The AllWISE MEP Database is a resource to explore the flux history of the objects found in the Catalog/Reject Table. It is not a catalog in the traditional sense of having one entry per object on the sky. The best way to use the MEP DB is to extract light curves of specific objects found in the Catalog or Reject Table using the unique identifiers source_id_mf, cntr_mf or Catalog/Reject Table equatorial position, rather than to make large area or statistical searches.
The MEP Database contains the Single-exposure flux measurements made on all framesets that were used in AllWISE Multiframe Processing. However, not all Single-exposure framesets were used in AllWISE processing, as described in V.1.b. Framesets that were evaluated to be of degraded quality or contained a large fraction of outlier pixels due to scattered light or other transient events, were omitted. W3 frames from the 3-Band Cryo phase were not used because of their greatly reduced sensitivity and highly variable calibration. The largest set of otherwise good quality measurements excluded from the MEP Database are those in the 73 Tiles near the ecliptic poles for which coverage was attenuated in AllWISE Multiframe processing. An example of this shown in Figure 1 which contains W1 and W2 light curves of a source near the north ecliptic pole that was observed over 5700 times during all WISE survey phases. The MEP Database contains a sparsely sampled light curve of 145 measurements for this object. All of the source extractions made on the Single-exposures during Scan/frame Pipeline processing, regardless of quality, can be found in the individual All-Sky, 3-Band and Post-Cryo Release Single-exposure Source Working Databases. However, please read the Cautionary Notes about using the Single-exposure Working Database photometry.
|Figure 1 - W1 (top) and W2 (bottom) light curves for WISEA J180018.63+663114.0. This source lies very close to the north ecliptic pole and was imaged 5751 times by WISE. The small black dots are the 5520 (quality>0) individual photometric measurements available from the WISE All-Sky, 3-Band Cryo and NEOWISE Post-Cryo Single-exposure Working Databases. The green circles are the 145 Single-exposure photometric measurements available in the MEP Database.|
In cases where Single-exposures in some WISE bands were not utilized in AllWISE Multiframe processing, the flux measurement values in those bands in the MEP Database will be null. This is most common for the W3 and W4 entries from 3-Band Cryo and Post-Cryo Single-exposures, and from 4-Band Cryo framesets that were taken within 2000 sec after the Si:As detectors were annealed.
As described in V.4, objects in the overlap region between Tiles or that are part of small-separation, same-Tile duplicate groups, may have been detected and extracted more than once during AllWISE Multiframe processing. All of the deep source extractions of these duplicate objects will have corresponding sets of photometry in the MEP Database. Because these sources are measured on the same sets of Single-exposures in all cases, the duplicated flux measurements in the MEP Database are redundant. Redundant, duplicate sources were identified and excluded from the AllWISE Catalog, but they remain in the MEP Database. The best way to distinguish between MEP Database entries corresponding to Catalog objects from those in Reject Table is via their unique identifiers, source_id_mf and cntr_mf.
As described in section VIII.3.e.iv of the NEOWISE Post-Cryo Release Explanatory Supplement, the Post-Cryo W1 and W2 profile-fit flux measurements of sources become systematically brighter than those the measurements of the same sources on 4-Band and 3-Band Cryo phase images beginning at the nominal saturation limits of W1<8 and W2<7 mag. This bias is illustrated in Figure 2. The systematic differences between Cryo and Post-Cryo Single-exposure measurements can give a false indication of flux variability. For this reason, the Post-Cryo Single-exposure measurements for sources brighter than the saturation limit are excluded from the flux distribution statistics that are accumulated for the deep source detections in the Catalog and Reject Table (V.3.b.v), and therefore are not used in the computation of the variability flag for those objects. The Post-Cryo flux measurements of saturated W1 and W2 sources are in the MEP Database, though, so use care when analyzing light curves for these sources.
|Figure 2 - The trimmed average differences in 0.25 mag bins between W1 (top panel) and W2 (bottom panel) profile-fit magnitudes of sources within five degrees of the north Galactic pole from the All-Sky Release Catalog and the Post-Cryo Single-exposure Source DB plotted as a function of Single-exposure magnitude. The error bars are the RMS of the trimmed distributions in each magnitude bin. The red horizontal line denotes the zero level. The upturn near W1~8 and W2~7 mag where Post-Cryo measurements become systematically brighter than All-Sky Catalog measurements occurs at the onset of saturation.|
The MEP Database contains the Single-exposure measurements for each source that was detected on the deep, coadded Atlas Images. Because the majority of the deep detections in the AllWISE Catalog and Reject Table are fainter than the Single-exposure sensitivity limits, their entries in the MEP Database will correspond to non-detections. Nonetheless, the instrumental flux measurements and uncertainties are available for the Single-exposure measurements. Note that when a Single-exposure flux measurement has SNR<2, the corresponding Single-exposure magnitude, w?mpro_ep, is computed from the 95% confidence flux upper limit and the magnitude uncertainty, w?sigmpro_ep, is null.
Extremely large profile-fit flux uncertainties, w?sigflux_ep, for some Single-exposure measurement in the MEP Database sometimes occur if the Single-exposure source images are contaminated by cosmic rays, scattered light or image artifacts, or only a small number of unmasked pixels are available for a fit. Be aware of these large uncertainties and exclude the corresponding fluxes in your analyses.
Although Single-exposure framesets of low quality were omitted from AllWISE Multiframe processing, measurements on any individual Single-exposure can still be contaminated or biased by charged particle strikes or scattered light from the moon. Several frameset quality metadata parameters are included in each MEP Database entry that can assist in identifying measurements that may be affected.
The intrinsic scatter in Single-exposure profile-fit photometric measurements can be larger than expected from the nominal brightness of the source for objects that are saturated, extended, blended or confused with nearby objects, or that are affected by image artifacts from bright stars. The elevated scatter in repeated measurements in these cases can give the false appearance of flux variability. Use care when analyzing MEP Database light curves of Catalog/Reject Table entries that have w?sat>0.01, ext_flg>0, nb>>1 or na>0, or cc_flags=[DHOP] in any band.
The ra and dec coordinates in each MEP Database entry are those of the corresponding deep source extraction in the Catalog or Reject Table. Positions are not re-derived for the profile-fit measurements on the individual Single-exposures during the AllWISE Multiframe processing. For sources that are bright enough to have been detected on the Single-exposure images, positions derived from the individual images are available from the All-Sky, 3-Band and Post-Cryo Release Single-exposure Source Working Databases.
Last Updated: 19 November 2013