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CUBIC (Cosmic Unresolved Background Instrument using CCDs) was [Photo of CUBIC instrument] an X-ray spectrometer built by the PSU X-ray astronomy group which was launched on November 4, 1996 on the US/Argentine satellite SAC-B by a Pegasus XL rocket (with HETE). Its purpose was to measure the spectrum of the soft X-ray diffuse background over the energy range 0.2 - 10 keV over a large part of the sky in order to improve our understanding of the multiple components of this background radiation. These data would have provided new insights into the cosmic X-ray background above 2 keV, which is dominated by emission from distant active galactic nuclei; the 3/4 keV diffuse galactic background, which is not well understood currently; and the 1/4 keV diffuse background, which is dominated by emission from the hot intersteller medium within a few hundred parsecs of the Sun. In this lowest energy regime, it would have complemented the data taken by the DXS instrument, which measured this spectrum with higher spectral resolution over a small part of the sky. Unfortunately, a failure of the Pegasus rocket resulted in the complete loss of the SAC-B and HETE satellites.

[Line diagram of CUBIC instrument] The CUBIC instrument consisted of a pair of X-ray CCDs operated in photon-counting mode which would have been exposed directly to the sky through an aperture that provided a 5 x 5 degree field of view below 1 keV and a 10 x 10 degree field of view above 3 keV. This unique and innovative collimator was designed to reduce calibration uncertainties associated with traditional mechanical collimators, and to provide an increased count rate above 2 keV where the sky is isotropic but the incident flux is dropping rapidly with increasing energy.

[Simulated CUBIC spectrum] [Simulated CUBIC spectrum of the NPS]
Two simulated spectra are shown above. The left-hand spectrum shows a simulation of a typical CUBIC observation, where the bottom curve shows the predicted particle background spectrum, the middle curve shows the predicted incident spectrum, based on proportional counter model fits, and the upper curve shows the predicted CUBIC pulse-height spectrum for a 50,000 second integration. Note that the expected count rate from the sky exceeds the expected particle background rate by more than an order of magnitude over nearly the complete instrumental bandpass. The right-hand plot shows a simulated spectrum for a 50,000 second observation of the North Polar Spur, one of the brightest features in the sky between 0.5 and 1.5 keV. CUBIC would have provided a wealth of information on the thermal state of the hot gas in the ISM, producing a similar revolution in our understanding of this important component of our galaxy to that produced by ASCA spectra of supernova remnants, active galactic nuclei, and other objects.

SAC-B and the piggyback HETE satellite were launched by "Maggie" (a Pegasus XL rocket) on 11/04/96. [Pegasus takeoff photo] Drop occurred at 17:08:56 GMT. Orbital insertion was about 10 minutes later. The Pegasus achieved a nominal orbit but failed to separate from the satellite due to a failure of the Pegasus transient power bus. The system is now flying with SAC-B, HETE and the Pegasus 3rd stage connected together as a single 650 kg spacecraft. The SAC-B satellite deployed its solar panels successfully and operated for about 12 hours before exhausting its battery power. The on-board software was modified by ground command to permit operation without a separation indication and the ACS system was placed in safe-hold mode in an attempt to gain control and point the solar panels to the sun. The combined "stack" was tumbling with the Z axis (the solar axis) ranging from about 55 degrees to about 95 degrees to the sun. The Pegasus 3rd stage was shadowing part of the solar array at all times, and during some portions of the oscillation none of the solar panels was illuminated. The average power was insufficient to recharge the batteries, even during the daylight part of the orbit, and the SAC-B satellite continued to lose power with each orbit. Finally, after more than 12 hours of orbital operation during which the satellite operation was completely nominal, the batteries became completely depleted and the satellite died. The Argentine Mission Operations and Control Center continues to attempt contacts with the satellite and has been able to turn its transmitter on briefly on several occasions, but has not received any telemetry data since the day of the launch. The consensus is that the spacecraft is occasionally obtaining enough power from its solar panels to turn on briefly, but that the average power received is less than the minimum spacecraft power consumption because of the incorrect attitude and shading of the solar panels by the Pegasus 3rd stage. The only data obtained from CUBIC was some thermal housekeeping data which showed that the thermal performance of our instrument was slightly better than predicted before launch.

[CUBIC Calibration Data] In addition to its potential scientific value for studies of the soft X-ray background, CUBIC was an innovative experiment in building a "faster, better, cheaper" instrument in a university environment. The entire CUBIC instrument was built in our lab by scientists, engineers, technicians, and graduate students of the Penn State X-ray astronomy group over a period of about 4 years for a budget of just over $2 million. Laboratory performance of the CUBIC instrument has been outstanding, as demonstrated by the calibration data shown here. The first plot shows a composite of X-ray preflight calibration data obtained with the CUBIC flight instrument. The second plot shows the CUBIC energy resolution derived from these data. [CUBIC Calibration Data] After launch, the CUBIC instrument was to have been operated from Penn State via a ground station outside of Buenos Aires. Spectral observations would have utilized typical integration times of 2-4 days per observation, with spectra of comparable quality to those shown above for each observation.

The scientific justification for CUBIC has not changed, as there is no other planned NASA mission capable of providing this spectral information on the diffuse X-ray background. The ROSAT All-Sky Survey has provided a tremendous improvement in our knowledge of the spatial structure of this diffuse background, and we now need a similar improvement in our knowledge of its spectral characteristics. CUBIC would have taken the first important step in this direction. We are now investigating options for reflying CUBIC. With all of the hardware and software design completed, and with a complete set of backup electronics in hand, we expect that CUBIC could be rebuilt very quickly and cheaply and could be ready for reflight in less than 2 years if a suitable platform can be identified to replace SAC-B. We are currently investigating the possibility of such a reflight on a Spartan-Lite.

I would like to take this opportunity to express my thanks to CONAE and INVAP for the excellent work they did on SAC-B. In its few hours of operation, SAC-B demonstrated that Argentina knows how to build a successful satellite. Unfortunately, we were never able to turn on the science instruments and demonstrate their capabilities. At the moment, CUBIC is only a memorial tribute to Mario Gulich, the original Argentine project manager, who died tragically in 1994. A plaque commemorating Mario's contributions to the creation of SAC-B now orbits aboard the CUBIC instrument.


More information is available on the following topics:

Pictures:

CUBIC photo gallery
SAC-B photo gallery


Bibliography

[DIR] Detailed description of the CUBIC instrument (July 1995), available as HTML or PostScript.

[DIR] CUBIC Instrument Handbook describing the CUBIC instrument in gory detail (10/23/96).

[HTML] First results of the CUBIC calibration of flight CCDs (July 1995 - HTML).

[PostScript] CUBIC: Preflight Calibration Results and Initial Operations (August 1996).

[PostScript] Poster paper presented at January 1997 AAS meeting (7 Mb).

[DIR] Poster paper presented at November 1997 HEAD meeting (9.7 Mb). This paper is also available in HTML format.



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Last updated December 12, 1997
Web page maintained by David Burrows ( burrows@astro.psu.edu )
Department of Astronomy and Astrophysics
Penn State University