More science results can be found at the high-energy astrophysics group web page.
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, Vladislav Loktev, Juri Poutanen, Andrei Berdyugin, Vadim Kravtsov ja 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
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., et al. 2022,
Black hole spin-orbit misalignment in X-ray binary MAXI J1820+070, Science, 375, 874-876
Neutron Star Equation of State
Neutron stars (NSs) contain the most extreme forms of matter available in the Universe. They also serve as astrophysical laboratories to study physics under extreme conditions of strong gravity, ultrahigh densities, super-strong magnetic fields and high radiation densities. The densities in NS interiors can exceed the density of saturated nuclear matter (about 3 x 1014 g/cm3) by a large factor. One of the goals of modern physics is to understand the nature of the fundamental interactions. NSs serve as excellent natural laboratories to study strong interactions and to determine the most important properties of matter in atomic nuclei and NSs. The theory of quantum chromodynamics, which can, in principle, treat strong interactions, is computationally intractable for multi-nucleon systems at NS densities. Instead, physicists have developed empirical models of nucleonic interactions, which make conflicting predictions. Laboratory experiments and NS observations are vital to test these theories and drive the progress. NSs observations can be used to place constraints on strong interactions because the forces between the nuclear particles set the stiffness of NS matter. This is encoded in the equation of state (EoS), the relation between pressure and density. For a given NS mass M, the EoS sets its radius R via the stellar structure equations. By measuring the M-R relation, we can recover the EoS at supranuclear densities, which is of major importance to both fundamental physics and astrophysics. It is central to understanding NSs, supernovae, and compact object mergers including at least one NS. To distinguish among the models of strong interactions one needs to measure NS M and R to a precision of a few per cent for several NSs. Our group has developed state-of-the-art neutron star atmosphere models and methods to measure NS parameters from the evolution of X-ray spectra observed during thermonuclear explosions at the NS surface known as X-ray bursts (Fig. 1), from pulse profiles of rotation-powered and accreting millisecond pulsars (AMPs), from X-ray polarization to be observed from AMPs by the Imaging X-ray Polarimeter Explorer (IXPE) to be launched in the end of 2021.
Figure 1: Top: Images of a neutron star rotating at 700 Hz. Upper panels: blackbody case of local isotropic intensity. Lower panels: electron-scattering dominated atmosphere. Panels from the left to the right correspond to the inclinations i= 0o, 45o, and 90o. The lines of constant latitudes (every 10o) and longitudes (every 15o) are shown in black. From Suleimanov V., Poutanen J., Werner K., 2020, Observational appearance of rapidly rotating neutron stars. X-ray bursts, cooling tail method and radius determination, A&A, 629, A33. Bottom: Constraints on the mass-radius of the neutron star in low-mass X-ray binary 4U 1702-429. From Nättilä J., Miller M.C., Steiner A.W., Kajava J.J.E., Suleimanov V.F., Poutanen J., 2017, Atmosphere model fits of thermonuclear X-ray burst cooling tail spectra: new neutron star mass and radius constraints using Bayesian hierarchical modeling, A&A, 608, A31.
X-ray Pulsars at Low Luminosities
X-ray pulsars is a subclass of accreting NS, possessing an extremely strong magnetic field (1012-1015G). Studies of these objects allow us to probe physical processes happening under conditions of the magnetic fields exceeding the values achieved by mankind by 10 orders of magnitude. Historically, due to the limited sensitivity of existing instrumentation, broadband X-ray spectra of accreting pulsars have been studied at comparatively high luminosities and accretion rates. Under these conditions, most X-ray pulsars appear to have quite similar spectra with a characteristic cutoff power-law continuum presumably associated with the comptonization of seed thermal emission within the optically thick emission region close to the neutron star magnetic poles. In a few exceptional cases that were known until recently, however, the origin of the observed spectra was not clear. The situation changed in 2019, when we were able to detect a dramatic spectral transition in transient X-ray pulsars GX 304-1 and A 0535+262. The typical cutoff power-law continuum observed at high fluxes transformed to a two-component spectrum peaking in the soft (20 keV) bands at low luminosity (see Fig. 2). We proposed the model where these spectral characteristics can be explained by the emission of cyclotron photons in the atmosphere of the neutron star caused by collisional excitation of electrons to upper Landau levels and further comptonization of the photons by electron gas. The latter is expected to be overheated in a thin top layer of the atmosphere.
Figure 2: Spectra of low-luminosity persistent X-ray pulsar X Per (green, based on the INTEGRAL data), and low-state NuSTAR spectra of 1A 0535+262 (blue/red) and GX 304-1 (gray) modeled with the same two-component comptonization model. From Tsygankov et al., 2019, Cyclotron emission, absorption, and the two faces of X-ray pulsar A 0535+262, MNRAS Letters, 487, L30.
Ultra-luminous X-ray sources
Ultraluminous X-ray sources (ULX) are the bright sources observed in the nearby galaxies and not associated with their nuclei. For long time it was believed that these are either intermediate mass black holes or stellar-mass black holes accreting at highly super-Eddington rates. Recently, using Nustar observatory coherent pulsations were discovered in a ULX X-2 in the galaxy M82. By 2018, pulsations were discovered in at least four more ULX. Also similarly bright outbursts were seen from X-ray pulsars in the Small Magellanic Cloud. Thus it became clear that a large fraction of ULX are in fact neutron stars with strong magnetic field. The emission from these neutron stars can exceed their Eddington limit (when radiation pressure balances gravitational attraction) by orders of magnitude. The main questions in this field are: how super-Eddington luminosities are produced, what is the geometry of the source and whether there is any beaming, how strong is the magnetic field? We have discovered strong evidence for magnetar-like magnetic fields in the first ULX-pulsar, source X-2 in M82. According to the Chandra data, the source shows strong variability in the X-ray by switching from the high to the low state different by a factor of 40 in luminosity and amazingly it does not show any intermediate state. We interpreted these data as evidence for the propeller effect, when the strong magnetic field of the neutron star starts to eject accreting gas when the accretion rate drops below some limit when the magnetospheric radius exceeds the corotational radius (where the disc rotates with exactly the same angular velocity as a neutron star). The observed relation between this limiting luminosity, the pulsar period and the magnetic field allowed us to measure neutron star magnetic field, which turned out about 3×1013-1014 G, depending on the model used to relate the magnetospheric radius to the accretion rate. Understanding the nature of ULX-pulsars and the physics of accretion onto strongly magnetized neutron stars is our ultimate goal.
Figure 3:
Upper figure: Chandra images of the galaxy M82. The source X-2 shows two states: the dim (on the left) and the bright (on the right). Lower figure: The light curve of M82 X-2 as seen by Chandra. The bimodality of the luminosities is clearly seen in the histogram shown in the middle panel with peaks at about 1040 and 3×1038 erg/s. The dependence of the X-ray luminosity on the accretion rate with the discontinuity associated with the propeller effect. From Tsygankov et al., 2016, Propeller effect in action in the ultraluminous accreting magnetar M82 X-2, MNRAS, 457, 1101.
Transitional Millisecond Pulsars
Transitional millisecond pulsars constitute a subclass of neutron stars that swing between the radio-pulsar state (when there is no active flow of matter from the companion star) and accretion state (when the matter dragged from the companion comes close to the neutron star). They provide strong evidence for the recycling scenario, where occasional accretion episodes can spin-up the neutron star to millisecond periods. Transitional millisecond pulsars also provide a unique set of observational data for understanding accretion at low rates onto magnetized neutron stars. For one of these sources, PSR J1023+0038, the pulsations at millisecond timescales have been found both in the X-rays and at optical wavelengths. This discovery challenged all previous low-rate accretion models, as the optical emission has to be coming from a very compact region. We proposed that the multiwavelength emission in this system originates from the area of interaction between the relativistic pulsar wind and the accretion stream (Fig. 4). Powerful wind is capable of stopping the stream, preventing its penetration towards the neutron star surface, and creating a standing shock. As the neutron star rotates, the shock slides along the matter leading to variations of the optical and X-ray fluxes with the period equal to half of the neutron star spin period, consistent with the observations. This scenario opens new prospects to study microphysical processes of interaction between the low-density relativistic beam with the matter.
Figure 4:
Geometry of the interaction between the pulsar wind and the accretion disc in two different modes. From Veledina et al., 2019, Pulsar Wind-heated Accretion Disk and the Origin of Modes in Transitional Millisecond Pulsar PSR J1023+0038, ApJ, 884, 144.
Optical Polarimetry of Compact Objects
High-precision optical polarimetry is a powerful tool to study geometry and orientation of astrophysical objects. It can be used for determination of inclination and orientation of binary orbit and helps to locate various gaseous structures, such as streams, disks and jets.
Tuorla Observatory possesses a unique expertise in developing high-precision instruments for optical polarimetry. Two such instruments, DIPol-2 and DIPol-UF (Fig. 5, top), have been built during the last decade in collaboration with the Institute of Solar Physics (Freiburg, Germany). Both polarimeters are currently employed for studying various polarization mechanisms, including polarization in exoplanets, binary systems and interstellar polarization with the accuracy up to 10-5 and better. Our latest results obtained with the DIPol-2 for the high-mass gamma-ray binary LS I+61o 303 revealed for the first time the presence of a variable, and synchronous with the orbital motion, linear polarization. From analysis of polarization data with the model of Thomson scattering by a cloud that orbits the Be star, we obtained new constraints on orbital parameters, including a small eccentricity (e < 0.2) and periastron phase 0.6, which coincides with the peaks in the radio, X-ray, and TeV emission (Fig. 5, bottom).
Figure 5:
Top: New high-precision optical polarimeter DIPol-UF built at the University of Turku and installed at the Nordic Optical Telescope (see Piirola et al., 2021, Double Image Polarimeter-Ultra Fast: Simultaneous Three-color (BVR) Polarimeter with Electron-multiplying Charge-coupled Devices, AJ, 161, 20.)
Bottom: Two possible geometries of the orbit of a compact object in LS I+61o 303 around Be star (yellow circle), which lies at the ellipse focus. The red dashed line is the major axis of the orbit. The black dots on the orbit are spaced by the phase interval Δφ=0.1. Phases of apoastron (0.12) and periastron (0.62) are shown by asterisks.
From Kravtsov et al., 2020, Orbital variability of the optical linear polarization of the gamma-ray binary LS I +61o 303 and new constraints on the orbital parameters, A&A, 643, A170.