AAS 207th Meeting, 8-12 January 2006
Session 198 Neutron Stars and Pulsars
Oral, Thursday, 10:00-11:30am, January 12, 2006, Balcony B

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[198.07] State Transition in Soft X-ray Transients

D. Maitra (Yale University)

Accreting compact binary systems are excellent laboratories for studying dynamics and stability of gas flows in accretion disks and jets and also allows testing relativity in strong-field limit. Soft X-ray transients (SXTs) are an interesting sub-class of compact binary systems. Their luminosity is generally very low, however occasionally they brighten up by several orders of magnitude at all wavelengths. The increase in flux is directly associated with an increase in the inferred mass accretion rate (\dot M). A host of different states, characterized by markedly different spectral and temporal features, are seen during an outburst of an SXT as it goes through the wide range of luminosities during the outburst.

While it is believed that change in \dot M is a principal factor in triggering state transitions in SXTs, we present compelling evidence from two systems, one neutron star (Aql X-1) and another black hole (V4641 Sgr), for the existence of factors other than \dot M that can trigger state transition. We have seen that during major outbursts of Aql X-1, transition from powerlaw photon dominated state to thermal dominated state during the rise of an outburst always occurs at a higher luminosity (and hence \dot M) than the transition back to powerlaw state during outburst decay, a clear evidence for the existence of factor(s) other than \dot M in operation. In contrast, during a low-intensity outburst of Aql X-1, we have seen that the usual correlation between optical and X-ray flux is not obeyed and the maximum optical flux is seen during the powerlaw dominated state. In V4641 Sgr, we find flux variation by factor of 5 or more within timescales of minutes, without any change in the state of the source. For V4641 Sgr we also find evidences for an enshrouded black hole with occasional outflows and very dynamic environment.

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