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Workshop on Accreting Neutron Stars
Max Planck Institut filr Physik and Astrophysik
Institut flr Extraterrestrische Physik
Garching bei Vdnchen, 18-23 July 1962
X-RAY TRANSIENTS AS SEEN BY VELA, 1969-1979
J. Terrell, W. C. Priedhoraky, R. D. Belian, J. P. Conner, and W. D. Evans
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Vela spacecraft 5A and 5B were launched into orbit in May 1969, to monitor
for nuclear tests in space. These spacecraft were among the first to be capable
of T-ray astronomy. One of these, Vela 5B, monitored the entire X-ray sky for
the unprecedented period of 10 years, from May 1969 to June 1979. Over the last
several years the data produced have been re-analysed to produce a series of
skymaps. These have now been made into a movie, in color, showing the changes
in the X-ray sky over the period 1969-1976.
The Vela spacecraft were put into orbit at 118000 km radius. Among the
many detectors aboard were collimated NaI detectors sensitive to 3-12 keV X-rays.
These scanned the sky over a 6.10 (FM) square field of view at 900 to the
earth-spacecraft axis, so that the entire X-ray sky was observed in 56 hours,
half of the orbital period. Any given source, when in view, was scanned every
64 sec (the rotation period) in two energy channels, with one-second count
accumulations.
During the 10-year lifetime of the X-ray counters, the Vela spacecraft
produced data on marny new types of sources, including X-ray transients, X-ray
bursts, and even gamma-ray bursts. Con X-4, an exceedingly bright X-ray
transient, was observed by Vela in the summer of 1969, shortly after launch
(.,onner, Evans, and Belian 1969). It did rot return to the X-ray sky until
May 1979. This source, with a very bright, hard precursor on 7 July 1969
(Belian, Conner, and Evans 1972), also produced the first of many X-ray bursts
detected by Vela. A number of other high-latitude X-ray bursts have been
detected, with no evidence of repeated bursts (Belian, Conner, and Evans 1976).
The Vela gamma-ray detectors revealed the new phenomenon of gamma-ray bursts
(lebesadel, Strong, and Olson 1973). It has recently been found that at least
two gamna-ray bursts were also detected by the X-ray counters (Terrell et al.
1982b). One of these events (GB720514 u GBS1128+77) was in a known direction
(Wheaton et al. 1973). The other (CB740723 i. GBS0228-75) was found, by means
of the collimated X-ray detectors, to lie in the direction of the Small
Magellanic Cloud. The time histories of thee two bursts as seen in X-rays
(with brief observations every 64 sea) are similar in that both showed evidence
of recurrences during the succeeding 1000 seconds.
The data produced by Vela 5B over the 7-year period May 1969-June 1976,
during which real-time tracking was reasonably complete, have been put into the
form of a series of 10-day skymaps. A 10-day interval is long enough to include
four scans of the entire sky, so that gaps in coverage are usually filled in.
Thir data base is well suited for determining the time histories of many sources,
and in part lar for Po-rier Enalyris. The Vela X-ray detectors were relatively
small (27 ameffective area) but were sensitive enough to yield good data on
Con A (IGO 5120) during the period 1973-1975 (Terrell 1982). During this period
Cen A was unusually bright and active (Beall at al. 1978). The new data show
that it often changed dramatically in intensity in less than 10 days.
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Terrell, J.; Priedhorsky, W. C.; Belian, R. D.; Conner, J. P. & Evans, W. D. X-ray transients as seen by Vela, 1969-1979, article, January 1, 1982; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1183198/m1/2/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.