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VELA5B - Vela 5B All-Sky Monitor Lightcurves



Data for these sources were obtained from the Vela 5B all-sky XC detector.

The Vela 5B nuclear test detection satellite was part of a program run jointly by the Advanced Research Projects of the U.S. Department of Defense and the U.S.Atomic Energy Commission, managed by the U.S. Air Force. It was placed in a nearly circular orbit at a geocentric distance of ~118,000 km on 23 May 1969; the orbital period was ~112 hours. The satellite rotated about its spin axis with a ~64-sec period. The X-ray detector was located ~90 degrees from the spin axis, and so covered the celestial sphere twice per satellite orbit. Data were telemetered in 1-sec count accumulations. Vela 5B operated until 19 June 1979, although telemetry tracking was poor after mid-1976.

The scintillation X-ray detector (XC) aboard Vela 5B consisted of two 1-mm-thick NaI(Tl) crystals mounted on photomultiplier tubes and covered by a 5-mil-thick beryllium window. Electronic thresholds provided two energy channels, 3-12 keV and 6-12 keV. In front of each crystal was a slat collimator providing a FWHM aperture of ~6.1x6.1 degrees. The effective detector area was ~26 sq-cm. Sensitivity to celestial sources was severely limited by the intrinsic detector background of ~36 cts/sec. The Vela 5B X-ray detector yielded ~40 cts/sec for the Crab, so 1 Vela ct/sec ~25 UFU~4.5E-10 ergs/sq-cm/sec in the 3-12 keV response band.


"The Ariel 5 and Vela 5B All-Sky Monitor Databases", by L. Whitlock, J. Lochner, and K. Rhode, Legacy, #2, November 1992.

Instrument Performance Characteristics

One important detector performance characteristic which affects the Vela 5B data is a gain variation due to a ~60 deg C satellite temperature change from one side of the orbit to the other. If the data for a source were taken when the satellite was at one of its temperature extremes, a profound modulation is introduced into the count rate at the 56-hour timescale between observation sequences of the source. Additionally, the amplitude of the effect is modulated by the ~300-day precession period of the Vela 5B orbit. Lack of pre-launch testing precludes any quantitative post-launch compensation. A temperature time history is available to HEASARC users in a FITS file (VELA_TEMP) so that they may check any suspicious source data against the known times of temperature extremes.

The time history of the Crab detected flux decreased by ~15% between 1969 and 1979. It is believed that this decrease is due to a gain change in the XC detector as it aged. No attempt to correct for this trend has been made in the data processing. Users who desire to do so, or who want to express detected source intensities in units of crabs, will have to access the FITS file containing the Crab data to extract the necessary information.


The date of creation of the light curve file.

The designation of the source.

Alternative designation for the source, if available.

The Right Ascension of the source.

The Declination of the source.

Galactic longitude of the source.

Galactic latitude of the source.

The average count rate in counts/sec.

Energy range is either 3-12 keV or 6-12 keV.

Bins are 1 second, 56 hour, or 112 hour.

The 1-second data are corrected to barycentric time. Results are known to be good to 1 part in 1000. No attempt was made to correct for the timing error introduced by the deterioration of the satellite orbit over the ten years of operation. The count rate of each 1-second observation was corrected for collimator response before being put into a bin.

The start time of the observation.

The end time of the observation.

The name of the data file online.

Contact Person

Questions regarding the VELA5B database table can be addressed to the HEASARC User Hotline.
Page Author: Browse Software Development Team
Last Modified: Thursday, 13-Sep-2012 22:36:56 EDT