English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We report the discovery with the RXTE/PCA of twin kHz QPOs in Cir X-1. Elevencases of simultaneous double QPOs occurred, with significances of up to 6.3 and5.5 sigma and centroid frequencies ranging between approximately 56-225 and230-500 Hz for the two QPO peaks, respectively, i.e., for the most part atfrequencies well below those of other sources. The QPO properties clearlyindicate that these double peaks are the kHz QPOs known from low magnetic fieldneutron stars, and not black-hole high-frequency QPOs, confirming that Cir X-1is a neutron star. The kHz QPO peak separation varies over a wide range,\~175-340 Hz, and increases with QPO frequency. This is contrary to what isseen in other sources but agrees with predictions of the relativisticprecession model and Alfven wave models; beat-frequency models requiremodification to accommodate this. In other observations single kHz QPOs can beseen down to frequencies as low as ~12 Hz, as well as a strong low-frequency(LF) QPO between 1 and 30 Hz. The relations between the frequencies of the kHzQPOs and the LF QPO are in good agreement with those found previously in Zsources, confirming that Cir X-1 may be a peculiar Z source. We suggest thatthe low frequencies of the kHz QPOs in Cir X-1 and to a lesser extent in(other) Z sources might be due to a relatively stronger radial inflow to theneutron star than in other kHz QPO sources.

Discovery of twin kHz QPOs in the peculiar X-ray binary Circinus X-1