Al in the inner Galaxy

Large-scale spectral characteristics derived with SPI/INTEGRAL

¹ Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany e-mail: rod@mpe.mpg.de
² Centre d'Étude Spatiale des Rayonnements and Université Paul Sabatier, 31028 Toulouse, France
³ DSM/DAPNIA/Service d'Astrophysique, CEA Saclay, 91191 Gif-Sur-Yvette, France
⁴ ESA/ESTEC, Science Operations and Data Systems Division (SCI-SD) 2201 AZ Noordwijk, The Netherlands
⁵ Space Sciences Lab., Berkeley, USA

Received: 4 October 2005
Accepted: 17 November 2005

Abstract

Aims.We performed a spectroscopic study of the 1809 keV gamma-ray line from ²⁶Al decay in the Galaxy using the SPI imaging spectrometer with its high-resolution Ge detector camera on the INTEGRAL observatory. Methods.We analyzed observations of the first two mission years, fitting spectra from all 7130 telescope pointings in narrow energy bins to models of instrumental background and the ²⁶Al sky. Instrumental background is estimated from independent tracers of cosmic-ray activation. The shape of the ²⁶Al signal is compared to the instrumental response to extract the width of the celestial line.Results.We detect the ²⁶Al line at 16σ significance. The line is broadened only slightly, if at all; we constrain the width to be below 2.8 keV (FWHM, 2σ). The average Doppler velocities of ²⁶Al at the time of its decay in the interstellar medium ( 1.04 My) therefore are probably around 100 km s^-1, in agreement with expectations from Galactic rotation and interstellar turbulence. The flux (3.3 (±0.4)  10^-4 ph cm^-2 s^-1 rad^-1) and spatial distribution of the emission are found consistent with previous observations. The derived amount of ²⁶Al in the Galaxy is 2.8 (±0.8) .