arXiv subjects

K. Kasprzak

Publications and source records attributed to K. Kasprzak.

5 recordsLinked to original sources

arXiv2026

Search for gamma-ray spectral lines from dark matter annihilation with the H.E.S.S. Inner Galaxy Survey

Spectral gamma-ray line features are expected as key signatures from dark matter (DM) annihilations of TeV-scale particle DM. Observations of the Galactic Centre with atmospheric Cherenkov telescopes are unique to probe thermal-relic TeV particle DM, well beyond the reach of direct detection and collider searches. We report here on the search for line signals in very-high-energy gamma rays using data from the Inner Galaxy Survey, consisting of 546 hours of H.E.S.S. observations of the inner few degrees of the Galactic Centre. No significant signal is detected. We then compute the exclusion limits on the annihilation line cross section $\langle σv \rangle_{\rm line}$, with a two-dimensional log-likelihood ratio test statistics, exploiting spectral and spatial features of the DM signal. Assuming an Einasto DM density profile for the Milky Way, our results provide the most constraining limits so far, reaching $\langle σv \rangle_{\rm line} = 2.3$ $\times$ $10^{-28}$ and $2.4 \times$ $10^{-27}$ cm$^3$s$^{-1}$ for DM masses of 1 and 10 TeV, respectively. The present limits are used to constrain the widely searched Wino, Higgsino and Quintuplet models. For the first time, thermal Higgsino DM is probed for DM Milky Way models.

astro-ph.HE↗

arXiv2026

H.E.S.S. observations of composite Seyfert-starburst galaxies

Context: Composite galaxies that contain both Seyfert and starburst components may produce very high-energy (VHE; >100 GeV) gamma-ray emission at a wide range of spatial scales, from a few Schwarzschild radii of a supermassive black hole to dimensions of kiloparsec-size jet-driven outflows. In addition to supernova remnants, various sources have been suggested to explain data collected on composite galaxies, including multi-messenger neutrino and ultra-high-energy cosmic-ray data. Aims: The closest composite Seyfert-starburst galaxies (NGC 1068, the Circinus galaxy, and NGC 4945) are observed with the High Energy Stereoscopic System (H.E.S.S.) to provide constraints on cosmic-ray populations in these systems. Methods: Data obtained in H.E.S.S. observations have been analyzed to search for VHE gamma-ray counterparts to the GeV gamma-ray signals detected with Fermi-LAT and for potential spectral components in the VHE range. Results: No significant signals have been found in these H.E.S.S. data. Upper limits on the VHE gamma-ray fluxes were applied to constrain theoretical models involving different spectral components.

astro-ph.HE↗

arXiv2026

The second H.E.S.S. gamma-ray burst catalogue: 15 years of observations with the H.E.S.S. telescopes

Recent observational efforts using imaging atmospheric Cherenkov telescopes (IACTs) have led to firm detections of very-high-energy (VHE) signals from bright gamma-ray bursts (GRBs), often at moderate redshifts. This work presents 15 years of H.E.S.S. GRB observations and examines their implications through population comparisons and selected modelling cases. GRBs are a key science target of the High Energy Stereoscopic System (H.E.S.S.). With a low-energy threshold ($\lesssim$100 GeV) and rapid repointing capabilities, H.E.S.S. can begin follow-up observations within tens of seconds after a GRB trigger, covering the late prompt or early afterglow phases. We report GRB follow-up observations with H.E.S.S. from 2004 to 2019, which resulted in no significant VHE signals (aside from the detections of GRB~180720B and GRB~190829A). The resulting upper limits comprise the largest set available for GRBs at VHE. A subset of bursts with favourable conditions were selected for X-ray analysis and emission modelling. Population studies were performed to compare detected and non-detected GRBs. The results indicate that VHE-detected GRBs are not a distinct population, but tend to feature luminous X-ray emission and favourable redshift and observing conditions. This highlights the potential of next-generation IACTs such as the Cherenkov Telescope Array Observatory (CTAO), whose lower energy threshold will enhance the detection of fainter and more distant GRBs.

astro-ph.HE↗

arXiv2024

Further improvement of medium temperature heat treated SRF cavities for high gradients

The application of heat treatments on 1.3 GHz TESLA type cavities in ultra-high vacuum at 250°C to 350°C is called medium temperature or mid-T heat treatment. In various laboratories such treatments on superconducting radio frequency (SRF) cavities result reproducible in three main characteristic features for the quality factor $Q_0$ in dependency of the accelerating electric field strength $E_{acc}$. First, comparing mid-T heat treatment with a baseline treatment, a significant increase of $Q_0$ up to $5\cdot10^{10}$ at 2K can be observed. Second, with increasing accelerating gradient $E_{acc}$ the $Q_0$ increases up to a maximum around 16 to 20 MV/m. This effect is known as anti-Q-slope. The third observation for a mid-T heat treatment compared to a baseline treatment is an often reduced maximum gradient $E_{acc}$. The appearance of a high-field-Q-slope (HFQS) was reported after mid-T heat treatments of 3 hours at 350°C or of 20 hours at 300°C at DESY. Using the heating temperature and the heating time taken from the temperature profile of the furnace effective oxygen diffusion lengths $l$ were calculated. In the follow-up study presented here, a set of three single-cell cavities with diffusion lengths $l$ above 1700 nm, showing HFQS, were treated with an additional so-called low-T bake of 24-48 hours at 120°C to 130°C. The subsequent reproducible Q(E) -performances results indicate that the low-T bake procedure cures the HFQS like for cavities treated with the EuXFEL recipe of EP and following low-T treatments. As presented in the following, Q values of more than $3\cdot10^{10}$ at 16 MV/m and accelerating gradients of 32 to 40 MV/m are achieved.

physics.acc-ph↗

arXiv2024

Correlation of srf performance to oxygen diffusion length of medium temperature heat treated cavities

This comprehensive study, being part of the European XFEL R\&D effort, elucidates the influence of medium temperature (mid-T) heat treatments between 250°C and 350°C on the performance of 1.3~GHz superconducting radiofrequency (SRF) niobium cavities. Utilizing a refurbished niobium retort furnace equipped with an inter-vacuum chamber and cryopumps at DESY, we have embarked on an investigation to enhance the state-of-the-art SRF cavity technology. Our research reveals that mid-T heat treatments significantly boost the quality factor ($Q_0$) of the cavities, achieving values between $2\cdot10^{10}$ to $5\cdot10^{10}$ at field strengths around 16~MV/m, while the maximum field strengths are limited to 25-35~MV/m and enhanced sensitivity to trapped magnetic flux is observed. Moreover, we delve into the effects of surface impurity concentration changes, particularly the diffusion of oxygen content, and its impact on performance enhancements. By categorizing treatments based on calculated diffusion lengths using the whole temperature profile, we recognize patterns that suggest an optimal diffusion length conducive to optimizing cavity performance. SIMS results from samples confirm the calculated oxygen diffusion lengths in most instances. Deviations are primarily attributed to grain boundaries in fine-grain materials, necessitating repeated measurements on single-crystal materials to further investigate this phenomenon. Investigations into cooling rates and the resulting spatial temperature gradients across the cavities ranging from 0.04 to 0.2~K/mm reveal no significant correlation with performance following a mid-T heat treatment. However, the increased sensitivity to trapped magnetic flux leads to new challenges in the quest for next-generation accelerator technologies since the requirement for magnetic hygiene gets stricter.

physics.acc-ph↗