eXtreme19

22 Jan 2019, 10:00 → 25 Jan 2019, 13:20 Europe/Rome

Polo di Psicologia - room 3I (3rd floor, building 2) (Padova)

Elisa Prandini (PD), Jonathan Biteau (IPNO, Univ. Paris-Sud, Univ. Paris/Saclay, CNRS/IN2P3), Luca Foffano (UNIPD - DFA)

eXtreme19_Sci_Intro.pdf

Slides_welcome_new.pdf

Tuesday, 22 January

Registration

Welcome from the organizers

eXtreme19_Sci_Intro.pdf

Slides_welcome_new.pdf

Introduction

Convener: Elisa Prandini

1 An Introduction to Extreme Blazars

Extreme blazars are characterized by the highest SED peak energies of the whole blazar population, either by synchrotron or inverse Compton emission (if it is inverse Compton at all), or both. Their peak energies surprised us reaching values up to or beyond 100 keV and 10 TeV, respectively, more than 2 orders of magnitude higher than run-of-the-mill HBLs. Over the past decade, observations with Cherenkov telescopes have revealed several of these objects, with hard VHE spectra. Although they allowed crucial limits to be derived for the EBL and intergalactic magnetic fields, their SED is still difficult to explain with standard scenarios. The origin of their gamma-ray emission remains elusive, but possibly indicative of different physical processes. With hindsight, I review what we know so far about this challenging type of blazars, in particular how they came to be recognized, their look and behaviour, why they are problematic and, last but not least, how to find them.

Speaker: Dr Luigi Costamante (ASI)

costamante_extreme19ok.pdf

12:30 Lunch

The population of blazars

Convener: Elisa Prandini

2 Extreme blazar samples: the case of the 3HSP catalogue

The number of known extreme blazars is still quite low, hampering a detailed study of this class of sources. I will first review what has been available so far and then present the extreme blazar sample from the 3HSP catalogue (Chang, Arsioli, Giommi, Padovani, & Brandt, 2019), which includes 344 sources with nu_peak > 10^17 Hz. I will then discuss in some detail their observational properties, with a look towards future facilities.

Speaker: Dr Paolo Padovani (ESO)

Padovani_extreme19.pdf

3 The 2BIGB gamma-ray catalog: Extreme & High Synchrotron Peak 3HSP Blazars newly detected over 10 years of Fermi-LAT observations

Here we present a large-scale likelihood analysis over all objects from the Third High Synchrotron Peak blazar (3HSP) sample, which includes 2011 sources. A total of ~1170 gamma-ray signatures are detectable down to a 3 sigma threshold, which includes ~180 new sources having no counterparts in previous Fermi-LAT catalogs, including the 1-2-3FGL, the 1-2-3FHL, and the most recent FL8Y. The gamma-ray binned analysis integrates over 10 yrs of Fermi-LAT observations at the 500 MeV to 500 GeV energy window. We build the gamma-ray spectral energy distribution for the entire sample, delivering a relevant high-energy spectral description of gamma-ray blazars, with a focus to HSP and Extreme HSP blazars. This sample is entitled as Second Brazil-ICRANet Gamma-ray Blazar catalog, holding the acronym 2BIGB, and follows as an all-sky implementation from previous works related to the 1BIGB catalog. All information will be at disposal via the Brazilian Science Data Center BSDC portal, including the broadband power-law fits and the spectral energy distribution (SED) data points.

Speaker: Dr Bruno Sversut Arsioli (Instituto de Fisica Gleb Wataghin, Univ. Estadual de Campinas, UNICAMP - IFGW, Campinas, Brazil)

BrunoArsioli-Extreme19.pdf

4 Radio core dominance in Fermi blazars

I would like to present our previous work on the radio core dominance in Fermi blazars. We compiled a sample of 4437 sources with available radio core-dominance parameters defined as the ratio of the core flux densities to extended ones, namely, R = Score/Sext., which includes 696 Fermi-detected sources respect to the catalog of FL8Y, the first eight years of science data from the Fermi Gamma-ray Space Telescope mission, and the rest of them are non-Fermi-detected sources. In our sample, 636 blazars are Fermi-detected and 1294 are not, and also consists of other subclasses such as Seyfert, Fanaroff-Riley I/II and normal galaxies. We investigate various different properties between Fermi-detected sources and non-Fermi-detected ones by using the core-dominance parameters as the previous study has shown that R is a good indication of beaming effect. We then calculate the radio spectral indices for whole sample and adopt γ-ray photon indices for Fermi sources from FL8Y catalog for discussing their different performances on different subclasses, and obtain the relation between core-dominance parameters and radio spectral indices for both Fermi and non-Fermi sources according to the two components model on radio band, which are consistent with our previous study. We found that the core-dominance parameters and radio spectral indices are quite different for different subclasses of AGNs, not only for Fermi sources but also non-Fermi sources, particularly, R for the former ones is averagely higher than later ones. We also adopt the same relation on core-dominance parameters and γ-ray photon indices for Fermi sources by taking the same assumption with two components model on γ-ray band, and obtain the fitting results indicating that the γ-ray emissions of Fermi blazars are mainly from the core component, which is perhaps associated with the beaming effect. Therefore, Fermi blazars are beamed.

Speaker: Zhiyuan Pei (PD)

Zhiyuan Pei.pdf

5 Search for high-redshift blazars with Fermi/LAT

High-redshift blazars (z ≥ 2.5) are one of the most powerful classes of gamma-ray sources in the Universe. These objects possess the highest jet powers and luminosities and have black-hole masses often in excess of 10^9 solar masses. In addition, high-redshift blazars are important cosmological probes and serve as test objects for blazar evolution models. Due to their large distance, their high-energy emission peaks are often downshifted to energies below the GeV range, which makes them difficult to study with Fermi/LAT and only the very brightest objects are detectable. Hence, only a small number of high-redshift blazars could be detected with Fermi/LAT so far. In this work, we present a strategy to significantly increase the detection statistics at redshift z ≥ 2.5 via a search for flaring events in high-redshift gamma-ray blazars whose long-term flux is just below the sensitivity limit of Fermi/LAT. Seven previously GeV undetected high-redshift blazars have been identified from their bright monthly outburst periods, while more detections are expected in the future.

Speaker: Mr Michael Kreter (North-West University)

high-z_blazars_kreter_V2.pdf

15:50 Coffee break

The population of blazars

Convener: Elisa Prandini

6 The gamma-ray properties of a sample of low-redshift BL-Lacs

With its discovery and characterization of several hundred sources, Fermi-LAT has revolutionized our knowledge and understanding of the BL Lac population with respect to other AGNs. The multi-wavelength picture has, however, not kept up the pace of the γ-ray observations. We have selected an unbiased sample of 42 nearby (z<0.2) BL Lacs located within the SDSS footprint, independent of their flux density. We have analysed data collected by the Fermi Large Area Telescope (LAT) during its first 8.5 years of operation in the energy range 0.1 − 300 GeV. We investigate the high-energy properties of the BL Lacs, and in particular the distribution of their γ-ray flux and photon index, and the connection between the γ rays and VLBI properties. The LAT-detected BL Lacs seem composed primarily of "classical" sources dominated by Doppler boosting and characterized by compact and bright radio emission as well as hard γ-ray spectra. However, three LAT-detected sources show non-classical properties for a γ-ray emitting BL Lac. Further details of the results will be presented.

Speaker: Silvia Raino' (BA)

Extreme19_SRaino.pdf

7 Te-REX: a sample of extragalactic TeV-emitting candidates

We present a reference sample of potential TeV emitters, likely to be found in the next CTA extragalactic survey, selected from the REX catalogue. The REX project (Radio-Emitting X-ray sources) is one of the largest and best blazar collection produced by combining radio and X-ray data. Specifically, we focus here on the high-energy peaked BL Lac (HBL) population which are the expected most numerous class of TeV emitting sources. To select these objects we have considered only the REX with the highest X-ray-to-radio flux ratio (αlpha_RX< 0.73) and B<21 (to exclude sources with z>0.5, where the EBL should preclude the possibility to detect them with CTA). We call this sub-sample Te-REX, for TeV- candidate REXs. We have been recently awarded of 35 hours at TNG/DOLORES to complete the spectroscopic identification of the Te-REX sample in the sky area that will be covered by the CTA extragalactic survey. Our sample constitutes an alternative and more complete approach with respect to other samples based on FERMI catalogues to carry out quantitative and unbiased studies of the TeV properties of this extreme class of blazars.

Speaker: Dr Barbara Balmaverde (INAF - Osservatorio astronomico di Brera)

balmaverde_ExtremePadova.pdf

8 Extreme blazars and their TeV gamma-ray emission: are they a unique population?

The spectral energy distribution of blazars is dominated by non-thermal emission from the jet and consists of two main broad humps. For the extreme blazars, these two components peak in the X-ray and GeV-TeV bands, respectively. Although the number of TeV detected extreme blazars is currently very limited, recent observations have revealed that in a few of them the energy of the second peak exceeds several TeV (e.g. 1ES 0229+200). In this contribution, we present a study of the TeV spectra of EHBLs, showing that a possible zoology could emerge in this category of objects. Furthermore, we show the results of a search for new EHBL candidates aimed at increasing their statistics. We selected them on the basis of hard X-ray and GeV gamma-ray emission, studying their detectability for current and future TeV gamma-ray telescopes.

Speaker: Luca Foffano (PD)

FOFFANO-extreme19.pdf

Poster session and welcome spritz

9 Fermi Source Classification with Machine Learning Methods

We report our study on the classification of the unassociated sources in 3FGL with Ensemble Machine Learning (EML) method. The two main objectives of our research are: 1)to categorize the unknown sources into AGN and PSR, 2)to identify BCUs to be BL Lacs and FSRQs. Our final purpose is to take advantage of the EML method to obtain a more complete category of the Fermi sources. The experiments demonstrate that our algorithms can effectively predict the 1010 unknown sources to be 867 AGNs and 143 PSRs, with an accuracy of 99.48%. The original 573 BCUs are clarified to be 341 BL Lacs and 232 FSRQs, the accuracy is 89.80%.

Speaker: Mr Hubing Xiao (Department of Physics and Astronomy, University of Padova)

10 Flares on minute time-scale in Blazars by mirroring plasmoids

Bright and fast gamma-ray flares have been recently detected from the Blazar 3C 279, with GeV luminosities up to 10^49 erg/s. The source is observed to flicker on timescales of minutes with no comparable optical-UV counterparts. Such observations challenge current models of high-energy emissions from Blazar sources that are dominated by relativistic jets along our line of sight with bulk Lorentz factors up to Gamma \sim 20$. We discuss a model based on a jet structure comprising strings of plasmoids as indicated by many radio observations. We follow the path of the Synchrotron radiations emitted in the optical - UV bands by relativistic electrons accelerated around the plasmoids to isotropic Lorentz factors gamma \sim 10^3. These primary emissions are partly reflected back by a leading member in the string that acts as a moving mirror for the approaching companions. In the inter-plasmoid, shrinking gaps transient overdensities of seed photons build up. The electrons then proceed to upscatter these seeds into the GeV range by the inverse Compton interactions. We show that such a combined process produces bright gamma-ray flares with little or none optical to X-ray enhancements. Main features of our model include: gamma-ray flares produced beyond the broad line region; Compton dominance at GeV energies by factors up to some 10^2; bright flares with risetimes as short as a few minutes, occurring at distances of order 10^18 cm from the central black hole; little reabsorption from local photon-photon interactions.

Speaker: Dr Valerio Vittorini (iaps inaf rome)

11 Modelling photon transport in the vicinity of neutrino source TXS 0506+056

In September 2017, the IceCube Neutrino Observatory detected an extremely-high-energy neutrino event, consisting of a muon coming from the bottom of the detector through the Earth, likely originated by a neutrino of energy of around 290 TeV. Promptly alerted, the Fermi LAT and MAGIC collaborations detected at more than 5σ a flare from the known gamma-ray blazar TXS 0506 +056, at a redshift ~0.34, within the region of sky consistent with the 50% probability region of the IceCube neutrino (of about one degree in size). The MAGIC detection allowed to determine a cutoff for the electromagnetic emission at ~400 GeV. The associated emission of gamma rays and neutrinos from the same source hints at an “hadronic mechanism” for the production of high energy cosmic rays.

In this paper we present a characterization of the photon fields and photon transfer in the vicinity of blazar TXS 0506+056 in order to characterize the properties of the emitting region. This was done under the assumptions of a true association between the detected neutrino and the active state blazar and that the neutrino and electromagnetic emission arise from the same spatial region.
The produced photon spectrum is expected to be similar to that of neutrinos, but photon energy is likely to degrade due to interactions, both with nearby fields and, along their path to Earth, with the extragalactic background light (EBL). Taking as a starting point the reported energy difference between the neutrino and electromagnetic components, a Monte-Carlo simulation of the evolution of an electromagnetic shower in the appropriate environment was implemented in order to extract the relevant features of the photon transport. The results can be used to constrain the size of the emitting region, a crucial parameter for many models of the electromagnetic emission of blazars.

Speaker: Laura Olivera-Nieto (Università di Padova, Università degli Studi di Roma "Tor Vergata")

12 Polarization maps from synchrotron sources associated with magnetohydrodynamic blazar jets

Very long baseline interferometry offers high resolution images of parsec scale structures of AGN jets. These maps along with systematic observations and polarization measurements, when associated with synchrotron radiation from highly energetic electrons, provide significant information on spatial and dynamical evolution of the emitting regions. By modeling the physical conditions in blazar jets it is possible to reproduce some of the polarization characteristics observed, attributed to opacity effects and field topology, and thus to make assumptions for significant physical parameters such as particle distribution and magnetic field structure. Using a light tracing code and taking into account absorption and propagation effects , we produce resolved maps from semi-analytical and simulated magnetohydrodynamic jets, considering linear and circular polarization.

Speaker: Mrs Stefania Kerasioti (National &amp; Kapodistrian University of Athens)

13 Results on blazar monitoring with HAWC

The High-Altitude Water Cherenkov (HAWC) observatory is a wide field-of-view instrument under operations since March 2015 and located in the state of Puebla, Mexico. HAWC observes two thirds of the sky daily at energies between 0.1 and 100 TeV with a duty cycle greater than 95%. This capability allows us to monitor unbiasedly known AGNs as the blazar Mrk 421 or the radio galaxy M87, to search blindly for TeV flares and to follow up on external alerts as neutrino ones from Ice Cube. In this work, highlights from our on-going monitoring program of AGNs is presented.

Speaker: Dr Maria Magdalena Gonzalez (Universidad Nacional Autonoma de Mexico)

14 Simulation of an IXPE observation of Centaurus A

X-ray polarimetry is entering in a golden age thanks to the technological maturity of polarimeters involving Gas Pixel Detectors and the study and development of dedicated space missions, such as IXPE by NASA and ASI and eXTP by CSA. A simulation of an IXPE observation of the radio galaxy Centaurus A is presented. Centaurus A is the nearest AGN and is included in the observing plan of IXPE that will provide for the first time the spatially resolved polarimetric study in the X-ray domain of its relativistic jet, enabling the study of the particle acceleration process. The polarimetric properties of different physical processes have been simulated and the results discussed in light of the Minimum Detectable Polarization. The outcome of this work will be presented in view of the IXPE observing plan in preparation.

Speaker: Dr Riccardo Ferrazzoli (INAF-IAPS)

15 Stability of relativistic magnetized jets

Astrophysical jets are collimated outflows extending in length several times their radii. Even though current -- driven and Kevin -- Helmholtz instabilities form along the flow, jets are observed to retain their shape almost intact. The jet dynamics are described by the ideal MHD equations and the stability is studied through a linear analysis, i.e., we introduce perturbations of the physical quantities to the MHD equations. We proceed with the numerical integration of the perturbed system, producing solutions which provide the characteristic growth rates of the perturbations. Finally, we will elaborate on the results and outline the main conclusions focusing on their dependence on the bulk Lorentz factor and the current distribution.

Speaker: Mr Charalampos Sinnis (Section of Astrophysics, Astronomy & Mechanics, Department of Physics, School of Sciences, National and Kapodistrian University of Athens)

Relativistic_jets_3col.pdf

16 Theoretical models to explain the TeV gamma-ray and X-ray correlations exhibited in blazars

The broadband spectral energy distribution (SED) of blazars has two well-separated peaks, one of low energy at soft X-rays and the other of high energy at hundreds of GeVs. The SED of blazars is generally understood through the standard one-zone synchroton self-Compton (SSC) model, where a strong correlation between X-ray and TeV gamma-ray fluxes is expected. During the last decade, a correlation among X-ray and TeV bands has been searched but not undoubtedly confirmed. Some studies have suggested serious deviations from the expected leptonic correlation. In this work, we propose a theoretical model to study the correlation between the TeV gamma-ray and X-ray emission. This leptonic model depends basically on the bulk Lorentz factor, the size of emitting region, the electron number density and the strength of the magnetic field. We explore regions of the parameter space where the current model can describe a unique correlation.

Speaker: Dr Nissim Fraija (Instituto de Astronomia - UNAM)

Wednesday, 23 January

Observational results

Convener: Dr Paolo Padovani

17 The extreme character of our closest VHE blazars, Mrk421 and Mrk501

Owing to their brightness and proximity (z=0.03), Mrk 421 and Mrk 501 are among the blazars that can be studied with the greatest level of detail; and hence a sort of astrophysical laboratories to study the blazar's phenomena. In the conference I will report recent highlight results obtained from multiwavelength campaigns on these two objects. I will show that they can change their personality from one season to the next, sometimes showing a remarkably extreme character in the broadband SED, as well as in the variability and Doppler factors that are measured or inferred from the data. I will report detailed observational and theoretical results related to the 2-week long highest X-ray activity in Mrk501 observed with Swift-XRT since its launch 14 years ago, which suggests the presence of narrow spectral components at TeV energies. And I will also show an unprecedented correlation study of VHE gamma-rays and X-rays for Mrk421, revealing a large degree of complexity in these relations when quantified on different temporal and energy bins, and supporting the presence of multiple components in the X-ray and VHE gamma-ray emission. These multi-instrument observations on Mrk421 and Mrk501 have yielded thought-provoking results, and demonstrate the importance of performing a continuous monitoring over multi-year timescales to fully characterise the dynamics of blazars.

Speaker: Dr David Paneque (SLAC/Kipac)

Extreme19_Paneque_Final.pdf

18 An unprecedentedly hard spectrum in VHE for the Extreme HBL 1ES 2344+51.4

The BL Lac object 1ES 2344+51.4 was one of the first sources to be included in the EHBL (Extreme High-peaked BL Lac) family: its broadband Spectral Energy Distribution (SED) features the synchrotron peak in the hard X-ray band, and from previous studies of the object in the VHE (Very-high-energy, E>100GeV) gamma-ray range, its Inverse Compton (IC) peak is expected at ~200GeV. 1ES 2344+51.4 has been object of several campaigns in the past due to its peculiar variability in the X-ray and VHE bands. In 2000, observations with Beppo-SAX revealed a large 0.1-10 keV flux variability on the timescales of a few hours, in particular during a strong flare. It was suggested that one electron population is responsible for the steady low energy synchrotron emission and another electron component producing higher energy X-rays with high time variability. The latter component should be responsible for the VHE emission via IC scattering. We present the most recent multi-wavelength study of the EHBL 1ES 2344+51.4, detected in an enhanced state in VHE band by the MAGIC and FACT telescopes in August 2016: high sensitivity observations were triggered by the monitoring carried out with FACT at TeV energies. The analysis of MAGIC, FACT and Fermi spectra constrains the IC peak of the SED, revealing an unprecedentedly hard spectrum in the HE and VHE bands and allowing us to study its variability at the highest energies. These data, together with simultaneous multifrequency data, (Swift-XRT, OVRO, KVA), will be used to model the emission of this peculiar EHBL during this particularly high state.

Speaker: Dr Marina Manganaro (University of Rijeka)

1ES2344 _v3.pdf

19 Variability study of extreme blazars with VERITAS

The VERITAS array of imaging atmospheric Cherenkov telescopes has collected nearly 5000 hours of active galactic nuclei (AGN) observations. It has detected 39 very-high-energy (VHE, >100 GeV) AGNs at redshifts up to z = 0.9, of which 24 are classified as high-frequency-peaked BL Lacertae objects (HBLs). VERITAS has obtained an extensive dataset of HBL and extreme HBL (xHBL) observations, with light-curves spanning up to 11 years, allowing the characterization of their long-term and short-term variability. The results of ~56 hours of 1ES 0033+595 observations are reported, including an estimate of its redshift and a multi-wavelength spectral analysis, confirming its classification as an xHBL. A study of the variability of 1ES 0033+595 and various other xHBLs/HBLs in the VERITAS dataset is presented and the correlation with other energy bands is tested. In particular, the short-term variability of xHBLs as a function of energy in the VHE band is examined. The study explores the presence of secondary gamma rays produced in cosmic-ray interactions with background photons.

Speaker: Orel Gueta (DESY)

eXtreme2019_variabilityStudy_veritas.pdf

20 All the MAGIC of extreme blazars

Extreme blazars (EHBLs) are generally characterized by a synchrotron peak located at energies > 1 keV in their SED, and by the hardness of their spectrum in the high-energy (HE, E > 100 MeV) gamma-ray range. So far, only a handful of these objects have been detected in very-high-energy (VHE, E > 100 GeV) gamma rays by imaging atmospheric Cherenkov telescopes (IACTs). Some such objects (e.g. 1ES 0229+200) have shown hard VHE spectra extending to several TeV, making their multi-wavelength emission difficult to interpret naturally with standard, one-zone synchrotron self-Compton models, unlike for non-extreme BL Lac objects. Their GeV-TeV spectra can also shed new light on the blazar sequence. Since 2010, eleven EHBLs have been observed in multi-wavelength campaigns involving the MAGIC IACTs, aiming to increase the number of known EHBL TeV-emitters. Five sources have been clearly detected by MAGIC. This contribution will present the results of these campaigns, including a comparison of the GeV-TeV behavior of these sources with that of the well-known, archetypal EHBL, 1ES 0229+200, as well as interpretations via models that invoke non-conventional mechanisms of electron acceleration.

Speaker: Mr Vandad Fallah Ramazani (Tuorla observatory)

EHBL_extreme19_V1.pdf

10:30 Coffee break

Observational results

Convener: Dr Paolo Padovani

21 On the the time-variable rotation measure in the core region of the TeV blazar Mrk 421

In this talk we will discuss the time variable rotation measure (RM) observed in the core region of the TeV blazar Markarian 421 (Mrk 421). These results are based on a one-year, multi-frequency (15, 24, and 43 GHz) Very Long Baseline Array monitoring campaign. We explore the possible connection between the RM and the accretion rate and we investigate the Faraday screen properties and its location with respect to the jet emitting region. Among the various scenarios, the jet sheath is the most promising candidate for being the main source of Faraday rotation. We interpret the two RM sign reversals observed during the one-year monitoring within the context of the magnetic tower models. We invoke the presence of two nested helical magnetic fields in the relativistic jet with opposite helicities, whose relative contribution produce the observed RM values. Additional scenarios are considered to explain the observed RM time evolution.

Speaker: Dr Rocco Lico (Max Planck Institute for Radio Astronomy (MPIfR))

Lico_Padova_2019.pdf

22 The magnetic field structure in CTA 102 from high resolution mm-VLBI observations during the high-energy flares in 2016-2017

Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to shed light on the crucial physical processes giving rise to the jet formation, as well as to their extraordinary radiation output up to gamma-ray energies. We study the variability and magnetic field structure of the quasar CTA 102 with 3 and 7 mm-VLBI polarimetric observations during a very active state of the source over the entire electromagnetic spectrum. Combining information from the polarization images with the Faraday rotation analysis between 3 and 7 mm we obtain the three-dimensional structure of the magnetic field in the innermost jet regions with unprecedented angular resolution. We compare the obtained rotation measure map with the polarization evolution in 7 mm VLBA images when a new superluminal component is ejected from the core and travels along the jet. At the same time, the source undergoes extraordinary multi-wavelength flares. We study the kinematics and variability at 7 mm and infer the physical parameters associated to the variability. We put the latter in context with the minimum Doppler factor value required to explain the observed high energy emission, as well as with the Doppler factor value needed to explain the increase of 6-7 magnitudes in optical reported in Raiteri C. et al., 2017, Nature, 552, 374.

Speaker: Dr Carolina Casadio (Max-Planck-Institut für Radioastronomie)

C.Casadio_Padova19.pdf

23 The jet parameters implied by the measured extreme brightness temperatures in BL Lac and 3C273

Recent observations of blazars with RadioAstron program revealed the jets with brightness temperatures exceeding the limiting equipartition value of 10^{11.5} K. This means that the jet physical parameters depart from their equipartition values. We propose that the non-equipartition jet parameters can be estimated by using the measurements of both the brightness temperature and the core shift effect. With these two measurements, we obtain the local magnetic field magnitude, particle number density, and magnetization of the flow. Results for BL Lac and 3C273 imply that the radiating plasma during the extreme flares must be magnetized very weakly.

Speaker: Dr Elena Nokhrina (Moscow Institute of Physics and Technology)

Nokhrina_Padova_2019.pdf

12:10 Lunch

Modeling

Convener: Dr Andrew Taylor

24 Particle acceleration in relativistic sources

The acceleration of charged particles to high energies in powerful astrophysical sources such as blazars represents a central question in modern multi-messenger astrophysics. Accelerated particles can interact with and radiate in ambient fields to produce secondary photon or neutrino fluxes, or escape from the source to become part of the cosmic ray spectrum. This presentation will discuss the physics of acceleration scenarios from first principles and address some of their consequences for blazar phenomenology.

Speaker: Dr Martin Lemoine (Institut d'Astrophysique de Paris)

Lemoine_eXtreme19.pdf

Lemoine_eXtreme19.pptx

25 Modeling extreme blazars - a review

The emerging class of extreme blazars, characterised by very hard high-energy spectra peaking around TeV energies, pose a challenge for the usual blazar emission models. While the multi-wavelength emission from high-frequency BL Lac type blazars can in generally be well interpreted with the single-zone SSC model, the very hard spectra of extreme blazars (or “ultra-high frequency peaked BL Lac objects”) push the model parameters to the limits of what is considered as physically acceptable. I will attempt to provide an overview over the leptonic and (lepto-)hadronic scenarios that are currently being proposed to interpret the observed emission from these objects without depending on parameters that are too extreme.

Speaker: Dr Andreas Zech (LUTH, Observatoire de Paris)

extreme2019_zech.pdf

26 Probing the dynamics of AGN jets with advanced semi-analytical modelling

The jets launched by accreting super-massive black holes can be some of the brightest sources in the high-energy sky; despite be- ing discovered decades ago, their physics and energetics are still poorly understood. The past decade has seen a dramatic improve- ment in the quality of available data, particularly in the X-ray and γ-ray bands. However, the semi-analytical modelling of jets has ad- vanced slowly, and simple one-zone models are still the preferred method of interpreting data, in particular for AGN jets. These models can roughly constrain the properties of jets but they can not unambiguously couple their emission to the launching regions and internal dynamics, which are usually probed with simulations. However, simulations are not easily comparable to observations be- cause they cannot yet self-consistently predict spectra. I will discuss our group’s ongoing efforts to develop an advanced semi-analytical model which accounts for the dynamics of the whole jet, starting from a simplified parametrization of Relativistic Magnetohydrody- namics in which the magnetic flux is converted into bulk kinetic energy. We apply our model to state-of-the-art datasets of the BL Lac PKS 2155−304 and of the radio galaxy M87 together with a thorough exploration of parameter space. Compared to previous modelling efforts this approach produces some of the best SED fits for these sources to date and provides stronger constraints on jet physics.

Speaker: Mr Matteo Lucchini (Anton Pannekoek Institute for Astronomy)

MatteoLucchini.pdf

27 Stationary shocks in HBLs, a solution to the bulk Lorentz factor crisis

Most of the TeV high-frequency-peaked BL Lacs (HBLs) show stationary or slow motions of their VLBI radio-knots, in stark contrast with their fast variability. This problem, known as "the bulk Lorentz factor crisis," indicates that the VLBI motions of these sources are not showing the real flow propagation of the jets. I will present evidence that these stationary radio knots are recollimation shocks, through which material is flowing with much larger velocity than inferred from apparent motion of the knots. Focusing of the well known HBL Mrk 421, I will describe an innovative way to measure the actual apparent flow speed from the study of long-term X-ray lightcurves. Finally, I will show the promising future applications of the multiple-shocks scenario, in terms of size and speed of jet perturbations and the possibility of particle re-acceleration.

Speaker: Dr Olivier Hervet (UC Santa Cruz)

19_01_shocks_HBLs_Hervet_Padova.pdf

16:00 Coffee break

Modeling

Convener: Dr Andrew Taylor

28 MHD Accretion Disk Winds and the Blazar Sequence

The launch of Fermi produced a significant number of AGN detections to allow statistical treatment of their properties. It confirmed the Blazar Sequence" established by EGRET and indicated a potentially novel one, namely that of theBlazar Divide" in FSRQs and BL Lacs according to their gamma-ray spectral index and luminosity. An MHD accretion wind model that describes the distribution of matter and magnetic fields in AGN over 5-6 decades in radius and provides a successful account of the Seyfert X-ray absorber properties, provides the vestiges of an account of the observed blazar classification in terms of a single parameter. We propose a model which reproduces in detail the broadband blazar spectra and their statistical properties in terms of the physical parameters of these MHD winds.

Speaker: Ms Stella Boula (National and Kapodistrian University of Athens)

Boula_eXtreme19.pdf

29 On the relation between Compton dominance and jet magnetization in blazars

Compton dominance, the observed ratio between the high-energy (assumed to be due to inverse Compton) and synchrotron components of the spectral energy distributions of blazars, is an indirect probe of magnetization of their relativistic jets. For most blazars, the Compton dominance is observed in the range 0.3-10, and in the framework of one-zone leptonic models it can be explained by mildly sub-equipartition magnetic fields (Nalewajko & Gupta 2017, A&A 606, A44). However, in the case of extreme BL Lacs, extremely low magnetizations and very high Doppler factors are inferred despite typical values of Compton dominance, mainly due to very inefficient IC scattering in deep Klein-Nishina regime (Tavecchio & Ghisellini 2016; Costamante et al. 2018). It is a mystery why should these blazars, observationally appearing to be a smooth continuation of classic BL Lacs, be powered by very different jets.

Speaker: Dr Krzysztof Nalewajko (Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences)

nalewajko_extreme19.pdf

30 Blazars under the lens of future X-ray polarimeters: perspectives for the IXPE mission

With the advent of the IXPE (Imaging X-ray Polarimetry Explorer) satellite
expected for launch in 2021, the study of the
X-ray polarization properties of several bright nearby sources
will become possible. Blazars are obvious candidates for X-ray
polarization observations.
For instance, a high degree of X-ray polarization is expected from
high-peaked blazars (HBL) because synchrotron radiation in an ordered
magnetic field
is naturally polarized. Moreover, X-ray polarization observations
have the potential of discriminating between different scenarios for the
origin
of the X-ray emission in low peaked blazars (LBL) objects.
In this talk, we discuss the perspectives of IXPE observations of blazars,
focusing on polarization modeling of different classes of BL Lac objects.

Speaker: Dr Laura Di Gesu (Italian Space Agency (ASI))

Extreme2019_230119_ldigesu.pdf

17:30 Social event: Museum of History of Physics

Thursday, 24 January

Modeling

Convener: Dr Fabrizio Tavecchio

31 Stochastic acceleration in blazars: theory and phenomenology with a focus on the spectral curvature and pile-up in the X-ray and TeV data

I will review some of the main phenomenological signatures of the stochastic acceleration acting in the relativistic jets of blazars. I will link predictions from Monte Carlo simulations and from the numerical solution of the diffusion equation in momentum space, to the spectral features observed in the multi-wavelength SED of blazars. In particular, I will focus on the spectral evolution and spectral curvature in the X-ray and in the TeV data. Finally, I will discuss the formation of pile-up during strong flares.

Speaker: Dr Andrea Tramacere (Astronomy Department of the University of Geneva,)

tramacere_extreme19_Padova_01_24_2019.pdf

32 Radiative Signatures of Diffusive Shock Acceleration in Blazar Jets

Diffusive shock acceleration (DSA) at mildly relativistic shocks has been argued to be an efficient mechanism for the acceleration of ultrarelativistic particles blazar jets. Here we present results of coupled time-dependent simulations of diffusive shock acceleration and production of synchrotron and Compton emission to model the expected spectral variability signatures of shocks in blazar jets. Test cases for both extreme HBLs and low-frequency peaked blazars are presented, including predictions for multi-wavelength light curves, inter-band cross-correlations and multi-wavelength spectral hysteresis.

Speaker: Prof. Markus Boettcher (North-West University)

eXtreme19_Boettcher_shocks.pdf

eXtreme19_Boettcher_shocks.pptx

33 Radiative Signatures of Relativistic Reconnection in Blazar Jets

Relativistic magnetic reconnection is a strong candidate for the acceleration of particles to high-energies and is a proposed mechanism for the multi-wavelength spectral and temporal variability observed in blazar jets. By coupling recent two-dimensional particle-in-cell simulations of relativistic reconnection with a time-dependent radiative transfer model, we compute the non-thermal emission from a chain of plasmoids, namely quasi-spherical blobs of plasma containing relativistic particles and magnetic fields formed during a reconnection event. Here, I will show that our derived photon spectra display characteristic features observed in blazar subclasses, with a differentiation achieved by varying the strength of the photon fields external to the jet, the jet magnetization, and the number of electron-positron pairs per proton contained within. For our modeling of BL Lac sources, we find an extended synchrotron component, with peak energies reaching a few keV, along with frequent gamma-ray flares with energies exceeding 1 TeV, with potential applications to extreme blazars. Finally, I will present several observational signatures of our model including the statistical properties of plasmoid-powered flares, the correlation of flaring events in multi-wavelength bands, and the power-spectral density of our reconnection driven light curves.

Speaker: Dr Ian Christie (Northwestern University)

xtreme_blazar_presentation_Ian_Christie.pdf

34 Application of hadronic supercriticalities to extreme Blazars

Hadronic models of high-energy AGN emission become more relevant than ever before after the latest IceCube results that associate a gamma-ray flaring AGN with high-energy neutrinos. Despite offering a unique framework for the production of high-energy neutrinos, hadronic scenarios have been criticized in the past for being radiatively inefficient. However, an overlooked property of hadronic models is that they can become ``supercritical'' by abruptly transforming the energy stored in relativistic protons into radiation. Supercriticality manifests itself when the proton density exceeds a critical value that depends on source properties. For even higher proton densities, we show that energetic high-energy flares can be produced within a few source light crossing times. For source radii and magnetic fields that are of relevance to high-frequency peaked blazars, and in particular, to extreme blazars, we first examine the parameter space that could drive the system to the supercritical regime. We then present results about the multi-wavelength flaring activity and discuss the expected neutrino signals.

Speaker: Ms Ioulia Florou (National & Kapodistrian University of Athens)

FLOROU_hadronic.pdf

10:20 Coffee break

Connection to neutrinos

Convener: Dr Fabrizio Tavecchio

35 How IceCube detects neutrinos and the first candidate source: a blazar.

Speaker: Elisa Resconi (Technical University of Munich)

RESCONI_Extreme2019.pdf

36 Implications of the observation of high-energy neutrinos in the direction of TXS 0506+056

The IceCube Collaboration recently announced the detection of a high-energy astrophysical neutrino consistent with arriving from the direction of the blazar TXS 0506+056 during an energetic gamma-ray flare, and a “neutrino flare” from the same direction during 2014-15. I will present some of the inferences drawn from these observations for neutrino emission from TXS 0506+056, in light of the characteristics of this source. Additionally, implications for neutrinos from blazar flares in general will be presented.

Speaker: Dr Foteini Oikonomou (ESO)

extreme19_oikonomou.pdf

37 Photo-Hadronically Produced IceCube Neutrinos from TXS 0506+056?

We present a systematic study of the feasibility of photo-hadronic production of IceCube neutrinos in the jet of the BL Lac object TXS 0506+056 during the extended 2014 - 2015 neutrino flare. Starting with the measured neutrino flux and spectrum, we study the effect of electromagnetic cascades initiated by pion decay, necessarily accompanying neutrino production. Depending on the relative energy densities of the co-moving target photon fields and the magnetic field, these cascades can either be synchrotron or Compton dominated. We find that the synchrotron-dominated case can be ruled out by existing observational constraints from TXS 0506+056, while in the case of Compton dominated cascades, the opacity of the emission region to gamma-gamma absorption is so high that any emanating gamma-ray flux is expected to be far below the observed Fermi spectrum. A further study of possible target photons for such a scenario indicates that it requires a UV - X-ray target photon field that is stationary in the rest frame of the AGN.

Speaker: Prof. Markus Boettcher (North-West University)

eXtreme19_Boettcher_TXS0506.pdf

eXtreme19_Boettcher_TXS0506.pptx

38 Exploring the activity of extreme Blazars and their observability through a neutrino telescope

Among the various categories of Active Galactic Nuclei (AGN), Blazars are the most promising sources of TeV/PeV neutrinos, due to their high luminosity at very high-energy (VHE) range during the active periods. After the multi-wavelength observation of the TXS0506+056, with a extreme high energy (EHE) event of IceCube followed-up by a significant flaring emission registered by Fermi-LAT and MAGIC observatories, this hypothesis became even more solid. Here we present a sample of Bl-lacs and flat spectrum radio quasars (FSRQ), from the 3FHL and 3FGL catalogs, spatially connected with the high energy starting events (HESE) track-like type and EHE events, for the study of their variability as well as the luminosity during their flaring activity. A specific focus is dedicated to the extreme Blazar TXS0506+056 with the analysis of the HE duty cycle and the spectral energy distribution (SED) features. The variability and the luminosity of these extreme sources are then correlated, through a hadronic emission process, with the possibility to observe them with a VHE neutrino telescope.

Speaker: Dr Antonio Marinelli (INFN Pisa)

A_Marinelli_presentation_extreme.pdf

12:40 Lunch

Connection to neutrinos

Convener: Dr Jonathan Biteau (IPNO, Univ. Paris-Sud, Univ. Paris/Saclay, CNRS/IN2P3)

39 3LAC counterparts to IceCube neutrinos above 100 TeV

High-fluence outbursts in coincidence with IceCube neutrino events have been found for the blazars PKS 1424-418 and TXS 0506+056. Here, we study all 179 3LAC blazars consistent with HESE IceCube neutrinos above 100 TeV and below 1 PeV. Adopting the IceCube neutrino spectral index, we compute the expected neutrino event numbers. We emphasize that the calorimetric photon energy fluxes obtained by integrating over the spectral energy distribution differ by a large margin from the Fermi-LAT fluxes alone. The resulting neutrino event numbers exceed the IceCube measurements. Therefore we find that if only 3% of the observed electromagnetic high-energy flux is of hadronic origin, blazars can explain all of the HESE events. We also note that the non-detection of many gamma-ray bright blazars is still in agreement with the hadronic blazar model.

Speaker: Dr Felicia Krauss (GRAPPA/API, University of Amsterdam)

krauss_e19.pdf

40 Calorimetric neutrino expectations from bright blazars

Blazar jets are extreme environments, in which relativistic proton interactions with a UV photon field could give rise to photopion production. High-confidence associations of individual high-energy neutrinos with blazar flares could be achieved via spatially and temporally coincident detections. Recently, the track-like, extremely high energy neutrino event IceCube-170922A was found to coincide with increased gamma-ray emission from the blazar TXS 0506+056, leading to the identification of the most promising neutrino source candidate so far. We test the chance coincidence of such events by calculating the expected number of neutrinos that can be detected by IceCube, based on a broadband parametrization of bright short-term blazar flares that were observed in the past 8-years by Fermi/LAT. We find that the integrated keV-to-GeV fluence of most individual blazar flares is far too small to yield a substantial Poisson probability for the detection of one or more neutrinos with IceCube. We show that the association of the IC170922A neutrino with TXS 0506+056 is energetically plausible at a significance level of about 3.5 sigma and discuss strategies to search for more significant associations in future data unblindings of IceCube and KM3NeT.

Speaker: Mr Michael Kreter (North-West University)

neutrino_flares_kreter.pdf

Connection to UHECR

Convener: Dr Jonathan Biteau (IPNO, Univ. Paris-Sud, Univ. Paris/Saclay, CNRS/IN2P3)

41 Cosmic Ray Production from (Extreme) Blazars

Extreme blazars prove themselves to be efficient particles accelerators, giving rise to some of the highest energy, and hardest spectral emission of all blazars. The nature of processes giving rise to this emission remains, with both leptonic and hadronic scenarios being put forward. I will try to balance up the arguments for and against these two scenarios, highlighting potential means by which they can be further tested. It will also be discussed how such sources may fit into the more general framework of blazars as cosmic ray sources.

Speaker: Dr Andrew Taylor (DESY)

ExtremeBlazars_UHECR.pdf

42 Ultra-high energy cosmic ray anisotropies measured with the Pierre Auger Observatory

Recently the Pierre Auger Observatory has observed a dipolar anisotropy of amplitude ~ 6.5% above 8 x 10^{18} eV that is unrelated to the Galactic plane. While the significance is largest above 8 x 10^{18} eV, the cosmic-ray data indicate a growth of the dipolar amplitude from 4 to 32 EeV and beyond. Furthermore, above 4 x 10^{19} eV, indications have been found for intermediate-scale anisotropies associated with extragalactic gamma-ray sources. Apart from those indications no significant anisotropies are seen. We review these observations and discuss possible astrophysical interpretations, in particular in the context of the sources discussed at this conference.

Speaker: Prof. Guenter Sigl (Universität Hamburg)

Sigl.pdf

43 The Telescope Array experiment

The Telescope Array (TA), located in Millard County, Utah, US (39.3° N, 112.9° W, 1400 m a.s.l.), is the largest cosmic ray detector in the Northern Hemisphere. Its main array for the study of cosmic rays with the highest energies (up to 100 EeV and beyond) consists of 507 surface detector (SD) stations on a 1.2 km-spacing square grid overlooked by 38 fluorescence detector (FD) telescopes at three sites at the edge of the array. Its sensitivity is extended to energies down to 1 PeV by the Telescope Array Low-energy Extension (TALE), consisting of 103 extra SD stations with tighter spacing overlooked by 10 extra FD telescopes with higher elevation, making it the first single experiment capable of measuring cosmic rays across five orders of magnitude in energy. In this presentation, I will review the latest results from TA, in particular on the distribution of UHECR arrival directions and possible associations with extragalactic sources. I will also describe TAx4, an extension of TA currently under construction, which is going to greatly increase the expected number of detected events and hence the statistical sensitivity to anisotropies in their distribution.

Speaker: Dr Armando di Matteo (ULB, Brussels, Belgium)

slides.pdf

16:00 Coffee break

Cosmology

Convener: Dr Jonathan Biteau (IPNO, Univ. Paris-Sud, Univ. Paris/Saclay, CNRS/IN2P3)

44 Characterizing cosmological fields with blazar observations: the EBL and IGMF

Recent progress in using very-high-energy gamma-ray observations to constrain the properties of the extragalactic background light and the intergalactic magnetic field is reviewed. Such studies capitalize on the scattering of gamma rays from distant emitters on diffuse photon fields, namely the extragalactic background light and cosmic microwave background. Observations of extreme blazars are particularly suited to these measurements due to their significant gamma-ray emission at multi-TeV energies. The latest measurements are discussed, as well as uncertainties pertaining to the gamma-ray emission and scattering processes.

Speaker: Elisa Pueschel (Deutsches Elektronen-Synchrotron (DESY))

EBL_IGMF_Padua_EPueschel.pdf

45 The Extragalactic Background Light in the Fermi-LAT Era

The light emitted by all galaxies across the history of the Universe is encoded in the intensity of the extragalactic background light (EBL), the diffuse cosmic radiation field at ultraviolet, optical, and infrared wavelengths. The EBL is a source of opacity for high-energy γ rays via the photon-photon interaction (γγ → e+e−), leaving a characteristic attenuation imprint in the spectra of distant γ-ray sources. In this talk, I will report on an unprecedented measurement of the EBL using data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, which has allowed us to derive the star-formation history of the Universe and estimate the number density of faint galaxies during the re-ionization epoch.

Speaker: Vaidehi Paliya (Clemson University)

PALIYA_EBL_extreme.pdf

46 Intergalactic electromagnetic cascades from extreme TeV blazars

Extreme TeV blazars (ETB) are active galactic nuclei that have intrinsic spectral energy distribution (SED) peaked at an energy E>1 TeV. ETB, defined solely by very high energy (VHE, E>100 GeV) gamma-ray properties, have much in common with extreme highly peaked BL Lac objects (EHBL) [1], defined by broadband (in particular, X-ray) properties. Indeed, some blazars such as 1ES 0229+200 and 1ES 0347−121 may be classified as both ETB and EHBL. Compared to well-known nearby blazars such as Mkn 501 and Mkn 421, ETB, as a rule, reveal hard observable spectra and weak, slow variability in the high energy (HE, E>100 MeV) and VHE spectral bands. These peculiar properties of ETBs allow for a possibility that a significant part of observable gamma-rays were in fact produced not inside the source, but as the result of electromagnetic cascade development in the intergalactic medium, where the extragalactic background light (EBL) and cosmic microwave background (CMB) dominate over intrinsic photon fields of the source, and the extragalactic magnetic field (EGMF) dominates over intrinsic magnetic fields. We discuss this “intergalactic cascade hypothesis” for ETBs in details and show that: 1) The “absorption-only model” that accounts for only the $\gamma\gamma\rightarrow e^{+}e^{-}$ pair production process and adiabatic losses and neglects secondary (cascade) flux faces significant difficulties; 2) All contemporary observations may be qualitatively accomodated in the framework of the “electromagnetic cascade model” that includes inverse Compton (IC) cascade flux; 3) The “basic hadronic cascade model” which assumes that all observable HE and VHE gamma-rays were produced in the intergalactic medium by primary ultra-high energy (UHE) cosmic rays (CR), is disfavoured due to a combination of source-intrinsic effects (such as the broadening of the primary CR beam in magnetic fields around the source) and the angular broadening of the observable gamma-ray image due to deflection of primary protons in filaments of the large scale structure. A description of our calculations and discussion of our results is available in [2-4]. References 1. G. Bonnoli et al., MNRAS, 451, 611 (2015) 2. T.A. Dzhatdoev et al., A&A, 603, A59 (2017) 3. T.A. Dzhatdoev et al., astro-ph/1808.06758 (2018) 4. T.A. Dzhatdoev et al., astro-ph/1810.06200 (2018)

Speaker: Timur Dzhatdoev (SINP MSU Moscow)

Extreme19-Cascades.pdf

47 Extreme high frequency peaked BL Lac objects to constrain the infrared background light

Extreme High Frequency Peaked BL Lacs (EHBL) are the cosmic sources able to produce the highest energy photons in the Universe. They make a relatively rare population of objects, difficult to identify also because they are quite faint at the energies probed by the Fermi surveys. Based on a hard X-ray selection, Foffano et al. have uncovered a small subset of such population. We discuss in the present contribution the implications of observing them with Cherenkov telescopes with the aim of constraining the extragalactic diffuse background at far-infrared wavelengths, where it has never been observed because of the overwhelming dominance of the local foreground emissions.

Speaker: Prof. Alberto Franceschini (UNIPD)

FranceschiniExtreme.pdf

20:00 Social dinner

Friday, 25 January

Fundamental physics

Convener: Elisa Pueschel

48 Fundamental physics with extreme BL Lacs

Extreme BL Lacs provide intense and hard beams of very-high energy photons that can be exploited to explore physical processes beyond those foreseen by the present standard model (SM) of particle physics. In particular, the potential extension of the spectrum of extreme BL Lacs well above the ~10 TeV, limit achieved by the current instruments, will provide the opportunity to test effects induced by the breaking of the Lorentz Invariance expected in several implementations of quantum gravity and to probe the existence of axion-like particles — small mass, neutral pseudo-scalar particles predicted by extensions of the SM. I will review the expected effects, the current limits and the perspective for the CTA.

Speaker: Dr Fabrizio Tavecchio (INAF-)AB)

ExtremeTavecchio.pdf

49 Extreme BL Lacs as a probe of axion-like particles

Axion-like particles (ALPs) are very light, neutral, pseudo-scalar bosons predicted by the String Theory and are supposed to interact with two photons. In the presence of an external magnetic field ALPs produce very interesting astrophysical effects in the very-high energy (VHE) band and above (10 GeV – 1000 TeV). ALPs can influence the emission of extreme BL Lacs (EHBLs): in particular, photon-ALP oscillations mediated by the jet magnetic field produce a sizable amount of ALPs already in the source neighborhood. Oscillations in the extragalactic space give rise to an increase of the transparency of the Universe to VHE photons partially preventing the γ-γ absorption due to the photons of the Extragalactic Background Light (EBL). Photon-ALP interactions in the Milky Way allow for a great part of still surviving ALPs to be converted back to photons and then to be detected. We investigate the propagation of the photon-ALP beam starting at the jet base of a BL Lac, where it is generated as pure photons. We follow the photon-ALP oscillations during their path up to us while crossing the BL Lac magnetic field, the intergalactic magnetic field – described by means of an improved model developed by us – and the Milky Way magnetic field. We find that the derived photon-ALP oscillations dramatically change BL Lac observed spectra and with our model we infer two features: (i) energy oscillations in the observed spectra, (ii) an unexpected peak at E = (10 − 30) TeV. The last prediction is particular important for EHBLs whose observed spectra reach energies above 10 TeV - which makes EHBLs an exceptional probe of ALPs. Our results can be checked both with the upcoming new generation of γ-ray observatories like CTA, HAWC, GAMMA-400, LHAASO and TAIGA-HiSCORE and with laboratory experiments like the upgrade of ALPS II at DESY, the planned IAXO and STAX experiments.

Speaker: Giorgio Galanti (INAF, Osservatorio Astronomico di Brera)

Galanti-eXtreme19.pdf

Future missions

Convener: Elisa Pueschel

50 High-performance gamma-ray astronomy and polarimetry with low-density homogeneous active targets: gas detectors.

Present sub-GeV gamma-ray telescopes are afflicted by the multiple scattering of the electron and of the positron in their tungsten converters. Due to the same reason, the gamma-ray polarimetry of cosmic sources has never been achieved. We have first characterised by simulations the potential of low-density homogeneous detectors for high-performance gamma-ray astronomy. We have shown that the single-track angular resolution is so good that polarimetry can be achieved. We have then designed, built and characterised on beam a high-pressure gas time-projection chamber (TPC) prototype with which we have demonstrated the polarimetry of a linearly-polarised MeV gamma-ray beam with an excellent dilution factor. En route, we have written an exact (five-dimensional) Monte Carlo event generator of the Bethe-Heitler differential cross section and have made it the first gamma-conversion Geant4 physics model that provides a target recoil-momentum distribution and a polarisation asymmetry compatible with the predictions of QED. (G4BetheHeitler5DModel, 10.5beta release) Gamma-ray polarimetry could enable the identification of the emitting particles in blazar jets (leptons, ionized matter), the emission mechanism in young pulsars (synchrotron or curvature radiation), and provide a smoking gun to dark matter annihilations in the Galactic center.

Speaker: Dr Denis Bernard (LLR Ecole Polytechnique)

eXtreme19_DBernard.pdf

11:00 Coffee break

Future missions

Convener: Elisa Pueschel

51 The next NASA SMEX Mission IXPE (the Imaging X-ray Polarimetry Explorer)

IXPE the Imaging X-ray Polarimetry explorer is the next SMEX NASA mission to be launched on 2021 with a large hardware contribution from ASI. It will re-open, after more than 40 years, the window of X-ray polarimetry for a number of celestial X-ray sources among almost all classes. In this talk we present the mission status and its astrophysical perspectives.

Speaker: Dr Paolo Soffitta (IAPS/INAF)

Soffitta_Extreme_2019_v1.pdf

52 LATTES - a new detector concept to monitor VHE gamma-ray sources

The era of Multi-Messenger Astrophysics has been officially inaugurated thanks to the detection of a High Energy neutrino event from a gamma-ray flaring Fermi-LAT blazar, together with the combined gamma-ray and gravitational signal from an identified cosmic source. Given the extreme and transient nature of the underlying processes, it has become clear that monitoring the sky in Very High Energy gamma-rays (VHE, E > 10 GeV) will provide a fundamental contribution to their interpretation. In this communication, we present a new detector concept, which can increase the performance of Extensive Air Shower (EAS) arrays, improving the sensitivity and resolution achieved so far and bridging the spectral gap that separates space-borne and ground-based observing facilities. This type of observatories will play a fundamental role in surveying the sky with a high duty-cycle and a wide field of view, which are mandatory requirements to detect transients. In addition, the large effective areas that they can cover make them ideal to survey a spectral energy range where satellite observations are very difficult to obtain, due to the intrinsically low fluxes of sources. Here, we discuss the contribution that an observatory based on an innovative hybrid detector will bring to the investigation of VHE emission from blazars with extreme SEDs, characterizing the spectral properties of these sources during flare events and placing constraints on the physics of the jet photon production in the VHE domain. We will describe a new approach to the gamma-ray initiated air shower analysis, in order to improve the suppression of cosmic-ray background, and we will finally summarize how the investigation of extreme gamma-ray emission from blazars will be affected by the ability to monitor the sky at VHE.

Speaker: Dr Giovanni La Mura (Laboratório de Instrumentação e Física Experimental de Partículas)

LaMura_eXtreme19_slides.pdf

53 The Cherenkov Telescope Array potential to probe the extreme extragalactic sky

The Fermi-LAT sources and specially the high-energy catalogues, 2FHL and 3FHL, provide the best unbiased proxy to the very-high energy (VHE, E > 100 GeV) extragalactic sky. Meaningful extrapolations can be derived therefrom, as to what are the expectations for future studies in this extreme observational window. Recent observational data in hard X-rays support the existence of a population of extreme high-energy synchrotron peaked blazar sources (EHSPs), composed of objects with a synchrotron peak frequency above 1E17 Hz. Observations with the current generation of VHE instruments, as well as Fermi-LAT seem to corroborate the existence of a population of EHSPs with inverse-Compton (IC) peak above 100 GeV in the γ-ray band. Current observations might nevertheless be probing only the low-energy side of the IC component, which could easily reach beyond the TeV range. As a result, these putative EHSPs are clearly undersampled by Fermi-LAT. In this talk, I will present the Cherenkov Telescope Array (CTA), whose first prototype telescope on site has just been inaugurated in La Palma last October. I intend to show that future VHE observations with CTA, covering the range between 20 GeV to 300 TeV, are in a good position to probe the poorly known class of extreme HSP sources, expanding our understanding of the blazar phenomenology and bringing new elements to a unified model of jetted active galactic nuclei.

Speaker: Dr Ulisses Barres de Almeida (Centro Brasileiro de Pesquisas Físicas)

CTA_Science_Extremes19_v2.pdf

Closure

Conveners: Dr Elisa Prandini (Padova University and INFN), Dr Jonathan Biteau (IPNO, Univ. Paris-Sud, Univ. Paris/Saclay, CNRS/IN2P3)