All Figures

$\begin{figure} \par\includegraphics[width=12cm]{8707fg01.eps} \end{figure}$ Figure 1: Orbital profile from 2687 individual science windows in the ISGRI energy range 20-40 keV for source offset angles < $12\hbox {$^\circ $ }$ (black squares) and $12\hbox {$^\circ $ }{-}15\hbox {$^\circ $ }$ (red dots). Though the performance of the IBIS instrument significantly decreases for angles larger than $10\hbox {$^\circ $ }$ , the count rate values for offset angles in the range $12\hbox {$^\circ $ }{-}15\hbox {$^\circ $ }$ are consistent with the count rates for angles below $12\hbox {$^\circ $ }$ . The sharp decrease near phase 0.55 is clearly visible.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{8707fg02.eps} \end{figure}$ Figure 2: Averaged orbital profile in the ISGRI energy band 20-40 keV. This profile was calculated from 1797 science windows for which the angular distance to the telescope axis was less than $12\hbox {$^\circ $ }$ (see text for description of error bars). The dashed line connects values giving the number of science windows (right axis) used for the respective averages.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{8707fg03.eps} \end{figure}$ Figure 3: Averaged orbital profile (ASM, 2-10 keV) plotted from quick-look results provided by the ASM/RXTE team. This curve is calculated from 12 830 single dwell values (sum band intensity) covering the used INTEGRAL observation period from MJD 52 668-53 946. The error bars are quadrature averages of the flux errors. The dashed line shows the number of dwells per data point (right axis).
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{8707fg04.eps} \end{figure}$ Figure 4: Determination of the centre of eclipse by fitting straight lines to ingress and egress. The dashed lines denote the width of the strips (0.03 phase units), which were used to determine the number of data points falling into the strips left and right to the fitted straight lines (see text). Only data points from SCWs with offset angles < $12\hbox {$^\circ $ }$ were used for this analysis (see Fig. 1).
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{8707fg05.eps} \end{figure}$ Figure 5: Period evolution of OAO 1657-415 as determined in this work from INTEGRAL ISGRI data (Table 1). The typical error of 0.004 s shown here was assumed for periods estimated by the epoch folding method. The actual errors for the plotted periods estimated by the phase connection method are in general about a factor of 10 lower. The $\dot{P}$ value for the second data point is indicated by the slope of a short line (whose length does not represent the time interval of the evaluated observations).
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In the text

$\begin{figure} \par\includegraphics[width=12cm,clip]{8707fg06.eps} \end{figure}$ Figure 6: Long-term period evolution of OAO 1657-415. The historic data from the literature (see Table 2) were corrected for binary motion. The BATSE data are from public archive (1, 3) and private communication by Mark Finger (2). The sinusoid with a period of 4.8 yr, superposed on a linear long-term spin-up, is to guide the eye to emphasise a quasi-periodic behaviour.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{8707fg07.eps} \end{figure}$ Figure 7: Observed pulse delay times for the second observation period, which covers about 2.5 orbital cycles. The solid line shows the best fit for the orbital parameters (see Sect. 3.4). The residuals (observed - calculated, lower panel) show some variation that may well be due to real pulse period variations on time scales shorter than one orbital period.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{8707fg08.eps} \end{figure}$ Figure 8: Energy-resolved pulse profiles. The relative intensity scale is normalised to the average count rate level (horizontal dashed line) in the corresponding energy interval. The percentage values indicate the pulsed fraction. The pulsed fraction of the highest energy interval was set to 100%, as the negative minimum flux values were set to zero. The top panel shows the hardness ratio (see text). The vertical dashed lines denote the four phase intervals for which spectra were extracted. The pulse phase limits of the four intervals are: a=0.025-0.250, b=0.25-0.50, c=0.50-0.75 and d=0.750-1.025.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{8707fg09.eps} \end{figure}$ Figure 9: Variation of the pulsed fraction with energy. Horizontal bars denote the energy range for which the pulsed fraction was calculated.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{8707fg10.eps} \end{figure}$ Figure 10: Phase-averaged broadband spectrum for ISGRI, JEM-X1, JEM-X2. The fitted model was HIGHECUT. To avoid artefacts in the residuals of the fit, the transition to the high energy cutoff was smoothed with an additional Gaussian absorption. The fit parameters are given in Table 4.
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In the text

$\begin{figure} \par\includegraphics[width=15cm,clip]{8707fg11.eps} \end{figure}$ Figure 11: Phase-resolved ISGRI spectra for the four phase intervals as defined in Fig. 8. The solid line in the upper panel is the model fit of the phase-averaged spectrum as shown in Fig. 10 (model A of Table 4). To enhance the differences between the spectra of the four phase intervals the lower panel gives the ratios between the four spectra and the model of the phase-averaged spectrum in a logarithmic scale. The dotted line in the lower panel corresponds to a fit of spectrum b with a line feature at 49.3 keV.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{8707fg02.eps} \end{figure}$	Figure 2: Averaged orbital profile in the ISGRI energy band 20-40 keV. This profile was calculated from 1797 science windows for which the angular distance to the telescope axis was less than $12\hbox {$^\circ $ }$ (see text for description of error bars). The dashed line connects values giving the number of science windows (right axis) used for the respective averages.
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{8707fg03.eps} \end{figure}$	Figure 3: Averaged orbital profile (ASM, 2-10 keV) plotted from quick-look results provided by the ASM/RXTE team. This curve is calculated from 12 830 single dwell values (sum band intensity) covering the used INTEGRAL observation period from MJD 52 668-53 946. The error bars are quadrature averages of the flux errors. The dashed line shows the number of dwells per data point (right axis).
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$\begin{figure} \par\includegraphics[width=12cm,clip]{8707fg06.eps} \end{figure}$	Figure 6: Long-term period evolution of OAO 1657-415. The historic data from the literature (see Table 2) were corrected for binary motion. The BATSE data are from public archive (1, 3) and private communication by Mark Finger (2). The sinusoid with a period of 4.8 yr, superposed on a linear long-term spin-up, is to guide the eye to emphasise a quasi-periodic behaviour.
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{8707fg07.eps} \end{figure}$	Figure 7: Observed pulse delay times for the second observation period, which covers about 2.5 orbital cycles. The solid line shows the best fit for the orbital parameters (see Sect. 3.4). The residuals (observed - calculated, lower panel) show some variation that may well be due to real pulse period variations on time scales shorter than one orbital period.
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$\begin{figure} \par\includegraphics[width=7.2cm,clip]{8707fg09.eps} \end{figure}$	Figure 9: Variation of the pulsed fraction with energy. Horizontal bars denote the energy range for which the pulsed fraction was calculated.
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$\begin{figure} \par\includegraphics[width=8.2cm,clip]{8707fg10.eps} \end{figure}$	Figure 10: Phase-averaged broadband spectrum for ISGRI, JEM-X1, JEM-X2. The fitted model was HIGHECUT. To avoid artefacts in the residuals of the fit, the transition to the high energy cutoff was smoothed with an additional Gaussian absorption. The fit parameters are given in Table 4.
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