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IRAS_SSC: IRAS Serendipitious Survey Catalogue


Table name: 	IRAS_SSC
Records: 	43886
Spectral Band: 	IRband
Kind of Object: IRASLog
Mode: 		Survey
Coordinates: 	B1950, J2000



In 1983 the sky was surveyed by the Infrared Astronomical Satellite (IRAS) in four broad spectral bands centered at 12, 25, 60 and 100 (mu)m. During the course of the 300 day mission, about two-thirds of the satellite time was used to carry out an unbiased sky survey wich led to the production of the IRAS Point Source Catalogue (IRAS/PSC) containing 245889 sources. That catalogue, along with many other aspects of the mission, are described in the IRAS Catalogues and Atlases Explanatory Supplement (1985). A portion of the remaining mission was used to conduct a series of Pointed Observations, usually directed at objects of interest, and using the various capabilities of the satellite. Many of these Pointed Observations were performed by repeatedly scanning the 0.5 deg. wide survey detector array over the selected object. These scans were combined to produce a sensitive map of area about one deg.x deg. around the selected source (or, in some cases, a targeted "blanck" field). By spatially coadding the repeated scans comprising each Pointed Observation a large data base was generated. Young et al.(1985) have described these data, the Pointed Observation program and the associated data processing at the Infrared Processing and Analysis Center (IPAC).

The IRAS Serendipitious Survey Catalogue (IRAS/SSC) has been constructed using the fortuitous observations at 12, 25, 60 and 100 (mu)m of 43886 point-like sources that happened to lie in 1813 of the individual fields included in the Pointed Observation program. Because the Pointed Observations resulted in longer integration times for each source in the field than was possible in the all-sky survey mode, the limiting sensivity for the IRAS/SSC is typically better than that of the IRAS/PSC by a factor of about 4. Also, the photometric accuracy, especially for faint sources, is improved relative to that of the IRAS/PSC. The amount of sky sampled in the SSC is nearly 1400 deg.x deg. but because of uneven sensitivity across the Pointed Observation fields, the effective sky coverage is 1108 deg.x deg.. Excluding certain areas of great scientific interest to the team who planned the observations (spacifically, the Galactic plane and the Magellanic Clouds), the Pointed Observations were widely distributed on the sky. Positional association of the SSC sources with those in the PSC reveal that 11129 are in both IRAS catalogues; also, 5470 SSC sources have been cataloged at other wavelenghts.

The positional accuracy of the SSC is not as high as that of the PSC, in part because of the restricted geometry of the observations themselves and also because of compromises made in processing the data. In those cases where more accurate positions are critically needed, it may be possible to obtain better results through re-processing the observations at one of the IRAS data centers.

The main objective of the Serendipitious Survey was to extend the detection threshold for point sources over a finite but significant portion of the sky so that those sources could be studied individually. No attempt has been made to construct an unbiased sample by removing the targeted sources or by correcting either for the non-random sky coverage or for the uneven depth of the survey corresponding to different characteristics of the several observing modes. Because most of the SSC sources are faint and because it was inherently difficult to achieve a uniform level of completeness in this program, it was decided to emphasize high reliability and photometric accuracy. These important SSC objectives were achieved by retaining only those sources wich were detected in each of two independent Pointed Observations and by averaging the two measurements. No fields were included where only a single Pointed Observation was available. This strategy also provided a means for rejecting some asteroids and other moving objects.

An important objective of this catalogue is to present a list of reliable sources found in all the selected Pointed Observation fields despite effect of high source density and/or complex backgrounds. In those heaviliy crowded fields where source confusion degrades the sensitivity, reliability, completeness, positional accuracy or photometric accuracy, the guiding philosophy has been as follows :

(a) accept some degradation,

(b) flag sources where confusion may have affected the confirmation process,

(c) indicate the local density of sources for each wavelength band,

(d) provide information on the effects of confusion so that the user is both warned and equipped to make valid scientific judgments.

This policy has the result that many of the SSC fields and sources must be analyzed with particular care by the user, but it also means that many faint sources are included in crowded regions of scientific interest.

In summary, the main features of the SSC relative to the PSC are as follows :


Name	Type	Meaning

name	A	IRAS/SSC Source Name (2)
r_a1950	I	B1950 Right Ascension
dec1950	I	B1950 Declination
r_a2000	I	J2000 Right Ascension
dec2000	I	J2000 Declination
lii	I	Galactic longitude
bii	I	Galactic latitude
angle	I	Position angle of SSC Source error box
                expressed in degrees East of North.
flux1	F	Averaged non-color corrected flux density
                at 12 (mu)m (1Jy = 10-26 W/m2/Hz) (2)
flux2	F	Averaged non-color corrected flux density
                at 25 (mu)m (1Jy = 10-26 W/m2/Hz) (2)
flux3	F	Averaged non-color corrected flux density
                at 60 (mu)m (1Jy = 10-26 W/m2/Hz) (2)
flux4	F	Averaged non-color corrected flux density
                at 100 (mu)m (1Jy = 10-26 W/m2/Hz) (2)
fqual1	I	Flux density quality for 12 (mu )m passband  
                high-quality=3, moderate quality=2, 
                upper limit =1 (2)
fqual2	I	Flux density quality  (see q_f1)
                for 25 (mu )m passband (2)
fqual3	I	Flux density quality  (see q_f1)
                for 60 (mu )m passband (2)
fqual4	I	Flux density quality  (see q_f1)
                for 100 (mu )m passband (2)
rgrid   I       Reference grid number
relunc1	I	Percent relative flux density uncertainty at 12 (mu )m (2)
relunc2	I	Percent relative flux density uncertainty at 25 (mu )m (2)
relunc3	I	Percent relative flux density uncertainty at 60 (mu )m (2)
relunc4	I	Percent relative flux density uncertainty at 100 (mu )m (2)
tlsnr1	I	10x(local Signal/Noise) for 12 (mu )m passband
tlsnr2	I	10x(local Signal/Noise) for 25 (mu )m passband
tlsnr2	I	10x(local Signal/Noise) for 60 (mu )m passband
tlsnr3	I	10x(local Signal/Noise) for 100 (mu )m passband
cc1	A	point-source correlation coefficient for 12 (mu )m 
                passband (2):

                    |  %    |%      |%      |%     |%     | %    |%     |
                    | 100 A | 99  B | 98  C | 97 D | 96 E | 95 F | 94 G |
                    |  93 H | 92  I | 91  J | 90 K | 89 L | 88 M | 87 N | 
                    |  86 O | 85  P | 84  Q | 83 R | 82 S | 81 T | 80 U |
                    |  79 V | 78  W | 77  X | 76 Y | 75 Z |

                    The quoted correlation coefficients come
                    from the reference or confirming grids, 
                    whichever is higher, for high quality sources.

cc2	A	point-source correlation coefficient for 25 (mu )m 
                passband (see cc1) (2)
cc3	A	point-source correlation coefficient for 60 (mu )m 
                passband (see cc1) (2)
cc4	A	point-source correlation coefficient for 100 (mu )m 
                passband (see cc1) (2)
trflux1 I       10x Fc/Fr (confirmed/reference) for 12 (mu )m passband
                SSC sources can have flux density ratios 
                0.5 < Fc/Fr < 2.0. 
trflux2 I       10x Fc/Fr (confirmed/reference) for 25 (mu )m passband
trflux3 I       10x Fc/Fr (confirmed/reference) for 60 (mu )m passband
trflux4 I       10x Fc/Fr (confirmed/reference) for 100 (mu )m passband
posdr12 I       Right Ascension Delta for 12 (mu )m passband
posdd12 I       Declination Delta for 12 (mu )m passband
posdr22 I       Right Ascension Delta for 25 (mu )m passband
posdd22 I       Declination Delta 25 (mu )m passband
posdr32 I       Right Ascension Delta for 60 (mu )m passband
posdd32 I       Declination Delta for 60 (mu )m passband
posdr42 I       Right Ascension Delta for 100 (mu )m passband
posdd42 I       Declination Delta for 100 (mu )m passband
pnearc1 I       Number of sources in confusion window
                for 12 (mu )m passband (2) (3)
pnearc2 I       Number of sources in confusion window
                for 25 (mu )m passband (2) (3)
pnearc3 I       Number of sources in confusion window
                for 60 (mu )m passband (2) (3)
pnearc4 I       Number of sources in confusion window
                for 100 (mu )m passband (2) (3)
nid     I       Number of positional associations (2)
idtype  I       Type of object (2)

(1) Sources are listed in order of increasing Right Ascension within each field.

(2) This quantity is listed in the printed version of the SSC.

(3) In regions of high source density, the Pointed Observation source extraction process, as well as the Serendipitous Survey Confirmation and Band Merging processing, can result in degraded positions and incorrectly band merged sources. PNEARC is 1-(number of confirmed sources in the confusion and band merge window).

Any value greater than zero is indicative of potential confusion in the processing and the resulting source information should be examined carefully, e.g. by inspection of the grids in question.