Analysis of the rich frequency spectrum of KIC 10670103 revealing the most slowly rotating subdwarf B star in the Kepler field

We analyse 2.75 yr of Kepler spacecraft observations of the pulsating subdwarf B star KIC 10670103. These 1.4 million measurements have an impressive duty cycle of 93.8 per cent, a frequency resolution of 0.017 μHz, and a 5σ detection limit of 0.1 parts-per-thousand (ppt). We detect 278 periodicities, making KIC 10670103 the richest pulsating subdwarf B star to date. Frequencies range from 23 to 673 μHz (0.4 and 11.8 h), with amplitudes from the detection limit up to 14 ppt. Follow-up spectroscopic data were obtained from which it was determined that KIC 10670103 does not show significant radial velocity variations. Updated atmospheric model fits determined Teff = 21 485 ± 540 K, log g = 5.14 ± 0.05, and log N(He)/N(H) = -2.60 ± 0.04. We identify pulsation modes using asymptotic period spacings and frequency multiplets. The frequency multiplets indicate a spin period of 88 ± 8 d. Of the 278 periodicities detected in KIC 10670103, 163 (59 per cent) have been associated with low-degree (ℓ ≤ 2) pulsation modes, providing tight constraints for model fitting. While the data are exquisite, amplitudes (and some frequencies) are not stable over the course of the observations, requiring tools which are non-standard for compact pulsators such as sliding Fourier transforms and Lorentzian fitting. Using the 163 identified pulsation modes, it is possible to make detailed examinations of the pulsation structure; including where the pulsation power is concentrated in radial order, over what frequency range mode trapping is inefficient, and how power switches between multiplet members