A Comprehensive Search for Gamma-Ray Lines in the First Year of Data from the INTEGRAL Spectrometer

We have carried out an extensive search for gamma-ray lines in the first year of public data from the spectrometer (SPI) on the INTEGRAL mission. INTEGRAL has spent a large fraction of its observing time in the Galactic plane with particular concentration in the Galactic center (GC) region (~3 Ms in the first year). Hence the most sensitive search regions are in the Galactic plane and center. The phase space of the search spans the energy range 20-8000 keV and line widths from 0 to 1000 keV (FWHM). It includes both diffuse and pointlike emission. We have searched for variable emission on timescales down to ~1000 s. Diffuse emission has been searched for on a range of different spatial scales from ~20° (the approximate field of view of the spectrometer) up to the entire Galactic plane. Our search procedures were verified by the recovery of the known gamma-ray lines at 511 and 1809 keV at the appropriate intensities and significances. We find no evidence for any previously unknown gamma-ray lines. The upper limits range from a few × 10-5 to a few × 10-2 cm-2 s-1 depending on line width, energy, and exposure; regions of strong instrumental background lines were excluded from the search. Comparison is made between our results and various prior predictions of astrophysical lines.