We performed X-ray observations of four accreting X-ray transients with ESA XMM-Newton and INTEGRAL, and NASA NuSTAR. We discovered two accreting ms pulsars, a significant addition to the 20 known (e.g. Sanna, Papitto et al. 2017, A&A), and performed the first high time resolution study of a well known accreting NS (Matranga, Papitto et al. 2017, A&A).
We cross-correlated gamma-ray (Fermi LAT) and X-ray (XMM-Newton) catalogues to identify three candidate transitional millisecond pulsars. A spin-off of this quest was the discovery of the brightest X-ray pulsars ever found in two ultra-luminous X-ray sources (ULX, Israel et al. 2016, Science; Israel, Papitto et al. 2017, MNRAS). Standard accretion models fail to explain their luminosity, even assuming beamed emission, but a strong multipolar magnetic field is needed.
We planned radio observations of the newly discovered accreting ms pulsars after they return to quiescence; only in one case this was possible but radio pulses could not be detected and their transitional nature remains to be determined. X-ray outbursts of three known accreting ms pulsars occurred during the project and were monitored with ESA’s XMM-Newton. We measured the spin and orbital evolution of these systems (e.g. Papitto et al. 2016, MNRAS). The results supported the scenario of a radio pulsar switching on during quiescence. Such a radio pulsar would be responsible for massive outflows of plasma, that in turn explain the fast orbital evolution observed.
We performed the first high time resolution study of the optical variability of a TMSP, discovering the first optical millisecond pulsar ever found (Ambrosino, Papitto et al. 2017, Nature Astronomy). The detection was achieved using the fast photometer SiFAP at INAF Galileo telescope and took place when the TMSP was surrounded by an accretion disk. This indicated that a rotationally-powered pulsar can be active even if surrounded by a disk. This outcome of the disk/magnetosphere interaction defies theoretical explanation according to current models. The wind of a rotationally powered pulsar also naturally provides an explanation for the outflow observed from TMSPs in the peculiar intermediate states. We also performed simultaneous optical (INAF Galileo) and X-ray (ESA XMM-Newton), high time resolution observations that showed us that optical and X-ray pulses appear simultaneously and both are most likely related to the same magnetospheric phenomenon (Papitto et al. 2018b, ApJ). We could also gave a 80-days long continuous look at the optical variability of this TMSP with the NASA Kepler telescope, unveiling a very frequent flaring behavior probably originated in the disk (Papitto et al. 2018a, ApJ).
The results obtained by the project were reported in 23 publications in top-tier astrophysical journals (two currently under refereeing), five as the first author, one in Nature Astronomy (as the corresponding and co-first author) and one in Science. Seven of these papers were the subject of press releases by the agencies involved. I presented the results obtained in talks given in 11 conferences, seven times as an invited speaker. I organized two international workshops devoted to the study and the dissemination of the results of observations of transitional millisecond pulsars.
