X-ray observations of Circinus X-1 in its low state

Ariel V experiment C repeatedly observed Cir X-1 in the low-intensity portion of its 16.6-day light curve. The spectrum of the source is found to be a power law with photon spectral index approximately 2.3 to 3.0 and hydrogen column density Nsub(H) approximately 1022 cm-2. A weak iron feature may be present. The observation of such a low column density for the low-intensity state runs counter to previous assumptions; the significance of this observation for the current model of the source, in which the modulation is explained by absorption of X-rays in the primary's stellar wind, is discussed. A spectrum of 4U 1510-59, in the same field as Cir X-1, is also reported. (author)