Not much is known about the variability of nearby AGN in the 912-1200 Å bandpass since observations at these wavelengths have been sparse. The Astro and ORFEUS shuttle missions have provided some insights, but FUSE promises to give the first real opportunity to study short and longer term variability in this band.
To date, the only repetitive observations of an AGN below 1200 Å have
been the HUT and ORFEUS observations of NGC 4151
(Kriss et al. 1992a;
Kriss et al. 1995;
Kriss et al. 1997;
Espey et al. 1998).
These showed that broad O VI emission responds immediately to continuum
changes, to within a resolution of 1-2 days.
The saturated broad absorption lines such as
Ly
and O VI show
little response to continuum changes, presumably because the changes in column
density required to produce noticeable changes in equivalent width are larger
than those induced by the varying continuum.
However, weaker absorption features such as C III
1176 and the
high-order Lyman series (which is optically thick at the Lyman limit in
these observations of NGC 4151) respond to continuum changes with a delayed
response of ~ 5 days, reflecting the recombination timescale of the
photoionized gas. Observations proposed with FUSE to monitor the Seyfert 1
galaxy NGC 3783 contemporaneously with HST and
Chandra grating
observations could give the first glimpse of the response of the gas in
a warm absorber to changes in the ionizing continuum at both X-ray and
far-UV wavelengths.
Potential future monitoring experiments such as Kronos could profit from
an ability to observe in the 912-1200 Å spectral range.
As noted earlier, O VI is bright in low-redshift AGN; it is nearly as
bright as C IV. While it probes a zone in the BLR similar in ionization
to He II, it is much brighter than He II
1640.
For studies of warm absorbers, the O VI lines and the higher-order
Lyman lines offer several advantages, as noted earlier.
Observing in this bandpass requires some observational trade-offs, however.
Good throughput requires LiF coatings on the optics and open-window detectors.
Both of these features entail significant costs in development and in
contamination management throughout the instrument development and
integration process.
Sealed detectors and MgF2 coatings are easier to manage, and their
use can improve the throughput at longer UV wavelengths (e.g., C IV
1549) by up to a factor of 2.