Sabrina Einecke

The origin of Galactic cosmic rays (CRs), particularly at sub-GeV energies, remains uncertain due to their difficulty in being traced. One possible tracer of low-energy CRs is ionized carbon ([C II]), which may be enhanced in regions affected by CR-induced ionization. We present observations of [C II] 158 $\mu$m line emission across the $\gamma$-ray-bright supernova remnant (SNR) RX J1713.7−3946 obtained with SOFIA. These data are compared with molecular and atomic gas from Mopra ($^{12}$CO), Nanten ($^{12}$CO) and SGPS (H i), and high-energy emission from TeV $\gamma$-ray H.E.S.S. and X-ray XMM–Newton. We find [C II] emission follows the atomic gas more closely than molecular gas, particularly in regions near the SNR shock front. Ratios of [C II] intensity to gas column density vary across the remnant, with peak values coinciding with regions of enhanced TeV $\gamma$-ray emission. To put the [C II] in a wider context, we examined pointings from the Herschel GOT C+ survey. Our analysis of the GOT C+ data shows no significant difference in the I[C II]/I[$^{12}$CO] ratio for observations towards star formation regions, H ii regions and SNRs. To assess the origin of carbon ionization, we model [C II] emissivity using the photoionization code CLOUDY. We find UV photons and CRs can produce comparable levels of [C II] emission, assuming low-energy CRs (< 1 GeV) are accelerated and trapped within the SNR shock. These results highlight the potential of [C II] as a tracer of CR ionization and the importance of combining multiwavelength observations to probe CR interactions in SNRs.

We report CO and H i studies of molecular and atomic gas toward the TeV gamma-ray source HESS J1646−458, widely considered to be associated with the young massive cluster Westerlund 1 (Wd1). We found that molecular clouds at V _LSR ∼ −32 km s ⁻¹ coincide with the arc-like structures seen at 8 μ m, likely illuminated by strong far-UV radiation from Wd1. ¹² CO( J = 3–2) emission at the same velocity reveals a cavity-like structure with an expansion velocity of ∼5 km s ⁻¹ toward the central region of Wd1, suggesting a recently formed wind-blown bubble driven by the cluster. We also identify a complementary spatial distribution between the V _LSR ∼ −55 and ∼−32 km s ⁻¹ clouds, connected by an intermediate-velocity component at V _LSR ∼ −44 km s ⁻¹ . These characteristics are consistent with signatures of triggered star formation through a cloud–cloud collision and imply that both clouds are physically associated with Wd1. On larger scales, the total interstellar proton column density at V _LSR ∼ −36 to −23 km s ⁻¹ shows a moderate spatial correspondence with the TeV gamma-ray shell. Together with this correlation, a substantial gas mass of ∼1.6 × 10 ⁶ M _⊙ , and the absence of bright synchrotron X-rays, the TeV gamma-ray emission surrounding Wd1 is consistent with a hadronic origin. The present finding allows us to calculate the total energy of accelerated cosmic-ray protons to be ∼6 × 10 ⁴⁹ erg.

We present a study of the interstellar medium associated with the two middle-aged supernova remnants (SNRs) W41 and G22.7–0.2, both detected in TeV gamma rays. Using high-angular-resolution ¹² CO( J = 1–0) data from the Nobeyama 45 m telescope and H i data from the Very Large Array, we investigated the spatial and kinematic properties of molecular and atomic gas that interact with the SNRs. We identified associated clouds in the velocity ranges of +50 to +80 km s ⁻¹ for W41 and +76 to +110 km s ⁻¹ for G22.7–0.2. Column density analysis indicates that target protons are dominated by molecular hydrogen, while atomic hydrogen contributes less than ∼10%–15% even after correction for self-absorption. The mean proton densities are ∼1.2 × 10 ³ cm ⁻³ for W41 and ∼5.3 × 10 ² cm ⁻³ for G22.7–0.2. From the gamma-ray luminosities, we estimate the total energy of accelerated cosmic-ray protons as W _p ∼ 3 × 10 ⁴⁷ erg for W41 and ∼1 × 10 ⁴⁸ erg for G22.7–0.2, corresponding to 0.03%–0.1% of the canonical supernova explosion energy. These W _p values agree with the decreasing trend in W _p observed in the middle-aged SNRs within the previously reported SNR age– W _p relation.

The results of the first extragalactic gamma-ray survey by the High Energy Stereoscopic System (H.E.S.S.) are presented. The survey comprises 2720 hours of very high-energy gamma-ray observations of the extragalactic sky, recorded with H.E.S.S. from 2004 up to the end of 2012. These data have been re-analysed using a common consistent set of up-to-date data calibration and analysis tools. From this analysis, a list of 23 detected objects, predominantly blazars, was obtained. This catalogue was assessed in terms of the source class populations that it contains. The level of source parameter bias for the blazar sources, probed by this observational dataset, was evaluated using Monte-Carlo simulations. Spectral results obtained with the H.E.S.S. data were compared with the Fermi-LAT catalogues to present the full gamma-ray picture of the detected objects. Lastly, this unique dataset was used to assess the contribution of BL Lacertae objects and flat-spectrum radio quasars to the extragalactic gamma-ray background light at several hundreds of gigaelectronvolts. These results are accompanied by the release of the high-level data to the astrophysical community.

Context. It is still unclear which fraction of cosmic rays above an energy of 1 PeV is accelerated by the observed Galactic PeVatron population. These sources of unknown physical origin are detected through their γ-ray emission, which also identifies them as particle accelerators. However, their γ-ray data are typically degenerate between hadronic and leptonic emission scenarios, which hinders their straightforward association with the mainly hadronic cosmic ray population. Aims. In this study, we aimed to distinguish between leptonic and hadronic particle acceleration scenarios for the PeVatron candidate HESS J1646−458, which is associated with the star cluster Westerlund 1 (Wd 1). To this end, we first studied the diffuse X-ray emission from Wd 1 to better understand if its origin is of thermal or nonthermal nature. In addition, we searched for X-ray synchrotron emission from the associated PeVatron candidate HESS J1646−458 to put new constraints on the magnetic field strength and the leptonic particle population of this source. Methods. We used data from the all-sky surveys 1 to 4 of the extended Roentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma orbital platform to spectrally analyze the diffuse emission from Wd 1 and HESS J1646−458. For Wd 1, we fitted and compared a purely thermal model and a model with a thermal and a nonthermal component. Next, we analyzed the spectra of four annuli around Wd 1 that coincide with HESS J1646−458 to search for synchrotron radiation. Results. We find that eROSITA data cannot distinguish between thermal and nonthermal source scenarios for the diffuse emission from Wd 1 itself. For a thermal source scenario, the observed X-ray flux can be explained in large part by unresolved pre-main sequence stars or by thermalized stellar wind shocks. In the case of the PeVatron candidate HESS J1646−458, we find no evidence of synchrotron emission. We estimated an upper confidence bound of the synchrotron flux up to 40 ′ around Wd 1 of 1.9 ⋅ 10⁻³ keV⁻¹ cm⁻² s⁻¹. We used this result to study the spectral energy distribution of the source. From that, we obtained an upper 1σ confidence bound on the magnetic field strength of HESS J1646−458 of 7 μG. Conclusions. Our upper bound on the magnetic field strength in HESS J1646−458 is compatible with a previous estimate in the literature for a fully leptonic source scenario. Therefore, a purely leptonic emission scenario is compatible with our results. The same is the case for hadronic and hybrid scenarios, for which even less synchrotron flux is expected compared to the leptonic scenario.

Context. The nearby elliptical galaxy M87 contains one of only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to γ-ray energies) took part in the second M87 EHT campaign. Aims. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. Methods. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high-energy (VHE) γ-rays as well as details of the individual observations and light curves. We also conducted phenomenological modelling to investigate the basic source properties. Results. We present the first VHE γ-ray flare from M87 detected since 2010. The flux above 350 GeV more than doubled within a period of ≈36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Conclusions. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE γ-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and it emphasises the need for combined image and spectral modelling.

Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g. box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.

Fukui et al. quantified the hadronic and leptonic gamma-rays in the young TeV gamma-ray shell-type supernova remnant (SNR) RX J1713.7-3946 (RX J1713), and demonstrated that gamma rays are a combination of hadronic and leptonic gamma-ray components with a ratio of ∼6: 4 in gamma-ray counts N _g. This discovery, which adopted a new methodology of multi-linear gamma-ray decomposition, was the first quantification of the two gamma-ray components. In the present work, we applied the same methodology to another TeV gamma-ray shell-type SNR RX J0852.0-4622 (RXJ0852) in 3D space characterized by (the interstellar proton column density N _p)-(the nonthermal X-ray count N _x)-[N _g], and quantified the hadronic and leptonic gamma-ray components as having a ratio of ∼5:5 in N _g. The present work adopted the fitting of two/three flat planes in 3D space instead of a single flat plane, which allowed suppression of the fitting errors. This quantification indicates that hadronic and leptonic gamma-rays are of the same order of magnitude in these two core-collapse SNRs, verifying the significant hadronic gamma-ray components. We argue that the target interstellar protons, in particular their spatial distribution, are essential in any attempts to identify the type of particles responsible for gamma-ray emission. The present results confirm that cosmic-ray (CR) energy ≲100 TeV is compatible with a scheme in which SNRs are the dominant source of these Galactic CRs.