1994A&A...285..903V

Aql X-1 has been detected in five observations with ROSAT, at X-ray luminosities varying from L_x(0.4-2.4 keV)∼4 x 10³² erg/s to ∼2 x 10³⁶ erg/s. The characteristic temperature of the X-rays varies from kT_bb≃0.31 keV at the lowest to kT_bb≃0.55 keV at the highest flux level observed. This moderate temperature variation excludes models that explain the large range in X-ray luminosity as the consequence mainly of a variation in temperature, and implies that accretion onto the neutron star drops by five orders of magnitude between outburst maximum (at L_x∼10³⁷ erg/s) and quiescence. Three other soft X-ray transients, all black hole candidates, were also observed with ROSAT. GS 2023+338 was detected at L_x(0.4-2.4 keV)≃1.5 x 10³³ erg/s (for a distance d = 3.5 kpc). For H 1705-25 an upper limit of L_x≲4.9 x 10³² erg/s (d=3 kpc) was obtained, and for GS 2000+25 an upper limit of L_x≲1.0 x 10³¹ erg/s