Is the INTEGRAL IBIS source IGR J17204-3554 a gamma-ray-emitting galaxy hidden behind the molecular cloud NGC 6334?

L. Bassani, A. De Rosa, A. Bazzano, A. J. Bird, A. J. Dean, N. Gehrels, J. A. Kennea, A. Malizia, M. Molina, J. B. Stephen, P. Ubertini, R. Walter

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11 Scopus citations

Abstract

We report on the identification of a soft gamma-ray source, IGR J17204-3554, detected with IBIS, the Imager on Board the INTEGRAL Satellite. The source has a 20-100 keV flux of ∼3 × 10-11 ergs cm -2 s-1 and is spatially coincident with NGC 6334, a molecular cloud located in the Sagittarius arm of the Milky Way. Diffuse X-ray emission has been reported from this region by ASCA and interpreted as coming from five far-infrared cores located in the cloud. However, the combined ASCA spectrum with a 9 keV temperature was difficult to explain in terms of emission from young pre-main-sequence stars known to be embedded in the star-forming regions. Detection of gamma rays makes this interpretation even more unrealistic and suggests the presence of a high-energy source in or behind the cloud. Follow-up observations with Swift and archival Chandra data allow us to disentangle the NGC 6334 enigma by locating an extragalactic object with the proper spectral characteristics to explain the gamma-ray emission. The combined Chandra-IBIS spectrum is well fitted by an absorbed power law with Γ = 1.2 ± 0.1, NH = (1.4 ± 0.1) × 1023 cm-2, and an unabsorbed 2-10 keV flux of 0.5 × 10-11 ergs cm-2 s-1. This column density is in excess of the Galactic value, implying that we are detecting a background galaxy concealed by the molecular cloud and further hidden by material located either in the galaxy itself or between IGR J17204-3554 and the cloud.

Original languageEnglish (US)
Pages (from-to)L21-L24
JournalAstrophysical Journal
Volume634
Issue number1 II
DOIs
StatePublished - Nov 20 2005

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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