Parallel Session: MESSENGERS - COSMIC RAYS (N)

Location: King Henry 0.02

N1 - Monday 14:00-15:40 (Matthew Baring)

Studies of ultra-high energy cosmic rays with the Pierre Auger Observatory


Henryk Wilczynski (Institute of Nuclear Physics, Polish Academy of Sciences)

The Pierre Auger Observatory has been collecting experimental data over the recent 15 years. The high accuracy of air shower measurements, combined with large statistics accumulated, enables determination of ultra-high energy cosmic ray (UHECR) properties with accuracy not attainable until now. A large-scale anisotropy of UHECR arrival directions was measured over more than three decades in energy. The energy spectrum was determined up to beyond 10^20 eV, confirming the flux suppression above ~5 x 10^19 eV. The UHECR mass composition determined using both fluorescence and surface array measurements indicates a mixed composition, rather than pure light or heavy nuclei. Such a result suggests that the spectrum suppression may be related to a limit of cosmic ray acceleration at sources, rather than to a propagation effect. However, the composition measurement is heavily influenced by uncertainties in hadronic interaction properties at ultra-high energies. Precise measurement of the muon component of air showers is the key to disentangling the cosmic ray composition from the influence of interaction properties, and thus to understanding the origin of the spectrum suppression. This is the main motivation for the ongoing upgrade of the Observatory.

Precision Measurement of Primary Cosmic Rays with Alpha Mgnetic Spectrometer on ISS


Qi Yan (LNS, MIT)

We present precision high statistics measurements of primary cosmic ray protons, helium, carbon and oxygen fluxes by Alpha Magnetic Spectrometer in the rigidity range from 2 GV to 3 TV. These measurements are based on 1 billion of protons, 125 million of Helium, 14 million of Carbon and 12 million of Oxygen nuclei collected by AMS during the first 7 years of operation aboard the International Space Station. The properties of these primary cosmic rays will be discussed.
We present high statistics fluxes of primary cosmic ray neon, magnesium and silicon measured by Alpha Magnetic Spectrometer in the rigidity range from 3 GV to 3 TV. These measurements are based on 5 million nuclei collected by AMS during the first 7 years of operation aboard the International Space Station. The unexpected new properties of these primary cosmic rays will be shown.

New properties of secondary cosmic rays measured by AMS on ISS


Laurent Derome (DPNC, AMS Collaboration /university of Geneva)

New properties of secondary cosmic rays Li, Be and B measured by AMS will be discussed.

Determining the Helicity of Intergalactic Magnetic Fields with Numerical Simulations in Astroparticle Physics


Andrey Saveliev (Immanuel Kant Baltic Federal University)

The origin of the first magnetic fields in the Universe is a standing problem in cosmology. Intergalactic Magnetic Fields (IGMFs) may be an untapped window to the primeval Universe, providing further constrains on magnetogenesis. We demonstrate the feasibility of using gamma rays from electromagnetic cascades originating from TeV blazars and Ultra-High-Energy Cosmic Rays (UHECRs) to constrain the helicity of IGMFs by performing simulations of their propagation in simple magnetic field and source configurations. We show that the arrival directions of the respective particles may be used to measure the absolute value of the helicity and its sign.

N2 - Monday 16:10-17:50 (Matthew Baring)

Results from the Telescope Array Experiment


Charles Jui (Physics and Astronomy, University of Utah)

The Telescope Array is the largest hybrid cosmic ray detector in the Northern hemisphere, designed to measure primary particles from 4 PeV to 100 EeV range. TA is a hybrid detector consisting of 507 plastic scintillation counters on a 1.2km square grid, and fluorescence detectors at three stations overlooking the sky above the surface detector array. A low energy extension (TALE) of ten high-elevation telescopes has been operating since 2014, and 80 SD units were deployed in 2017 to form an infill array with 400m and 600 m spacing to complement the FD. These additions lower the energy threshold of TA down to 4 PeV. We are also half way through deploying the TAx4 SD counters which will expand the coverage of TA from 700 to 2800 square-km at the highest energies. In this talk, we will present the most recent results of TA, including spectrum, composition, and anisotropy, as well as review the current status and discuss future prospects for the experiment.

Why there is no simultaneous detection of Gamma rays and x-rays from x-ray bright galaxy clusters? A hydrodynamical study on the manufacturing of cosmic rays in the evolving dynamical states of galaxy clusters.


Reju Sam John (Inter-University Centre for Astronomy and Astrophysics)

Galaxy clusters are known to be reservoirs of Cosmic Rays (CRs), as inferred from theoretical calculations or detection of CR-derived observables. CR acceleration in clusters is mostly attributed to the dynamical activity that produces shocks. Shocks in clusters emerge out of merger or accretion, but which one is more effective in producing CRs? at which dynamical phase? and why? To this aim, we study the production or injection of CRs through shocks and its evolution in the galaxy clusters using cosmological simulations with the {sc enzo} code. Particle acceleration model considered here is primarily the Diffusive Shock Acceleration (DSA) of thermal particles, but we also report a tentative study with pre-existing CRs. Defining appropriate dynamical states using the concept of virialization, we studied a sample of merging and non-merging clusters. We report that the merger shocks (with Mach number $mathcal{M}sim2-5$) are the most effective CR producers, while high-Mach peripheral shocks (i.e. $mathcal{M}>5$) are mainly responsible for the brightest phase of CR injection in clusters. Clusters once merged, permanently deviate from CR and X-ray mass scaling of non-merging systems, enabling us to use it as a tool to determine the state of merger. Through a temporal and spatial evolution study, we found a strong correlation between cluster merger dynamics and CR injection. We observed that the brightest phase of X-ray and CR injection from clusters occur respectively at about 1.0 and 1.5 Gyr after every mergers. This is the reason for simultaneous non-detection of Gamma rays and x-rays from x-ray bright galaxy clusters.

New Properties of Primary Cosmic Rays Measured by AMS on ISS


Yao Chen (DPNC, Université de Genève)

New properties of the primary cosmic rays from Z=1 to Z=16 measured by the Alpha Magnetic Spectrometer on the International Space Station will be presented.

Cosmic Ray Isotopes measured by AMS


Fernando Barao (AMS Collaboration/CIEMAT)

The properties of cosmic ray isotopes of H, He and Li, measured by AMS will be presented

N3 - Tuesday 14:00-15:40 (Matthew Baring)

Have charge, will travel: What HAWC has to say about the origin and diffusion of Galactic cosmic rays


Henrike Fleischhack (Department of Physics, Michigan Technological University)

More than a century after the discovery of cosmic rays, there are still many unresolved questions relating to their composition, their origins and their propagation in the Galaxy. Gamma-ray astronomy has and will play an important part in answering these questions.

HAWC, the High Altitude Water Cherenkov Observatory, has been continuously scanning the northern TeV gamma-ray sky for more than three years, providing an unprecedented view of gamma-ray sources in the Galaxy. In this presentation, I will highlight some of the recent results by the HAWC collaboration related to Galactic cosmic rays. In particular, I will touch on the following questions: What are the sources of Galactic cosmic rays up to the knee region? (How) do pulsars and pulsar wind nebulae affect cosmic ray diffusion in their vicinity?

I will also discuss future prospects in ground-based gamma-ray astronomy and how they can improve our understanding of Galactic cosmic rays.

Can Supernova Remnants Accelerate Particles to PeV energies?


Vikram Dwarkadas (Astronomy and Astrophysics, University of Chicago)

Particles up to the `knee’ in the cosmic-ray spectrum are presumed to arise from Galactic sources. Supernova Remnants (SNRs) are one of the prime candidates for producing particles at energies greater than a PeV. However, to date not a single observed SNR has been shown to have accelerated particles to PeV energies and beyond. In this presentation we investigate, using analytic and semi-analytic models, the maximum energies that particles could be accelerated to in SNRs, and the various factors that contribute to this. We show that young supernovae, days to weeks after outburst, are more likely to accelerate particles to PeV energies than older SNRs, and discuss prospects for the upcoming CTA to detect these supernovae.

Detection of a gamma-ray halo around Geminga with the Fermi-LAT and implications for the positron flux


Silvia Manconi (Institute for Theoretical Particle Physics and Cosmology, RWTH Aachen )

An excess in the flux of cosmic positrons at Earth above 10 GeV has been measured by Pamela, Fermi-LAT and with unprecedented precision by AMS-02. The observed flux cannot be explained by the production of positrons in the spallation reaction of hadronic cosmic rays with the interstellar medium. Various interpretations have been invoked to explain this excess, such as the production in Galactic supernova remnants and pulsar wind nebulae (PWNe) or, intriguinly, in the dark matter halo of the Milky Way. Recently, Milagro and HAWC experiments reported the detection of an extended gamma-ray emission from Geminga and Monogem PWNe at TeV energies. These nearby and powerful PWNe have been widely considered as the main candidates to contribute to the cosmic positrons at Earth. Severe constraints for a significant PWNe contribution to the positron excess can be derived from this gamma-ray emission, which has been interpreted as coming from the electrons and positrons accelerated in the PWNe and undergoing inverse Compton scattering in the interstellar medium. Moreover, the size of extension of these halos suggests that the diffusion around PWNe is about two orders of magnitude less intense than the value assumed to fit the cosmic-rays measured by AMS-02.
In this contribution we report the first detection of a significant emission from the Geminga halo at GeV energies in Fermi-LAT data, derived by including the proper motion of its pulsar. We present a detailed study of the gamma-ray halo around Geminga and Monogem, and show the constraints found for the contribution of these PWNe to the positron excess, combining Milagro and HAWC data with measurements from the Fermi-LAT for the first time. The size of
extension and the consequences for the diffusion coefficient in these halos at GeV energies are also explored. We demonstrate that using gamma-ray data from the LAT is of central importance to provide a precise estimate for the PWN contribution to the cosmic positron flux, and discuss some perspectives for the search of extended GeV halos around other Galactic PWNe.

The Important Role of Cosmic-Ray Re-Acceleration


Martina Cardillo (IAPS-INAF)

The improvement of high-energy instruments, both from Earth and Space, has provided a great amount of data from Supernova Remnants (SNRs). These data were interpreted in the context of Cosmic Ray (CR) acceleration with hadronic and leptonic models, in order to find a direct proof of CR acceleration in correspondence of the shocks of these objects. However, in the last years, the role of pre-existing CR re-acceleration was found to be not negligible and it was considered in several gamma-ray models in order to explain some particle and gamma-ray CR spectral features. Here we briefly summarize the main results obtained in the study of CR re-acceleration, stressing its fundamental contribution in the middle-aged SNR shocks and, likely, in the forward shock (FS) of stellar wind. Our aim is to fix the importance of re-energization of pre-existing CRs in the Galaxy and the need to take it into account in the explanation of detected particle and gamma-ray CR spectrum.

N4 - Tuesday 16:10-17:50 (Matthew Baring)

Massive Stars as Major Factories of Galactic Cosmic Rays


Emma de Ona Wilhelmi (DESY / ICE)

The identification of major contributors to the locally observed fluxes of Cosmic Rays (CRs) is a prime objective towards the resolution of the long-standing enigma of CRs. We report on a compelling similarity of the energy and radial distributions of multi-TeV CRs extracted from observations of very high energy (VHE) γ-rays towards the Galactic Center (GC) and two prominent clusters of young massive stars, Cyg OB2 and Westerlund 1. This resemblance we interpret as a hint that CRs responsible for the diffuse VHE γ-ray emission from the GC are accelerated by the ultracompact stellar clusters located in the heart of GC. The derived 1/r decrement of the CR density with the distance from a star cluster is a distinct signature of continuous, over a few million years, CR injection into the interstellar medium. The lack of brightening of the γ-ray images toward the stellar clusters excludes the leptonic origin of γ-radiation. The hard, −2.3 type power-law energy spectra of parent protons continues up to PeV energies. The efficiency of conversion of kinetic energy of stellar winds to CRs can be as high as 10 percent implying that the young massive stars may operate as proton PeVatrons with a dominant contribution to the flux of highest energy galactic CRs.

Multiwavelength Variability Signatures of Relativistic Shocks in Blazar Jets


Markus Boettcher (Centre for Space Research, North-West University)

Mildly-relativistic shocks in colliding magnetohydrodynamic flows
are prime sites for relativistic particle acceleration and production
of strongly variable, polarized multi-wavelength emission from relativistic
jet sources such as blazars and gamma-ray bursts. The principal energization
mechanisms at these shocks are diffusive shock acceleration and shock drift
acceleration. In recent work, we had self-consistently coupled shock acceleration
and radiation transfer simulations in blazar jets in a basic one-zone scenario.
These one-zone models revealed that the observed spectral energy distributions
(SEDs) of blazars strongly constrain the nature of hydromagnetic turbulence near
the shock. In this paper, we expand our previous work by including full time
dependence and treating two zones, one being the site of acceleration, and the
second being a larger emission zone. This construction is applied to multiwavelength
flares of the flat spectrum radio quasar 3C 279 and the high-frequency peaked BL Lac
object Mrk 501, fitting snap-shot SEDs and generating light curves that are consistent
with observed variability timescales. The model predicts correlated variability across
all wavebands, but cross-band time lags depending on the type of blazar (FSRQ vs. BL Lac),
as well as distinctive spectral hysteresis patterns in all wavelength bands, from mm
radio waves to gamma-rays. These evolutionary signatures serve to provide diagnostics
on the competition between acceleration and radiative cooling.

Studying the Extreme Behaviour of 1ES 2344+514


Axel Arbet-Engels (Institute for Particle Physics and Astrophysics , ETH Zürich)

The BL Lac type object 1ES 2344+514 was one of the first sources to be included in the EHBL (extreme high-peaked BL Lac) family. EHBLs are characterised by a broadband spectral energy distribution (SED) featuring the synchrotron peak above ~1 keV. From previous studies of 1ES 2344+514 in the very-high-energy (VHE, E>100GeV) gamma-ray range, its inverse Compton (IC) peak is expected around 200 GeV.
1ES 2344+514 was first detected in VHE by Whipple in 1995 during a very bright outburst showing around 60% of the flux of the Crab Nebula above 350 GeV. In 2000, observations with Beppo-SAX revealed a large 0.1-10 keV flux variability on timescales of a few hours, during another strong flare in the X-ray band.
The extreme behaviour of the source triggered several multi-wavelength campaigns, during most of which the source appeared to be in a low state, not showing extreme behaviour again.
In August 2016, FACT detected 1ES 2344+514 in a high state and triggered multi-wavelength observations. The VHE observations show a flux level similar to the historical maximum of 1995. The combination of MAGIC, FACT and Fermi-LAT spectra provides an unprecedented characterisation of the IC peak for this object during a flaring episode. We find an atypically hard spectrum in the VHE as well as a hard X-ray spectrum, revealing a renewed extreme behaviour.