R. C. G. Chaves

A search for VHE counterparts of Galactic Fermi bright sources and MeV to TeV spectral characterization

Very high-energy (VHE; E > 100 GeV) γ-rays have been detected in a wide range of astronomical objects, such as pulsar wind nebulae (PWNe), supernova remnants (SNRs), giant molecular clouds, γ-ray binaries, the Galactic center, active galactic nuclei (AGN), radio galaxies, starburst galaxies, and possibly star-forming regions. At lower energies, observations using the Large Area Telescope (LAT) onboard Fermi provide a rich set of data that can be used to study the behavior of cosmic accelerators in the MeV to TeV energy bands. In particular, the improved angular resolution of current telescopes in both bands compared to previous instruments significantly reduces source confusion and facilitates identification of associated counterparts at lower energies. In this paper, a comprehensive search for VHE γ-ray sources that are spatially coincident with Galactic Fermi/LAT bright sources is performed, and the available MeV to TeV spectra of coincident sources compared. It is found that bright LAT GeV sources are correlated with TeV sources, in contrast to previous studies using EGRET data. Moreover, a single spectral component seems unable to describe the MeV to TeV spectra of many coincident GeV/TeV sources. It has been suggested that γ-ray pulsars may be accompanied by VHE γ-ray emitting nebulae, a hypothesis that can be tested with VHE observations of these pulsars.

Exploring the Galaxy at TeV energies: Latest results from the H.E.S.S. Galactic Plane Survey

The High Energy Stereoscopic System (H.E.S.S.) is an array of four imaging atmospheric-Cherenkov telescopes located in Namibia and designed to detect extensive air showers initiated by gamma-rays in the very-high-energy domain. It is an ideal instrument for surveying the Galactic plane in search of new sources, thanks to its location in the Southern Hemisphere, its excellent sensitivity, and its large field-of-view. The efforts of the H.E.S.S. Galactic Plane Survey, the first comprehensive survey of the inner Galaxy at TeV energies, have contributed to the discovery of an unexpectedly large and diverse population of over 60 sources of VHE gamma rays within its current range of l=250 to 65 degrees in longitude and |b|<=3.5 degrees in latitude. The population of VHE gamma-ray emitters is dominated by the pulsar wind nebula and supernova remnant source classes, although nearly a third remain unidentified or confused. The sensitivity of H.E.S.S. to sources in the inner Galaxy has improved significantly over the past two years, from continued survey observations, dedicated follow-up observations of interesting source candidates, and from the development of advanced methods for discrimination of gamma-ray-induced showers from the dominant background of hadron-induced showers. The latest maps of the Galaxy at TeV energies will be presented, and a few remarkable new sources will be highlighted.

The H.E.S.S. Galactic Plane Survey

The H.E.S.S. (High Energy Stereoscopic System) Galactic plane survey (HGPS) was performed with the H.E.S.S. I Cherenkov telescope array in Namibia from 2004 to 2013. In total ∼ 2700 hours of high-quality observations of the Galactic plane are available in the Galactic longitude range of 250° to 65° and Galactic latitude range of −3.5° 0.1 TeV) gamma-ray sources. In this contribution, we will show the latest survey maps, describe the source catalog construction method and further results from the upcoming HGPS paper such as source population statistics and the association of H.E.S.S. sources with known pulsar wind nebulae and highly energetic pulsars, supernova remnants, binary systems and GeV sources detected ...

Discovery of VHE gamma-ray emission from the SNR G15.4+0.1 with H.E.S.S

Supernova remnants (SNRs) have emerged as one of the largest source classes in very-high-energy (VHE; E>0.1,TeV) astronomy. Many of the now known VHE gamma-ray emitting SNRs have been discovered by the H.E.S.S. imaging Cherenkov telescope array, thanks to its unique access to the inner galaxy. Statistically-significant emission of VHE gamma rays has now been detected from the direction of the supernova remnant G15.4+0.1. While the centroids of the H.E.S.S. source and the shell-type SNR are compatible, the VHE morphology suggests a center-dominated source at TeV energies, something which is at odds with the shell-like morphology observed at radio frequencies. This suggests that H.E.S.S. may be observing TeV emission from a previously unknown pulsar wind nebula (PWN) located within the boundaries of the radio shell. If this interpretation is correct, G15.4+0.1 would in fact be a composite SNR, the first case in which an SNR is identified as a composite on the basis of VHE gamma-ray observations. Archival data from MAGPIS gives exciting hints that there is radio emission from the central parts of the remnant, giving support to this hypothesis. Unfortunately, image artefacts from a nearby strong radio source produce considerable uncertainties in the radio analysis. Additional observations in both the radio and X-ray are needed to confirm the composite nature of G15.4+0.1 suggested by H.E.S.S.

Evidence for a spectral turnover in the broadband gamma-ray emission from SNR Puppis A revealed by H.E.S.S. observations

The 4 kyr-old supernova remnant (SNR) Puppis A shows strong evidence of interaction between the forward shock and a molecular cloud. The results from Fermi -LAT indicate extended high- energy gamma-ray emission with a 0.2-100 GeV spectrum that does not significantly deviate from a power law, in contrast to most of the GeV-bright SNRs known to be interacting with molecular clouds. In order to characterize the position of a spectral feature at higher energies, very-high- energy (E > 0.1 TeV) gamma-ray observations of Puppis A were carried out with the High Energy Stereoscopic System (H.E.S.S.) telescope array. The analysis of the H.E.S.S. data did not reveal any significant emission towards Puppis A. The upper limits on the di erential photon flux imply that its broadband gamma-ray spectrum must exhibit a spectral break, estimated to be below 280 GeV, or alternatively a cuto below 450 GeV or 280 GeV when assuming exponential or sub- exponential cuto in the power law spectrum, respectively. These results provide insights into our understanding of the processes accelerating particles in the shock front of Puppis A. The details of this work have been published in [ 1 ].

Search for new supernova remnant shells in the Galactic plane with H.E.S.S.

Amongst the population of TeV gamma-ray sources detected with the High Energy Stereoscopic System (H.E.S.S.) in the Galactic plane, clearly identified supernova remnant (SNR) shells constitute a small but precious source class. TeV-selected SNRs are prime candidates for sources of efficient cosmic-ray acceleration. In this work, we present new SNR candidates that have been identified in the entire H.E.S.S. phase I data set of the Galactic plane recorded over the past ten years. Identification with a known SNR shell candidate was successful for one new source, HESS J1534-571. In other cases, TeV-only shell candidates are challenging to firmly identify as SNRs due to their lack of detected non-thermal emission in lower energy bands. We will discuss how these objects may present an important link between young and evolved SNRs, since their shell emission may be dominated by hadronic processes.

Discovery of VHE gamma-rays from the vicinity of the shell-type SNR G318.2+0.1 with H.E.S.S.

The on-going H.E.S.S. Galactic Plane Survey continues to reveal new sources of VHE gamma-rays. In particular, recent re-observations of the region around the shell-type supernova remnant (SNR) G318.2+0.1 have resulted in the discovery of statistically-significant very-high-energy (VHE) gamma-ray emission from an extended region. Although the source remains unidentified, archival observations of CO12 in the region provide an opportunity to investigate a potential SNR/molecular cloud interaction. The morphological properties of this newly-discovered VHE gamma-ray source HESSJ1457-593 are presented and discussed in light of the multi-wavelength data available.