GRB 980425

Stephen Holland, Johan Fynbo, Bjarne Thomsen (University of Aarhus),
Michael Andersen (University of Oulu),
Gunnlaugur Bjornsson (University of Iceland),
Jens Hjorth (University of Copenhagen),
Andreas Jaunsen (University of Oslo),
Priya Natarajan (University of Cambridge, & Yale), and
Nial Tanvir (University of Hertfordshire)

	We have obtained 1240 seconds of STIS images with the 50CCD
(clear) aperture and 1185 seconds with the F28X50LP (long pass)
aperture of the host galaxy of GRB 980425.  This data was taken as
part of the Survey of the Host Galaxies of Gamma-Ray Bursts (Holland
et al. GCN 698
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 ) approximately 778 days after the burst.  Combined
images are now available at
"http://www.ifa.au.dk/~hst/grb_hosts/data/index.html" and a GIF image
of the host galaxy is available at
"http://www.ifa.au.dk/~hst/grb_hosts/data/grb980425_colour.gif".

	An astrometric solution from VLT V-, R-, and I-band images
(P.I., B. Libundgut) suggests that the supernova is located at X = 987
+/- 2, Y = 1064 +/- 2 on our drizzled long pass image.  This is in an
extended object with AB magnitudes of 26.2 +/- 0.1 in the STIS clear
filter and 26.3 +/- 0.1 in the STIS long pass filter.  An
extrapolation of the V-band late-time light curve of SN 1998bw
(McKenzie & Schaefer 1999, PASP, 111, 964) suggests that the SN
remnant will have a magnitude of ~28.4 in the clear filter, which is
near the detection limit of our data.  The extended object has a
full-width at half-maximum, in the long pass image, of 0.13
arcseconds.  When the resolution limit (0.088 arcsec) is taken into
account the object has a diameter of, at most, ~0.09 arcsec.  For z =
0.0085, and a cosmology with H0 = 65, Omega_matter = 0.2 and
Omega_lambda = 0, this corresponds to a diameter of less than ~17 pc.
The colour and size of the object is consistent with it being a young
star cluster.  The extended object is embedded in a large extended
feature (possibly an HI region) with a diameter of ~75 pc and an
estimated colour of V-R ~= 0.7 +/- 0.3.  A GIF image of the region
containing the supernova is available at
"http://www.ifa.au.dk/~hst/grb_hosts/data/grb980425cd_sfr.gif".  A
detailed analysis of these images is in progress (Holland et al. 2000,
in preparation).

Bradley E. Schaefer and Eric H. McKenzie (Yale University) report:

"We have obtained 139 photometric observations in B, V, and I of the late
time light curve for SN1998bw associated with GRB980425.  These
demonstrate a strikingly linear decline in magnitude versus time.  

Our data was taken with the Yale 1.0-m telescope on Cerro Tololo between
27 June and 28 October 1998.  We used standard IRAF reduction and the
comparison stars of Galama et al.
(http://www.astro.uva.nl/~titus/grb980425/grb980425chart.html).  During
the time interval of our observations, the light from the underlying
galaxy (ESO 184-G82) was insignificant in our photometry.
	
Our first and last data nights had measured photometry as follows:
  JD2450992.9   B=16.68+-0.03, V=15.79+-0.02, I=15.16+-0.03
  JD2451115.6   B=18.35+-0.06, V=18.09+-0.05, I=17.33+-0.06
Between these two nights, all our photometry is perfectly consistent with
an exactly linear decline in magnitudes (hence an exponential decline in
luminosity).  Our measured uniform decline rates are as follows:
  B   0.0141+-0.0002 mag/day for equivalent half-life of 53.4+-0.8 days
  V   0.0184+-0.0003 mag/day for equivalent half-life of 40.9+-0.7 days
  I   0.0181+-0.0003 mag/day for equivalent half-life of 41.6+-0.7 days

The observed exponential decline is in contradiction to the theoretical
predictions of Iwamoto et al. (1998, Nature, 395, 672) and Iwamoto (1999,
ApJ, astro-ph/9810400), which claims that the decline will be as a power
law.  The observed exponential decline has a rate similar to that expected
from the decay of radioactive cobalt as modified by the effects due to the
expansion of the shell.  So it is reasonable to conclude that the
late-time light curve of SN1998bw is being powered by radioactive cobalt.
This then implies that the underlying explosion mechanism must create
large masses of radioactive cobalt.

The light curve of SN1998bw is significantly different from all
previously known supernovae.  For a comparison with Type Ia events, the
decline rate of SN1998bw is the same in B but not V and I as for Type Ia
events, while SN1998bw does not show the bump in the I band light curve
from 20-50 days after peak.  A comparison with Type Ic events is difficult
since their lights curves are not well defined, yet the late time decline
rate of Type Ic events is substantially smaller than for SN1998bw.
	
Our observations of SN1998bw are continuing with the Yale 1.0-m
telescope.  However, since the source has come out from behind the Sun,
the light from the galaxy  provides an increasingly significant
obstacle to accurate photometry.  Perhaps future accurate photometry must
await the complete fading of SN1998bw to allow for subtraction of the
galaxy light."

GRB980425
 
 
E. Pian, ITESRE, CNR, Bologna, Italy, L. A. Antonelli, Osservatorio di
Monteporzio, Rome, Italy, L. Piro, and M. Feroci, IAS, CNR, Rome, Italy
 
report:  "The field of GRB980425 (IAUC 6884) was re-observed with the
BeppoSAX Narrow Field Instruments on 1998 November 10.754-12.004 UT.
Preliminary analysis of the MECS data shows that the source
1SAXJ1935.0-5248 reported in the GCN N. 61 and 69, the position of
which (see updated coordinates in GCN N. 155) is consistent with that
of the supernova SN1998bw (Galama et al. 1998, Nature 395, 670), has
a count rate of (1.8 +- 0.4)x10E-03 cts/s in the 1.6-10 keV range,   
and therefore has decreased by approximately a factor of two with    
respect to the average level observed in April-May (see GCN N. 61 and
69). This suggests the presence of variable X-ray emission from the
supernova.
This preliminary analysis shows also that the source 1SAXJ1935.0-5248
is slightly extended.
 
The source 1SAXJ1935.3-5252 (see GCN N. 61, 69, 155) is not detected
down to a 3-sigma limit of 1.4E-03 cts/s.

J. P. Halpern, Columbia University, reports:

I obtained optical spectra of objects in the error circle of the variable
X-ray source 1SAXJ1935.3-5252 (Pian et al. GCN 61
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 ,69
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 ) using the CTIO 1.5m 
telescope on Oct. 17-21.  A total of 24 objects in the original error circle
(Pian et al. GCN 61
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 ) and the revised error circle (Piro et al. GCN 155
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 ) were
observed, completing this spectroscopic survey to a limiting magnitude of 18.
No compelling candidate for identification was found at this level.  Thus,
1SAXJ1935.3-5252 remains a plausible candidate for the afterglow of
GRB980425.

The brightest galaxy in this field, of approximate magnitude 17.5, has narrow 
emission lines of H-alpha and [N II] 658 nm at z = 0.142, but no evidence of 
nuclear activity.  Its position is RA = 19h35m27.6s, Dec = -52o53'00" (J2000), 
which is within both the old and the new error circles.

Deeper spectroscopy in this field using larger telescopes would be worthwhile,
since the optical counterpart of 1SAXJ1935.3-5252 could easily be fainter than 
the limit surveyed here.   If a definitive identification cannot be obtained,
more X-ray observations of 1SAXJ1935.3-5252 are needed to determine if this
X-ray source recurs.

GRB980425: revised SAX-NFI positions

L. Piro, IAS/CNR, Rome, Italy, R.C. Butler, ASI, Rome, Italy, F. Fiore, A.
Antonelli, BeppoSAX SDC, Roma, E. Pian, ITesre, Bologna, Italy, report: 

The majority of BeppoSAX observations were performed with the star tracker
Z (i.e., the one co-aligned with the NFI) in the attitude control loop.
However, a few observations could not be performed in this configuration,
but had to be carried out with the X and Y star trackers only.  We have
found out that, in these very infrequent cases, the attitude
reconstruction could have an error of up to 3'.  This problem does not
affect observations accomplished after May 5, 1998.  Prior to this date,
the only GRB which was observed by the NFI in this configuration is
GB980425.  The revised coordinates (equinox 2000.0) of the two sources
found in the WFC error box (Pian et al., GCN 61
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 ) are:
1SAXJ1935.0-5248, RA = 19h35m05.9s, Dec =-52o50'03";  
1SAXJ1935.3-5252, RA = 19h35m22.9s, Dec =-52o53'49",
with a conservative error radius of 1.5'.  Both updated positions are
still within the WFC error box (Soffitta et al., IAU Circ. No. 6884). 
However, 1SAXJ1935.0-5248 is at 50" from SN1998bw (Galama et al. 1998,
Nature, in press, astro-ph/9806175;  Kulkarni et al., Nature, in press,
astro-ph/9807001) and therefore consistent with it, while the variable
source 1SAXJ1935.3-5252 is 3' away, i.e., inconsistent.

M. H. Wieringa, R. M. Wark, J. L. Higdon, Australia Telescope National
Facility, D. A. Frail, National Radio Astronomy Observatory,
S. R. Kulkarni, J. S. Bloom, California Institute of Technology
report:

Monitoring of the time-variable radio source (GCN 63
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 , IAUC 6896, GCN
70
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 ) coincident with a supernova candidate proposed by Galama et
al. (GCN 60
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 , IAUC 6895) has continued with the Australia Telescope
Compact Array at 20, 13, 6 and 3 cm.  The radio source may have reached
a peak on May 7 1998 at 6 and 3 cm of 45 and 49 mJy, respectively.  A
radio light curve, spectra, and individual maps are available at
http://www.narrabri.atnf.csiro.au/public/grb/grb980425.html

S. R. Kulkarni, J. S. Bloom, California Institute of Technology,
D. A. Frail, National Radio Astronomy Observatory, R. Ekers,
M. Wieringa, R. Wark, J. L. Higdon, Australian Telescope National
Facility report:

Within the localization of GRB 980425 (IAUC 6884) Galama et al. (IAUC
6895) reported a possible supernova candidate for which Wieringa et
al.  (IAUC 6896) saw a brightening radio source.  The object appears to
be an unusual supernova based on its spectrum (IAUC 6895).  The
continued brightening in the optical (IAUC 6899) suggests that the
supernova is young and is compatible with an explosion on or around
April 24, 1998, the epoch of GRB 980425.  For an assumed expansion speed
of 20,000 km/s and a distance of 44 Mpc to the host galaxy of the
supernova (from the redshift given in IAUC 6896) we derive a brightness
temperature of 3x10^14 K from the observed 39 mJy at 6 cm on May 5
(IAUC 6896). This is in excess of the usual Compton limit of 10^12 K.
Despite this, no X-ray emission is seen (GCN #69
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 ).  Thus we are forced
to invoke relativistic expansion speed which results in a larger source
size and correspondingly smaller brightness temperature.  We suggest
that the radio emission arises in a relativistic shock and the optical
emission in a standard low velocity shock. The model predicts that the
radio source should not exhibit diffractive scintillation.  We urge
observers to carry out higher frequency radio observations and IR
observations as these directly measure the particle spectrum that gives
rise to the radio emission.  The urgency is that the radio emission may
cease once the relativistic shock runs into denser ambient gas.
Parenthetically, we note that it is possible that such a fast moving
shock could generate an initial burst of gamma-rays."

GRB980425


E. Pian, F. Frontera, Istituto TESRE, CNR, Bologna, Italy, 
L.A. Antonelli, SAX-SDC, Rome, Italy, L. Piro, IAS, CNR, Rome, Italy, 
also on behalf of the BeppoSAX team

report:  "A second Target-of-Opportunity observation of the BeppoSAX Wide
Field Camera (WFC) error box of GRB980425 (IAUC 6884) was done with the
BeppoSAX Narrow Field Instruments (NFI) in May 2.604-3.646 UT. Preliminary
analysis of the MECS data shows that the source 1SAXJ1935.0-5248 reported in
GCN N. 61 has a count rate of (3.0 +- 0.5)x10E-3 cts/s in the 1.6-10 keV range
(1.9x10E-13 erg sE-1 cmE-2 in 2-10 keV), and therefore has decreased only with
marginal significance with respect to the TOO observation performed in April
26.31-28.16 UT.  A 3-sigma upper limit of 1.5x10E-3 cts/s in the 1.6-10 keV
range is found for 1SAXJ1935.3-5252.  If one assumes for the X-ray emission of
this source a power-law temporal decay f ~ (t - t0)^{-alpha}, the measured
upper limit is consistent with the alpha >0.4 fading observed between the
first and second part of the first TOO NFI pointing (GCN N. 61), as well as
with the alpha ~1.4 decrease between the 2-10 keV flux preliminarily estimated
from the WFC detection (~2.6x10E-08 erg sE-1 cmE-2, Pian et al. 1998, in
preparation) and that observed in the first 27.7 hours of the first NFI
pointing (GCN N. 61). This variability behavior is typical of X-ray afterglows
of GRBs so far detected.

GRB 980425: BATSE Observations

R. M. Kippen (University of Alabama in Huntsville) reports on behalf
of the BATSE GRB team:

GRB 980425 (IAUC 6884) was detected by BATSE on Apr. 25.90913 UT as
trigger number 6707.  The event consisted of a single pulse lasting
about 40 s with little resolvable structure.  Its peak flux (50-300
keV; integrated over 1 s) and fluence (> 20 keV) are 0.96 (-/+ 0.05)
photons cmE-2 sE-1 and 4.0 (-/+ 0.6) x 10E-6 erg cmE-2, respectively,
ranking it near the middle of the BATSE burst flux/fluence distribution.
The BATSE location is consistent with that of the BeppoSAX-WFC and the
reported SAX-NFI x-ray counterpart (GCN 61
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 ).  Please note that this report
has been delayed due to an interruption of BATSE data delivery from
GSFC.  Bursts are normally fully processed within a few days.

The BATSE lightcurve and location map can be found at
http://www.batse.msfc.nasa.gov/~kippen/batserbr/

This message is citeable.

P.M. Vreeswijk, T.J. Galama, P.J. Groot (University of Amsterdam);
F. Frontera, E. Palazzi, N. Masetti (CNR, Bologna); L. Nicastro
(IFACI-CNR, Palermo); E. Costa, L. Piro (CNR, Roma); R.G. Strom (NFRA
and U of Amsterdam); C. Kouveliotou (USRA/MSFC); J. van Paradijs (U of
Amsterdam and U of Alabama in Huntsville); J.F. Gonzalez, V. Doublier
(ESO, Chile) report: "Comparison of R band images of the error box of
GRB 980425 (Soffitta et al. 1998; IAUC 6884) taken at the ESO NTT
telescope on April 28.37 UT (900s) and May 1.33 UT (900s) shows no
variation > 0.3 mag down to 22.8 mag at the location of the transient
BeppoSAX NFI X-ray source 1SAXJ1935.3-5252 (Pian et al. 1998; GCN #61
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 )."

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Optical Observations of GRB 980425

J.S. Bloom, S.R. Kulkarni, S.G. Djorgovski (Caltech), P. McCarthy
(OCIW) and D.Frail (NRAO) report on behalf of the Caltech-OCIW GRB
effort: "Kaspar von Braun (U.  Mich) imaged the field of the X-ray
source reported in IAUC 6884 (Soffitta et al.) on the 40-inch at Las
Campanas Observatory in Chile.  Two sets of images were obtained in the
R band at April 27.4 UT and April 28.4 UT.  We scrutinized the images
in the vicinity of the two BeppoSAX NFI sources reported by L. Piro and
Pian et al. (GCN #61
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 ).  In the 1-arcmin error circle of  the fading
X-ray source SAXJ1935.3-5252 we found no source with variability
greater than 0.2 mag for R < 21 mag.

We thank Kaspar von Braun and Mario Mateo for their help on these
observations. "

This report may be cited.

GRB 980425: Radio Observations

M. Wieringa (ATNF), D.A. Frail (NRAO), S.R. Kulkarni (Caltech),
J.L. Higdon (ATNF), R. Wark (ATNF), and the BeppoSAX GRB Team report:

Radio observations were made with the Australian Telescope Compact
Array beginning on April 28.73 and April 28.98 UT at 6cm and 3cm,
centered on the position of 1SAXJ1935.0-5248, an X-ray source detected
by the NFI on BeppoSAX (Pian et al. 1998; GCN #61
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 ). In addition, radio
observations of the transient BeppoSAX NFI source 1SAXJ1935.3-5252
were made (also at 6cm and 3cm) on April 29.79 and April 30.00 UT

No radio sources were detected in the 1' error radius of the two NFI
sources. Typical three-sigma limits were 0.26 mJy and 0.3 mJy at 3cm
and 6cm, respectively.

Three radio sources are detected in the 8' error radius of the
BeppoSAX WFC. There is one object just outside the error circle of
1SAXJ1935.0-5248 at RA 19:34:56.49, Dec -52:49:04.9 (J2000) with a
flux density of 2.4 mJy at 6cm and 1.1 mJy at 3cm. The brightest radio
source in the field is at RA 19:35:03.31, Dec -52:50:44.7 (posn. error
+/-0.1"). It coincides within the likely optical astrometric errors to
the candidate supernova proposed by Galama et al. (1998, GCN# 60) in
the barred spiral Galaxy ESO 184-82. Approximate flux densities
(corrected for primary beam attenuation) on April 28 were 9.3 mJy and
17 mJy at 6cm and 3cm, respectively.

Further observations of 1SAXJ1935.3-5252 are planned.

This message is citable.

T.J. Galama, P.M. Vreeswijk, P.J. Groot (University of Amsterdam);
E. Pian, F. Frontera, E. Palazzi, N. Masetti (CNR, Bologna);
L. Nicastro (IFACI-CNR, Palermo); M. Feroci (CNR, Roma); R.G. Strom
(NFRA and U of Amsterdam); C. Kouveliotou (USRA/MSFC); J. van Paradijs
(U of Amsterdam and U of Alabama in Huntsville) report: "Comparison of
red band images of the error box of GRB 980425 (Soffitta et al. 1998; 
IAUC 6884) taken at the 50in telescope at the Australian National
University's Mt. Stromlo Observatory on April 26.63 UT (1800s) and April
28.68 UT (1800s) shows no variation > 0.2 mag down to about 21 mag at 
the location of the transient BeppoSAX NFI X-ray source 1SAXJ1935.3-5252
(Pian et al. 1998; GCN #61
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 )."

This work is based partly upon images obtained by the MACHO Project
with the 50in telecope at the Australian National University's Mt. Stromlo 
Observatory.

This message is citeable.

GRB980425


E. Pian, Istituto TESRE, CNR, Bologna, Italy, L. A. Antonelli, M. R.
Daniele, S. Rebecchi, SAX-SDC, Rome, Italy, V. Torroni, SAX-SOC, Rome,
Italy, G. Gennaro, SAX-OCC, Rome, Italy, M. Feroci, L. Piro, IAS, CNR,
Rome, Italy,

report:  "The BeppoSAX Wide Field Camera error box of GRB980425 (IAUC
6884) was observed with the BeppoSAX Narrow Field Instruments (NFI) 
starting about 10.0 hr after the burst in the period April 26.31-28.16 UT. 
Preliminary analysis of the data shows two previously unknown X-ray
sources in the LECS and MECS:  1SAXJ1935.0-5248 and 1SAXJ1935.3-5252,
located at RA = 19h35m04s, Dec = -52o48'33'' and at RA = 19h35m21s, Dec =
-52o52'19'' (equinox 2000.0), respectively, with an error radius of 1'
each.  These positions are 2' and 4'.7 away from the centroid of the WFC
error box, respectively.  During the first 27.7 hours of observation, the
average count rates in the two MECS units (1.6-10 keV) were (4.5 +-
0.7)x10E-3 cts/s, corresponding to (3.0 +- 0.4)x10E-13 erg sE-1 cmE-2 in
the 2-10 keV range, for 1SAXJ1935.0-5248 and (2.4 +- 0.5)x10E-3 cts/s for
1SAXJ1935.3-5252, equivalent to (1.6 +- 0.3)x10E-13 erg sE-1 cmE-2 (2-10
keV). The measurements during the subsequent 16.6 hours showed that
1SAXJ1935.0-5248 did not vary significantly from the first epoch. 
1SAXJ1935.3-5252 was not detected at the second epoch, with a 3 sigma
upper limit of 1.8x10E-3 cts/s.  This indicates a decay by a factor of 1.3
or more in 22 hours.  A further BeppoSAX observation is already planned to
monitor the behavior of both sources."

T.J. Galama, P.M. Vreeswijk, P.J. Groot, B. Stappers (University of
Amsterdam); E. Pian, F. Frontera, E. Palazzi, N. Masetti (CNR,
Bologna); L. Nicastro (IFACI-CNR, Palermo); M. Feroci (CNR, Roma);
R.G. Strom (NFRA and U of Amsterdam); C. Kouveliotou (USRA/MSFC);
J. van Paradijs (U of Amsterdam and U of Alabama in Huntsville)
report: "Comparison of Digitized Sky Survey (DSS) and ESO NTT images
obtained on April 28.37 UT (1998) shows a point source in the BeppoSAX
WFC error box of GRB 980425 (IAUC 6884) which is not visible in the
DSS.  The object is also detected in red and blue band images taken at
the 50in telecope at the Australian National University's Mt. Stromlo
Observatory on April 26.63 UT (1800s), April 26.81 UT (1800s) and Apr
28.68 UT (1800s). It is located at RA 19:35:03.17, DEC -52:50:46.1
(J2000), offset from the nucleus of the barred spiral galaxy ESO
184-82 (in the DN 1931-529 group of galaxies; Duus & Newall, ApJS 35,
209, 1977) and coincident with the spiral arms. The object varies by
less than 0.2 magnitudes between April 26.63 and 28.83 and has a blue
magnitude of 16.2 +/- 0.5. It is therefore not clear whether the
source is related to GRB 980425, or whether it is, e.g., a
supernova. Images of the object and the SB galaxy can be found at
http://www.astro.uva.nl/titus.  Comparison of the Mt. Stromlo April
26.63 UT and Apr 28.68 UT red band images at the location of the X-ray
source (BeppoSAX GRB MAIL N. 98/11) shows no variation > 0.2 mag down
to about 21 mag."

This work is based partly upon images obtained by the MACHO Project
with the 50in telecope at the Australian National University's Mt. Stromlo 
Observatory.


This message is citeable.