The diffuse interstellar band around 8620 Å I. Methods and application to the GIBS data set

Context. Diffuse interstellar bands (DIBs) are interstellar absorption features that widely exist in the optical and near-infrared wavelength range. DIBs play an important role in the lifecycle of the interstellar medium and can also be used to trace the Galactic structure.Aims. We developed a set of procedures to automatically detect and measure the DIB around 8620 angstrom (the Gaia DIB) for a wide range of temperatures. The method was tested on 5000 spectra from the Giraffe Inner Bulge Survey (GIBS) that has a spectral window similar to that of the Gaia-RVS spectra. Based on this sample, we studied the correlation between the equivalent width (EW) of the Gaia DIB and the interstellar reddening E(J - K-S) toward the inner Galaxy, as well as the DIB intrinsic properties. Methods. Our procedure automatically checks and eliminates invalid cases, and then applies a specific local normalization. The DIB profile is fit with a Gaussian function. Specifically, the DIB feature is extracted from the spectra of late-type stars by subtracting the corresponding synthetic spectra. For early-type stars we applied a specific model based on the Gaussian process that needs no prior knowledge of the stellar parameters. In addition, we provide the errors contributed by the synthetic spectra and from the random noise. Results. After validation, we obtained 4194 reasonable fitting results from the GIBS database. An EW versus E(J - K-S) relation is derived as E(J - K-S) = 1.875 (0.152) x EW - 0.011 (+/- 0.048), according to E(B - V)/EW = 2.721, which is highly consistent with previous results toward similar sightlines. After a correction based on the Vista Variables in the Via Lactea (VVV) database for both EW and reddening, the coefficient derived from individual GIBS fields, E(J - K-S)/EW = 1.884 +/- 0.225, is also in perfect agreement with literature values. Based on a subsample of 1015 stars toward the Galactic center within - 3 degrees < b < 3 degrees and - 6 degrees < l < 3 degrees, we determined a rest-frame wavelength of the Gaia DIB as 8620.55 angstrom. Conclusions. The procedures for automatic detection and measurement of the Gaia DIB are successfully developed and have been applied to the GIBS spectra. A Gaussian profile is proved to be a proper and stable assumption for the Gaia DIB as no intrinsic asymmetry is found. A tight linearity of its correlation with the reddening is derived toward the inner Milky Way, which is consistent with previous results.