Highlights

The 2024 Bruno Rossi Prize (10.1.2024)

The 2024 Bruno Rossi Prize has been awarded to Dr. Martin Weisskopf, Dr. Paolo Soffitta, and the IXPE team for their development of the Imaging X-ray Polarimetry Explorer whose novel measurements advance our understanding of particle acceleration and emission from astrophysical shocks, black holes and neutron stars. 

This is the highest honor specifically in high-energy astrophysics. Members of our group are active members of the IXPE collaboration.

Bessel Research Award (29.11.2023)

Sergey Tsygankov has received the prestigious Friedrich Wilhelm Bessel Research Award. 

Sergey Tsygankov has made fundamental contributions to the understanding of accretion phenomena on neutron stars characterised by the strongest magnetic fields in Nature, so called X-ray pulsars. More recently, he has been one of the pioneers in X-ray polarimetry studies of compact objects. Dr. Tsygankov currently continues his active involvement in the analysis of the data coming from the recently launched NASA’s Imaging X-ray Polarimetry Explorer (IXPE). He contributes also to the development of new avenues in studies of neutron stars, which will advance our understanding of the physics and astrophysics of these fascinating celestial objects. Much of the research by Dr. Tsygankov that led to the prize was done in collaboration with researchers from Universities of Tübingen in Germany. There is a hope that this prize will contribute to strengthen the collaboration between the Universities of Turku and Tübingen and will foster the relation between Finland and Germany in the field of high-energy astrophysics.

The award is named after German astronomer and mathematician Friedrich Wilhelm Bessel (1784–1846) and funded by the German Federal Ministry of Education and Research. The Alexander von Humboldt Foundation presents approximately 20 Friedrich Wilhelm Bessel Research Awards annually to internationally renowned academics from abroad in recognition of their outstanding accomplishments in research.

ERC Starting Grant Awarded to Yannis Liodakis (27.9.2023)  

Yannis Liodakis, our group member and currently a NASA research fellow at the Marshall Space Flight Center, has been awarded the prestigious European Research Council (ERC) Starting Grant. The Grant of 1.5M euro will be used to build a group at the Institute of Astrophysics in Creete.

Yannis Liodakis aims to use Skinakas Observatory to uncover the origin of violent and explosive phenomena around supermassive black holes. He will use optical and X-ray polarimetry to study the magnetic fields in the plasma surrounding supermassive black holes. He aims aims to uncover the processes related to the formation of accretion disks around supermassive black holes and how particles are energized in the relativistic jet. His group will rely on the optical polarimeter Robopol at the Skinakas Observatory, as well as, an assemble of different telescopes in space and on the ground to collect a unique set of data and produce the first comprehensive picture of these explosive phenomena.

Discovery of X-ray polarization in the Black Hole Transient Swift J1727.8−1613 (27.9.2023)

We report the first detection of the X-ray polarization of the bright transient Swift J1727.8-1613 with the Imaging X-ray Polarimetry Explorer. The observation was performed at the beginning of the 2023 discovery outburst, when the source resided in the bright hard state. We find a time- and energy-averaged polarization degree of 4.1%+/-0.2% and a polarization angle of 2.2+/-1.3 deg (errors at 68% confidence level; this translates to about 20-sigma significance of the polarization detection). This finding suggests that the hot corona emitting the bulk of the detected X-rays is elongated, rather than spherical. The X-ray polarization angle is consistent with that found in sub-mm wavelengths. Since the sub-mm polarization was found to be aligned with the jet direction in other X-ray binaries, this indicates that the corona is elongated orthogonal to the jet.

The research paper was published in Veledina A., Muleri F., Dovciak M., Poutanen J., et al. (incl. Kravtsov V., Nitindala A.P., Tsygankov S.S.), Discovery of X-ray Polarization from the Black Hole Transient Swift J1727.8−1613, 2023, ApJ Letters, 958, L16 (authors from UTU are in boldface)

ERC Starting Grant Awarded to Joonas Nättilä (5.9.2023)  

The European Research Council (ERC) Starting Grant (2.2M euro) has been awarded to our previous PhD student Joonas Nättilä.

The project “Illuminating neutron stars with radiative plasma physics (ILLUMINATOR)” aims to study the generation mechanism of eruptions, explosions and flare from the magnetized neutron stars using state-of-the-art particle-in-cell simulations. Neutron stars are extremely dense remnants of collapsed stars whose magnetic fields are repeatedly observed to emit very bright flashes of light at radio and X-ray frequencies. The origin of the flashes remains a mystery.

Discovery of complex variations of X-ray polarization in the X-ray pulsar LS V +44 17 / RX J0440.9+4431 (3.6.2023)  

We report on Imaging X-ray polarimetry explorer (IXPE) observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 made at two luminosity levels during the giant outburst in January-February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in supercritical and subcritical states with significantly different emission-region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase-resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on the pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations in the spin axis inclination, the position angle, and the magnetic colatitude by tens of degrees within the space of just a few days. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of an unpulsed polarized component in addition to the polarized radiation associated with the pulsar itself. We then show that the observed PA phase dependence in both observations can be explained with a single set of RVM parameters defining the pulsar’s geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation in the equatorial disk wind.

The research paper was published in Doroshenko V., Poutanen J., Heyl J., Tsygankov S.S. et al. (IXPE collaboration, incl. Veledina A., Forsblom S.V., Loktev V., Suleimanov V.F., Salganik A., Berdyugin A.V., Kravtsov V., Nitindala A.P.),
Complex variations of X-ray polarization in the X-ray pulsar LS V +44 17 / RX J0440.9+4431,
2023, A&A, 677, A57 (group members are in boldface)

NASA’s IXPE Helps Solve Black Hole Jet Mystery

Some of the brightest objects in the sky are called blazars. They consist of a supermassive black hole feeding off material swirling around it in a disk, which can create two powerful jets perpendicular to the disk on each side. A blazar is especially bright because one of its jets of high-speed particles points straight at Earth. For decades, scientists have wondered: How do particles in these jets get accelerated to such high energies? NASA’s Imaging X-Ray Polarimetry Explorer, or IXPE, has helped astronomers get closer to an answer. In a new study in the journal Nature, authored by a large international collaboration, including a number of researchers from the University of Turku, astronomers find that the best explanation for the particle acceleration is a shock wave within the jet.

“This is a 40-year-old mystery that we’ve solved,” said Yannis Liodakis, lead author of the study. “We finally had all of the pieces of the puzzle, and the picture they made was clear.” Launched Dec. 9, 2021, the Earth-orbiting IXPE satellite, a collaboration between NASA and the Italian Space Agency, provides a special kind of data that has never been accessible from space before. This new data includes the measurement of X-ray light’s polarization, meaning IXPE detects the average direction and intensity of the electric field of light waves that make up X-rays. Information about the electric field orientation in X-ray light, and the extent of polarization, is not accessible to telescopes on Earth because the atmosphere absorbs X-rays from space.

The new study used IXPE to point at Markarian 501, a blazar in the constellation Hercules. This active black hole system sits at the center of a large elliptical galaxy. IXPE watched Markarian 501 for three days in early March of 2022, and then again two weeks later. During these observations, astronomers used other telescopes in space and on the ground to gather information about the blazar in a wide range of wavelengths of light including radio, optical, and X-ray. Particularly important were the observations in optical polarization conducted at the Nordic Optical Telescope – NOT, owned by the University of Turku, that allowed the X-ray observations to be put into perspective. “NOT has always been a corner stone in polarization studies of blazars”, said Jenni Jormanainen, PhD student at the University of Turku, who led the analysis of the NOT observations. Further information was obtained with the in-house-built optical polarimeter DIPol-2 installed on the 60cm Tohoku telescope at Haleakala observatory in Hawaii that is operated remotely from Turku. “This instrument has been a workhorse in our program of high-precision polarimetric measurements of various astronomical sources“, pointed out Vadim Kravtsov, PhD student at the University of Turku, in charge of the DIPoL-2 observations.

Figure: This illustration shows the IXPE spacecraft, at right, observing blazar Markarian 501, at left. A blazar is a black hole surrounded by a disk of gas and dust with a bright jet of high-energy particles pointed toward Earth. The inset illustration shows high-energy particles in the jet (blue). When the particles hit the shock wave, depicted as a white bar, the particles become energized and emit X-rays as they accelerate. Moving away from the shock, they emit lower-energy light: first visible, then infrared, and radio waves. Farther from the shock, the magnetic field lines are more chaotic, causing more turbulence in the particle stream. Image: Pablo Garcia (NASA/MSFC)

Scientists found that X-ray light is more polarized than optical, which is more polarized than radio. But the direction of the polarized light was the same for all the wavelengths of light observed and was also aligned with the jet’s direction. After comparing their information with theoretical models, the team of astronomers realized that the data most closely matched a scenario in which a shock wave accelerates the jet particles. A shock wave is generated when something moves faster than the speed of sound of the surrounding material, such as when a supersonic jet flies by in our Earth’s atmosphere. The study was not designed to investigate the origins of shock waves, which are still mysterious. But scientists hypothesize that a disturbance in the flow of the jet causes a section of it to become supersonic. This could result from high-energy particle collisions within the jet, or from abrupt pressure changes at the jet boundary.  “As the shock wave crosses the region, the magnetic field gets stronger, and energy of particles gets higher,” said Alan Marscher, an astronomer at Boston University who leads the group studying giant black holes with IXPE. “The energy comes from the motion energy of the material making the shock wave.” As particles travel outward, they emit X-rays first because they are extremely energetic. Moving farther outward, through the turbulent region farther from the location of the shock, they start to lose energy, which causes them to emit less-energetic light like optical and then radio waves. This is analogous to how the flow of water becomes more turbulent after it encounters a waterfall – but here, magnetic fields create this turbulence.

Scientists will continue observing the Markarian 501 blazar to see if the polarization changes over time. IXPE will also investigate a broader collection of blazars during its two-year prime mission, exploring more longstanding mysteries about the universe. “It is part of humanity’s progress toward understanding nature and all of its exoticness,” Marscher said.

The research paper was published in Liodakis I.,  et al. (IXPE collaboration, incl. Berdyugin A.V., Kravtsov V., Poutanen J., Tsygankov S.S.),
Polarized Blazar X-rays imply particle acceleration in shocks, 2022, Nature, 611, 677-681 (group members are in boldface)

Polarized X-rays reveal shape and orientation of extremely hot matter around black hole

The researchers observed the X-ray radiation from the matter around a black hole. According to the researchers the shape and orientation of the X-ray glow support the theory, that the X-rays come from the disc-shaped material flowing into the black hole which is perpendicular to previously imaged relativistic outflows of matter called jets. These findings give a better understanding about the inner workings of black holes and how they consume mass.

It is well known that massive stars, which weigh over 25 Suns, end their lives in a highly compact remnant, the black hole.

If such a star has a nearby companion, at some stage of the binary evolution the black hole will consume matter from that. As the matter is pulled by the strong gravity of the black hole, it is heated to millions of degrees and we are able to observe the bright X-ray glow from these systems, which are called X-ray binaries.

The configuration of this matter, however, has been a matter of debate, and these sources are too distant: distinguishing them in the sky is like trying to discern a hair on the surface of the Moon. No current technologies are capable of doing an image of such a system.

“It is not even clear whether we see the last sights of matter before it goes beyond the event horizon – the imaginary surface, beyond which no information can reach the distant observer – or, instead, if it is a cry of joy of a small fraction of material which escapes the system, ” says Juri Poutanen, Professor of Astronomy at the University of Turku. “There have been suggestions that the X-ray glow of matter we see forms a highly compact sphere. Other alternatives included an elongate structure, either as a slab entering the black hole or in a shape of the cone, pointing away from the black hole.”

Recently, astrophysicists found a way to get insights into the configuration of the X-ray emitting gas. They used the special property of light called polarization. The light can be thought of as a set of coming waves. But unlike the waves in the ocean, which can oscillate only in the vertical direction, up and down, the light waves may have these oscillations in any direction. Polarization is the appearance of one preferential direction of these oscillations.

X-ray polarization can be produced in scattering, when the photons bounce from one particle to another. In this case, its orientation is related to the symmetry axis of the system. Detecting this orientation from the ultrashort X-ray wavelengths is a highly challenging task; this became feasible with the launch of the Imaging X-Ray Polarimetry Explorer (IXPE) space mission, an international collaboration between NASA and the Italian Space Agency (ASI).

The first massive stellar remnant studied with this mission was the prototypical black hole X-ray binary Cygnus X-1, which is also the first X-ray source discovered in Cygnus constellation during a rocket flight in 1964, and the first source that was widely accepted to be a black hole.

Polarimetric measurements from Cyg X-1, reported on November 3 in the journal Science, reveal that the X-ray emitting gas extends perpendicular to a two-sided, pencil-shaped plasma outflow, or jet, imaged in earlier radio observations.

“The new data rule out models in which the X-ray emitting gas forms a compact substance or a cone along the jet axis, instead lending strong support to the hypothesis that the X-ray glow comes from the inflow of matter,” explains Alexandra Veledina, one of the leading authors of the publication. “A better understanding of the plasma geometry can reveal much about the inner workings of black holes and how they consume mass.”

Figure: Artist’s impression of the Cygnus X-1 system, with the black hole appearing in the center and its companion star on the left. Image credit: John Paice.

Furthermore, IXPE observations reveal that the inflow is seen more edge-on than previously thought. This may indicate the misalignment between the black hole spinning axis and the axis of the orbit in which the black hole is rotating around the massive companion.

“In order to verify this assumption, we performed an observational campaign with the high-precision optical polarimeter DIPol-2, which can say about the direction of the orbital axis on the plane of the sky,” says Andrei Berdyugin, senior researcher at the University of Turku and a co-developer of high-precision optical polarimetric instruments.

“However, unlike in another black hole binary system, where we have been able to identify a large misalignment seen in the plane of the sky, the binary orbit in Cyg X-1 is very well aligned with the black hole spin axis,” he adds.

“Exciting finding opens new prospects to study the curved spacetime and laws of gravity in unprecedented detail. We have already found a lot of surprises in a wide range of sources, from the ultra-dense stellar remnants called neutron stars to the supermassive black holes in the centers of galaxies,” concludes Poutanen. “We are thrilled to be part of this wave of new scientific discoveries.”

The paper Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1 is published in Krawczynski H. et al. 2022, Science, 378, 650–654. The authors from the University of Turku are Alexandra Veledina (a corresponding author), Vladislav Loktev, Juri Poutanen, Andrei Berdyugin, Vadim Kravtsov and Sergey Tsygankov from Tuorla observatory.

Getting grips on a strongly magnetized neutron star geometry

Researchers from the University of Turku determined geometrical parameters of a neutron star floating in the Galaxy 21,000 light years away. The finding confirms old ideas that this star precesses like a whirligig.

X-ray pulsars are strongly magnetized neutron stars powered by accretion of gas from a nearby companion star and are among most prominent sources in the X-ray sky. A new perspective on these objects is now provided by the recently launched International X-ray Polarimeter Explorer (IXPE) space observatory which started operations in the end of 2021. IXPE measures polarization of X-rays and was used to measure polarization from an X-ray pulsar for the first time, which allowed to constrain its basic geometry.

“Hercules X-1 was the first X-ray pulsar observed by IXPE, and a very low polarization we observed came as a big surprise and that is something we still do not fully understand”, says Victor Doroshenko from the University of Tuebingen in Germany, the lead author of the Nature Astronomy paper.

The average degree of the polarization of about 9% measured by IXPE with very high accuracy turned out to be much lower than optimistically expected 80% by some theoreticians.

“Such a large discrepancy implies that existing models of radiative transport in strongly magnetized plasma confined at the poles of a neutron star and our ideas regarding geometry and structure of the emission region in Hercules X-1 and possibly other pulsars will need to be substantially revised in light of IXPE results”, adds Juri Poutanen from the University of Turku in Finland, leader of the IXPE’s working group studying accreting neutron stars.

Looking at variations of the polarization angle over the spin phase, it was possible to measure the angle between the spin and magnetic dipole axes – a piece of information elemental to any modeling of emission from such objects. Joint modeling of new X-ray polarimetric observations and older optical polarimetric measurements obtained at the Nordic Optical Telescope allowed also to unambiguously show that the spin axis of the pulsar is misaligned with the orbital angular momentum, which is a strong indication that the neutron star precesses like a whirligig.

Free precession of the neutron star was previously invoked to explain observed semi-regular variations of pulsars flux and pulse profile shape with period of ~35 days, and has some important consequences for our understanding of internal structure of neutron stars, but up to now only indirect evidence for this hypothesis is available. The ultimate proof is expected to arrive later when IXPE observes Hercules X-1 at another phase of the precession cycle.

“IXPE is just starting to explore the new observational window, X-ray polarimetry, and we are continuing observations of objects of all kinds, so stay tuned for more surprising discoveries!”, summarizes Sergey Tsygankov from the University of Turku, one of the leading authors of the publication.

IXPE was launched on a Falcon 9 rocket from the Cape Canaveral in December 2021, and now orbits 600 kilometers above Earth’s surface. The mission is a collaboration between NASA and the Italian Space Agency with partners and science collaborators in 13 countries including Finland.

Figure: Artist impression of a precessing X-ray pulsar close to an ordinary star. @Alexander Mushtukov.

The research paper was published in Doroshenko V., Poutanen J., Tsygankov S.S., et al. 2022,
Determination of X-ray pulsar geometry with IXPE polarimetry, Nature Astronomy, 6, 1433–1443 (authors from UTU are in boldface)

Death spiral: a black hole spins on its side

Our group found that the axis of rotation of a black hole in a binary system MAXI J1820+070 is tilted more than 40 degrees relative to the axis of stellar orbit. The finding challenges current theoretical models of black hole formation.

Often for the space systems with smaller objects orbiting around the central massive body, the own rotation axis of this body is to a high degree aligned with the orbital axis of its satellites. This is true also for our solar system: the planets orbit around the Sun in a plane, which roughly coincides with the equatorial plane of the Sun. The inclination of the Sun rotation axis with respect to orbital axis of the Earth is only seven degrees.

“The expectation of alignment, to a large degree, does not hold for the bizarre objects such as black hole X-ray binaries. The black holes in these systems were formed as a result of a cosmic cataclysm − collapse of a massive star. Now we see the black hole dragging matter from the nearby, lighter companion star orbiting around it. We see bright optical and X-ray radiation as the last sigh of the infalling material, and also radio emission from the relativistic jets expelled from the system.” – says Juri Poutanen, Professor of Astronomy at the University of Turku and the lead author of the publication.

By following these jets, the researchers were able to determine the direction of the axis of rotation of the black hole very accurately. As the amount of gas falling from the companion star to the black hole later began to decrease, the system dimmed, and much of the light in the system came from the companion star. In this way, the researchers were able to measure the orbit inclination using spectroscopic techniques, and it happened to nearly coincide with the inclination of the ejections.

“To determine the 3D orientation of the orbit, one additionally needs to know the position angle of the system on the sky (i.e. how the system is turned with respect to the direction to the North on the sky). This was measured using polarimetric techniques.” – says Juri Poutanen.

The results published in Science magazine open interesting prospects towards studies of black hole formation and evolution of such systems, as such extreme misalignment is hard to get in many black hole formation and binary evolution scenarios.

“The difference of more than 40 degrees between the orbital axis and the black hole spin was completely unexpected. Scientists have often assumed this difference to be very small when they have modeled the behavior of matter in a curved time space around a black hole. The current models are already really complex, and now the new findings force us to add a new dimension to them”, Poutanen states.

The key finding was made using the polarimetric instrument DIPol-UF, built by the Tuorla Observatory in collaboration with the Leibniz Institute for Solar Physics (Germany) and deployed at the Nordic Optical Telescope, which is owned by the University of Turku jointly with the Aarhus University in Denmark.

Figure: Artist impression of the X-ray binary system MAXI J1820+070 containing a black hole (small black dot at the center of the gaseous disk) and a companion star (red). A narrow jet is directed along the black hole spin axis, which is strongly misaligned from the axis of the orbit.  Video produced with Binsim (credit: R. Hynes).

The research paper was published in Poutanen J., Veledina A., Berdyugin A.V., Berdyugina S.V., Jermak H., Jonker P.G., Kajava J.J.E., Kosenkov I.A., Kravtsov V., Piirola V., Shrestha M., Torres M.A.P., Tsygankov S.S., 2022,
Black hole spin-orbit misalignment in X-ray binary MAXI J1820+070, Science, 375, 874-876 (authors from UTU are in boldface)

Astro-polarimetry is an important method in astronomical observation, aimed at detecting and measuring polarisation of light emitted, reflected, or scattered by astronomical objects such as asteroids, planets, nebulae, stars and exoplanets. The astro-polamiter built with the lead of a research group from the University of Turku made its first observations in July 2019 at the Nordic Optical Telescope on the Canary Islands. In October 2019, the University of Turku becomes the joint owner of the Telescope.

In space, the degree and direction of polarisation depends on spatial distribution and density of matter that reflects or scatters light, the properties of light-scattering particles, and physical conditions in stellar and planetary atmospheres. An astro-polarimeter is an instrument that allows astronomers to obtain this information by studying the polarisation of various celestial objects.

– By studying the polarisation of light, we can obtain information on the structure and composition of stellar and planetary atmospheres as well as on dust and gas disks surrounding young stars, out of which new planets will form. When observing high-density objects, such as white dwarfs, neutron stars and black holes, we can study how matter behaves in extremely strong gravitational and magnetic fields, says Senior Researcher, Docent Andrei Berdyugin from the University of Turku.

Tuorla Observatory at the Department of Physics and Astronomy of the University of Turku is known in the astronomical community for building high-quality, high-precision instruments for measuring polarisation in the optical wavelengths. The new Dipol-UF astro-polarimeter was designed, built and commissioned with the lead of the research group at the University of Turku and in co-operation with the Leibniz Institute for Solar Physics in Germany. In addition to Berdyugin, the research group at the University of Turku includes Docent Emeritus Vilppu Piirola and Doctoral Candidate Ilia Kosenkov.

– The first observations with the instrument were made in July 2019 at the Nordic Optical Telescope at La Palma. During two nights, Dipol-UF measured polarisation from a number of stars with an accuracy better than one per hundred thousand in the blue, green and red parts of spectrum. High accuracy is needed so we can differentiate the weaker polarisation signal of an exoplanet from that of the star it’s orbiting, for instance. Because of the instrument’s high sensitivity, we can also study the interstellar magnetic field in proximity of the Sun, says Berdyugin.

The instrument is equipped with three high-speed ANDOR EM CCD cameras which are capable of recording images of the sky with the impressive rate of 56 frames per second. The instrument employs three industrial grade mini-PCs and special optical layout to record images in polarised light simultaneously in three colours.

– The process of observations with Dipol-UF is highly automated and all observations can in principle also be done remotely from a few thousand kilometres away, describes Berdyugin.

Figure: Left: Dipol-UF mounted at the bottom of the 2.56 m Nordic Optical Telescope. Middle: Normal image of the area of stellar sky. Right: The same area of sky as seen by the polarimeter. A special optical element, a plane-parallel calcite plate, inserted into optical path splits each stellar image in two orthogonally polarized ones. By measuring relative intensities of these images, one can measure polarization of the stellar light.