3. Analysis and results
3.1. Long-term X-ray light curve
The X-ray light curve of XTE J1748-288 from the IXAE observations for 2 - 18 keV energy range was constructed by using the summed count rates from PPC-1 and PPC-3, averaged over each orbit of the satellite. The background count rates obtained by observing a nearby source-free region were subtracted from the count rates obtained during the source observations. The data were also corrected for vignetting by using aspect information from the star tracker. The event processing time for each PPC is about 20 µs which leads to a dead time correction of less than 1% for the maximum observed count rates of 200 counts s-1 for each PPC. Therefore dead time correction was not done. The X-ray light curve for the entire period of IXAE observations (1998 June 14-25) is shown in Fig. 1a for 2 - 18 keV energy band. The continuous line in Fig. 1a is the exponential fit to the light curve which gives an e-folding time of 19.0 1.8 days. Fig. 1b is the light curve of the source from one day averaged data in 1.3 - 12 keV energy range obtained from the All Sky Monitor(ASM) on the RXTE. This light curve was obtained from the publicly available ASM data archive. The IXAE observations, which were made during the declining phase of the source are indicated by two vertical lines in the light curve for ASM data in Fig. 1b. A gradual decrease in the X-ray intensity of the source with time can be clearly seen in both the light curves. The summed PPC count rate at the beginning of observation e.g. June 14, was about 550 counts s-1 in 2 - 18 keV band which declined to about 260 counts s-1 on June 25. From the ASM light curve, it is clear that the intensity of the source was maximum on 1998, June 6 with a count rate of 38 ASM counts s-1 (Crab = 75 ASM counts s-1) After staying at the maximum for 8 days, the intensity decayed exponentially over the next 30 days. An exponential fit to the ASM data gives a decay time of 15.6 0.4 days. The decay time obtained from the PPCs agrees with the ASM data within errors. Using 2 - 6 keV and 6 - 18 keV count rates from PPC-3, the hardness ratio (counts in 6 - 18 keV band / counts in 2 - 6 keV band) was computed for all the observations. PPC-1 data are not used for computing the hardness ratio as data for 2 - 6 keV energy channel for some of the observations were not available due to data readout problems. The hardness ratio computed from the PPC-3 data is shown in Fig. 2 for the entire observation period. There is no significant short term variation in the hardness ratio in the individual observations. From Fig. 2, it can be noticed that the average hardness ratio which was 1.75 0.013 during June 14 - 19, changed to 1.50 0.012 after June 19, indicating a softening of the spectrum during the decay phase. Similar spectral softening has been reported by Revnivtsev et al. (1999) from PCA observations when the source made transition from a Bright Hard State (BHS) to a High State (HS).
3.2. Light curves of individual observations
The X-ray light curves for individual observations of XTE J1748-288 were constructed after correcting the data for the background and vignetting, in the energy band 2-18 keV using 1 s and 0.1 s bin data. No significant variability was detected over time scales of 0.1 s to a few seconds. To investigate variability on a longer time scale, 1 second data bins were added to generate light curve in 10 sec bins. There is suggestion of irregular intensity variations in individual observations of the source on time scale of 10 s and longer. A constant intensity fit to the 10 s light curves gives a reduced 2 2 for 50 or more degrees of freedom.
3.3. The power density spectrum
The timing behaviour of XTE J1748-288 was studied by taking the fast Fourier transform of the 1 s and 0.1 s time resolution data. We generated count rate profiles of XTE J1748-288 in the 2-18 keV bands with a time resolution of 1 s and also for 0.1 s. The PDS are obtained for the individual data segments of the observation and then co-added and the final PDS is produced. The data from the two PPCs are added to obtain the PDS for improving the statistics. The PDS are normalised to the mean count rate.
It may be noticed from the PDS for 2 - 18 keV energy range, in Fig. 3, that it is flat and featureless in the frequency range of 0.01 Hz to 0.5 Hz and goes up below 0.01 Hz. The PDS in the frequency range 0.003 Hz to 0.5 Hz fits well with a model comprising of a power law component with index 1.7 and a constant giving reduced 2 of 0.41 for 14 degrees of freedom. The Poisson fluctuation level is subtracted during the normalisation of the PDS to (rms/mean)2/Hz. The rms fractional variation of the PDS in Fig. 3 is calculated for different frequency ranges. It is found to be 1.6% in 0.002 - 0.01 Hz range, 2.6% in 0.01 - 0.1 Hz and 5.9% in the higher frequency range 0.1 - 0.5 Hz.
A similar PDS is also obtained using data of 0.1 s mode for the combined observations. This PDS is also flat with no indication of any QPO in the frequency range of 0.002 to 5 Hz. A search for the presence of regular X-ray pulsations in the period range of 5 to 100 s gave negative results.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000