Discovery of very high energy gamma-ray emission in the W 28 (G6.4−0.1) region, and multiwavelength comparisons

H.E.S.S. observations of the old-age (>10 yr; ∼ 0.5◦ diameter) composite supernova remnant (SNR) W 28 reveal very high energy (VHE) γ-ray emission situated at its northeastern and southern boundaries. The northeastern VHE source (HESS J1801−233) is in an area where W 28 is interacting with a dense molecular cloud, containing OH masers, local radio and X-ray peaks. The southern VHE sources (HESS J1800−240 with components labelled A, B and C) are found in a region occupied by several HII regions, including the ultracompact HII region W 28A2. Our analysis of NANTEN CO data reveals a dense molecular cloud enveloping this southern region, and our reanalysis of EGRET data reveals MeV/GeV emission centred on HESS J1801−233 and the northeastern interaction region. Introduction & H.E.S.S. Results The study of shell-type SNRs at γ-ray energies is motivated by the idea that they are the dominant sites of hadronic Galactic cosmic-ray (CR) acceleration to energies approaching the knee (∼ 10 eV) and beyond, e.g. [34]. CRs are then accelerated via the diffusive shock acceleration (DSA) process (eg. [3, 31]). Gamma-ray production from the interaction of these CRs with ambient matter and/or electromagnetic fields is a tracer of such particle acceleration, and establishing the hadronic or electronic nature of the parent CRs in any γ-ray source is a key issue. Already, two shell-type SNRs, RX J1713.7−3946 and RX J0852.0−4622, exhibit shell-like morphology in VHE γ-rays [16, 18, 19] to 20 TeV and above. Although a hadronic origin of the VHE γ-ray emission is highly likely in the above cases, an electronic origin is not ruled out. W 28 (G6.4−0.1) is a composite or mixedmorphology SNR, with dimensions 50x45 and an estimated distance between 1.8 and 3.3 kpc (eg. [4, 33]). It is an old-age SNR (age 3.5×10 to 15×10 yr [11]), thought to have entered its radiative phase of evolution [33]. The shell-like radio emission [27, 23] peaks at the northern and northeastern boundaries where interaction with a molecular cloud [2] is established [35, 36]. The X-ray emission, which overall is well-explained by a thermal model, peaks in the SNR centre but has local enhancements in the northeastern SNR/molecular cloud interaction region [25]. Additional SNRs in the vicinity of W 28 have also been identified: G6.67−0.42 and G7.06−0.12 [21]. The pulsar PSR J1801−23 with spin-down luminosity Ė ∼ 6.2 × 10 erg s and distance d ≥ 9.4 kpc [28], is at the northern radio edge. Given its interaction with a molecular cloud, W 28 is an ideal target for VHE observations. This interaction is traced by the high concentration of 1720 MHz OH masers [6, 5, 29], and also the location of very high-density (n > 10 cm) shocked gas [36, 35]. Previous observations of the W 28 region at VHE energies by the CANGAROO-I teleSLAC-PUB-12932 arXiv:0710.2017[astro-ph] Contributed to 30th International Cosmic Ray Conference (ICRC 2007), 07/03/2007--7/11/2007, Merida, Yucatan, Mexico scope revealed no evidence for such emission [13] from this and nearby regions. The High Energy Stereoscopic System (H.E.S.S.: see [24] for details and performance) has observed the W 28 region over the 2004, 2005 and 2006 seasons. After quality selection, a total of ∼42 hr observations were available for analysis. Data were analysed using the moment-based Hillas analysis procedure employing hard cuts (image size >200 p.e.), the same used in the analysis of the inner Galactic Plane Scan datasets [15, 17]. An energy threshold of ∼ 320 GeV results from this analysis. The VHE γ-ray image in Fig. 1 shows that two source of VHE γ-ray emission are located at the northeastern and southern boundaries of W 28. The VHE sources are labelled HESS J1801−233 and HESS J1801−240 where the latter can be further subdivided into three components A, B, and C. The excess significances of both sources exceed ∼8σ after integrating events within their fitted, arcminute-scale sizes. Similar results were also obtained using an alternative analysis [7]. W 28: The Multiwavelength View We have revisited EGRET MeV/GeV data, including data from the CGRO observation cycles (OC) 1 to 6, which slightly expands on the dataset of the 3rd EGRET catalogue (using OCs 1 to 4; [9], revealing the source 3EG J1800−2338. We find a pointlike E > 100 MeV source in the W 28 region, labelled GRO J1801−2320 in Fig 1. The 68% and 95% location contours of GRO J1801−2320 match well the location of HESS J1801−233. However we cannot rule out a connection to HESS J1800−240 due to the degreescale EGRET PSF. Fig. 2 presents CO (J=1–0) observations from the NANTEN [1] Galactic Plane survey [12] covering the line-of-sight velocity ranges VLSR= 0 to 10 km s and 10 to 20 km s. These ranges represent distances 0 to ∼2.5 kpc and 2.5 to ∼4 kpc respectively and encompass the distance estimates for W 28. We cannot rule out however, distances ∼4 kpc for the VLSR >10 km s cloud components. It is clear that molecular clouds coincide well with the VHE sources. The northeastern cloud VLSR <10 km s component near HESS J1801−233, is already well-studied [35, 36]. Contributions from the VLSR >10 km s cloud components are also likely. The molecular cloud to the south of W 28 coincides well with HESS J1800−240 and its three VHE components. The VLSR <10 km s component of this cloud coincides well with HESS J1800−240B, and may represent the dense molecular matter surrounding the ultra-compact HII region W 28A2. This cloud also extends to VLSR ∼20 km s and thus, similar to HESS J1801−233, the total VHE emission in HESS J1800−240 may result from several molecular cloud components in projection. Fig. 3 compares the radio (left panel — VLA 90 cm [8]), infrared and X-ray views (right panel MSX 8.28 μm and ROSAT PSPC 0.5 to 2.4 keV [25]) with the VHE results. HESS J1801−233 overlaps the northeastern shell of the SNR, coinciding with a strong peak in the 90 cm continuum emission. Additional SNRs G6.67−0.42 and G7.06−0.12 [21, 10] are indicated. The non-thermal radio arc G5.71−0.08 is a SNR candidate [8], and is possibly a counterpart to HESS J1801−240C. The distances to G6.67−0.42 and G5.71−0.08 are presently unknown. The unusual, ultracompact HII region W 28A2, is positioned within 0.1 of the centroid of HESS J1800−240B. W 28A2, at a distance d ∼2 kpc, exhibits energetic bipolar molecular outflows [30, 26, 14] and may therefore be an energy source for particle acceleration in the region. The other HII regions G6.1−0.6 [32] and 6.225−0.569 [22] are also associated with radio emission. The X-ray morphology (Fig. 3 right panel) shows the central concentration of X-ray emission. A local X-ray peak or Ear is seen at the northeastern W 28 boundary. The HII regions, W 28A2 and G6.1−0.6 are prominent in the MSX 8.28 μm image (Fig. 3 right panel), indicating that a high concentration of heated dust still surrounds these very young stellar objects. Discussion and Conclusions H.E.S.S. and NANTEN observations reveal VHE emission in the W 28 region spatially coincident with molecular clouds. The VHE emission and 18h 18h03m -2 4 o -2 3 o 30 ́ -2 3 o 0 20 40 60 80 H.E.S.S. W 28 (Radio Boundary) W 28A2 G6.1-0.6 6.225-0.569 GRO J1801-2320 PSR J1801-23