Effects of X-ray stars on VLF signal phase

Abstract

Stellar X-ray effects from Sco XR-1, Cen XR-2 and Cen XR-4 have, been found on several of the numerous VLF phase recordings collected on a world-wide basis since 1965. Only marginal VLF phase anomalies could be associated with Tau XR-1, and none with the Sco XR-1 flares of 3 June 1967. Stellar VLF phase effects are more pronounced at higher signal frequencies and on VLF paths with long nighttimes. Scattered L α radiation after sunset and before sunrise reduces the sidereal shift of VLF effects on E ↔ W paths; and the closeness of the signal path position to the solar terminator at local midnight can shift the date when a maximum stellar VLF phase anomaly is observed. A detectability factor is defined which compares expected stellar effects on arbitrary signal paths to the established effect of Sco XR-1 on the 22.3 kHz signal from NWC to Tananarive. VLF data show: In 1968, Sco XR-1 caused a maximum phase advance of ~0.6 μsec/Mm on NWC (22.3 kHz)—Tananarive; Cen XR-2 and Cen XR-4 increased their X-ray flux to about that of Sco XR-1 on or about 7 March 1967 and 6 July 1969, respectively; Cen XR-2 flux peaked ~27 March and possibly ~10 April and decreased rapidly between 20 April and 10 May 1967. It is concluded that 1. (a) presently known stellar X-ray sources are not detrimental to the accuracy of the OMEGA navigational system; 2. (b) the permanent X-ray stars may give rise to a seasonal variation of nighttime phase at frequencies above 15 kHz; 3. (c) continuous VLF phase tracking at frequencies above 20 kHz can provide some valuable astrophysical information complementing space probe data.