OSSE observations of gamma-ray emission from Centaurus A

R. L. Kinzer*, W. N. Johnson, C. D. Dermer, J. D. Kurfess, M. S. Strickman, J. E. Grove, R. A. Kroeger, D. A. Grabelsky, W. R. Purcell, M. P. Ulmer, G. V. Jung, K. Mcnaron-Brown

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

49 Scopus citations

Abstract

Results are reported on hard X-ray and soft gamma-ray observations of Centaurus A during 1991-1994 with the Oriented Scintillation Spectrometer Experiment (OSSE) on NASA's Compton Gamma Ray Observatory (CGRO). Long-term intensity changes by a factor of 2-3 were measured over this period. Short-term variations by as much as 25% on timescales as short as 12 hr were detected in most of the observations. No evidence for gamma-ray line emission was detected at sensitivities much improved over previous observations. An intensity-dependent evolution of the 50-1000 keV spectral shape was observed which can be characterized as an exponentially cut-off power law with an intensity-dependent cutoff energy ranging from ∼300 keV at the highest observed level to ∼700 keV at the lowest levels. A broken power-law model with an intensity-correlated break above ∼120 keV describes the observed spectral changes equally well. Best-fit spectral breaks vary from ΔΓ ∼ 0.25 ± 0.1 at the lowest levels to ΔΓ ∼ 0.7 ± 0.15 at the highest observed intensity. Below 120 keV the spectra at all intensities can be fitted with photon spectral index Γ ∼ 1.70 ± 0.08, in agreement with prevous X-ray measurements. Contemporaneous measurements by the EGRET instrument on CGRO require additional spectral softening in the MeV region relative to broken power-law extrapolations of the OSSE measurements. At the maximum observed intensity, the luminosity of Cen A appears to peak at ∼ 200 keV with Ly(50-1000 keV) ≈ 7 × 1042 ergs s-1, implying a minimum black hole mass of only ≈ 5 × 104 solar masses for Eddington-limited accretion.

Original languageEnglish (US)
Pages (from-to)105-118
Number of pages14
JournalAstrophysical Journal
Volume449
Issue number1
DOIs
StatePublished - Aug 10 1995

Keywords

  • Galaxies: individual (NGC 5128)
  • Gamma rays: observations
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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