1. Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W?28 (G6.4-0.1) field
- Author
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Aharonian, F., Akhperjanian, A., Bazer-Bachi, A., Behera, B., Beilicke, M., Benbow, W., Berge, D., Bernl?hr, K., Boisson, C., Bolz, O., Borrel, V., Braun, I., Brion, E., Brown, A., B?hler, R., Bulik, T., B?sching, I., Boutelier, T., Carrigan, S., Chadwick, P., Chounet, L.-M., Clapson, A., Coignet, G., Cornils, R., Costamante, L., Degrange, B., Dickinson, H., Djannati-Ata?, A., Domainko, W., Drury, L., Dubus, G., Dyks, J., Egberts, K., Emmanoulopoulos, D., Espigat, P., Farnier, C., Feinstein, F., Fiasson, A., F?rster, A., Fontaine, G., Fukui, Y., Funk, Seb., Funk, S., F??ling, M., Gallant, Y., Giebels, B., Glicenstein, J., Gl?ck, B., Goret, P., Hadjichristidis, C., Hauser, D., Hauser, M., Heinzelmann, G., Henri, G., Hermann, G., Hinton, J., Hoffmann, A., Hofmann, W., Holleran, M., Hoppe, S., Horns, D., Jacholkowska, A., de Jager, O., Kendziorra, E., Kerschhaggl, M., Kh?lifi, B., Komin, Nu., Kosack, K., Lamanna, G., Latham, I., Le Gallou, R., Lemi?re, A., Lemoine-Goumard, M., Lenain, J.-P., Lohse, T., Martin, J., Martineau-Huynh, O., Marcowith, A., Masterson, C., Maurin, G., McComb, T., Moderski, R., Moriguchi, Y., Moulin, E., de Naurois, M., Nedbal, D., Nolan, S., Olive, J.-P., Orford, K., Osborne, J., Ostrowski, M., Panter, M., Pedaletti, G., Pelletier, G., Petrucci, P.-O., Pita, S., P?hlhofer, G., Punch, M., Ranchon, S., Raubenheimer, B., Raue, M., Rayner, S., Reimer, O., Renaud, M., Ripken, J., Rob, L., Rolland, L., Rosier-Lees, S., Rowell, G., Rudak, B., Ruppel, J., Sahakian, V., Santangelo, A., Saug?, L., Schlenker, S., Schlickeiser, R., Schr?der, R., Schwanke, U., Schwarzburg, S., Schwemmer, S., Shalchi, A., Sol, H., Spangler, D., Stawarz, ?., Steenkamp, R., Stegmann, C., Superina, G., Takeuchi, T., Tam, P., Tavernet, J.-P., Terrier, R., van Eldik, C., Vasileiadis, G., Venter, C., Vialle, J., Vincent, P., Vivier, M., V?lk, H., Volpe, F., Wagner, S., and Ward, M.
- Abstract
Aims. Observations of shell-type supernova remnants (SNRs) in the GeV to multi-TeV ?-ray band, coupled with those at millimetre radio wavelengths, are motivated by the search for cosmic-ray accelerators in our Galaxy. The old-age mixed-morphology SNR W?28 (distance?~2?kpc) is a prime target due to its interaction with molecular clouds along its northeastern boundary and other clouds situated nearby.Methods. We observed the W?28 field (for ~40?h) at very high energy (VHE) ?-ray energies (E > 0.1?TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET E > 100?MeV data was also undertaken. Results from the NANTEN?4?m telescope Galactic plane survey and other CO?observations were used to study molecular clouds. Results. We have discovered VHE ?-ray emission (HESS?J1801-233) coincident with the northeastern boundary of W?28 and a complex of sources (HESS?J1800-240A, B and C) ~0.5? south of W?28 in the Galactic disc. The EGRET source (GRO?J1801-2320) is centred on HESS?J1801-233 but may also be related to HESS?J1800-240 given the large EGRET point spread function. The VHE differential photon spectra are well fit by pure power laws with indices ? ~2.3 to 2.7. The spectral indices of HESS?J1800-240A, B, and C are consistent within statistical errors. All VHE sources are ~10?in intrinsic radius except for HESS?J1800-240C, which appears pointlike. The NANTEN 12CO(J = 1-0) data reveal molecular clouds positionally associating with the VHE emission, spanning a ~15?km?s-1range in local standard of rest velocity. Conclusions. The VHE/molecular cloud association could indicate a hadronic origin for HESS?J1801-233 and HESS?J1800-240, and several cloud components in projection may contribute to the VHE emission. The clouds have components covering a broad velocity range encompassing the distance estimates for W?28 (~2?kpc) and extending up to ~4?kpc. Assuming hadronic origin and distances of 2 and 4?kpc for cloud components, the required cosmic-ray density enhancement factors (with respect to the solar value) are in the range ~10 to ~30. If situated at 2?kpc distance, such cosmic-ray densities may be supplied by SNRs like W?28. Additionally and/or alternatively, particle acceleration may come from several catalogued SNRs and SNR candidates, the energetic ultra compact HII?region W?28A2, and the HII?regions M?8 and M?20, along with their associated open clusters. Further sub-mm observations would be recommended to probe in detail the dynamics of the molecular clouds at velocites > 10?km?s-1and their possible connection to W?28.
- Published
- 2008