The largest single optical telescope in the Southern Hemisphere and one of the largest worldwide, SALT received the award in the category of International STI Partnership Achieving Excellence in Global Science. Known affectionately as Africa’s Giant Eye on the Sky, SALT is a powerful symbol of international scientific cooperation, with eight current shareholder partners including leading universities, science organisations, and funding agencies from Africa, India, Europe, and North America. It is based in Sutherland, Northern Cape, at the South African Astronomical Observatory (NRF-SAAO), a facility of the National Research Foundation. The NRF, funded and supported by the Department of Science, Technology and Innovation (DSTI), is the principal shareholder and is responsible for the maintenance and operations of SALT through NRF-SAAO. This multinational partnership continues to strengthen SALT’s scientific output through shared expertise, postgraduate training opportunities, student exchange programmes, and wider societal impact realised through its SALT Collateral Benefits Programme.

This latest accolade for SALT comes hot on the heels of the 20th anniversary celebrations of its astronomical excellence and breakthroughs on 10 November. For over two decades, SALT has delivered groundbreaking global research and accelerated South Africa’s contribution to modern astrophysics. The facility has produced more than 600 peer-reviewed scientific publications, including a milestone of over 80 publications in 2024 alone.

Among numerous science highlights, in August 2017, SALT produced one of the world’s first optical spectra associated with a gravitational wave event—marking the dawn of multi-messenger astronomy. This breakthrough continues to shape the South African National Multiwavelength Strategy and future scientific direction for the next decade and beyond.

“I would like to congratulate the SALT team for this well-deserved award. SALT’s success story is one of national pride and aspiration. Its iconic silver dome rising above the Karoo hills symbolises South Africa’s determination to look upward and forward. Through its education, outreach, and research programmes, SALT has inspired countless young people to pursue careers in science, technology, engineering, and mathematics (STEM). The telescope’s visibility in media, tourism, and education continues to capture the public imagination. It is a living example that science belongs to everyone and that major discoveries can emerge from African soil,” said Dr Mlungisi Cele, Director-General, DSTI.

Receiving the award, Prof Rosalind Skelton, SALT board member and Managing Director of NRF-SAAO, said, “SALT is a catalyst for SA’s leadership in astronomy, enabling world-class research and fostering meaningful partnerships across borders in pursuit of shared scientific discovery.”

SALT’s influence reaches well beyond its scientific achievements. It has helped cultivate a vibrant community of African astronomers, engineers, and data scientists, laying the groundwork for sustained excellence in the region. Its success was pivotal in securing South Africa’s role as host of the SKA-Mid telescope—a testament to the country’s growing stature in global big science and its commitment to collaborative innovation.

The SFSA Science Diplomacy Awards celebrate individuals, organisations and institutions demonstrating outstanding commitment to advancing science diplomacy—those whose work contributes meaningfully to the intersection of scientific research, international cooperation and partnerships, policy development, evidence-based decision-making, and diplomacy with a footprint in the regional and global science community.

NRF-SAAO MD Dr Rosalind Skelton receiving the award.

Artist’s impression of a supersoft X-ray source: the accretion disk around a white dwarf star is made mainly of helium. © schematics: F. Bodensteiner; background image: ESO

White dwarfs can only explode as supernovae when their mass exceeds a certain limit. One of the primary aims of this research was to understand the process by which the mass of a white dwarf can grow to this point, known as the Chandrasekhar limit. The team found that [HP99] 159, unlike other known Super Soft Sources (SSS), was overflowing and burning helium, not hydrogen, which had not been observed before in such detail.

Using SALT’s two spectrographs, the team was able to determine that the optical spectra of the system were entirely consistent with helium accretion. The measured luminosity suggests that the mass of the white dwarf is growing more slowly than previously thought possible, which could potentially help understand the number of supernovae caused by exploding white dwarfs.

“This demonstrated the uniqueness of this object, but also the capability of SALT’s two spectrographs allowing for the required detailed follow-up optical observations, confirming the nature of the source ”, said Dr Itumeleng Monageng.

Low-resolution optical spectrum of [HP99] 159, taken with the SALT/RSS spectrograph, with labels for the main emission lines, which are all due to helium. (The two ‘bkg’ labels are residuals of removing sky lines). The insets demonstrate that at two wavelengths, where He- and H-lines are close together, the signal results from He II and not hydrogen.
© MPE

The unique properties of [HP99] 159 suggest that it could end up in a subclass of SN Ia, known as SN Iax, which have weaker explosions and therefore less helium is blown away. The team hopes to find dozens of similar sources in the two Magellanic Clouds with the eROSITA telescope, which could help further constrain the conditions for SN Ia progenitors.

“This is another excellent example of the productive collaborations between our team and the German eROSITA team in discovering and studying new and interesting transient phenomena,” said Prof David Buckley at the South African Astronomical Observatory (SAAO), who leads the SALT transient programme for which the optical observations were obtained, and with, Dr Monageng, is one of the paper’s co-authors.

Original press release

https://www.mpe.mpg.de/7938487/news20230322

Reference

J. Greiner, C. Maitra, F. Haberl, R. Willer, J. M. Burgess, N. Langer, J. Bodensteiner, D. A. H. Buckley, I. M. Monageng, A. Udalski, H. Ritter, K. Werner, P. Maggi, R. Jayaraman & R. Vanderspek: A helium-burning white dwarf binary as a supersoft X-ray source. Nature 615, 605-609 (2023) (View)

Trajectories within the quadruple stellar system HD 74438: the two close pairs, having orbital periods of 21 and 4 days, orbit around each other in 6 years. At some point, the two pairs could merge into white dwarfs giving rise to a thermonuclear supernova, as illustrated in the bottom right. (Image Credit: Merle, A)

Stars like to be in company, and unlike our Sun, most of the stars in the Galaxy have one or more stellar companions. Binary stars are now recognized to play a major role in a large range of astrophysical events, such as the 2017 gravitational wave emission detection. In addition, binary stars allow us to derive fundamental stellar parameters like masses, radii and luminosities with better accuracy compared to single stars. They represent the gems on which various astrophysics studies rely.

While binaries have received much attention so far, higher-order stellar systems have remained aside until recently, despite the fact that they show a wide variety of interactions, especially in tight systems. Stellar quadruples only represent a marginal fraction (a few percent) of all multiple systems. The complex evolution of such high-order multiples involves mass transfer and collisions, leading to mergers that are also possible progenitors of thermonuclear supernovae. These supernovae represent standard candles for fixing the Universe distance scale, despite the fact that the evolutionary channel(s) leading to the progenitors of such supernova explosions are still highly debated.

A spectroscopic quadruple (HD 74438) was discovered in 2017 in the Gaia-ESO Survey. The Gaia-ESO Survey is a public spectroscopic survey providing a detailed overview of the stellar content of the Milky Way by characterizing more than 100 000 stars. Subsequent follow-up spectroscopic observations of HD 74438 were obtained with high-resolution spectrographs at the University of Canterbury Mount John Observatory in New Zealand and at the Southern African Large Telescope in South Africa. These observations allowed us to determine that this stellar quadruple is made of 4 gravitationally-bound stars: a short-period binary orbiting another short-period binary on a longer orbital period (2+2 configuration). Its membership in the open cluster IC 2391 makes it the youngest (43 million years) spectroscopic quadruple discovered so far and among the quadruple systems with the shortest outer orbital period (6 years).

Thanks to the spectroscopic analysis it was possible to show that this quadruple system is undergoing dynamic effects on long time scales compared to the orbital periods. Indeed, one of the inner binaries should have evolved into a circular orbit whereas it has an eccentric one. This is explained by the gravitational effect of the distant binary companion which can pump up the eccentricity. State-of-the-art simulations of this system’s future evolution show that such gravitational dynamics can lead to one or multiple collisions and merger events producing white dwarfs with masses just below the Chandrasekhar limit.

A star like our Sun will end its life as a white dwarf, and the mass of white dwarfs cannot go above the so-called Chandrasekhar limit. If it does, as a result of mass transfer or merger events, it does collapse and produces a thermonuclear supernova. Interestingly, 70 to 85% of all thermonuclear supernovae are now suspected to result from the explosion of white dwarfs with sub-Chandrasekhar masses. The evolution of stellar quadruples such as HD 74438 thus represents a new promising channel to form them.

Reference

1. Merle, A. S. Hamers, S. Van Eck, A. Jorissen, M. Van der Swaelmen, K. Pollard, R. Smiljanic, D. Pourbaix, Tomaž Zwitter, G. Traven, G. Gilmore, S. Randich, A. Gonneau, A. Hourihane , G. Sacco and C. C. Worley

A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar type Ia supernovae 

Nature Astronomy, Letters https://doi.org/10.1038/s41550-022-01664-5

Optical image of the first galaxy found with quasi-periodic eruptions in the eROSITA all-sky data, the NICER X-ray light-curve is overlayed in green. The galaxy was identified as 2MASS 02314715-1020112 at a redshift of z~0.05, determined by SALT. The peak-to-peak separation of the X-ray outbursts is about 18.5 hours. Credit: MPE; optical image: DESI Legacy Imaging Surveys/D. Lang (Perimeter Institute)

Quasars or “active galactic nuclei” (AGN) are often called the lighthouses of the distant universe. The brightness of their central region, where a very massive black hole accretes large amounts of material, can be thousands of times higher than that of a galaxy like our Milky Way.

“In the eROSITA all-sky survey, we have now found two previously dormant galaxies with huge, almost periodic sharp pulses in their X-ray emission,” says Riccardo Arcodia, PhD student at the Max Planck Institute for Extraterrestrial Physics (MPE), who is the first author of the study now published in Nature. These kinds of objects are fairly new: only two such sources were known before, found either serendipitously or in archival data in the past couple of years. “As this new type of erupting sources seems to be peculiar in X-rays, we decided to use eROSITA as a blind survey and immediately found two more,” he adds.

Optical image of the second galaxy found with quasi-periodic eruptions in the eROSITA all-sky data, the XMM-Newton X-ray light-curve is overlayed in magenta. The galaxy was identified as 2MASX J02344872-4419325 at a redshift of z~0.02, determined by SALT. This source shows much narrower and more frequent eruptions with a mean peak-to-peak separation of only about 2.4 hours. Credit: MPE; optical image: DESI Legacy Imaging Surveys/D. Lang (Perimeter Institute)

The eROSITA telescope currently scans the entire sky in X-rays and the continuous data stream is well suited to look for transient events, such as these eruptions. Both the new sources discovered by eROSITA showed high-amplitude X-ray variability within just a few hours, which was confirmed by follow-up observations with the XMM-Newton and NICER X-ray telescopes. Contrary to the two previously known similar objects, the host galaxies of these new sources found by eROSITA show no signs of previous black hole activity.

“These were normal, average low-mass galaxies with inactive black holes,” explains Andrea Merloni at MPE, principal investigator of eROSITA. “Without these sudden, repeating X-ray eruptions we would have ignored them.” The optical observations of the two galaxies concerned were obtained with SALT through a collaboration between the German eROSITA Consortium and SALT transient follow-up teams. “This is an exciting new result”, says David Buckley of the South African Astronomical Observatory, principal investigator of the SALT transient programme. “It shows that energetic X-ray emission from black hole interactions is not just confined to the nuclei of active galaxies. ”

Scientists now have the chance to explore the vicinity of supermassive black holes with relatively low masses of 100 000 to 10 million times the mass of our Sun.

Quasi-periodic emission, such as the one discovered by eROSITA, is typically associated with binary systems. If these eruptions are indeed triggered by the presence of an orbiting object, its mass has to be much smaller than the black hole’s – maybe of the order of a star or even a white dwarf, which might be partially disrupted by the huge tidal forces close to the black hole at each passage.

“We still do not know what causes these X-ray eruptions,” admits Arcodia. “But we know that the black hole’s neighbourhood was quiet until recently, so a pre-existing accretion disk, as  present in active galaxies, is not required to trigger these phenomena.” Future X-ray observations will help to constrain or rule out the “orbiting object scenario” and to monitor possible changes in the orbital period. This scenario could also make these kinds of objects observable via both electromagnetic and gravitational wave signals, thus opening up new possibilities with multi-messenger astrophysics.

“This is the first major result from the SALT-eROSITA collaboration”, says Buckley. “Due to the unprecedented sensitivity of eROSITA, coupled with its repeated scanning of the entire sky, we are continuing to make new discoveries. These help to reveal the nature of variable X-ray sources in our Universe”.

Original publication 

X-ray Quasi-Periodic Eruptions from two previously quiescent galaxies

Arcodia, A. Merloni, K. Nandra, et al.

Nature, published  29 April 2021

Link / DOI: https://dx.doi.org/10.1038/s41586-021-03394-6

The scientists observed an ‘accreting’ neutron star as it entered an outburst phase in an international collaborative effort involving five groups of researchers, seven telescopes (five on the ground, two in space), and 15 collaborators. 

The telescopes involved include two space observatories: the Neil Gehrels Swift X-ray Observatory, and the Neutron Star Interior Composition Explorer (NICER) on the International Space Station; as well as the ground-based Las Cumbres Observatory network of telescopes and the Southern African Large Telescope (SALT). 

“Optical spectra obtained with SALT were crucial in demonstrating the powering-up,” said the SALT Transient programme lead, Dr David Buckley. “We observed on 6 occasions during August 2019. The first two observations showed it was very faint, while by the time of the third observation, on 6 August, is was clearly in outburst. This demonstrates the importance of flexible telescope scheduling that can quickly react to changing circumstances,” he concluded. SALT has made many advances in the study of compact objects, like neutron stars and black holes, including studies of the most energetic events in the Universe, like gamma-ray bursts and gravitational wave events.

It is the first time such an event has been observed in this detail – in multiple frequencies, including high-sensitivity measurements in both optical and X-ray. 

This illustration shows a neutron star — the core of a star that exploded in a massive supernova. This particular neutron star is known as a pulsar because it sends out rotating beams of X-rays that sweep past Earth like lighthouse beacons.

The physics behind this ‘switching on’ process has eluded physicists for decades, partly because there are very few comprehensive observations of the phenomenon. 

The researchers caught one of these accreting neutron star systems in the act of entering outburst. They witnessed the onset of the outburst, from the first sign of optical activity to the beginning of X-ray emission, all the way to the end of the outburst. 

The observations revealed that it took 12 days for material to swirl inwards and collide with the neutron star, substantially longer than previously thought. 

“These observations allow us to study the structure of the accretion disk, and determine how quickly and easily material can move inwards to the neutron star,” said study lead PhD candidate Adelle Goodwin from the Monash School of Physics and Astronomy in Melbourne, Australia. 

“Using multiple telescopes that are sensitive to light in different energies we were able to trace that the initial activity happened near the companion star, in the outer edges of the accretion disk, and it took 12 days for the disk to be brought into the hot state and for material to spiral inward to the neutron star, and X-rays to be produced,” she said.

In an ‘accreting’ neutron star system, a pulsar which is a dense remnant of an old star, strips material away from a nearby star, forming an accretion disk of material spiralling in towards the pulsar, where it releases extraordinary amounts of energy – about the total energy output of the sun in 10 years, over the period of a few short weeks. 

This is so energetic that most of the radiation is released in the highest energy portion of the electromagnetic spectrum: in X-rays. 

Some accreting neutron stars are not always active and can spend years in a quiet state, known as quiescence, where they emit barely any light at all and accrete at a very low rate. They can suddenly go into outburst and become extremely bright in X-rays for around a month. 

The pulsar in a binary system is named SAX J1808.4−3658, and spins at a staggering rate of 400 times per second. 

What the researchers saw was unexpected: it took 12 days from the first sign of increased optical activity before any high energy X-ray emission was observed. 

This is longer than anyone thought it would take, with most theories suggesting there should be only a two- to three-day delay. 

“This work enables us to shed some light on the physics of accreting neutron star systems, and to understand how these explosive outbursts are triggered in the first place, which has puzzled astronomers for a long time,” said New York University researcher, Dr David Russell, one of the study’s co-authors. 

Accretion disks are usually made of hydrogen, but this particular object has a disk that is made up of 50% helium, more helium than most disks. The scientists think that this excess helium may be slowing down the heating of the disk because helium “burns” at a higher temperature, causing the “powering up” to take 12 days.

The research, led by PhD candidate Adelle Goodwin from the Monash School of Physics and Astronomy in Melbourne, Australia, was featured at the recent 236th virtual meeting of the American Astronomical Society meeting (1-3 June 2020), before it is published in Monthly Notices of the Royal Astronomical Society. Adelle leads a team of international researchers, including her supervisor, Monash University Associate Professor Duncan Galloway, and Dr David Russell from New York University Abu Dhabi and Dr David Buckley from the South African Astronomical Observatory.  

The preprint of the submitted paper is available on the Astro-PH arXiv here: https://arxiv.org/abs/2006.02872

When it comes to galaxies, how fast is fast? The Milky Way, an average spiral galaxy, spins at a speed of 210 kilometres per second in our Sun’s neighbourhood. New research has found that the most massive spiral galaxies spin faster than expected. These “super spirals,” the largest of which weigh about 20 times more than our Milky Way, spin at a rate of up to 570 kilometres per second.

Super spirals are exceptional in almost every way. In addition to being much more massive than the Milky Way, they’re also brighter and larger in physical size. The largest span 450,000 light-years compared to the Milky Way’s 100,000-light-year diameter. Only about 100 super spirals are known to date. Super spirals were discovered as an important new class of galaxies while studying data from the Sloan Digital Sky Survey as well as the NASA/IPAC Extragalactic Database (NED).

The top row of this mosaic features Hubble images of three spiral galaxies, each of which weighs several times as much as the Milky Way. The bottom row shows three even more massive spiral galaxies that qualify as “super spirals,” which were observed by the ground-based Sloan Digital Sky Survey. Super spirals typically have 10 to 20 times the mass of the Milky Way. The galaxy at lower right, 2MFGC 08638, is the most massive super spiral known to date, with a dark matter halo weighing at least 40 trillion Suns. Astronomers have measured the rotation rates in the outer reaches of these spirals to determine how much dark matter they contain. They found that the super spirals tend to rotate much faster than expected for their stellar masses, making them outliers. Their speed may be due to the influence of a surrounding dark matter halo, the largest of which contains the mass of at least 40 trillion suns. Credits: Top row: NASA, ESA, P. Ogle and J. DePasquale (STScI). Bottom row: SDSS, P. Ogle and J. DePasquale (STScI)

“Super spirals are extreme by many measures,” says Patrick Ogle of the Space Telescope Science Institute in Baltimore, Maryland. “They break the records for rotation speeds.”
Ogle is the first author of a paper that was published October 10, 2019, in the Astrophysical Journal Letters. The paper presents new data on the rotation rates of super spirals collected with the Southern African Large Telescope (SALT), the largest single optical telescope in the southern hemisphere. Additional data were obtained using the 5-meter Hale telescope of the Palomar Observatory, operated by the California Institute of Technology. Data from NASA’s Wide-field Infrared Survey Explorer (WISE) mission was crucial for measuring the galaxy masses in stars and star formation rates.

“This work beautifully illustrates the powerful synergy between optical and infrared observations of galaxies, revealing stellar motions with SDSS and SALT spectroscopy, and other stellar properties — notably the stellar mass or ‘backbone’ of the host galaxies — through the WISE mid-infrared imaging,” says Tom Jarrett of the University of Cape Town, South Africa.

Theory suggests that super spirals spin rapidly because they are located within incredibly large clouds, or halos, of dark matter. Dark matter has been linked to galaxy rotation for decades. Astronomer Vera Rubin pioneered work on galaxy rotation rates, showing that spiral galaxies rotate faster than if their gravity were solely due to the constituent stars and gas. An additional, invisible substance known as dark matter must influence galaxy rotation. A spiral galaxy of a given mass in stars is expected to rotate at a certain speed. Ogle’s team finds that super spirals significantly exceed the expected rotation rate.

Super spirals also reside in larger than average dark matter halos. The most massive halo that Ogle measured contains enough dark matter to weigh at least 40 trillion times as much as our Sun. That amount of dark matter would normally contain a group of galaxies rather than a single galaxy.

“It appears that the spin of a galaxy is set by the mass of its dark matter halo,” Ogle explains.
The fact that super spirals break the usual relationship between galaxy mass in stars and rotation rate is a new piece of evidence against an alternative theory of gravity known as Modified Newtonian Dynamics, or MOND. MOND proposes that on the largest scales like galaxies and galaxy clusters, gravity is slightly stronger than would be predicted by Newton or Einstein. This would cause the outer regions of a spiral galaxy, for example, to spin faster than otherwise expected based on its mass in stars. MOND is designed to reproduce the standard relationship in spiral rotation rates, therefore it cannot explain outliers like super spirals. The super spiral observations suggest no non-Newtonian dynamics is required.
Despite being the most massive spiral galaxies in the universe, super spirals are actually underweight in stars compared to what would be expected for the amount of dark matter they contain. This suggests that the sheer amount of dark matter inhibits star formation. There are two possible causes: 1) Any additional gas that is pulled into the galaxy crashes together and heats up, preventing it from cooling down and forming stars, or 2) The fast spin of the galaxy makes it harder for gas clouds to collapse against the influence of centrifugal force.

“This is the first time we’ve found spiral galaxies that are as big as they can ever get,” Ogle says.
Despite these disruptive influences, super spirals are still able to form stars. Although the largest elliptical galaxies formed all or most of their stars more than 10 billion years ago, super spirals are still forming stars today. They convert about 30 times the mass of the Sun into stars every year, which is normal for a galaxy of that size. By comparison, our Milky Way forms about one solar mass of stars per year.

Ogle and his team have proposed additional observations to help answer key questions about super spirals, including observations designed to better study the motion of gas and stars within their disks. After its 2021 launch, NASA’s James Webb Space Telescope could study super spirals at greater distances and correspondingly younger ages to learn how they evolve over time. And NASA’s WFIRST mission may help locate more super spirals, which are exceedingly rare, thanks to its large field of view.

The Space Telescope Science Institute is expanding the frontiers of space astronomy by hosting the science operations centre of the Hubble Space Telescope, the science and operations centre for the James Webb Space Telescope, and the science operations centre for the future Wide Field Infrared Survey Telescope (WFIRST). STScI also houses the Mikulski Archive for Space Telescopes (MAST) which is a NASA-funded project to support and provide to the astronomical community a variety of astronomical data archives and is the data repository for the Hubble, Webb, Kepler, K2, TESS missions and more.

Zoomed in image of the supernova in Figure 2 below, 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

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, https://www.darkenergysurvey.org/) 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.

“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.

An example image depicting one of the supernovae that SALT took spectra of. Left: Image of a galaxy before the supernova, Right: Image of the same galaxy after the supernova, with the explosion clearly visible in the left of the galaxy. Image credit: SALT/SAAO

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”.

A spectrum of the Type Ia supernova “DES15S2ocv” taken by SALT on the 9th of January 2016. Over-plotted in red is a template spectrum used to classify the supernova. Image credit: SALT/SAAO

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 Arxiv.org, 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.

https://arxiv.org/search/astro-ph?query=Bassett+kasai&searchtype=author&abstracts=show&order=-announced_date_first&size=50

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.
Picture credit: NASA/CXC/M.Weiss

A new binary star system has been discovered where 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.

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 ??
http://dx.doi.org/10.1038/s41550-018-0694-z

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.

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.

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)

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.

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)

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).

The article by Shen et al. was published in the Astrophysical Journal on 20 September 2018. Additional information is available at https://www.cosmos.esa.int/web/gaia/iow_20181119.

The Hourglass Nebula as viewed by the Hubble Space Telescope in the light of ionized nitrogen (represented by red), hydrogen (green), and doubly-ionized oxygen (blue). A remarkable new SALT discovery has proven the ionizing star to be a binary system. Image credit: Raghvendra Sahai and John Trauger (JPL), the WFPC2 science team, and NASA.

SALT has discovered a binary star system in The Hourglass Nebula, one of the most famous nebulae captured by the Hubble Space Telescope.

The Hourglass Nebula consists of two hourglass-shaped lobes of gas and what appears to be an eye staring right back at us. Shells of gas form the eye surrounding the hot central star that illuminates the nebula like a neon sign. Astronomers have long suspected the peculiar nebula to be formed by two interacting stars in a binary system, but until now no one could prove it. The SALT discovery of two stars orbiting each other every 18.15 days in The Hourglass Nebula firmly settles the matter and gives new insights into how a wide variety of close binary stars and hourglass-shaped nebulae may form.

An international team of astronomers, led by SALT Astronomer Dr Brent Miszalski at the SAAO, used SALT to peer into the “sparkle” of the eye of the Hourglass Nebula – its central star.

Dr Miszalski says, “A total of 26 SALT measurements were taken that detected the small movements of the central star towards or away from us caused by the gravity of a second companion star. This Doppler or “wobble” method, that can also be used to find planets around other stars, revealed a hidden companion orbiting the central star every 18.15 days.”

Co-author of the study, Mr Rajeev Manick, formerly a Masters student at SAAO and The University of Cape Town, and now completing his PhD at The Katholieke Universiteit Leuven in Belgium, analysed the SALT measurements and found that the companion must be a small, cool star about 5 times lighter than the Sun.

Another surprise came with the binary – the relatively wide separation between the two stars is remarkable. Co-author Professor Joanna Mikołajewska of the Nicolaus Copernicus Astronomical Center in Warsaw, Poland, a major partner in SALT, Prof. Mikołajewska says, “Previous authors have suggested that a nova explosion could explain many aspects of The Hourglass Nebula, but curiously we found the stars were too far apart for this to have ever been possible.”

Instead of a nova explosion, the orbital period indicates the Hourglass Nebula formed through an interaction that many close binary stars experience – a so-called common-envelope stage. In this scenario, the cooler companion spirals into the atmosphere of its larger companion and helps ejects the shared atmosphere which we now see as the nebula. The Hourglass Nebula is one of very few such examples to show an orbital period above 10 days, making it helpful to improve our understanding of this brief phase that many types of binary stars experience during their lifetime.

The new SALT discovery may help astronomers understand how a wide variety of other hourglass-shaped or butterfly-shaped nebulae form. The picture shows several examples of such nebulae captured by the Hubble Space Telescope, with The Hourglass Nebula visible at the lower left corner. Image Credit: ESA/Hubble.

While astronomers still do not quite understand how hourglass-shaped nebulae form, the discovery of a binary in The Hourglass Nebula considerably strengthens the long suspected, but difficult to prove, connection between binary stars and hourglass-shaped nebulae. A famous example is the nebular remnant of Supernova 1987A that is often compared against The Hourglass Nebula because of its very similar shape. It is thought to have resulted from the merger of two massive stars before the supernova event. This process shares similarities with that which formed The Hourglass Nebula, hinting at some shared physics resulting in two of the most unusual nebulae in the sky.

Dr Miszalski says, “The combination of SALT’s enormous 11-metre mirror, highly sensitive instrumentation and flexible queue-scheduled operations was fundamental to making this difficult, cutting-edge discovery. We will continue searching other nebulae for new binary systems to gain more insights into their complex origins.”

The study entitled “SALT HRS discovery of the binary nucleus of the Etched Hourglass Nebula MyCn 18” was recently accepted for publication in the Publications of the Astronomical Society of Australia (PASA) journal and is available from https://arxiv.org/abs/1805.07602. The work is the result of a collaboration between astronomers at SAAO and SALT in South Africa, the Nicolaus Copernicus Astronomical Center in Poland, and The Katholieke Universiteit Leuven in Belgium.