Tag Archives: #method

Australian Invention To Make It Easier To Find ‘New Earths’ (Astronomy)

Photonics combined with AI will help decipher the ‘twinkle’ of stars

University of Sydney scientists have developed a sensor that will help decipher the ‘twinkle’ of stars and allow for ground-based exploration of exoplanets. Their invention will be deployed in one of the world’s largest telescopes at Mauna Kea, Hawaii.

Australian scientists have developed a new type of sensor to measure and correct the distortion of starlight caused by viewing through the Earth’s atmosphere, which should make it easier to study the possibility of life on distant planets.

Using artificial intelligence and machine learning, University of Sydney optical scientists have developed a sensor that can neutralise a star’s ‘twinkle’ caused by heat variations in the Earth’s atmosphere. This will make the discovery and study of planets in distant solar systems easier from optical telescopes on Earth.

“The main way we identify planets orbiting distant stars is by measuring regular dips in starlight caused by planets blocking out bits of their sun,” said lead author Dr Barnaby Norris, who holds a joint position as a Research Fellow in the University of Sydney Astrophotonic Instrumentation Laboratory and in the University of Sydney node of Australian Astronomical Optics in the School of Physics.

“This is really difficult from the ground, so we needed to develop a new way of looking up at the stars. We also wanted to find a way to directly observe these planets from Earth,” he said.

The team’s invention will now be deployed in one of the largest optical telescopes in the world, the 8.2-metre Subaru telescope in Hawaii, operated by the National Astronomical Observatory of Japan.

“It is really hard to separate a star’s ‘twinkle’ from the light dips caused by planets when observing from Earth,” Dr Norris said. “Most observations of exoplanets have come from orbiting telescopes, such as NASA’s Kepler. With our invention, we hope to launch a renaissance in exoplanet observation from the ground.”

The research is published today in Nature Communications.

Novel Methods

Using the new ‘photonic wavefront sensor’ will help astronomers directly image exoplanets around distant stars from Earth.

Over the past two decades, thousands of planets beyond our solar system have been detected, but only a small handful have been directly imaged from Earth. This severely limits scientific exploration of these exoplanets.

Making an image of the planet gives far more information than indirect detection methods, like measuring starlight dips. Earth-like planets might appear a billion times fainter than their host star. And observing the planet separate from its star is like looking at a 10-cent coin held in Sydney, as viewed from Melbourne.

Research team: (left to right) Alison Wong, Christopher Betters, Barnaby Norris, Sergio Leon-Saval from the School of Physics. © University of Sydney.

To solve this problem, the scientific team in the School of Physics developed a ‘photonic wavefront sensor’, a new way to allow the exact distortion caused by the atmosphere to be measured, so it can then be corrected by the telescope’s adaptive optics systems thousands of times a second.

“This new sensor merges advanced photonic devices with deep learning and neural networks techniques to achieve an unprecedented type of wavefront sensor for large telescopes,’ Dr Norris said.

“Unlike conventional wavefront sensors, it can be placed at the same location in the optical instrument where the image is formed. This means it is sensitive to types of distortions invisible to other wavefront sensors currently used today in large observatories,” he said.

Professor Olivier Guyon from the Subaru Telescope and the University of Arizona is one of the world’s leading experts in adaptive optics. He said: “This is no doubt a very innovative approach and very different to all existing methods. It could potentially resolve several major limitations of the current technology.

“We are currently working in collaboration with the University of Sydney team towards testing this concept at Subaru in conjunction with SCExAO, which is one of the most advanced adaptive optics systems in the world.”

Wider applications

The scientists have achieved this remarkable result by building on a novel method to measure (and correct) the wavefront of light that passes through atmospheric turbulence directly at the focal plane of an imaging instrument. This is done using an advanced light converter, known as a photonic lantern, linked to a neural network inference process.

“This is a radically different approach to existing methods and resolves several major limitations of current approaches,” said co-author Jin (Fiona) Wei, a postgraduate student at the Sydney Astrophotonic Instrumentation Laboratory.

The Director of the Sydney Astrophotonic Instrumentation Laboratory in the School of Physics and member of Sydney Nano at the University of Sydney, Associate Professor Sergio Leon-Saval, said: “While we have come to this problem to solve a problem in astronomy, the proposed technique is extremely relevant to a wide range of fields.

“It could be applied in optical communications, remote sensing, in-vivo imaging and any other field that involves the reception or transmission of accurate wavefronts through a turbulent or turbid medium, such as water, blood or air.”

Provided by University Of Sydney

Cooling Tail Method, Is The New Method For Accurate Determination Of The Radii Of Neutrons Stars (Astronomy)

Guys, we all know neutron stars are the smallest and densest astrophysical objects with visible surfaces in the Universe. They form after gravitational collapses of the iron nuclei of massive (with masses about ten solar masses) stars at the end of their nuclear evolution. We can observe these collapses as supernovae explosions.

The masses of neutron stars are typical for normal stars, about one and half solar masses, but their radii are extremely small in comparison with normal stars—they are between ten and fifteen kilometers. For comparison, the radius of the Sun is about 700,000 km. It means that the average matter density of neutron stars is a few times larger than the density of atomic nuclei, namely about 1 billion tons per cubic centimeter.

The neutron star matter consists mainly of close up neutrons, and the repulsive forces between neutrons prevent neutron stars from collapsing into a black hole. Theoretical quantitative description of these repulsive forces is not possible at the moment, and it is a fundamental problem of the nuclear physics and astrophysics. This problem is also known as the equation of state of the superdense cold matter problem. Astrophysical observations of neutron stars can limit the existing different theoretical models of the equation of state, because the neutron star radii depend on the repulsive forces.

One of the most suitable astrophysical objects for neutron star radii measurements are X-ray bursting neutron stars. They are components of close binary systems, so called low-mass X-ray binaries. In such systems, the secondary component, which is a normal solar-like star, losses its matter, and the neutron star accretes the matter. The matter flows from the normal star onto the surface of the neutron star. The surface gravity on a neutron star is very high, hundred billion times higher than on the Earth’s surface. As a result, the conditions for exploding thermonuclear burning arise on the bottom of the fresh accreted matter. It’s these explosions that we observe as X-ray flashes in low-mass X-ray binaries.

Durations of the most X-ray flashes are about 10 to 100 seconds. After the maximum, the X-ray brightness decays almost exponentially. An X-ray bursting neutron star emits as a black body with some temperature (about ten million degrees), and this temperature decreases together with the brightness decreasing. But the connection between the brightness and the temperature is not fixed. It depends on the physical structure of the upper layers of the emitting neutron star envelope (the atmosphere). The model atmospheres of X-ray bursting neutron stars can be computed for various masses and radii of, as well as for a given X-ray flash brightness, and some time ago the co-authors computed the extended grid of such model atmospheres.

The comparison of joint observational decreasing of the temperature and the X-ray brightness in some X-ray flashes with the model predictions allows to find the mass and radius of a neutron star. This method, which was named the cooling tail method, was suggested more than ten years ago. The authors of this method are Valery Suleimanov, Juri Poutanen, Mike Revnivtsev, and Klaus Werner, three of whom are the co-authors of this current publication. Further development of this approach and its application to the many X-ray flashes allowed them to limit the neutron star radii in the range from 11 to 13 km. All the following determinations, including an observation of the merging of two neutron stars by gravitational wave detectors, gave values inside of this range.

In the method, the researchers assumed that the neutron star is not rotating and has a spherical shape with a uniform temperature distribution over the surface. But the neutron stars in the considered binary systems can rotate rapidly with the typical period a few milliseconds.

In particular, the fastest rotating neutron star in the system 4U 1608-52 has a spin period of 0.0016 seconds. Shapes of such rapidly rotating neutron stars are far from spherical. They have larger radii at the equators than at the poles, and the surface gravity and the surface temperature are larger at the poles than at the equators. Therefore, there are systematic uncertainties in the method of the neutron star masses and radii determination. The obtained neutron star radii can be systematically overestimated due to their rapid rotation.

Recently Valery Suleimanov, Juri Poutanen, and Klaus Werner developed a fast approximate approach for computing the emergent radiations of rapidly rotating neutron stars. They extended the cooling tail method for thermonuclear flashes on the rapidly rotating neutron star surfaces. This extended method was applied to the X-ray burst on the surface of the neutron star in the system SAX 1810.8-2609, which is rotating with the period of about 2 milliseconds.

The study showed that the radius of this neutron star can be overestimated on the value in the range from one to a half kilometer depending on the inclination angle of the rotation axis to the line of sight. It means that the systematic corrections are not crucial and can be ignored in the first approximation. The plan is to apply this method to the fastest rotating neutron star in the system 4U 1608-52.


References: Valery F. Suleimanov, Juri Poutanen and Klaus Werner, ‘Observational appearance of rapidly rotating neutron stars’, A&A 639, A33 (2020) link: https://www.aanda.org/articles/aa/abs/2020/07/aa37502-20/aa37502-20.html