In this discretionary time TOO proposal we outline a strategy for systematic
ISO observations of future gamma-ray bursts (GRBs) in order to detect the
infrared emission associated with the burst sources. The recent remarkable
discoveries of fading (time scale of days) long wavelength afterglows (X-ray
through to optical and radio) from a few GRBs have radically improved our
understanding of these enigmatic sources. The direct redshift measurement of
the optical transient (OT) of GRB970508 has yielded a value of z=0.835 from
both emission and absorption spectral features, providing the first direct
evidence for the cosmological origin of GRBs. These discoveries were made
possible by the capability of the Wide Field Camera (WFC) aboard the BeppoSAX
X-ray satellite, to rapidly localise GRBs to within a ~3' radius error circle.
The fact that ISO and SAX, both European missions, are in orbit at the same
time, provides an unprecedented opportunity to exploit this capability to the
utmost. We request TOO observations of future GRBs in the light of these new
discoveries which could not have been anticipated at the time of the
supplemental call. We anticipate that ISOCAM will detect transient emission at
15microns, once the observations are made within a few days of the GRB and that
ISOPHOT will detect the host galaxy, which may be difficult to detect in the
optical due to its likely dusty nature. As part of our strategy, we request
further observations of GRB970508 (requiring ~1 hour), in order to confirm the
PHT detection of a faint 60um source consistent with the OT position and to
further characterise the nature of this host galaxy. Observations of a number
of different GRBs with ISO will be crucial in determining the nature of the
host galaxies and establishing commonalities between them and in the detection
and monitoring of the mid to far infra-red afterglow. This consortium has
formed from a number of groups who have been carrying out successful ground
and satellite-based campaigns to study GRBs at many frequencies and who have
previously been heavily involved in ISO GRB proposals. The team members
include instrument and mission specialists on the SAX and ISO teams and
scientists who have made some of the key new observations of GRBs.