Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field

F. Aharonian; A. G. Akhperjanian; A. R. Bazer-Bachi; B. Behera; M. Beilicke; W. Benbow; D. Berge; K. Bernlöhr; C. Boisson; O. Bolz; V. Borrel; I. Braun; E. Brion; A. M. Brown; R. Bühler; T. Bulik; I. Büsching; T. Boutelier; S. Carrigan; P. M. Chadwick; L. M. Chounet; A. C. Clapson; G. Coignet; R. Cornils; L. Costamante; B. Degrange; H. J. Dickinson; A. Djannati-Ataï; W. Domainko; L. O.C. Drury; G. Dubus; J. Dyks; K. Egberts; D. Emmanoulopoulos; P. Espigat; C. Farnier; F. Feinstein; A. Fiasson; A. Förster; G. Fontaine; Y. Fukui; Seb Funk; S. Funk; M. Füßling; Y. A. Gallant; B. Giebels; J. F. Glicenstein; B. Glück; P. Goret; C. Hadjichristidis; D. Hauser; M. Hauser; G. Heinzelmann; G. Henri; G. Hermann; J. A. Hinton; A. Hoffmann; W. Hofmann; M. Holleran; S. Hoppe; D. Horns; A. Jacholkowska; O. C. De Jager; E. Kendziorra; M. Kerschhaggl; B. Khélifi; Nu Komin; K. Kosack; G. Lamanna; I. J. Latham; R. Le Gallou; A. Lemière; M. Lemoine-Goumard; J. P. Lenain; T. Lohse; J. M. Martin; O. Martineau-Huynh; A. Marcowith; C. Masterson; G. Maurin; T. J.L. McComb; R. Moderski; Y. Moriguchi; E. Moulin; M. De Naurois; D. Nedbal; S. J. Nolan; J. P. Olive; K. J. Orford; J. L. Osborne; M. Ostrowski; M. Panter; G. Pedaletti; G. Pelletier; P. O. Petrucci; S. Pita; G. Pühlhofer; M. Punch; S. Ranchon; B. C. Raubenheimer; M. Raue; S. M. Rayner; O. Reimer; M. Renaud; J. Ripken; L. Rob; L. Rolland; S. Rosier-Lees; G. Rowell; B. Rudak; J. Ruppel; V. Sahakian; A. Santangelo; L. Saugé; S. Schlenker; R. Schlickeiser; R. Schroder; U. Schwanke; S. Schwarzburg; S. Schwemmer; A. Shalchi; H. Sol; D. Spangler; Stawarz; R. Steenkamp; C. Stegmann; G. Superina; T. Takeuchi; P. H. Tam; J. P. Tavernet; R. Terrier; C. Van Eldik; G. Vasileiadis; C. Venter; J. P. Vialle; P. Vincent; M. Vivier; H. J. Völk; F. Volpe; S. J. Wagner; M. Ward

doi:10.1051/0004-6361:20077765

Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field

F. Aharonian^*, A. G. Akhperjanian, A. R. Bazer-Bachi, B. Behera, M. Beilicke, W. Benbow, D. Berge, K. Bernlöhr, C. Boisson, O. Bolz, V. Borrel, I. Braun, E. Brion, A. M. Brown, R. Bühler, T. Bulik, I. Büsching, T. Boutelier, S. Carrigan, P. M. ChadwickL. M. Chounet, A. C. Clapson, G. Coignet, R. Cornils, L. Costamante, B. Degrange, H. J. Dickinson, A. Djannati-Ataï, W. Domainko, L. O.C. Drury, G. Dubus, J. Dyks, K. Egberts, D. Emmanoulopoulos, P. Espigat, C. Farnier, F. Feinstein, A. Fiasson, A. Förster, G. Fontaine, Y. Fukui, Seb Funk, S. Funk, M. Füßling, Y. A. Gallant, B. Giebels, J. F. Glicenstein, B. Glück, P. Goret, C. Hadjichristidis, D. Hauser, M. Hauser, G. Heinzelmann, G. Henri, G. Hermann, J. A. Hinton, A. Hoffmann, W. Hofmann, M. Holleran, S. Hoppe, D. Horns, A. Jacholkowska, O. C. De Jager, E. Kendziorra, M. Kerschhaggl, B. Khélifi, Nu Komin, K. Kosack, G. Lamanna, I. J. Latham, R. Le Gallou, A. Lemière, M. Lemoine-Goumard, J. P. Lenain, T. Lohse, J. M. Martin, O. Martineau-Huynh, A. Marcowith, C. Masterson, G. Maurin, T. J.L. McComb, R. Moderski, Y. Moriguchi, E. Moulin, M. De Naurois, D. Nedbal, S. J. Nolan, J. P. Olive, K. J. Orford, J. L. Osborne, M. Ostrowski, M. Panter, G. Pedaletti, G. Pelletier, P. O. Petrucci, S. Pita, G. Pühlhofer, M. Punch, S. Ranchon, B. C. Raubenheimer, M. Raue, S. M. Rayner, O. Reimer, M. Renaud, J. Ripken, L. Rob, L. Rolland, S. Rosier-Lees, G. Rowell, B. Rudak, J. Ruppel, V. Sahakian, A. Santangelo, L. Saugé, S. Schlenker, R. Schlickeiser, R. Schroder, U. Schwanke, S. Schwarzburg, S. Schwemmer, A. Shalchi, H. Sol, D. Spangler, Stawarz, R. Steenkamp, C. Stegmann, G. Superina, T. Takeuchi, P. H. Tam, J. P. Tavernet, R. Terrier, C. Van Eldik, G. Vasileiadis, C. Venter, J. P. Vialle, P. Vincent, M. Vivier, H. J. Völk, F. Volpe, S. J. Wagner, M. Ward

^*Corresponding author for this work

Research output: Contribution to journal › Article › peer-review

246 Scopus citations

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 ( 0.1$]]> TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET 100$]]> 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 CO() data reveal molecular clouds positionally associating with the VHE emission, spanning a ∼15 km s range 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 and their possible connection to W 28.

Original language	English (US)
Pages (from-to)	401-410
Number of pages	10
Journal	Astronomy and Astrophysics
Volume	481
Issue number	2
DOIs	https://doi.org/10.1051/0004-6361:20077765
State	Published - Apr 2008
Externally published	Yes

Keywords

Gamma rays: observations

ASJC Scopus subject areas

Astronomy and Astrophysics
Space and Planetary Science

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10.1051/0004-6361:20077765

Cite this

Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R., Behera, B., Beilicke, M., Benbow, W., Berge, D., Bernlöhr, K., Boisson, C., Bolz, O., Borrel, V., Braun, I., Brion, E., Brown, A. M., Bühler, R., Bulik, T., Büsching, I., Boutelier, T., Carrigan, S., ... Ward, M. (2008). Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field. Astronomy and Astrophysics, 481(2), 401-410. https://doi.org/10.1051/0004-6361:20077765

@article{5bd84e002213488bbcc078dd6f096ce7,

title = "Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field",

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 ( 0.1$]]> TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET 100$]]> 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 CO() data reveal molecular clouds positionally associating with the VHE emission, spanning a ∼15 km s range 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 and their possible connection to W 28.",

keywords = "Gamma rays: observations",

author = "F. Aharonian and Akhperjanian, {A. G.} and Bazer-Bachi, {A. R.} and B. Behera and M. Beilicke and W. Benbow and D. Berge and K. Bernl{\"o}hr and C. Boisson and O. Bolz and V. Borrel and I. Braun and E. Brion and Brown, {A. M.} and R. B{\"u}hler and T. Bulik and I. B{\"u}sching and T. Boutelier and S. Carrigan and Chadwick, {P. M.} and Chounet, {L. M.} and Clapson, {A. C.} and G. Coignet and R. Cornils and L. Costamante and B. Degrange and Dickinson, {H. J.} and A. Djannati-Ata{\"i} and W. Domainko and Drury, {L. O.C.} and G. Dubus and J. Dyks and K. Egberts and D. Emmanoulopoulos and P. Espigat and C. Farnier and F. Feinstein and A. Fiasson and A. F{\"o}rster and G. Fontaine and Y. Fukui and Seb Funk and S. Funk and M. F{\"u}{\ss}ling and Gallant, {Y. A.} and B. Giebels and Glicenstein, {J. F.} and B. Gl{\"u}ck and P. Goret and C. Hadjichristidis and D. Hauser and M. Hauser and G. Heinzelmann and G. Henri and G. Hermann and Hinton, {J. A.} and A. Hoffmann and W. Hofmann and M. Holleran and S. Hoppe and D. Horns and A. Jacholkowska and {De Jager}, {O. C.} and E. Kendziorra and M. Kerschhaggl and B. Kh{\'e}lifi and Nu Komin and K. Kosack and G. Lamanna and Latham, {I. J.} and {Le Gallou}, R. and A. Lemi{\`e}re and M. Lemoine-Goumard and Lenain, {J. P.} and T. Lohse and Martin, {J. M.} and O. Martineau-Huynh and A. Marcowith and C. Masterson and G. Maurin and McComb, {T. J.L.} and R. Moderski and Y. Moriguchi and E. Moulin and {De Naurois}, M. and D. Nedbal and Nolan, {S. J.} and Olive, {J. P.} and Orford, {K. J.} and Osborne, {J. L.} and M. Ostrowski and M. Panter and G. Pedaletti and G. Pelletier and Petrucci, {P. O.} and S. Pita and G. P{\"u}hlhofer and M. Punch and S. Ranchon and Raubenheimer, {B. C.} and M. Raue and Rayner, {S. M.} and O. Reimer and M. Renaud and J. Ripken and L. Rob and L. Rolland and S. Rosier-Lees and G. Rowell and B. Rudak and J. Ruppel and V. Sahakian and A. Santangelo and L. Saug{\'e} and S. Schlenker and R. Schlickeiser and R. Schroder and U. Schwanke and S. Schwarzburg and S. Schwemmer and A. Shalchi and H. Sol and D. Spangler and Stawarz and R. Steenkamp and C. Stegmann and G. Superina and T. Takeuchi and Tam, {P. H.} and Tavernet, {J. P.} and R. Terrier and {Van Eldik}, C. and G. Vasileiadis and C. Venter and Vialle, {J. P.} and P. Vincent and M. Vivier and V{\"o}lk, {H. J.} and F. Volpe and Wagner, {S. J.} and M. Ward",

year = "2008",

month = apr,

doi = "10.1051/0004-6361:20077765",

language = "English (US)",

volume = "481",

pages = "401--410",

journal = "Astronomy and Astrophysics",

issn = "0004-6361",

publisher = "EDP Sciences",

number = "2",

}

Aharonian, F, Akhperjanian, AG, Bazer-Bachi, AR, Behera, B, Beilicke, M, Benbow, W, Berge, D, Bernlöhr, K, Boisson, C, Bolz, O, Borrel, V, Braun, I, Brion, E, Brown, AM, Bühler, R, Bulik, T, Büsching, I, Boutelier, T, Carrigan, S, Chadwick, PM, Chounet, LM, Clapson, AC, Coignet, G, Cornils, R, Costamante, L, Degrange, B, Dickinson, HJ, Djannati-Ataï, A, Domainko, W, Drury, LOC, Dubus, G, Dyks, J, Egberts, K, Emmanoulopoulos, D, Espigat, P, Farnier, C, Feinstein, F, Fiasson, A, Förster, A, Fontaine, G, Fukui, Y, Funk, S, Funk, S, Füßling, M, Gallant, YA, Giebels, B, Glicenstein, JF, Glück, B, Goret, P, Hadjichristidis, C, Hauser, D, Hauser, M, Heinzelmann, G, Henri, G, Hermann, G, Hinton, JA, Hoffmann, A, Hofmann, W, Holleran, M, Hoppe, S, Horns, D, Jacholkowska, A, De Jager, OC, Kendziorra, E, Kerschhaggl, M, Khélifi, B, Komin, N, Kosack, K, Lamanna, G, Latham, IJ, Le Gallou, R, Lemière, A, Lemoine-Goumard, M, Lenain, JP, Lohse, T, Martin, JM, Martineau-Huynh, O, Marcowith, A, Masterson, C, Maurin, G, McComb, TJL, Moderski, R, Moriguchi, Y, Moulin, E, De Naurois, M, Nedbal, D, Nolan, SJ, Olive, JP, Orford, KJ, Osborne, JL, Ostrowski, M, Panter, M, Pedaletti, G, Pelletier, G, Petrucci, PO, Pita, S, Pühlhofer, G, Punch, M, Ranchon, S, Raubenheimer, BC, Raue, M, Rayner, SM, Reimer, O, Renaud, M, Ripken, J, Rob, L, Rolland, L, Rosier-Lees, S, Rowell, G, Rudak, B, Ruppel, J, Sahakian, V, Santangelo, A, Saugé, L, Schlenker, S, Schlickeiser, R, Schroder, R, Schwanke, U, Schwarzburg, S, Schwemmer, S, Shalchi, A, Sol, H, Spangler, D, Stawarz, Steenkamp, R, Stegmann, C, Superina, G, Takeuchi, T, Tam, PH, Tavernet, JP, Terrier, R, Van Eldik, C, Vasileiadis, G, Venter, C, Vialle, JP, Vincent, P, Vivier, M, Völk, HJ, Volpe, F, Wagner, SJ & Ward, M 2008, 'Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field', Astronomy and Astrophysics, vol. 481, no. 2, pp. 401-410. https://doi.org/10.1051/0004-6361:20077765

TY - JOUR

T1 - Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field

AU - Aharonian, F.

AU - Akhperjanian, A. G.

AU - Bazer-Bachi, A. R.

AU - Behera, B.

AU - Beilicke, M.

AU - Benbow, W.

AU - Berge, D.

AU - Bernlöhr, K.

AU - Boisson, C.

AU - Bolz, O.

AU - Borrel, V.

AU - Braun, I.

AU - Brion, E.

AU - Brown, A. M.

AU - Bühler, R.

AU - Bulik, T.

AU - Büsching, I.

AU - Boutelier, T.

AU - Carrigan, S.

AU - Chadwick, P. M.

AU - Chounet, L. M.

AU - Clapson, A. C.

AU - Coignet, G.

AU - Cornils, R.

AU - Costamante, L.

AU - Degrange, B.

AU - Dickinson, H. J.

AU - Djannati-Ataï, A.

AU - Domainko, W.

AU - Drury, L. O.C.

AU - Dubus, G.

AU - Dyks, J.

AU - Egberts, K.

AU - Emmanoulopoulos, D.

AU - Espigat, P.

AU - Farnier, C.

AU - Feinstein, F.

AU - Fiasson, A.

AU - Förster, A.

AU - Fontaine, G.

AU - Fukui, Y.

AU - Funk, Seb

AU - Funk, S.

AU - Füßling, M.

AU - Gallant, Y. A.

AU - Giebels, B.

AU - Glicenstein, J. F.

AU - Glück, B.

AU - Goret, P.

AU - Hadjichristidis, C.

AU - Hauser, D.

AU - Hauser, M.

AU - Heinzelmann, G.

AU - Henri, G.

AU - Hermann, G.

AU - Hinton, J. A.

AU - Hoffmann, A.

AU - Hofmann, W.

AU - Holleran, M.

AU - Hoppe, S.

AU - Horns, D.

AU - Jacholkowska, A.

AU - De Jager, O. C.

AU - Kendziorra, E.

AU - Kerschhaggl, M.

AU - Khélifi, B.

AU - Komin, Nu

AU - Kosack, K.

AU - Lamanna, G.

AU - Latham, I. J.

AU - Le Gallou, R.

AU - Lemière, A.

AU - Lemoine-Goumard, M.

AU - Lenain, J. P.

AU - Lohse, T.

AU - Martin, J. M.

AU - Martineau-Huynh, O.

AU - Marcowith, A.

AU - Masterson, C.

AU - Maurin, G.

AU - McComb, T. J.L.

AU - Moderski, R.

AU - Moriguchi, Y.

AU - Moulin, E.

AU - De Naurois, M.

AU - Nedbal, D.

AU - Nolan, S. J.

AU - Olive, J. P.

AU - Orford, K. J.

AU - Osborne, J. L.

AU - Ostrowski, M.

AU - Panter, M.

AU - Pedaletti, G.

AU - Pelletier, G.

AU - Petrucci, P. O.

AU - Pita, S.

AU - Pühlhofer, G.

AU - Punch, M.

AU - Ranchon, S.

AU - Raubenheimer, B. C.

AU - Raue, M.

AU - Rayner, S. M.

AU - Reimer, O.

AU - Renaud, M.

AU - Ripken, J.

AU - Rob, L.

AU - Rolland, L.

AU - Rosier-Lees, S.

AU - Rowell, G.

AU - Rudak, B.

AU - Ruppel, J.

AU - Sahakian, V.

AU - Santangelo, A.

AU - Saugé, L.

AU - Schlenker, S.

AU - Schlickeiser, R.

AU - Schroder, R.

AU - Schwanke, U.

AU - Schwarzburg, S.

AU - Schwemmer, S.

AU - Shalchi, A.

AU - Sol, H.

AU - Spangler, D.

AU - Stawarz,

AU - Steenkamp, R.

AU - Stegmann, C.

AU - Superina, G.

AU - Takeuchi, T.

AU - Tam, P. H.

AU - Tavernet, J. P.

AU - Terrier, R.

AU - Van Eldik, C.

AU - Vasileiadis, G.

AU - Venter, C.

AU - Vialle, J. P.

AU - Vincent, P.

AU - Vivier, M.

AU - Völk, H. J.

AU - Volpe, F.

AU - Wagner, S. J.

AU - Ward, M.

PY - 2008/4

Y1 - 2008/4

N2 - 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 ( 0.1$]]> TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET 100$]]> 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 CO() data reveal molecular clouds positionally associating with the VHE emission, spanning a ∼15 km s range 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 and their possible connection to W 28.

AB - 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 ( 0.1$]]> TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET 100$]]> 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 CO() data reveal molecular clouds positionally associating with the VHE emission, spanning a ∼15 km s range 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 and their possible connection to W 28.

KW - Gamma rays: observations

UR - http://www.scopus.com/inward/record.url?scp=41449087562&partnerID=8YFLogxK

U2 - 10.1051/0004-6361:20077765

DO - 10.1051/0004-6361:20077765

M3 - Article

AN - SCOPUS:41449087562

SN - 0004-6361

VL - 481

SP - 401

EP - 410

JO - Astronomy and Astrophysics

JF - Astronomy and Astrophysics

IS - 2

ER -

Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field

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