Workshop Summary :

by Chryssa Kouveliotou

About 80 scientists gathered at the island of Elba for a two day meeting, to
discuss the latest exciting developments on the field of Gamma-Ray Bursts
(GRBs). At the center of the discussion was the discovery of  X-ray
counterparts for GRB970228, GRB970402 and GRB970508, of optical counterparts
for the first and third of these events and of a radio counterpart of the last
event.  The meeting was balanced between observational reports, theoretical
models that were newly developed in view of these results and discussions on
future strategy for detection of more counterparts.

Observations: 

The launch of  BeppoSAX has clearly started a new era in the GRB field. The
satellite combines the unprecedented capability of GRB detection AND location
to very  small error boxes, of the order of 3' within hours of  trigger. The
computation of these locations has improved through a series of events and the
latest GRB was available to the community in less than 4 hours after trigger,
as opposed to 21 hours for GRB970111.  How does BeppoSAX do that?  The mission
scientist, Luigi Piro, described in his talk the labour intensive series of
events that take place as soon as a GRB has been confirmed until the location
is produced. It was evident that a lot of coordination and hard work from the
BeppoSAX team is needed and in fact, has worked in each case.  The first
ingredient is the search of the GRB Monitor data from the duty scientist at the
BEppoSAX data center in Rome. The instrument has an 80% false trigger rate;
itself alone would not suffice to detect and confirm a GRB (F. Frontera). Enter
the Wide Field Cameras (J. Heise): their false trigger rate is practically
negligible. If the event is in the field of view of one of the two cameras, it
is detected and thus confirmed, unambiguously. The detailed data set is then
transmitted to the ground, and sent to Utrecht for further analysis by the WFC
team. Within a few hours, they return a 3' error radius location in Rome.
Recently, the decision has been made by the BeppoSAX Steering Committee to make
this location available to the community immediately thereafter.  

The 3' location is enough to get the optical observers going. But there is
more: BeppoSAX is equiped with a series of X-ray telescopes (Narrow Field
Instruments, NFI) that have one degree field of view. The satellite is
repointed within a few hours to observe the GRB location. As a result, in three
out of four cases, an X-ray counterpart has been detected at the 3' error box
of the GRB (E. Costa, L. Piro, F. Frontera). The NFIs thus further constrain
the location of the GRB to an error radius of 50". Time for the powerful
telescopes to jump in the game!  Hubble Space Telescope, Keck and VLA are major
players from that point on.

Not so, for the actual detection of the optical counterparts! The first
counterpart was detected with a combination of good luck and international
collaboration, using the 4.2 m William Herschel Telescope in  La Palma of the
Canary Islands (J. van Paradijs), which had a 7'x7' field of view, to observe
the initial 3' field of the WFC. The first image was taken ~21 hours after the
burst triggered BeppoSAX and it was compared with another image taken 8 days
later with the same telescope. A variable source was found at 21.3 mag that lay
within the error box of the 50" NFI location. In the mean time the NFI reported
the detection of an X-ray source (E. Costa) at an initial peak flux of ~
0.1mCrab, that faded by a factor of 20 three days later. The source's optical
light curve shows an exponential decay with a constant of  -1.4 days. The decay
rate leveled off about 8 days after the trigger to a constant of ~25 days. No
radio counterpart was detected from this source.

The second optical counterpart was detected with an even smaller telescope at
Kitt Peak: Howard Bond detected a variable star of ~20th mag, using the 36"
telescope, about 5 hours after the trigger of GRB970508.  Mark Metzger using
Keck went on the source immediately after and determined a spectral redshift of
0.835 from the source direction that put a lower limit to its distance of about
1Gpc. An upper limit of z=2.3 was given by the non-detection of redshifted
Lyman alpha lines in the Keck spectrum. If this is indeed the GRB optical
counterpart, these observations are the first direct evidence for a
cosmological origin hypothesis of the GRB sources.

Other observational facts that were reported in the meeting are:

*** E. Costa reported on the long-term decay of the soft X-ray  emission from
GRB970228 since trigger. A plot combining the data from the WFC, the two NFI
observations and ASCA is nicely following a power-law with -1 index. The same
trend was recorded in the X-ray decay (NFI data only) of GRB970402, while the
last even seemed to have first increased in intensity and then somehow to have
remained constant (within statistics) during the following NFI observations.
Also, the ratio of X-rays to Gamma-rays in the first event was the highest ever
reported: 12% (for the WFC range of 2-10 keV over the GRBM range of 40-700
keV).

*** J. Heise made the point that all these events are selected ONLY after they
are detected with the WFC. They are thus, all, X-ray selected events, ie events
that have intrinsically a high content in X-rays. This may give us a clue why
not all GRBs have optical counterparts.

*** J. Greiner reported that the ROSAT position of GRB970228 ( 10" accuracy)
coincides with theoptical counterpart, and that the source intensity falls well
on top of the E. Costa plot progressing the time scale a bit further.

*** J. van Paradijs compared the light curves of the two optical counterparts.
The first light curve was decaying steadily for 8 days and leveled off, while
the second clearly peaked and declined only afterwards. He also noted that a
good  understanding of the GRB afterglow might require the follow-up of many 
events.

*** Guarnieri and Peddichini, reported that they had data starting 16.5 hours
after the trigger of GRB970228, and that they had detected the optical
counterpart at the same level or fainter than the Groot et al first point. This
may be a hint of a rise in the light curve for this counterpart as well.

*** H. Pedersen reported that his data show that the light curve of GRB970508
start at ~5 hours after the trigger and they clearly show a rise to be followed
by a decay and then a rise to a second peak. This second peak is the one picked
up by most observers who started observing the second night after the trigger.

*** S. Kulkarni reported the detection by his group of a radio counterpart for
GRB970508 (Frail et al) in 8.4 GHz over 5 days after the trigger. Their first
observations in 1.4 GHz, starting 3.5 hours after the trigger, did not reveal
any source. S. Kulkarni also reported on the detection of a variable radio
source outside the error box of GRB970111; the object was intriguing enough to
make it difficult for him to accept the accuracy of the error box!

*** D. Macchetto and P. Caraveo, both reported on the HST observations of
GRB970228: the latter detects and the former does not detect proper motion in
the pointlike source. This is one of the biggest controversies currently that
has to be resolved. The extended source properties of GRB970228 (decay or not
decay) is another one. D. Lamb reports that his simulations show that the two
HST images are consistent with each other but not consistent with the Keck
results that show a fading of the extended source. Golden also reported that
his analysis of the HST data supports the Caraveo results, insofar that he also
detects a proper motion but of half the amplitude.

Theory:

The scientists are converging here on the fireball model for the GRB origin. M.
Rees, P. Meszaros and T. Piran gave very interesting reviews and several
speakers elaborated on particular variants (R. Wijers, Pietri, E. Waxman, M.
Tavani, I. Wasserman, T. Bulik).  The main idea was that internal shocks are
most probably responsible for the gamma-rays and external shocks for the
optical and the x-rays. This idea is supported by the complexity of the
structure expected from the former and the smoothness expected from the latter,
which fits well the observations. It was also pointed out that the optical and
the gamma-ray emission may not come from the same beam direction; this opens
the question: how many optical counterparts without detectable GRBs are
flashing around ?


Future:

It was pointed out that ther wide field instruments can attempt to raster the
larger error boxes provided with BATSE (of the order of 2 degrees radius), to
enhance the sample of the optical counterparts. LOTIS and ROTSE I and II are
some of the best candidates for that follow-up observation (Park, McKay).R.
Hudec, J.-L. Atteia and T. Cline reported on new instruments, ideas, and
proposals that are submitted for future instruments (BART/OTM, Tarot, BASIS). A
new project was reported which combines two NASA satellites (CGRO/RXTE) to
derive small GRB error boxes: BATSE two degree error boxes are communicated to
RXTE, which slews and scans the whole error region. A serendipitous detection
of an X-ray source will be distributed with BACODINE to the community
immediately and thus enable follow-up observations (C. Kouveliotou, F.
Marshall, S. Barthelmy).

The policy of the data dissemination was discussed by a group of 6 speakers,
who suggested that the data become immediately public, be it from BeppoSAX or
GRO/XTE. The HST representative also assured the participants that future HST
observations will become public immediately.

The meeting was closed with a concise summary by L. Woltjer.

Semi-Final Program:

Recent Developments Towards Understanding of Gamma ray Burst

MAY 26  -  MORNING SESSION

8:45	SERGIO DE JULIO, 	Welcome address
	President of ASI

9:00	G. FISHMANN		General review of gamma-ray
	(M.S.F.C.)			bursts

9:30	L. PIRO			The Sax mission
	(I.A.S. - Frascati)

10:00	E. COSTA			Sax observations of gamma-ray
	(I.A.S. - Frascati)		bursts and their X-ray afterglow


10:30	Coffee break


11:00	F. FRONTERA		The Sax gamma burst monitor
	(ITESRE and U. of Ferrara)

11:20	J. HEISE			The Sax wide field camera and the
	(Sron, Utrecht)		positioning of the bursts

11:40	J. GREINER			Rosat observations
	(Potsdam)
	
11:50	A. YOSHIDA		Asca observations
	(Riken)

12:00	J. VAN PARADIJS		Optical observations
	(Amsterdam and Alabama U.)

12:20	A. GUARNIERI		Optical observations
	(Univ. of Bologna)

12:30	A. DI PAOLA		Optical observations
	(Observatory of Roma)

12:40	J. GOROSABEL		Optical observations
	(LAEFF-INTA, Madrid)

12:50	H. PEDERSEN		Optical observations
	(Univ. of Copenhagen)

MAY 26  -  AFTERNOON SESSION

3:00	A. CASTRO-TIRADO	ISO observations
	(LAEFF-INTA, Madrid)

3:10	S. KULKARNI		Optical, IR, radio observations
	(Caltech, Pasadena)

3:30	D. MACCHETTO		HST observations
	(ST SCI Institute)

3:50	P. CARAVEO		Proper motion of bursting sources
	(IFCTR, Milano)

4:00	T. CLINE			The distance of the burst 970228
	(G.S.F.C.)


4:10	Coffee break


4:40	M. REES			Models for the sources
	(I.O.A., Cambridge)

5:20	R. WIJERS			The blast-wave afterglow
	(I.O.A., Cambridge)

5:35	M. VIETRI			Hydrodynamics of the afterglow
	(University of Rome III)

5:50	E. WAXMAN		Models for the afterglow
	(I.A.S., Princeton)

6:05	D. LAMB			Nebular reflection models
	(University of Chicago)

6:20	M. TAVANI			Models of the afterglow
	(IFCTR, Milano)


MAY 27  -  MORNING SESSION

9:00	I. WASSERMANN		Models for bursts
	(Cornell)

9:15	T. PIRAN			Models for the afterglow
	(The Hebrew Univ.)

9:30	T. BULIK			Distance scale
	(Copernicus Center)

9:45	J.P. NORRIS		Time dilation effects
	(G.S.F.C.)

9:55	J. BLOOM			Extragalactic hosts
	(I.O.A., Cambridge)

10:10	Coffee break


10:45	S. BARTHELMY		Bacodine
	(G.S.F.C.)

10:55	H.S. PARK			Lotis
	(Livermore)

11:05	R. BUTLER			High time resolution with Triffid 2
	(Galway)

11:15	R. HUDEC			Search of counterparts
	(Ondrejov)

11:25	T. CLINE			Basis Mission
	(G.S.F.C.)

11:35	K. HURLEY			Sax-Ulysses triangulations
	(Berkeley)

11:45	T. MCKAY		Rotse
	(Michigan)

11:55	F. MARSHALL		XTE
	(G.S.F.C.)


MAY 27  -  AFTERNOON SESSION
(Chairman L. Woltjer)


14:30	General discussion of the scientific strategy for the near future

	SUMMARY OF THE MEETING BY L. WOLTJER
 

First Announcement