Highlight Science Talks

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MeV Astronomy Instrumentation Development in Taiwan
Hsiang-Kuang Chang (NTHU)

I will talk about science goals and current status of three projects: the Compton Spectrometer and Imager (COSI), the Compton Polarimeter (Compol), and the Gamma-ray Transient Monitor (GTM). These are all for building instruments to detect photons in the MeV energy range, which is relatively unexplored in the whole electromagnetic spectrum because of difficulties in detection techniques.

COSI is a general-purpose, next-generation Compton telescope based on high-purity
germanium detectors. It is led by Space Sciences Lab, UC Berkeley, and has been selected by NASA SMEX program for phase A study. Compol is a small Compton polarimeter based on scintillator detectors to fly on a CubeSat for observing polarization of soft gamma rays from Cyg X-1.
Compol is still looking for CubeSat opportunities. GTM is, similar to Compol, based on scintillator detectors. It is mainly for GRB monitoring and may also detect bright transients
from other sources. It has been selected by NSPO to fly on Formosat 8B, planned to launch in 2024. GTM was also selected by NSPO for feasibility study to fly in a lunar orbit.

High-Energy Radiation of Classical Novae
Kwan Lok Li (NCKU)

Since the first Fermi-LAT detection of V407 Cyg in 2010, we have known that novae are not only bright optical transients, but can be very luminous in gamma-rays. Recent studies also suggested that these gamma-rays should be shock-powered, and can be closely related to the hard X-ray emission (and the reprocessed optical emission) of the novae. In this talk, I will briefly tell you our current understanding of this relatively new gamma-ray source class.

Galactic PeVatrons in the multi-wavelength context
Alison Mitchell (Adelaide/Zurich)

The all particle cosmic ray spectrum exhibits two spectral breaks, known as the “knee” and the “ankle" (at ~10^15 eV and ~10^18 eV respectively). Cosmic Rays with energies up to the knee are thought to have Galactic origins -  yet conclusive evidence of particle acceleration to PeV energies (10^15 eV) is still lacking. The search for Galactic PeVatrons - accelerators of PeV particles - is a highly active area of research encompassing many complementary methods. Within the next few years, several facilities with improved sensitivity to PeVatrons will come online, further aiding the search.  Candidate PeVatron accelerators include supernova remnants, stellar clusters and pulsar wind nebulae.  To build a complete picture of these accelerators, multi-wavelength and multi-messenger information is necessary, together with modelling of the underlying physical processes. This talk will explore the various pieces of evidence that indicate PeVatron activity, or to otherwise determine system properties.  We will take a look at how gathering data from multiple inputs could build a convincing case to confirm an accelerator as a PeVatron.

Exploring starburst origins of the extra-galactic gamma-ray background 
Ellis Owen (NTHU)

In recent years, high-energy gamma-ray emission has been detected from star-forming galaxies in the local universe, including M82, NGC 253, Arp 220 and M33. The bulk of this emission is thought to be of hadronic origin, arising from the interactions of cosmic rays (CRs) with the interstellar medium of their host galaxy.  More distant star-forming galaxies would also presumably be bright in gamma-rays, but these would not be resolved as point sources. Instead, they contribute gamma-rays as unresolved sources to the extra-galactic gamma-ray background (EGB). However, despite the wealth of high-quality all-sky EGB data from the Fermi-LAT gamma-ray space telescope collected over more than a decade of operation, the exact contribution of SFGs to the EGB and the signatures their emission would imprint on the gamma-ray sky remains unsettled. I will discuss how this can be assessed by modelling the gamma-ray emission from SFG populations above 1 GeV. I will demonstrate that such emission can be characterised by just a small number of key physically-motivated parameters, and outline how source populations would leave anisotropic signatures in the EGB. I will present possible theoretical and statistical approaches that may offer potential to facilitate proper signal extraction from the EGB, and discuss how this could yield information about the underlying properties and evolution of SFGs over cosmic time. I will also show that development of such a statistical and theoretical framework is timely, as detailed EGB signatures will soon be accessible with the ongoing operation of current gamma-ray instruments and detection prospects will be further improved by near-future up-coming facilities, in particular the Cherenkov Telescope Array.

Observing large-scale structures in the gamma-ray sky

Pooja Surajbali (MPIK)

The High Altitude Water Cherenkov (HAWC) gamma-ray observatory has a wide field of view and is currently a unique detector to study extended sources (>2 degree) at energies above a few TeV. This talk constitutes the development and application of novel data analysis and modelling techniques to observations from HAWC and is organised in three main parts

- defining a novel discriminator to distinguish between gamma-ray-induced and proton-induced atmospheric showers;

- generating a new background model which incorporates the cosmic-ray anisotropy;

- describing a profile likelihood approach to calculate the significance and flux from any region of the sky.

With the above tools, I performed blind searches for large-scale structures in the TeV gamma-ray sky and found a candidate source region with significance up to 5.30σ at 16° integration scale. Finally, with no significant signal from the north Fermi bubble (one of the largest structures in the gamma-ray sky) and its base, I compute their integral flux upper limits, at 95% confidence level and present a hadronic model with an estimated proton cut-off energy at 85 TeV.