We present a near-infrared study of the candidate star cluster Mercer 81, located at the centre of the G338.4+0.1 H ii region and close to the TeV gamma-ray source HESS 1640-465. Using Hubble Space ...Telescope/NICMOS imaging and VLT/ISAAC spectroscopy, we have detected a compact and highly reddened cluster of stars, although the bright stars in the centre of the field are in fact foreground objects. The cluster contains nine stars with strong Pα emission, one of which we identify as a Wolf-Rayet (WR) star, as well as an A-type supergiant. The line-of-sight extinction is very large, A
V
∼ 45, illustrating the challenges of locating young star clusters in the Galactic plane. From a quantitative analysis of the WR star, we argue for a cluster age of 3.7
Myr, and, assuming that all emission-line stars are WR stars, a cluster mass of ≳104 M⊙. A kinematic analysis of the cluster's surrounding H ii region shows that the cluster is located in the Galactic disc at a distance of 11 ± 2 kpc. This places the cluster close to where the far end of the Bar intersects the Norma spiral arm. This cluster, as well as the nearby cluster DBS2003179, represents the first detections of active star cluster formation at this side of the Bar, in contrast to the near side which is well known to have recently undergone a ∼106 M⊙ starburst episode.
We outline the science prospects for gamma-ray bursts (GRBs) with the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory operating at energies above few tens of ...GeV. With its low energy threshold, large effective area and rapid slewing capabilities, CTA will be able to measure the spectra and variability of GRBs at multi-GeV energies with unprecedented photon statistics, and thereby break new ground in elucidating the physics of GRBs, which is still poorly understood. Such measurements will also provide crucial diagnostics of ultra-high-energy cosmic ray and neutrino production in GRBs, advance observational cosmology by probing the high-redshift extragalactic background light and intergalactic magnetic fields, and contribute to fundamental physics by testing Lorentz invariance violation with high precision. Aiming to quantify these goals, we present some simulated observations of GRB spectra and light curves, together with estimates of their detection rates with CTA. Although the expected detection rate is modest, of order a few GRBs per year, hundreds or more high-energy photons per burst may be attainable once they are detected. We also address various issues related to following up alerts from satellites and other facilities with CTA, as well as follow-up observations at other wavelengths. The possibility of discovering and observing GRBs from their onset including short GRBs during a wide-field survey mode is also briefly discussed.
The imaging atmospheric Cherenkov technique provides potentially the highest angular resolution achievable in astronomy at energies above the X-ray waveband. High-resolution measurements provide the ...key to progress on many of the major questions in high energy astrophysics, including the sites and mechanisms of particle acceleration to PeV energies. The huge potential of the next-generation CTA observatory in this regard can be realised with the help of improved algorithms for the reconstruction of the air-shower direction and energy.
Hybrid methods combining maximum-likelihood-fitting techniques with neural networks represent a particularly promising approach and have recently been successfully applied for the reconstruction of astrophysical neutrinos. Here, we present the FreePACT algorithm, a hybrid reconstruction method for IACTs. In this, making use of the neural ratio estimation technique from the field of likelihood-free inference, the analytical likelihood used in traditional image likelihood fitting is replaced by a neural network that approximates the charge probability density function for each pixel in the camera.
The performance of this improved algorithm is demonstrated using simulations of the planned CTA southern array. For this setupFreePACT provides significant performance improvements over analytical likelihood techniques, with improvements in angular and energy resolution of 25% or more over a wide energy range and an angular resolution as low as 40′′ at energies above 50TeV for observations at 20° zenith angle. It also yields more accurate estimations of the uncertainties on the reconstructed parameters and significantly speeds up the reconstruction compared to analytical likelihood techniques while showing the same stability with respect to changes in the observation conditions. Therefore, the FreePACT method is a promising upgrade over the current state-of-the-art likelihood event reconstruction techniques.
Very-high-energy
γ
-ray astronomy based on the measurement of air shower particles at ground-level has only recently been established as a viable approach, complementing the well established air ...Cherenkov technique. This approach requires high (mountain) altitudes and very high surface coverage particle detectors. While in general the properties of air showers are well established for many decades, the extreme situation of ground-level detection of very small showers from low energy primaries has not yet been well characterised for the purposes of
γ
-ray astronomy. Here we attempt such a characterisation, with the aim of supporting the optimisation of next-generation
γ
-ray observatories based on this technique. We address all of the key ground level observables and provide parameterisations for use in detector optimisation for shower energies around 1 TeV. We emphasise two primary aspects: the need for large area detectors to effectively measure low-energy showers, and the importance of muon identification for the purpose of background rejection.
A deep X-ray observation of the unidentified very high energy (VHE) gamma-ray source HESS J1702
$-$
420, for the first time, was carried out by Suzaku. No bright sources were detected in the XIS ...field of view (FOV), except for two faint point-like sources. The two sources, however, are considered not to be related to HESS J1702
$-$
420, because their fluxes in the 2–10 keV band (
$\sim\ $
10
$^{-14}\ $
erg s
$^{-1}\ $
cm
$^{-2}$
) are
$\sim\ $
3 orders of magnitude smaller than the VHE gamma-ray flux in the 1–10 TeV band (
$F_{\rm TeV}$
$=$
3.1
$\times$
10
$^{-11}\ $
erg s
$^{-1}\ $
cm
$^{-2}$
). We compared the energy spectrum of diffuse emission, extracted from the entire XIS FOV with those from nearby observations. If we consider the systematic error of background subtraction, no significant diffuse emission was detected with an upper limit of
$F_{\rm X}$
$<$
2.7
$\times$
10
$^{-12}\ $
erg s
$^{-1}\ $
cm
$^{-2}$
in the 2–10 keV band for an assumed power-law spectrum of
$\Gamma$
$=$
2.1 and a source size same as that in the VHE band. The upper limit of the X-ray flux is twelve-times as small as the VHE gamma-ray flux. The large flux ratio (
$F_{\rm TeV}/F_{\rm X}$
) indicates that HESS J1702
$-$
420 is another example of a “dark” particle accelerator. If we use a simple one-zone leptonic model, in which VHE gamma-rays are produced through inverse Compton scattering of the cosmic microwave background and interstellar far-infrared emission, and the X-rays via the synchrotron mechanism, an upper limit of the magnetic field (1.7
$\ \mu$
G) is obtained from the flux ratio. Because the magnetic field is weaker than the typical value in the galactic plane (3–10
$\ \mu$
G), the simple one-zone model may not work for HESS J1702
$-$
420 and a significant fraction of the VHE gamma-rays may originate from protons.
After decades of development, multi-messenger astronomy, the combination of information on cosmic sources from photons, neutrinos, charged particles and gravitational waves, is now an established ...reality. Within this emerging discipline we argue that very-high-energy gamma-ray observations play a special role. We discuss the recent progress on explosive transients, the connections between neutrino and gamma-ray astronomy, and the search for dark matter. Finally, the experimental prospects for the next decade in the VHE gamma-ray field are summarised.
Context.
Young massive star clusters (YMCs) have increasingly become the focus of discussions on the origin of galactic cosmic rays (CRs). The proposition that CRs are accelerated inside superbubbles ...(SBs) blown by the strong winds of these clusters avoids issues faced by the standard paradigm of acceleration at supernova remnant shocks.
Aims.
We provide an interpretation of the latest TeV
γ
-ray observations of the region around the YMC Westerlund 1 taken with the High Energy Stereoscopic System (H.E.S.S.) in terms of diffusive shock acceleration at the cluster wind termination shock, taking the spectrum and morphology of the emission into account. As Westerlund 1 is a prototypical example of a YMC, this study is relevant to the general question about the role of YMCs for the Galactic CR population.
Methods.
We generated model
γ
-ray spectra, characterised particle propagation inside the SB based on the advection, diffusion, and cooling timescales, and constrained key parameters of the system. We considered hadronic emission from proton-proton interaction and subsequent pion decay and leptonic emission from inverse Compton scattering on all relevant photon fields, including the cosmic microwave background, diffuse and dust-scattered starlight, and the photon field of Westerlund 1 itself. The effect of the magnetic field on cooling and propagation is discussed. Klein-Nishina effects are found to be important in determining the spectral evolution of the electron population.
Results.
A leptonic origin of the bulk of the observed
γ
-rays is preferable. The model is energetically plausible, consistent with the presence of a strong shock, and allows for the observed energy-independent morphology. The hadronic model faces two main issues: confinement of particles to the emission region, and an unrealistic energy requirement.
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