The Ariel 5 and Vela 5B
All-Sky Monitor Databases
Data from past all-sky monitor (ASM) experiments are now being archived and made available at the HEASARC. This article is intended to provide HEASARC users with an overview of the ASM archive. The first section of the article reviews the primary purposes and value of ASM experiments. Subsequent sections give details concerning the first two datasets to be archived, Ariel 5 and Vela 5B; both of these experiments are described, and information given about the processing of the data and their conversion to FITS (Flexible Image Transport System) format. Finally, a list is provided of those sources for which data are currently available (or nearly so) and a brief discussion given of the future plans for the ASM database.
X-ray sources are known to exhibit luminosity variations on timescales ranging from milliseconds to decades. The different timescales are the results of different physical phenomena. For example, thermonuclear flashes on neutron stars can produce flares on timescales of milliseconds to minutes, while binary precessional variations are on timescales of tens of days, and some transient sources recur on timescales of years (see for example, Rappaport and Joss 1983; White 1985). Additionally, the temporal variability can be strictly periodic, quasi-periodic, or even aperiodic. To date, most temporal studies have concentrated on the coherent periodicities of compact X-ray sources (e.g., neutron star rotational periods and binary orbital periods), as they are the easiest to study observationally and to interpret theoretically. Today, we also realize that phenomena which are not strictly periodic and coherent over time can also be successfully used to probe the physics and properties of compact X-ray binaries. This point has been emphatically made by the discovery, and subsequent study, of the sub-second X-ray quasi-periodic oscillations (QPOs) of the class of accreting neutron stars known as the low-mass X-ray binaries (e.g., van der Klis 1989). The QPOs have shed much light on the properties of these previously enigmatic sources. Lastly, there is a third type of X-ray temporal variability, namely, aperiodic phenomena. We use the term "aperiodic" in the sense that the phenomena are what are referred to as "transient sources" of X-ray emission, not necessarily that they are free of coherent periodicities. We leave open the possibility that these sources can exhibit periodic behavior. The outburst from a transient X-ray binary is most likely due to an accretion flow instability, thermonuclear burning instability, global instability of the compact object, or an instability of the envelope of the mass-losing star. As such, transients offer the possibility of probing the properties of a wide range of thermal and dynamical instabilities of astrophysical systems.
The function of an all-sky monitor is two-fold (see Holt and Priedhorsky 1987; Doty 1988). First, it is to record the photometric history of known objects. Such observations of the long-term behavior of sources have provided insights into many of the fundamental characteristics of X-ray sources such as were described above. The second function of an ASM is to detect transient sources. In this capacity, the ASM also serves as an alarm for other on-orbit detectors and to ground-based observers in order to allow for simultaneous multi-wavelength observations. As satellites have become more complex in recent years, this has been the primary function for an ASM included on a mission with a larger area, higher time-resolution detector on-board.
In the ~30 years of X-ray astronomy, there have been only a handful of true all-sky monitors. Data from two of these, Ariel 5 and Vela 5B, are now being included in the HEASARC and will be described in detail below. These two satellites provide the longest continual records of source behavior to date (Ariel 5 for ~5.5 years , Oct. 1974 - Mar. 1980; Vela 5B for ~ 10 years, May 1969 - June 1979). These datasets will be made available through the HEASARC On-line Service, and in due course distributed on CD-ROM.
The Vela nuclear test detection satellites were 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. Six sets of Vela satellites were launched into nearly circular orbits at a geocentric distance of ~118,000 km. The satellites were launched in pairs (Figure 1) and deployed 180° apart in a given orbit. The two Vela 5 satellites (designated 5A and 5B) were put into orbit on 23 May 1969. Vela 5A and 5B experiment design and implementation were done by W.D. Evans, J.P. Connor, R.D. Belian, J.A. Bergey, H.C. Owens, and E.R. Tech of Los Alamos National Laboratory and the staff of Sandia National Laboratory.
The orbital period of Vela 5B was ~112 hours. The satellite rotated about its
spin axis, actively controlled to point toward Earth center, with a ~64-sec
period. The X-ray detector was located
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 ~ 6.1° x 6.1°. The effective detector area was ~26 cm2.
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~6.x10-10 ergs/cm2 in the 3-12 keV response band. The wide collimation leads to source confusion in many interesting regions of the sky. However, good aspect information allows the deconvolution of individual source intensities in many cases.
Figure 1Tandem Spacecraft and Booster Adapter Assembly
The 10-year, all-sky characteristics of the Vela 5B X-ray database make it unique to date. Many remarkable results have already emerged from this modest instrument. These include:
One important detector performance characteristic which affects the Vela 5B data is a gain variation due to a ~60ºC satellite temperature change from one side of the orbit to the other (Figure 2). Thus, if the data for a source were taken when the satellite was at one of its temperature extremes, there is a profound modulation (Figure 3) introduced into the count rate at the 56-hour timescale (~1/2 satellite orbit) between observation sequences of the source. Additionally, the magnitude of the 112-hr modulation is modulated by the ~ 300-day precession period of the Vela 5B orbit (Figure 4). The effect on the data due to this temperature fluctuation follows the expected "highest gain for lowest temperature" trend. However, lack of pre-launch testing precludes any quantitative post-launch compensation. A temperature time history will be available to HEASARC users in a FITS file so that they may check any suspicious source data against the known times of temperature extremes.
Figure 2 Satellite temperature, as determined from on-board monitor, showing variation over satellite orbit.
Figure 3Light curve (~1/2 orbit bins) for A0620-00 showing effects of satellite temperature variation on data.
Figure 4Satellite temperature variation modulated by orbit precession.
The light curves in the HEASARC were generated by accessing the Vela 5B coordinate-ordered database. In creating this database, the data were subjected to the following:
The light curves for the single sources are put into bins based on the natural observation sequences of the satellite. That is, each time the satellite observed the source for some number of hours as it moved about its orbit, these data were binned together. So for a given source, there are two bins per satellite orbit. Typically, these bins are 56 hours apart. However, if the source is near the satellite pole (such as Cen A), it is observed more often and for longer stretches. Bins for these sources were set by searching for gaps between data points of greater than eight hours. The time assigned to the bin is the mid-point time between the first and last 1-sec observations in the data span. The average count rate is determined as the average of the 1-sec count rates weighted by their associated errors.
The light curves for sources in confused regions of the sky will be binned into 112-hour averages. The reason for the longer binning time is the need to have complete maps of the region on which to perform a 2-D deconvolution to determine the appropriate contributions of each source to the total count rate seen by Vela 5B. Typically, a complete map is not achieved by a 56-hour span. Figure 5 shows an example of the data before deconvolution by the fitting program. In the figure, we see the light curve for the Crab confused by the nearby source A0535+26. In Figure 6, we see the light curves for those two respective sources after processing by the fitting program. In both Figures 5 and 6, data are displayed in ~10-day bins. Readily apparent in the time history of the Crab is the ~15% decrease in the detected flux 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 access to the FITS file containing the Crab data to extract the necessary information.
The binned light curves described above are intended for access with BROWSE. The 1-sec data files, which will also become available, are too large for use in the BROWSE program. Software tools to access these files and tools to perform simple analyses of these data are being developed or modified.
Software to access the coordinate-ordered database (so the user can search locations of the sky not included in the light curve products) will eventually be available. This will also allow users who do not like the way the data for a source were processed to go back to the raw count rates and perform their own processing.
Figure 5Light curve for the Crab confused by the nearby source A0535+26.
Figure 6Light curves for the Crab and A0535+26 after deconvolution by the 2-D fitting program.
The All Sky Monitor was one of six X-ray instruments on the Ariel 5 satellite. The satellite was launched into a low inclination (2.8º), nearly circular orbit (altitude 520 30 km) on 15 October 1974. Ariel 5 was actively pointed so that objects of interest could be observed by the four instruments aligned along its spin axis. The ASM was mounted 90º from the spin axis; the satellite had a spin period of 6 seconds. The ASM operated from October 18, 1974 to March 10, 1980.
The ASM instrument, built by the Laboratory for High Energy Astrophysics at NASA-Goddard Space Flight Center (see Holt 1976; Holt, et al. 1979; Holt, et al. 1979a), provided continuous coverage of the entire sky, except for a 20º band straddling the satellite's equator. The ASM was intended to act as an early detection system for transients, and to monitor the variability of bright ( > 0.2 Crab) galactic sources. The instrument consisted of a pair of X-ray pinhole cameras, each covering opposite halves of the sky, with gas-filled imaging proportional counters. Position determination of sources was accomplished through position-sensitive anode wires and satellite rotation. Each camera had a 1-cm2 aperture. Overall telemetry constraints limited the duty cycle for any given source to 1 percent. With the low telemetry rate provided for this instrument (1 bit/s), temporal and spectral information were sacrificed for the sake of all-sky coverage. Hence, spectral information was limited to a single 3 - 6 keV bandpass, and temporal resolution was limited to the satellite orbital period, ~100 minutes.
To monitor the sky, X-ray events were assigned locations in the spacecraft coordinate system. These locations were binned into 16 latitude elements and 32 longitude elements, with each element ~ 10º x 10º. The origin of these spacecraft coordinates was the Sun, and hence the coordinate system moved 1 degree per day. Dead bands existed at the spacecraft equator, as mentioned above, and at the spacecraft poles, due to poor spatial resolution. With typical source sizes of 3º x 5º, flux from a given source may be divided into as many as four elements. (The ASM also operated in an Octant mode, in which the 512 elements were devoted to only 1/16 of the sky. The greater sensitivity was used for monitoring a particular source in outburst, or more crowded regions of the sky. The Octant mode data will not be included in the HEASARC database).
Background contributions were from the diffuse X-ray sky, and the internal detector background. The contribution from the sky background is determined by the detector geometry, and is approximately 2 cts / element. The internal background arising from high energy charged particles was minimized by detector design features, including active anti-coincidence rejection of events at the anode ends and radiation monitors. The background measured in flight was found to be 7 cts/element near the satellite pole and 1-2 cts/element just outside the equatorial exclusion zone.
Overall, the detector efficiency and gain were stable throughout the 5.5 years of operation. However, long-term gain variation, combination of data from the "north" and "south" counters, and occasional low count rates due to spatial offsets may affect the observed source intensities. A closer examination of constant sources like the Crab illustrate the gain difference. Figure 7 shows the half-day averages for the Crab obtained by the process described below. The outburst of A0535+26 is prominent at MJD 2442516 - 2442553. Near MJD 2442718, a gain change of 10 percent occurred in the "south" counter (Figure 8, based on Kaluzienski 1977), causing a decrease in the average Crab count rate from 1.28 cts s-1 cm-2 (computed with the A0535+26 outburst removed) to 1.11 cts s-1 cm-2. In August 1976, the gain in the south counter was adjusted.
Figure 7 Ariel 5 ASM light curve of the Crab from Oct 1974 to March 1980. Outburst of the neighboring transient A0535+26 is noted.
Light curves were obtained by fitting multiple satellite orbit accumulations (up to ~ 1/2 day) with the locations and intensities of known sources. Large differences (> 3 ) between observed and expected count rates were flagged and investigated as new sources or outbursts of persistent sources. The sky catalog used was the Third Uhuru (3U) Catalog, which has some differences in source positions from their true positions.
The light curves in the HEASARC database were obtained from Dr. Steve Holt's final production tape, which covers all the data for all sources in the 3U catalog. The brightest sources and known transients were chosen to be included here. The algorithm for extracting the light curves from the tape used two flags: an occultation correction flag and a data quality flag. The occultation flag came about because Holt noticed, when analyzing the Cyg X-1 data, a yearly variation attributable to solar X-rays scattering or fluorescing off the Earth's atmosphere. This served to give counts for sources occulted by the Earth. Holt determined the average correction empirically from a few such sources. Both occultation-corrected and uncorrected light curves are included in the HEASARC database. The second flag is a data quality flag. Light curves included here corresponded to the `Best' combination of selection criteria for accepting a given datum. These selection criteria include omitting data taken near the pole, the dead zone, or the Sun, and requiring that the source be detected only in the element where it is expected to appear.
Figure 8 Examination of the first 900 days of Ariel 5 data for the Crab. Observations by the "north" and "south" detectors are noted, as well as the change in gain near MJD 2442718.
Like the other data sets being archived at the HEASARC, the Ariel 5 and Vela 5B data will be provided in FITS format. One of the objectives of the HEASARC is to design and promote FITS format standards for high energy data products. To this end, a HEASARC standard for light curves and event lists is being developed (Pence 1992). The FITS files for the ASM data sets will consist of an empty primary array, followed by a binary table extension containing observed flux rates tabulated at different times.
The Ariel 5 binary extension will have five columns:
TIME observation time FLUX flux, in photons/cm2/s ERR-FLUX error on the flux FLUX-CORR occultation-corrected flux, in photons/cm2/s ERR-FLUX-CORR error on the occultation-corrected flux DQF integer data quality flagThe occultation-corrected flux is simply the flux value with the earth occultation correction applied. The earth occultation correction, as well as the data quality flag, were described in the previous section of this article.
The format for the binary extension in the Vela 5B files is still being developed. As it stands now, the binary extension in the data files will have five columns: time; channel 1 count rate and corresponding error; and channel 2 count rate and error. If, as is sometimes the case, there is no data for one of the channels at a particular time, then "Not a number" will appear in the appropriate column. It may be necessary, because of their size, to split the data from each energy channel into separate FITS files for the 1-sec data sets. This matter is still being investigated.
In conclusion, work to bring the long-term lightcurves of various X-ray sources as seen by the Ariel 5 and Vela 5B all-sky monitors into the HEASARC has led to the establishment of an ASM database. Table 1 gives a list, in no particular order, of the sources currently available for these two ASMs. The 3U source names are given because the Ariel 5 source fits were done based on the the 3U positions of the objects. Blanks in the columns for either monitor indicate that that source is not included in the available HEASARC files. It should be noted here that, for Ariel 5, this is a final and complete listing of all sources for which we were able to recover the data. The Vela 5B sources, denoted by an 'S', includes only those sources found to be isolated in the ~ 6º x 6º field-of-view (or sources which dominated any other nearby source). Thus, lightcurves for such sources were generated assuming that they are the "Single" source of the signal. In the future, it is expected that ~ 65 additional sources will be added to the Vela 5B list as a result of the on-going processing with the 2-D fitting program. A catalog of these sources will be published in a future issue of Legacy .
It should be noted that the appearance of a source in the list of Ariel 5 and/or Vela 5B does not imply that there is an actual detection of that source in the data. It means only that data from the appropriate location in the sky have been extracted and made into a lightcurve.
Additionally, other satellites with all-sky (or nearly so) capabilities such as Uhuru, HEAO 1, etc. may also be included in the HEASARC ASM databases as manpower is available based on prioritization by the HEASARC Users Group. Current and future missions which have all-sky monitors, such as GRO BATSE, the X-ray Timing Explorer (XTE) and the Monitoring X-ray Experiment (MOXE) are also expected to be included as they become available.
The inclusion of these ASM data bases was made possible by the help and cooperation of many people. We would like to thank Dr. Steven S. Holt of NASA-GSFC for providing us with his Ariel 5 light curve tape and programs. The Vela 5B data base was made available to us by the staff members of SST 9 at Los Alamos National Laboratory, especially Dr. William C. Priedhorsky. Additionally, invaluable assistance in the transfer of this data base was provided by Mr. Donald Salazar, Mr. Steve Blair, and the staff of SST 10 at Los Alamos.
Belian, R.D., Conner, J.P., and Evans, W.D., 1972, "A Probable Precursor
to the X-Ray Nova Centaurus XR-4", Ap. J. Lett., 171, L87.
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