Parameters of the angular distribution of photoelectrons <em>e</em>₂ (3) emitted in sequential 2PDI of the argon 3p shell (left) and neon 2p shell (right), summed over intermediate fine-structure ion states A⁺ (<em>n</em>p⁵ ²P_{1/2, 3/2})

<p><strong>Figure 3.</strong> Parameters of the angular distribution of photoelectrons <em>e</em>₂ (<a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824eqn03" target="_blank">3</a>) emitted in sequential 2PDI of the argon 3p shell (left) and neon 2p shell (right), summed over intermediate fine-structure ion states A⁺ (<em>n</em>p⁵ ²P_{1/2, 3/2}). Curves as in figures <a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824f1" target="_blank">1</a> and <a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824f2" target="_blank">2</a>.</p> <p><strong>Abstract</strong></p> <p>As an extension to recent developments in the theory of non-dipole effects in nonlinear photoprocesses in the XUV/x-ray wavelength regime, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically and compared to similar processes in the 2p shell of neon. The calculations predict distinct non-dipole effects in the region of the Cooper minimum in argon at photon energies around 50 eV, where they are of similar importance as at photon energies of more than 500 eV. The non-dipole effects should therefore be clearly observable in experiments performed at the present x-ray free electron lasers.</p

Photoionization dipole (<em>E</em>1) and quadrupole (<em>E</em>2) cross sections

<p><strong>Figure 2.</strong> Photoionization dipole (<em>E</em>1) and quadrupole (<em>E</em>2) cross sections. Left: 3p ionization of Ar and unpolarized Ar⁺(3p⁵ ²P); right: 2p ionization of Ne and unpolarized Ne⁺(2p⁵ ²P). Curves for different LS multiplet states of the <em>n</em>p⁴ configuration as in figure <a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824f1" target="_blank">1</a>. Dotted curves refer to neutral Ar and Ne.</p> <p><strong>Abstract</strong></p> <p>As an extension to recent developments in the theory of non-dipole effects in nonlinear photoprocesses in the XUV/x-ray wavelength regime, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically and compared to similar processes in the 2p shell of neon. The calculations predict distinct non-dipole effects in the region of the Cooper minimum in argon at photon energies around 50 eV, where they are of similar importance as at photon energies of more than 500 eV. The non-dipole effects should therefore be clearly observable in experiments performed at the present x-ray free electron lasers.</p

Non-dipole parameter γ + 3δ of the photoelectron angular distribution in the single-photon 3p ionization of Ar and unpolarized Ar⁺ as a function of the photoelectron energy

<p><strong>Figure 1.</strong> Non-dipole parameter γ + 3δ of the photoelectron angular distribution in the single-photon 3p ionization of Ar and unpolarized Ar⁺ as a function of the photoelectron energy. Inset shows the region of energies around the Cooper minimum. Ionization of Ar⁺ to different multiplet states of the residual doubly charged ion: Ar^{2 +} (3p⁴ ³P) (solid); Ar^{2 +} (3p⁴ ¹S) (dashed); Ar^{2 +} (3p⁴ ¹D) (chain). Ionization of Ar: this work (dotted); RPA [<a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824bib27" target="_blank">27</a>, <a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824bib28" target="_blank">28</a>] (crosses).</p> <p><strong>Abstract</strong></p> <p>As an extension to recent developments in the theory of non-dipole effects in nonlinear photoprocesses in the XUV/x-ray wavelength regime, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically and compared to similar processes in the 2p shell of neon. The calculations predict distinct non-dipole effects in the region of the Cooper minimum in argon at photon energies around 50 eV, where they are of similar importance as at photon energies of more than 500 eV. The non-dipole effects should therefore be clearly observable in experiments performed at the present x-ray free electron lasers.</p

Asymmetry (4) for θ = 45° as a function of the photon energy for different final multiplet states of Ar⁺⁺(3p⁴)

<p><strong>Figure 5.</strong> Asymmetry (<a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824eqn04" target="_blank">4</a>) for θ = 45° as a function of the photon energy for different final multiplet states of Ar⁺⁺(3p⁴). The results are summed over the intermediate ionic fine-structure states Ar⁺(3p⁵ ²P_{1/2, 3/2}). Curves as in figure <a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824f1" target="_blank">1</a>. Insets show angular distribution of electrons at the photon energies of 46.2 eV and 1 keV for the final ionic state Ar⁺⁺(3p⁴ ³P) in the units [σ₂/4π] (see equation (<a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824eqn03" target="_blank">3</a>)). Dashed lines in the insets correspond to the dipole approximation.</p> <p><strong>Abstract</strong></p> <p>As an extension to recent developments in the theory of non-dipole effects in nonlinear photoprocesses in the XUV/x-ray wavelength regime, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically and compared to similar processes in the 2p shell of neon. The calculations predict distinct non-dipole effects in the region of the Cooper minimum in argon at photon energies around 50 eV, where they are of similar importance as at photon energies of more than 500 eV. The non-dipole effects should therefore be clearly observable in experiments performed at the present x-ray free electron lasers.</p

Same as figure 3 for the 2PDI of the argon 3p shell in the region of the Cooper minimum

<p><strong>Figure 4.</strong> Same as figure <a href="http://iopscience.iop.org/0953-4075/46/16/164014/article#jpb463824f3" target="_blank">3</a> for the 2PDI of the argon 3p shell in the region of the Cooper minimum. Solid: sequential 2PDI via the fine-structure Ar⁺ (3p⁵ ²P_3/2) state; dashed: summed over the unresolved Ar⁺ (3p⁵ ²P_{1/2, 3/2}) fine structure states.</p> <p><strong>Abstract</strong></p> <p>As an extension to recent developments in the theory of non-dipole effects in nonlinear photoprocesses in the XUV/x-ray wavelength regime, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically and compared to similar processes in the 2p shell of neon. The calculations predict distinct non-dipole effects in the region of the Cooper minimum in argon at photon energies around 50 eV, where they are of similar importance as at photon energies of more than 500 eV. The non-dipole effects should therefore be clearly observable in experiments performed at the present x-ray free electron lasers.</p

Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-energy Emission from Prompt to Afterglow

We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. The prompt gamma-ray emission was detected by the Fermi GRB Monitor (GBM), the Fermi Large Area Telescope (LAT), and the Swift Burst Alert Telescope (BAT) and the long-lived afterglow emission was subsequently observed by the GBM, LAT, Swift X-ray Telescope (XRT), and Swift UV Optical Telescope. The early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed by the XRT at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to observe the transition from internal-shock- to external-shock-dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment. We estimate the initial bulk Lorentz factor using the observed high-energy spectral cutoff. Considering the onset of the afterglow component, we constrain the deceleration radius at which this forward shock begins to radiate in order to estimate the maximum synchrotron energy as a function of time. We find that even in the LAT energy range, there exist high-energy photons that are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high-energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy-loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process

Detection of extended γ -ray emission around the Geminga pulsar with H.E.S.S.

Geminga is an enigmatic radio-quiet γ-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy γ-ray emission around the pulsar was discovered by Milagro and later confirmed by HAWC, which are both water Cherenkov detector-based experiments. However, evidence for the Geminga pulsar wind nebula in gamma rays has long evaded detection by imaging atmospheric Cherenkov telescopes (IACTs) despite targeted observations. The detection of γ-ray emission on angular scales ≳2° poses a considerable challenge for the background estimation in IACT data analysis. With recent developments in understanding the complementary background estimation techniques of water Cherenkov and atmospheric Cherenkov instruments, the H.E.S.S. IACT array can now confirm the detection of highly extended γ-ray emission around the Geminga pulsar with a radius of at least 3° in the energy range 0.5-40 TeV. We find no indications for statistically significant asymmetries or energy-dependent morphology. A flux normalisation of (2.8 ± 0.7)×10-12 cm-2 s-1 TeV-1 at 1 TeV is obtained within a 1° radius region around the pulsar. To investigate the particle transport within the halo of energetic leptons around the pulsar, we fitted an electron diffusion model to the data. The normalisation of the diffusion coefficient obtained of D0 = 7.6-1.2+1.5×1027 cm2 s-1, at an electron energy of 100 TeV, is compatible with values previously reported for the pulsar halo around Geminga, which is considerably below the Galactic average

Time-resolved hadronic particle acceleration in the recurrent nova RS Ophiuchi

Recurrent novae are repeating thermonuclear explosions in the outer layers of white dwarfs, due to the accretion of fresh material from a binary companion. The shock generated when ejected material slams into the companion star's wind can accelerate particles. We report very-high-energy (VHE; [Formula: see text]) gamma rays from the recurrent nova RS Ophiuchi, up to 1 month after its 2021 outburst, observed using the High Energy Stereoscopic System (H.E.S.S.). The temporal profile of VHE emission is similar to that of lower-energy giga-electron volt emission, indicating a common origin, with a 2-day delay in peak flux. These observations constrain models of time-dependent particle energization, favoring a hadronic emission scenario over the leptonic alternative. Shocks in dense winds provide favorable environments for efficient acceleration of cosmic rays to very high energies

EChO

A dedicated mission to investigate exoplanetary atmospheres represents a major milestone in our quest to understand our place in the universe by placing our Solar System in context and by addressing the suitability of planets for the presence of life. EChO—the Exoplanet Characterisation Observatory—is a mission concept specifically geared for this purpose. EChO will provide simultaneous, multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. The use of passive cooling, few moving parts and well established technology gives a low-risk and potentially long-lived mission. EChO will build on observations by Hubble, Spitzer and ground-based telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. However, EChO’s configuration and specifications are designed to study a number of systems in a consistent manner that will eliminate the ambiguities affecting prior observations. EChO will simultaneously observe a broad enough spectral region—from the visible to the mid-infrared—to constrain from one single spectrum the temperature structure of the atmosphere, the abundances of the major carbon and oxygen bearing species, the expected photochemically-produced species and magnetospheric signatures. The spectral range and resolution are tailored to separate bands belonging to up to 30 molecules and retrieve the composition and temperature structure of planetary atmospheres. The target list for EChO includes planets ranging from Jupiter-sized with equilibrium temperatures T ₑq up to 2,000 K, to those of a few Earth masses, with T ₑq 3c 300 K. The list will include planets with no Solar System analog, such as the recently discovered planets GJ1214b, whose density lies between that of terrestrial and gaseous planets, or the rocky-iron planet 55 Cnc e, with day-side temperature close to 3,000 K. As the number of detected exoplanets is growing rapidly each year, and the mass and radius of those detected steadily decreases, the target list will be constantly adjusted to include the most interesting systems. We have baselined a dispersive spectrograph design covering continuously the 0.4–16 μm spectral range in 6 channels (1 in the visible, 5 in the InfraRed), which allows the spectral resolution to be adapted from several tens to several hundreds, depending on the target brightness. The instrument will be mounted behind a 1.5 m class telescope, passively cooled to 50 K, with the instrument structure and optics passively cooled to 3c45 K. EChO will be placed in a grand halo orbit around L2. This orbit, in combination with an optimised thermal shield design, provides a highly stable thermal environment and a high degree of visibility of the sky to observe repeatedly several tens of targets over the year. Both the baseline and alternative designs have been evaluated and no critical items with Technology Readiness Level (TRL) less than 4–5 have been identified. We have also undertaken a first-order cost and development plan analysis and find that EChO is easily compatible with the ESA M-class mission framework

Sensitivity of the Cherenkov Telescope Array to a dark matter signal from the Galactic centre

We provide an updated assessment of the power of the Cherenkov Telescope Array (CTA) to search for thermally produced dark matter at the TeV scale, via the associated gamma-ray signal from pair-annihilating dark matter particles in the region around the Galactic centre. We find that CTA will open a new window of discovery potential, significantly extending the range of robustly testable models given a standard cuspy profile of the dark matter density distribution. Importantly, even for a cored profile, the projected sensitivity of CTA will be sufficient to probe various well-motivated models of thermally produced dark matter at the TeV scale. This is due to CTA's unprecedented sensitivity, angular and energy resolutions, and the planned observational strategy. The survey of the inner Galaxy will cover a much larger region than corresponding previous observational campaigns with imaging atmospheric Cherenkov telescopes. CTA will map with unprecedented precision the large-scale diffuse emission in high-energy gamma rays, constituting a background for dark matter searches for which we adopt state-of-the-art models based on current data. Throughout our analysis, we use up-to-date event reconstruction Monte Carlo tools developed by the CTA consortium, and pay special attention to quantifying the level of instrumental systematic uncertainties, as well as background template systematic errors, required to probe thermally produced dark matter at these energies