Brandt, Søren

Chenevez, Jérôme

Lund, Niels

Westergaard, Niels Jørgen Stenfeldt

A new imaging method has been developed for JEM-X. The flux from each sky pixel is obtained from a fit to the observed shadowgram rather than from a back projected image. The fitting method is more direct than the standard back projection method used in the public OSA software and allows better possibilities for elimination of systematic image artifacts. An improvement of more than a factor two for the signal-to-noise of weak sources in mosaic images has been obtainedat low energies near strong sources

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Improved JEM-X imagingLund, Niels; Westergaard, Niels Jørgen Stenfeldt; Chenevez, Jérôme; Brandt, SørenPublished in:P o S - Proceedings of SciencePublication date:2010Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Lund, N., Westergaard, N. J. S., Chenevez, J., & Brandt, S. (2010). Improved JEM-X imaging. P o S -Proceedings of Science.PoS(INTEGRAL 2010)070Improved JEM-X imagingNiels Lund∗, Niels J. Westergaard, Jérôme Chenevez and Søren BrandtDTU Space - Juliane Maries Vej 30, DK-2100 Copenhagen Ø, DenmarkE-mail: nl@space.dtu.dk, njw@space.dtu.dk, jerome@space.dtu.dk,sb@space.dtu.dkA new imaging method has been developed for JEM-X. The flux from each sky pixel is obtainedfrom a fit to the observed shadowgram rather than from a back projected image. The fittingmethod is more direct than the standard back projection method used in the public OSA softwareand allows better possibilities for elimination of systematic image artifacts. An improvement ofmore than a factor two for the signal-to-noise of weak sources in mosaic images has been obtainedat low energies near strong sources.8th INTEGRAL Workshop "The Restless Gamma-ray Universe"- Integral2010,September 27-30, 2010Dublin Ireland∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(INTEGRAL 2010)070Improved JEM-X imaging Niels Lund1. IntroductionAs the INTEGRAL mission [1] progresses more and more archieval data become available forsystematic surveys and time variability studies. However, systematic noise features are present inthe JEM-X [2] images and becomes increasingly visible as more data are combined into mosaicimages. This noise is currently the limiting factor for the ultimate instrument sensitivity. The sameproblem has been recognized already for several years in the analysis of ISGRI data [3], and mucheffort has been invested by the ISGRI team in suppressing these effects. We are now addressingthis issue in the JEM-X analysis.2. JEM-X image reconstructionThe public analysis software for JEM-X data available through the OSA software [4] providesthe j_ima_iros–tool for the generation of sky images based on data from single science windows.It also provides the j_ima_mosaic–tool [5] for combining multiple such sky images into a singleimage covering a larger sky field and/or increasing the effective observation time associated with agiven sky field.j_ima_iros uses a back projection technique to generate the sky image for each science win-dow. The images constructed with this technique contains a number of known systematic noisefeatures for which we have already developed correction algorithms. Statistical noise then do-minates the resulting single science window images. However, when combining hundreds of suchimages into a mosaic image, additional systematic features emerge for which we did not find effec-tive cures in the past.All the analyses presented in this paper are based on data from 1800 science windows coveringthe sky field around the strong source GRS 1915+105. The peak effictive exposure time is about700 ks. Figure 1 is a 13o diameter mosaic centered on GRS 1915+105 and analyzed with the currentOSA software. The color scales used for this and the subsequent figures are individually normalizedto half the peak level for the weak source IGR J19140+0950 (For source identifications see Figure5). With this choice for the color scales the noise levels in each figure are directly comparablerelative to the weak sources in the field.We expected that a better image reconstruction with less systematic noise could be obtainedthrough the use of a fitting procedure for the flux from each sky pixel. The j_ima_iros tool alreadyuses such a fitting procedure to determine source fluxes in the ’iterative removal of sources’–loop.The fitting functions used are ’pixel illumination functions’ (PIF’s) for the sources found in theback projected images. Much more instrument description detail can be incorporated in the PIF’sthan in the back projection algorithm. We have also found that the source fluxes obtained throughfitting are more stable than those derived from an analysis of the images.Calculating a PIF value for every sky pixel is much more time consuming than running a backprojection. But with the new image generation process we can facilitate the subsequent mosaickingoperations by using a fixed set of sky pixels for all science windows. Then the mosaicking step willbe quite fast. The old back projected images are generated in coordinates fixed to the instrumentand the mosaicking of many such images is a relatively slow process - and also causes some loss of2PoS(INTEGRAL 2010)070Improved JEM-X imaging Niels Lundresolution and consequently a reduction in the final sensitivity achievable with a given input dataset.Figure 1: Back projection image mosaic13o diameter mosaic centered on GRS 1915Figure 2: Fitted image mosaic3. Analysis of the new JEM-X imagesFigure 2 shows a mosaic of the GRS 1915 region based on the new, fitted images. The noiselevel is reduced relative to that of Figure 1, but not by a dramatic amount: the signal–to–noise (S/N)ratios for the weak sources in the field is improved by about 20%.Some of the remaining noise is just the unavoidable Poisson noise caused by the high photonflux from GRS 1915. But there are also clear systematic noise features such as the circular ridgeswhich are conspicuous in both figures.4. Selective image generationClose inspection of the noise features in Figure 2 convinced us that in addition to the obvious,but relatively harmless, ring structures there were other systematic artifacts which were more trou-blesome, because they could easily be mistaken for weak sources. These, apparently, are caused byremaining correlations between the strong source and the image point in question. Nominally thecorrect application of the PIF’s should eliminate such correlations, but obviously the reality doesnot conform to our models here.We have therefore added a further condition to the image generation process: namely eli-mination of all shadowgram pixels where the total of contribution from all sources detected byj_ima_iros is predicted to be more than 50 % of the background signal in the pixel. The sourceand background contribution for the pixels are estimated based on the PIF values and the fittedsource strengths as evaluated by j_ima_iros. In other words, we eliminate pixels which are well3PoS(INTEGRAL 2010)070Improved JEM-X imaging Niels Lundilluminated by known, strong sources. We call this imaging method "selective imaging". It is herean advantage that the JEM-X masks are only 25% transparent, meaning that at most a quarter ofthe shadowgram pixels are affected by any given source.As shown in Figure 3 the selective imaging process turns out to be quite efficient for reducingnoise. The weak source S/N-ratios in Figure 3 are improved by about 60% relative to those ofFigure 2.Figure 3: PIF selected image mosaic Figure 4: Ring cleaned mosaicFinally we have implemented a smoothing of the ring features in the mosaic. This is done as asimple background subtraction along circular segments centered on GRS 1915. Figure 4 shows thefinal result. We see a further 15% improvement of the S/N ratios for the weak sources.This last process can be said to be cosmetic only, since it is done on the image itself and notat the shadowgram level. Therefore it will not suppress point like artifacts in the mosaics, but itGRS 1915+105IGR J19140+0951AX J1911.0+090H 1907+097Figure 5: Elimination of shadowgram pixels with strong source illumination suppresses artifacts and andimproves the visibility of real sources. Left: All pixel image, right: PIF selected and ring cleaned image4PoS(INTEGRAL 2010)070Improved JEM-X imaging Niels LundSignal-to-noise improvement relative to current OSA mosaicsEnergy range Back projection Fitted image Selective Ring(keV) image mosaic mosaic imaging cleaning3 to 7 1.00 1.20 1.85 2.107 to 11 1.00 1.08 1.52 1.6311 to 20 1.00 1.10 1.34 1.4020 to 35 1.00 1.01 1.09 1.04Table 1: Analysis based on JEM-X1 data from 1800 science windows (revolution 170-735).The improvement factors are calculated for the source IGR J19140+0951 relative to the averageimage noise in a 13o diameter region centered on GRS 1915+105.does allow weak sources (and point like artifacts) to stand out more clearly. Figure 5 illustratesthe improvement in the quality of the mosaic images by comparing a zoom of Figure 2 with thecorresponding region in Figure 4.5. ConclusionsNew methods have been developed for noise reduction in mosaic images based on data fromthe JEM-X instruments. A significant reduction in the image noise has been achieved. The im-provement is most significant for energies below 10 keV where the count rate from bright sourcesoften dominates over the background count rate. Table 1 shows the S/N improvements achieved infour energy bands.The improved methods will be used in the future for detection and spectral characterization offaint sources. As far as possible the new techniques will be implemented in future OSA releases.References[1] C. Winkler et al., “The INTEGRAL Mission,” A&A 411, pp. L1–L6, Nov. 2003.[2] N. Lund et al., “JEM-X: The X-ray monitor aboard INTEGRAL,” A&A 411, pp. L231–L238, Nov.2003.[3] P. Ubertini et al., “IBIS: The Imager on-board INTEGRAL,” A&A 411, pp. L131–L139, Nov. 2003.[4] T. Courvoisier et al., “The INTEGRAL Science Data Centre (ISDC),” A&A 411, pp. L53–L58, Nov.2003.[5] J. Chenevez et al., “Improved Mosaicking Capabilities with JEM-X,” ESA-SP 522, pp. 837–840, 2004.5

Improved JEM-X imaging

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Improved JEM-X imaging

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