Engineering topics

Zylstra, G.

Publications and source records attributed to Zylstra, G..

4 records

NASA NTRS1989

Vela X-1 pulse timing. II - Variations in pulse frequency

The pulsed X-ray emission of Vela X-1 during May 1978 and December-January 1978-1979 is investigated analytically on the basis of published satellite observations. The data are compiled in tables and graphs and discussed in detail, with reference to data for the entire 1975-1982 period. Variations in pulse frequency are identified on time scales from 2 to 2600 days; the lower nine octaves are characterized as white noise (or random walk in pulse frequency), while the longer-period variations are attributed to changes in neutron-star rotation rates.

Deeter, J. E.↗

NASA NTRS1987

Apsidal advance in Vela X-1

An upper bound on the rate of apsidal advance in Vela X-1 is determined, taking into account the effect of the red noise in pulse frequency reported by Boynton et al. (1986). A new, more precise mean orbit is computed which provides substantially smaller upper bounds on both the rate of apsidal advance and the rate of change of the orbital period than previous solutions. A new, hybrid method for measuring the rate of apsidal advance is described that uses eclipse observations as well as pulse timing. The new method is potentially as useful as the direct measurement from pulse-timing alone for systems that allow more precise determinations of eclipse ingress and egress than does Vela X-1.

Deeter, J. E.↗

NASA NTRS1986

Vela X-1 pulse timing. I - Determination of the neutron star orbit

Analysis and interpretation of observations of the pulsed X-ray emission from Vela X-1 are presented. The interrelated tasks of determining the neutron-star orbit and calculating pulse emission times in the rest frame of the source are considered, with the underlying motivation to utilize this time series for analysis of fluctuations in the pulse emission frequency, which are interpreted as noise in the neutron-star rotation rate. However, the redness and strength of this noise in Vela X-1 pose a nontrivial complication, because the parameters of the neutron-star orbit are estimated by pulse timing and hence are themselves affected by fluctuations in the pulse emission frequency. A second challenge is presented by substantial fluctuations in the Vela X-1 pulse shape that, if ignored, would significantly reduce both the precision of the orbital solution and the ability to study fluctuations in the rotation rate. The report emphasizes the analysis techniques that have been developed to obtain the most precise orbital solution possible in the face of these difficulties. These techniques are general and may be applied to other pulsars that exhibit substantial fluctuations in pulse shape and frequency, such as Her X-1 and Cen X-3.

Boynton, P. E.↗

NASA NTRS1984

New evidence on the nature of the neutron star and accretion flow in Vela X-1 from pulse timing observations

It is believed that variations in the intrinsic pulse frequencies of neutron star X-ray sources reflect changes in the rotation rate of the stellar crust produced by torque originating outside and inside the star. The external torque depends on the flow pattern of the accreting plasma, while the internal torque depends on the state of the interior and its coupling to the crust. Information about the accretion flow and the star itself can, therefore, be obtained on the basis of a study of intrinsic frequency variations. The present investigation is concerned with the results of a detailed study of the 283 s pulsing X-ray star Vela X-1 using pulse timing techniques. It is found that the frequency variations in this source are consistent with the occurrence of a succession of temporally unresolved events which can be characterized mathematically as second-order red noise in the pulse phase.

Boynton, P. E.↗