Subject: Transient science
From: Fiona Harrison
Submitted: Thu, 7 Aug 2003 15:52:22 -0700 (PDT)
Message number: 160
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next: 161
up: Index)
To those interested in transient science;
I volunteered to write a first draft of the transient followup
section. In sitting down to think about it, a number of questions
emerge (see below). If you have any thoughts, please distribute
them. Chris (Stubbs) - I hear you have some ideas on some of
these and other issues. If you send them along, I'll incorporate
them into the first draft.
Fiona
-----------------------------------
The science argument (some thoughts):
Rather than targeting a particular type of transient (e.g. orphan GRB
afterglows), I think the best argument we can make is maximizing
discovery space. We should be clear about what we define as a
transient, and what we want to emphasize as scientifically most
interesting. On defining transient, do we mean catastrophic events, or
do we include objects that have recurrent 'flaring' activity? On the
issue of what is scientifically most interesting - I would vote for
searching for catastrophic events such as collapsars, mergers, etc. --
events which have compact objects as the endpoint, and which may have
transient lightcurves peaking in optical rather than gamma-ray or
X-ray. A peak in the optical could be due to e.g. lower ejecta
Lorentz factors resulting from more entrained material, or something
intrinsic to the explosion itself. Orphan GRB afterglows are of
somewhat secondary interest compared to something really new.
Phase space for discovery:
Since LSST is the trigger, the discovery phase space will be determined
by the cadence and filters. The parameters relevant to discovery are:
timescale of transient (length of time in the 'on' state)
spectrum (colors)
brightness
Detecting a transient requires three images; two with the transient
'on', and at least one with it 'off'. The minimum timescale that
can be detected therefore depends on the time between successive
exposures, which should be a minute or even less. The third
exposure can be any time later, however measuring short timescales
would dictate that the three exposures be closely spaced in time.
Followup:
It seems clear that significant followup efforts will be required to
identify the nature of detected events. The first problem is
filtering out trash (variable stars, etc), which can be done to some
extent be done using colors (?). The hard problem is figuring out
what the objects are and connecting them to astrophysical phenomena.
For this, the following will be essential:
lightcurves (recurrence)
distance (redshift)
host properties
counterparts at other wavelengths (radio,x-ray)
Issues to resolve for the writeup:
Phase space for transient detection is dictated by number and
frequency of images (as many as closely spaced as possible to
the limit that sensitivity is not significantly compromised).
What can we assume reasonably (limited by other lsst goals)?
Note that since we need two images with the transient 'on',
if we want to sample minute-long transients, we need on average
30s exposures.
Having multiple colors could help sort out candidates for
followup. This requires (for a 1-telescope design) rapid
switching of filters. We should think through how helpful
this will be in reality. For example, GRB afterglows are
blue - unless they are extincted or at high-Z, in which
case they are red. So, to really do reasonable selection
requires a number of colors - is it really feasible?
How much is it worth pushing the observing
Should a lightcurve followup program be planned for LSST
for interesting events, or is this better done with a
dedicated small telescope (or two)? Should some dedicated
spectroscopy program be planned in advance, and be part
of the 'mission', or should we assume that the community
will react to interesting triggers?
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