I. Georgantopoulos et al., ASCA observations of deep ROSAT fields - IV. Infrared and hard X-ray observations of an obscured high-redshift QSO, M NOT R AST, 305(1), 1999, pp. 125-131
We use UKIRT and ASCA observations to determine the nature of a high-redshi
ft (z = 2.35) narrow-line AGN, previously discovered by Almaini et al. The
UKIRT observations show a broad Her line while no HP line is detected. This
together with the red colour (B - K = 5.4) suggests that our object is a m
oderately obscured QSO (A(V) > 3), at optical wavelengths. The ASCA data su
ggest a hard spectrum, probably the result of a large obscuring column, wit
h Gamma = 1.93(-0.46)(+0.62), N-H similar to 10(23) cm(.)(-2) The combined
ASCA and ROSAT data again suggest a heavily obscured spectrum (N-H similar
to 10(23) cm(-2) Or A(V) similar to 100). In this picture, the ROSAT soft X
-ray emission may arise from electron scattering, in a similar fashion to l
ocal Seyfert 1.9 galaxies. Then, there is a large discrepancy between the m
oderate reddening witnessed in the infrared and the large X-ray absorbing c
olumn. This could be possibly explained on the basis of, e.g., high gas met
allicities, or by assuming that the X-ray absorbing column is inside the du
st sublimation radius. An alternative explanation can be obtained when we a
llow for variability between the ROSAT and ASCA observations. Then the best
-fitting spectrum is still flat, Gamma = 1.35(-0.14)(+0.16), but with low i
ntrinsic absorption in better agreement with the IR data, while the ROSAT n
ormalization is a factor of 2 below the ASCA normalization. This object may
be one of the bright examples of a type II QSO population at high redshift
, previously undetected in optical surveys. The hard X-ray spectrum of this
object suggests that such a population could make a substantial contributi
on to the X-ray background.