A report on Type II X-ray bursts from SMC X-1

We study RXTE PCA data for the high mass X-ray binary source SMC X-1 between 2003-10 and 2003-12 when the source was in its high states. The source is found to be frequently bursting which can be seen as flares in lightcurves that occur at a rate of one every 800 s, with an average of 4-5 Type II X-ray bursts per hour. We note that typically a burst was short, lasting for a few tens of seconds in addition to a few long bursts spanning more than a hundred seconds that were also observed. The flares apparently occupied 2.5% of the total observing time of 225.5 ks. We note a total of 272 flares with mean FWHM of the flare similar to 21 s. The rms variability and aperiodic variability are independent of flares. As observed, the pulse profiles of the lightcurves do not change their shape, implying that there is no change in the geometry of an accretion disk due to a burst. The hardness ratio and rms variability of lightcurves exhibit no correlation with the flares. The flare fraction shows a positive correlation with the peak-to-peak ratio of the primary and secondary peaks of the pulse profile. The observed hardening or softening of the spectrum cannot be correlated with the flaring rate but may be due to the interstellar absorption of X-rays as evident from the change in hydrogen column density (n(H)). It is found that the luminosity of the source increases with the flaring rate. Considering that the viscous timescale is equal to the mean recurrence time of flares, we fixed the viscosity parameter alpha similar to 0.16.