planet orbiting it and blocking the light. Usually, only dips of greater than 1% are picked up by ground-based searches, but the NGTS telescopes can pick up a dip of just 0.1%. With a dip almost that small, this exoplanet is, by a long way, the shallowest transiting planet ever discovered by a ground-based survey (the transit is less than 0.2%).

Dr Burleigh explained “Since this transit is so shallow, NGTS-4b wasn’t initially one of our top priority targets. But thanks to the excellent telescopes at SAAO in Sutherland, we were able to detect and confirm the transit, convincing ourselves the planet is real. We then set in motion many more observations to measure its mass and size.”

It is the first exoplanet of its kind to have been found in the Neptunian Desert, which is the region close to stars where, up until now, no Neptune-sized planets have been found. This area receives strong irradiation from the star, meaning the planets do not retain their gaseous atmosphere as they evaporate leaving just a rocky core. However, NGTS-4b still has its atmosphere of gas leading researchers to believe the planet may have moved into the Neptunian Desert recently, in the last
one million years or that it was originally very big and the atmosphere is still evaporating.

High-res image available at
The full paper at

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The changing landscape of South African astronomy

South Africa is looking forward to hosting the International Astronomical Union (IAU) General Assembly in 2024 — the first on the African continent. The meeting will come at a time of burgeoning scientific prosperity for the growing community of indigenous South African and African astronomers.

SAAO Astronomer, David Buckley, today published an article in the prestigious Nature Astronomy journal on the changing landscape of South African astronomy, highlighting the huge strides made in the development of astronomy in South Africa and the continent as a whole in recent decades, and providing some perspectives on the future of astronomy on the continent:

“the African continent has seen enormous strides in astronomy development over the last ~2 decades, not least being the construction of three major internationally recognized facilities. First came the European-led High Energy Stereoscopic System (HESS), a ground-based Cherenkov TeV gamma-ray telescope array in Namibia, which began operations in 2002. The 10-m class Southern African Large Telescope (SALT), still, the largest single optical telescope in the Southern Hemisphere, was completed in 2005 and is situated at the Sutherland site of the South African Astronomical Observatory (SAAO), which itself was established in 1972. Finally, the MeerKAT radio telescope array situated near the remote Karoo town of Carnarvon, one of the Square Kilometre Array (SKA) pathfinders and now a precursor to SKA Phase 1, was inaugurated only a month before the IAU General Assembly, in July 2018.”

Read the full article here:

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The South African Astronomical Observatory Celebrates the life of an Astronomy Giant, Professor Michael William Feast

Professor Michael William Feast died peacefully early on Monday morning, 1 April 2019, aged 92. He is survived by his wife Connie, three children and eight grand-children. Prof. Feast was an Honorary Professor in the Astronomy Department at the University of Cape Town, a former director of the South African Astronomical Observatory (SAAO), a Founding […]

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Declaration of the South African Astronomical Observatory as a National Heritage Site

Image Credit: Illustrated London News, 21 March 1857/Ian Glass

On December 21, 2018, the South African Heritage Resources Agency(SAHRA) officially declared the South African Astronomical Observatory as a National Heritage Site.

This is a very exciting development for the SAAO, recognising the incredible achievements and their significance over the past two centuries, and will ensure this heritage is preserved. The declaration is made with the understanding that we are to remain a working site and that the Heritage status will not curtail our primary function as a world-class observatory.

SAHRA released the following statement along with a statement of significance, please find attached the entry in the Government Gazette:

“SAHRA identified the site as having qualities so exceptional that it is of special national significance and warrants declaration as a National Heritage Site.”

Statement of Significance

The South African Astronomical Observatory in Cape Town has played a highly significant scientific role over time as the oldest permanent observatory in the Southern Hemisphere. The site offers an overview of the history of astronomy both locally and internationally. It is a “living site” with almost 200 years of history while still retaining its prominence in the international astronomical community.

Contributions to astronomy from the site range from some of the first accurate measurements of the distance to a star(Alpha Centauri), first catalogues of the principal southern stars, the first photographic survey of the sky, accurate measurements of the distance to the Sun( a value that became the benchmark to measure all other cosmic distance and represented a paradigm shift in astronomy), development of spectroscopy, remeasurement of Lacaille’s Arc of Meridian, establishment of the true shape of the Earth in the Southern hemisphere and the first accurate geodetic surveys of southern Africa.

Architecturally, there are several buildings of historical value and not only reflect the changing architectural styles over the nineteenth century but also have a considerable scientific value due to their contributions to the field of astronomy. The Main Building(a Georgian Building) – designed by the British naval architect, John Rennie, and completed in 1828, the heliograph – the oldest dome on the site and which runs on cannon balls, and the McClean Telescope Building – designed by Herbert Baker.

The range of scientific object related to the observatory as a collection is integral to the scientific value of the site. Some of the instruments within structures have been used with varying degrees of continuity and consistency for over 180 years.

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SAAO contributes to MU69 exploration

On the 1st of January 2019, NASA’s New Horizon’s spacecraft performed a very exciting flyby of the most distant object ever explored, 2014 MU69, nicknamed Ultima Thule. MU69 is located in the Kuiper belt, the icy disk in the outer solar system that contains leftover material from the formation of the Solar System 4.5 billion years ago. In order to prepare for this flyby, and learn as much as possible about this object, astronomers at SAAO and around the world have been performing Earth-based observations from as early as 2017.

This image taken by the Long-Range Reconnaissance Imager (LORRI) is the most detailed of MU69 returned so far by the New Horizons spacecraft. It was taken at 5:01 Universal Time on January 1, 2019, just 30 minutes before closest approach from a range of 28,000 kilometres, with an original scale of 140 meters per pixel.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The New Horizons spacecraft, which initially visited Pluto in 2015, was then re-directed to explore an even more distant object. For this, scientists chose MU69, a body around 30km in size in the outer reaches of our solar system, and aimed to learn about its surface composition, its structure, and whether it hosts moonlets, a coma, or rings.MU69 offered a rare opportunity to learn more about objects in our outer solar system and how they may have formed.

However long before the 2019 flyby, observations of MU69 had already begun. Astronomers, using what are known as occultation observations, had already learned a fair amount about 2014 MU69. An occultation observation is performed as the object in question moved in front of a background star, briefly blocking the star’s light:

As 2014 MU69 passes in front of a background star, this occultation can be observed from Earth with carefully placed lines of ground telescopes or with SOFIA, an airborne observatory. [NASA]

In a recently published study led by Eliot Young (Southwest Research Institute), and including SAAO Astronomers, scientists searched for evidence of rings around MU69 using these occultation observations. Observations were performed on three dates in 2017: on 3 June, from SAAO and Argentina, on 10 June, from the airborne observatory SOFIA, and on 17 July, from Argentina. On these days, telescopes were set up with the goal of catching 2014 MU69 as it crossed in front of a background star.

The light curves produced by these occultations allowed the team to explore whether MU69 is encircled by additionally light-blocking rings. An example of candidate rings (red ellipses) ruled out by occultation observations on 17 July 2018 (yellow lines and map in the right panel). [Young et al. 2018]

The authors compared predicted light curves to the actual dimming of the background star captured during MU69’s occultations and concluded that MU69 was highly unlikely to host any rings. This lack of rings is consistent with the newly obtained low-resolution images from New Horizons’s flyby.

“Limits on a Ring System at 2014 MU69 from Recent Stellar Occultations,” Eliot F. Young et al 2018 Res. Notes AAS 2 224.


Occultations Suggest No Rings for Ultima Thule

Original Publication:


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A mystery in the outer Solar System

Stellar occultations are a powerful technique that allows characterization of foreground objects by watching them block light from a distant star. This technique is particularly useful for studying small and/or distant objects in the Solar System, which are difficult to see from the ground. In March 2017, a team of astronomers from the South African Astronomical Observatory and the Massachusetts Institute of Technology observed a stellar occultation that was predicted for the large, Trans-Neptunian Object (TNO) Orcus (Fig. 1, left).

Prior to this observation, Orcus was known to be approximately 900 km in size and had one satellite, Vanth, with a size of roughly 280 km. The Orcus system is in a 3:2 orbital resonance with Neptune, similar to Pluto’s orbit. Although not expected to maintain an atmosphere, Orcus’ features could hint toward a possible resurfacing mechanism, such as cryovolcanism. Goals of the occultation observation included accurately constraining Orcus’ size, measuring any atmosphere around Orcus, and detecting rings or debris in the system.

Five sites in North America attempted the observation, with locations spanning from Chile to northern California. Two sites successfully detected the star disappearing: NASA’s 3-m Infrared Telescope Facility (IRTF) on Mauna Kea, Hawai’i and a 1-m Las Cumbres Observatory (LCO) telescope at McDonald Observatory, Texas (Fig. 2). However, a quick analysis of the geometry and the timing showed that data did not align with the expectation of one star being occulted by one outer Solar System object. After months of careful study, the best guess was that one site observed an occultation by Vanth of the target star while the second site observed an occultation by Orcus of an unknown secondary star.

An external reviewer recommended obtaining high-resolution images in order to definitively solve the mystery. So, a proposal was submitted to the 8.1-m Gemini South telescope to observe the star using the Differential Speckle Survey Instrument (DSSI). The DSSI speckle images easily detected a companion star 250 milliarcsec away from the brighter primary (Fig. 3). Armed with confirmation of the location of the second star, the occultation data were reanalyzed to reveal that both detections were of Vanth, each blocking one of the two separate stars (Fig. 1, right).

The data allowed Vanth’s size to be measured at 443±10 km in diameter. They also placed a constraint of a few microbars on any global Vanth atmosphere and did not detect any material in the two star paths down to a limit of a few km in extent.

Click here for the Gemini Observatory press release, “Outer Solar System Object has Astronomers Seeing Double”.

Publication details:
“A Stellar Occultation by Vanth, a satellite of (90482) Orcus”
Amanda A. Sickafoose, Amanda S. Bosh Stephen E. Levine, Carlos A. Zuluaga, Anja Genade, Karsten Schindler, Tim Lister, and Michael J. Person
Icarus, volume 319, pages 657-668

Preprint available at

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SALT plays key role in the global hunt for Dark Energy

The Southern African Large Telescope (SALT), the largest optical telescopes in the Southern Hemisphere, has played an important role as part of the international Dark Energy Survey’s (DES, quest to pin down dark energy, the mysterious force accelerating the expansion of the universe.

As part of the hunt, SALT conducted follow-up spectroscopy of supernovae – stars that explode at the end of their lives – discovered by DES. Supernovae are so bright that they can be seen on the other side of the Universe and astronomers can accurately calculate the distances to a small subclass of them – the so-called Type Ia supernovae. Once their distances are known, Type Ia supernovae can be used to measure the acceleration of the expansion of the universe. Sorting through the chaff of variable objects to find and classify the Type Ia jewels was the important role undertaken by SALT and several other of the world’s biggest telescopes.

Zoomed-in image of the supernova, highlighting it in one of the host galaxy’s spiral arms. For a brief time, the supernova can be as bright as the 100 billion stars of the host galaxy.
Image Credit: SALT/SAAO

“To measure the acceleration of the universe’s expansion by studying stars that died hundreds of millions of years ago takes the most powerful telescopes in the world, combined with meticulous analysis. SALT has provided a key contribution to the international Dark Energy Survey, the most sophisticated study of dark energy with supernovae yet” said Dr. Eli Kasai, former PhD student at SAAO, now lecturer at the University of Namibia, and Principal Investigator for the South African Astronomical Observatory DES program from 2014 to the end of the survey in 2018.

“We need to control systematic uncertainties to very high precision so that we have confidence in our conclusions. SALT, with its massive mirror and ability to rapidly target exciting new candidates, allows us to take a confirming spectrum when the supernova candidate is at its brightest. This translates into clean answers to exactly what kind of exploding star we are looking at.”, said Dr. Mathew Smith, who is based at Southampton University and was the PI of the SALT spectroscopic follow-up program of DES supernova candidates from 2013 to 2014.

DES began science observations in 2013, in Chile, South America, with an overall goal of measuring the expansion history of the Universe in order to place tight constraints on the quantity and properties of dark energy at an accuracy of about 1%. DES employs several methods of constraining dark energy, of which supernova observations, are a primary tool.

“20 years ago we discovered Dark Energy and the acceleration of the universe by carefully observing supernovae. Today, two decades later, dark energy is still one of the great mysteries of our time. These results, with the purest sample of supernovae to date, confirm yet again that dark energy is real, and will be a key target of investigation for the Square Kilometre Array (SKA), that will be built primarily in South Africa.” said Professor Bruce Bassett, a member of the SALT DES supernova follow-up program, astronomer at SAAO and head of the Data Science group at the South African Radio Astronomical Observatory (SARAO). SARAO has recently completed construction of the MeerKAT radio telescope that will form an important part of the Square Kilometre Array.

“Dark Energy is perceived to exist in the vast empty spaces between galaxies in the Universe known as voids and we believe that it is responsible for making galaxies move away from each other at ever-increasing speeds. In other words, it is responsible for the accelerated expansion of the Universe that we observe” said Prof. Roy Maartens, an SKA Chair in Astrophysics and Cosmology at the University of the Western Cape and a member of the SALT DES supernova follow-up program. He went on saying “observing more and more supernovae in many galaxies gives us a handle to quantify the properties of dark energy and also provides us insight into the true nature of supernovae”.

SALT consistently played a pivotal role of classifying into various types the discovered supernova candidates and successfully determining how far they were from Earth, two important parameters that were key to the success of the DES experiment, which came to an end at the end of February 2018. Spectral observations of the discovered SN candidates by SALT and other spectroscopic capable telescopes in DES played a crucial role in helping algorithms that could classify the discovered supernova candidates and determine their redshift using only the images in which such candidates were discovered. This type of classification and redshift determination is less accurate in comparison to that performed with spectroscopic data from SALT and other spectroscopic capable telescopes.

“The DES team has independently confirmed the existence of dark energy by combining four different cosmic probes: (1) supernova observations, (2) baryonic acoustic oscillations, (3) weak gravitational lensing and (4) galaxy clustering”, said Dr Eli Kasai. He continued by saying “The conclusions from DES from combining these four probes mean that for the first time we have been able to find strong evidence for cosmic acceleration and dark energy from a single experiment, instead of combining results from many different telescopes and different analyses.”

The past three weeks have seen the release of 8 DES papers to, reporting the findings of the analyses of DES supernova data observed over the first three years of the survey. The papers made use of the survey’s spectroscopic data including that taken with SALT.

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SALT contributes to the discovery of a bingeing white dwarf

A new binary star system has been discovered in which a small white dwarf star is cannibalising its larger Sun-like companion. Such objects are actually quite common, but for this new object, the white dwarf binged on its neighbour at a prodigious rate, heating part of it to nearly a million degrees. The object, named ASASSN-16oh, was found on 2 December 2016 by the All-Sky Automated Survey for Supernovae (ASASSN), a network of about 20 optical cameras distributed around the globe which automatically surveys the entire sky every night in search of transient events, objects which suddenly appear. ASASSN-16oh was found to be in the Milky Way’s satellite galaxy, the Small Magellanic Cloud, at a distance of ~200,000 light years.

An artist’s impression of the supersoft X-ray binary system, ASASSN-16oh, with a small white dwarf star (left) accreting hot gas from its Sun-like companion (right), through an accretion disk. The stream of gas from the companion forms a flattened accretion disk and the gas gradually spirals down to the white dwarf, getting hotter as it does so. Eventually, the accreted gas impacts the equator of the white dwarf, heating it up to nearly a million degrees, emitting in soft X-rays.
Image credit: NASA/CXC/M.Weiss

Optical follow-up observations were conducted by the Southern African Large Telescope (SALT), the Polish OGLE telescope in Chile and the Las Cumbres Observatory (LCO) telescope network. It was also discovered to be a so-called “supersoft” X-ray source by the NASA Neil Gehrels Swift Observatory and the Chandra X-ray Observatory, produced by gas at temperatures of ~900,000 degrees. Such supersoft systems have previously always been associated with a thermonuclear runaway explosion on the surface of a white dwarf, as occurs in a hydrogen bomb, brought on by the accumulation of hot and dense accreted gas which eventually reaches a critical explosive limit.

“Supersoft sources are a really interesting class of transient events, and ASASSN-16oh is no exception”, says David Buckley, the Principal Investigator of the SALT Large Science Programme on transients, who is based at the South African Astronomical Observatory. “We were fortunate to be able to react quickly to its discovery and undertake crucial observations during the outburst phase”, he said. “Our SALT spectra showed all the hallmarks of a highly energetic system, with an intensely strong emission line from ionized helium which changed in velocity from night-to-night”, says Buckley. In addition, robotic observations were triggered with the LCO telescopes in South Africa, Chile and Australia, allowing for monitoring over a 34 hour period, beginning on Christmas Day 2016. “A nice Christmas present courtesy of the LCO Director who granted the time”, quipped Buckley. The SALT and LCO data were then quickly analysed by another member of the SALT transients collaboration, Andry Rajoelimanana, at the University of the Free State, in Bloemfontein, South Africa.

It became clear after the optical and X-ray observations were analyzed that ASASSN-16oh was no normal thermonuclear powered supersoft source. “In the past, the supersoft sources have all been associated with nuclear burning on the surface of white dwarfs,” said lead author Tom Maccarone, a professor in the Texas Tech Department of Physics & Astronomy, lead author of the ASASSN-16oh discovery paper that has just appeared in the December 3rd issue of Nature Astronomy.

If nuclear fusion is the cause of the supersoft X-rays from ASASSN-16oh then it should begin with an explosion and the emission should come from the entire surface of the white dwarf. However, the optical light does not increase quickly enough to be caused by an explosion and the Chandra X-ray data show that the emission is coming from a region smaller than the surface area of the white dwarf. The source is also a hundred times fainter in optical light than white dwarfs known to be undergoing fusion on their surface. These observations, plus the lack of evidence for gas expelled away from the white dwarf, provide strong arguments against fusion having taken place on the white dwarf.

Because no signs of nuclear fusion are present, the authors present a different scenario. As with the fusion explanation, the white dwarf pulls gas from its companion star, a red giant, in a process called disk accretion. The gas forms a large flattened rotating disk surrounding the white dwarf, becoming hotter as it spirals inwards, as shown in our illustration. The gas then falls onto the white dwarf, producing X-rays along an equatorial belt where the disk meets the star. The rate of inflow of matter through the disk varies by a large amount and when the rate of mass loss from the companion increases, the X-ray and optical brightness of the system becomes much higher.

“The transfer of mass is happening at a higher rate than in any system we’ve caught in the past,” added Maccarone. If the white dwarf keeps gaining mass it may reach a mass limit and destroy itself in a Type Ia supernova explosion, a type of event which was used to discover that the expansion of the universe is accelerating. The team’s analysis suggests that the white dwarf is already unusually massive, so ASASSN-16oh may be relatively close – in astronomical terms – to exploding as a supernova.

“Our result contradicts a decades-long consensus about how supersoft X-ray emission from white dwarfs is produced,” said co-author Thomas Nelson from the University of Pittsburgh. “We now know that the X-ray emission can be made in two different ways: by nuclear fusion or by the accretion of matter from a companion.”

Also involved in the study were scientists from Texas A&M University, NASA Goddard Space Flight Center, University of Southampton, University of the Free State in the Republic of South Africa, the South African Astronomical Observatory, Michigan State University, Rutgers State University of New Jersey, Warsaw University Observatory, Ohio State University and the University of Warwick.

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.

Publication Details:

“Unconventional origin of supersoft X-ray emission from a white dwarf binary”

Thomas J. Maccarone, Thomas J. Nelson, Peter J. Brown, Koji Mukai, Philip A. Charles, Andry Rajoelimanana, David Buckley, Jay Strader, Laura Chomiuk, Christopher T. Britt, Saurabh W. Jha, Przemek Mróz, Andrzej Udalski, Michal K. Szymański, Igor Soszyński, Radosław Poleski, Szymon Kozłowski10, Paweł Pietrukowicz, Jan Skowron, Krzysztof Ulaczyk

Nature Astronomy, Volume 2, No. 11, Article Number 2397-3366

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SALT chases hypervelocity stars

The Southern African Large Telescope(SALT) was recently involved in the identification of a hypervelocity star, flung across the galaxy by a supernova explosion that occurred 90 000 years ago. The discovery of this star and two others may help to solve a decades-old debate on how supernovae occur.

Video animation produced using Gaia Sky showing the path of the hypervelocity star candidate by Shen et al (2018), with its movement exaggerated by a factor of 300 billion times. The path of the candidate is first shown moving forward in time and subsequently going into the past, back to the supernova remnant. The circle indicates the location of the remnant of the supernova G70.0-21.5. Credits: S. Jordan, T. Sagrista

Credits: ESA/Gaia/DPAC, K. Shen, S. Jordan, T. Sagrista

Type Ia supernovae are the thermonuclear explosions of white dwarfs in binary star systems. They are one of the most common types of supernovae and have a fundamental importance as cosmological distance indicators. Despite this, the nature of the binary system and the details of the explosion has remained a mystery. Many theoretical models have arisen over the past few decades to explain how these stars explode, but there have been few pieces of direct evidence that any of these scenarios actually succeeds in nature.

One model, dubbed the “dynamically driven double-degenerate double-detonation” (D6) scenario, predicts the possibility that the other star in the binary system is another white dwarf that can survive the explosion of its companion. Such a surviving star would be flung away from the system when the gravitational pull of its companion disappeared and would continue zipping away at speeds between 1000 – 2500 km/s.

Shen et al. (2018) searched for such hypervelocity survivors in Gaia’s second data release in April 2018 and discovered three likely candidates. These stars were followed up with ground-based telescopes, including the Southern African Large Telescope (SALT), and found to possess many of the predicted features for survivors of D6 Type Ia supernovae: a lack of hydrogen and strong signatures of carbon, oxygen, and magnesium, as well as luminosities and temperatures unlike almost all other stars.  Furthermore, the past location of one of the stars is spatially coincident with a known supernova remnant, making it highly probable that it was ejected from a system that underwent a supernova.

The combination of Gaia, which precisely measured the high-speed motion of these hypervelocity stars in the plane of the sky, and the ground-based spectroscopic observations, which provided a measurement of the radial component of the stars’ motion, has shown that these are among the fastest freely moving stars in our Milky Way Galaxy.

Much follow-up work remains to be done to ascertain precise characteristics of these stars and the explosions that gave birth to their hypervelocity natures. However, it is very likely that these stars represent the first discoveries of surviving companions to Type Ia supernovae, and that they confirm the success of the D6 “dynamically driven double-degenerate double-detonation” model.

The team undertaking the observations and data analysis was led by Ken Shen, a researcher at the University of California at Berkeley (USA). The SALT observations were taken by Marissa Kotze at the Southern African Astronomical Observatory as part of a program led by Saurabh Jha from Rutgers, the State University of New Jersey (USA).


Figure 1: Orbital solution of the second white dwarf candidate for the D6-scenario, overlaid with H-alpha images from the Virginia Tech Spectral Line Survey (VTSS, Dennison et al. 1998). The blue trajectory extends 90,000 years into the past, the red trajectory extends the same amount of 90,000 years into the future. The green circle indicates the remnant of the supernova G70.0-21.5. Image credit: Shen et al. (2018)

Figure 2: The three hypervelocity candidates are shown here in the colour-magnitude diagram with the green, blue and orange circles. Some other white dwarfs are indicated in this diagram as well. The black circles and colored regions show the reliably measured stars from Gaia. Image credit: Shen et al. (2018)

The article by Shen et al. was published in the Astrophysical Journal on 20 September 2018.

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