A Bragg Spectrometer for Stellar X-Ray Astronomy

Extract

In the last year there have been several proportional counter observations of Sco X-1 which give an indication of an emission feature at around 7 keV (Holt et al., 1969; Acton et al., 1970). The emission lines of Fe⁺²⁴ and Fe⁺²⁵, which are very strong in radiation from solar flares of comparable plasma temperature, may be responsible for this feature. Two factors, poor energy resolution and the steeply varying continuum spectrum whose shape is not known in detail, make it impossible to obtain with proportional counters an unambiguous identification of emission features. For this reason we have constructed a Bragg crystal spectrometer to scan the spectrum of Sco X-1. The instrument, flown successfully aboard an Aerobee-170 rocket in April 1970 in conjunction with the Kitt Peak National Observatory, scanned the spectral range from 2.4–2.9 keV which includes the Lyman-α line of hydrogenic sulphur (2.62 keV). The spectrum obtained shows no feature at this energy. The sensitivity of the instrument may be judged from the upper limit (3σ) of 0.08 photon/cm²/s which we are able to place on the line intensity. This value can be compared with the flux of 0.55 photon/cm²/s predicted by Tucker (1967) for the S⁺¹⁵ line intensity from an isothermal, optically thin plasma at 5× 10⁷K. If the line had had this theoretical intensity, we would have observed a 21-σ signal.