The Density and Location of the X-ray Absorbing Gas in NGC 3516
| Authors:
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Netzer, H.1,
Chelouche, D.1,
George, I.M.2,3,
Turner, T.J.2,3,
Crenshaw, D.M.4,
Kraemer, S.5,
Nandra, K.2,6
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| Affiltns:
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1Tel Aviv University,
2 LHEA,
NASA/GSFC,
3 JCA, UMBC
4 Georgia State University,
5 Catholic University of America,
6 USRA
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| Journal:
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ApJ
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| Publn Date:
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in press
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Summary
A new Chandra observation
and archival observations by ASCA
are used to investigate spectral variations in the Seyfert 1 galaxy NGC 3516
over a period of 7 years.
A large change in flux (factor approx 50 at 1 keV) is observed
between an ASCA observation in 1994 and the Chandra
observation
in 2000, with the source close to the
all-time maximum and minimum X-ray flux states, respectively.
We find the variations in the observed flux and spectra at these
epochs to be consistent with a constant column density of line-of-sight
material reacting to changes in the ionizing continuum.
The data from the two epochs are consistent with a simple
decrease (by a factor 8-10) in the luminosity of a constant
0.5-50 keV slope source
and a line-of-sight
absorber with an equivalent hydrogen column density of
1021.9 cm-2.
Intermediate
luminosities, sampled during other ASCA observations,
are all fitted by the
same model with a very small change in spectral index
(well below Delta Gamma = 0.2).
In addition, analysis of the long (360 ks) ASCA observation in
1998 shows clear
``color'' variations that are entirely consistent with this model and are
interpreted as due to changes in the
opacity of the absorbing gas. The data allow us to put a conservative upper
limit of 60 ks on the recombination time which translates
to a lower limit of about 2.4 x 106 cm-3
on the density of the recombining gas and an upper
limit of about
6 x 1017 h75-2 cm
on its distance from the central source. These
are the best limits obtained so far on the density and location of the X-ray
absorbing gas in a type-1 Active Galactic Nucleus (AGN).
They indicate that the absorbing gas
%absorber probably forms a
%thin shell (or shells) of gas outflowing from the center and
is different, in
terms of its density and location, from the ionized gas commonly observed
in type-II AGN.
The Chandra ACIS/LETGS data also reveals a strong (EW=290
eV), unresolved 6.4 keV iron line, a strong
OVII line and a
marginally detected NVI line. The former is interpreted as originating in a
large column of gas of lower state of ionization seen in ``reflection'',
and is consistent with the spectrum at high
energies at all epochs.
The two others emission lines are probably emitted by the gas also responsible
for the line-of-sight absorption.
Figures:
Fig1
Fig2
Fig3a
Fig3b
Fig4
Full Text
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See Also
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