Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Blank Field X-Ray Sources   The X-ray sky is not as well known as is sometimes thought. We report on our
search of minority populations (Kim & Elvis 1999). One of the most intriguing
is that of `blank field sources', i.e. bright ROSAT sources (F(X)>1e-13
erg/cm2/s) with no optical counterpart on the Palomar Sky Survey (to O=21.5)
within their 39"(99%) radius error circle. The nature of Blank Field sources is
unknown: no known extragalactic population has such extreme X-ray to optical
ratios (f(X)/f(V) >60). Moreover blank field source tend to have much flatter
PSPC spectra compared to radio-quiet AGN. Both properties suggest obscuration.
`Blank field sources' could be: Quasar-2s, low-mass AGNs, isolated neutron
stars, high redshift clusters of galaxies, failed clusters, AGNs with no big
blue bump. Identification with any of these populations would be an interesting
discovery.

27
The categories of this paper are: comp-gas
9
The categories of this paper are: nlin
22
The categories of this paper are: chao-dyn
28
The categories of this paper are: alg-geom
18
The categories of this paper are: econ
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
21
The categories of this paper are: adap-org
29
The categories of this paper are: funct-an
32
The categories of this paper are: atom-ph
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
13
The categories of this paper are: quant-ph
37
The categories of this paper are: supr-con