The first observation of AX J1749.1-2733 in a bright X-ray state—Another fast transient revealed by INTEGRAL

An intense outburst of hard radiation (with a peak flux of ∼50 mCrab) was detected from the X-ray transient AX J1749.1-2733 by the IBIS/ISGRI gamma-ray telescope onboard the INTEGRAL observatory when the Galactic center field was monitored on September 8 10, 2003. Previously, this source had never been observed in a bright X-ray state. During the outburst, the source’s radiation spectrum was gently sloping and hard (extended to ∼100 keV), followed a power law in the standard X-ray energy range, and had an exponential cutoff above 40 50 keV. The spectral hardness decreased with increasing flux. These and other properties described here and the shortness of the outburst (<2 days) allow the source to be attributed to the group of fast X-ray transients many representatives of which have an early O-B supergiant as their optical counterpart. Possible causes of the outbursts of fast transients are discussed. We show that accretion from the supergiant’s stellar wind should have led to intense persistent radiation from transients. The absence of radiation can be explained by the ejection of accreting matter from the system (propeller effect) during its contact with the magnetosphere of a rapidly rotating neutron star. Transient outbursts could originate in sources of this type if the spin period of their neutron star P _s differed only slightly from the critical period P <Stack> _s ^* </Stack> ≃ 3 s at which the propeller effect is still possible. The outburst is triggered by an insignificant rise in the local stellar wind density, by a factor of (P <Stack> _s ^* </Stack>/P _s)^7/3. The entire outburst profile cannot be explained by an individual inhomogeneity in the wind, but is the reflection of a long-term (∼2 days for AX J1749.1-2733) change in the rate of wind outflow from the supergiant’s surface facing the compact source. The rate of wind outflow could be enhanced through X-ray heating of the supergiant’s surface.