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Astron. Astrophys. 330, 175-180 (1998)

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3. Results

3.1. The 1E 2259+586 spectrum

LECS and MECS spectra were obtained centered on the pulsar position using an extraction radius of [FORMULA]. The 1E 2259+586 count rates above background are 0.28 and 0.55 s-1 in the LECS and MECS, respectively. Examination of the LECS spectrum reveals that the pulsar is only detected between 0.5 and 8.0 keV and data outside this range are excluded. Similarly, the MECS fit is restricted to the energy range 1.65-10 keV.

[FIGURE] Fig. 2. The best-fit power-law and blackbody fit to the pulsar spectrum (top) and the best-fit power-law model fitted to the pulsar spectrum with 0.19 of the SNR shell spectrum subtracted (bottom). The smaller panels give the residuals in counts s-1  keV-1. The models are described in the text and summarized in Table 1

The combined LECS and MECS spectrum of the pulsar was first fit using a power-law model with [FORMULA] and low-energy absorption of [FORMULA]  atoms cm-2 yielding a [FORMULA] of 449 for 266 degrees of freedom (dof). The photo-electric absorption coefficients of Morisson & McCammon (1983) and the solar abundances of Anders & Grevesse (1989) were used for all fits. Examination of the residuals reveals significant structure below [FORMULA] 1.0 keV. A better fit ([FORMULA] of 271 for 264 dof) is obtained when a blackbody component is added. The spectral parameters listed in Table 1 are similar to those obtained using the same spectral model with ASCA spectra by Corbet et al. (1995) and with a combination of ASCA, ROSAT, and BBXRT spectra by Rho & Petre (1997). The remaining residuals near 1 keV may indicate an even more complex spectral shape.


Table 1. Spectral fit results for 1E 2259+586. Uncertainties are given at 68% confidence

In order to investigate an alternative explanation for the low energy residuals, the spectrum of the SNR shell multiplied by a scale factor, F, was subtracted from the 1E 2259+586 LECS spectrum (the SNR hardly contributes in the MECS energy range, see Fig. 4). The fit with a single power-law was then repeated and the best-fit value of F of [FORMULA] determined by minimizing [FORMULA]. This gives a [FORMULA] of 296 for 265 dof, and the amplitude of the residuals [FORMULA] 1 keV is similarly reduced as with the power-law and blackbody fit (Fig. 2). The best-fit value of [FORMULA] is [FORMULA] and N [FORMULA] is [FORMULA]  atoms cm-2. The ratio of the areas of the pulsar and SNR shell extraction regions (39 and 361 arcmin2, respectively) is 0.11. This is comparable with the value obtained for F of 0.19, given the observed variations in surface brightness of the SNR, the presence of the X-ray lobe, and the extended (2-3 [FORMULA] radius) emission around 1E 2259+586 reported in Rho & Petre (1997). Although the fit quality is not as good as with the power-law and blackbody model, we cannot exclude the possibility that some, or all, of the residuals below [FORMULA] 1 keV are caused by the contribution of the SNR that lies within the pulsar's extraction region. The 1E 2259+586 spectral fit results are summarized in Table 1. The 2-10 keV luminosity of 1E 2259+586 is [FORMULA]  erg s-1 for a distance of 4 kpc, identical to the value in Corbet et al. (1995).

3.2. 1E 2259+586 pulse timing and phase resolved spectroscopy

The MECS counts were used to determine the 1E 2259+586 pulse period, after correction of their arrival times to the solar system barycenter. The data were divided into 14 time intervals (each with [FORMULA] 2000 counts) and for each interval the relative phase of the pulsations determined. This was performed by folding the counts at half the pulse period value, in order to obtain light curves with a stronger modulation. The phases of the 14 time intervals were then fitted with a linear function giving a best-fit period of [FORMULA]  s. The 1E 2259+586 light curve (Fig. 3) shows a double-peaked profile with the amplitude of the second peak about half that of the main peak.

[FIGURE] Fig. 3. The pulse profile (top panel) of 1E 2259+586 in the 0.5-10 keV energy range. The bottom panel shows the values of [FORMULA]. The values are repeated for clarity

[FIGURE] Fig. 4. The G109.1-1.0 shell spectrum, together with the best-fit single-component NEI model

A set of four phase-resolved spectra of the pulsar were accumulated, approximately coinciding with the peaks and valleys of the pulse profile. These spectra were fit with the same power-law plus blackbody model as used in Sect.  3.1, with N [FORMULA] fixed at the phase-averaged best-fit value. There are insufficient counts to simultaneously constrain both the power-law and blackbody components. Initially, the blackbody spectral parameters were fixed at their phase-averaged best-fit values and only the power-law parameters allowed to vary. The fits were then repeated with the power-law parameters fixed while the blackbody parameters were allowed to vary. With the latter approach, the two fits in the valleys are unacceptable with [FORMULA] 's of 2.9 and 2.3 for 117 dof. This is because the power-law component contributes too much flux, even if the contribution from the blackbody is set to zero.

The best-fit values of [FORMULA] obtained with the first approach are shown in Fig. 3 and reveal a small phase dependence. This variation corresponds to a [FORMULA] of 9.4 for 3 dof with respect to a constant value. Although this is significant at [FORMULA] 99% confidence, we cannot be certain that there are real variations in [FORMULA], since there is no clear correlation with the flux (as might be expected). In addition, small uncertainties in background subtraction, or a contribution from the SNR could cause such an effect, and so we prefer to set a limit of [FORMULA] to any phase-dependent change in [FORMULA].

3.3. The G109.1-1.0 spectrum

In order to accumulate the spectrum of the SNR shell a complex shaped extraction region consisting of a circle, delimited by two straight lines with the pulsar and jet-like X-ray lobe cut out was used (see Fig. 1). Since the spectra of both the shell and the lobe are soft (there is little flux [FORMULA] 2 keV), only LECS data were used. The effective area of the LECS depends on source position within the FOV and the appropriate response matrix was determined by using 11 point sources whose position and relative intensities were chosen to mimic the observed count distribution. Examination of the extracted spectrum shows that the SNR is only detected between 0.25-4.5 keV and data outside this range are excluded.

The spectrum of the SNR shell was fit with the Non-Equilibrium Ionization (NEI) plasma emission model (plus absorption) implemented in V. 1.10 of the SPEX package. Fits were performed with freely varying O, Ne, Mg, Si, S, Fe and Ni abundances. A one-component NEI spectrum satisfactorily describes the spectrum with a [FORMULA] of 65 for 66 dof (see Table 2). The spectrum of the X-ray lobe was separately extracted and analyzed, using the region indicated in Fig. 1. Since the size of the extraction region is comparable to the standard LECS extraction region, no correction for source extent was applied to the response matrix. A background spectrum was obtained from the standard blank field exposures using the same extraction region as for the source spectrum. When the X-ray lobe spectrum is fit with the same NEI model, the best-fit values are in all cases within [FORMULA] of those of the shell spectrum. This is in agreement with the results of Rho & Petre (1997) and supports the view that the two regions are physically related.


Table 2. Spectral parameters for the one-component NEI fit to the SNR shell spectrum. Uncertainties are given at 90% confidence ([FORMULA]). A distance of 4 kpc is assumed. The X-ray lobe has consistent spectral parameters, except for an emission measure ([FORMULA]) of [FORMULA] cm-3

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998