Subject: APO search for additional images of the GRB?

From: elt@astro.Princeton.EDU

Submitted: Mon, 25 Jan 1999 16:38:36 -0500 (EST)

Message number: 29 (previous: 28, next: 30 up: Index)

As those of you following GRB 990123 no doubt know (see GCN Circulars
206, 219 and 221 for details), there appears to be a significant chance
that this burst has been strongly gravitationally lensed and that 
additional images of the event will appear on a time scale of days to
months depending on the details of the lensing mass distribution and
alignment.  In a message attached below, James Rhoads suggests that
the APO 3.5-meter might be particularly well suited to monitor the
field looking for the additional optical image(s) to occur.  Needless to
say, successful detection would be of major scientific importance and a
high profile contribution to this competitive field and to our understanding
of this extraordinary GRB.

Even before receiving James's message, I was thinking along very similar
lines.  However, it does not seem to me that adding this target to the
Princeton-APO Grav. Lens Monitoring Project list (PU02 in 1Q1999) is the
way to go because 1) PU02 is not currently scheduled for any time
during the remainder of 1Q1999 and 2) it seems to me that the ARC GRB
community, as opposed to the PU02 team, is a more logical choice to
carry out such a project.  Thus, I would suggest that we immediately
instigate frequent monitoring of the GRB 990123 field as a Director's
Discretionary Time Program (DD02) with the data immediately available
to all interested parties in ARC.

Just to be clear, the goal of this program would *not* be to build up
a light curve of the fading afterglow of the 990123 event but rather
to detect the appearance of one or more additional images following the
differential lensing time delay.  This would have the very important
practical consequence that we would not need to change instruments to
get the needed data.  Rather, DD02 would just take the last 5 or 10
min of each night when DIS or SPIcam (or maybe GRIM2...can it go deep
enough in a few min to have a reasonable shot at detecting such an
event?) were in use to get an image or two of the field.  These data
could be obtained by the Observing Specialist and quick visual inspection
might well be sufficient to see if a new image of the event has apperared.
Organizing the program in this way would minimize its impact on other
scheduled programs and on operations.

The purpose of this message is to solicit your input on this plan.
Any and all comments welcome.  Is it scientifically justified by the
current level of evidence of the lensing hypothesis and by the potential
scientific return?  Is this the right, or at least a good, way to do it?
Are you interested in participating, e.g, by looking at the data?  Etc.



PS - UW08 had small allotments of time at the end of several nights in
     the coming weeks (using GRIM2).  Taking a 5-10 minutes from it
     would have a much bigger impact than taking it from programs otherwise
     having the whole second half.  Thus, we could either skip the UW08
     nights (including tonight 1/25) or not, depending on the preferences
     of the UW08 investigators who are, fortunately, GRB'ers also/anyway.

>Hi Ed,
>	Have you seen the GCN circulars #206 and 219?
>Djorgovski et al are suggesting, based on the observed redshift of GRB
>990123 (z >= 1.60; Kelson et al) and the presence of a nearby low-z galaxy
>(visible on POSS plates) that this source is lensed.  Hjorth et al
>agree with this idea and further estimate that there should be a second
>event 2-3" north of GRB 990123 in a few months, with possible additional
>events if the lens is quadruple.  Hjorth et al confirm the GRB redshift
>>=1.60 and also find emission lines at z = 0.286, presumably from the
>foreground (candidate lens) galaxy.
>	This is still speculative, but it would be a really excellent source
>for the Princeton Lens Monitoring project.  The decay is power-law, but
>the turn on from zero flux to peak is very sharp, so with decent weather
>one could expect delay measurements with an error of a couple of days
>(i.e., equal to the sampling time).  Magnification ratios would be harder
>to get from the optical, though they might be forthcoming from gamma ray
>experiments (depending on whether we are lucky or not with earth occultations
>of BATSE).  Good observations of the lensing galaxy will be possible
>once the GRB afterglow dies away, and I think we might hope for HST
>astrometry of the field.
>	I think we should seriously consider adding this source 
>to the lens monitoring target list, or (if that's not feasible)
>monitoring it with DD time until a new program can be proposed and
>scheduled.  The source currently transits an hour after morning twilight,
>and is at dec=+44 degrees, so could easily be monitored for the next 9
>months or so.  I see that PU02 is not scheduled in February or March, so
>probably DD time would be necessary to make this work.  What do you think?
>					Best regards,  James.

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