XG EXPERIMENT

The XG EXPERIMENT was a balloon mission to study the transient X-ray pulsar A0535+26 during its 1980 October outburst, with a particular focus on its pulse profile, time variability, and energy dependence in the hard X-ray range above 27 keV. The intention was to investigate how the accretion processes onto the neutron star influenced pulse structure, timing stability, and luminosity, as well as to search for chaotic short-time variability in the emission. The detectors were developed as part of an extensive program of balloon-borne hard X-Ray observations by the Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri, Consiglio Nazionale Ricerche, located in Bologna, Italy, and the Istituto Astrofisica Spaziale, Consiglio Nazionale Ricerche, located in Frascati, Italy.

The instrument was composed of an array of sixteen independent square NaI(Tl) scintillation detectors, each with a frontal surface of 105 × 105 mm. Fourteen crystals had a thickness of 10 mm, while the remaining two were 3 mm thick. Each detector was optically coupled to a photomultiplier tube through a lead glass optical guide, and each had its own mechanical square collimator that restricted the field of view to 9.2° × 9.2° (FWHM). The total effective geometric area was 1455 cm². The nominal common energy band was 20–210 keV, though during the flight this range shifted slightly due to gain changes across the detection units. To suppress background charged particles, the array was surrounded by a plastic scintillator active shield, leaving only the X-ray entrance window open. In-flight calibration was secured by four ^139Ce radioactive sources that scanned all detectors every 2000 seconds. The instrument was mounted in an alt-azimuth configuration on a platform stabilized in azimuth within about 20 arcminutes. Pointing was controlled by an onboard computer from the ground. The data acquisition system provided pulse height analysis in 64 channels with 2 s integration and telemetry, along with high-time-resolution data in three broad energy ranges (20–60, 60–120, 120–210 keV). Time tagging of events was ensured by a high-stability onboard clock.

Balloon launched on: 10/4/1980 at 23:02 utc
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 727.460 m3 (15.24 Microns - Stratofilm) - SF 404.85-060-NSCR-01
Balloon serial number: W26.59-2-01
Flight identification number: 1220P
End of flight (L for landing time, W for last contact, otherwise termination time): 10/5/1980 at 13:18 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 15 h
Landing site: 17 Miles N of Meridian, Mississippi, US
Payload weight: 1387 kgs

The balloon was launched on 1980 October 4 from Palestine, Texas, and maintained a stable float altitude for about 11 hours at residual pressures between 2.80 and 2.95 mbar. The observation of A0535+26 began on October 5 at 09:18 UT and ended at 12:12 UT. The strategy included three 10-minute drift scans at fixed elevation and one continuous tracking of the source for about 110 minutes, complemented by background measurements at offset azimuths. The total effective on-source time was 8400 s. Although the main flight goal was a long observation of the Crab Nebula, located 4.4° away, the pointing approach also allowed the transient pulsar to be well observed during its outburst.

The payload was recovered 17 Miles N of Meridian, Mississippi.

• Balloon program for hard-X-ray astronomy IL Nuovo Cimento C Novembre–Dicembre 1984, Volume 7, Issue 6, pp 648-655
• Hard X-ray emission from the recurrent transient A0535 + 26 during its 1980 high-intensity outburst Advances in Space Research vol. 3, no. 10-12, 1984, p. 47
• Hard X-ray observation of galactic X-ray sources Advances in Space Research - Volume 5, Issue 3, 1985, Pages 125–128
• Hard X-ray observation of the transient X-ray pulsar A0535+26 Max-Planck Inst. Phys. and Astrophys. Accreting Neutron Stars p 112-116
• National Scientific Balloon Facility Annual Report FY 1981 National Center for Atmospheric Research, April 1982
• The Pulsed Spectrum of the Transient Source A0535+26 in the Hard X-Ray Range Physica Scripta. Vol. T7, 11 1-1 12, 1984
• The X-ray pulsar A0535 + 26 - Pulse profile and its time variability in hard X-rays Astrophysical Journal, Part 1 vol. 298, Nov. 15, 1985, p. 585
• The X-ray pulsar A0535+26 in hard X-rays - Average spectrum, pulse-phase spectroscopy, and spectral time variability Astrophysical Journal, Part 1 vol. 331, Aug. 1, 1988, p. 313