The INTEGRAL satellite (see Chapt. 3) was designed mainly for the study of galactic hard X-ray sources, and in particular for the detection of transientγ-ray emitting sources.
Therefore, a significant part of the exposure time is spent in the Galactic center and Galac-tic plane regions, where the density of sources is much higher. In parGalac-ticular, the central region of our Galaxy (within 20◦ from the Galactic center) has been observed for more than 10 Ms (∼5 months) during the first 5 years of the mission. The Ophiuchus cluster is located 9 degrees away from the Galactic center, which implies that it has been observed for a very long time since the launch of INTEGRAL. The detection of the source with the IBIS/ISGRI instrument was first reported by Revnivtsev et al. (2004) in a first 2 Ms survey of the Galactic center region, at a flux level of 4.2±0.2 mCrab in the 18-60 keV band. The detection significance of the source was 20σ. The source was then reported in allINTEGRAL source catalogs (e.g. Bird et al. (2007)). We (Eckert et al. (2008), see the full paper in Sect. 5.7) performed a detailed analysis of INTEGRALdata on Ophiuchus.
In this chapter, I will present the results of our analysis of the cluster in the 3-80 keV band with JEM-X and ISGRI.
5.2.1 ISGRI data analysis
The main problem regarding the analysis of the Ophiuchus cluster withINTEGRALis its location, 9 degrees from the Galactic center and 14 degrees from the very bright low-mass X-ray binary Scorpius X-1. Indeed, uncertainties in the exact shape of the IBIS mask, mainly due to the screws which are used to attach the mask to the supporting structure (NOMEX), as well as the intrinsic periodicity of the mask, introduce important artifacts in standard deconvolved ISGRI sky images (the so-called “ghosts”, see Appendix A.2).
Therefore, one must be extremely careful during the analysis to properly take into ac-count the artifacts introduced in the sky images.
For our analysis, we considered only Science Windows (ScWs) for which the Ophiuchus cluster is less than 8 degrees off-axis. Indeed, the intensity of the ghosts increases with the off-axis angle, and the effective area of the instrument decreases, so it is safer to consider only pointings where the source is clearly within the FOV. Therefore, we analyzed 1580 ScWs of ISGRI data where Ophiuchus is at most 8 degrees off-axis, for a total exposure time of slightly more than 3 Ms. To construct mosaic images, we used the mosaicking method described in Appendix A.2, which partially takes the imperfections of the mask into account. The resulting mosaic image in the 20-24 keV band is shown in Fig. 5.2. The source is clearly detected in the mosaic image, with a significance of 16σ. The source is clearly detected in all energy bands between 14 and 40 keV.
For spectral extraction, the standard OSA 7.0 software works in the detector space by fitting shadow patterns (PIFs) to the corrected detector shadowgram, and hence it does not suffer from the systematic effects reported above. However, for a correct spectral extraction, it is necessary to take all sources brighter or similar to the analyzed source in order to avoid contamination from other sources to the spectrum. In the case of the Ophi-uchus cluster, selecting the catalog of sources which should be used for spectral extraction
5.2. INTEGRALobservations of the Ophiuchus cluster 79
Figure 5.2: INTEGRAL/ISGRI mosaic image of the Galactic center region. The Ophi-uchus cluster is shown, as well as a number of bright sources in the same field, including the Galactic center “sausage”, which is a blend of many sources in the direction of the Galactic center, in the bottom left corner of the image.
is a bit tricky. Indeed, the field is very crowded (see Fig. 5.2), and many of the sources in the field are transient, so a number of usually faint sources could become very bright during short time periods, and significantly influence the results of the spectral extraction procedure if they are not taken into account by the fitting procedure. On the other hand, for the stability of the results, it is not recommended to have more than 30 sources in the input source catalog used by the standard OSA spectral extraction procedure. Therefore, to select the sources to include in the input catalog, we used the total mosaic created before, and selected all sources which are brighter or similar to the Ophiuchus cluster. We also searched the complete data set for transient sources which appear as faint sources in the mosaic image, but experienced a strong outburst during a short time period. In total, the final source catalog included the Ophiuchus cluster, plus 18 other sources which must be taken into account for the spectral fitting procedure.
5.2.2 JEM-X data analysis
Although the main goal of the analysis was to search for a hard tail in the spectrum of the cluster, for which ISGRI is clearly the required instrument, it is also important to have a good knowledge of the low-energy part of the spectrum to fix the thermal component as well as possible, and to extend the spectrum to an energy band as broad as possible.
Therefore, we also analyzed data from the JEM-X monitor (see Sect. 3.1.3), which has the necessary sensitivity down to 3 keV. Since the FOV of the JEM-X instrument (4.5 degrees in diameter full-response) is much smaller than that of IBIS, the exposure time on the cluster is much smaller (∼250 ksec), but thanks to the brightness of the source at low energy, and given the very important improvements in the JEM-X imaging software brought by OSA 7.0, it should be detectable for JEM-X. Moreover, the angular resolution of JEM-X (3.8 arcmin FWHM) is three times better than that of ISGRI, so it should be possible to resolve the source spatially.
Unlike the IBIS mask, which was designed to have specific geometrical properties, the JEM-X mask has a random pattern. The advantage of the random pattern is that the shadow pattern it casts on the detector is not periodic, and therefore, reconstructed sky images are not affected by ghosts of strong sources. Moreover, it is now possible to com-bine mosaic images obtained with the two JEM-X instruments for better signal-to-noise.
Consequently, we performed a completely standard analysis of the JEM-X data with the OSA software, version 7.0. We extracted mosaic images of the source in the 3-5, 5-7, 7-10 and 10-18 keV bands in order to study the dependence of the morphology with energy.
The corresponding mosaics are shown in Fig. 5.3. The source is clearly detected (detec-tion significance >10σ) in all energy bands. A comparison with the other source in the FOV (the Cataclysmic Variable V2400 Oph) clearly shows that the source is extended.
No obvious energy dependence of the morphology is seen.
For spectral extraction, we used the mosaic spec tool to extract the spectrum from the mosaic images. Indeed, the standard JEM-X spectral extraction tool fits the data with a point-source model, which results in an under-estimation of the source flux. On the other hand, mosaic spec can fit the extent of the source directly from the mosaic, and therefore