The optimum parameters for the generation of synchrotron radiation in ultraintense laser pulse interactions with planar foils are investigated with the application of Bayesian optimization, via Gaussian process regression, to 2D particle-in-cell simulations. Individual properties of the synchrotron emission, such as the yield, are maximized, and simultaneous mitigation of bremsstrahlung emission is achieved with multi-variate objective functions. The angle-of-incidence of the laser pulse onto the target is shown to strongly influence the synchrotron yield and angular profile, with oblique incidence producing the optimal results. This is further explored in 3D simulations, in which additional control of the spatial profile of synchrotron emission is demonstrated by varying the polarization of the laser light. The results demonstrate the utility of applying a machine learning-based optimization approach and provide new insights into the physics of radiation generation in laser–foil interactions, which will inform the design of experiments in the quantum electrodynamics (QED)-plasma regime.

As TeV gamma-ray astronomy progresses into the era of the Cherenkov Telescope Array (CTA), there is a desire for the capacity to instantaneously follow up on transient phenomena and continuously monitor gamma-ray flux at energies above $10^{12}\,\mathrm{eV}$ . To this end, a worldwide network of Imaging Air Cherenkov Telescopes (IACTs) is required to provide triggers for CTA observations and complementary continuous monitoring. An IACT array sited in Australia would contribute significant coverage of the Southern Hemisphere sky. Here, we investigate the suitability of a small IACT array and how different design factors influence its performance. Monte Carlo simulations were produced based on the Small-Sized Telescope (SST) and Medium-Sized Telescope (MST) designs from CTA. Angular resolution improved with larger baseline distances up to 277 m between telescopes, and energy thresholds were lower at 1 000 m altitude than at 0 m. The ${\sim} 300\,\mathrm{GeV}$ energy threshold of MSTs proved more suitable for observing transients than the ${\sim}1.2\,\mathrm{TeV}$ threshold of SSTs. An array of four MSTs at 1 000 m was estimated to give a 5.7 $\sigma$ detection of an RS Ophiuchi-like nova eruption from a 4-h observation. We conclude that an array of four MST-class IACTs at an Australian site would ideally complement the capabilities of CTA.

We present results of the propagation of high-energy cosmic rays (CRs) and their secondaries in the intracluster medium (ICM). To this end, we employ three-dimensional cosmological magnetohydrodynamical simulations of the turbulent intergalactic medium to explore the propagation of CRs with energies between 1014 and 1019 eV. We study the interaction of test particles with this environment considering all relevant electromagnetic, photohadronic, photonuclear, and hadronuclear processes. Finally, we discuss the consequences of the confinement of high-energy CRs in clusters for the production of gamma rays and neutrinos.

A characteristic pattern of solar hard X-ray emission, first identified in SOL1969-03-30 by Frost & Dennis (1971), turns out to have a close association with the prolonged high-energy gamma-ray emission originally observed by Forrest et al. (1985). This identification has become clear via the observations of long-duration γ-ray flares by the Fermi/LAT experiment, for example in the event SOL2014-09-01. The distinctive features of these events include flat hard X-ray spectra extending well above 100 keV, a characteristic pattern of time development, low-frequency gyrosynchrotron peaks, CME association, and gamma-rays identifiable with pion decay originating in GeV ions. The identification of these events with otherwise known solar structures nevertheless remains elusive, in spite of the wealth of EUV imagery available from SDO/AIA. The quandary is that these events have a clear association with SEPs in the high corona, and yet the gamma-ray production implicates the photosphere itself, despite the strong mirror force that should focus the particles away from the Sun We discuss the morphology of these phenomena and propose a solution to this problem.

Solar hard X-ray and gamma-ray emission was measured by BDRG instrument, the part of set of instruments operated on board the Russian satellite Lomonosov from April 2016 until now (solar-synchronous orbit with altitude 490 km, inclination of 97.6 degrees). Lomonosov measurements (11 flares with the X-ray energy more than 10 keV, and more than half of them have class in soft X-rays less than C2) were compared to the data obtained by RHESSI and Fermi space observatories as well as the Nobeyama Radioheliograph operating at the same time. The quasi-periodicity with different periods were found in some of them.

The Fermi-LAT gamma-ray data in the inner Galaxy region show several prominent features possibly related to the past activity of the Milky Way’s super massive black hole. At a large, 50 deg scale, the Fermi LAT revealed symmetric hour glass structures with hard energy spectra extending up to 100 GeV (and dubbed ‘the Fermi bubbles’). More recently and closer to the Galactic centre, at the 10 deg scale, several groups have claimed evidence for excess gamma-ray emission that appears symmetric around the Galactic center and has an energy spectrum peaking at few GeVs. We explore here the possibility that this emission originates in inverse Compton emission from high-energy electrons produced in a short duration, burst-like event injecting 1052 − 1053 erg, roughly 106 yrs ago. Several lines of evidence suggest that a series of ‘burst like’ events happened in the vicinity of our black hole in the past and gamma-ray observations may offer a new view of that scenario.

The possibility is considered that core emission in pulsars results from secondary e± pairs generated through linear acceleration emission (LAE), rather than the conventional curvature emission (CE). It is found that this is possible in principle, but only under conditions that appear not to be satisfied in practice.

During late October 2014, active region NOAA 2192 caused an unusual high level of solar activity, within an otherwise weak solar cycle. While crossing the solar disk, during a period of 11 days, it was the source of 114 flares of GOES class C1.0 and larger, including 29 M- and 6 X-flares. Surprisingly, none of the major flares (GOES class M5.0 and larger) was accompanied by a coronal mass ejection, contrary to statistical tendencies found in the past. From modeling the coronal magnetic field of NOAA 2192 and its surrounding, we suspect that the cause of the confined character of the flares is the strong surrounding and overlying large-scale magnetic field. Furthermore, we find evidence for multiple magnetic reconnection processes within a single flare, during which electrons were accelerated to unusual high energies.

Diogeness was an uncollimated scanning flat crystal spectrometer observing solar flare X-ray spectra in four narrow wavelength bands in the vicinity of Ca xix, S xv and Si xiii He-like line ‘triplets’ around 3.18 Å, 5.04 Å and 6.65 Å. In two of the spectral channels, emission lines around the Ca xix 3.178 Å resonance line were scanned in opposite directions, being diffracted from precisely adjusted identical Quartz crystals mounted on a common shaft in a so-called Dopplerometer (tachometer) configuration. Observations of solar X-ray spectra made by Diogeness provide a direct diagnostic information on plasma characteristics during the impulsive flare energy release. We present a sample of events which occurred during the Diogeness operation time from August 16, 2001 to September 17, 2001.

The Fermi Large Area Telescope (LAT) is the most sensitive instrument ever deployed in space for observing gamma-ray emission >100 MeV. This has also been demonstrated by its detection of quiescent gamma-ray emission from pions produced by cosmic-ray protons interacting in the solar atmosphere, and from cosmic-ray electron interactions with solar optical photons. The Fermi-LAT has also detected high-energy gamma-ray emission associated with GOES M-class and X-class solar flares, each accompanied by a coronal mass ejection and a solar energetic particle event, increasing the number of detected solar flares by almost a factor of 10 with respect to previous space observations. During the impulsive phase, gamma rays with energies up to several hundreds of MeV have been recorded by the LAT. Emission up to GeV energies lasting several hours after the flare has also been detected by the LAT. Of particular interest are the recent detections of three solar flares whose position behind the limb was confirmed by the STEREO satellites. While gamma-ray emission up to tens of MeV resulting from proton interactions has been detected before from occulted solar flares, the significance of these particular events lies in the fact that these are the first detections of >100 MeV gamma-ray emission from footpoint-occulted flares. We will present the Fermi-LAT, RHESSI and STEREO observations of these flares and discuss the various emission scenarios for these sources.

Current Cherenkov Telescopes for VHE gamma ray astrophysics are pointing instruments with a field of view up to a few tens of deg2. We propose to build an array of two non-steerable telescopes with a FoV of 5×60 deg2 oriented along the meridian. Roughly half of the sky drifts through this FoV in a year. We have performed a MC simulation to estimate the performance of this instrument, which we dub MACHETE. The sensitivity that MACHETE would achieve after 5 years of operation for every source in this half of the sky is comparable to the sensitivity that a current IACT achieves for a specific source after a 50 h devoted observation. The analysis energy threshold would be 150 GeV and the angular resolution 0.1 deg. For astronomical objects that transit over MACHETE for a specific night, it would achieve an integral sensitivity of 12% of Crab in a night. This makes MACHETE a powerful tool to trigger observations of variable sources at VHE or any other wavelengths.

ChemiX is a Bragg crystal spectrometer that will fly on the two Interhelioprobe spacecraft due for launch in 2025 and 2026. The spacecraft perihelion will be only 0.3 a.u. and the orbit inclination up to 30°, and so instruments on board will have a close view of solar active regions and flares and regions near each solar pole. The ChemiX X-ray spectrometer, built by a consortium of groups led by the Space Research Centre, Polish Academy of Sciences, will fly on each of the spacecraft, and observe X-ray spectra in the 1.5 - 9 Å range. Spectral lines in this range include resonance lines of helium-like and hydrogen-like ions of elements such as Fe, Ca, Ar, S, and Si, with less abundant elements such as K and Cl represented by weaker lines which the high sensitivity of ChemiX should be able to detect. The free–free and free–bound continua should also be detected since instrumental background will be eliminated. Three of the seven channels of ChemiX will be in a “dopplerometer” arrangement by which spatial and spectral shifts present in flare impulsive stages can be disentangled.

Solar flare X-ray loop top sources (LTSs) are known since early observations from Skylab.They are always present accompanying long duration events (LDE), but their nature remains unclear. One of the main unknowns is spatial structure of LTSs. Observations indicate that LTSs are large and diffuse, but several authors suggested that there is present a fine, internal structure within them which should be resolved even using present space instrumentation.We present an example for the solar flare well observed by SDO/AIA and RHESSI. The LTS was dynamic with an episode of splitting into two sources. We performed detailed analysis of X-ray sources morphology using restored RHESSI images. Moreover, we conducted a number of simulations taking into account various possible spatial distributions of X-ray emission. We have found that in case of the flare investigated RHESSI should be capable to detect the fine structure if present. However it is not visible on the restored images. Some of our simulations suggest that we can also miss large, diffuse sources in the X-ray observations which may eventually lead to misinterpretation of the observed X-ray features.

The High Altitude Water Cherenkov (HAWC) Observatory is a wide-field gamma-ray detector sensitive to primary energies between 100 GeV and 100 TeV. The array is being built at an altitude of 4100 m a.s.l. on the Sierra Negra volcano near Puebla, Mexico. Data taking has already started while construction continues, with the completion projected for early 2015. The design is optimized to detect extended air showers induced by gamma rays that pass through the array and to reconstruct the directions and energies of the primary photons. With a duty cycle close to 100% and a daily coverage of ~8 sr of the sky, HAWC will perform a survey of TeV emissions from many different sources. The northern active galactic nuclei will be monitored for up to 6 hours each day, providing unprecedented light curve coverage at energies comparable to those of imaging air Cherenkov telescopes. HAWC has been in scientific operation with more than 100 detector modules since August 2013. Here we present a preliminary look at the first results and discuss the efforts to integrate HAWC in multi-wavelength studies of extragalactic jets.

The subject of our work is the review and comparison of solar activity during the last two solar minima that occurred between recent activity cycles. We use the soft X-ray global solar corona observations covering the two nine-months long time intervals in 1997/98 and 2009. Data from RF15-I multichannel photometer are used for the penultimate minimum. For the last unusually deep and prolonged solar activity minimum in 2009 the data from SphinX spectrophotometer are used. Comparison of measurements from both minima takes place in the overlapping energy range 2–15 keV. We focus on the active region formation, evolution and flaring productivity during respective minima.