Newsgroups: sci.astro
Subject: Re: GRBs identified?  (in some respects, probably)
Organization: Princeton University Dept of AstroPhysics


  Tom Womack asks:

> I read one or two papers recently suggesting that gamma-ray bursters
> were cosmological - someone was able to get time on a large telescope
> immediately after the burst was detected, and produced images of a
> rapidly-fading galaxy around z=0.8; another burst was detected
> recently with the same sort of results, and the light curve was
> consistent with a massive pulse of gamma rays interacting with the
> interstellar media in the distant galaxy.

  There have been 3 gamma-ray bursts (GRBs) in recent months which
have inspired optical observers to search for optical counterparts.
The Dutch BeppoSAX satellite is able to detect GRBs, then point
an X-ray telescope with narrow field of view at the source within
about 8 hours.  It can pinpoint the X-ray emission (if there is any)
to within about 1 arcminute.  That means that, for the first time,
astronomers know where to look _quickly_, and there's hope for 
seeing optical emission.  

  The Feb 28 burst (which occurred only about 15 degrees from the
galactic plane) was seen from the ground by many groups over several
weeks, and by HST on two occasions.  There is an point source near the 
position of the GRB which is embedded in a very faint nebulosity,
which is just an arcsecond or two in size.  Only HST can resolve the
point source from the "fuzz" clearly.  Measurements from the ground
show that the point source + fuzz faded by several magnitudes over
several weeks, consistent with HST measurements of the point source
alone.

  The Apr 2 burst (which occurred at a far southern declination,
only 5 or so degrees from the galactic plane) did not produce
any detectable optical emission, down to R=22 or so.

  The most recent burst (May 8) occurred at Dec = +79, far from
the galactic plane.  A variable point source has been detected
within the error circle of BeppoSAX by ground-based optical
observers.  Strangely enough, the point source appears to brighten
slightly for about a day, then start to fade slightly ...
just as some models for the optical emission predict.  
A spectrum of the object shows several absorption lines which
may be Fe and Mg lines which _normally_ would appear in the
ultraviolet, but which could be shifted to the optical if the
redshift of the absorbing material were 0.8 or so.  

  This latest case (just a few days old) is very exciting.  We MAY
be seeing light from the GRB pass through an unassociated, intervening
galaxy, or the GRB may be in a galaxy at z=0.8; on the other
hand, it is still _possible_ that the variable object is a distant
active galaxy which has nothing to do with the GRB.

  The hallways here at Princeton are filled with conversations
about the new burst.  It's very exciting.  I have heard it said,
second-hand, that Don Lamb -- a proponent of the idea that
GRBs reside in our own Milky Way -- has publicly declared
that he was wrong, and that GRBs are at cosmological distances.
It may be a bit premature for such a reaction ... but not very
premature.

  BeppoSAX is producing one good position for a GRB per 6 weeks.
If it keeps up the pace for another few months, I do think that
we'll at last know whether GRBs occur closeby or far away.

                                Michael Richmond