Spectral lags of flaring events in LS I+61°303 from RXTE Observations

Abstract

This work reports the first discovery of (negative) spectral lags in X-ray emission below 10keV from the gamma ray binary LS I+61° 303 during large flaring episodes using Rossi X-ray Timing Explorer (RXTE) observations. It is found from the RXTE data that during the flares, low energy (3–5 keV) variations lead the higher energy (8–10 keV) variations by a few tens of seconds whereas no significant time lag is observed during the non-flaring states. The observed spectral lag features for flaring events suggest that inverse Compton scattering may be operating, at least in some part of the system. Another possibility is that the sites of particle acceleration may be different for flaring and non-flaring events such as in the microquasar model, in which the flaring radiation may come from hot spots sitting above the black hole while steady state emissions are due to the jets.