Science Meeting 2023-2024
Past science meeting calendars, abstracts, and slides can be found at the following links:
February - July 2019
September 2019 - July 2020
September 2020 - July 2021
September 2021 - July 2022
September 2022 - June 2023
Ecogia Science Meetings 2023-2024
Schedule
Meetings are at 14 in the Pavillon meeting room at Ecogia and on Zoom
We conduct a comprehensive analysis of the accreting X-ray pulsar, Hercules X-1, utilizing data from IXPE and NuSTAR. IXPE performed five observations of Her X-1, consisting of three in the Main-on state and two in the Short-on state. Our time-resolved analysis uncovers a linear correlation between the flux and polarization degree (PD). Besides, we also identify a linear anti-correlation between the pulsed fraction (PF) and PD, signifying the dominance of a low-PD pencil beam pattern and the potential contribution of a high-PD component, possibly originating from scattering within the disk wind. Geometry parameters are rigorously constrained by fitting the phase-resolved modulations of Cyclotron Resonance Scattering Feature (CRSF) and polarization angle (PA) with a simple dipole model and Rotating Vector Model (RVM) respectively, yielding consistent results. During Main-on observations, we observe a linear correlation between chi_p and phi_35, indicative of system precession. Furthermore, a linear association between CRSF and PA implies the prevalence of a dominant dipole magnetic field, and their phase-resolved modulations are likely arising from viewing angle effects.
Following the successful launch at the beginning of July, Euclid has been carrying out an intense programme of commissioning and performance verification activities. I will give a summary of how things have gone so far (though sadly limited to what is in the public domain), and some of the data input products we produced for our pipeline before Euclid data existed.
2023-10-30 00:00:00 Skipped
2023-11-06 00:00:00 Skipped
Euclid released its first color images as part of the Early Release Observations (ERO) on November 7. These images uncover astonishing details at a resolution of up to 0.15 arcseconds, covering an area of 0.7 by 0.7 degrees for each target. As a member of the Euclid ERO programme committee, I will share with you the developments of the past 9 months and explain how we achieved these ERO images.
Supergiant High Mass X-ray Binaries (SgXBs) are fundamental laboratories to understand the physics of accretion, when this takes place via stellar winds onto strongly magnetized neutron stars. The bulk of SgXBs hosts, indeed, a neutron star orbiting around an OB supergiant companion, which posses a strong and dense wind with terminal velocities reaching up to few thousands of km/s. It was recognized in the early 00s that SgXBs are also crucial in order to advance our understanding of massive star winds, as the compact object in these binary systems works as an in-situ probe of the wind and can help narrowing down the still existing large uncertainties on the physical properties of this fast moving medium. Massive star winds are a key ingredients in many hot topics of the modern Astronomy and Astrophysics, as these winds regulate the chemical enrichment of the galaxies and consequently control part of the evolution of the Universe also on large scales. In this talk, I will review the status of our understanding of accretion in SgXBs and the associated uncertainties when we try to reverse-engineer the X-ray observations of these systems into physical parameters of the massive star winds. I will discuss the currently most pursued efforts in the field and provide an overview of future perspectives by describing the expected improvements foreseen with the next generation of X-ray instruments in space.
Gaia DR3 contains 1.8 billion sources with G-band photometry, 1.5 billion of which with G_BP and G_RP photometry, complemented by positions on the sky, parallax, and proper motion. We present a summary of the supervised classification of Galactic and extra-galactic objects that are detected as variable by Gaia across the whole sky consisting of 12.4 million sources which include about 9 million variable stars classified into 22 periodic and non-periodic variable types (pulsating, eclipsing, rotating, eruptive, cataclysmic, stochastic, and microlensing) with amplitudes from a few milli-magnitudes to several magnitudes. Additionally it contains a few thousand supernova, about a million active galactic nuclei, and almost 2.5 million galaxies.
The identification of galaxies was made possible by the artificial variability of extended objects as detected by Gaia, which also affects close pairs. These so-called "scan-angle dependent signals" can propagate in the derived photometry, astrometry and radial velocity signals and is due to Gaia's way of acquiring observations and the subsequent calibration steps. We will discuss the effect, origin, ways of detecting it in the current data, and how we can possibly treat it in future data releases.
Reaching microarcsecond resolutions in visible light is not possible with any modern existing standalone telescope or any under construction. Such resolution is required to resolve the structure of an accretion disc in AGNs or galactic objects (microquasars, cataclysmic variables). The structure of such disks has imprints of information about the blackhole mass, spin, gas feeding rate and variability. Milliarcseconds resolution is reachable in radio, using the phase interferometry technique with very long baselines, while practically impossible in visible light due to significantly shorter wavelengths. Intensity interferometry is a technique that originates from experiments of Hanbury Brown & Twiss (HBT) in the 1950s. Unlike traditional interferometry where the phase of light waves is measured, the intensity interferometry technique, as the name suggests, focuses on intensities. Spatially separated detectors observe the same source and the acquired intensities are correlated allowing one to extract information about the observed object. The size of the object can be measured with just two detectors (a single baseline). To constrain the spatial structure of the source many baselines are required.
In pursuit of developing a new instrument for intensity interferometry, we started building one based on state-of-the-art single-photon avalanche diode (SPAD) photo-detectors. SPADs allow to detection of individual photons with pico-second time accuracy. The setup we built in the lab employs two SPAD detectors to perform a classical HBT experiment using thermal and quasi-thermal sources. The same instrument is then adapted to be used on a Telesto telescope and later scaled to many telescopes to perform intensity interferometry.
I will mostly focus on the recent hardware details for the lab and telescope setups, but also discuss the physical prerequisites for intensity interferometry and possible astrophysical applications of this technique.
2023-12-11 00:00:00 Skipped
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2023-12-25 00:00:00 Holidays
2024-01-01 00:00:00 Holidays
2024-01-08 00:00:00 Holidays
Current and upcoming weak lensing surveys require accurate estimation of the point spread function (PSF) for precise measurements of galaxy shapes. With a wavelength-dependent PSF model in a diffraction-limited telescope like Euclid, knowledge of each galaxy spectral energy distribution (SED) is essential. However, even in rich multi-band data fields like COSMOS, the SEDs lack adequate wavelength coverage, primarily in the red end of the spectrum, leading to potential biases in the estimates of cosmological parameters. Therefore, in order to perform WL analyses, sufficiently accurate knowledge of the SED is needed. In this study we test different galaxy SED reconstruction methods against the requirements imposed on the Euclid PSF model. We employ both physics and data-driven approaches, focusing on template fitting and Gaussian Processes (GP). To test their precision we introduce an SED metric whose bias translates into inaccuracies on the PSF quadrupole moments. Additionally, we investigate the impact on the SED reconstruction of a simulated evenly-spaced medium-band filter design. Our findings demonstrate that GP meet the requirements in specific redshift intervals, while template fitting exhibits robustness in others. A hybrid approach, leveraging both methods, is proposed. This solution proves effective in meeting the Euclid requirements across various redshift bins. Simulations based on evenly-spaced medium-band filters show promising results, ensuring accurate SED reconstructions and meeting all the mission requirements.
I will present some preliminary results regarding the connection between the statistical properties of the observed optical and X-ray synchrotron polarization and the possible connection with statistical properties of turbulence and magnetic reconnection.
I will show how the statistical properties of the turbulence/fragmentation of the magnetic cells can reproduce the X-ray degree of polarization, observed by IXPE for BL Lacs with the synchrotron peak above the UV, and how they can reproduce the observed correlation between the optical degree of polarization with the peak frequency of the synchrotron bump, reported by ROBOPOL, for a large sample of BL Lacs. This scenario hints at a connection among the PDF of the geometrical sizes of the magnetic cells, the acceleration efficiency, and the frequency-dependent degree of polarization.
We just received the news that our proposition (with Pablo Santos Diaz) "A millimeter-accurate Lunar Positioning System with laser technology" to ESA was selected for evaluation. I will describe this project.
Here is the proposal
The cosmic X-ray background (CXB) is produced by the emission of active galactic nuclei (AGN), thus providing key information about the properties of the primary and reprocessed emission components of the AGN population. Equally important, X-ray surveys of AGN provide more constraints on their properties, such as their luminosity and obscuration. Until now, such constraints have not been self-consistently addressed by intrinsically linking obscuration and reflection. Here we perform numerical simulations with our ray-tracing code, RefleX, which allows the self-consistent modelling of the X-ray emission of AGN with flexible circumnuclear geometries. Using the RefleX-simulated emission of an AGN population, we attempt to simultaneously reproduce the CXB and obscuration properties obtained in the X-rays, i.e. the observed NH distribution and fraction of absorbed AGN. We sample an intrinsic X-ray luminosity function and construct gradually more complex physically motivated geometrical models. We examine how well each model can match all observational constraints using a simulation-based inference (SBI) tool. Finally, we explore the relationship between reflection and obscuration and derive the intrinsic fraction of Compton-thick AGN.
2024-02-12 00:00:00 Skipped
The role played by AGN feedback on galaxy formation is one of the key scientific puzzles in modern astrophysics. Galaxy groups are an intriguing resource to address this question, as their bindind energy is comparable to the one supplied by the central AGN. The XMM Group AGN Project (X-GAP) is a large program dedicated to follow up observations of 49 galaxy groups selected after cross-matching the ROSAT all-sky survey (RASS) with the Sloan Digital Sky Survey (SDSS).
In sight of a comparison of this dataset to hydro-dynamical simulations, understanding the sample selection is essential. This can be achieved with accurate end-to-end mocks, processing the simulations exactly like the real data and analyzing the detection probability.
In this talk, I will present the mock sky we are developing to mimick the X-GAP selection in RASS and SDSS. We start from the cosmological Uchuu simulation and use accurate models to populate dark matter haloes with galaxies and active galactic nuclei. We develop a new model for galaxy clusters and groups to predict the temperatures and emissivity profiles as a function of mass and redshift. We account for spatial variations of the X-ray background following the real RASS background maps.
To conclude, I will give an outlook of the project: we will run an X-ray source detection algorithm on the mock RASS and cross-match it with Friends-of-Friends optical groups of mock galaxies. We will measure the X-GAP detection probability and model its selection function.
The general picture of X-ray emission from accreting X-ray binaries is well established. However, the complexity of the average folded light curves (pulse profiles) is a challenge for any detailed model and several individual studies have tackled this issue, without reaching a consensus.
We developed a standardized and versatile method to obtain optimally spaced energy-dependent pulse profiles of neutron-star X-ray binaries and we plan apply it to the full sample of sources which present a cyclotron line and have been observed by NuSTAR, whose spectral resolution and energy range is particularly suited for this kind of study.
In this presentation, we will focus on a sample of prototypical sources and discuss several reduced quantities such as the puled fraction, aka the fractional variability as function of energy, the amplitude and phase of the the first harmonics, and the self correlation/lag spectrum.
We show quantitatively that there are generally two regions with different trends for the pulsed fraction, below and above 10-15 keV, that at an energy corresponding to the iron line or the cyclotron scattering feature, there are dips or excesses in the pulsed fraction.
Even if the extreme heterogeneity of pulse profiles poses a challenge, the comparison of individual cases obtained with a standard method is able to enhance our understanding of emission mechanisms.
The Advanced X-ray Imaging Satellite (AXIS) is a proposed probe-class space telescope sensitive to X-ray energies (0.510 keV) whose primary features are a point-spread function of 12 arcsec across a 24-arcmin-diameter field of view, an effective area up to several times that of current missions, and a low detector background. These characteristics make it well suited for performing deep extragalactic surveys, imaging diffuse emission in fine detail, and for mapping the most crowded regions of the Milky Way. After presenting a selection of the key science questions to be answered by AXIS, I will describe the Galactic Plane Survey (GPS) which will map 104 square degrees of our Galaxy's center, bulge, and inner spiral arms to a limiting sensitivity of 1E15 (1E16) erg/cm2/s for 5 (100) ks of exposure. Extrapolation of the number-flux relations from Chandra to these flux limits suggests that $\sim$ 1 million X-ray sources reside within the GPS footprint which are too faint for Chandra and XMM-Newton but which can be detected by AXIS.
2024-03-11 00:00:00 Skipped
Classification is an important step in the photometric redshift
pipeline of Euclid, and it aims to identify stars, galaxies, and QSOs
in the observed source catalogs, based on their colors. Machine learning methods appear to be very well suited to the task. In this talk, I describe the supervised method employed in the Euclid pipeline, that is the Probabilistic Random Forest (PRF) method. This is a modification of the Random Forest algorithm that takes into account uncertainties in the features and labels. Including
information on the signal-to-noise ratio of data can in fact produce significant improvement in the source classification of deep surveys like Euclid.
Finally, another critical aspect of the classification process is discussed: the training set for the PRF classifiers. In particular, I describe how the training set has been built based on available observational data.
2024-03-25 00:00:00 Skipped
2024-04-01 00:00:00 Holidays
2024-04-08 00:00:00 Skipped
Machine learning applications to physical sciences have seen a huge diffusion in recent years, with cosmology not being immune to the appealing features of such powerful and expressive models. Such algorithms promise to be the only possible answer to the big-data challenges that lie in front of us thanks to facilities like Euclid, the Vera Rubin Observatory and the James Webb Space Telescope. In this talk, I will present recent applications of deep learning techniques to accelerate the generation of expensiveN-body simulations, as well as to emulate summary statistics like cosmological power spectra in the context of Bayesian inference, with a critical focus on the advantages and limitations of such methods over traditional techniques. In particular, I will discuss the black-box nature of deep learning models, and explain how we can interpret them combining representation learning and information theory.
The study of galaxy clusters provides invaluable data on key parameters that describe our standard model of cosmology (CDM). It has long been known from simulations and observed behaviours that the dark matter component that governs these systems does not always follow a spherically symmetric geometry, but more that of a concentric triaxial density profile. However, analytic models such as the standard NFW and Einasto profiles tend to work under the spherical symmetryassumption. We model a set of 252 triaxial systems to study the effect this triaxiality can induce to the observables of these systems. We calculate the potential and analyse its properties. Quantifying the axial ratios of the generated potential surfaces and showing strong dependencies on the concentration of the mass density distribution and a strong deviance from its geometry. The 3D thermodynamical properties of the ICM gas that populates these clusters have been produced and are within reason with current observations except for when cooling astrophysical processes are dominant. The 3D distributions of the observables have been projected, the mass density to obtain the lensing observable and the ICM thermodynamical values to find the projected SZ effect, the X-ray emissivity, and the measured temperature.
Recent measurements concerning cosmological models present tensions between values determined by CMB measurements and those determined by late epoch observations which could potentially challenge the current theoretical model, in particular for the clumpiness parameter 8 which is related to the normalization of the primordial matter power spectrum.
In this project, we are trying to investigate this tension by adding a measure of this parameter with galaxy cluster counts and with data from galaxy clusters of the XXL survey. In addition we investigate the cosmological parameter m, representing the density of matter in the Universe, and which is correlated to the parameter 8 and also the mass-luminosity scaling relation.
Jointly investigating these parameters allows us to marginalize the obtained results for one parameter on the uncertainties of others. For this purpose, we implement a pipeline based on forward cosmological modeling of the XXL sample. We then use SBI to train this pipeline and find the parameters that best reproduce the observed sample.
Model parameter estimation is one of the fundamental steps in scientific research. The field has evolved enormously, from chi-2 to generalized likelihood, to Bayesian statistics. I will quickly retrace the path through a model parameter estimation journey, explaining the problems new approaches, enabled by the increase in computing power, have sought to address, culminating with simulation-based inference. I will then give a short SBI tutorial.
The Perseus cluster provided the first and most spectacular example to date of radio-mode AGN feedback from a BCG on the surrounding intra-cluster medium. Such feedback is expected to be ubiquitous among galaxy clusters throughout the Universe, but is challenging to identify in large numbers. The Euclid high-resolution imaging opens up the possibility of teasing out point-source AGN emission from host galaxies, and thereby building a census of AGN activity in the Euclid survey. We explore the reliability of this AGN-host decomposition on the members of the Perseus cluster, including the hosts of 13 known X-ray point sources which serve as a reference sample.
For the structural decomposition we carefully account for sources of bias, making sure that our measurements are robust against degeneracies in the multi-dimensional parameter space of galaxy structural properties. We perform multiple fits of increasing complexity, combining one or multiple Srsic profiles and PSF components, using our newly developed python package named GingaFit.
Accordion to the preliminary results, the addition of a point-source component significantly improves the fitting model, whilst also accounting for the point-source residual. We also analyse the implied point-source luminosity for each cluster member galaxy, integrated over the VIS band, against their host's absolute magnitudes from the bulge and disc components. We are able to isolate two sub-samples, one within the same luminosity-magnitude range of the X-ray sources, and another low-mass sample with high luminosity. The measurement reliability and nature of the emission of all luminous point sources will be determined in the future steps of the project.
2024-05-20 00:00:00 Holidays
Black hole X-ray binaries (BHXRBs) provide a unique opportunity to study intricate radiation processes characterized by diverse temporal, spectral, and flux variations. X-ray polarimetry with the Imaging X-ray Polarimetry Explorer (IXPE) offers new insights into these phenomena. The recent transient BHXRB candidate, Swift J1727.81613, has experienced significant outbursts, making it an ideal object for investigating its polarization signatures and evolution. In this talk, I will present IXPE's polarimetric views of Swift J1727.81613, focusing on our work on the first quasi-periodic oscillation phase resolved spectro-polarimetric analysis, which reveals discrepancies with predictions of prevailing models.
The recent discovery of the Gaia BH3 with its mass of 33 solar mass (and orbital period of 12 years) provides
us a first sample of an expected Galactic population of such massive stellar remnants. The existence of such
population was already inferred from extragalactic gravitational wave mergers but no electromagnetically
observable example was known so far. I'll go over the properties of the Gaia BH3 binary system and the
plentitude of information that can be derived from Gaia data alone.
2024-06-10 00:00:00 TBD
2024-06-17 00:00:00 TBD
2024-06-24 00:00:00 TBD
