Optical Studies of Companions to Millisecond Pulsars

Optical observations of the companions of pulsars can help determine the properties of the binaries, as well as those of their components, and give clues to the preceding evolution. In this review, we first describe the different classes of binary pulsars, and we present a table with a summary of what is known about their optical counterparts. Next, we focus on the class of pulsars that have low-mass, helium-core white dwarf companions. We discuss attempts to determine the masses of both components using optical spectroscopy, and compare the pulsar spin-down ages with cooling ages of the white dwarfs. We confirm that, for a given age, the lowest-mass white dwarfs are much hotter than the more massive ones, consistent with recent evolutionary models, although with one glaring exception. We discuss the case of PSR B0820+02, where the cooling age indicates a braking index less than 3, and we conclude by describing how cooling ages can be used to test formation scenarios for PSR J1911−5958A, a pulsar binary in the outskirts of NGC 6752. 1. Binary Pulsars and Their Evolutionary Histories In Table 1, we list all pulsars in binaries outside of globular clusters. One sees that their properties vary widely, but one can identify different types on the basis of the spin and orbital properties. For instance, systems separate in clusters by inferred companion mass and orbital period, as can be seen in Figure 1. Below, we briefly describe the different groups and their evolutionary histories (for reviews, see Phinney & Kulkarni 1994; Stairs 2004). PSR+OB(e) Pulsars with massive stellar companions, which formed in binaries where one star went supernova. The pulsars are like young, isolated pulsars. Probably, there are many more PSR+OB(e) systems in which the pulsar is hidden by the companion’s stellar wind. There should also be pulsars with lower-mass companions,