My identification strategy was to search the X-ray error circle
for a counterpart until a plausible counterpart was found.
Plausible means an object with either
a characteristic signature (color, spectrum) for a counterpart
to an X-ray source or where the X-ray to optical flux ratio was
within the typical ratio for a given source type. AGN spectra,
broad Balmer lines and forbidden [OIII] emission were always taken
as a strong indicator for the X-ray counterpart, regardless of
the presence of radio emission. M dwarf spectra with or without
emission were also strong counterpart candidates.
In a few cases bright stars are found just outside the error circle.
These are quite likely the counterparts to the X-ray sources, as
other identification programs have shown.
The analysis of the X-ray data and the multi-wavelength follow-up program led to the identification of the majority of sources in my sample. Out of 89 sources 54 have firm identifications; for the remaining 34 sources the current status of the identification process is presented. More spectroscopic data would be required to resolve the ambiguous cases with several optical counterparts inside the error box. However, in most cases the most likely counterpart has already been identified. Table 2 (click here) summarizes the results of this program. Previously known objects are listed with their names. In many cases optical spectra were required to resolve ambiguities, see Fig. 8 (click here). I created finding charts for all objects that I took spectra of, see Fig. 9. Arrows on the finding charts mark the objects. These finding charts are based on the APM scans of the POSS I plates. CCD frames are reproduced for all fields where no good APM or DSS finding chart was available, Fig. 10. The APM finding charts, the CCD frames and the identification spectra are all referenced to Table 2 (click here) through their internal ID number which is easier to handle than the full X-ray position. Table 3 (click here) finally lists remarks on the rationale for each individual identification.
Figure 7 (click here) presents the distribution of the separation between the X-ray source and the identified counterpart for my sample. The distribution is shown first as the number of identifications as a function of the separation. A second panel shows the same distribution but normalized by the radius. In this normalization the increasing sampling area for each radius bin has been removed from the distribution. The distribution confirms my initial assumption that most sources will have counterparts within 30 arcseconds of the X-ray source position. In a few cases very bright stars that burn out the point spread function are nominally found at larger distances.
Of the 54 firm identifications which are cataloged objects or objects with optical identification spectra, 34 are stars and 20 are extragalactic objects. Of the remaining sources, all but two have several counterparts in their error box. A few of the sources at the faint flux limit are expected to be spurious detections due to the lower likelihood threshold that I used in the source detection process. No unidentified source shows the X-ray characteristics that I expect from a soft thermal source nor is any of them luminous enough to indicate an unusual X-ray to optical flux ratio in the presence of the counterparts inside the error box. The only soft and bright source in the entire 89 source sample, 49, stands out in its X-ray characteristics and was readily identified with a DC white dwarf star (see identification spectrum). A considerably larger amount of telescope time would have to be invested to resolve all ambiguities at the end of which I expect that most of the suggested candidates would be confirmed.
| ID | Remark | |
| 01 | no object on POSS, 3 red objects detected in CCD frames, object B is most prominent in I band, tentative identification: B | |
| 02 | + | bright star |
| 03 | + | faint object shows broad Balmer lines |
| 04 | + | 5 objects visible in error circle, all show late stellar spectra, A is by far the brightest (K6-7) |
| 05 | no radio source, no POSS source, faint red object in 30 s cosmic pointing | |
| 06 | + | K star just on error circle (nothing else in 600 s 60-inch R) |
| 07 | + | 2 objects within error circle. Brightest object shows AGN spectrum |
| 08 | + | radio source right at X-ray position, no POSS object, no object in 60-inch frames but in Cosmic frame (60 s) point like object discovered in B, V and R at radio position, object A |
| 09 | 1 object, no spectrum | |
| 10 | + | 2 objects, brighter is star, fainter shows AGN lines |
| 11 | 1 extended object (A), no spectrum | |
| 12 | + | 2 objects, fainter object (R) coincides with radio source |
| 13 | 1 bright object (A), no spectrum, B is reddest object | |
| 14 | 1 brighter object (A), no spectrum | |
| 15 | only A is detected in B image, in R and I a faint object is visible next to A | |
| 16 | + | bright SAO star |
| 17 | + | 1 object, AGN spectrum |
| 18 | A and B are detected in R and I frame, A is brightest in I | |
| 19 | + | bright star, K3 |
| 20 | + | brightest star of 6 objects is K star, apparently extended object is a blend of 2 |
| 21 | A deep in emission cloud, pre-main sequence object? | |
| 22 | + | bright K0 star BD+30 547 |
| 23 | 2 objects, no spectra | |
| 24 | + | brightest object is M dwarf with emission lines |
| 25 | + | A is K0III star, position of B is compatible with radio source (faint and red), X-ray source is hard, so identify B as counterpart |
| 26 | + | 1 bright star inside error circle (B) a brighter star north-west from that (A) both are K3 stars (pick B as counterpart) |
| 27 | + | radio source coincides with R. A is a nebula that shows
no significant line emission (no no [OIII]) |
| 28 | A is detected in B, R and I frames | |
| 29 | bright star south-east of X-ray position (S) is likely counterpart | |
| 30 | + | 2 objects, brighter on is late type star, fainter one coincides with radio source, shows AGN lines, B is counterpart |
| 31 | + | MS Source, type QSO, not rediscovered in radio pointing, only southern part of close pair (A) is detected in B |
| 32 | + | bright star |
| 33 | + | 2 objects both diametrically opposite on the error circle, B is late type star, A is M-dwarf with strong H-alpha emission |
| 34 | + | T-Tauri |
| 35 | several objects, C is reddest | |
| 36 | + | K star binary not resolved on POSS |
| 37 | + | Einstein source, bright M-star |
| 38 | no data | |
| 39 | + | Quasar HB89 |
| 40 | + | A coincides with radio source |
| ID | Remark | |
| 42 | + | bright star |
| 43 | + | bright SIMBAD object, low mass star |
| 44 | + | bright star |
| 45 | 2 faint (B and C) objects in CCD frame | |
| 46 | + | bright star |
| 47 | + | bright star |
| 48 | + | bright star |
| 49 | + | 1 blue object within error box, DC white dwarf |
| 50 | A is just outside the error circle, bluish | |
| 51 | B is red, likely extragalactic | |
| 52 | + | 2 objects, brighter (A) is K0III, fainter (B) is K3, claim A as counterpart |
| 53 | several red objects, no blue object | |
| 54 | + | bright star |
| 55 | A shows AGN spectrum and is counterpart, B is a serendipitously detected carbon star | |
| 56 | + | CCD frames shows red object (B) coincident with radio position, A is 20 arcseconds off from radio source |
| 57 | no obvious counterpart | |
| 58 | A is detected in B, V, R, and I | |
| 59 | A is brightest object, E is a galaxy | |
| 60 | + | A is brightest object, AGN |
| 61 | + | POSS object, galaxy spectrum with break? no radio source down
to 615  Jy |
| 62 | + | bright SAO star. |
| 63 | A is brightest object. | |
| 64 | no object on 300 s 60-inch frame. star S is close. | |
| 65 | + | 2 objects, A is late type star, B is AGN @ z=0.214 |
| 66 | + | bright star, M dwarf |
| 67 | + | bright star, M dwarf |
| 68 | + | K3-5 star (just outside error box) |
| 69 | radio source coincides with A | |
| 70 | + | binary system unresolved on APM chart, each component shows
strong lines |
| 71 | + | bright star |
| 72 | + | 2 objects, brighter is late type star, fainter object, closer to X-ray position coincides with radio source |
| 73 | + | bright M-dwarf |
| 74 | A is brightest object | |
| 75 | A is brightest object and is red | |
| 76 | + | 2 objects. brighter shows AGN lines. z=0.14 |
| 77 | + | AGN @ z = 0.47 |
| 78 | A is brightest object. | |
| 79 | + | bright star |
| 80 | + | emission line star |
| 81 | + | A is bright G star, B is faint late type star, Abell cluster 0496 is 1.5 arcminutes off |
| 82 | 1 POSS object | |
| 83 | + | bright star |
| 84 | A is brightest object | |
| 85 | crowded field | |
| 86 | + | bright star |
| 87 | + | bright star |
| 88 | + | bright star |
| 89 | too far south | |
| 90 | too far south. | |
Figure 8:
Optical spectra. The internal ID and the spectral classification
are given on top of each spectrum. The x-axis shows the wavelength
in Angstrom and the y-axis the flux in arbitrary units.
Remarks:
07: AGN, z=0.29, [NeIII], 
, [OIII] 4959 Å,
5007 Å. 10: AGN, z=0.32, , [OIII] 4959 Å,
5007 Å; 17: AGN, z=0.22, 
, , [OIII]
4959 Å, 5007 Å
Figure 8: continued.
Remarks:
24: dM1.5:e, and in emission;
33: dM1.5:e, strong emission, some .
49: DC white dwarf, the feature close to 5000 Å is probably due
to a mismatch between the red and the blue side of the spectrograph;
52: K0III, feature is due to red/blue mismatch;
55-A: AGN, z=0.074, [0III] 5007 Å, HeI, , [SII]
6731 Å
Figure 8: continued.
Remarks:
70-1: binary system of two emission line M-dwarfs
Figure 8: continued.
Remarks:
70-2: second component of 70-1;
76: AGN, z=0.14, , [OIII] 4959 Å, 5007 Å,
;
77: AGN, z=0.47, 
, , [OIII] 4363
Å, , [OIII] 4959 Å, 5007 Å