Hard X-ray morphology of the cluster

We processed the full set of data (see Table 4.1) with the Offline Scientific Analysis (OSA) software version 6.0, and created sky images in the 18-30, 30-50 and 50-100 keV bands. It must be noted that the status of the energy calibration in OSA 6.0 was not satisfactory enough below 22 keV, and therefore, it was difficult to extract quantitative information in this band. We chose to use the data below 22 keV anyway for the imaging analysis to ensure a sufficient signal-to-noise in this band. To increase the photon statistics, we combined the resulting sky images to create a single mosaic for each energy band. While the source was clearly detected (at the 11σlevel) in the 18-30 keV band, we were not able to detect the source above 30 keV.

Figure 4.3 shows the OSA 6.0 ISGRI mosaic image of the Coma cluster in the 18-30 keV band with 3−10σ significance contours. The yellow crosses represent the position of the two central cD galaxies, NGC 4874 and NGC 4889, and of the quasar EXO 1256+281, a background object which is quite bright in X-rays. The centroid of the 1-10 keV emission is located close to NGC 4874. A first look at the image indicates that the source is extended for ISGRI (see the image of the point source NGC 4388 in the inset for comparison), and that the morphology of the source is elongated in the North-East/South-West direction.

To compare the hard X-ray morphology of the source with the soft X-ray profile, we an-alyzed existing XMM-Newton data. In June 2000, XMM-Newton (Jansen et al. 2001) performed a mosaic observation of the Coma cluster, in order to cover the largest possible area. We analyzed the corresponding data of the EPIC/PN camera on board XMM-Newton (Str¨uder et al. 2001) with the SAS software version 6.5 to extract images and spectra. The main problem when analyzing extended sources with the standard SAS software is the background determination. Indeed, in the case of point sources, the SAS software requires the input of the user to determine a source-free region in the same FOV, where the background level can be measured. However, in the case of Coma, it is not possible to use this method, since the FOV of the EPIC instrument (27×27 arcmin²) is too small compared to the extension of the source. To alleviate this problem, we used blank-sky pointings as described in Read & Ponman (2003) to estimate the background level. Finally, we created a mosaic from all the different pointings. The resulting mosaic in the 1-10 keV band can be found in the left panel of Fig. 4.4. The lines which cross the

4.3. Analysis ofINTEGRAL data with OSA 6.0 49

Figure 4.3: Standard OSA 6.0 significance image of the Coma cluster with ∼1.1 Ms of data, in the 18-30 keV energy band. Significance contours from 3 to 10σ in steps of 1σ are overlaid in red. The position of the 3 brightest X-ray point sources is also displayed.

For comparison, the inset in the bottom right corner shows a mosaic image of a known point source in the same field, NGC 4388.

center of the image are artifacts due to the gaps between the different modules. Similarly, the excesses which can be seen around the lines are induced by the mosaicking procedure.

The angular resolution of the EPIC instrument (6.6 arcsec FWHM) is much better than that of ISGRI, and therefore it is not possible to compare directly the images extracted with the two instruments. To compare the hard X-ray morphology of the source with the lower energy thermal profile, we smoothed theXMM-Newtonmosaic image and convolved it with a Gaussian of half-width 6 arcmin, which corresponds to the angular resolution of ISGRI. After appropriate re-binning, the resulting image can be directly compared with the hard X-ray image obtained with ISGRI. The right panel of Fig. 4.4 shows the con-volved PN image with 18-30 keV ISGRI contours overlaid. On this image, one can clearly see that the centroid of the hard X-ray emission is displaced towards the South-West (SW) direction compared to the 1-10 keV emission. To make the displacement more obvious, we rebinned the smoothed PN image to the same binning as the ISGRI mosaic, and then subtracted the rebinned PN profile from the ISGRI image. For the subtraction, the PN image was re-normalized such that the difference between the two images would cancel at the peak of the 1-10 keV emission, between NGC 4874 and NGC 4889. Figure 4.5 shows the resulting image, in units of σ. Significant residuals (∼6σ) are found South-West of the cluster core.

To quantify this effect, we fitted the hard X-ray profileI(~r) with several models for the morphology of the source, i.e.

Figure 4.4: Left: XMM-Newton/PN mosaic image of the center of the Coma cluster extracted from the Coma mosaic observation. The lines are artifacts due to the gaps between the modules of the detector. The positions of the two central cD galaxies, NGC 4874 and NGC 4889, as well as the quasar EXO 1256+281 are displayed. Right: Same as previous, but convolved with a two-dimensional Gaussian to match the angular resolution of ISGRI. The green contours represent the ISGRI 18-30 keV brightness contours extracted with OSA version 6.0.

Figure 4.5: ISGRI significance image after the subtraction of the properly renormalized PN image (see text). Significant excess (∼ 6σ) are found SW of the cluster core. The green contours show the 1-10 keV surface brightness profile extracted from the smoothed PN mosaic.

4.3. Analysis ofINTEGRAL data with OSA 6.0 51 1. Model 1: A single point source,

I(~r) =Aexp

i.e. a Gaussian with a standard deviation σ = 4.8 arcmin fixed to the PSF of the instrument and a central position~r₀ left free while fitting;

2. Model 2: A double point source, with the position of the two sources, ~r₁ and~r₂, left free while fitting;

3. Model 3: An ellipse-shaped extended source, given by I(~r) =Aexp

where ~n is the unit vector in the direction of the major axis of the ellipse (given by the ellipse inclination angleθ),~r₃ is the position of the centroid of the ellipse, andσ₁, σ₂ are the standard deviations of the major, respectively minor axes of the ellipse. All these parameters are left free while fitting.

4. Model 4: A superposition of an extended source with the morphology of the core of the Coma cluster in the 1-10 keV band, plus an additional point source. The 1-10 keV surface brightness profile is described by a Gaussian convolved with the ISGRI PSF, i.e. a Gaussian with a half-width of 10 arcmin,

I(~r) =Aexp the position of the additional point source which is left free while fitting, andσ is fixed to the half-width of the PSF in the same way as in model 1.

Fitting the ISGRI profile with a single point source (Model 1), we find that the data are poorly described by this model. In this case, we find that the centroid of the source is displaced compared to the centroid of the 1-10 keV emission by 4.5 arcmin, which confirms the visual impression from the right panel of Fig. 4.4. The best fit is achieved with an ellipse-shaped extended source (Model 3), whereσ₁ = 16.8^′±0.5^′ and σ₂ = 11.7^′±0.4^′. The ellipse inclination angle is θ = 61±4^◦. It is interesting to note that the θ angle corresponds roughly to the direction towards the falling sub-cluster around NGC 4839.

Acceptable fits are also achieved with Models 2 and 4. In the case of Model 4, the additional source is found at RA=194.71±0.01 and DEC=27.87±0.01, which is located 6.1 arcmin away from the nearest X-ray point source, the quasar EXO 1256+281. Therefore, association of the SW excess with this objected can be rejected at the 90% level.