Ecogia Science Meetings
Past Ecogia Science Meetings (2019)
Ecogia Science Meetings 2019
Schedule
Meeting are at 14 local time in the "Castle" meeting room at Ecogia.
| Date | Speaker | Title |
| 2019-03-18 | Dominique Eckert | The empirical evidence for dark matter
The purpose of this talk is twofold. First, I would like to illustrate a possible way for people to use the science meeting as a way of passing along useful information to the group by reviewing the state of the art in a given topic. Second, and most importantly, I would like to use this opportunity to answer common criticisms from fellow scientists working in different fields when they learn that I am spending most of my time studying the astrophysical imprint of dark matter. Typical reactions include "dark matter is the new aether" or "personally I believe in MOND". I think that most of the time these reactions are bourne out of an incomplete knowledge of the existing phenomenology. I will start with a brief historical overview of how the missing mass problem was discovered. I will then review some of the numerous lines of evidence that exist today for the existence of dark matter. These include the CMB power spectrum, the mass profiles of halos, dissociative cluster mergers, gravitational lensing, baryon fractions, and the recently uncovered class of ultra-diffuse dwarf galaxies. Any theory aiming at explaining away the effects attributed to dark matter through a modification of gravity needs to be able to explain self-consistently all of these empirical lines of evidence, and I think it is fair to say that so far, none of the attempts ever came close.
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| 2019-03-25 | Nicolas Produit | Radiation damages
The principle of energy loss of charged particle will be reminded and consequences in electronic damage and biological damage will be discussed.
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| 2019-04-01 | Carlo Ferrigno | The continuum modelling of magnetized X-ray pulsars
I will review our current understanding of the spectral continuum in X-ray binary pulsars endowed in a high magnetic field (~10^12 G). In particular, the appearence of broad spectral features both in absorption and in emission is now considered as a key ingredient to describe any observation.
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| 2019-04-08 | Luciano Burderi [External Seminar] | The HERMES project (High Energy Rapid Modular Ensemble of Satellites)
I discuss how several of the proposed models for space-time quantization predict an energy dependent speed for photons. Although the predicted discrepancies with the general speed of light are minuscule, I discuss how it is possible to detect this intriguing signature of space-time granularity with a new concept of modular observatory for photons in the energy band 10 keV -- 30 MeV. This observatory may consist of a swarm of micro/nano- satellites on low orbits. Sub-microsecond time resolution and wide energy band allows to probe tiny energy dependent delays, expected to be the signature of the granular structure of space- time in several of the proposed theories of Quantum Gravity. Moreover this kind of experiment allows to perform temporal triangulation of high signal to noise impulsive events with positional accuracies of few arcseconds, making an observatory like that a promising hunter for the elusive electromagnetic counterparts of Gravitational Waves.
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| 2019-04-15 | Andrea Tramacere | Self-consistent modeling of particle acceleration and evolution of spectral distribution in blazars based on the JetSeT code
JetSeT is an open source Python framework with a C numerical engine, to reproduce radiative and accelerative processes acting in relativistic jets, allowing to fit the numerical models to observed data (git repo: https://jetset.readthedocs.io/en/latest/ doc: https://github.com/andreatramacere/jetset). I have developed this code during the last 15 years of my research activity on blazars, and now it is released as a public open source project. I will use JetSeT to review some of the main phenomenological signatures of the stochastic acceleration acting in the relativistic jets of blazars. In particular I will link predictions from Monte Carlo simulations and from the numerical solutions of the diffusion equation in momentum space, to the spectral features observed in the multi-wavelength SED of blazars. I will focus on the spectral evolution and spectral curvature in the X-ray/hard X-ray data and in the TeV data, and the implications on the acceleration mechanisms. Finally, I will discuss the formation of pile-up during strong flares.
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| 2019-04-22 | Holidays | |
| 2019-04-29 | Etienne Lyard | Neural networks for the Gamma/Hadron Separation of the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) will be the largest ground-based gamma-ray observatory. CTA will detect the signature of gamma rays and cosmic rays hadrons and electrons interacting with the Earth's atmosphere. Making the best possible use of this facility requires to be able to separate events generated by gamma rays from the particle-induced background. Deep neural networks produced encouraging results, but so far there has been no evaluation of their performance for gamma/hadron separation with respect to well established approaches. In this talk I will briefly introduce deep learning and neural networks and compare convolutional neural networks with a standard analysis technique, namely boosted decision trees. We will use simulated observation data to do so and look at the Receiver Operating Characteristics (ROC) curves produced by the two approaches.
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| 2019-05-06 | Tom Orjollet—Lacomme | Gas and Dark Matter Distribution in Galaxy Clusters observed by XMM-Newton
The ΛCDM model (Λ Cold Dark Matter) is a model that aims to describe the history and the evolution of the universe. Its main hypothesis is the existence of a non-baryonic matter called dark matter, that only interacts gravitationally with baryonic matter. From this hypothesis, predictions can be made, one in particular is the validity of Navarro-Frenk-White (NFW) model which describes the dark matter distribution in a dark matter halo. To test this prediction, a galaxy cluster named PSZ2 G066.68+68.4 was studied. A galaxy cluster is an accumulation of hot gas and galaxies, composed of baryonic matter (15%) and non-observable matter (85%) which is supposed to be dark matter in the ΛCDM model. The goal was to calculate the mass profile then the dark matter profile (or the baryonic mass profile) and to compare it with the NFW model. The method used to calculate those mass profiles was to study the emission of the plasma of the cluster. With this information, temperature and density of the gas were calculated, which permitted to resolve Euler equation for the cluster and find the mass profile. The mass profile and baryonic mass profile found are coherent with the literature on the subject, and the dark matter profile was proven to be qualitatively close to a NFW model. Therefore, it is an argument in favor of the validity of the ΛCDM model.
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| 2019-05-13 | Christos Panagiotou | Reflection from AGN disk and surroundings, unification and the CXB
Compton thick (CT) AGN are predicted by unified models, which attribute most of the AGN diversity to their inclination, and play an important role for the understanding of the growth of black holes in the early Universe. The fraction of CT AGN at low redshift can be derived from the observed CXB spectrum assuming AGN spectral templates and luminosity functions. We show that high signal-to-noise average hard X-ray spectra, derived from Swift/BAT and NuSTAR, imply that mildly obscured Compton thin AGN feature a strong reflection and contribute massively to the CXB. Thus, a population of CT AGN larger than that effectively detected is not required. The strong reflection observed in mildly obscured AGN, even in individual NuSTAR observations, suggests that the covering fraction of the gas and dust surrounding their central engines is a key factor in shaping their appearance. In addition, NuSTAR observations of AGN show clearly that the reflection behaviour varies with the obscuration. The disk is found to be the main reflector in unobscured sources. Instead, obscured objects feature a correlation between reflection and column density, a characteristic of a clumpy reprocessing region located far away.
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| 2019-05-20 | Stephane Paltani | Self-consistent modeling of the environment of supermassive black holes
Supermassive black holes in active galactic nuclei are surrounded by matter organized in different structures like an accretion disk, a torus, etc. Radiation interacts with these structures essentially through the mechanisms of absorption, scattering and fluorescence, making X-rays in principle a fantastic probe of the geometry of these objects. The presence of several structures makes however the processes of up- and down-scattering particularly complex and geometry-dependent; such complexity cannot be captured in models. We present a general ray-tracing simulation framework, RefleX, which is able to simulate arbitrary geometries in a fully consistent way. RefleX includes most radiation processes taking place in the soft-to-hard X-ray regime. We demonstrate the capability of RefleX by determining the X-ray properties of a system consisting of an accretion disk and a torus simulated in a self-consistent way, starting from the thermal emission of the accretion disk.
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| 2019-05-27 | Canceled | |
| 2019-06-03 | Simonamichela Mazzola (visitor) | Decade-long analysis of spectral state changes in neutron star low mass X-ray binaries.
Low mass X-ray binaries (LMXBs) are binary systems in which a compact object, a black hole (BH) or a neutron star (NS), and a late-type companion star orbit each other around a common centre of mass. The spectral study of their X-ray emission is fundamental to understand the physics and the behaviour of the hosted compact object. Furthermore, it gives important information about the geometry of the systemas well as the physics of accretion processes. Since the total emitted spectrum of LXMB sources results in an extremely complex combination of several phenomena, the broad-band spectral analysis is one of the most powerful methods of investigation to study their physical scenario.
Most of these sources show several spectral variations over time, related to the variation in luminosity and, most substantially, in the mass accretion rate. Generally, we distinguish two main classes of spectral states, called soft and hard. The spectral state of a NS-LMXB source moves from hard to soft (and vice-versa) over time and the detection and recognition of any transitional states is actually a challenging task. Recently, an hysteresis-like pattern was observed in the hardness-intensity diagram and rms variability-intensity diagram of some of these sources. These systems, in fact, seem to evolve from hard state to soft state and come back to the former state following a specific evolution track. Since these sources seem to follow similar pattern, we can expect to recover the same cyclical behaviour studying their spectral variability on long temporal baseline. Then, we used the INTEGRAL data collected in the last sixteen years (from 2002 to 2018) to reproduce the HID for a sample of persistent NS-LMXBs. We extracted a broadband spectrum from several regions of the hardness-intensity diagram for each source in order to understand their spectral variations and highlight the different spectral states which each source goes through, searching the average properties in the different states for each one. Our first results encourage us to extend this study to all the persistent NS-LMXBs (about 30 sources) detected by INTEGRAL. |
| 2019-06-10 | Holidays | |
| 2019-06-17 | Margaret Lazzarini (visitor) | Multiwavelength Studies of High Mass X-ray Binaries in Nearby Galaxies
Combining NuSTAR, Chandra, and HST imaging of the X-ray binary population in nearby galaxies, we are able to infer compact object type, spectral type of the donor star, and age using multiwavelength observations from NuSTAR, Chandra, and the Hubble Space Telescope (HST). The hard X-ray colors and luminosities from NuSTAR permit the tentative classification of accreting X-ray binary systems by compact object type, distinguishing black hole from neutron star systems. We identify UV-bright optical counterparts spatially associated with X-ray sources using high quality HST imaging. We perform spectral energy distribution (SED) fitting for the most likely optical counterparts to the HMXB candidates to infer the most likely stellar mass and temperature for the companion stars. We can also obtain high quality age estimates using spatially resolved star formation histories. We plan to combine this multiwavelength analysis for three nearby galaxies: M31, M33 and the SMC in order to probe a wide range of star forming environments; both star formation histories and metallicities. We have already completed this analysis for a subset of the star forming disk of M31, are currently working on deep NuSTAR observations of three fields in the SMC, and plan to add analysis of new NuSTAR and HST observations of M33 to our sample.
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| 2019-06-24 | Vitalii Sliusar | 5.5 years multi-wavelength variability of Mrk 421: evidence of leptonic emission from radio to TeV
Mrk 421 is a high-synchrotron-peaked blazar featuring bright and persistent GeV and TeV emission. We use multi-wavelength light-curves of Mrk 421 spanning 5.5 years with FACT (TeV) and Fermi LAT (GeV) in the gamma rays, Swift BAT, Swift XRT and MAXI in the X-rays, together with optical and radio data and investigate the physical processes driving the emission and variability. Observations by FACT are continuous and not triggered, so the source was found in a wide range of flux states and more than 30 flares were identified. The TeV and X-ray light-curves feature very similar flares with rise and decay times of a few days and zero lag, characteristic of electron processes. At least two parameters per flare, the amplitude and the cut-off energy, are required to explain the observed variability. The GeV light-curve leads and is strongly correlated with the optical and radio light-curves as expected from SSC emitting shock propagating in a conical jet.
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| 2019-07-01 | Laurent Eyer Krzysztof Nienartowicz |
The variable celestial objects detected by Gaia and their data platform
We will present the data analysis which is done in the consortium on time series analysis and the associated supplementary observations from ground based telescopes. We present recent results from the published DR2 data on the variability of white dwarfs. Some insight on a unique data platform developed and used by CU7/DPCG, with on a parallel-database Postgres-XL at core, will be given.
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