Up: High X-ray-to-optical flux ratio
Subsections
The alternative flux ratio constraint stems from the statistics to known X-ray
sources. Here known sources are the correlation results between RASS-BSC and
several catalogs of known objects available in computer-readable format,
including catalogs of AGN (Véron-Cetty & Véron 1996), white dwarfs
and NGC, the Bright Star Catalog, NED
and SIMBAD.
To evaluate X-ray flux for every source in RASS-BSC will cost much time.
B magnitude of counterparts is not
available either
. We establish
restrict on X-ray count-rate to E
magnitude to do preselection from RASS-BSC
instead, where E-magnitude can
be derived from the Digitized Sky Survey
(DSS) image (Cao et al. 1997).
We should point out that not taking Galactic
absorbing material into account
will reduce precision of the flux ratio
constraint and then the efficiency of this preselection method.
We all know that the relationship between magnitude (m) and flux (f) is:
|  |
(1) |
For E magnitude, the relation can be written as:
|  |
(2) |
Despite Galactic absorption material
count rate (c) is roughly
proportional to X-ray flux
,
so with Eq. (2):

We can see that for fixed
,
= constant.
B or V magnitudes of known X-ray sources can be drawn from those catalogs
that we have mentioned. They must be changed to E magnitude at first. A
relation was derived by Humphreys et al. (1991) between the Johnson BVR
photometry and the E photographic magnitude on the Palomar plates:
then
Typical color indexes, (V-R) and (B-V), for various types of objects can be
drawn from literatures. Now for a known X-ray source,
its count rate and E magnitude are available. A plot of
, versus apparent optical magnitude, E magnitude, is
shown in Fig. 1. Only O-M spectral type stars, white dwarfs, emission line
AGN and BL Lacertae objects are shown in this graph.
 |
Figure 1:
Plot of vs. E-magnitude for known sources |
There is an apparent gap between Galactic stars and extragalactic
objects except white dwarfs. Emission line
AGN concentrate on the discrete area between line A (
= 4.9)
and line B (
= 6.4). It looks like that BL Lacertae objects
have been scatted between
and
randomly. Nearly half
number of BL Lacertae objects (45 to 105) located at the region above
line B, however emission line AGN in this region are only
about
of its total amount included in the statistic. So taking
as high X-ray-to-optical
flux ratio criteria can avoid too much normal emission line AGN being
included in the sample.
The high X-ray-to-optical flux ratio sample is defined by following
criteria:
- 1.
- Exclude known objects. The known objects mean the same as that
in Sect. 2.1.
- 2.
- Located in the northern hemisphere. In practice we choose
(epoch 1950), because we can evaluate E magnitude
from DSS images only for those objects with declination larger than 3
.
- 3.
- High Galactic latitude
, in order to keep away from low
Galactic sky area where mainly stars are located.
- 4.
- There are counterparts within the circle with radius D=D1 + 5'',
where D1 is the ROSAT X-ray position error given in the RASS-BSC.
- 5.
- The E magnitude of the counterpart is less than
, because it
is hard to get high enough signal to noise ratio spectrum with a 2 m telescope
for objects fainter than that limit, and meets the high X-ray-to-optical
ratio criterion:
Thirty-four X-ray sources with Right Ascension between
and
were
selected from RASS-BSC. Table 1 lists the X-ray information for the X-ray
sources.
There are several cases which have two or more counterparts in the cross-circle.
Only those satisfying the above criteria are listed in the
Table 1
. Objects are identified by ROSAT name (Col. 1).
The optical position
(epoch 2000.0, Col. 2 and Col. 3) are derived from the DSS image and should be
accurate better than 2''. The X-ray information (e.g.
position error, count-rate, hardness 1, and extent in
arcsecond) is given in Col. 4 to Col. 7 respectively. These parameters
are all given in RASS-BSC and useful for identification procedure. The
E magnitude, derived from the DSS images, is given in Col. 8.
Up: High X-ray-to-optical flux ratio
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