Spin Frequencies and Magnetic Fields of Neutron Stars: Implications of the kHz QPOs Recently Discovered in Circinus X-1

Details Spin Frequencies and Magnetic Fields of Neutron Stars: Implications of the kHz QPOs Recently Discovered in Circinus X-1 Spin Frequencies and Magnetic Fields of Neutron Stars: Implications of the kHz QPOs Recently Discovered in Circinus X-1

Apr 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aps..apr.l8004b&link_type=abstract

American Physical Society, 2008 APS April Meeting and HEDP/HEDLA Meeting, April 11-15, 2008, abstract #L8.004

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Scientific paper

The paired kilohertz quasi-periodic oscillations (kHz QPOs) recently discovered in the X-ray emission of Cir X-1 are generally similar to those seen in disk-accreting neutron stars with relatively weak magnetic fields, establishing that the compact object in the Cir X-1 system is such a star. Periodic oscillations have not yet been detected from Cir X-1, so its spin rate has not yet been measured directly. In many stars that produce kHz QPOs, the frequency separation δν of the QPO pair is equal or roughly equal to the stellar spin rate νs or to νs/2. The involvement of the stellar spin in producing δν indicates that the magnetic fields of these stars are dynamically important. If the mechanism that produces the kilohertz QPOs is similar in all stars, the value of δν provides a rough estimate of the star's spin rate. In Cir X-1, δν varies by 167 Hz, from ˜230 Hz to ˜500 Hz, the largest variation seen so far in any neutron star. The frequency νu of the upper kHz QPO in Cir X-1 is up to a factor of three smaller than is typical. The low observed values of νu and the large variation of δν challenge current models for the generation of kHz QPOs. We discuss the implications of the observed properties of the Cir X-1 kHz QPOs for mechanisms for generating the kHz QPOs in Cir X-1 and other accreting neutron stars. This research was supported in part by NASA grant NAG 5-12030, NSF grant AST 0709015, and funds of the Fortner Endowed Chair at Illinois.

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