sub-Keplerian Rotation Research Articles

Spin‐up of Low‐Luminosity, Low‐Mass X‐Ray Binaries

We examine the spin-up of low-luminosity, low-mass X-ray binaries (LMXBs) to millisecond pulsars (MSPs). In the conventional spin-up model of the Ghosh & Lamb type, where the stellar magnetic field interacts with the Keplerian accretion disk, MSPs could be produced from LMXBs if the magnetic field B*108/1016 g s-1) G, where is the mass accretion rate. However, for <c~1016 g s-1, accretion is likely to occur via a quasi-spherical flow with a sub-Keplerian rotation. The sub-Keplerian rotation rate is smaller than the Keplerian rate by a factor ~2-10. As a consequence, the spin up of LMXBs produces pulsars with spin periods longer by a factor of ~2-10 than those with a Keplerian accretion disk. The observed MSPs could be produced only for B* < 107 G even when ~c~1016 g s-1. This suggests that the low-luminosity LMXBs with <c would not be able to spin-up to the observed MSPs. This rules out any undetected populations of persistent, low-luminosity LMXBs and a potentially significant fraction of the atoll sources as a possible source population of the observed MSPs. If a large number of undetected, persistent, low-luminosity LMXBs do exist, they could produce MSPs near the pulsar death line with intrinsic electromagnetic luminosity 1030 ergs s-1. The observed MSPs could possibly arise from a population of soft X-ray transients (SXTs) containing neutron stars, although this is not supported by current estimates of the outburst rate or numbers of such systems. Accretion-induced collapse of low magnetic field white dwarfs remains a possible channel for MSP formation.

VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068

We have made the first VLBI synthesis images of the H2O maser emission associated with the central engine of the Seyfert galaxy NGC 1068. Emission extends about ±300 km s-1 from the systemic velocity. Images with sub-milliarcsecond angular resolution show that the redshifted emission lies along an arc to the northwest of the systemic emission. (The blueshifted emission has not yet been imaged with VLBI.) Based on the maser velocities and the relative orientation of the known radio jet, we propose that the maser emission arises on the surface of a nearly edge-on torus, where physical conditions are conducive to maser action. The visible part of the torus is axially thick, with comparable height and radius. The velocity field indicates sub-Keplerian differential rotation around a central mass of ~1 × 107 M☉ that lies within a cylindrical radius of about 0.65 pc. The estimated luminosity of the central engine is about 0.5 of the Eddington limit. There is no detectable compact radio continuum emission near the proposed center of the torus (TB < 5 × 106 K on size scales of ~0.1 pc), so that the observed flat-spectrum core cannot be direct self-absorbed synchrotron radiation.