New estimates of the gamma-ray line emission of the Cygnus region from INTEGRAL/SPI observations

¹ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 avenue colonel Roche, BP44346, 31028 Toulouse Cedex 4, France e-mail: martinp@mpe.mpg.de
² Université de Toulouse (UPS), CNRS, UMR 5187, France
³ Max Planck Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
⁴ Geneva Observatory, 1290 Sauverny, Switzerland

Received: 25 March 2009
Accepted: 11 June 2009

Abstract

Context. The Cygnus region harbours a huge complex of massive stars at a distance of 1.0–2.0 kpc from us. About 170 O stars are distributed over several OB associations, among which the Cyg OB2 cluster is by far the largest with about 100–120 O stars. As a consequence of their successive nuclear-burning episodes, these massive stars inject large quantities of radioactive nuclei into the interstellar medium such as ²⁶Al and ⁶⁰Fe. The gamma-ray line signal from the latter is a solid tracer of ongoing nucleosynthesis.

Aims. We want to compare the decay emission from the Cygnus region with predictions of recently improved stellar models. As a first step, we establish observational constraints upon the gamma-ray line emission from ²⁶Al and ⁶⁰Fe, with particular emphasis placed on separating the emission due to the Cygnus complex from the foreground and background mean Galactic contributions.

Methods. We used the high-resolution gamma-ray spectrometer INTEGRAL/SPI to analyse the ²⁶Al and ⁶⁰Fe decay signal from the Cygnus region. The weak gamma-ray line emissions at 1809 and 1173/1332 keV have been characterised in terms of photometry, spectrometry, and source morphology.

Results. The 1809 keV emission from Cygnus is centred on the position of the Cyg OB2 cluster and has a typical size of 9° or more. The total 1809 keV flux from the Cygnus region is (6.0 ± 1.0) 10^-5 ph cm^-2 s^-1, but the flux really attributable to the Cygnus complex reduces to (3.9 ± 1.1) 10^-5 ph cm^-2 s^-1. The 1809 keV line centroid agrees with expectations considering the direction and distance of the Cygnus complex, and the line width is consistent with typical motions of the interstellar medium. No decay emission from ⁶⁰Fe has been observed from the Cygnus region and an upper limit of 1.6 10^-5 ph cm^-2 s^-1 was derived.

Conclusions. The determination in a coherent way of the gamma-ray line emission of the clustered OB population of the nearby Cygnus complex will allow a more accurate comparison of observations with theoretical expectations based on the most recent stellar models.