FERMI LARGE AREA TELESCOPE THIRD SOURCE CATALOG

Evidence for PeV Proton Acceleration from Fermi-LAT Observations of SNR G106.3+2.7

The existence of a "knee" at energy ∼1  PeV in the cosmic-ray spectrum suggests the presence of Galactic PeV proton accelerators called "PeVatrons." Supernova remnant (SNR) G106.3+2.7 is a prime candidate for one of these. The recent detection of very high energy (0.1-100 TeV) gamma rays from G106.3+2.7 may be explained either by the decay of neutral pions or inverse Compton scattering by relativistic electrons. We report an analysis of 12 years of Fermi-LAT gamma-ray data that shows that the GeV-TeV gamma-ray spectrum is much harder and requires a different total electron energy than the radio and x-ray spectra, suggesting it has a distinct, hadronic origin. The nondetection of gamma rays below 10 GeV implies additional constraints on the relativistic electron spectrum. A hadronic interpretation of the observed gamma rays is strongly supported. This observation confirms the long-sought connection between Galactic PeVatrons and SNRs. Moreover, it suggests that G106.3+2.7 could be the brightest member of a new population of SNRs whose gamma-ray energy flux peaks at TeV energies. Such a population may contribute to the cosmic-ray knee and be revealed by future very high energy gamma-ray detectors.

Pulsar Wind Nebulae and Unidentified Galactic Very High Energy Sources

The riddle of the origin of Cosmic Rays (CR) has been an open question for over a century. Gamma ray observations above 100 MeV reveal the sites of cosmic ray acceleration to energies where they are unaffected by solar modulation; recent evidence supports the existence of hadronic acceleration in Supernova Remnants (SNR), as expected in the standard model of cosmic ray acceleration. Nevertheless, the results raise new questions, and no final answer has been provided thus far. Among the suggested possible alternative accelerators in the Very High Energy (VHE) gamma ray sky, pulsar wind nebulae (PWNe, which together with dark matter are the main candidates to explain the local positron excess as well) are the dominant population among known Galactic sources. However, the most numerous population in absolute terms is represented by unidentified sources (~50% of VHE gamma ray sources). The relationship between PWNe and unidentified sources seems very close; in fact, in a PWN, the lifetime of inverse Compton (IC) emitting electrons not only exceeds the lifetime of its progenitor pulsar, but also exceeds the age of the electrons that emit via synchrotron radiation. Therefore, during its evolution, a PWN can remain bright in IC such that its GeV-TeV gamma ray flux remains high for timescales much larger than the lifetimes of the pulsar and the X-ray PWN. In addition, the shell-type remnant of the supernova explosion in which the pulsar was formed has a much shorter lifetime than the electrons responsible for IC emission. Hence, understanding PWNe and VHE unidentified sources is a crucial piece of the solution to the riddle of the origin of cosmic rays. Both theoretical aspects (with particular emphasis on the ancient pulsar wind nebulae scenario) and their observational proofs are discussed in this paper. Specifically, the scientific cases of HESS J1616-508 and HESS J1813-126 are examined in detail.

Gamma-Ray Cosmology and Tests of Fundamental Physics

The propagation of gamma-rays over cosmological distances is the subject of extensive theoretical and observational research at GeV and TeV energies. The mean free path of gamma-rays in the cosmic web is limited above 100 GeV due to the production of electrons and positrons on the cosmic optical and infrared backgrounds. Electrons and positrons cool in the intergalactic medium while gyrating in its magnetic fields, which could cause either its global heating or the production of lower-energy secondary gamma-rays. The energy distribution of gamma-rays surviving the cosmological journey carries observed absorption features that gauge the emissivity of baryonic matter over cosmic time, constrain the distance scale of ΛCDM cosmology, and limit the alterations of the interaction cross section. Competitive constraints are, in particular, placed on the cosmic star-formation history as well as on phenomena expected from quantum gravity and string theory, such as the coupling to hypothetical axion-like particles or the violation of Lorentz invariance. Recent theoretical and observational advances offer a glimpse of the multi-wavelength and multi-messenger path that the new generation of gamma-ray observatories is about to open.

The Crab Pulsar and Nebula as Seen in Gamma-Rays

Slightly more than 30 years ago, Whipple detection of the Crab Nebula was the start of Very High Energy gamma-ray astronomy. Since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast variability, pulsed emission up to unexpectedly high energy, and the very recent detection of photons with energy exceeding 1 PeV. In this article, we review the impact of gamma-ray observations on our understanding of this extraordinary accelerator.

Gamma Rays as Probes of Cosmic-Ray Propagation and Interactions in Galaxies

Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in unprecedented detail the properties of galactic cosmic rays. A variety of scales and environments are now accessible to us, from the local interstellar medium near the Sun and the vicinity of cosmic-ray accelerators, out to the Milky Way at large and beyond, with a growing number of gamma-ray emitting star-forming galaxies. Gamma-ray observations have been pushing forward our understanding of the life cycle of cosmic rays in galaxies and, combined with advances in related domains, they have been challenging standard assumptions in the field and have spurred new developments in modelling approaches and data analysis methods. We provide a review of the status of the subject and discuss perspectives on future progress.

Galactic Center Excess in a New Light: Disentangling the γ-Ray Sky with Bayesian Graph Convolutional Neural Networks

A fundamental question regarding the Galactic Center excess (GCE) is whether the underlying structure is pointlike or smooth, often framed in terms of a millisecond pulsar or annihilating dark matter (DM) origin for the emission. We show that Bayesian neural networks (NNs) have the potential to resolve this debate. In simulated data, the method is able to predict the flux fractions from inner Galaxy emission components to on average ∼0.5%. When applied to the Fermi photon-count map, the NN identifies a smooth GCE in the data, suggestive of the presence of DM, with the estimates for the background templates being consistent with existing results.

Spurious Point Source Signals in the Galactic Center Excess

We reexamine evidence that the Galactic Center Excess (GCE) originates primarily from point sources (PSs). We show that in our region of interest, non-Poissonian template fitting evidence for GCE PSs is an artifact of unmodeled north-south asymmetry of the GCE. This asymmetry is strongly favored by the fit (although it is unclear if this is physical), and when it is allowed, the preference for PSs becomes insignificant. We reproduce this behavior in simulations, including detailed properties of the spurious PS population. We conclude that the non-Poissonian template fitting evidence for GCE PSs is highly susceptible to certain systematic errors and should not at present be taken to robustly disfavor a dominantly smooth GCE.

Testing the Sensitivity of the Galactic Center Excess to the Point Source Mask

The Fermi Large Area Telescope (Fermi-LAT) Collaboration has an updated point source catalog, referred to as 4FGL. We perform the first template fit using a mask based on this new catalog and find that the excess in gamma rays detected at the Galactic Center in Fermi-LAT data persists. On the other hand, we find that a search for point sources is highly sensitive to the use of the 4FGL catalog: no sizable excess of bright pixels is apparent in the inner Galaxy when we mask out 4FGL point sources. Combining these observations restricts the ability of point sources to contribute to the Galactic Center excess. After identifying which bright sources have no known counterpart, we place strong constraints on any point source luminosity function capable of explaining the smooth emission identified in the template fit.

Gamma-Ray Dark Matter Searches in Milky Way Satellites—A Comparative Review of Data Analysis Methods and Current Results

If dark matter is composed of weakly interacting particles with mass in the GeV-TeV range, their annihilation or decay may produce gamma rays that could be detected by gamma-ray telescopes. Observations of dwarf spheroidal satellite galaxies of the Milky Way (dSphs) benefit from the relatively accurate predictions of dSph dark matter content to produce robust constraints to the dark matter properties. The sensitivity of these observations for the search for dark matter signals can be optimized thanks to the use of advanced statistical techniques able to exploit the spectral and morphological peculiarities of the expected signal. In this paper, I review the status of the dark matter searches from observations of dSphs with the current generation of gamma-ray telescopes: Fermi-LAT, H.E.S.S, MAGIC, VERITAS and HAWC. I will describe in detail the general statistical analysis framework used by these instruments, putting in context the most recent experimental results and pointing out the most relevant differences among the different particular implementations. This will facilitate the comparison of the current and future results, as well as their eventual integration in a multi-instrument and multi-target dark matter search.

Detection of Cross-Correlation between Gravitational Lensing and γ Rays

In recent years, many γ-ray sources have been identified, yet the unresolved component hosts valuable information on the faintest emission. In order to extract it, a cross-correlation with gravitational tracers of matter in the Universe has been shown to be a promising tool. We report here the first identification of a cross-correlation signal between γ rays and the distribution of mass in the Universe probed by weak gravitational lensing. We use data from the Dark Energy Survey Y1 weak lensing data and the Fermi Large Area Telescope 9-yr γ-ray data, obtaining a signal-to-noise ratio of 5.3. The signal is mostly localized at small angular scales and high γ-ray energies, with a hint of correlation at extended separation. Blazar emission is likely the origin of the small-scale effect. We investigate implications of the large-scale component in terms of astrophysical sources and particle dark matter emission.

Deciphering Residual Emissions: Time-dependent Models for the Nonthermal Interstellar Radiation from the Milky Way

Cosmic rays (CRs) in the Galaxy are an important dynamical component of the interstellar medium (ISM) that interact with the other major components (interstellar gas and magnetic and radiation fields) to produce broadband interstellar emissions that span the electromagnetic spectrum. The standard modeling of CR propagation and production of the associated emissions is based on a steady-state assumption, where the CR source spatial density is described using a smoothly varying function of position that does not evolve with time. While this is a convenient approximation, reality is otherwise, where primary CRs are produced in and about highly localized regions, e.g., supernova remnants, which have finite lifetimes. In this paper, we use the latest version of the galprop CR propagation code to model time-dependent CR injection and propagation through the ISM from a realistic 3D discretized CR source density distribution, together with full 3D models for the other major ISM components, and make predictions of the associated broadband nonthermal emissions. We compare the predictions for the discretized and equivalent steady-state model, finding that the former predicts novel features in the broadband nonthermal emissions that are absent for the steady-state case. Some of the features predicted by the discretized model may be observable in all-sky observations made by WMAP and Planck, the recently launched eROSITA, the Fermi-LAT, and ground-based observations by HESS, HAWC, and the forthcoming CTA. The nonthermal emissions predicted by the discretized model may also provide explanations of puzzling anomalies in high-energy γ-ray data, such as the Fermi-LAT north/south asymmetry and residuals like the so-called "Fermi bubbles."

Revival of the Dark Matter Hypothesis for the Galactic Center Gamma-Ray Excess

Statistical evidence has previously suggested that the galactic center GeV excess (GCE) originates largely from point sources, and not from annihilating dark matter. We examine the impact of unmodeled source populations on identifying the true origin of the GCE using non-Poissonian template fitting (NPTF) methods. In a proof-of-principle example with simulated data, we discover that unmodeled sources in the Fermi bubbles can lead to a dark matter signal being misattributed to point sources by the NPTF. We discover striking behavior consistent with a mismodeling effect in the real Fermi data, finding that large artificial injected dark matter signals are completely misattributed to point sources. Consequently, we conclude that dark matter may provide a dominant contribution to the GCE after all.

Development and performance verification of a 3-D position-sensitive Compton camera for imaging MeV gamma rays

In gamma-ray astronomy, the 1-10 MeV range is one of the most challenging energy bands to observe owing to low photon signals and a considerable amount of background contamination. This energy band, however, comprises a substantial number of nuclear gamma-ray lines that may hold the key to understanding the nucleosynthesis at the core of stars, spatial distribution of cosmic rays, and interstellar medium. Although several studies have attempted to improve observation of this energy window, development of a detector for astronomy has not progressed since NASA launched the Compton Gamma Ray Observatory (CGRO) in 1991. In this work, we first developed a prototype 3-D position-sensitive Compton camera (3D-PSCC), and then conducted a performance verification at NewSUBARU, Hyogo in Japan. To mimic the situation of astronomical observation, we used a MeV gamma-ray beam produced by laser inverse Compton scattering. As a result, we obtained sharp peak images of incident gamma rays irradiating from incident angles of 0° and 20°. The angular resolution of the prototype 3D-PSCC was measured by the Angular Resolution Measure and estimated to be 3.4° ± 0.1° (full width at half maximum (FWHM)) at 1.7 MeV and 4.0° ± 0.5° (FWHM) at 3.9 MeV. Subsequently, we conceived a new geometry of the 3D-PSCC optimized for future astronomical observations, assuming a 50-kg class small satellite mission. The SΩ of the 3D-PSCC is 11 cm²sr, anticipated at 1 MeV, which is small but provides an interesting possibility to observe bright gamma-ray sources owing to the high intrinsic efficiency and large field of view (FoV).

Gamma-Ray Sensitivity to Dark Matter Subhalo Modelling at High Latitudes

Searches for “dark” subhaloes in gamma-ray point-like source catalogues are among promising strategies for indirect dark matter detection. Such a search is nevertheless affected by uncertainties related, on the one hand, to the modelling of the dark matter subhalo distribution in Milky-Way-like galaxies, and, on the other hand, to the sensitivity of gamma-ray instruments to the dark matter subhalo signals. In the present work, we assess the detectability of dark matter subhaloes in Fermi-LAT catalogues, taking into accounts uncertainties associated with the modelling of the galactic subhalo population. We use four different halo models bracketing a large set of uncertainties. For each model, adopting an accurate detection threshold of the LAT to dark matter subhalo signals and comparing model predictions with the number of unassociated point-sources in Fermi-LAT catalogues, we derive upper limits on the annihilation cross section as a function of dark matter mass. Our results show that, even in the best-case scenario (i.e., DMonly subhalo model), which does not include tidal disruption from baryons, the limits on the dark matter parameter space are less stringent than current gamma-ray limits from dwarf spheroidal galaxies. Comparing the results obtained with the different subhalo models, we find that baryonic effects on the subhalo population are significant and lead to dark matter constraints that are less stringent by a factor of ∼2 to ∼5. This uncertainty comes from the unknown resilience of dark matter subhaloes to tidal disruption.

Relativistic Jets in Gamma-Ray-Emitting Narrow-Line Seyfert 1 Galaxies

Before the launch of the Fermi Gamma-ray Space Telescope satellite only two classes of active galactic nuclei (AGN) were known to generate relativistic jets and thus to emit up to the γ -ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The discovery by the Large Area Telescope (LAT) on-board the Fermi satellite of variable γ -ray emission from a few radio-loud narrow-line Seyfert 1 galaxies (NLSy1) revealed the presence of an emerging third class of AGN with powerful relativistic jets. Considering that NLSy1 are usually hosted in late-type galaxies with relatively small black hole masses, this finding opened new challenging questions about the nature of these objects, the disc/jet connection, the emission mechanisms at high energies, and the formation of relativistic jets. In this review, I will discuss the broad-band properties of the γ -ray-emitting NLSy1 included in the Fourth Fermi LAT source catalog, highlighting major findings and open questions regarding jet physics, black hole mass estimation, host galaxy and accretion process of these sources in the Fermi era.

Blazar Optical Polarimetry: Current Progress in Observations and Theories

Polarimetry has been a standard tool to probe the active galactic nucleus (AGN) jet magnetic field. In recent years, several optical polarization monitoring programs have been carried out, bringing in many exciting new results and insights into jet dynamics and emission. This article discusses current progress in blazar optical polarimetry. The main focus is the variability of polarization signatures, which has spurred a lot of theoretical studies. These novel developments have provided unique constraints on the blazar flares and emphasized the role of the magnetic field in jet evolution. Optical polarimetry will continue to act as an essential component in the multi-messenger study of AGN jets, in particular with the upcoming high-energy polarimetry. Comparing to first-principle numerical simulations, future multi-wavelength polarimetry can shed light on jet dynamics, particle acceleration, and radiation processes.

The High Energy View of FR0 Radio Galaxies

A new class of low-power compact radio sources with limited jet structures, named FR 0, is emerging from recent radio-optical surveys. This abundant population of radio galaxies, five times more numerous than FR Is in the local Universe (z < 0.05), represent a potentially interesting target at high and very-high energies (greater than 100 GeV), as demonstrated by a single case of Fermi detection. Furthermore, these radio galaxies have been recently claimed to contribute non-negligibly to the extra-galactic γ -ray background and to be possible cosmic neutrino emitters. Here, we review the radio through X-ray properties of FR 0s to predict their high-energy emission (from MeV to TeV), in light of the near-future facilities operating in this band.

Search for Gamma-ray Emission from p-wave Dark Matter Annihilation in the Galactic Center

Indirect searches for dark matter through Standard Model products of its annihilation generally assume a cross-section which is dominated by a term independent of velocity (s-wave annihilation). However, in many DM models an s-wave annihilation cross-section is absent or helicity suppressed. To reproduce the correct DM relic density in these models, the leading term in the cross section is proportional to the DM velocity squared (p-wave annihilation). Indirect detection of such p-wave DM is difficult because the average velocities of DM in galaxies today are orders of magnitude slower than the DM velocity at the time of decoupling from the primordial thermal plasma, thus suppressing the annihilation cross-section today by some five orders of magnitude relative to its value at freeze out. Thus p-wave DM is out of reach of traditional searches for DM annihilations in the Galactic halo. Near the region of influence of a central supermassive black hole, such as Sgr A*, however, DM can form a localized over-density known as a "spike". In such spikes the DM is predicted to be both concentrated in space and accelerated to higher velocities, thereby allowing the γ-ray signature from its annihilation to potentially be detectable above the background. We use the Fermi Large Area Telescope to search for the γ-ray signature of p-wave annihilating DM from a spike around Sgr A* in the energy range 10 GeV-600 GeV. Such a signal would appear as a point source and would have a sharp line or box-like spectral features difficult to mimic with standard astrophysical processes, indicating a DM origin. We find no significant excess of γ rays in this range, and we place upper limits on the flux in γ-ray boxes originating from the Galactic Center. This result, the first of its kind, is interpreted in the context of different models of the DM density near Sgr A*.

The Large Magellanic Cloud with the Cherenkov Telescope Array

The Large Magellanic Cloud (LMC) is a spiral galaxy, satellite of the Milky way with a high star formation activity. It represents a unique laboratory for studying an extended and spatially resolved star-forming galaxy through gamma-ray observatories. Therefore, the LMC survey is one of the key science projects for the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory. In this document we present the work performed over the last year by the CTA working group dedicated to the LMC, in order to offer a first characterization of the LMC at TeV energies. We have performed detectability forecasts based on the expected CTA performance for all sources in the region of interest of the LMC with known emission at GeV energies and above. Based on previous observations made by Fermi-LAT and H.E.S.S. we have characterized all point sources, extended sources and diffuse emission produced by cosmic-ray propagation, extrapolating their spectra to CTA energies. Finally, we have characterized the signal expected by different annihilation mechanisms of dark matter (DM) particles within the LMC, computing the detection sensitivity curve for this target in the cross-section-to-mass plane.

Monitoring and Multi-Messenger Astronomy with IceCube

IceCube currently is the largest neutrino observatory with an instrumented detection volume of 1 km3 buried in the ice-sheet close to the antarctic South Pole station. With a 4 π field of view and an up-time of >99%, it is continuously monitoring the full sky to detect astrophysical neutrinos. With the detection of an astrophysical neutrino flux in 2013, IceCube opened a new observation window to the non-thermal Universe. The IceCube collaboration has a large program to search for astrophysical neutrinos, including measurements of the energy spectrum of the diffuse astrophysical flux, auto- and cross-correlation studies with other multi-messenger particles, and a real-time alert and follow-up system. On 22 September 2017, the IceCube online system sent out an alert reporting a high-energy neutrino event. This alert triggered a series of multi-wavelength follow-up observations that revealed a spatially-coincident blazar TXS 0506+056, which was also in an active flaring state. This correlation was estimated at a 3 σ level. Further observations confirmed the flaring emission in the very-high-energy gamma-ray band. In addition, IceCube found an independent 3.5 σ excess of a time-variable neutrino flux in the direction of TXS 0506+056 two years prior to the alert by examining 9.5 years of archival neutrino data. These are the first multi-messenger observations of an extra-galactic astrophysical source including neutrinos since the observation of the supernova SN1987A. This review summarizes the different detection and analysis channels for astrophysical neutrinos in IceCube, focusing on the multi-messenger program of IceCube and its major scientific results.

Progress in Multi-Wavelength and Multi-Messenger Observations of Blazars and Theoretical Challenges

This review provides an overview of recent advances in multi-wavelength and multi-messenger observations of blazars, the current status of theoretical models for blazar emission, and prospects for future facilities. The discussion of observational results will focus on advances made possible through the Fermi Gamma-Ray Space Telescope and ground-based gamma-ray observatories (H.E.S.S., MAGIC, VERITAS), as well as the recent first evidence for a blazar being a source of IceCube neutrinos. The main focus of this review will be the discussion of our current theoretical understanding of blazar multi-wavelength and multi-messenger emission, in the spectral, time, and polarization domains. Future progress will be expected in particular through the development of the first X-ray polarimeter, IXPE, and the installation of the Cherenkov Telescope Array (CTA), both expected to become operational in the early to mid 2020s.

Monitoring the Extragalactic High Energy Sky

Blazars are jetted active galactic nuclei with a jet pointing close to the line of sight, hence enhancing their intrinsic luminosity and variability. Monitoring these sources is essential in order to catch them flaring and promptly organize follow-up multi-wavelength observations, which are key to providing rich data sets used to derive e.g., the emission mechanisms at work, and the size and location of the flaring zone. In this context, the Fermi-LAT has proven to be an invaluable instrument, whose data are used to trigger many follow-up observations at high and very high energies. A few examples are illustrated here, as well as a description of different data products and pipelines, with a focus given on FLaapLUC, a tool in use within the H.E.S.S. collaboration.

Searches for IceCube neutrinos from blazar flares using correlations with gamma-ray lightcurves

Blazars have long been considered as accelerator candidates for cosmic rays. In such a scenario, hadronic interactions in the jet would produce neutrinos and gamma rays. Correlating the astrophysical neutrinos detected by IceCube with the gamma-ray emission from blazars could therefore help elucidate the origin of cosmic rays. In our method we focus on periods where blazars show an enhanced gamma-ray flux, as measured by Fermi-LAT, thereby reducing the background of the search. We present results for TXS 0506+056, using nearly 10 years of IceCube data and discuss them in the context of other recent analyses on this source. In addition, we give an outlook on applying this method in a stacked search for the combined emission from a selection of variable Fermi blazars.

High-energy gamma- and cosmic-ray observations with future space-based GAMMA-400 gamma-ray telescope

The future space-based GAMMA-400 gamma-ray telescope will be installed on the Navigator platform of the Russian Astrophysical Observatory. A highly elliptical orbit will provide observations for 7-10 years of many regions of the celestial sphere continuously for a long time (~ 100 days). GAMMA-400 will measure gamma-ray fluxes in the energy range from ~ 20 MeV to several TeV and electron + positron fluxes up to ~ 20 TeV. GAMMA-400 will have an excellent separation of gamma rays from the background of cosmic rays and electrons + positrons from protons and an unprecedented angular (~ 0.01° at Eγ = 100 GeV) and energy (~ 1% at Eγ = 100 GeV) resolutions better than for Fermi-LAT, as well as ground-based facilities, by a factor of 5-10. Observations of GAMMA-400 will provide new fundamental data on discrete sources and spectra of gamma-ray emission and electrons + positrons, as well as the nature of dark matter.

Dissecting the region around IceCube-170922A: the blazar TXS 0506+056 as the first cosmic neutrino source

On MJD 58018 the IceCube neutrino observatory detected a highlyenergetic, well-reconstructed neutrino, IceCube-170922A, at a distance of 0:1° to a γ-ray flaring blazar, TXS 0506+056. Follow-up searches in archival data additionally revealed a larger flare of neutrinos from the same direction. In order to complete the picture we present here a full multi-wavelength study of the region around IceCube-170922A. While we identify also other non-thermal counterpart candidates, we show that all the evidence points to TXS 0506+056 as the dominant neutrino emitter. Additionally, an analysis of all the available Fermi-LAT data indicates a hard spectrum/low flux of TXS 0506+056 during the neutrino flare in contrast to a soft spectrum/high flux at the arrival time of IceCube-170922A. Putting all the pieces together we conclude that the SED of TXS 0506+056 can be energetically reconnected for both neutrino observations.

AGILEγ-ray sources coincident with cosmic neutrino events

Using data obtained by theγ-ray imager on board of the AGILE satellite, we systematically searched for transientγ-ray sources above 100 MeV in temporal and spatial coincidence with high-energy neutrino IceCube events. We found some significantγ-ray transients possibly associated with HESE neutrinos recently detected by the IceCube experiment. In this talk, we review the AGILE results and we will show that the probability of this association to be by chance is very low. One of the sources detected by AGILE is related to the blazar AGN TXS 0506+056. For the other sources, there are no obvious known counterparts.

GRAINE project: precise gamma-ray observations with balloon-borne emulsion telescope

Observation of cosmic gamma rays is important in the understanding of high-energy objects or phenomena in the universe. Since 2008, the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope (Fermi-LAT) has surveyed the sub-GeV/GeV gamma-ray sky and achieved high statistics measurements. However, observation at low galactic latitudes remains difficult owing to the lack of angular resolution, and new issues following the operation of Fermi-LAT have arisen. We devised a precise gamma-ray observation project, Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE), using balloon-borne emulsion gammaray telescopes to realize high angular resolution, polarization-sensitive, and large-aperture observations in the 10 MeV–100 GeV energy region. Following basic developments on the ground, we performed three balloon-borne experiments with upgraded instruments. In this paper, we present results from the second balloon experiment in 2015, a report on the latest balloon experiment conducted on April 26, 2018, and a recent study on hadronic interactions using proton beams.

Detection of a flaring blazar coincident with an IceCube high-energy neutrino

In September 22, 2017, IceCube released a public alert announcing the detection of a 290 TeV neutrino track event with an angular uncertainty of one square degree (90% containment). A multi-messenger follow-up campaign was initiated resulting in the detection of a GeV gamma-ray flare by the Fermi Large Area Telescope positionally consistent with the location of the known Bl Lac object, TXS 0506+056 , located only 0.1 degrees from the best-fit neutrino position. The probability of finding a GeV gamma-ray flare in coincidence with a high-energy neutrino event assuming a correlation of the neutrino flux with the gamma-ray energy flux in the energy band between 1 and 100 GeV was calculated to be 3σ (after trials correction). Following the detection of the flaring blazar the imaging air Cherenkov telescope MAGIC detected the source for the first time in the > 100 GeV gamma-ray band. The activity of the source was confirmed in X-ray, optical and radio wavelength. Several groups have developed lepto-hadronic models which succeed to explain the multi-messenger spectral energy distribution.

Molecular Ionization Rates and Ultracompact Dark Matter Minihalos

Molecular ionization in the central molecular zone of our Galaxy is enhanced over the typical galactic value by an order of magnitude or more. This cannot be easily explained for dense Galactic center molecular complexes in the absence of embedded sources of low energy cosmic rays. We provide such a source in the form of ultracompact minihalos of self-annihilating dark matter for a variety of annihilation channels that depend on the particle mass and model. Such sources are motivated for plausible inflationary power spectrum parameters, and while possibly subdominant in terms of the total dark matter mass within the Galactic bulge, might also account for, or at least not be in tension with, the Fermi Galactic center γ-ray excess.

Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501

Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been proposed to explain these data. Here, we collect some of the recent developments on this topic using the well-known blazar Mrk 501 as example of complex and interesting aspects coming from its flux distribution in different energy ranges and at different timescales. The observational data we refer to are those collected in a complementary manner by Fermi-LAT over multiple years, and by the First G-APD Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence of the bright flare of June 2014.

Detection of Periodic Radio Signal from the Blazar J1043+2408

The search for periodic signals from blazars has become a widely discussed topic in recent years. In the scenario that such periodic changes originate from the innermost regions of blazars, the signals bear imprints of the processes occurring near the central engine, which are mostly inaccessible to our direct view. Such signals provide insights into various aspect of blazar studies, including disk-jet connection, magnetic-field configuration and, more importantly, strong gravity near the supermassive black holes and release of gravitational waves from binary supermassive-black-hole systems. In this work, we report the detection of a periodic signal in the radio light curve of blazar J1043+2408 spanning ∼10.5 years. We performed multiple methods of time-series analysis, namely, epoch folding, Lomb–Scargle periodogram, and discrete autocorrelation function. All three methods consistently revealed a repeating signal with a periodicity of ∼560 days. To robustly account for the red-noise processes usually dominant in the blazar variability and other possible artefacts, a large number of Monte Carlo simulations were performed. This allowed us to estimate high significance (99.9% local and 99.4% global) against possible spurious detection. As possible explanations, we discuss a number of scenarios, including binary supermassive-black-hole systems, Lense–Thirring precession, and jet precession.

A gamma-ray determination of the Universe's star formation history

The light emitted by all galaxies over the history of the Universe produces the extragalactic background light (EBL) at ultraviolet, optical, and infrared wavelengths. The EBL is a source of opacity for gamma rays via photon-photon interactions, leaving an imprint in the spectra of distant gamma-ray sources. We measured this attenuation using 739 active galaxies and one gamma-ray burst detected by the Fermi Large Area Telescope. This allowed us to reconstruct the evolution of the EBL and determine the star formation history of the Universe over 90% of cosmic time. Our star formation history is consistent with independent measurements from galaxy surveys, peaking at redshift z ~ 2. Upper limits of the EBL at the epoch of reionization suggest a turnover in the abundance of faint galaxies at z ~ 6.

Fermi: Monitoring the Gamma-Ray Universe

Since 2008, the Large Area Telescope and the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope have been monitoring the entire sky at energies from about 8 keV to more than 1 TeV. Photon-level data and high-level data products are made publicly available in near-real time, and efforts continue to improve the response time. This long-duration, all-sky monitoring has enabled a broad range of science, from atmospheric phenomena on Earth to signals from high-redshift sources. The Fermi instrument teams have worked closely with multiwavelength and multi-messenger observers and theorists to maximize the scientific return from the observatory, and they look forward to continued cooperative efforts as Fermi moves into its second decade of operation.

Radio Galaxies at VHE Energies

Radio Galaxies have by now emerged as a new γ-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe the nature and physics of active galactic nuclei (AGN) in general. This review provides a compact summary of their observed characteristics at very high γ-ray energies (VHE; greater than 100 GeV) along with a discussion of their possible physics implications. A particular focus is given to a concise overview of fundamental concepts concerning the origin of variable VHE emission, including recent developments in black hole gap physics.

A 34.5 day quasi-periodic oscillation in γ-ray emission from the blazar PKS 2247-131

Since 2016 October, the active galaxy PKS 2247−131 has undergone a γ-ray outburst, which we studied using data obtained with the Fermi Gamma-ray Space Telescope. The emission arises from a relativistic jet in PKS 2247−131, as an optical spectrum only shows a few weak absorption lines, typical of the BL Lacertae sub-class of the blazar class of active galactic nuclei. Here we report a ≃34.5 day quasi-periodic oscillation (QPO) in the emission after the initial flux peak of the outburst. Compared to one-year time-scale QPOs, previously identified in blazars in Fermi energies, PKS 2247−131 exhibits the first clear case of a relatively short, month-like oscillation. We show that this QPO can be explained in terms of a helical structure in the jet, where the viewing angle to the dominant emission region in the jet undergoes periodic changes. The time scale of the QPO suggests the presence of binary supermassive black holes in PKS 2247−131.

PKS 2247–131 is an active galaxy that has been emitting gamma-ray flares since October 2016. Here, the authors used data obtained with the Fermi Gamma-ray Space Telescope to find that PKS 2247–131 presents a relatively short, month-like flux oscillation at gamma-ray energies of 0.1–300 GeV.

Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A

Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera–electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray–emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.