New unidentified H.E.S.S. Galactic sources

📝 Abstract

H.E.S.S. is one of the most sensitive instruments in the very high energy (VHE; > 100 GeV) gamma-ray domain and has revealed many new sources along the Galactic Plane. After the successful first VHE Galactic Plane Survey of 2004, H.E.S.S. has continued and extended that survey in 2005-2008, discovering a number of new sources, many of which are unidentified. Some of the unidentified H.E.S.S. sources have several positional counterparts and hence several different possible scenarios for the origin of the VHE gamma-ray emission; their identification remains unclear. Others have so far no counterparts at any other wavelength. Particularly, the lack of an X-ray counterpart puts serious constraints on emission models. Several newly discovered and still unidentified VHE sources are reported here.

💡 Analysis

H.E.S.S. is one of the most sensitive instruments in the very high energy (VHE; > 100 GeV) gamma-ray domain and has revealed many new sources along the Galactic Plane. After the successful first VHE Galactic Plane Survey of 2004, H.E.S.S. has continued and extended that survey in 2005-2008, discovering a number of new sources, many of which are unidentified. Some of the unidentified H.E.S.S. sources have several positional counterparts and hence several different possible scenarios for the origin of the VHE gamma-ray emission; their identification remains unclear. Others have so far no counterparts at any other wavelength. Particularly, the lack of an X-ray counterpart puts serious constraints on emission models. Several newly discovered and still unidentified VHE sources are reported here.

📄 Content

arXiv:0907.0574v1 [astro-ph.HE] 3 Jul 2009 PROCEEDINGS OF THE 31st ICRC, Ł ´OD´Z 2009 1 New unidentified H.E.S.S. Galactic sources Omar Tibolla∗†, Ryan C. G. Chaves∗, Okkie de Jager‡, Wilfried Domainko∗, Armand Fiasson§ Nukri Komin¶, Karl Kosack∥ on behalf of the H.E.S.S. collaboration ∗Max-Planck-Institut f¨ur Kernphysik, P.O. Box 103980, D-69029 Heidelberg, Germany †Landessternwarte, Universit¨at Heidelberg, K¨onigstuhl, D 69117 Heidelberg, Germany ‡Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa §Laboratoire d’Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue - BP 110 F-74941 Annecy-le-Vieux Cedex, France ¶CEA, Irfu, SPP, Centre de Saclay, F-91191 Gif-sur-Yvette, France ∥CEA, Irfu, SAP, Centre de Saclay, F-91191 Gif-sur-Yvette, France Abstract. H.E.S.S. is one of the most sensitive instruments in the very high energy (VHE; > 100 GeV) gamma-ray domain and has revealed many new sources along the Galactic Plane. After the successful first VHE Galactic Plane Survey of 2004, H.E.S.S. has continued and extended that survey in 2005– 2008, discovering a number of new sources, many of which are unidentified. Some of the unidentified H.E.S.S. sources have sev- eral positional counterparts and hence several dif- ferent possible scenarios for the origin of the VHE gamma-ray emission; their identification remains unclear. Others have so far no counterparts at any other wavelength. Particularly, the lack of an X- ray counterpart puts serious constraints on emission models. Several newly discovered and still unidentified VHE sources are reported here. Keywords: Astronomical Observations; High En- ergy Gamma rays; Cosmic Rays. I. INTRODUCTION Very high energy (VHE, > 1011 eV) particles can be traced within our Galaxy by a combination of non- thermal X-ray emission and VHE gamma-ray emis- sion via leptonic (i.e. Inverse Compton scattering of electrons, Bremsstrahlung and synchrotron radiation) or hadronic (i.e. the decay of charged and neutral pions, due to interactions of energetic hadrons) processes. H.E.S.S. detects VHE γ-rays above an energy thresh- old of ∼100 GeV and up to ∼100 TeV with a typi- cal energy resolution of 15% per photon. The angular resolution is ∼0.1◦per event, allowing a positional error better than 40” for a point source detected with a statistical significance 6 σ. The H.E.S.S. field of view is almost 5◦in diameter with a point source sensitivity of < 2.0 × 10−13 ergs cm−2 s−1 (∼1% of the Crab Nebula) for a 5 σ detection in 25 hours of observations [1]. II. NEW UNIDENTIFIED H.E.S.S. SOURCES After the successful first survey of 2004 [2], H.E.S.S. extended the survey in 2005-2008 [3], leading to the discovery of several new VHE gamma-ray sources. Of these, several have been associated with Supenovae Remnants (SNRs; such as CTB 37A, CTB 37B, RCW 86, Kes 75), some of them are candidate PWNe (such as HESS J1356-654 and HESS J1849-000), however the rest remain unidentified. In this section, five recently discovered VHE gamma- ray sources, that are still unidentified, will be discussed, showing their morphology, their spectrum and providing evidence for possible counterparts. One of the sources, HESS J1507-622, is reported for the first time, whereas the others were covered in recent publications [5], [4], [6], [7]. A. HESS J1507-622 HESS J1507-622 (Figure 1) is one of the most in- teresting newly discovered sources. HESS J1507-622 is among the brightest (∼8% of Crab Flux) newly discovered sources and it is so far without plausible counterparts (similar to HESS J1427-608, HESS J1708- 410, HESS J1858+020 [8] and HESS J1616-508 [9]). While all unidentified VHE sources that have been discovered in the H.E.S.S. Galactic Plane Survey so far are located in a quite narrow angular band of ±1◦around the Galactic plane, HESS J1507-622 is unique in this respect since it lies ∼3.5◦offset from the Galactic plane, ∼3◦away from RCW 86 and from MSH 15-52. If truly offset from the plane and not simply a nearby source, it would be even more surprising to not find any trace of counterparts, considering the comparably lower nH at 3.5◦off the plane and, hence, the lower Galactic absorp- tion in X-rays and the reduced background emission. The discovery peak significance, calculated following the method of Eq. (17) in [10], is 8.2 σ (employing a 0.22◦oversampling radius, which is the standard radius used in source searches in the H.E.S.S. Galactic Plane Survey). The results on HESS J1507-622 are still preliminary: Figure 1 shows the uncorrelated excess count map (smoothed with Gaussian of 0.07◦), using 2 O. TIBOLLA et al. NEW UNIDENTIFIED H.E.S.S. SOURCES Fig. 1. Excess count map (smoothed with Gaussian of 0.07◦radius) of HESS J1507-622. The contours show 3σ, 4σ, 5σ, 6σ of significance respectively for an integration radius of 0.12◦. The white circles represent the position of three faint RASS [12] sources: X1 indicates 1RXS J150841.2-621006, X2 indicates 1RXS J150639.1-615704 and X3

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