A new class of millisecond pulsars?

2.9 A new class of millisecond pulsars?

One of the most remarkable recent discoveries is the binary pulsar J1903+0327 [72

]. Found in an on-going multibeam survey with the Arecibo telescope [294

, 86

], this 2.15-ms pulsar is distinct from all other millisecond pulsars in that its 95-day orbit has an eccentricity of 0.43! In addition, timing measurements of the relativistic periastron advance and Shapiro delay in this system (see Section 4.4), show the mass of the pulsar to be $1.74 ± 0.04M ⊙$ and the companion star to be $1.051 ± 0.015 M ⊙$ . Optical observations show a possible counterpart which is consistent with a $1 M ⊙$ star. While similar systems have been observed in globular clusters (e.g. PSR J0514–4002A in NGC 1851 [119 ]), presumably a result of exchange interactions, the standard recycling hypothesis outlined in Section 2.6 cannot account for pulsars like J1903+0327 in the Galactic disk.

How could such an eccentric binary millisecond pulsar system form? One possibility is that the binary system was produced in an exchange interaction in a globular cluster and subsequently ejected, or the cluster has since disrupted. Statistical estimates [72 ] of the likelihood of both these channels are in the range 1–10%, implying that a globular cluster origin cannot be ruled out.

Another possibility is that the pulsar is a member of an hierarchical triple system with a one solar mass white dwarf in the 95-day orbit, and a main sequence star in a much wider and highly inclined orbit which has so far not been revealed by timing. The origin of the high eccentricity is through perturbations from the outer star, the so-called Kozai mechanism [199 ]. Formation estimates based on observational data on stellar multiplicity [293 ] find that around 4% of all binary millisecond pulsars are expected to be triple systems [72 ]. The existence of a single triple system among the current sample of millisecond pulsars appears to be consistent with this hypothesis.

If future observations of the proposed optical counterpart confirm it as the binary companion through spectral line measurements of orbital Doppler shifts, the above triple-system scenario will be ruled out. Such an observation would favour a hybrid scenario suggested by van den Heuvel [384 ] in which the white dwarf and pulsar merge due to gravitational radiation losses. Tidal disruption of the white dwarf in the inspiral would produce an accretion disk and induce an eccentricity in the orbit of the outer star leaving behind an eccentric binary system. This idea could naturally account for the high pulsar mass observed in this system which could arise from accretion of a white-dwarf debris disk following coalescence. Alternatively, as suggested by Champion et al. [72 ], the millisecond pulsar might have ablated the white dwarf companion in a triple system leaving only the unevolved companion in an elliptical orbit.