Polarized X-ray reflection from a star

Ahlberg Varpu, University of Turku

The orbital variability of polarization in X-ray binaries can be explained with a reflecting medium that is distributed unevenly around the compact object. A clear example of this is reflection from the stellar companion. In the soft X-rays the stellar reflection is strongly linearly polarized, as most of the light is scattered only once. The variability can be used to study the orbital parameters of the binary, which could help constrain mass of the compact object. We developed an analytical single-scattering model for the stellar X-ray reflection both for simplified geometries and for a star filling its Roche lobe. Our model shows that the observed polarization degree is typically less than one percent unless the direct emission is obscured. We fitted with our model the X-ray polarimetric data obtained with the Imaging X-ray Polarimetry Expoler of the accreting weakly magnetized neutron star GS 1826–238. We find that the observed orbital variability appears to be too large to be explained by the stellar reflection alone.

The Array for Gigahertz Observations (ARGOS)

Antoniadis John, FORTH IA

Astronomy is being transformed by multi-messenger surveys performed with instruments capable of searching the sky with high speed and sensitivity, while delivering science-ready datasets to the community. While radio astronomy is not yet fully participating in this revolution, it is clear that an instrument following the same philosophy is urgently needed. ARGOS is a concept for a leading-edge, low-cost, sustainable European astronomical facility that will finally realise this ambition, directly addressing multiple fundamental scientific questions, from the nature of dark matter and dark energy to the origin of fast radio bursts and the properties of extreme gravity, thereby satisfying urgent needs of the community. ARGOS will enable, for the first time, continuous wide-field monitoring of the sky at centimetre wavelengths, while publicly distributing science-ready data and alerts in real time. In this talk, I will describe the project and its main science and engineering objectives, as well as ARGOS-pathfinder, a 10-dish prototype that is being deployed as part of the project.

Mercury regolith modeling using MESSENGER spectrophotometry

Björn Vesa, University of Helsinki

Mercury can be modeled as an atmosphereless Solar System body. Such objects are covered by a regolith which affects how they scatter light. To deduce physical properties of Mercury's regolith, we use spectrophotometry from the MDIS (Mercury Dual Imaging System) instrument of NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) mission. The data comes in eight colors (Domingue et al., Icarus 257, 477, 2015) between the wavelengths of 433.2 nm and 996.2 nm, with phase angles from 20 to 130 degrees. There are 37752 data points, of which we use 27618 that are at incidence and emergence angles below 70 degrees. Theoretical Lommel-Seeliger and particulate medium (PM) models are used to interpret the observed reflectance. The PM model includes a shadowing correction that depends on three geometry parameters of the regolith. The first parameter is the packing density, i.e., the ratio of the particles' volume to the total volume. The other two parameters describe the regolith's roughness as a fractional Brownian motion (fBm) surface: the Hurst exponent in the horizontal and the amplitude in the vertical direction. The numerical implementation of the PM model includes a set of discrete parameter values (Wilkman et al., Planet. Space Sci. 118, 250, 2015). However, using trilinear interpolation, we extend the parameters to have arbitrary values within the range of the discrete values, which are 0.15--0.55 for the packing density, 0.20--0.80 for the Hurst exponent, and 0.00--0.10 for the amplitude (in units of the width of the simulated medium). We optimize the model parameters in least-squares sense using the Nelder--Mead simplex method, followed by Markov chain Monte Carlo (MCMC) sampling that uses proposed parameter values based on virtual least-squares solutions. The model parameters are solved for all wavelengths simultaneously, which means that the result is physically consistent. In our study, the size of the regolith particles follows a uniform distribution between 0.0006 and 0.003, in units of the medium width. Our preliminary results indicate that Mercury's regolith has a packing density of about 0.54, and an fBm surface with a Hurst exponent of 0.53 and an amplitude of 0.098. Such a regolith is densely packed with moderate horizontal and large height variations. The MCMC solution allows us to predict the spectrophotometry for differing viewing geometries. Future work includes improving the implementation of the PM model by increasing the range of the parameters and by modifying the size distribution of the regolith particles. The results of our study can be utilized in the BepiColombo mission.

Polarimetric properties of the weakly magnetized neutron stars: theory and observations

Bobrikova Anna, University of Turku

Observations show that the X-ray emission of the accreting weakly magnetized neutron stars is polarized. The polarization degree of the observed sources exceeds the expectations derived from the previous models of these sources. In this talk, I introduce a new theoretical model, where we assume the emission of the accreting neutron star coming from the spreading layer, the extension of the boundary between the disk and the neutron star surface onto the surface. We then calculate the Stokes parameters of the emission accounting for relativistic aberration and gravitational light bending in the Schwarzschild metric. I present the main results of our simulations. I show how our results provide new insights into the X-ray polarization from weakly magnetized neutron stars observed with the Imaging X-ray Polarimetry Explorer (IXPE). I specifically focus on the X-ray burster GX 13+1.

The Merger of the Milky Way & Andromeda

Delhomelle Jehanne, University of Helsinki / Helsinki Institute of Physics

The impending merger of the Milky Way and Andromeda galaxies is often viewed as a foregone conclusion. We test this assuming combining simple orbit models and N-body simulations that take into account the uncertainties of the relevant parameters. We discuss the main sources of uncertainty in the orbital evolution, and determine the most likely - and some less likely - future scenarios for the Local Group.

Distance estimation of gamma-ray emitting BL Lac objects from imaging observations

Fallah Ramazani Vandad, Finca

In optical and near-infrared bands, the emission from BL Lac objects is dominated by the non-thermal emission from a relativistic jet. In other words, their optical spectra usually show no emission line from the host galaxy. Therefore, the direct determination of their redshift is highly challenging. We aim to overcome this difficulty by attempting to detect the host galaxy and derive redshift constraints of a sample of BL Lac objects. After obtaining optical images with long exposure time under good seeing conditions, we see if the host galaxy is detected and then derive the imaging redshift by using the host galaxy as a standard candle using two different methods. We determine imaging redshift for 9 out of 17 sources in our sample. The redshift range of these targets is 0.28-0.60 and the two methods used to derive the redshift give very consistent results within the uncertainties. We also performed a detailed comparison of the imaging redshifts with those obtained by other methods. We show that the constraints from different methods are consistent and that for example in the case of J2156.0+1818, which is the most distant source for which we detect the host galaxy, combining the three constraints narrows down the redshift to 0.63 < z < 0.71.

Characterising very small asteroids as building blocks of the Solar System

Fedorets Grigori, Finnish Centre of Astronomy with ESO (FINCA)

The taxonomic distribution of asteroids serves as the footprint of the evolution of the Solar System. The resolution for this footprint is most accurate in the domain of the understudied population of very small asteroids (less than 10 m in diameter) . While these objects are very faint, and their observational windows are narrow, thanks to recent advancements, such observations have recently become feasible. We present first results of physical characterisation observations of very small asteroids at the Nordic Optical Telescope.

Tidal disruption of asteroids increases the rate of impacts with the Earth

Granvik Mikael, University of Helsinki / Luleå University of Technology

Tidal disruptions of gravitational aggregates during very close and slow encounters with terrestrial planets was first seriously considered in the late 1990's. The observational evidence for such events remained rather sparse for more than two decades. Granvik & Walsh (2024) showed that non-negligible signatures in the orbital and size distributions of near-Earth asteroids can be explained as a result of tidal disruptions. The exact nature of these events is still under study, but one of the key outcomes is that the fragments produced by tidal disruptions can at least partly explain the long-standing discrepancy between impact-rate estimates based on data from telescopic surveys and direct measurements of fireball rates, where the latter suggests a higher rate.

Spot Transits in Kepler and TESS Photometry of Late Main-Sequence Stars

Haris Andras, University of Helsinki

Transit mapping is an effective method for detecting small starspots that may go unnoticed with other techniques used to study active stars' surfaces. The Kepler and TESS space telescopes provide high-precision transit light curves for a large number of stars. The accuracy of these observations allows us to identify spot occultations that occur during planetary transits. We conduct a systematic search for spot-transiting events using a Markov Chain Monte Carlo (MCMC) approach on all 60s and 120s cadence Kepler and TESS light curves of late-type stars with known transiting exoplanets. A more comprehensive occurrence statistics for small spots are presented for the Kepler and TESS sample. The geometric properties of the identified spots can be used to constrain the parameters of the stellar dynamo on these planet-hosting stars, which has implications for the atmospheric evolution and habitability of their planets.

Catching the formation of the ultra-diffuse galaxy Hydra-UDG32 in the act

Hartke Johanna, FINCA, UTU

Ultra-diffuse galaxies (UDGs) are extremely low-surface brightness galaxies with a size of several kpc, i.e. comparable to that of the Milky Way, but with at least 100 times smaller stellar masses. In the scope of the LEWIS large programme with VLT-MUSE, we have targeted a complete sample of 32 UDG candidates in the 50-Mpc distant Hydra I cluster. In this presentation, I will focus on UDG 32, a galaxy that has been hypothesised to have formed from material stripped from the nearby spiral galaxy NGC 3314a. Our new MUSE data shows that NGC 3314a's filaments extend to unprecedented distances, completely engulfing the UDG and could confirm that the UDG and the filaments are indeed co-spatial based on their line-of-sight velocities. UDG 32 may thus be one of the first ultra-diffuse galaxies where we catch the formation from ram-pressure stripped gas in the act.

Masses of the superclusters

Heinämäki Pekka, Tuorla Observatory

Superclusters are the largest mass and density concentrations of the Universe. With fractional overdensities of just a few times the mean density of the Universe superclusters are remained in the linear regime until recent times and finally around z ∼ 0.5 halted at current scales. So, the development of superclusters as a whole has not been significantly influenced by nonlinear processes, and therefore, the mass function of superclusters could potentially be used as a cosmological probe. We present in this talk some preliminary results on the masses of superclusters using both observational data and theoretical results.

Resolving supermassive black hole jets in near-infrared wavelengths

Hovatta Talvikki, FINCA

Relativistic jets launched by supermassive black holes are bright emitters over the entire electromagnetic spectrum. The Doppler-beamed parsec-scale jets that are responsible for most of their extreme variability have so far been spatially resolved only in radio and millimeter wavelengths. In modeling their spectral energy distributions often a single, compact emission region is assumed in addition to potential thermal emission components, which however are typically swamped by the jet emission in high activity states of these sources. I will describe our near-infrared observations utilizing the VLTI-GRAVITY instrument where we attempt to spatially resolve parsec-scale jets in these short wavelengths for the first time. The angular resolution of 4 milliarcseconds allows us to model the emission on parsec scales and potentially distinguish multiple emission components.

Late-time evolution of the interacting supernova 2017dio

Humina Christina, University of Turku

The majority of massive stars end their lives in a violent explosion called a supernova (SN). In some cases, the stars may have lost their hydrogen and helium layers before the explosion, producing a type Ic SN. SN 2017dio is a type Ic SN that exploded in a hydrogen-rich circumstellar medium (CSM). The CSM is usually produced by the mass loss of the progenitor, but in the case of SN 2017dio, the hydrogen-rich CSM suggests that the CSM originates from a companion star instead, indicating a very complex evolution and interaction process. SN 2017dio has proven to be a challenging case for stellar evolution theory on how to produce the progenitors of this kind of SN. We present late-time spectra and light curves of SN 2017dio, which are dominated by the ejecta interacting with CSM. Our goal is to determine the properties of the extended CSM by modelling the lightcurve and analysing the hydrogen emission line profiles. We identify an infrared excess in the light curves, which may have originated from infrared echo produced by pre-existing dust. By modelling this echo, we can derive the CSM properties at even greater distances. By deriving the CSM properties at different distances we can study the radial distribution of the CSM. This, in turn, tells us about the time evolution of the progenitor mass-loss history, which can shed light on the mass-loss mechanism and evolution of massive stars in binary systems.

Resolving the complex dynamical evolution of supermassive black holes in cosmological simulations using the KETJU code

Johansson Peter, University of Helsinki

Traditional numerical simulations employing gravitational softening are unable to resolve the small-scale dynamics and gravitational wave emission from supermassive black hole binaries. Instead, the parsec-scale dynamics is typically modelled by postprocessing the simulations using either semi-analytic methods based on orbit-averaged equations or by resimulating the core regions of the merged galaxies using an altogether separate N-body code. An alternative is to use a hybrid approach, such as the KETJU code, recently developed in our group. The KETJU code includes algorithmically regularised regions around every supermassive black hole (SMBH). This allows for simultaneously following global galactic-scale dynamical and astrophysical processes, while solving accurately the dynamics of SMBHs at sub-parsec scales. In addition, the KETJU code also includes post-Newtonian terms in the equations of motions of the SMBHs, which allows us to directly calculate the expected gravitational wave signal from the motion of the resolved SMBH binary in mergers of massive galaxies. We demonstrate here how KETJU can be used to model the dynamics of SMBHs in a full cosmological setting and thus provide more accurate predictions for the upcoming LISA gravitational wave observatory. Finally, we also present a new public version of the KETJU supermassive black hole (SMBH) dynamics module, as implemented into the GADGET-4 code.

The long 2016 polarisation angle rotation of OJ 287 viewed through radio, mm-radio and optical observations

Jormanainen Jenni, FINCA, UTU

OJ 287 is a bright and well-known blazar that has been observed for over a century, and it is one of the strongest candidates to host a supermassive black hole binary to date. In 2016, a long polarisation angle rotation was observed from this source. Curiously, the rotation was seen most clearly in the higher radio frequencies, but it appeared to disappear in the lowest frequency bands. A few competing models have been suggested to describe the long rotation, namely, in Myserlis et al. 2018 they attributed the rotation to a moving emission feature in a helical, bent jet where the rotation disappears in the lower frequencies due to opacity effects. Later, Cohen & Savolainen 2020 demonstrated that such rotations could result from phase effects between the typical steady and variable polarisation components. Conclusive evidence to differentiate between these models suffer from the lack of high cadence sampling of the polarisation data. As an attempt to extract information from both optical and radio bands, we compare the multiwavelength optical information across the optical bands down to the radio bands, including the intermediate mm-wavelengths observed with ALMA. We also collect optical data from several instruments to get a clearer picture of the polarisation curve than used in the previous studies. With this study we hope to demonstrate the necessity for a denser sampling (as supported by past studies) of the polarisation degree and angle as well as the use of multiband information in similar attempts to deduce the true model behind the observed polarisation signatures.

GPU-assisted analysis of interstellar medium

Juvela Mika, University of Helsinki

The study of interstellar medium is based on the comparison of numerical models and observations from optical to radio wavelengths. The work includes many computationally heavy tasks, especially in simulations but also in the basic processing of measurement data. I will discuss how the use of graphics processing units (GPUs) has led to quantitative and qualitative improvements in this work, taking examples from the radiative transfer modelling and the analysis of dust continuum and molecular spectral line observations.

Supermassive black hole dynamics with KETJU and a dynamical friction subgrid model

Keitaanranta Atte, University of Helsinki

Modern cosmological simulations often use a subgrid model in order to take the unresolved effect of dynamical friction into account. We study the dynamics of supermassive black holes (SMBHs) in galaxy simulations using either the regularized integrator KETJU or a dynamical friction subgrid model. The dynamics of a SMBH binary can be resolved to sub-parsec scales with KETJU as long as the ratio of a black hole and stellar particle mass is sufficient. The subgrid model is less dependent on the mass resolution, but the dynamics are modeled only until dynamical friction becomes ineffective. We then combine the two models in a way that KETJU is used when a SMBH reaches a mass limit. This hybrid model is used in a cosmological zoom-in simulation run until redshift 2. We focus on the SMBHs which reach the mass limit for KETJU integration and on two mergers of these SMBHs.

Driving infant transient discovery with the Gravitational-wave Optical Transient Observer

Killestein Tom, University of Turku

Time-domain astrophysics is in an unprecedented growth period, driven by the current generation of sky surveys that have unveiled new corners of transient parameter space: by tracing superluminous supernovae in the more distant Universe, having adequate cadence to sample the rise and fall of an emerging zoo of 'fast' transients, and the survey areas to spot tidal disruption events in the nuclei of galaxies. This growth has been underpinned by machine learning -- without which the volumes of data are simply too much for human filtering to bear. With the Vera Rubin Observatory due next year promising a factor 10 increase in candidates, current techniques will be strained. Upcoming surveys like the Rubin Observatory require new approaches - not only by improving existing source classification approaches, but active prioritisation of the candidates we do discover. I will present novel work using Gravitational-wave Optical Transient Observer data on both of these tasks, enhancing not only recovery of nuclear transients but novel 'contextual classification' approaches to automatically identify the hosts of transient explosions - as well as some of the unique discoveries made within the collaboration as a result.

Feature detection, butterfly diagrams, and analysis for Metsähovi solar observations on 37 GHz

Kivistö Sami, Aalto University (Department of Computer Science, Metsähovi Radio Observatory)

Metsähovi Radio Observatory (MRO) has recorded solar intensity maps since year 1978, mostly with frequency 37 GHz. When looking at the radial intensity profile on these maps, we observe a limb brightening as we go away from the center of the disk towards the limb, followed by a continuous transition from limb intensity values to the coronal values. This transition profile is convoluted by the 14 m dish at MRO, which has a beam diameter of ca. 2.4 arc minutes for this frequency. Methods and models have been developed to compensate for these effects in order to see the true intensity profile of the solar disk. We can then obtain information from the polar regions, which are always seen at the limb from Earth, and construct butterfly diagrams which show features also at high latitudes. The limb profile obtained from the maps is used to improve the centering and intensity normalization when processing the raw solar maps, iteratively converging into an optimal limb profile and solar radius at 37 GHz. These vary by solar cycle and phase. As new data is accumulated continously, we can monitor the time variability of the limb. Solar maps contain bright and dim features, which are automatically detected from the solar maps. Sometimes the feature contains fine structure, such as multiple local maxima or minima. A database is created from these features, which preserves the fine structure as a an hierarchy of sub-objects. We link subsequent observations of a particular feature based on its slowly migrating position at the rotating heliographic surface. This allows tracking the movement of the features, and building statistics for differential rotation of these features.

More is more: The extreme coronal line emitter AT 2022fpx

Koljonen Karri, NTNU

Supermassive black holes are known to disrupt passing stars, triggering tidal disruption events (TDEs). TDEs have garnered recent attention due to their unique dynamics and emission processes, which are still not fully understood. Optical TDEs, particularly, are intriguing as they often display delayed or obscured X-ray emissions from the accretion disk, making the origin of the prompt emission unclear. This presentation focuses on AT 2022fpx, a recent optical TDE candidate. I will explore its distinctive optical, ultraviolet, and X-ray characteristics, such as highly-ionized iron emission lines in its optical spectra, indicative of extreme coronal line emitters, alongside variable, low-polarized optical emission and late-time X-ray flaring behavior. The overall outburst decay of AT 2022fpx deviates from typical TDEs, resembling Bowen fluorescence flares more closely. These observations suggest AT 2022fpx may serve as a connection point for various long-lived TDE scenarios. In this presentation, I will delve into these findings and discuss their implications for our understanding of TDE dynamics and emission mechanisms.

Are blazars high energy neutrino factories?

Kouch Pouya M., UTU and Metsähovi

Identifying the most likely sources for high-energy neutrino emission has been one of the main topics in high-energy astrophysics ever since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active galactic nuclei (AGNs) with relativistic jets pointing close to our line of sight, blazars, have been considered to be one of the main candidates due to their ability to accelerate particles to high energies. In our earlier study, we investigated the connection between radio emission and IceCube neutrino events using data from the Owens Valley Radio Observatory and Metsähovi Radio Observatory blazar monitoring programs. While not all neutrinos arrive during strong radio flares, our results suggest that when they do, it is unlikely to be a random coincidence. In this talk, I will give an update on these results using three years of additional data. I will also show the first results from our study investigating the connection between optical flares and neutrino arrival times.

Using high-precision optical polarimetry to constrain geometry of black hole X-ray binaries

Kravtsov Vadim, University of Turku

Black hole X-ray binary spectrum is a product of a complex interplay between various components, such as an optical companion, accretion disk, hot accretion flow, jet, etc. Identifying different spectral components and studying their radiative properties are essential for understanding the mechanisms that trigger the outbursts. The contribution of different components to the total spectrum has been studied utilizing a variety of methods, with polarimetry often overlooked and undervalued. In this talk, using several black hole binary systems as examples, we will show that high-precision polarimetry is a powerful technique for this purpose. We will show how modeling of orbital variations of the polarization allowed us to impose constraints on the geometry of the accretion disk in Cyg X-1. In addition, we will discuss the detection of the significant optical polarization, variable with the orbital phase, in low mass X-ray binary 1A 0620-00 that, combined with a broadband IR to UV spectrum, allowed us to shed light on the nature of the quiescent emission of this source.

Radiative-transfer coherent-backscattering modelling for photometric and polarimetric phase curves of Galilean satellites

Leppälä Ari, University of Helsinki

Photometric phase curves of airless Solar System objects exhibit a distinctive opposition effect, characterized by nonlinear brightening as phase angles approach backscattering. At phase angles less than approximately 20 degrees, polarimetric phase curves predominantly display a negative degree of linear polarization. These phenomena are inadequately explained by radiative transfer (RT) models alone but incorporating coherent backscattering (CB) provides a comprehensive modelling solution. In our study, we employed radiative-transfer coherent-backscattering (RT-CB, [1]) modelling with an ensemble-averaged scattering matrix. With this approach, parameterized phase matrix elements were utilized to replicate the observed low-phase-angle polarimetric phase curves for Io and icy moon Ganymede [2], as well as Europa [3]. We adjusted the scattering matrix until the computations closely matched the observed data, resulting in an ensemble-averaged scattering matrix for modelling both photometric and polarimetric phase curves for these satellites. Io and Europa have similar geometric albedos (A_g) 0.63 and 0.67, but negative polarization branch (NPB) shapes differ. The NPB of Ganymede (A_g = 0.43) is morphologically similar to that of Europa, although it is described by different parameter values for scattering function. This is likely due to the compositions of their surfaces: Europa’s with H_2O ice, Ganymede’s with H_2O ice and silicates, and Io’s with sulfuric/silicate. Polarimetric observations show only slight or no dependence on the wavelength, indicating wide particle size distributions with different real parts of refractive index, for Europa and Ganymede (Re(m) ∼ 1.3) and Io Re(m) >∼ 1.4). Numerical computations using the RT-CB method demonstrated a match to the polarimetric observations and to the geometric albedos. For Ganymede, the single-scattering albedo ω and mean free path length kl = 2πl/λ_eff, were ≈ 0.943 and ≈ 150, respectively, where λ_eff is the wavelength. For Io’s regolith, ω ≈ 0.979 and kl ≈ 40.

MAGIC Telescopes - 20 years of very high energy gamma-ray observations of violent phenomena of the universe

Lindfors Elina, Department of Physics and Astronomy

MAGIC Telescopes have been observing the Very High Energy gamma-ray sky now for 20 years. In these 20 years, we have published 200 scientific papers on violent phenomena of the universe. Many of the violent phenomena are variable or transient in nature and therefore many of the discoveries have resulted from the target of opportunity programs, which use multiwavelength information from other wavelengths both from optical telescopes from the ground as well as X-ray and gamma-ray satellites from the space. In this talk, I will show some of the recent highlights from such observations.

A Spectro-Polarimetric view into acccretion Plasma Physics

Loktev Vladislav, University of Turku

We introduce an analytical ray-tracing technique for polarized light, called artpol, designed for the rapid extraction of spinning black hole parameters from their spectro-polarimetric signa- tures. Such signatures, found in the accretion disks of X-ray binaries and active galactic nuclei, offer insights into the masses and spins of central black holes, as well as the plasma dynamics and geometry of matter in proximity to these compact objects. Our technique serves as a swift alternative to the traditionally cumbersome numerical ray-tracing methods for analyzing optically thick or geometrically thin accretion flows. Our method delivers precise results for dimensionless spin parameters a ≤ 0.94. Using Schwarzschild approximation for lightbending the technique achieves a speed enhancement of over four orders of magnitude compared to direct ray-tracing calculations. This method paves the way for efficient direct fitting of spectro- polarimetric data obtained from the Imaging X-ray Polarimetry Explorer (IXPE), advancing our understanding of accretion plasma physics in the vicinity of black holes.

Cloud evolution in the high-energy molecular ring Lambda Orionis

Mannfors Emma, University of Helsinki

[Background]: High-mass stars contribute significantly to the evolution of the Galactic environment, both during their lifetime through feedback and outflows, and at their death through supernovae. Due to their short lifespans and rarity, the interaction between high-mass stars and the interstellar medium (ISM) is difficult to observe. This leads to a lack of understanding on what causes stars to form, and how the gas within galaxies evolves through time. The nearby (d∼400 pc) molecular ring Lambda Orionis is the perfect region to study both the effects of ongoing feedback and a recent supernova on the ISM. [This work]: We present 12CO and 13CO molecular line observations of three dense regions in lambda Orionis, which have been observed as part of the B-Fields and staR fOrmation across Scales with TRAO (B-FROST) survey (Montillaud et al., in prep.) using the 13-m TRAO telescope located in Korea. The purpose of this survey is to explain how magnetic fields interact with molecular clouds across a wide range of Galactic environments. We use a combination of data, including Planck polarization and infrared continuum observations, WISE continuum observations, and molecular lines to achieve a full picture of the Lambda Orionis region, and to understand how magnetic fields and stellar feedback drive cloud evolution. Catalogs of protostars from the GAIA telescope are used to study the possibility of triggered star formation in the ring.

Metsähovi Compact Array -- commissioning a new radio interferometer

McKay Derek, Metsähovi Radio Observatory

Metsähovi Compact Array (MCA) is a radio telescope system, comprising at least three 5.5-metre parabolic dish antennas and operating at frequencies from 4 to 8 GHz. It is being built at the Aalto University Metsähovi Radio Observatory (MRO) in Kirkkonummi, Finland. When complete, it will provide an interferometer for scientific research, university-level hands-on training, and schools' experience. The first antenna, MCA-1, was recommissioned in 2022 and has been used for telescope methodology studies and student education projects. The second antenna, MCA-2, has now been assembled, fitted-out with its motors, controllers, receiver and the necessary infrastructure, and has started service in early 2024. With two antennas operational, and a baseline of 47 metres, development of the interferometer itself is underway. Site preparation work for MCA-3 has also commenced, with assembly expected later this year. This presentation introduces the project, reports on the current status, and describes the plans and capabilities.

Detecting a gravitational wave background from early universe phase transitions with LISA

Minkkinen Tiina, University of Helsinki

In the next decade or so, the first spaceborne gravitational wave observatory LISA (Laser Interferometer Space Antenna) will start probing the universe. It has the potential to observe a gravitational wave background created by early universe events, such as first order phase transitions happening right after the Big Bang. Their existence could help fill in the gaps in our current Standard Model of particle physics. Here we explore different phase transition models to see which ones give rise to gravitational wave backgrounds in LISA’s sensitivity range, where there are also realistic galactic binary systems and instrument noises interfering with our observations. We use the Deviance Information Criterion to establish a range of detectable phase transition models, and see if we can use the expected annual modulation of the galactic foreground to improve phase transition detectability.

Scattering of light in planetary regoliths using Mueller matrix decomposition

Muinonen Karri, University of Helsinki

Scattering matrices of cosmic particulate ensembles are analytically decomposed into sums of pure Mueller matrices. The ensembles are assumed to have equal numbers of nonspherical particles and their mirror particles, both in random orientation. In the general case, there are four pure Mueller matrices in the decomposition. Of these four matrices, there is a single matrix that qualifies as a pure scattering matrix, whereas the remaining three matrices represent other classes of Mueller matrices. For ensembles of spherical particles, there are two pure Mueller matrices in the decomposition. Again, there is a single matrix qualifying as a pure scattering matrix. The decomposition allows for radiative-transfer coherent-backscattering (RT-CB) computations for discrete random media of nonspherical particles. It turns out that the full decomposition must be incorporated in RT-CB computations for both spherical and nonspherical particles, securing the proper treatment of depolarization. First results are shown for decomposition modeling of photometric and polarimetric phase curves of atmosphereless planetary objects.