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The Evolving XUV Absorber in NGC3516
Smita Mathur, Belinda Wilkes & Thomas Aldcroft
Harvard­Smithsonian Center for Astrophysics, 60 Garden St.,
Cambridge, MA 02138, USA
Abstract. For NGC3516 we find that the X­ray warm absorption and
the broad UV associated absorption features can be produced by the same
absorbing material. We argue that the evolution of the XUV absorber
from pre­1992 to 1995 is consistent with expectation for an expanding,
outflowing material.
1. NGC3516: X­ray and UV Observations
NGC3516 contains the strongest UV absorption system known in a Seyfert
1 galaxy. This system contains at least two distinct components: a broad
(FWHM¸2000 km/s) variable component and a narrow (¸500 km/s) non­variable
component. Both the broad and the narrow systems contain high as well as low
ionization absorption lines. Recent observations have shown that the broad high
ionization absorption lines have disappeared since ¸1992 (Koratkar et al. 1996
and references therein, Kriss et al. 1996)
We analyze a high signal­to­noise (S/N) ROSAT PSPC archival spectrum
of NGC3516 obtained in 1992. The high S/N allows the strong detection of both
OVII and OVIII edges independently, in spite of the limited spectral resolution
of the PSPC. A warm absorber fit to the data shows that the absorber is highly
ionized (U= 10 +2:6
\Gamma2:1 ), and has a large column density NH ¸ 10 22 cm \Gamma2 .
2. The XUV Absorber
In several AGN, the X­ray and the UV absorbers were found to be one and
the same (the `XUV' absorbers, Mathur et al. 1994, 1995). Is it also true for
NGC3516? The absorption systems in NGC3516 are clearly complex with mul­
tiple components (Kriss et al. 1996). It is the high ionization, broad absorption
system that is most likely to be associated with the X­ray warm absorber. Inves­
tigation of this question is tricky, however, because the broad absorption lines
have disappeared. Here we argue that the XUV absorption picture is consis­
tent with the presence of a highly ionized X­ray absorber and the current non­
detection of CIV and NV broad absorption lines (see Fig. 1). The X­ray absorber
MUST have a UV signature showing OVI absorption lines (Fig. 1). Since there
were no simultaneous ROSAT & far­UV observations in 1992, this cannot be
directly determined. However, we note that in the 1995 HUT observations, OVI
doublets are unresolved, but consistent with being broad (FWHM=1076\Sigma146
km/s).
1

2 Mathur et al.
5 10 15 20
­5
­4
­3
­2
­1
0
OVIII
OVII
OVI
CIV
NV
1992
Pre­1992
1995
Figure 1. Ionization fractions f of OVI, OVII, OVIII, CIV and NV
as a function of ionization parameter, U (with CLOUDY, Ferland
1991). The vertical lines define the range of U for which the ratio
f OVII /f OVIII lies within the observed ROSAT range. The arrows on
the CIV and NV curves indicate the lower limits of f CIV ?
¸ 3 \Theta 10 \Gamma4
and f NV ?
¸ 3:1 \Theta 10 \Gamma4 based on the published IUE data. The + mark
corresponds to the HUT data in Kriss et al. 1996.
We argue that the XUV absorber in NGC3516 has evolved with time (Fig.
1). Pre­1992: It showed broad, high ionization CIV and NV absorption lines
and an X­ray ionized absorber (U !
¸ 7). As it evolves, outflowing and expanding,
the density falls and the ionization parameter increases. 1992: CIV and NV
absorption lines disappeared; X­ray absorber is still present with OVI lines in the
UV (U¸10) (No UV data available to verify). 1995: CIV and NV absorption line
remain absent; X­ray absorber is present. OVI lines are present, and detected
with HUT (U¸13.5). Post­1996: We predict that the OVI absorption lines
will disappear as the ionization parameter increases further (U ?
¸ 20). The OVIII
edge will continue to strengthen relative to the OVII edge. Eventually, even the
X­ray absorber will also disappear.
Acknowledgments. SM gratefully acknowledges the financial support of
NASA grant NAGW­4490 (LTSA) and BW, TA of NASA contract NAS8­39073
(ASC).
References
Ferland, G. F. 1991 ``HAZY'', OSU Astronomy Department Internal Report
Kriss, G. et al. 1996, ApJL, in press
Koratkar, A. et al. 1996, ApJ, in press
Mathur, S., Wilkes, B., Elvis, M. & Fiore, F. 1994, ApJ, 434, 493
Mathur, S., Elvis, M. & Wilkes, B. 1995, ApJ, 452, 230