Implications of kHz QPOs for the Spin Frequencies and Magnetic Fields of Neutron Stars: New Results from Circinus X-1

Abstract

Detection of paired kilohertz quasi‐periodic oscillations (kHz QPOs) in the X‐ray emission of a compact object is compelling evidence that the object is an accreting neutron star. In many neutron stars, the stellar spin rate is equal or roughly equal to Δν, the frequency separation of the QPO pair, or to 2Δν. Hence, if the mechanism that produces the kilohertz QPOs is similar in all stars, measurement of Δν can provide an estimate of the star's spin rate. The involvement of the stellar spin in producing Δν indicates that the magnetic fields of these stars are dynamically important.We focus here on the implications of the paired kHz QPOs recently discovered in the low‐mass X‐ray binary (LMXB) system Cir X‐1 [3]. The kHz QPOs discovered in Cir X‐1 are generally similar to those seen in other stars, establishing that the compact object in the Cir X‐1 system is a neutron star. However, the frequency νu of its upper kHz QPO is up to a factor of three smaller than is typical, and Δν varies by about a factor 2 (167 Hz, the largest variation so far observed). Periodic oscillations have not yet been detected from Cir X‐1, so its spin rate has not yet been measured directly. The low values of νu and the large variation of Δν challenge current models of the generation of kHz QPOs. Improving our understanding of Cir X‐1 will improve our knowledge of the spin rates and magnetic fields of all neutron stars.