Parallel Session: MESSENGERS - GAMMA RAYS (O)

Location: King Henry 0.02

O5 - Wednesday 14:00-15:40 (Emma de Oña-Wilhelmi)

High-Energy Astrophysics with AGILE


Carlotta Pittori (INAF-OAR and ASI-SSDC)

I will present the main AGILE results, including some recent updates related to gravitational waves, neutrinos and the hunt for their electromagnetic counterparts.
AGILE is an Italian Space Agency (ASI) space mission devoted to gamma-ray observations in the 30 MeV – 50 GeV energy range, with simultaneous X-ray imaging in the 18-60 keV band. Launched in April 2007, the AGILE satellite is in its 13th year of operations in orbit, and it is substantially contributing to improve our knowledge of the high-energy gamma-ray sky.
Gamma-ray emission from cosmic sources at energies above 100 MeV is intrinsically non-thermal, and the study of the wide variety of observed Galactic and Extragalactic gamma-ray sources provides a unique opportunity to test theories of particle acceleration and radiation processes in extreme conditions, and it may help to shed light on the foundations of physics itself.

The role of Fermi-GBM in the era of multi-messenger Astronomy


Christian Malacaria (NASA-MSFC/USRA)

We review 10 years of continuous monitoring of the high-energy sky with the Gamma-ray Burst Monitor (GBM), the softer-energy all-sky monitoring instrument aboard the Fermi Gamma-ray Space Telescope.
The excellent combination of timing, spectral and full-sky coverage capabilities of GBM make it a unique instrument for the study of transient sources.
Over 10 years of operation, GBM helped to study a pletora of astrophysical phenomena, from Gamma-ray Bursts (GRBs), to Terrestrial Gamma-ray Flashes (TGFs), to accreting X-ray Pulsars (XRPs), to finish with Gravitational Wave counterparts, of which GBM observed the only GRB unambiguously associated with merging neutron stars, GRB 170817A, thus opening the era of multi-messenger astronomy.
As of today, GBM is the most prolific detector of short GRBs, with about 40 per year triggered on board and an additional 80 candidates in ground searches each year.
Recently, sub-threshold, targeted on-ground searches have been integrated to increase the sensitivity around GW events confirmed by the LIGO/Virgo collaboration, thus resulting in a few potential associations (currently under investigations).
The ground-based searches are also used to search for counterparts to neutrinos and fast radio bursts in the GBM data.

Extracting Insights from Time Domain in Relativistic Sources


Nachiketa Chakraborty (High Energy Astrophysics / Data Assimilation Research Centre, MPIK, Heidelberg / University of Reading)

Despite extensive observational and theoretical research, some fundamental properties of the most luminous particle accelerators and variable sources like AGNs, GRBs, etc are not clearly known. Location and mechanisms of particle acceleration such as shocks vs reconnection, leptonic vs hadronic emission and origin of flares and relation to the long-term quiescent behaviour are open questions. Complexity of environments and processes make it hard to disentangle different scenarios. Studying variability of these relativistic sources statistically gives us a way to do this. These variations range from slow, quasi-periodic oscillations to fast, episodic outbursts. With the help of time-series simulations, insights and inputs from physical models and indeed some novel approaches such as causality measures from information theory, we can probe deeper and potential distinguish between different models and scenarios. This presentation seeks to demonstrate the potential of combinations of these to break degeneracies between competing particle acceleration and radiative models.

The Transient High-Energy Sky and Early Universe Surveyor (THESEUS)


Lorenzo Amati (OAS Bologna, INAF)

The Transient High-Energy Sky and Early Universe Surveyor (THESEUS) is a space mission
concept currently under Phase A study by ESA as candidate M5 mission, aiming at exploiting
Gamma-Ray Bursts for investigating the early Universe and at providing a substantial
advancement of multi-messenger and time-domain astrophysics. Through an unprecedented
combination of X-/gamma-rays monitors, an on-board IR telescope and automated fast slewing
capabilities, THESEUS will be a wonderful machine for the detection, characterization and
redshift measurement of any kind of GRBs and many classes of X-ray transients. In addition
to the full exploitaiton of high-redshift GRBs for cosmology (pop-III stars, cosmic
re-ionization, SFR and metallicity evolution up to the “cosmic dawn”), THESEUS will allow
the identification and study of the electromagnetic counterparts to sources of
gravitational waves which will be routinely detected in the late ’20s / early ’30s by next
generation facilities like aLIGO/aVirgo, LISA, KAGRA, and Einstein Telescope (ET), as well
as of most classes of transient sources, thus providing an ideal sinergy with the large
e.m. facilities of the near future like LSST, ELT, TMT, SKA, CTA, ATHENA.

COSI: MeV measurements of antimatter annihilation, element formation, compact objects, and extreme explosions


Terri Brandt (NASA Goddard Space Flight Center)

COSI, the Compton Spectrometer and Imager, measures gamma-rays from 0.2-5MeV using Germanium detectors (GeDs) in a Compton telescope. Thus COSI is a sensitive imaging, spectroscopy, and polarization instrument, making it ideal for measuring Galactic positron annihilation emission and other nuclear lines, constraining Gamma-ray Burst (GRB) polarization, and studing energetic objects such as the Crab Nebula and accreting black holes. With an instantaneous field of view for the GeDs of pi and nearly 2-pi for the scintillator shields, which are also sensitive to gamma-rays, COSI also has the capability to detect emission from counterparts of gravitational waves and neutrinos, making it a natural multimessenger mission for the coming decade. Following a very successful 46-day flight in 2016, COSI will return to Wanaka, New Zealand for its second superpressure balloon flight, with launch planned for 1 April 2020. The COSI payload has been reintegrated and is undergoing calibrations in preparation for the next flight. In this presentation, we will summarize anticipated outcomes of the 2020 flight, review 2016 measurements, including the 511keV Galactic positron annihilation spectrum, and discuss technical advances, including those which will enable a successful satellite mission, such as COSI-SMEX, proposed to the NASA 2019 small Explorer mission opportunity.

O6 - Wednesday 16:10-17:50 (Emma de Oña-Wilhelmi)

Fermi Large Area Telescope Observations of the Fast-dimming Crab Nebula in 60–600 MeV


Paul K. H. Yeung (Department of Physics, University of Hamburg)

The Crab pulsar and its nebula are the origin of relativistic electrons which can be observed through their synchrotron and inverse Compton emission. The transition between synchrotron and inverse Compton dominated emission takes place at $approx 10^9$~eV.
The short-term (weeks to months) flux variability of the synchrotron emission of the most energetic electrons is investigated with data from ten years of observations with the textit{Fermi} Large Area Telescope (LAT) in the energy range from 60~MeV to 600 MeV.
The off-pulse light-curve has been reconstructed from phase-resolved data. The light-curve is used to identify different flux-states and its statistical significance is estimated with dedicated simulations of mock light-curves. The energy spectra for different flux states are reconstructed.
We confirm the presence of flaring-states which follow a log-normal flux distribution. Additionally, we discover a low-flux state where the flux drops to less than 16.8~% of the time-averaged flux and stays there for several weeks. The transition time is observed to be as short as $t_{1/2}=(1.4pm 1)$~days (halving time). The energy spectrum during the low flux state follows the extra-polation of the inverse Compton spectrum measured at energies beyond several GeV energy.
The low-flux states found here and the transition time of at most few days indicates that the bulk ($>75$%) of the synchrotron emission above $10^8$~eV originates in a compact volume with apparent angular size of $theta=0.4”$ for a variability time-scale of five days. We tentatively identify the so-called inner knot feature as the origin of the bulk of the gamma-ray emission as predicted by Komissarov & Lyutikov (2011).

Relativistic Astrophysics in Non-Relativistic Protostellar Jets


Anabella Araudo (ELI Beamlines, Institute of Physics, Czech Academy of Sciences )

Synchrotron radio emission from non-relativistic jets
powered by massive protostars has been reported, indicating the presence of relativistic electrons
and magnetic fields of about 1 mG. We study diffusive shock acceleration and
magnetic field amplification in protostellar jets with velocities
of about 1000 km/s and ionization degree between 0.1-1. We show that the magnetic field in the
synchrotron emitter can be significantly amplified by the Bell
instability excited by the streaming of cosmic rays.
The maximum energy of electrons is determined by synchrotron and
escape losses, whereas the maximum energy of protons
is constrained by the escape of particles upstream of the shock.
In both cases and for a large range of parameters, maximum energies are in the TeV domain.
TeV electrons and protons can emit gamma rays in their interaction with photon and matter fields. In particular, proton-proton collisions with matter in the dense molecular cloud where protostellar jets are embedded can produce detectable levels of gamma rays by the forthcoming Cherenkov Telescope Array. The detection of this radiation will open a new window to study the formation of massive stars, as well as
diffusive shock acceleration and magnetic field amplification in low velocity shocks and high density plasmas.

Lepto-hadronic emission from the jets of Cygnus X-1


Dimitrios Kantzas (API/GRAPPA, UvA)

Dimitrios Kantzas, Sera Markoff, A. Chhotray, M. Lucchini, C. Ceccobello & CHOCBOX collaboration

Cygnus X-1 is the first Galactic source confirmed to host a black hole. It has been observed across the whole electromagnetic spectrum and recently in the GeV band by Fermi/LAT. The source’s radio through near-infrared radiation is thought to originate from the relativistic jets because of the flat radio to sub-millimeter spectrum and of the orbital-period dependence found in the high energy light curve. The processes that lead to this non-thermal emission are still debated, with both leptonic and hadronic deemed to be viable. In this work, we use a multi-zone jet model parameterizing the dynamical properties, such as the jet velocity profile, the magnetic field, and the energy density, combined with a self-consistent secondary cascade and radiation calculations. We revisit the source with the first-ever high-quality simultaneous multiwavelength data-set obtained from the CHOCBOX campaign trying to break the degeneracy. We compare our new models to prior work and discuss the contribution of secondary electrons as well as the physical conditions of the source.

Binary population synthesis models for core-collapse gamma-ray burst progenitors


Ashley Chrimes (Department of Physics, University of Warwick)

Long-duration gamma-ray bursts (GRBs) are thought to arise from the core-collapse of rapidly spinning, massive, stripped envelope stars. However, the evolutionary pathways leading to these progenitors, and their physical properties at core-collapse, remain uncertain. Single stars are disfavoured as plausible candidates, given the difficulty in maintaining angular momentum while losing the stellar envelope through winds. An alternative pathway is through the spin-up of a star by accretion from a binary companion, and subsequent quasi-homogeneous evolution (QHE). However, this pathway is predicted by population synthesis models to be rare and confined to very low metallicities. We explore the possibility that massive stars in binaries can have their envelopes stripped, either by winds or a companion, whilst retaining sufficient spin to launch relativistic jets at core-collapse. To investigate, we apply tidal interactions to the output stellar evolution models of BPASS (Binary Population and Spectral Synthesis), and identify rapidly spinning, stripped envelope stars. Through a Bayesian analysis, we show that a dual pathway model of QHE and tidally spun progenitors can explain the observed GRB rate as a function of redshift. The model can produce bursts up to Solar metallicity, and reproduces theoretical estimates for the minimum core specific angular momentum required for relativistic jet production. We conclude with a summary of our predictions for the physical properties of GRB progenitor systems.

O7 - Thursday 14:00-15:40 (Emma de Oña-Wilhelmi)

Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei


Frank Rieger (ITA, ZAH Heidelberg University)

The detection of extreme gamma-ray variability in AGN has generated new interest in near black-hole particle acceleration and emission scenarios. I will review the phenomenological status, highlight the occurrence and
relevance of magnetospheric gaps, and discuss the adequacy of different (steady) model realisations to account
for the observed characteristics. Current analysis suggests that the variable (day-scale)VHE activity seen in the
radio galaxy M87 could be compatible with a magnetospheric origin, while such an origin appears less likely for
the (minute-scale) VHE activity in IC 310. I will discuss recent developments and challenges in modelling gap physics.

– Katsoulakos & Rieger 2018, ApJ 852, 112
– Rieger & Levinson 2018, Galaxies 6, 116 (review)

Extreme blazars: a challenge for the next decade


Elisa Prandini (Physics and Astronomy, Padova U.)

Extreme blazars are closely aligned, jetted active galactic nuclei whose double-humped emission is peaked at extremely high energies, exceeding the keV and/or the multi-TeV domain. Recent observations in the X-ray and very-high energy bands (VHE, E>100 GeV) point to a complex picture for this class of objects, suggesting that there are three ways of “behaving extreme” for a blazar: i) showing a persistent hard X-ray spectrum up to 1 keV, ii) showing a persistent hard VHE gamma-ray spectrum up to 1 TeV; iii) featuring one of the previous mentioned behaviours only during enhanced emission states. In this contribution the state-of-the-art of the experimental and theoretical researches aimed at understanding the emission of these extreme accelerators is presented, with special emphasis on open points and challenges. The impact on cosmic ray physics, cosmology, and fundamental physics is also discussed. Finally, the observational and theoretical strategies that should guide the research of the next decade are outlined.

11 years of observations of M87 with the VERITAS telescopes.


Mireia Nievas Rosillo (Gamma Ray Astronomy, DESY)

Radio galaxies are a type of radio-loud Active Galactic Nuclei powered by a super-massive black hole (SMBH). They are a rare class of very-high-energy (VHE, E>100GeV) gamma-ray source with remarkable properties in terms of rapid variability and unusual spectra. Five radio galaxies are currently known to emit TeV radiation. Among them, M87 stands out as one of the best laboratories to study AGN physics due to its proximity (16 Mpc, z=0.0043). It harbors one of the most massive SMBH known to be associated with a TeV source. Finally, the jet can be spatially resolved in radio, optical and X-rays, allowing detailed studies that test the connection between gamma-ray activity and changes in the jet structure. M87 has been systematically monitored by Cherenkov telescopes for more than 15 years since its discovery as a VHE source in 2003 by HEGRA. In this contribution, we present one of the longest gamma-ray datasets of the source to date, which includes 11 years of observations of M87 with the VERITAS telescopes and 10 years with Fermi-LAT. We present the gamma-ray spectrum of the source over 5 decades of energy and its flux evolution in a multi-wavelength context. We finally discuss the implications of the collected data in the spectral modeling of the source and the prospects to use its spectrum to constrain the mid-infrared component of the EBL.

VHE gamma-ray emission of PKS 1510-089 – from persistent low-state emission to giant flares


Julian Sitarek (Department of Astrophysics, University of Lodz)

PKS 1510-089 is the only Flat Spectrum Radio Quasar that has been detected in the very-high-energy gamma-ray (VHE, E>100 GeV) domain in all possible states: from persistent low-state emission, through high states up to bright flares of different time scale and intensity.
In this presentation we will report on the results of the multi-year monitoring and target-of-opportunity gamma-ray observations of PKS 1510-089 with the MAGIC and H.E.S.S. Cherenkov telescopes focusing on the variability patterns of the emission.
The characterization of the low state as well as the violent variability during flares will be presented.
We will discuss how those observations, combined with multiwavelength information, shed light on the gamma-ray production locations and mechanisms in Flat Spectrum Radio Quasars.

A blazar with extreme shift of the synchrotron peak frequency: multi-wavelength study of the BL Lac 1ES 1215+303


Janeth Valverde (Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France)

Blazars are known for their variability on a wide range of timescales at all wavelengths. Their classification into flat spectrum radio quasars, low-, intermediate- or high-frequency-peaked BL Lac (FSRQ, LBL, IBL, HBL, respectively) is based on broadband spectral characteristics that do not account for possible different activity states of the source. Recently, it was proposed that blazars could be classified according to the kinematics of their radio features. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares, due to the scarcity of observational campaigns or due to the relatively young existence of specialized, sensitive enough detectors.

Thanks to a decade of observations from the Fermi-LAT and VERITAS, we present an extensive study of the long-term multi-wavelength variability of the blazar 1ES 1215+303. This unprecedented data set reveals multiple strong gamma-ray flares and a long-term increase in the gamma-ray and optical flux baseline of the source over the ten-year period, which results in a linear correlation between these two energy bands over a decade.
Typical HBL behaviors were identified in the radio morphology of the source. However, analyses of the broadband SED at different flux states, unveils an extreme shift in energy of the synchrotron peak frequency from IR to soft X-rays. This evidences that the source exhibits IBL characteristics during quiescent states and HBL behavior during high states. A two-component synchrotron self-Compton model is used to describe this dramatic change.

The different methods applied and presented in this work provide a complete and detailed panorama of the intricate nature of blazars, and possibly even challenge our current classification scheme. Moreover, this work demonstrates the rewarding potential of blazars long-term studies that will be accessible, and potentially improved, thanks to future imaging atmospheric instruments, such as the Cherenkov Telescope Array (CTA).

Authors: Janeth Valverde, Olivier Hervet, Deirdre Horan, Denis Bernard, Stephen Fegan

Co-authors: M. L. Lister, Y. Y. Kovalev, A. B. Pushkarev, T. Savolainen, B. G. Piner, P. G. Edwards, S. Kiehlmann, W. Max-Moerbeck, A. C. S. Readhead, M. Tornikoski, A. Lahteenmaki, E. Lindfors, L. Takalo, K. Nilsson, V. Fallah Ramazani, T. Hovatta, and J. Jormanainen

for the Fermi-LAT, VERITAS, Tuorla, VLBA, MOJAVE, OVRO and Metsahovi collaborations.

O8 - Thursday 16:10-17:50 (Emma de Oña-Wilhelmi)

Search for high-redshift blazars with Fermi/LAT


Michael Kreter (North-West University)

High-z blazars (z > 2.5) are among the most powerful gamma-ray sources in the Universe.
These objects have black-hole masses often in excess of 10^9 solar masses, which makes them important test objects to constrain cosmological black hole growth. Due to their large distance, their high-energy emission peak is often downshifted in energies below the GeV range, which makes high-z blazars difficult to study with Fermi/LAT and only a handful of
the very brightest objects are detected so far.
In this work, we studied the long-term gamma-ray emission of a sample of 176 blazars, which have not been reported as known gamma-ray sources by Fermi/LAT. We calculated monthly Fermi/LAT light curves for a large sample of blank sky positions and derived the amount of random fluctuations on various test statistic (TS) levels.
Four previously unknown blazars up to a redshift of 3.6 have been significantly (TS > 25) detected during individual monthly flares. Half of our sample shows gamma-ray activity at a TS > 9 level, including the most distant blazar seen at GeV energies at a distance of z ~ 5.2. By comparing the amount of observed gamma-ray flares to the amount of expected random fluctuations, we were able to almost double the number of known high-z gamma-ray blazars. A detailed study of these blazars will be the scope of our future work.

H.E.S.S. Observations of Gamma-ray Bursts at Very High Energies


Robert Parsons (Max-Planck-Institut für Kernphysik)

Gamma ray bursts (GRBs) are some of the most luminous and energetic events in the Universe. These events represent a powerful cosmic ray acceleration site (potentially of the most energetic cosmic rays) and should also be bright at gamma-ray energies above 100 GeV. After several decades of searching with Imaging Atmospheric Cherenkov Telescopes, within the last two years three GRBs have been detected, with energetic gamma-rays being detected in the prompt, early-afterglow and deep-afterglow phases.

In this contribution we will present the status of the GRB observation programme of the H.E.S.S. array of gamma-ray telescopes. The highlight result being the detection of the bright and relatively nearby (z=0.65) GRB 180720B, remarkably achieved over 10 hours after the end of GRB prompt emission phase when the X-ray flux had already decayed by four orders of magnitude. We will detail the gamma-ray flux level and spectrum of the GRB in context with data at longer wavelengths, discussing the possible gamma-ray emission mechanisms and outlining the implications on the GRB detection rates for the next generation VHE observatories.

GRB afterglows and the ends of the particle spectrum


Don Warren (iTHEMS, RIKEN)

The afterglows of gamma-ray bursts (GRBs) reveal a great deal of information about the bursts: their distances, progenitor environments, and more. These afterglows are produced by a population of energetic electrons accelerated by a relativistic shock wave. The microphysics of particle acceleration by such shocks can be treated using particle-in-cell (PIC) simulations. However, PIC simulations are far too computationally expensive to extend to the time and space scales needed to fully model an afterglow. Instead, I use the results of PIC simulations—primarily the intense, small-scale magnetic field turbulence—to guide Monte Carlo simulations of electron shock acceleration. The turbulent magnetic field has consequences at both the low-energy and high-energy ends of the electron distribution. I then place these Monte Carlo-derived electron distributions in the context of a traditional GRB afterglow, and illustrate the resultant changes to the “standard” afterglow model—and the inferred GRB parameters.