Spectra from Gaia's Radial Velocity Spectrometer (RVS) of Gaia Photometric Science Alerts are processed at the Mullard Space Science Laboratory (MSSL) using the Gaia Data Processing and Analysis (DPAC) Co-ordination Unit (CU) 6 Spectroscopic Processing software running within a bespoke MSSL framework called Odyssey. The straylight and wavelength calibrations are run in the CU6 daily pipeline at the CU6 Data Processing Centre CNES (DPCC). The RVS transits corresponding to the triggering alert transit, or subsequent transits of a source that previously triggered an alert, are regularly searched for in the outputs of the DPAC Initial Data Treatment (IDT). The majority of photometric alerts do not have corresponding RVS spectra because the RVS limiting magnitude is Grvs = 16.2 mag. The Grvs magnitude provided is measured on board Gaia from the flux in the samples in RP spectra that correspond to the RVS wavelength range. For B0V and K4V stars, this approximately corresponds to G = 15.9 and 17.4 mag respectively, whereas the Gaia photometric limit is G = 20.7 mag. The RVS CCDs are on four of the seven Gaia CCD rows only, which is another reason why alerts that are bright enough do not always have corresponding RVS spectra.
If all three RVS windows from the three strips of RVS CCDs, constituting an RVS transit, are not overlapping with those of other stars, the corresponding IDT-processed RVS spectra and their auxiliary data are downloaded using the MDBExplorer tool. The straylight and wavelength calibrations are downloaded from DPCC. The data are then ingested into Odyssey. The typical uncertainty in correcting the straylight background is approximately 3 electrons per 0.025nm wavelength bin.
Wavelength calibrations are derived as a function of field angles, which are the angular celestial positions of sources in Gaia's Field of View Reference System. The DPCC daily wavelength calibration is derived using field angles calculated with positions from the One Day Astrometric Solution (ODAS). If the calibration is applied to field angles based on ODAS, the accuracy of the wavelength scale is ~1 km/s. The accuracy of the wavelength scale is reduced to ~10 km/s if only the IDT-derived positions are available. (Note that radial velocities published in formal data releases will be based on DPCC global wavelength calibrations, derived using field angles calculated with positions from the Astrometric Global Iterative Solutions (AGIS). In this case the accuracy of the wavelength scale is ~0.3 km/s.)
The CU6 pipeline running in Odyssey produces wavelength-calibrated spectra, corrected for bias non-uniformity, straylight, cosmic rays and the RVS filter response. Transits can be rejected from the Odyssey pipeline if: (a) one or more of the windows contains column defects or; (b) IDT cross match issues can produce an erroneous wavelength scale, which causes the removal of the RVS filter response to fail. A bespoke RVS alert pipeline running at MSSL then takes the successful outputs of Odyssey and completes the processing. The RVS spectra is convolved with a Gaussian with a full width half maximum set to the RVS spectral resolution element. This is measured by CU6 software, developed at Observatoire Paris-Site de Meudon, running daily in the IDT-Detailed First Look (IDT-DFL) pipeline at the European Space Astronomy Centre (ESAC). The measurement is manually retrieved from the automated Detailed First Look Evaluation (DFLE) report. The Gaia-centric wavelength scale of each spectrum is converted to barycentric using Gaia's velocity along the line of sight. Each spectrum is interpolated onto a linear 0.025 nm wavelength scale between 846 and 870 nm. The combined spectrum is the average of the three spectra, unless stated otherwise. The three vertical dashed lines indicate the vacuum rest wavelengths of the calcium triplet absorption lines to help identify these lines and give an indication of the source's radial velocity: at 846 and 870 nm, 0.025 nm corresponds to 8.6 and 8.9 km/s respectively.