Detection of nuclear gamma radiation from Centaurus A

Abstract

Centaurus A (NGC 5128) was observed in the low-energy gamma-ray region (.33 MeV-12.25 MeV) on 1974 April 2 U.T. by a balloon-borne detector. The detector is an actively collimated scintillation counter with a sensitive area of 182 cm^ and a 13° FWHM (.4 steradians) field-of-view. The instrumental field-of-view encompassed the entire extent of the Cen A radio lobes without contamination from other dis¬ crete X-ray sources. The detector was launched from Rio Cuarto, Argentina, and the observations were made at an atmospheric depth of 4 gm/cm . To measure the background flux the detector was periodically rotated ^18° in azimuth from the Cen A orientation. To derive the source spectrum the background fluxes were subtracted from the fluxes observed when the detector was pointed towards Cen A. The residual flux was then cor¬ rected for instrumental effects and atmospheric absorption. Pointing difficulties during the experiment required incorporation of off-axis attenuation correction factors. Occasional, random telemetry-saturating count rates were observed. These anomalous counts are probably due to impact of high-energy particles on the detector. To avoid distortion by these counts, they were deleted from the data. Statistical anomalies in the data required a variance analysis of the data and modifications of the data reduction. The anomalies were isolated to the breakdown of the statistical methods used when working with few counts per channel. The computed Cen A spectrum shows a ten-fold increase since 1971 in hard X-ray flux. The best fit to the intensified continuum detected in the 1974 observation is (.86 +_ .17)E[keV] 1*9+.4 photons-cm ^-sec ^-keV^. Gamma-ray spectral lines were also detected. These are the first nuclear lines detected from what is apparently an extragalactic source. The lines are at 1.6 MeV and 4.5 MeV. The 1.6 MeV line is 3.3a above the continuum and is 24.51 FWHM broadened. The 4.5 MeV line is also 3.3o above the continuum but is not broadened. The lines are most likely from nuclear deexcitation of 24Mg*(1.37 MeV), 2Ne*(1.63 MeV), and Si*(1.78 MeV) for the broad 1.6 MeV feature and deexcitation of ^2C*(4.43 MeV) for the 4.5 MeV line. No positron annihilation line (.511 MeV) was detected. The nuclear lines may be due to low-energy cosmic rays interacting with interstellar gas in Cen A, from nucleosynthesis in the galaxy NGC 5128, or, possibly, from line-of-sight contamination by low-energy cosmic rays within our Galaxy.