PAPITTO, ALESSANDROALESSANDROPAPITTODE MARTINO, DomitillaDomitillaDE MARTINOBELLONI, Tomaso Maria MelchiorreTomaso Maria MelchiorreBELLONIBURGAY, MARTAMARTABURGAYPELLIZZONI, ALBERTO PAOLOALBERTO PAOLOPELLIZZONIPOSSENTI, ANDREAANDREAPOSSENTITorres, D. F.D. F.Torres2020-03-232020-03-2320150035-8711http://hdl.handle.net/20.500.12386/23459AP and DFT acknowledge support from the grants AYA2012-39303 and SGR 2014-1073. AP is supported by a Juan de la Cierva fellowship. DdM acknowledges financial support from ASI/INAF I/037/12/0. TMB acknowledges support from INAF PRIN 2012-6. Based on observations obtained with XMM – Newton , an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. We gratefully thank Dr Norbert Schartel and the ESAC staff for their help in obtaining the XMM–Newton data.We present the first detection of X-ray coherent pulsations from the transitional millisecond pulsar XSS J12270-4859, while it was in a sub-luminous accretion disc state characterized by a 0.5-10 keV luminosity of 5 × 10<SUP>33</SUP> erg s<SUP>-1</SUP> (assuming a distance of 1.4 kpc). Pulsations were observed by XMM-Newton at an rms amplitude of (7.7 ± 0.5) per cent with a second harmonic stronger than the fundamental frequency, and were detected when the source is neither flaring nor dipping. The most likely interpretation of this detection is that matter from the accretion disc was channelled by the neutron star magnetosphere and accreted on to its polar caps. According to standard disc accretion theory, for pulsations to be observed the mass inflow rate in the disc was likely larger than the amount of plasma actually reaching the neutron star surface; an outflow launched by the fast rotating magnetosphere then probably took place, in agreement with the observed broad-band spectral energy distribution. We also report about the non-detection of X-ray pulsations during a recent observation performed while the source behaved as a rotationally-powered radio pulsar.STAMPAenArticle10.1093/mnrasl/slv0132-s2.0-84926436168000353555000006http://arxiv.org/abs/1412.4252v3https://academic.oup.com/mnrasl/article/449/1/L26/10283642015MNRAS.449L..26PFIS/05 - ASTRONOMIA E ASTROFISICAScienze Fisiche Settori ERC (ERC) di riferimento::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation