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II. AllWISE Source Catalog and Reject Table

II.1. General Properties

The AllWISE Source Catalog contains accurate positions, motion measurements, photometry and ancillary information for 747,634,026 objects that were detected on the deep, coadded AllWISE Atlas Images. Once detected, sources positions and fluxes were measured by fitting PSF templates simultaneously to the "stack" of all Single-exposure images in all WISE bands that cover their locations. Aperture flux measurements made on the coadded images are also provided for each source.

The AllWISE Reject Table contains 428,787,253 position, flux and motion measurements of the deep source detections made on the Atlas images that but that do not meet the criteria for inclusion in Catalog. Reject Table sources may be duplicate, redundant extractions of real objects already in the AllWISE Source Catalog, faint sources below the SNR limits for the Catalog, or spurious detections of image artifacts or residual transient events.

The AllWISE Source Catalog is a significant improvement over the WISE All-Sky Release Catalog.

The WISE All-Sky Release Catalog may provide better photometry than in the AllWISE Catalog for objects brighter than the nominal W1 and W2 saturation limits (W1<8 and W2<7 mag) that were observed during the NEOWISE Post-Cryo survey phase. These saturated sources have systematically larger photometric uncertainties in the AllWISE Catalog because the measurement of saturated sources in the Post-Cryo images is not as well characterized as it was in the 4-Band Cryo data.

This section provides a high-level description of the format, information content and properties of the AllWISE Source Catalog. More detailed discussions of the photometric and astrometric properties of the Catalog are given in sections II.3 and II.5, respectively. The Catalog completeness and reliability are described in II.4.a and II.4.b, respectively. The quality and interpretation of apparent motion measurements are discussed in II.6 and in Kirkpatrick et al. (2014). The AllWISE Source Catalog is a highly reliable and complete representation of compact sources of mid-infrared radiation over the entire sky. But as with any very large Catalog, there known features and issues of which you should be aware. Please read the Cautionary Notes to make the best use of the AllWISE Source Catalog in your research.

II.1.a Format and Column Descriptions

II.1.b. Catalog Selection Criteria - The Difference Between the AllWISE Source Catalog and Reject Table

AllWISE data processing generates a Working Database (WDB) of 1,176,421,279 deep source extractions detected on the coadded Atlas Images. The full AllWISE WDB contains extractions of real astrophysical objects, as well as duplicate, redundant extractions of objects that fall within the Tile overlap regions or are cases of small-separation, same Tile (SSST) sources, and spurious detections of image artifacts from bright sources, faint noise excursions, and residual transient events not suppressed during image coaddition.

The AllWISE Source Catalog is the subset of entries in the WDB that are single, unique detections for each object, that lie farther than 50 arcsec from a Tile edge, and that satisfy the following reliability criteria in at least one band:

There are some special exceptions to these criteria to compensate for the effect of W1 saturation during the early part of the 3-Band Cryo survey. These exceptions and all of the AllWISE Catalog source selection criteria are described in detail in V.4.

747,634,026 WDB entries satisfy these criteria, and constitute the AllWISE Source Catalog. The 428,787,253 WDB entries that do not meet these criteria are contained in the AllWISE Reject Table.

II.1.c. Catalog Statistics

II.1.d. Photometric Information

II.1.d.i Different Photometric Measurements in the Source Catalog

Several different sets of calibrated PSF profile-fit and aperture source flux measurements in the four WISE bands are provided for each deep detection in the AllWISE Source Catalog and Reject Table. All include magnitudes and magnitude uncertainties, along with other information specific to the type of photometry. A brief description of the different photometry algorithms, their associated parameters, and when each is appropriate are listed below.

Note that for all types of photometry, if the flux measurement has SNR<2; the magnitude quoted is derived from the 95% confidence flux upper limit, and the magnitude uncertainty will be null. The one exception to this is for the small number of sources severely affected by W1 saturation in the early 3-Band Cryo observations. These objects, identified in the Catalog by having a photometric quality flag value in W1 of 'Z' (ph_qual[1]='Z'), usually have valid W1 Deep Detection profile-fit magnitude that is not an upper limit, but have w1sigmpro="null".

Comparisons between AllWISE Catalog deep detection profile-fit magnitudes and the average Single-exposure profile-fit and standard aperture magnitudes are shown in Figures 9a-d. The Deep Detection and Single-exposure profile-fit measurements generally agree well over the full brightness range well, with the exception of the "glitch" that appears in the W1 and W2 deep detection photometry in the range 6.5<W1<8 mag and 5.5<W2<7 mag. The Deep Detection photometry briefly becomes ~10% brighter than the Single-exposure profile-fit measurements in this range. This discrepancy may be related to how the specific sections of PSF wings changed during the Post-Cryo survey phase. The Standard Aperture photometry increasingly underestimate the flux of bright sources brightward of the nominal saturation levels in the fours bands. Aperture measurements also become systematically brighter than the profile-fit measurements towards fainter magnitudes because of contamination of nearby sources within the measurement apertures.

Figure 9a - W1 Figure 9b - W2 Figure 9c - W3 Figure 9d - W4
Differences between AllWISE Catalog Deep-detection profile-fit magnitudes and Average Single-exposure profile-fit (top panels) and Standard Aperture magnitudes (bottom panels), plotted as a function of Deep-detection profile-fit magnitude for sources around the north Galactic pole. Black dots are individual sources. The solid green lines are the trimmed average differences in 0.2 mag wide bins, and dashed green lines are the ±1 RMS difference contours.

II.1.d.ii Photometric Properties

Characterizations of the photometric sensitivity and accuracy of the AllWISE Source Catalog are presented in II.3.a and II.3.b.

In regions that are not confusion limited, AllWISE achieves SNR=5 flux at 54, 71, 730 and 5000 mJy (16.9, 16.0, 11.5 and 8.0 mag) in W1, W2, W3 and W4, respectively. The W1 and W2 sensitivity is improved relative to that in the All-Sky Release Catalog. The W3 and W4 sensitivity is also slightly improved relative to that in the All-Sky Catalog because of the Single-exposure recalibration and improved local background estimation algorithms used for AllWISE data processing.

The differences between AllWISE Catalog and WISE All-Sky Release Catalog profile-fit photometry for the same objects are shown plotted as a function of AllWISE magnitude in Figures 10a-d. These are the for sources in the same Ecliptic plane field that is illustrated in Figures 6a-d.

AllWISE photometry is increasingly brighter than All-Sky Release Catalog photometry for W1<14, W2<13, W3&;lt;8 and W4<6 mag. because the faint source flux underestimation bias that affected the All-Sky Release Catalog photometry has been greatly reduced in AllWISE. This improvement results from the recalibration of the Single-exposure image pixels done prior to AllWISE Multiframe processing. AllWISE photometry of faint sources is now much more consistent with that measured by Spitzer.

W1 W2
W3 W4
Figures 10a-d - The difference between AllWISE Catalog and WISE All-Sky Release Catalog profile-fit photometry as a function of AllWISE magnitude for the same ecliptic plane field shown in Figures 6a-d. The small black dots are individual sources. The larger green points and error bars are the trimmed average and RMS photometric residuals in 0.2 magnitude wide bins (W1-W3) and 0.5 magnitude wide bins in W4.

II.1.d.iii. Flux Variability

AllWISE Source Catalog and Reject Table entries that have a statistically significant probability that their their Single-exposure flux distributions varied over the course of the WISE observations are identified by the var_flg. Derivation of the var_flg is described in Var_flg values ≥5 in any band indicate probable flux variability.

In addition to the var_flg, each AllWISE source record includes a number of statistics derived from the Single-exposure profile-fit flux distributions that are useful when classifying variables sources. These statistics are in the w?k (Stetson K), w?mlq (~100 × Stetson L), w?ndf, rho12, rho23, rho34, q12, q23, and q34 columns.

Single-exposure profile-fit photometry light curves for every object in the AllWISE deep detection Source Catalog and Reject Table are available in the AllWISE Multiepoch Photometry Database.

II.1.e. Astrometric Information

II.1.e.i. Positions in the Source Catalog and Reject Table

J2000 equatorial source positions are provided for each entry in the AllWISE Source Catalog and Reject Table. Position reconstruction was done using bright 2MASS point sources as the astrometric reference. For AllWISE, the proper motion of the reference stars in the 11 years separating the WISE and 2MASS surveys has been integrated into the solutions to improve the absolute astrometric accuracy of the Catalog and Atlas (see V.2.b).

Two equatorial positions and associated uncertainties are provided for each source. A brief description of the different positions is given below, along with suggestions for their use.

II.1.e.ii. Astrometric Accuracy

The astrometric accuracy of the AllWISE Source Catalog is described in II.5. AllWISE position reconstruction is improved relative to that in the All-Sky Release Catalog because proper motions of the 2MASS astrometric reference stars has been taken into account, and the relative positions of sources detected many times in overlapping Single-exposure images have been used to rigidize the astrometric frame.

The positions of non-saturated, high SNR AllWISE Catalog sources (8<W1<12 mag) in non-confused region are accurate to ~50 mas radially with respect to those of the quasars that define the ICRF. AllWISE position accuracy gradually decreases with decreasing source brightness.

II.1.e.iii. Apparent Motions

A major new feature of the AllWISE Source Catalog is the measurement of apparent motion for nearly every deep detection. The motion measurements are the result of having added small, linear motion to the profile-fit source extraction measurement model. The motion-fit measurements and uncertainties are given in pmra, pmdec, sigpmra, sigpmdec.

Despite the "pm" prefix, the AllWISE motions do not distinguish between proper motion and parallax. They are derived from total apparent motion over the 6 or 12 months spanned by WISE and NEOWISE observations, and can be influenced significantly by parallax for nearby objects. Sensitivity to motion is a function of source brightness, the number of independent Single-exposure observations covering and source, and the total epoch spanned by the observations. AllWISE motion measurements may be corrupted by source confusion, contamination by image artifacts, extended emission and noise.

Detailed discussions of how to interpret AllWISE measurements, the accuracy and limits of the motion measurements, and examples of true and spurious motion measurements are given in II.6 and in Kirkpatrick et al. (2014). For non-saturated, high SNR point sources with two epochs of coverage, AllWISE measures motions down to ~0.2 arcsec/year per coordinate axis, as illustrated in Figure 11. Motion measurement uncertainties increase with decreasing source brightness. The motion of Barnard's star, the object with the largest known proper motion (10.4 arcsec/year), is recovered with reasonable accuracy despite the fact that it heavily saturated in the WISE images, and has significant parallax. AllWISE can in principle detect larger motions, but no high confidence detection of a non-Solar System object moving faster than Barnard's star has been found yet.

Figure 11 - AllWISE Catalog RA (top) and Dec (bottom) motions measured for sources in Atlas Tile 1914p545_ac51. Black points are individual sources and green dots are the trimmed average motions in 0.2 mag wide bins. The green lines show contours connecting the 5*RMS value in each bin. Because most sources have zero motion measurable by WISE, the green lines give a reasonable estimate of the minimum measurable motion as a function of magnitude for this Tile. The red point is the measured motion of LHS 342, and the red star represents the actual motion reported by Monet et al. (1992).

II.1.f. Additional Information

II.1.f.1. 2MASS PSC and XSC Associations

Entries in the AllWISE Source Catalog and Reject Table have been positionally cross-correlated with the 2MASS Point and Extended Source Catalogs (PSC and XSC).

A search radius of 3 arcsec was used for the AllWISE/2MASS PSC correlation. If more than one 2MASS PSC object was found within 3 arcsec, the closest association was recorded. No correction was made for proper motion between the epochs of 2MASS and WISE. AllWISE Catalog and Reject Table source records include the tmass_key, which is the unique identifier of the associated 2MASS PSC source, n_2mass, which is the number of 2MASS PSC sources found within 3 arcsec of the WISE position, the associated 2MASS source J, H and Ks photometry and uncertainties, and the amplitude and position angle of the vector from the WISE source to the 2MASS source, in degrees East of North, r_2mass and pa_2mass.

Associations between AllWISE Catalog and Reject Table entries and the 2MASS XSC were searched using a matching radius equal to 1.1 times the Ks isophotal radius size of the 2MASS XSC source. This will identify WISE extractions that are both identically the 2MASS XSC source, as well as some that are fragments of or foreground objects superimposed on the extended object. The separation between the AllWISE and 2MASS XSC positions is given by xscprox. Classification of AllWISE-2MASS XSC associates is given by the ext_flg.

2MASS source information included in the AllWISE source records is an association not an identification. Although the position accuracies of the two Catalogs are excellent, there is a non-zero probability of chance associations between physically unrelated objects, as well as missed associations between the Catalogs. You should always confirm associations by reviewing the entries from both Catalogs.

Last Updated: 7 October 2015

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