Mass accretion and ejection in transitional millisecond pulsars

Project Information

TMSP

Grant agreement ID: 660657

DOI

10.3030/660657

Project closed

EC signature date 14 April 2015

Start date 4 April 2016

End date 3 April 2018

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 168 277,20

EU contribution

€ 168 277,20

Coordinated by

ISTITUTO NAZIONALE DI ASTROFISICA
Italy

Periodic Reporting for period 1 - TMSP (Mass accretion and ejection in transitional millisecond pulsars)

Reporting period: 2016-04-04 to 2018-04-03

Summary of the context and overall objectives of the project

Neutron stars (NSs) are prime laboratories to investigate the behaviour of matter at extreme densities and the most violent phenomena seen in the Universe, such as accretion of matter and relativistic outflows. When they are part of a binary system, and matter is transferred from their companion star, several phenomena occur which have shed light on the physics of mass accretion and NS magnetospheres, as well as the production of high energy radiation and pulsations. A new class of pulsars spinning at a period of few milliseconds and switching between plasma accretion and ejection was discovered in 2013, and dubbed transitional millisecond pulsars (TMSPs). They alternate between radio and X-ray pulsar regimes as a result of the interaction between the in-flowing plasma and the outward pressure exerted by their magnetosphere and radiation. They experience transitions from a rotationally powered regime, in which they behave like radio pulsars, to a regime in which they accrete matter and emit intense high-energy radiation, like standard X-ray binary systems. The project aims at exploiting the new and complex phenomenology shown by TMSPs to (i) determine how outflows of plasma are launched and how they are coupled to the accretion process; (ii) discover TMSPs and measure their mass and spin; (iii) study the formation of accretion disks, and how a pulsar behaves when surrounded by matter. The multi-wavelength observations (radio, optical, X-ray and gamma-rays) of TMSPs performed during the project increased significantly our knowledge of how TMSPs and related systems work. The discovery of the first optical millisecond pulsar (Ambrosino, Papitto et al. 2017, Nature Astronomy) is the highest impact result, as it indicates that a rotationally powered pulsar is active in these systems and is responsible for the outflows observed.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The discovery of optical pulsations from a TMSP opened a brand new observing window for the study of quickly spinning NSs. It demonstrated that also ms pulsars can produce optical pulses. Such a radiation is most likely powered by acceleration of charges in the magnetosphere, but was detected when the pulsar was surrounded by a disk. This unexpected outcome will prompt a readjustment of the theories describing the pulsar/disk interaction. The phenomenology observed from this TMSP might be very common, and explain a large fraction of the unidentified gamma-ray sources.

Another far-reaching impact of this discovery is the impetus it gives to high-time-resolution optical observations. Searching for ms pulsars in the optical band has the huge advantages of very large photon count rates with respect to observations at higher energies (X-rays, gamma-rays). This open the possibility of searching for fast coherent signals in the optical band from sources that have not been detected at other wavelengths so far (e.g. candidate continuous gravitational waves sources). Pulses observed from such a TMSP are the fastest optical pulsations ever detected (by a large factor) and will motivate more searches, and the development of further instrumentation to allow the study and characterization of fast optical variability. This has to be viewed in the context of the design of instruments for the class of very large telescopes now under construction (such as the European Extremely Large Telescope), and thus to open a new window on the high-energy Universe.

On the other hand, the detection of the brightest X-ray pulsars in ultra-luminous X-ray sources demonstrated that accreting neutron stars can achieve extreme luminosity not foreseen in current accretion models. Such high luminosities are often displayed by many ULXs that have previously been classified as accreting black holes. A multicomponent strong magnetic field is necessary to account for the properties of these pulsars. This and similar discovery are challenging the scenario that interpreted ULXs as massive accreting black holes, and prompt a rethinking of the census of compact objects.

Artistic impression of the press release of the discovery of the optical ms pulsar