Transitional millisecond pulsars

Abstract

Millisecond pulsars in tight binaries have recently opened new challenges in our understanding of physical processes governing the evolution of binaries and the interaction between astrophysical plasma and electromagnetic fields. Transitional systems that showed changes from rotation-powered to accretion powered states and vice versa have bridged the populations of radio and accreting millisecond pulsars, eventually demonstrating the tight evolutionary link envisaged by the recycling scenario. A decade of discoveries and theoretical efforts have just grasped the complex phenomenology of transitional millisecond pulsars from the radio to the gamma-ray band. This review summarises the main properties of the three transitional millisecond pulsars discovered so far, as well as of candidates and related systems, discussing the various models proposed to cope with the multifaceted behaviour.

On the torque reversals of 4U 1626--67

“…2), it was shown through analytic calculations within a new torque model that a steady SP phase can be established with r in much smaller than r A (Ertan 2017(Ertan , 2018. Applying this model to the transitional millisecond pulsars (tMSP), the critical Ṁin level for the SP/WP transition was found to be consistent with the estimated accretion rates and the torque variations corresponding to transitions between the radio pulsar and X-ray pulsar states (Archibald et al 2009;Papitto et al 2013;Bassa et al 2014;Jaodand et al 2016;Papitto & de Martino 2020). The extended version of this model, including the SU phase as well, can also account for the general torque-reversal characteristics summarized above (Ertan 2021).…”

“…In the WP phase, most of the matter from the inner disk is accreted onto the star, while the star still slows down. In the SU phase, all the matter arriving at the inner disk flows onto the star along the closed magnetic field lines, and the star spins up (Bildsten et al 1997;Papitto & de Martino 2020).…”

“…, where G is the gravitational constant, and M is the mass of the star (Davidson & Ostriker 1973;Lamb et al 1973). With r in = r ξ = ξr A and 0.5 ≤ ξ ≤ 1 (Illarionov & Siuniaev 1975;Ghosh & Lamb 1979b;Arons 1993;Ostriker & Shu 1995), these models encounter the following difficulties in explaining the characteristic X-ray luminosity, L X , and rotational behavior of neutron stars in LMXBs (Bildsten et al 1997;Archibald et al 2009;Papitto et al 2013;Bassa et al 2014;Jaodand et al 2016;Papitto & de Martino 2020): (1) There is ongoing accretion with rates much smaller than the rate corresponding to the r A = r co condition. (2) For a large range of accretion rates Ṁin (up to 2 orders of magnitude depending on the ξ, η, µ and P parameters described in Sect.…”

“…2), spin-down continues while there is mass accretion onto the star. (3) The transition to the SP phase takes place at very low Ṁin when r A r co (Bassa et al 2014;Archibald et al 2015;Papitto et al 2015;Papitto & de Martino 2020). ( 4) Transitions occur between the spin-up and spin-down regimes with similar torque magnitudes without a significant change in L X (Deeter et al 1989;Bildsten et al 1997;Chakrabarty et al 1997b,a;İnam et al 2009;Camero-Arranz et al 2010;Takagi et al 2016).…”

Magnetic angle evolution in accreting neutron stars

“…On one hand, accretion is often claimed to interfere with pulsar emission mechanisms (e.g. Shvartsman 1971;Melatos & Mastrano 2016;Papitto & de Martino 2020). On the other hand, independently of the physical conditions in the magnetosphere, open magnetic field lines should still provide a torque determined by the magnetic stresses at the light cylinder (Contopoulos & Spitkovsky 2006).…”