H.E.S.S. observations of the old-age (>10^4yr; ~0.5deg diameter) composite
supernova remnant (SNR) W 28 reveal very high energy (VHE) gamma-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.Comment: 4 pages, 3 figures, proceedings of the 30th ICRC, Merida, Mexico,
  200

Brion, E.

Collaboration, H. E. S. S.

Djannati-Ataï, A.

Fukui, Y.

Funk, S.

Moriguchi, Y.

Reimer, O.

Rowell, G.

English

arXiv

H.E.S.S. observations of the old-age (&gt;10{sup 4} yr; {approx} 0.5 degree diameter) composite supernova remnant (SNR)W28 reveal very high energy (VHE) {gamma}-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 labeled 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 centered on HESS J1801-233 and the northeastern interaction region

Djannati-Atai, A.

UNT Digital Library

Work supported in part by US Department of Energy contract DE-AC02-76SF00515Discovery of very high energy gamma-ray emission in the W 28 (G6.4−0.1) region,and multiwavelength comparisonsG. ROWELL1†, E. BRION2† , O. REIMER3† , Y. MORIGUCHI4 , Y. FUKUI4 , A. DJANNATI-ATAI¨5†, S.FUNK3†1 School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia2 DAPNIA/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex, France3 Stanford University, HEPL & KIPAC, Stanford, CA 94305-4085, USA4 Department of Astrophysics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan5 APC, 11 Place Marcelin Berthelot, F-75231 Paris Cedex 05, France† for the H.E.S.S. Collaboration www.mpi-hd.mpg.de/hfm/HESSgrowell@physics.adelaide.edu.auAbstract: H.E.S.S. observations of the old-age (>104 yr; ∼ 0.5◦ diameter) composite supernova rem-nant (SNR) W 28 reveal very high energy (VHE) γ-ray emission situated at its northeastern and southernboundaries. The northeastern VHE source (HESS J1801−233) is in an area where W 28 is interact-ing with a dense molecular cloud, containing OH masers, local radio and X-ray peaks. The southernVHE sources (HESS J1800−240 with components labelled A, B and C) are found in a region occupiedby several HII regions, including the ultracompact HII region W 28A2. Our analysis of NANTEN COdata reveals a dense molecular cloud enveloping this southern region, and our reanalysis of EGRET datareveals MeV/GeV emission centred on HESS J1801−233 and the northeastern interaction region.Introduction & H.E.S.S. ResultsThe study of shell-type SNRs at γ-ray energies ismotivated by the idea that they are the dominantsites of hadronic Galactic cosmic-ray (CR) acceler-ation to energies approaching the knee (∼ 1015 eV)and beyond, e.g. [34]. CRs are then acceleratedvia the diffusive shock acceleration (DSA) process(eg. [3, 31]). Gamma-ray production from the in-teraction of these CRs with ambient matter and/orelectromagnetic fields is a tracer of such particleacceleration, and establishing the hadronic or elec-tronic nature of the parent CRs in any γ-ray sourceis a key issue. Already, two shell-type SNRs,RX J1713.7−3946 and RX J0852.0−4622, exhibitshell-like morphology in VHE γ-rays [16, 18, 19]to 20 TeV and above. Although a hadronic originof the VHE γ-ray emission is highly likely in theabove cases, an electronic origin is not ruled out.W 28 (G6.4−0.1) is a composite or mixed-morphology SNR, with dimensions 50′x45′ and anestimated distance between 1.8 and 3.3 kpc (eg.[4, 33]). It is an old-age SNR (age 3.5×104 to15×104 yr [11]), thought to have entered its ra-diative phase of evolution [33]. The shell-like ra-dio emission [27, 23] peaks at the northern andnortheastern boundaries where interaction with amolecular cloud [2] is established [35, 36]. TheX-ray emission, which overall is well-explainedby a thermal model, peaks in the SNR cen-tre but has local enhancements in the northeast-ern SNR/molecular cloud interaction region [25].Additional SNRs in the vicinity of W 28 havealso been identified: G6.67−0.42 and G7.06−0.12[21]. The pulsar PSR J1801−23 with spin-downluminosity E˙ ∼ 6.2 × 1034 erg s−1 and distanced ≥ 9.4 kpc [28], is at the northern radio edge.Given its interaction with a molecular cloud, W 28is an ideal target for VHE observations. This in-teraction is traced by the high concentration of1720 MHz OH masers [6, 5, 29], and also the loca-tion of very high-density (n > 103 cm−3) shockedgas [36, 35]. Previous observations of the W 28 re-gion at VHE energies by the CANGAROO-I tele-SLAC-PUB-12932arXiv:0710.2017[astro-ph]Contributed to 30th International Cosmic Ray Conference (ICRC 2007), 07/03/2007--7/11/2007, Merida, Yucatan, Mexicoscope 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 ob-served the W 28 region over the 2004, 2005 and2006 seasons. After quality selection, a total of∼42 hr observations were available for analysis.Data were analysed using the moment-based Hillasanalysis procedure employing hard cuts (imagesize >200 p.e.), the same used in the analysis ofthe inner Galactic Plane Scan datasets [15, 17].An energy threshold of ∼ 320 GeV results fromthis analysis. The VHE γ-ray image in Fig. 1shows that two source of VHE γ-ray emission arelocated at the northeastern and southern bound-aries of W 28. The VHE sources are labelledHESS J1801−233 and HESS J1801−240 wherethe latter can be further subdivided into three com-ponents A, B, and C. The excess significances ofboth sources exceed ∼8σ after integrating eventswithin their fitted, arcminute-scale sizes. Simi-lar results were also obtained using an alternativeanalysis [7].W 28: The Multiwavelength ViewWe have revisited EGRET MeV/GeV data, in-cluding data from the CGRO observation cycles(OC) 1 to 6, which slightly expands on the datasetof the 3rd EGRET catalogue (using OCs 1 to4; [9], revealing the source 3EG J1800−2338.We find a pointlike E > 100 MeV source inthe W 28 region, labelled GRO J1801−2320 inFig 1. The 68% and 95% location contoursof GRO J1801−2320 match well the location ofHESS J1801−233. However we cannot rule out aconnection to HESS J1800−240 due to the degree-scale EGRET PSF.Fig. 2 presents 12CO (J=1–0) observations fromthe NANTEN [1] Galactic Plane survey [12] cov-ering the line-of-sight velocity ranges VLSR= 0 to10 km s−1 and 10 to 20 km s−1. These ranges rep-resent distances 0 to ∼2.5 kpc and 2.5 to ∼4 kpcrespectively and encompass the distance estimatesfor W 28. We cannot rule out however, distances∼4 kpc for the VLSR >10 km s−1 cloud com-ponents. It is clear that molecular clouds coin-cide well with the VHE sources. The northeast-ern cloud VLSR <10 km s−1 component nearHESS J1801−233, is already well-studied [35,36]. Contributions from the VLSR >10 km s−1cloud components are also likely. The molecularcloud to the south of W 28 coincides well withHESS J1800−240 and its three VHE components.The VLSR <10 km s−1 component of this cloudcoincides well with HESS J1800−240B, and mayrepresent the dense molecular matter surroundingthe ultra-compact HII region W 28A2. This cloudalso extends to VLSR ∼20 km s−1 and thus, simi-lar to HESS J1801−233, the total VHE emission inHESS J1800−240 may result from several molec-ular cloud components in projection.Fig. 3 compares the radio (left panel — VLA90 cm [8]), infrared and X-ray views (right panelMSX 8.28 µm and ROSAT PSPC 0.5 to 2.4 keV[25]) with the VHE results. HESS J1801−233overlaps the northeastern shell of the SNR, co-inciding with a strong peak in the 90 cm con-tinuum emission. Additional SNRs G6.67−0.42and G7.06−0.12 [21, 10] are indicated. Thenon-thermal radio arc G5.71−0.08 is a SNRcandidate [8], and is possibly a counterpart toHESS J1801−240C. The distances to G6.67−0.42and G5.71−0.08 are presently unknown. Theunusual, ultracompact HII region W 28A2,is positioned within 0.1◦ of the centroid ofHESS J1800−240B. W 28A2, at a distanced ∼2 kpc, exhibits energetic bipolar molecular out-flows [30, 26, 14] and may therefore be an energysource for particle acceleration in the region. Theother 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) showsthe central concentration of X-ray emission. A lo-cal X-ray peak or Ear is seen at the northeasternW 28 boundary. The HII regions, W 28A2 andG6.1−0.6 are prominent in the MSX 8.28 µm im-age (Fig. 3 right panel), indicating that a high con-centration of heated dust still surrounds these veryyoung stellar objects.Discussion and ConclusionsH.E.S.S. and NANTEN observations reveal VHEemission in the W 28 region spatially coincidentwith molecular clouds. The VHE emission and18h18h03m-24o-23o30´-23o020406080H.E.S.S.W 28 (Radio Boundary)W 28A2G6.1-0.66.225-0.569GRO J1801-2320PSR J1801-23HESS J1801-233HESS J1800-240A BCRA J2000.0 (hrs)Dec J2000.0 (deg)Figure 1: H.E.S.S. VHE γ-ray excess counts,corrected for exposure and Gaussian smoothed(with 4.2′ std. dev.). Solid green contours rep-resent excess significance levels of 4, 5, and6σ, for an integrating radius θ=0.1◦. The VHEsources HESS J1801−233 and a complex ofsources HESS J1800-240 (A, B & C) are in-dicated. The thin-dashed circle depicts the ap-proximate radio boundary of the SNR W 28 [23,8]. Additional objects indicated are: HII regions(black stars); W 28A2, G6.1−0.6 6.225−0.569;The 68% and 95% location contours (thick-dashed yellow lines) of the E > 100 MeVEGRET source GRO J1801−2320; the pulsarPSR J1801−23 (white triangle). The inset de-picts a pointlike source under identical analysisand smoothing as for the main image.molecular clouds are found at the northeasternboundary, and ∼ 0.5◦ south of W 28 respectively.The SNR W 28 may be a source of power for theVHE sources, although there are additional poten-tial particle accelerators in the region such as otherSNR/SNR-candidates, HII regions and open clus-ters. Further details concerning these results anddiscussion are presented in [20].AcknowledgementsThe support of the Namibian authorities and of the University of Namibiain facilitating the construction and operation of H.E.S.S. is gratefullyacknowledged, as is the support by the German Ministry for Educationand Research (BMBF), the Max Planck Society, the French Ministryfor Research, the CNRS-IN2P3 and the Astroparticle InterdisciplinaryProgramme of the CNRS, the U.K. Particle Physics and AstronomyResearch Council (PPARC), the IPNP of the Charles University,the Polish Ministry of Science and Higher Education, the SouthAfrican Department of Science and Technology and National ResearchFoundation, and by the University of Namibia. We appreciate theexcellent work of the technical support staff in Berlin, Durham,Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in theconstruction and operation of the equipment. The NANTEN project isfinancially supported from JSPS (Japan Society for the Promotion ofScience) Core-to-Core Program, MEXT Grant-in-Aid for ScientificResearch on Priority Areas, and SORST-JST (Solution OrientedResearch for Science and Technology: Japan Science and TechnologyAgency). We also thank Crystal Brogan for the VLA 90 cm image.References[1] Mizuno A. and Fukui Y. ASP Conf. Proc., 317:59, 2004.[2] Wootten A. ApJ, 245:105, 1981.[3] Bell A.R. MNRAS, 182:443, 1978.[4] Goudis C. Ap&SS, 40:91, 1976.[5] Claussen M.J.and Frail D.A., Goss W.M., and Gaume R.A. ApJ,489:143, 1997.[6] Frail D.A., Goss W.M., and Slysh V.I. ApJ, 424:L111, 1994.[7] de Naurois M. arXiv:astro-ph/0607247, 2006.[8] Brogan C.L. et al. ApJ, 639:L25, 2006.[9] Hartman R.C. et al. ApJ (Supp), 123:79, 1999.[10] Helfand D.J. et al. AJ, 131:2525, 2006.[11] Kaspi V.M. et al. ApJ, 409:L57, 1993.[12] Matsunaga K. et al. PASJ, 53:1003, 2001.[13] Rowell G.P et al. A&A, 359:337, 2000.[14] Sollins P.K. et al. 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ApJ, 522:349,1999.[30] Harvey P.M. and Forveille T. A&A, 197:L19, 1988.[31] Blandford R.D. and Ostriker J.P. ApJ, 221:L29, 1978.[32] Kuchar T.A. and Clark F.O. ApJ, 488:224, 1997.[33] Lozinskaya T.A. Sov. Astron. Lett., 7:17, 1981.[34] Ginzburg V.L. and S.I. Syrovatskii. The Origin of Cosmic Rays.(New York: Macmillan), 1964.[35] Reach W.T., Rho J., and Jarrett T.H. ApJ, 618:297, 2005.[36] Arikawa Y., Tatematsu K., Sekimoto Y., and Takahashi T., 1999.18h18h03m-24o-23o30´-23o020406080100NANTEN 12CO(J=1-0)  0-10 km/sl=+6.0ol=+7.0ob=-1.0ob=0.0oW 28 (Radio Boundary)W 28A2G6.1-0.66.225-0.569GRO J1801-2320HESS J1801-233HESS J1800-240A BCRA J2000.0 (hrs)Dec J2000.0 (deg)18h18h03m-24o-23o30´-23o020406080100NANTEN 12CO(J=1-0)  10-20 km/sl=+6.0ol=+7.0ob=-1.0ob=0.0oW 28 (Radio Boundary)W 28A2G6.1-0.66.225-0.569GRO J1801-2320HESS J1801-233HESS J1800-240A BCRA J2000.0 (hrs)Dec J2000.0 (deg)Figure 2: Left: NANTEN 12CO(J=1-0) image (linear scale in K km s−1) for VLSR=0 to 10 km s−1 withVHE γ-ray significance contours overlaid (green) — levels 4,5,6σ and other features as in Fig. 1. Right:NANTEN 12CO(J=1-0) image for VLSR=10 to 20 km s−1 (linear scale and same maximum as for leftpanel).Figure 3: Left: VLA 90cm radio image [8] in Jy beam−1. The VHE significance contours (green) fromFig. 1 are overlaid along with the HII regions (white stars) and the additional SNRs and SNR candidates(with yellow circles indicating their location and approximate dimensions) discussed in text. Right: ROSATPSPC image — 0.5 to 2.4 keV (smoothed counts per bin [25]). Overlaid are contours (cyan — 10 linearlevels up to 5×10−4 W m−2 sr−1) from the MSX 8.28 µm image. Other contours and objects are as for theleft panel. The X-ray Ear representing a peak at the northeastern edge is indicated.

Discovery of Very High Energy Gamma-Ray Emission in the W 28 (G6.4-0.1) Region and Multiwavelength Comparisons

Submitted to Cornell University’s online archive www.arXiv.org in 2007 by Gavin Rowell.H.E.S.S. observations of the old-age (>10^4yr; ~0.5deg diameter) composite
supernova remnant (SNR) W 28 reveal very high energy (VHE) gamma-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

Adelaide Research &amp; Scholarship

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

https://digital.library.unt.edu/ark:/67531/metadc881791/m2/1/high_res_d/918972.pdf

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

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

Abstract

Similar works

Full text

Available Versions

UNT Digital Library

Adelaide Research &amp; Scholarship

Adelaide Research & Scholarship