Abstract for Proposal 090245
Tackling unresolved questions on transient obscuring outflows in AGN
Transient obscuring outflows are remarkably different from the commonly
seen warm-absorber outflows in AGN. They exhibit large columns of
high-velocity gas near the accretion disk/BLR. They shield much of
the X-ray radiation, which can have important implications for the
surrounding gas and the launching of AGN outflows. Joint ToO observations
with XMM-Newton, NuSTAR, and HST, triggered using Swift monitoring, have
been vital for ground-breaking results on transient obscuration events
to come into light in recent years. We propose to study a new event
to address the outstanding questions on the link between obscuration
and AGN outflows and broaden our understanding of these events. We thus
request joint ToO observations with XMM-Newton (60 ks), NuSTAR (30 ks),
and HST/COS (2 orbits).
Details on Observing Strategy and Trigger Criteria
We use our weekly Swift monitoring of the AGN sample to find a 'transient
obscuration event'. When an increase in the Swift/XRT hardness ratio
(R) meets our criteria for an obscuration event we will submit the
trigger request for our ToO XMM/NuSTAR/HST observations to be taken.
The R triggering limit value of each target is provided in Table 1 and
Fig. 3 of the proposal, which corresponds to a transient obscuration
with NH > 3e+21 cm^-2. We note that our XMM/NuSTAR/HST ToO is NOT
a rapid request. Our ToO program can be carried out any time within
14 days after the trigger request. We ask exposures with XMM-Newton,
NuSTAR, and HST to be taken within +/- 2 days from each other to provide
contemporaneous X-ray/UV coverage of the obscuration event. Our previous
triggers (NGC 3783, NGC 3227, MR 2251-178) were successfully carried out.
The probability that one of our targets is triggered in AO-21 is 0.5
as explained in the proposal. This triggering probability is consistent
with the frequency of our previous successful triggers.