AIP/XMM/WFI Release (#1)

April 2009

Technical Summary

CURRENT RELEASE
Survey ..................... XMM-Newton fields
Telescope .................. ESO/MPG 2.2m
Instrument ................. WFI
Program IDs ................ 68.A-0473; 69.A-0615; plus four programmes in Max-Planck time
Regions..................... Selected XMM-Newton Serendipitous Survey
Number of Fields ........... 79 (11 not genuine XMM)
Passbands .................. B; V; R; I; Z (variable per field)
Release Date ............... April 2009
Release prepared by ........ AIP team: A. Schwope, A. Staude, R. Schwarz, M. Müller, A. Kolodzig, A. Rabitz

PRODUCTS
Product Type ............... Catalogs and stacked Images
Number of catalogs/images .. 162

Abstract

Recognizing that the large light collecting power of the XMM-Newton satellite uncovers X-ray sources mainly beyond the limits of existing optical surveys, a dedicated optical imaging program was initiated by the XMM-Newton SSC (Survey Science Centre). Part of that program was performed as a public imaging survey by the ESO EIS-team. Data were obtained on 12 XMM fields in filters BVRI and released via the ESO web-pages . A complementary survey on partially the same fields with deeper exposures in B and R filters was performed by a group of scientiest from Bonn University (Schneider, Dietrich). The data were used to construct catalogues of clusters of galaxies which were published in Dietrich et al. (2007), A&A 821, 834. The images and catalogues of this survey are released in an accompanying release. A more comprehensive survey covering additional fields was performed by the AIP-node of the SSC, again with the ESO/MPG 2.2m telescope. The data obtained during this latter program from the body of the current release and were provided by either AIP-observers in visitor mode or ESO staff in service mode. Data were provided by ESO programmes 68.A-0473 and 69.A-0615 (PI A. Schwope) and by four additional programmes performed in Max-Planck time (PIs Hasinger & Schwope).

The present release consists of single-passband (SPB) images and object catalogs extracted from ``final'' stacked images produced for all AIP-observed XMM fields. The list comprises fields at high and low galactic latitudes; it covers essentially all RA and declinations below +17 degrees. The catalogs were extracted using the SExtractor program (Bertin & Arnouts 1996) using the same parameter files as used for the XMM-EIS data release.

For more information about the terminology and conventions used in this document refer to the WEB README pages.

Contents of this Release and retrieving the data products

This is the first release of reduced data for the AIP-led XMM-Newton follow-up survey. The release consists of 162 images obtained in 79 fields. The catalogs were extracted using SExtractor from fully calibrated ESO/MPG 2.2m WFI stacked images in B-, V-, R-, I-, and Z-passbands. The filter coverage is inhomogeneous, some fields were observed through just one filter, others through the whole suite of filters. The choice of filters results from the need to just generate a finding chart deeper than the DSS in some cases to colour-based selction in other cases, optimized/truncated by the available telescope time when running the programme in visitor mode.

The list of observed fields actually contains targets which were no XMM-Newton targets. These (images obtained in the Ic-band of GSC stars) were retrieved from the ESO archive and pipeline reduced with the aim to achieve a better correction of fringing pattern in proper XMM-Newton fields. Once in the pipeline these were also included in the final database.

The stacked images, the weight maps, and the catalogues can be retrieved via this web page.

This long index gives row-by-row information per individual stacked image. It is sorted in alphabetical order, re-sorting (in ascending or descending order) can be achieved by clicking on any of the column labels. These are centroid positions of the fields (in equatorial coordinates), the galactic coordinates, the filter used, the date of the first frame obtained in that field and used for the final stack, the number of frames that entered the final stack, the time span between the mid-exposure times of the first and the last frame that entered the final stack, the total exposure time in the final stack, the average seeing, the sky brightness, the file name of the stacked image, and the data of pipeline reduction.

Object names were those read from the original file headers and are typically associated with the original target name of the XMM-Newton observation.

The compact index contains similar information listing all filters per target field. Once again, sorting in ascending/descending order is possible for object name, equatorial or galactic coordinates and date of observation (the epoch of the first frame of the first filter listed for the given field).

Each of the pages linked to the Index or the Compact Index has a third link labeled Standard which informs about photometric solutions per day and filter. Different solutions with different levels of complexity were automatically generated by the pipeline scripts, the one applied to the data was chosen manually after visual inspection of the solutions. There were nights without any standard star observation, others with too few standard star observations. In all those cases a default photometric solution determined by the instrument team was used for catalog creation. Each date on the standard star page is linked to a page visualizing the different solutions. The one-parameter solution re-determined the photometric zeropoint (labeled 'Nullpunkt (1.P)'), the two-parameter solution determined the zeropoint and the colour term (Nullpunkt (2.P), and Farbterm (2.P)), the three-parameter solution determined zeropoint, colour term and extinction coefficient. The default solution is always listed.

Information per stacked image

The 'Index' and 'Compact Index' are linked to pages describing the stacked image per field and filter in more detail. Upon clicking on one of the filter identifiers a page is opened, which gives in the top section a list of all the accessible fields via their XMM-names and below an image of the field in png-format. When clicking on the image (which is essentially a 'thumb-nail image' the full png-image is displayed. These images were created with very small dynamic range around the background level to make immediately visible any imperfection of the image. A grey box to the right of the image gives more information on the content of the stacked image and gives access to the data products:

1. Field identifier (typically the name of the original target identifier of the XMM-Newton observations);

2. Equatorial coordinates of that field;

3. Galactic coordinates of that field;

4. Links to the stacked image, the corresponding weight image, and the SExtractor source catalog (all in FITS-format);

5. Available images/filters for that field, the currently displayed is highlighted with a boldface black letter;

6. A date for the stacked image (mean epoch of the individual contributing frames);

7. Total integration time of the stacked image from which the catalog was created;

8. Time span (time difference between first and last contributing observation;

9. Number of contributing frames;

10. Average seeing in arcsec (arithmetic mean of seeing measured in contributing images);

11. Average background level (in counts per sec per pixel, arithmetic mean of contributing images);

12. Average airmass (arithmetic mean of airmass of contributing images);

13. A list with values for seeing, sky background, airmass and exposure time of the contributing frames. The last column informs about the photometric solution applied to the individual contributing frame before stacking. The leading digit encodes the type of photometric solution used (0 - default solution determined by the instrument team; 1,2,3 - one/two/three parametric fits to standard star observations obtained in that night), the following number gives the photometric zero point for the chosen solution. Hyperlinks to graphs of the photometric solutions are given per frame.

14. A list of contributing frames, the name encoding the observation dates.

15. Grade (A to D, from good to bad) assigned to the catalog. Not used at the moment. Grades for images and catalogues are tabulated here.

Data Reduction

This release represents a set of images and catalogs extracted from WFI images and created systematically using the AIP-adapted version of the GaBoDS wide-field imaging reduction pipeline described in detail in Erben et al.(2005). The system worked largely in an un-supervised manner with two exceptions, (1) masking of residual satellite tracks or similar defects on individual contributing images before stacking, and (2) choice of the most appropriate photometric solution for the individual contributing frame before stacking. The input images for the catalog creation are final stacked images produced from the nightly reduced images.

Catalogs were extracted using the SExtractor program and a common configuration file, with the option of using the weight-map associated to each image.

Magnitudes are given in the Vega system and were not corrected for galactic extinction.

Astrometry

The accuracy of the astrometric calibration relies on the astrometric calibration of the reduced images and the reference catalog used. Since we are following a very similar approach as the ESO-EIS team the figure described in the corresponding release of EIS-XMM images, applies here too, astrometry is typically better than 250 mas.

Photometry

The reduced images were calibrated to the Vega magnitude system based on observations of Landolt (1992) standard stars as described in ESO/EIS release number 19. Based on these photometric calibrations the photometric zeropoints of the stacked images were derived, as described in the ESO/EIS release number 22 and in Dietrich et al. (2006), where more details concerning the accuracy of the photometric calibration can be found.

General Features

1. The production of catalogs was done completely un-supervised. The XMM selected fields include a broad range of galactic and extragalactic fields of varying density of objects, bright galaxies and stars. In this sense this survey is a useful benchmark for the problems one must deal in extracting science grade catalogs, making it difficult to have a unique set of parameters to cope to the large variety of situations encountered. Finding compromises is the main challenge to enable the automatic, un-supervised generation of catalogs.

2. The inspection of the projected distribution of objects, strongly suggests that the automatic masking of satellite tracks has worked remarkably well as no prominent linear features, a signature of this type of problem, are seen on the inspected catalogs.

3. One of the main criteria to assess the quality of a catalog is to minimize the number of false positives. The parameters of extraction adopted in the production of the catalogs being released, in general, work well. However, there are certain unavoidable situations, among which: 1) the presence of ghost images near bright stars. Their location and size vary with position and magnitude making it difficult to deal with in an automatic way; 2) the presence of bright galaxies since the algorithm for automatic masking does not work well in this case; 3) residual fringing on the image; 4) the presence of stray light, in particular, associated with bright objects just outside the observed field.

4. A number of tests have been made for the EIS-XMM data release to find an adequate compromise for the scaling factor used in the calculation of the size of the automatic masks. The parameters found there were also applied here.

5. The criteria adopted in the procedure used to define the ``trimmed'' region sometimes overshoots because of the nature of the weight map at the corners and because in some cases the image is slightly tilted. This has as a consequence that large number of spurious objects at the corners of the image are not flagged appropriately.

Data Quality Assessment

Grade

Images and catalogs overlayed on the images were examined by eye and graded by the AIP team, with the grade range being from A (best) to D (worst). The grade is meant as a subjective indication of the quality of the image and the catalog derived from it. It summarizes information on fringing, mosaicing, blending, and catalogue incompleteness which may occur for various reasons.

Photometric Calibration

The quality of the photometric calibration of the reduced and stacked images have been assessed for the XMM-EIS survey in the papers by Dietrich et al. (2006, 2007). The problems described there are even more severe for this current release with sometimes not well-defined or unknown observing conditions, lack of sufficient standard star observations, likely different observing conditions for standard star and science observation etc. Attempts were made to control and document those uncertainties by correlating subsets of sources drawn from the WFI-catalogues with external reference catalogues (XMM-OM with B and V filters, catalogues from digitized POSS and UKST images as made available by the SuperCOSMOS Sky Survey). Aperture magnitudes (through apertures of size three times the mean seeing value of the image) of point-like objects in a mid brightness range were compared, outliers were clipped and a mean offset between both catalogues was determined. The results are listed in a table and hyperlinked to graphs showing the offset magnitudes and the regression.
Offset magnitudes in the range -2.1 up to +3.5 were found, which in cases might be resulting from problematic photometry of the reference catalogue (CenB R-filter, AB Dor I-filter, CF Tuc R-filter, NGC5846 B-filter). Users are strongly discouraged to blindly apply the mean offset to measured magnitudes without inspecting the graph of the offset magnitudes. The number of useful stars varied strongly from field to field and is strongly dependent on the reference (OM: few stars, but reliable photometry). The error given for the offset magnitude is the sigma of the distribution and not the error of the mean.

References