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Astron. Astrophys. 337, 815-818 (1998)

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3. Results

3.1. Periodic pulsations

Pulsations with 221 s period as reported by Takeshima et al. (1998) are clearly seen in the light curves (Fig. 1). We have used all four observation stretches and obtained the pulsation period by [FORMULA] maximising method. The period obtained thus is [FORMULA] s. The pulsation period could not be determined very accurately because of only about 7000 s of useful data and a large pulse period. Barycentric corrections were not applied which is about one order of magnitude smaller than the error in period estimation. All the light curves were folded with this period and the resultant pulse profiles in two energy bands are shown in the top two panels of Fig. 2 for two cycles. The pulse profile is single peaked and nearly sinusoidal as reported earlier by Takeshima et al. (1998). The pulse fraction (defined as the ratio of pulsed flux to total flux) in the higher energy band ([FORMULA] 7 keV) is 20%, significantly larger than that of 10% in the lower energy band (1.3-7 keV). There is indication of significant change in the spectrum with the pulse phase, as the hardness ratio, shown in the bottom panel of Fig. 2, varies by about 15% during the pulsation. A detailed analysis of pulse phased spectrum is in progress.

[FIGURE] Fig. 2. The pulse profiles of XTE J1858+034 folded at a period of 220.7 s are shown in two energy bands along with the hardness ratio. The profiles are repeated for 2 cycles for clarity.

3.2. Power density spectrum

We generated power density spectrum (PDS) from the 1 s time resolution data. The light curves were broken into segments of length 512 s and the PDS obtained from each of these segments were averaged to produce the final PDS as shown in Fig. 3. A broad QPO feature around 0.1 Hz is very prominent in the PDS. The PDS in the frequency range of 0.006 to 0.6 Hz fits well with a model consisting of a power-law type spectrum and a Gaussian ([FORMULA] of 1.4 for 68 degrees of freedom). The value of [FORMULA] is 2.3 when only a power law is used indicating that the presence of the feature at 0.11 Hz is very significant. The power-law index for the best fit model is found to be -0.95 and the Gaussian, representing the QPO feature is centered at 0.11 [FORMULA] 0.01 Hz with a width of 0.02 Hz. While fitting the PDS to this model, the region below 0.006 Hz were excluded to avoid the power due to the regular pulsations at 0.0045 Hz. The rms variability in the QPO feature is 6.5%. PDS were also generated in the two energy bands of 1.3-7 keV and [FORMULA] 7 keV. The PDS in the two energy bands are found to be identical in shape, comprising of one power law component of index -0.95 and a QPO feature. The rms variation in the high energy band is generally higher and rms in the QPOs is much more in the higher energy band (7.8%) compared to the same in the lower energy band (3.7%). There was no detectable difference in the QPO frequency in the four data sets.

[FIGURE] Fig. 3. The power density spectrum of XTE J1858+034 generated from the light curve over the entire energy band of the PCA. The line represents the best fitted model in the frequency range of 0.006-0.6 Hz comprising a power-law type spectrum and a Gaussian centered at the QPO frequency.

3.3. Energy spectrum

We have generated the count spectrum in 129 binned channels of the PCA detectors from the observation A (see Table 1). The background was generated using the "pcabackest" model provided by the XTE guest observer facility (GOF). Data from all the 5 detectors were added together to produce the spectrum. One low energy channel and channels corresponding to energy greater than 50 keV were ignored because of low signal to noise ratio. The new pulsar XTE J1858+034 is in the Galactic ridge (l [FORMULA] [FORMULA] and b [FORMULA] [FORMULA]). The background subtraction model that we have used takes care of the diffuse cosmic X-ray emission and the internal background, but not the emission from extended source like the Galactic ridge. From a detailed observation of the Galactic ridge obtained using the PCAs (Valinia & Marshall, 1998) we estimate that about 10% of the observed flux can be accounted for by the Galactic ridge emission. We have attempted a spectral fitting for the pulse averaged spectrum of XTE J1858+034 by explicitly taking the Galactic ridge emission as a sum of a Raymond-Smith plasma and a power-law, with parameters constrained to be within the range obtained for the ridge emission in Valinia & Marshall (1998). The residual spectrum in 1.7-50 keV range is found to be very hard which can be described as a cut-off power law with a power-law photon index close to 1, cut-off energy of 21 keV, along with a neutral absorption with an equivalent Hydrogen column density of 6 [FORMULA] 1022 cm-2. A narrow emission line, which can be ascribed to atomic Iron inner shell emission, was also found at 6.6 keV, with an equivalent width of 165 eV. Though an acceptable value of [FORMULA] was obtained (82 for 83 degrees of freedom), parameters values could not be constrained due to the large number of free parameters involved. The total incident flux in the 1.3 - 100 keV band is 6.5 [FORMULA] 10-10 erg cm-2 s-1 for the pulsar and 0.5 [FORMULA] 10-10 erg cm-2 s-1 for the Galactic ridge emission. The best fit spectrum with the parameters mentioned above is shown in the top panel of Fig. 4 along with the observed spectrum deconvolved through the detector response function. The observed spectrum and the folded model along with the residual to the model fit are shown in the middle and the bottom panel of the same figure.

[FIGURE] Fig. 4. The X-ray spectrum of XTE J1858+034 is fitted with a cut-off power-law of index 1 and Gaussian line at 6.6 keV of equivalent width 165 eV. The Galactic ridge emission is modeled as a Raymond-Smith plasma of temperature 2.6 keV and a power-law of index 1.7.

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© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998