Scientific background: transitional pulsars and disk accretion onto neutron stars

The nature of the coupling between accretion of plasma onto astrophysical objects (from young stellar objetcs to supermassive black holes), and outflows such as jets and winds, is one of the key questions of current astrophysical research. The interaction between magnetic fields and the plasma in an accretion disk plays a crucial role in this respect, and a progress in its understanding can provide a breakthrough in our knowledge of the processes which lead to the formation and growth of structures, such as galaxies, stars and planetary systems.

Binary systems composed by a magnetized neutron star (NS) orbiting a companion star are prime astrophysical laboratories for such a study. Like isolated NS, t he rotation of the huge magnetic fields of these compact object s usually yields the observation of a pulsed emission spanning the whole electromagnetic spectrum, from the radio to the gamma-ray band. However, if the two stars of a binary systems are sufficiently close, matter can be transferred from the companion star towards the compact object, driving a bright X-ray emission and quenching the emission powered by the rotation of the NS field. According to theory and simulations, the in-flowing plasma can be channeled to the magnetic poles of the NS (yielding X-ray pulses) or being ejected by the fast rotating magnetosphere, depending on the balance between the matter in-falling towards the NS, and the rotating magnetosphere of the NS.

Simulations of channelling of the in-flowing plasma onto the magnetic poles of a NS (left), and outflows cast by a rapidly rotating magnetosphere (right). From Romanova et al. 2014.

Among binary systems, transitional millisecond pulsar (TMSP) have been recently discovered (Archibald et al. 2009, Science, 324, 1411; Papitto et al. 2013, Nature, 501, 517). These NS rotate at a spin period of few milliseconds, and are unique in undergoing fast transitions between all the states accessible to these systems:

• a bright (>1E36 erg/s) X-ray pulsar regime, powered by the accretion onto the NS surface of matter transferred by the companion star through an accretion disk;
• a radio (and possibly gamma-ray) pulsar regime, powered by the energy losses due to the rotation of the NS magnetic field, in which the system is faint in X-rays (~1E32 erg/s) and ejects the matter transferred by the companion in the surrounding;
• an intermediate (LX~1E34 erg/s) accretion phase in which an accretion disk is present around the NS, but matter is presumably prevented from accreting onto the NS surface by the onset of a magnetospheric centrifugal barrier.

So far, one system has been observed to go through all these three phases (Papitto et al. 2013), while two more were observed to switch between the radio pulsar and the intermediate accretion state (Stappers et al. 2013, arXiv:1311.7506, Patruno et al. 2014, ApJ, 781, L3, Papitto et al. 2014, MNRAS, Bassa et al. 2014, arXiv:1402.0765, Bogdanov et al. 2014, arXiv:1402.6342). Transitional pulsars experience a wide range of physical phenomena, and are then perfectly suited to tackle a number of astrophysical issues by merging the knowledge of observers and theoreticians experts of such systems.

The interaction between the neutron star magnetic field and the accretion disk

The observed transitions take place on short time-scales of less than a few weeks, and are generally interpreted in terms of variations of the inward pressure exerted by the in-flowing matter, which cause the disk-magnetosphere boundary to expand and contract. However, theory and simulations show that the picture is much more complex than such a simple pressure balance, and depends on how much the field lines penetrate through the disk, and are bent by differential rotation. Also, the driver of the variations of the mass in-flow rates has not been identified yet. Being the only systems known which experience a wide range of states, and possess many diagnostic probes which provide key information about their properties (especially pulsations observed in different bands, spectral features, the broad-band spectral energy distribution), transitional pulsars offer new and ideal observational benchmark of theory and modelling of the field-disk interaction.

Variability of the X-ray and radio emission of these objects on very short intervals (down to less than a second) suggest even faster transitions to a regime in which most of the in-falling matter is ejected from the system by centrifugal forces of the fast rotating magnetosphere. Transitional pulsars could be then used to probe how the accretion/ejection cycles observed in many astrophysical context work in a system whose parameters are well known (such as the spin and the field of the central object, the rate of mass in-flow).

During the intermediate accretion state an enhanced emission of gamma-rays is observed, possibly interpreted in terms of residual acceleration of leptons along magnetospheric field lines even in presence of a disk, or through a Fermi process taking place at the disk-magnetosphere boundary. Understanding the process underlying this emission would then assess whether a new class of accreting gamma-ray sources with a low mass companion star could be established.

In the context of the proposed team, observers who have been involved in the early characterization of these systems in the relevant energy bands (radio, X-ray, gamma-ray), will be able to present the main problems and opportunities raised by observations of these new systems to the theoreticians involved. At the same time, observers will be given an input to select ideal systems to spot, and devise the best multi-wavelength approach to highlight features predicted by different models. The over-arching goal of is the assessment of how the field-plasma interaction determine accretion and ejection of plasma in similar systems, and try to extend the results obtained to systems of different scales, such as supermassive black holes and young stellar objects.

The physics of the accretion disks

Transitional pulsars also offer ideal conditions to test the formation of accretion disks. As a matter of fact, while during the rotation-powered regime these pulsars are assumed to eject the transferred matter from the vicinity of the surface of the companion star, a state transition marks the onset of the formation of an accretion disk. Prospects of a close monitoring of transitional pulsars to obtain an accurate measure of the time needed to form an accretion disk will be discussed by the team, as well as the possibility of obtaining an accurate measure of the disk viscosity in the different states of the source. In particular, the observation of sources lying in an intermediate accretion state for a decade or more was unexpected (Papitto et al. 2014, MNRAS, 438, 2105), and requests the formulation of new disk models capable to cope with a source which propel away the in-flowing matter over such long time-scales.

Neutron stars evolutionary scenarios

Transitional pulsars bridge two classes of sources, accreting NS with a low-mass companion star and radio millisecond pulsars, which were long thought to share a common evolutionary history. Old, spinning down millisecond radio pulsars are in fact assumed to be the descendants of a previous Gyr-long evolutionary phase in which the NS accreted mass from the companion star, being spun up to a spin period of few milliseconds. The discovery of transitional systems finally proved this link, showing that in the course of the secular evolution of these systems an intermediate phase exists in which transitions in both directions take place. However, it is largely uncertain how long this phase lasts, and which systems are affected. Possibly, most of the accreting and rotation-powered NS in close binary systems could be transitional systems observed so far in one of the two possible states. The meetings will serve to address these scenarios and to devise the best possible strategies to increase the known population of transitional pulsars. Also, the determination of the properties of these systems (such as the spin and orbital evolution and the rate of mass accreted and ejected) will be implemented in codes which simulate the evolution of these binaries. This will be aimed at determining how the transitional phase influence the evolution of accreting NS towards the maximum mass and spin rates that can be attained in the process.