Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Chandra Observations and Optical Identification of Hard X-ray Sources
  Discovered with ASCA   We present the first results of the Chandra and optical follow-up
observations of hard X-ray sources detected in the ASCA Medium Sensitivity
Survey (AMSS). Optical identifications are made for five objects. Three of them
show either weak or absent optical narrow emission lines and are at low
redshift <z>~0.06. One of them is a broad line object at z=0.910 and one is a
z=0.460 object with only narrow lines. All the narrow line objects show strong
evidence for absorption in their X-ray spectra. Their line ratios are
consistent with a Seyfert II/LINER identification as are the line widths. The
three low redshift objects have the colors of normal galaxies and apparently
the light is dominated by stars. This could be due to the extinction of the
underlying nuclear continuum by the same matter that absorbs X-rays and/or due
to the dilution of the central source by starlight. These results suggest that
X-ray sources that appear as ``normal'' galaxies in optical and near-IR bands
significantly contribute to the hard X-ray background. This population of
objects has a high space density and probably dominates the entire population
of active galaxies.

19
The categories of this paper are: eess
17
The categories of this paper are: q-fin
31
The categories of this paper are: ao-sci
9
The categories of this paper are: nlin
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
29
The categories of this paper are: funct-an
1
The categories of this paper are: cs
35
The categories of this paper are: mtrl-th
3
The categories of this paper are: physics
23
The categories of this paper are: solv-int
7
The categories of this paper are: hep-th
28
The categories of this paper are: alg-geom
22
The categories of this paper are: chao-dyn
27
The categories of this paper are: comp-gas
8
The categories of this paper are: hep-ex