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Register a new userDiscovery of extended VHE gamma-ray emission from the vicinity of the young massive stellar cluster Westerlund 1
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Results obtained in very-high-energy (VHE; E > 100 GeV) gamma-ray observations performed with the H.E.S.S. telescope array are used to investigate particle acceleration processes in the vicinity of the young massive stellar cluster Westerlund 1 (Wd 1). Imaging of Cherenkov light from gamma-ray induced particle cascades in the Earths atmosphere is used to search for VHE gamma rays from the region around Wd 1. Possible catalogued counterparts are searched for and discussed in terms of morphology and energetics of the H.E.S.S. source. The detection of the degree-scale extended VHE gamma-ray source HESS J1646-458 is reported based on 45 hours of H.E.S.S. observations performed between 2004 and 2008. The VHE gamma-ray source is centred on the nominal position of Wd 1 and detected with a total statistical significance of ~20sigma. The emission region clearly extends beyond the H.E.S.S. point-spread function (PSF). The differential energy spectrum follows a power law in energy with an index of Gamma=2.19 pm 0.08_{stat} pm 0.20_{sys} and a flux normalisation at 1 TeV of Phi_0 = (9.0 pm 1.4_{stat} pm 1.8_{sys}) x 10^{-12} TeV^{-1} cm^{-2} s^{-1}. The integral flux above 0.2 TeV amounts to (5.2 pm 0.9) x 10^{-11} cm^{-2} s^{-1}. Four objects coincident with HESS J1646-458 are discussed in the search of a counterpart, namely the magnetar CXOU J164710.2-455216, the X-ray binary 4U 1642-45, the pulsar PSR J1648-4611 and the massive stellar cluster Wd 1. In a single-source scenario, Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent population would be accelerated within the stellar cluster. Beside this, there is evidence for a multi-source origin, where a scenario involving PSR J1648-4611 could be viable to explain parts of the VHE gamma-ray emission of HESS J1646-458.
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Westerlund 1 (Wd 1) is the most massive stellar cluster in the Galaxy and associated with an extended region of TeV emission. Here we report the results of a search for GeV gamma-ray emission in this region. The analysis is based on ~4.5 years of Fermi-LAT data and reveals significantly extended emission which we model as a Gaussian, resulting in a best-fit sigma of sigma_S = (0.475 +/- 0.05) deg and an offset from Wd 1 of ~1 deg. A partial overlap of the GeV emission with the TeV signal as reported by H.E.S.S. is found. We investigate the spectral and morphological characteristics of the gamma-ray emission and discuss its origin in the context of two distinct scenarios. Acceleration of electrons in a Pulsar Wind Nebula provides a reasonably natural interpretation of the GeV emission, but leaves the TeV emission unexplained. A scenario in which protons are accelerated in or near Wd 1 in supernova explosion(s) and are diffusing away and interacting with molecular material, seems consistent with the observed GeV and TeV emission, but requires a very high energy input in protons, ~10^51 erg, and rather slow diffusion. Observations of Wd 1 with a future gamma-ray detector such as CTA provide a very promising route to fully resolve the origin of the TeV and GeV emission in Wd 1 and provide a deeper understanding of the high-energy (HE) astrophysics of massive stellar clusters.
Massive stellar clusters have recently been hypothesised as candidates for the acceleration of hadronic cosmic rays up to PeV energies. Previously, the H.E.S.S. Collaboration has reported about very extended $gamma$-ray emission around Westerlund 1, a massive young stellar cluster in the Milky Way. In this contribution we present an updated analysis that employs a new analysis technique and is based on a much larger data set, allowing us to constrain better the morphology and the energy spectrum of the emission. The analysis technique used is a three-dimensional likelihood analysis, which is especially well suited for largely extended sources. The origin of the $gamma$-ray emission will be discussed in light of multi-wavelength observations.
Massive stars and their stellar winds are important for a number of feedback processes. The mass lost in the stellar wind can help determine the end-point of the star as a NS or a BH. However, the impact of mass-loss on the post-Main Sequence evolutionary stage of massive stars is not well understood. Westerlund 1 is an ideal astrophysical laboratory in which to study massive stars and their winds in great detail over a large range of different evolutionary phases. Aims: We aim to study the radio emission from Westerlund 1, in order to measure radio fluxes from the population of massive stars, and determine mass-loss rates and spectral indices where possible. Methods: Observations were carried out in 2015 and 2016 with the Australia telescope compact array (ATCA) at 5.5 and 9 GHz using multiple configurations, with maximum baselines ranging from 750m to 6km. Results: 30 stars were detected in the radio from the fully concatenated dataset, 10 of which were WRs (predominantly late type WN stars), 5 YHGs, 4 RSGs, 1 LBV star, the sgB[e] star W9, and several O and B supergiants. New source detections in the radio were found for 5 WR stars, and 5 OB supergiants. These detections have led to evidence for 3 new OB supergiant binary candidates, inferred from derived spectral index limits. Conclusions: Spectral indices and index limits were determined for massive stars in Westerlund 1. For cluster members found to have partially optically thick emission, mass-loss rates were calculated. Under the approximation of a thermally emitting stellar wind and a steady mass-loss rate, clumping ratios were then estimated for 8 WRs. Diffuse radio emission was detected throughout the cluster. Detections of knots of radio emission with no known stellar counterparts indicate the highly clumped structure of this intra-cluster medium, likely shaped by a dense cluster wind.
Globular clusters are old stellar systems which exhibit very-high stellar densities in their cores. The globular cluster Terzan 5 is characterized by a high stellar encounter rate and hosts the largest detected population of millisecond pulsars. It also features bright GeV gamma-ray emission and extended X-ray radiation. However, no globular clusters have been detected in very-high-energy gamma rays (VHE, E> 100 GeV) so far. In order to investigate this possibility Terzan 5 has been observed with the H.E.S.S. telescope array in this energy band. The discovery of a source of VHE gamma rays from the direction of this globular cluster will be reported. The results of the VHE analysis and a multi-wavelength view of Terzan 5 will be presented in this contribution. No counterpart or model can fully explain the observed morphology of the detected VHE gamma-ray source.
We report the discovery of very high energy gamma-ray emission from the direction of the SNR G54.1+0.3 using the VERITAS ground-based gamma-ray observatory. The TeV signal has an overall significance of 6.8$sigma$ and appears point-like given the 5$^{arcminute}$ resolution of the instrument. The integral flux above 1 TeV is 2.5% of the Crab Nebula flux and significant emission is measured between 250 GeV and 4 TeV, well described by a power-law energy spectrum dN/dE $sim$ E$^{-Gamma}$ with a photon index $Gamma= 2.39pm0.23_{stat}pm0.30_{sys}$. We find no evidence of time variability among observations spanning almost two years. Based on the location, the morphology, the measured spectrum, the lack of variability and a comparison with similar systems previously detected in the TeV band, the most likely counterpart of this new VHE gamma-ray source is the PWN in the SNR G54.1+0.3. The measured X-ray to VHE gamma-ray luminosity ratio is the lowest among all the nebulae supposedly driven by young rotation-powered pulsars, which could indicate a particle-dominated PWN.
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