Manuel Skalka, M.Sc.

Since 01/01/2026:
PhD candidate at the Institute for Theoretical Physics and Astrophysics / Chair of Astronomy of the Julius-Maximilians-University, Würzburg, Germany
Supervisor: Dr. Thomas Siegert

01/09/2021 – 29/01/2025:
Physics studies (M.Sc.) at Julius-Maximilians-University, Würzburg, Germany

15/09/2017 – 31/08/2021:
Physics studies (B.Sc.) at Julius-Maximilians-University, Würzburg, Germany

30/05/2014:
Fach-Abitur, Franz-Oberthür-Schule, Würzburg, Germany

19/09/1994:
Born in Würzburg, Germany

Investigating the 511 keV Electron-Positron Annihilation and 2.223 MeV Neutron Capture Lines

511 keV line

My PhD research focuses on the 511 keV electron-positron annihilation line and its correlation
with the 2.223 MeV neutron capture line to address the long-standing “Positron Puzzle” (Siegert
2025). While this radiation is naturally occurring in our Galaxy, the exact distribution of positron
sources remains ambiguous.
By investigating Globular Clusters (CSs) – which act as point-like sources in the overall galactic
picture – I aim to distinguish localized emission from the diffuse Galactic background. This aims
to at least solve initial problems concerning the “Positron Puzzle” and consequently provide
more accurate models for positron transport in the interstellar medium. Furthermore, this can
contribute in the search for dark matter and the solar coronal heating problem.
During my PhD I will focus on this topic based on the 511 keV line by investigating the gamma-ray
spectra of GCs and distinguish them from the weak gamma-ray spectra and diffuse Galactic
gamma-ray background. GCs are ideal candidates for observation since they appear as point-
like hotspots (see Fig. 1, right) which enables them to be analyzed as a single population source.
Since most of the 511 keV emission in our galaxy is diffuse, studying GCs for this project can
significantly improve the statistics. Furthermore, I will investigate the proportion of positron
sources contributed by flare events. For this project I will use 20 years of INTEGRAL/SPI
observation data.

2.2 MeV line

A critical part of my research involves the 2.223 MeV line, which has shown a strong physical
correlation with the 511 keV line (Mittal et al. 2025). This emission results from neutron capture
on hydrogen, a process occurring in the stellar atmospheres of donor stars within X-ray binary
systems.
I will investigate this correlation in detail by creating an all-sky map of the 2.223 MeV line and
determine the luminosity ratios of the 511 keV and 2.223 MeV emissions in both GCs and the
Galaxy at large.
My work will build on the upcoming COSI (Compton Spectrometer and Imager) mission, which
will investigate the 0.2 to 5 MeV range with significantly improved imaging sensitivity and higher
spectral resolution from which our data sets and research will benefit.

Fig. Correlation between the prompt 511 keV line from electron-positron annihilation and the
2.2 MeV line from neutron capture in solar flares measured with SMM, RHESSI, INTEGRAL/SPI,
Fermi/GBM, and others. Figure from Mittal et al. 2025.

Fig. Reconstructed gamma-ray sky in the 511 keV line with 20 years of observational data from
INTEGRAL/SPI. The distribution of globular clusters is shown as turquoise circles. The closer the
source, the larger the circle. Many GC are clearly located in the bright bulge region of the galaxy.
The solid, dashed and dotted lines show the observation times of at least 20 Ms, 2 Ms, and 0.2
Ms, respectively (Yoneda et al. 2025). If GC or stellar flares in general account for a significant
portion of the galactic 511 keV radiation, one would expect a corresponding flux at 2.2 MeV,
which has not yet been investigated with INTEGRAL/SPI. Figure from Yoneda et al. 2025.