arXiv subjects

Kasun Wimalasena

Publications and source records attributed to Kasun Wimalasena.

3 recordsLinked to original sources

arXiv2026

Design of a mission to measure the shape and substructure of the 511 keV gamma-ray line from the center of the Milky Way

The 511 keV electron-positron annihilation feature near the galactic center has been detected for more than half a century, yet its origin remains a mystery. In this paper, we describe a concept for a balloon-borne 511 keV $γ$-ray mission called the 511-Spectrometer Mission. The mission will use Transition-Edge Sensor (TES) arrays with thick metal absorbers that are thermally coupled to the TES. The strength of the approach is a projected energy resolution of 200 eV Full Width Half Maximum (FWHM) at 511 keV, enabling detailed studies of the shape and substructure of the 511 keV emission from the galactic center region. A first mission equipped with 8,192 $γ$-ray detectors and a fully active shield and collimator could detect the galactic center with ~35 $σ$ statistical significance. We present the mission concept as well as first results obtained with a prototype detector equipped with $1.35\times1.35\times2$ mm$^{3}$ Bi absorbers. The detector has a quantum efficiency of 15% for 511 keV photons in photoelectric effect interactions. In tests with a $^{137}$Cs source, these prototype detectors show an energy resolution of 525 eV FWHM at 662 keV. We end with a discussion of follow-up missions that use coded mask imaging, or use concentrating or focusing optics to scrutinize the sources of 511 keV $γ$-rays on smaller angular scales.

astro-ph.IM↗

arXiv2025

Low-temperature Performance of $\mathrm{Gd_3(Ga, Al)_5O_{12}}$:Ce Scintillators

The last years have seen the first cryogenic detectors to be proposed for usage on balloon-borne missions. In such missions, the instrument will be exposed to the high radiation environment of the upper atmosphere. This radiation can induce a significant background to the measurements, something which can be mitigated through the use of an anti-coincidence shield. For hard X-ray and gamma-ray detectors such a shield typically consists photomultiplier tubes or, more recently, silicon photomultipliers coupled to scintillators placed around the detector. When using cryogenic detectors, the shield can be placed around the entire cryostat which will make it large, heavy and expensive. For the ASCENT (A SuperConducting ENergetic x-ray Telescope) mission, which uses Transition Edge Detectors, it was therefore considered to instead place the shield inside. This comes with the challenge of operating it at cryogenic temperatures. For this purpose, we tested the performance of 2 different types of GAGG:Ce scintillators down to 15 mK for the first time. Although significant variations of both the decay time and the light yield were found when varying the temperature, at 4 K its performance was found to be similar to that at room temperature. Furthermore, unexpected behavior around 2 K was found for both types of GAGG:Ce, leading to more in depth studies around these temperatures. Overall, the studies show that the combination of materials will allow to produce a functional anti-coincidence shield at several Kelvin.

physics.ins-det↗

arXiv2025

XL-Calibur Polarimetry of Cyg X-1 Further Constrains the Origin of its Hard-state X-ray Emission

The balloon-borne hard X-ray polarimetry mission XL-Calibur observed the Black Hole X-ray Binary (BHXRB) Cygnus X-1 (Cyg X-1) during its nearly six-day Long Duration Balloon (LDB) flight from Sweden to Canada in July 2024. The XL-Calibur observations allowed us to derive the most precise constraints to date of the Polarization Degree (PD) and Polarization Angle (PA) of the hard X-ray emission from a BHXRB. XL-Calibur observed Cyg X-1 in the hard state and measured a $\sim$19-64 keV PD of ($5.0^{+2.7}_{-3.0}$)% at a PA of $-28^{\circ}\pm 17^{\circ}$, with an 8.7% chance probability of detecting larger PDs than the one observed, given an unpolarized signal. The XL-Calibur results are thus comparable to the 2-8 keV PD and PA found by IXPE, with a similar agreement between the hard X-ray PA and the radio jet direction. We also discuss the implications of our polarization measurements in the context of models describing the origin of the broadband X-ray and $γ$-ray emission, to which XL-Calibur provides independent constraints on any proposed emission modeling.

astro-ph.HE↗