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ICRANet Newsletter
June-July 2020

1. Comunicado à imprensa ICRA-ICRANet-UNLP-UIS: Geodesic motion of S2 and G2 as a test of the fermionic dark matter nature of our Galactic core

The anomalous perihelion precession of Mercury around our Sun led to the greatest change of paradigm of physics thanks to the conception of General Relativity by Albert Einstein. The multi-year, high-quality data recording the motion of the closest objects around the compact source at the Galactic center, Sgr A*, led to the verification of the predicted gravitational redshift, to the anomalous precession of S2, as well as to the anomalous fly-by of G2. This heralds a fermionic dark matter dense core interpretation of the nature of Sgr A*, traditionally interpreted as a black hole. A new neutral fermion of 56 keV, a dark matter "ino", for short a "darkino", is basic to this alternative approach. New perspectives are open 1) to the understanding of the predominance of dark matter in our Galaxy and in the large scale of the Universe, 2) to formulate a new paradigm for identifying the seed for the formation of ten-billion-solar-masses black holes in active galactic nuclei, and 3) to address the fundamental physics of the darkinos which, together with the neutrinos, appear to have a fundamental role in accounting for a large portion of the Universe mass-energy. These results are presented in the new article appearing on 9 September 2020, in Astronomy & Astrophysics, co-authored by E.A. Becerra-Vergara, C.R. Argüelles, A. Krut, J.A. Rueda, and R. Ruffini [1].
Harvesting the detailed analysis made possible by the Einstein Theory of General Relativity, the article shows that the motion of the S2 star and the G2 cloud around Sgr A*, traditionally interpreted as due to a black hole of about 4 million solar masses, is instead better explained by the fermionic dark matter dense core of nearly the same mass (see Figs. 1-4 on pages 2-5). The core is composed of darkinos of 56 keV rest mass-energy, roughly 9 times lighter than electrons. This dark matter component extends, from the core, to the entire Galaxy, creating the stable gravitational cradle where all the stars rotate. The dense core formed of these "darkinos" becomes unstable giving origin to a black hole when it reaches a mass of about 100 million solar masses. This represents the lowest black hole seed mass for the growth of supermassive ten-billion-solar-masses black holes in active galactic nuclei.
This discovery has been made possible thanks to some of the largest observational facilities ever achieved in the history of our planet. The data of S2 (see Fig. 1 on page 2 and Fig. 3 on page 4) are taken from the SINFONI ( and NACO ( instruments of the Very Large Telescope (VLT) (, operated by the European Southern Observatory (ESO) located on Cerro Paranal in the Atacama Desert in Chile (, the Keck I ( and Keck II ( Telescopes, operated by the W. M. Keck Observatory located in Hawaii (, the Gemini North Telescope (, operated by the Gemini Observatory located in Hawaii (, as well as from the Subaru Telescope (, operated by the National Astronomical Observatory of Japan at the Mauna Kea Observatory on Hawaii ( The observational data of G2 (see Fig. 2 on page 3 and Fig. 4 on page 5) are taken from the SINFONI and NACO instruments of the VLT.
This approach is rooted in the work of Enrico Fermi who introduced the fermions in particle physics. Remo Ruffini recalls: "Eugene Wigner, Nobel laureate colleague of Einstein and Fermi, often stated: the Thomas-Fermi model works better than it should. This model has been leading for 93 years the description of all atoms: a gas of electrons, negatively-charged fermions, attracted electromagnetically by a positively-charged nucleus. In 1973, in Princeton, I addressed the gravitational analog of a Thomas-Fermi atom. Many neutral self-gravitating fermions characterized by their mass and spin, kept in equilibrium by their collective self-gravitation [2]. This idea was developed for years in ICRA and ICRANet, leading to a new approach to neutron stars (see [3] and references therein), and to the dark matter distribution in galaxies in the RAR model [4, 5], here applied to the dark matter galactic cores".

[1] (A&A forthcoming article).
[2] R. Ruffini and S. Bonazzola, Physical Review 187, 1767 (1969).
[3] J. A. Rueda, R. Ruffini, and S. S. Xue, Nucl. Phys. A 872, 286 (2011), 1104.4062.
[4] C. R. Argüelles, A. Krut, J. A. Rueda, and R. Ruffini, Phys. Dark Universe 21, 82 (2018), 1810.00405.
[5] C. R. Argüelles, A. Krut, J. A. Rueda, and R. Ruffini, Phys. Dark Universe 24, 100278 (2019).

Figure. 1. Taken from [1]. Theoretical (central BH and RAR models) and observed orbit of S2 around Sgr A*. The left panelshows the orbit, i.e. the right ascension (X) vs. declination (Y) angular positions, and the right panel shows the X and Y positions as a function of the observation time, and the corresponding residuals of the best-fit for the BH (blue) and the RAR(red) models. The theoretical models are calculated by solving the equations of motion of a test particle in the gravitationalfield of: 1) a Schwarzschild BH of 4.075 million solar masses (blue-dashed curves), and 2) the DM distribution obtained fromthe extended RAR model for 56 keV-fermions (red curves). The mass of the quantum core in the RAR model is 3.5 millionsolar masses. Figure available at:

Figure 2. Taken from [1]. Same as Fig. 1, but for G2. Figure available at:

Figure 3. Taken from [1]. Same as Fig. 1, but for the line-of-sight radial velocity of S2 (i.e. the redshift functionz). Figure available at:

Figure 4. Taken from [1]. Same as Fig. 3, but for G2. Figure available at:

• Link ao Comunicado de imprensa no site da ICRANet:
• Link ao Comunicado de imprensa no site de A&A:
• Link ao Comunicado de imprensa no site da CONICET:
• Link ao Comunicado de imprensa no site do INAF website:

2. Últimas notícias sobre o Quarto Encontro virtual Zeldovich de 7 a 11 de Setembro de 2020

É com grande prazer que anunciamos a lista completa dos palestrantes do Quarto Encontro virtual Zeldovich.
Mais detalhes disponíveis no site da conferência:

Marika Asgari Marika Asgari
Royal Observatory, Edimburgo, Reino Unido

Talk: "Weak lensing and the Kilo-Degree Survey"
Abhay Ashtekar Abhay Ashtekar
Institute for Gravitation & the Cosmos, Penn State University, EUA

Talk: "Loop Quantum Cosmology"
Artem Burdanov Artem Burdanov
Massachusetts Institute of Technology, EUA

Talk: "Search for exoplanets in ultracold dwarfs"
Rong-Gen Cai Rong-Gen Cai
Institute of Theoretical Physics, Chinese Academy of Sciences, China

Talk: "Gravitational waves from the early Universe"
Daniela Calzetti Daniela Calzetti
University of Massachusetts Amherst, EUA

Talk: "The astrophysical implications of the Starburst Attenuation Curve"
Jens Chluba Jens Chluba
Jodrell Bank Centre for Astrophysics, University of Manchester, Reino Unido

Talk: "CMB spectral distortions"
Alexander Dolgov Alexander Dolgov
Novosibirsk State University and ITEP, Rússia

Talk: "Primordial black holes and modification of Zeldovich-Novikov mechanism"
Jaan Einasto Jaan Einasto
Tartu Observatory, Estônia

Talk: "The biasing phenomenon"
Katherine Freese Katherine Freese
University of Texas, Austin, EUA
Chris Fryer Chris Fryer
Los Alamos National Laboratories, EUA

Talk: "Supernova explosions"
Stefan Gillessen Stefan Gillessen
Max Planck Institute for Extraterrestrial Physics, Alemanha

Talk: "GRAVITY scientific results"
Luca Izzo Luca Izzo
DARK - Niels Bohr Institute, Dinamarca

Talk: "Recent progresses on the connection between GRBs and type-Ic broad-lined supernovae"
Joanna Kiryluk Joanna Kiryluk
Stony Brook University, EUA

Talk: "Results from ICECube"
Claus Lämmerzahl Claus Lämmerzahl
ZARM, Alemanha

Talk: "Tests of general relativity"
Vladimir Lipunov Vladimir Lipunov
Moscow State University, Rússia

Talk: "Central GRB Engine from Early Multimessanger observations"
Andrea Merloni Andrea Merloni
Max-Planck Institute fuer Extraterrestrische Physik, Garching, Alemanha

Talk: "Mapping the hot Universe: the first year of operations of eROSITA on SRG"
Felix Mirabel Felix Mirabel
CEA Saclay, França

Talk: "Black holes in the universe"
Razmik Mirzoyan Razmik Mirzoyan
Max Planck Institute for Physics, Alemanha
Slava Mukhanov Slava Mukhanov
Ludwig-Maximilians-Universität München, Alemanha

Talk: "The final state of nonsigular evaporating black hole"
Piero Rosati Piero Rosati
University of Ferrara, Itália

Talk: "Cosmography and tests of the LCDM paradigm with high-precision strong lensing modelling of galaxy clusters"
Jorge Rueda Jorge Rueda
ICRANet, Itália

Talk: "An update of the binary-driven hypernova scenario"
Remo Ruffini Remo Ruffini
ICRANet, Itália

Talk: "The discovery of the moment of formation of the black hole in GRB 190114C"
Nikolay Shakura Nikolay Shakura
Sternberg Astronomical Institute of the Moscow State University, Rússia

Talk: "On the nature of 35-day cycle in Her X1/HZ Her"
Joseph Silk Joseph Silk
Oxford University, Reino Unido
Ignas Snellen Ignas Snellen
University of Leiden, Países Baixos

Talk: "Exoplanets and the search for extraterretrial life"
Dmitry Sokoloff Dmitry Sokoloff
Moscow State University, Rússia

Talk: "Dynamo in accretion discs"
Alexey Starobinsky Alexey Starobinsky
Landau institute for theoretical physics, RAS, Rússia

Talk: "Inflation"
Rashid Sunyaev Rashid Sunyaev
Max Planck Institute for Astrophysics, Alemanha

Talk: "Results of SRG Orbital Observatory with eRosita and ART-XC X-Ray telescopes aboard"
Amaury Triaud Amaury Triaud
University of Birmingham, Reino Unido

Talk: "Exoplanet atmospheres"
Ye-Fei Yuan Ye-Fei Yuan
Univ. of Sci. and Tech. of China, China

Talk: "Probes of strong gravity: SgrA* and M87*"
Shuang Nan Zhang Shuang Nan Zhang
Institute of High Energy Physics, Chinese Academy of Sciences, China

Talk: "HSTC mission"

3. Últimas publicações

M. Haghighat, S. Mahmoudi, R.Mohammadi, S. Tizchang, S.S Xue, Circular polarization of cosmic photons due to their interactions with Sterile neutrino dark matter, submitted for publication on Phys. Rev. D 101, 123016 (2020).
In this paper, we explore the possibility of the polarization conversion of a wide energy range of cosmic photons to the circular polarization through their interactions with right handed Sterile neutrinos as a candidate for dark matter. By considering the Sterile neutrino in the seesaw mechanism framework and right-handed current model, we examine the Faraday conversion Δϕ\tiny{FC} of gamma ray burst (GRB) photons at both the prompt and afterglow emission levels as well as the radio photons emitted from our galaxy and extra-galactic sources interacting with the Sterile neutrinos. Consequently, for the Sterile neutrino with mixing angle θ2≲10-2 motivated by models with a hidden sector coupled to the sterile neutrino, the Faraday conversion can be estimated as Δϕ\tiny{FC}≲10-3-10-18 rad for GRB, Δϕ\tiny{FC}≲10-6-10-11 rad for radio emission source from our galaxy and Δϕ\tiny{FC}≲10-6-10-15 rad for extra-galactic sources. We also examine the V-mode power spectrum CVl of the cosmic microwave background (CMB) at the last scattering surface. We show that the circular polarization power spectrum at the leading order is proportional to the linear polarization power spectrum Cpl and the mixing angle where for θ2≲10-2 leads to CVl≲0.01 Nano-Kelvin squared.

Becerra-Vergara, E. A.; Argüelles, C. R.; Krut, A.; Rueda, J. A.; Ruffini, R., The geodesic motion of S2 and G2 as a test of the fermionic dark matter nature of our galactic core, accepted for publication on July 22, 2020 in Astronomy & Astrophysics.
[Abridged] The S-stars motion around the Galactic center (Sgr A*) implies the existence of a compact source with a mass of about 4×106M, traditionally assumed to be a massive black hole (BH). Important for any model is the explanation of the multiyear, accurate astrometric data of S2 and the challenging G2: its post-pericenter velocity decelerates faster than expected from a Keplerian orbit around the putative BH. This has been reconciled in the literature by acting on G2 a drag force by an accretion flow. Alternatively, we show that the S2 and G2 motion is explained by the "core-halo" fermionic dark matter (DM) profile of the fully-relativistic Ruffini-Argüelles-Rueda (RAR) model. It has been already shown that for 48-345 keV fermions, it accurately fits the rotation curves of the Milky-Way halo. We here show that, for a fermion mass of 56 keV, it explains the time-dependent data of the position (orbit) and light-of-sight radial velocity (redshift function z) of S2 and G2, the latter without a drag force. We find the RAR model fits better the data: the mean of reduced chi-squares of the orbit and z data are, for S2, ⟨χ¯ 2S2,RAR≈3.1 and ⟨χ¯ 2S2,BH≈3.3 while, for G2, ⟨χ¯ 2G2,RAR≈20 and ⟨χ¯ 2G2,BH≈41. For S2 the fits of the z data are comparable, χ¯ 2z,RAR≈1.28 and χ¯ 2z,BH≈1.04, for G2 only the RAR model fits, χ¯ 2z,RAR≈1.0 and χ¯ 2z,BH≈26. In addition, the critical mass for the gravitational collapse of a degenerate 56 keV-fermion DM core into a BH is ∼108M, which may be the initial seed for the formation of the observed central supermassive BH in active galaxies, such as M87.

M. A. Prakapenia, G.V.Vereshchagin, Pauli blocking effects in thermalization of relativistic plasma, accepted for publication on June 14,2020 in Physics Letters A.
We investigate the effects of Pauli blocking on thermalization process of relativistic plasma by solving relativistic Uehling-Uhlenbeck equations with QED collision integral for all binary and triple processes. With this purpose we consider nonequilibrium initial state of plasma to be strongly degenerate. We found that when electron-positron annihilation is efficient, initial plasma degeneracy is quickly destroyed. As a result in a wide range of final temperatures ranging from nonrelativistic to mildly relativistic 0.1mec2≤kBT≤10ec2 thermalization is not affected by Pauli blocking. Conversely, when electron-positron annihilation process is inefficient, thermalization process in such degenerate plasma is strongly affected by Pauli blocking. This is possible either in a nonrelativistic plasma, with equilibrium temperature kBT≤0.3mec2, or in photon-electron plasma. In these cases all reaction rates are strongly suppressed by Pauli blocking and thermalization does not occur until electrons can populate energy states above the Fermi energy. Soon after this happens thermalization proceeds suddenly in an avalanche-like process. Such rapid thermalization can be a unique footprint of strongly degenerate plasma.

N. Sahakyan, Broadband Study of High-Synchrotron-Peaked BL Lac Object 1ES 1218+304, accepted for publication in MNRAS on July 2020.
The origin of the multiwavelength emission from the high-synchrotron-peaked BL Lac 1ES 1218+304 is studied using the data from Swift UVOT/XRT, NuSTAR and Fermi-LAT. A detailed temporal and spectral analysis of the data observed during 2008-2020 in the γ-ray (>100 MeV), X-ray (0.3-70 keV), and optical/UV bands is performed. The γ-ray spectrum is hard with a photon index of 1.71 ± 0.02 above 100 MeV. The Swift UVOT/XRT data show a flux increase in the UV/optical and X-ray bands; the highest 0.3 - 3 keV X-ray flux was (1.13 ± 0.02) × 10-10erg cm-2 s-1. In the 0.3-10 keV range the averaged X-ray photon index is >2.0 which softens to 2.56 ± 0.028 in the 3-50 keV band. However, in some periods, the X-ray photon index became extremely hard (<1.8), indicating that the peak of the synchrotron component was above 1 keV, and so 1ES 1218+304 behaved like an extreme synchrotron BL Lac. The hardest X-ray photon index of 1ES 1218+304 was 1.60 ± 0.05 on MJD 58489. The time-averaged multiwavelength spectral energy distribution is modeled within a one-zone synchrotron self-Compton leptonic model using a broken power-law and power-law with an exponential cutoff electron energy distributions. The data are well explained when the electron energy distribution is E-2.1eEe-2.1 extending up to γbr/cut ≃ (1.7 - 4.3) × 105, and the magnetic field is weak (B ∼ 1.5 × 10-2 G). By solving the kinetic equation for electron evolution in the emitting region, the obtained electron energy distributions are discussed considering particle injection, cooling, and escape.

MAGIC Collaboration, Studying the nature of the unidentified gamma-ray source HESS J1841-055 with the MAGIC telescopes, accepted for publication in MNRAS on July 2020.
We investigate the physical nature and origin of the gamma-ray emission from the extended source HESS J1841-055 observed at TeV and GeV energies. We observed HESS J1841-055 at TeV energies for a total effective time of 43 hours with the MAGIC telescopes, in 2012 and 2013. Additionally, we analysed the GeV counterpart making use of about 10 years of Fermi-LAT data. Using both Fermi-LAT and MAGIC, we study both the spectral and energy-dependent morphology of the source for almost four decades of energy. The origin of the gamma-ray emission from this region is investigated using multi-waveband information on sources present in this region, suggested to be associated with this unidentified gamma-ray source. We find that the extended emission at GeV-TeV energies is best described by more than one source model. We also perform the first energy-dependent analysis of the HESS J1841-055 region at GeV-TeV. We find that the emission at lower energies comes from a diffuse or extended component, while the major contribution of gamma rays above 1 TeV arises from the southern part of the source. Moreover, we find that a significant curvature is present in the combined observed spectrum of MAGIC and Fermi-LAT. The first multi-wavelength spectral energy distribution of this unidentified source shows that the emission at GeV-TeV energies can be well explained with both leptonic and hadronic models. For the leptonic scenario, bremsstrahlung is the dominant emission compared to inverse Compton. On the other hand, for the hadronic model, gamma-ray resulting from the decay of neutral pions (π0) can explain the observed spectrum. The presence of dense molecular clouds overlapping with HESS J1841-055 makes both bremsstrahlung and π0-decay processes the dominant emission mechanisms for the source.

MAGIC Collaboration, An intermittent extreme BL Lac: MWL study of 1ES 2344+514 in an enhanced state, in MNRAS, Volume 496, Issue 3, pp.3912-3928.
Extreme high-frequency BL Lacs (EHBL) feature their synchrotron peak of the broad-band spectral energy distribution (SED) at νs ≥ 1017 Hz. The BL Lac object 1ES 2344+514 was included in the EHBL family because of its impressive shift of the synchrotron peak in 1996. During the following years, the source appeared to be in a low state without showing any extreme behaviours. In 2016 August, 1ES 2344+514 was detected with the ground-based γ-ray telescope FACT during a high γ-ray state, triggering multiwavelength (MWL) observations. We studied the MWL light curves of 1ES 2344+514 during the 2016 flaring state, using data from radio to very-high-energy (VHE) γ-rays taken with OVRO, KAIT, KVA, NOT, some telescopes of the GASP-WEBT collaboration at the Teide, Crimean, and St. Petersburg observatories, Swift-UVOT, Swift-XRT, Fermi-LAT, FACT, and MAGIC. With simultaneous observations of the flare, we built the broad-band SED and studied it in the framework of a leptonic and a hadronic model. The VHE γ-ray observations show a flux level of 55 per cent of the Crab Nebula flux above 300 GeV, similar to the historical maximum of 1995. The combination of MAGIC and Fermi-LAT spectra provides an unprecedented characterization of the inverse-Compton peak for this object during a flaring episode. The Γ index of the intrinsic spectrum in the VHE γ-ray band is 2.04 ± 0.12stat ± 0.15sys. We find the source in an extreme state with a shift of the position of the synchrotron peak to frequencies above or equal to 1018 Hz.

MAGIC Collaboration, Testing two-component models on very-high-energy gamma-ray emitting BL Lac objects, accepted for publication in A&A on June 2020.
Context. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very-high-energy (VHE) gamma-ray emitting blazars. While two-component models are performing better, they are difficult to constrain due to the large number of free parameters. Aims. In this work, we make a first attempt to take into account the observational constraints from Very Long Baseline Interferometry (VLBI) data, long-term light curves (radio, optical, and X-rays) and optical polarisation to limit the parameter space for a two-component model and test if it can still reproduce the observed spectral energy distribution (SED) of the blazars. Methods. We selected five TeV BL Lac objects based on the availability of VHE gamma-ray and optical polarisation data. We collected constraints for the jet parameters from VLBI observations. We evaluated the contributions of the two components to the optical flux by means of decomposition of long-term radio and optical light curves as well as modeling of the optical polarisation variability of the objects. We selected eight epochs for these five objects, based on the variability observed at VHE gamma rays, for which we constructed the SEDs that we then modeled with a two-component model. Results. We found parameter sets which can reproduce the broadband SED of the sources in the framework of two-component models considering all available observational constraints from VLBI observations. Moreover, the constraints obtained from the long-term behavior of the sources in the lower energy bands could be used to determine the region where the emission in each band originates. Finally, we attempted to use optical polarisation data to shed new light on the behavior of the two components in the optical band. Our observationally constrained two zone model allows explanation of the entire SED from radio to VHE with two co-located emission regions.

MAGIC Collaboration, Unraveling the Complex Behavior of Mrk 421 with Simultaneous X-Ray and VHE Observations during an Extreme Flaring Activity in 2013 April, in APJ Supplement Series, Volume 248, Issue 2, id.29.
We report on a multiband variability and correlation study of the TeV blazar Mrk 421 during an exceptional flaring activity observed from 2013 April 11 to 19. The study uses, among others, data from GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT), Swift, Nuclear Spectroscopic Telescope Array (NuSTAR), Fermi Large Area Telescope, Very Energetic Radiation Imaging Telescope Array System (VERITAS), and Major Atmospheric Gamma Imaging Cherenkov (MAGIC). The large blazar activity and the 43 hr of simultaneous NuSTAR and MAGIC/VERITAS observations permitted variability studies on 15 minute time bins over three X-ray bands (3-7 keV, 7-30 keV, and 30-80 keV) and three very-high-energy (VHE; >0.1 TeV) gamma-ray bands (0.2-0.4 TeV, 0.4-0.8 TeV, and >0.8 TeV). We detected substantial flux variations on multi-hour and sub-hour timescales in all of the X-ray and VHE gamma-ray bands. The characteristics of the sub-hour flux variations are essentially energy independent, while the multi-hour flux variations can have a strong dependence on the energy of the X-rays and the VHE gamma-rays. The three VHE bands and the three X-ray bands are positively correlated with no time lag, but the strength and characteristics of the correlation change substantially over time and across energy bands. Our findings favor multi-zone scenarios for explaining the achromatic/chromatic variability of the fast/slow components of the light curves, as well as the changes in the flux-flux correlation on day-long timescales. We interpret these results within a magnetic reconnection scenario, where the multi-hour flux variations are dominated by the combined emission from various plasmoids of different sizes and velocities, while the sub-hour flux variations are dominated by the emission from a single small plasmoid moving across the magnetic reconnection layer. * Contact MAGIC Collaboration ( for queries. Corresponding authors are D. Paneque, A. Babic, J. Finke, T. Hassan, and M. Petropoulou.

MAGIC Collaboration, Broadband characterization of the very intense TeV flares of the blazar 1ES 1959+650 in 2016, in Astronomy & Astrophysics, Volume 638, id.A14, 16 pp.
1ES 1959+650 is a bright TeV high-frequency-peaked BL Lac object exhibiting interesting features like "orphan" TeV flares and broad emission in the high-energy regime that are difficult to interpret using conventional one-zone Synchrotron Self-Compton (SSC) scenarios. We report the results from the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) observations in 2016 along with the multi-wavelength data from the Fermi Large Area Telescope (LAT) and Swift instruments. MAGIC observed 1ES 1959+650 with different emission levels in the very-high-energy (VHE, E > 100 GeV) γ-ray band during 2016. In the long-term data, the X-ray spectrum becomes harder with increasing flux and a hint of a similar trend is also visible in the VHE band. An exceptionally high VHE flux reaching ∼3 times the Crab Nebula flux was measured by MAGIC on the 13 and 14 of June, and 1 July 2016 (the highest flux observed since 2002). During these flares, the high-energy peak of the spectral energy distribution (SED) lies in the VHE domain and extends up to several TeV. The spectrum in the γ-ray (both Fermi-LAT and VHE bands) and the X-ray bands are quite hard. On 13 June and 1 July 2016, the source showed rapid variations in the VHE flux within timescales of less than an hour. A simple one-zone SSC model can describe the data during the flares requiring moderate to large values of the Doppler factors (δ ≥ 30-60). Alternatively, the high-energy peak of the SED can be explained by a purely hadronic model attributed to proton-synchrotron radiation with jet power Ljet ∼ 1046 erg s-1 and under high values of the magnetic field strength (∼100 G) and maximum proton energy (∼few EeV). Mixed lepto-hadronic models require super-Eddington values of the jet power. We conclude that it is difficult to get detectable neutrino emission from the source during the extreme VHE flaring period of 2016.

Giommi P., Glauch T., Padovani P., Resconi E., Turcati A., Chang Y.L., Dissecting the regions around IceCube high-energy neutrinos: growing evidence for the blazar connection, published on July 16, 2020 in MNRAS.
The association of two IceCube detections, the IceCube-170922A event and a neutrino flare, with the blazar TXS 0506+056, has paved the way for the multimessenger quest for cosmic accelerators. IceCube has observed many other neutrinos but their origin remains unknown. To better understand the reason for the apparent lack of neutrino counterparts we have extended the comprehensive dissection of the sky area performed for the IceCube-170922A event to all 70 public IceCube high-energy neutrinos that are well reconstructed and off the Galactic plane. Using the multi-frequency data available through the Open Universe platform, we have identified numerous candidate counterparts of IceCube events. We report here the classification of all the γ-ray blazars found and the results of subsequent statistical tests. In addition, we have checked the 4LAC, 3FHL and 3HSP catalogues for potential counterparts. Following the dissection of all areas associated with IceCube neutrinos, we evaluate the data using a likelihood-ratio test and find a 3.23 σ (post-trial) excess of HBLs and IBLs with a best-fit of 15 ± 3.6 signal sources. This result, together with previous findings, consistently points to a growing evidence for a connection between IceCube neutrinos and blazars, the most energetic particle accelerators known in the Universe.

Giommi P., Padovani P., Oikonomou F., Glauch T., Paiano S., Resconi E., 3HSP J095507.9+355101: a flaring extreme blazar coincident in space and time with IceCube-200107A, accepted for publication in A&A Letters.
The uncertainty region of the highly energetic neutrino IceCube200107A includes 3HSP J095507.9+355101 (z~=~0.557), an extreme blazar, which was detected in a high, very hard, and variable X-ray state shortly after the neutrino arrival. Following a detailed multi-wavelength investigation, we confirm that the source is a genuine BL Lac, contrary to TXS 0506+056, the first source so far associated with IceCube neutrinos, which is a "masquerading" BL Lac. As in the case of TXS0506+056, 3HSP J095507.9+355101 is also way off the so-called "blazar sequence". We consider 3HSP J095507.9+355101 a possible counterpart to the IceCube neutrino. Finally, we discuss some theoretical implications in terms of neutrino production.

Giommi P., Chang Y.~L., Turriziani S., Glauch T., Leto C., Verrecchia F., Padovani P., et al., The Open Universe survey of Swift-XRT GRB fields: a complete sample of HBL blazars, accepted for publication in A&A Letters.
We have analysed all the X-ray images centred on Gamma Ray Bursts generated by Swift over the last 15 years using automatic tools that do not require any expertise in X-ray astronomy, producing results in excellent agreement with previous findings. This work, besides presenting the largest medium-deep survey of the X-ray sky and a complete sample of blazars, wishes to be a step in the direction of achieving the ultimate goal of the Open Universe Initiative, that is to enable non expert people to fully benefit of space science data, possibly extending the potential for scientific discovery, currently confined within a small number of highly specialised teams, to a much larger population. We have used the Swift_deepsky Docker container encapsulated pipeline to build the largest existing flux-limited and unbiased sample of serendipitous X-ray sources. Swift_deepsky runs on any laptop or desktop computer with a modern operating system. The tool automatically downloads the data and the calibration files from the archives, runs the official Swift analysis software and produces a number of results including images, the list of detected sources, X-ray fluxes, SED data, and spectral slope estimations. We used our source list to build the LogN-LogS of extra-galactic sources, which perfectly matches that estimated by other satellites. Combining our survey with multi-frequency data we selected a complete radio flux-density limited sample of High Energy Peaked (HBL) blazars.

Petropoulou M., Oikonomou F., Mastichiadis A., Murase K., Padovani P., Vasilopoulos G., Giommi P., Comprehensive Multimessenger Modeling of the Extreme Blazar 3HSP J095507.9+355101 and Predictions for IceCube, accepted for publication in ApJ.
3HSP J095507.9+355101 is an extreme blazar which has been possibly associated with a high-energy neutrino (IceCube-200107A) detected one day before the blazar was found to undergo a hard X-ray flare. We perform a comprehensive study of the predicted multimessenger emission from 3HSP J095507.9+355101 during its recent X-ray flare, but also in the long term. We focus on one-zone leptohadronic models, but we also explore alternative scenarios: (i) a blazar-core model, which considers neutrino production in the inner jet, close to the supermassive black hole; (ii) a hidden external-photon model, which considers neutrino production in the jet through interactions with photons from a weak broad line region; (iii) a proton synchrotron model, where high-energy protons in the jet produce γ-rays via synchrotron; and (iv) an intergalactic cascade scenario, where neutrinos are produced in the intergalactic medium by interactions of a high-energy cosmic-ray beam escaping the jet. The Poisson probability to detect one muon neutrino in ten years from 3HSP J095507.9+355101 with the real-time IceCube alert analysis is ∼1% (3%) for the most optimistic one-zone leptohadronic model (the multi-zone blazar-core model). Meanwhile, detection of one neutrino during the 44-day-long high X-ray flux-state period following the neutrino detection is 0.06%, according to our most optimistic leptohadronic model. The most promising scenarios for neutrino production also predict strong intra-source γ-ray attenuation above ∼100 GeV. If the association is real, then IceCube-Gen2 and other future detectors should be able to provide additional evidence for neutrino production in 3HSP J095507.9+355101 and other extreme blazars.

Paiano S., Falomo R., Padovani P., Giommi P., Gargiulo A., Uslenghi M., Rossi A., et al., The redshift and the host galaxy of the neutrino candidate 4FGL J0955.1+3551 (3HSP J095507.9+355101) in MNRAS, 495, L108.
The BL Lac object 4FGL J0955.1+3551 has been suggested as a possible source of ultra-energetic neutrinos detected by the IceCube observatory. The target was observed in 2020 January at the Large Binocular Telescope. Our spectroscopy (4100-8500 Å) yields a firm redshift z = 0.557 as deduced by the absorption lines of the host galaxy. The upper limit of the minimum equivalent width on emission lines is ∼0.3 Å. From the source image, we are able to resolve the host galaxy for which we measure an absolute magnitude M(R) = -22.9 and Re = 8 kpc, which are values which are typical of the host galaxies of BL Lacs.

Fermi collaboration. Abdollahi S., Acero F., Ackermann M., Ajello M., Atwood W. B.,Axelsson M., Baldini L., et al., Fermi Large Area Telescope Fourth Source Catalog, in ApJS, 247, 33.
We present the fourth Fermi Large Area Telescope catalog (4FGL) of γ-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range from 50 MeV to 1 TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse γ-ray emission, and two sets of light curves (one-year and two-month intervals). The 4FGL catalog includes 5064 sources above 4σ significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall, 358 sources are considered as identified based on angular extent, periodicity, or correlated variability observed at other wavelengths. For 1336 sources, we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars.
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