Hard X-ray monitoring of Centaurus A by SIGMA during 1990-94: definition of a low luminosity state.

Details Hard X-ray monitoring of Centaurus A by SIGMA during 1990-94: definition of a low luminosity state. Hard X-ray monitoring of Centaurus A by SIGMA during 1990-94: definition of a low luminosity state.

Mar 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...307..708b&link_type=abstract

Astronomy and Astrophysics, v.307, p.708-714

Astronomy and Astrophysics

Astrophysics

22

Galaxies: Active, Galaxies: Ngc 5128, Galaxies: Nuclei, X-Rays: Galaxies

Scientific paper

We present the results of a 4year monitoring campaign of the active galaxy Cen A carried out by the hard X-ray imaging telescope SIGMA during 1990-94. The accumulated statistics in the imaging data allow a precise position measurement of the hard X-ray source. On comparing the SIGMA data with other hard X-ray measurements during this period, we can infer that the source brightened and dimmed on at least 3 occasions between 1990 July and 1992 July. The 40-120keV energy transfer during each episode was at least ~10^42^erg/s. From 1992 July to 1994 July, the source was repeatedly found at flux levels among the lowest ever recorded. A comparison of the SIGMA data with all available hard X-ray flux measurements since 1969 allows the identification of a low luminosity state in addition to the previously identified high state which occurred during 1973-76. The low luminosity of Cen A recorded by SIGMA during 1992-94 probably represents the stable quiescent state of the hard X-ray source in Cen A.

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