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Astron. Astrophys. 321, 288-292 (1997)

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COMPTEL detection of low-energy gamma rays from the HVC Complex M and A region?

J.J. Blom 2, H. Bloemen 2, 5, A.M. Bykov 4, 2, W.B. Burton 5, Dap Hartmann 6, W. Hermsen 2, A.F. Iyudin 1, J. Ryan 3, V. Schönfelder 1, A.W. Strong 1 and Yu.A. Uvarov 4

1 Max-Planck-Institut für extraterrestrische Physik, P.O. Box 1603, D-85740 Garching, Germany
2 SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3 Space Science Center, Univ. of New Hampshire, Durham NH 03824, USA
4 A.F. Ioffe Institute of Physics and Technology, 194021 St. Petersburg, Russia
5 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
6 Center for Astrophysics, 60 Garden St., MS 72, Cambridge, MA 02138, USA

Received 22 August 1996 / Accepted 7 October 1996


We report the first observational indication for extended MeV emission that may be associated with high-velocity clouds (HVCs). Based on a combination of fifteen COMPTEL observations made during the period 1991-1995 we find evidence for intense [FORMULA] -radiation at 0.75-3 MeV from the general direction of the HVC Complexes M and A, adjacent to the Lockman et al. hole. The [FORMULA] -ray excess roughly covers a region of exceptionally low HI column densities and appears to consist of a time-variable source and persistent, likely diffuse, emission. The (enhanced) diffuse soft X-rays from this HVC area seen by ROSAT may be closely related to the [FORMULA] -ray emission. We show that the X-ray and [FORMULA] -ray emission can both be attributed to non-thermal electron bremsstrahlung arising from HVC interactions with the galactic disk or lower halo.

Key words: gamma rays: observations – ISM: clouds – ISM: individual objects: HVC Complex M, Complex A

Send offprint requests to: J.J. Blom (H.Blom@sron.ruu.nl)

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998