Sudden Changes in the Intensity of High Energy X-Rays from Sco X-1

Abstract

WE report the observation of sudden changes in the intensity of Sco X-1 by a factor of about three in the energy interval 29.9–52.3 keV on December 22, 1968, between 0427 h and 0553 h UT. The observation was made with an X-ray telescope in a balloon flight from Hyderabad (latitude 17.6° N, longitude 78.5° E), India. The telescope consisted of an NaI(Tl) crystal with an area of 97.3 cm² and thickness 4 mm surrounded by both active and passive collimators. The passive collimator was a cylindrical graded shield of lead, tin and copper and the active collimator was a plastic scintillator surrounding the shield. The field of view of the telescope at f.w.h.m. was 18.6°. The axis of the telescope was inclined at an angle of 32° to the zenith. The telescope was mounted on an oriented platform programmed to look in four specified directions successively, spending about 4 min in each direction during a cycle of period about 16 min. The four specified directions were N(ϕ = 0), SW(ϕ=110°, with the convention ϕ = 90° being due west), S(ϕ = 180°, due south) and NE(ϕ = 310°). In this flight the orienter performed satisfactorily and the telescope picked up Sco X-1 in the south direction, Cyg X-1 in the direction NE(ϕ = 310°) and a new source in the direction SW(ϕ = 110°). In the north direction (ϕ = 0) there was no source during the period of observation and therefore the information on background X-rays was obtained from this direction. The observations of the short term changes of intensity of Sco X-1 are discussed here—the results from the other data will be published separately. A pair of crossed flux gate magnetometers provided information every 8.2 s on the azimuth of the telescope. The pulse heights from the X-ray detector were sorted into nine contiguous channels extending from 10 to 120 keV. Counting rates of several channels were combined for analysis to improve the statistics. A ²⁴¹Am source came into the field of view of the telescope once in 15 min for about 30 s to provide in-flight calibration of the detector and this indicated that there was stable operation and no drift in the channel positions during the flight. The balloon was launched at 0200 h UT and reached the ceiling altitude at 0435 h. Between 0435 and 0530 h, the ceiling altitude was 7.6 ± 0.3 g cm⁻². Just before the balloon reached the ceiling Sco X-1 was in the field of view of the telescope for 3 min 41 s when the balloon was ascending from 9.2 to 8.3 g cm⁻². After the balloon reached ceiling, Sco X-1 was in the field of view on five occasions between 0443 and 0553 h. During the last observation, 0551–0553 h, however, the floating altitude of the balloon was a little lower, 8.5 g cm⁻². The meridian transit of Sco X-1 was at 0454 h and the flight was terminated at 0615 h.