GRB 980326

P.J. Groot, P.M. Vreeswijk, T.J. Galama (University of Amsterdam); 
E. Pian, F. Frontera, E. Palazzi (CNR, Bologna); M. Feroci (CNR, Roma);
C. Kouveliotou and C. Robinson (USRA at NASA/MSFC) on behalf of the 
CTIO GRB collaboration; J. van Paradijs (U of Amsterdam and
U of Alabama in Huntsville); C. Tinney, Phillips, Driver, Smith,
Jones, Windridge (AAO); M. Keane, P. Hall, M. Smith (NOAO at CTIO);
J.F. Gonzalez, C. Lidman (ESO) report:
"Comparison of optical R-band observations of the error box of 
GRB980326 (IAUC 6851) taken at the AAT at Mar 27.42 UT (480s), 
the CTIO 4m-telescope at Mar 28.04 UT (600s) and the 
ESO 3.5m NTT at Mar 28.01 UT (ESO1, 600s) and Mar 28.17 UT 
(ESO2, 600s) shows a clearly variable object located at RA 08:36:34.28, 
Decl -18:51:23.9 (J2000).  Differential photometry with respect
to three local stars shows a decrease between the AAT and ESO1 
of 1.2(1) mag, AAT and CTIO of 1.6(1) mag and AAT and ESO2 of 1.7(1) mag. 
The three local standards have coordinate end-figures: 1) 36:35.41, 51:41.6; 
2) 36:31.00, 51:17.3; 3) 36:34.03, 50:53.2.  Their R-band magnitudes are: 
1) 18.4(1); 2) 21.7(1); 3) 20.3(1), as determined with respect to 
Landolt SA 98, no. 626, 624, 634, 590.  At the time of the AAT observation 
(Mar. 27.42 UT) the R-band magnitude of the variable was R=21.7, 
equal to that of comparison star 2. 
A finding chart of the variable can be found at:
http://www.astro.uva.nl/paulgr/grb/grb980326.html.

This message is citeable."

GRB 980326: Optical Transient Confirmed:

A. C. Eichelberger, S. R. Kulkarni, S. G. Djorgovski (California
Institute of Technology) report on behalf of the Caltech-Keck GRB
effort:

On UT March 27, 1998, A. V. Filippenko, D. C. Leonard and A. G. Reiss
(U. C. Berkeley), observed the WFC localization region (IAUC 6851) with
the Low Resolution Imaging Spectrograph (LRIS) on the Keck II
telescope.  On UT March 28, 1998, H. Spinrad, A. Bunker, D. Stern (U.
C. Berkeley), A. Dey (JHU) and S. A. Stanford (IGPP), observed the
same field with LRIS.  For both nights images were obtained in the R band.

We report the following magnitudes for the variable object reported by
Groot et al. (IAUC 6852; also previous GCN note #32):

      R(UT 1998 March 27.31) = 21.04 mag.
      R(UT 1998 March 28.25) = 23.54 mag.

The zero point was set by assuming R=21.7 magnitude for the object
designated as "comparison star 2" by Groot et al.  (IAUC 6852; R=21.7).

Combining the two Keck points with the R magnitude from the AAT
observations (IAUC 6852) we find the variable source exhibits a power
law decay with index = -2.0 This is considerably steeper than those
exhibited by optical transients of previous GRBs.

This note is citeable.

Bruce Grossan, Robert Knop, Saul Perlmutter (Lawrence Berkeley National
Laboratory and CfPA, U.C. Berkeley), and Isobel Hook(ESO) report that the
optical transient counterpart to GRB980326 was observed with 900 seconds of
exposure at Keck II with the LRIS instrument on UT 1998 March 30.2.  Relative
photometric measurements were made with respect to the second and third stars
given in IAUC6582, and the average of these two relative measurements yielded
approximately R=24.5 mag.  A 1.5" aperture was used in 0.9" seeing conditions
for these preliminary results.

Addendum to GCN #34
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Bruce Grossan, Robert Knop, Saul Perlmutter (Lawrence Berkeley
National Laboratory and CfPA, U.C. Berkeley), and Isobel Hook(ESO)
report that in the observations of UT Mar 30.2 using the LRIS
instrument on Keck II (see GCN#34
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 ), OT980326 (see IAUC6582) is
irregular in shape, with some elongation roughly NE to SW.  We have
therefore analyzed the OT photometry with a 1.5" aperture to include
all the light up to the point where the curve-of-growth flattens.  The
OT, with 900 seconds of exposure, was delta R= 3.1 and 4.7 mag fainter
than stars 2 and 3 of IAUC6582, where these reference stars were
measured with a larger 3.0" aperture.  This yields R = 24.8 or 25.0
mag for the OT using the absolute calibrations for stars 2 and 3. Note
that the OT has a neighboring object currently 0.7 mag brighter,
approximately 1.4" E, 0.7" S of the OT.

GRB 980326
     A. Guarnieri, C. Bartolini, A. Piccioni and G. Valentini
University of Bologna, communicate:  "A preliminary analysis 
of two combined 30 min. CCD frames of the GRB 980326 field 
obtained with the Bologna University 1.5-m telescope in R band
on March 27.84 UT with a seeing of 2".8 does not show the OT
(mag. limit of the plates about 22).  We can state that it was 
fainter than the second comparison star measured by Groot et al. 
(IAUC 6852), so its R magnitude was > 21.7.

This report is citable.

K. Hurley (UCB) on behalf of the Ulysses GRB team,
M. Feroci (IAS/CNR, Rome) and F. Frontera (ITESRE/CNR, Bologna) 
on behalf of the BeppoSAX/GRBM Team report:

We have derived a preliminary IPN annulus for this burst using
Ulysses and the BeppoSAX Gamma-Ray Burst Monitor.  The annulus
is described by a circle of radius 40.771 degrees centered at
RA(2000) = 154.714 degrees, Declination (2000) = 13.166 degrees.
The half-width of the annulus is 0.042 degrees.  The annulus
intersects the WFC error circle (IAUC 6851) and reduces its area.  
The intersection points are:

		RA(2000)		Decl (2000)
     129.194 o = 8 H 36 M 47 S     -18.981 o =-18 o 58 ' 53 "
     128.988 o = 8 H 35 M 57 S     -18.817 o =-18 o 49 ' 01 "
     129.242 o = 8 H 36 M 58 S     -18.911 o =-18 o 54 ' 38 "
     129.054 o = 8 H 36 M 13 S     -18.761 o =-18 o 45 ' 38 "

This error box includes the optical transient (GCN #32
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 , IAUC 6852).
Further processing will reduce the annulus width.  An image may be
found at http://ssl.berkeley.edu/ipn3/980326/.

This message is citable.

Frank Valdes, Buell Jannuzi, and James Rhoads (on behalf of the NOAO
Deep Widefield survey team and the KPNO GRB followup team) report:
We observed the location of the GRB 980326 optical transient on 1998
April 7.15 UT using the KPNO 4 meter telescope and the CCD Mosaic
camera.  The total integration time was 3300 seconds.

The final coadded image shows no source at the location of the optical
transient.  The flux obtained in a 5 pixel (1.30 arcsecond) radius aperture
and calibrated by comparison to Groot et al's reference star 1 (IAU Circular
6852) corresponds to -0.44 +- 0.33 microJansky at R band.  The corresponding
3 sigma limiting magnitude is R > 24.3 ; the 2 sigma limit is R > 25.3.  For
a 7 pixel aperture, the flux is -0.52 +- 0.38 microJansky.

We also placed an aperture on the location of Grossan et al's nearby source
(GCNC #35).  The fluxes in 5 and 7 pixel apertures are 0.14 +- 0.32
microJansky and 0.51 +- 0.37 microJansky respectively.  Corresponding 3 sigma
limits are R > 23.6 and 23.2 ; two sigma limits are R > 24.0 and 23.4
respectively.  These results are marginally consistent with Grossan et al's
magnitude measurement of R=24.1 or 24.3 for the neighboring source.

More detail, including a link to a 104 by 91 arcsecond section of the
final image, is available at http://www.noao.edu/noao/grb/980326.html .
The Mosaic camera has a 36 arcminute field of view, and so covers
the entire BeppoSAX error box (16 arcminute diameter).  We can make
data for this full region available, with the caveat that further data
processing might be required.  Contact one of us if interested.




[GCN Operator's Note:  Due to a programming error this notice
was distributed with a FROM:-line that had incorrect information
(XXXX at XXXX <rene@hep.uchicago.edu>)  The correct information
has been included in this archived copy of the notice.]

S. G. Djorgovski, S. R. Kulkarni, P. Cote, J. Blakeslee, J. S. Bloom, and 
S. C. Odewahn (Caltech), on behalf of the Caltech GRB collaboration, report:

We obtained R band images of the field of GRB 980326, using Keck-II 10-m
telescope, on the night of 17 April 1998 UT.  We clearly detect a galaxy
coincident to within the astrometric errors (less than 0.3 arcsec) with
the previously reported optical transient (cf. Groot et al. IAUC 6852,
and Kulkarni et al. GCN Circ. 33
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 ).

Using the magnitudes of stars 2 and 3 from Groot et al., we derive for
this galaxy R = 25.5 +- 0.5 mag (preliminary reductions).  This is about
0.6 magnitudes fainter than the tentative detection by Grossan et al.
(GCN Circ. 35
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 ), and is consistent with the upper limits reported by 
Valdes et al. (GCN Circ. 56
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 ).

We interpret this object as the host galaxy of GRB 980326, probably with 
a negligible contamination from the OT itself.  The difference in 
brightness from the measurements by Grossan et al. on 30 March 1998 UT 
suggests that some contribution from the OT was still present at that time, 
with the estimated OT magnitude of R ~ 25.8 at that epoch.

Images of the field can be found at:
http://astro.caltech.edu/~george/grb/grb980326.html

Further observations of this field are in progress.

This note can be cited.

F.E. Marshall (Goddard Space Flight Center) and T. Takeshima
(USRA at GSFC) report:

X-ray emission from GRB980326 was not detected during two observations with the 
Proportional Counter Array (PCA) on the Rossi X-ray Timing
Explorer (RXTE). The first observation began at 05h31m UT
on March 27th (about 8.2 hours after the burst) and lasted about
1850 s. The second observation began 12 days later and lasted about
1600 s. During both observations, the PCA alternated between
viewing the GRB and a nearby background region. Assuming that
any X-ray afterglow was negligible during the second observation
and that there were no other variable sources in the field-of-view,
the observed change in counting rate from the direction of the 
apparent optical counterpart of the GRB (IAUC 6852) of 
-0.03 +/- 0.15 (90% confidence) counts/sec/detector is a measure of the
intensity of the X-ray afterglow 8.5 hours after the burst. An upper limit of
0.15 counts/sec/detector corresponds to a flux of 1.6e-12 ergs/s/cm**2 
in the 2-10 keV band for an assumed Crab-like spectrum.

This note can be cited.

J. S. Bloom and S. R. Kulkarni report on behalf of the  Caltech GRB
effort:

"On the nights of 17 and 18, December 1998 we carried out imaging
observations of the field of GRB 980326 with LRIS instrument on Keck
II.  In the R-band we obtained 8 images each with an integration time
of 300 s. These images were debiased, flat-fielded, registered and
median stacked. In the resulting image we do not detect the host galaxy
at the position of the optical transient of this GRB (Groot et al., ApJ
502, L123, 1998). We determined the zero-point calibration using the
reference stars quoted in Groot et al. (1998). The faintest object that
was readily detected and in the general vicinity of the OT was ~27.3
mag (with aperture radius of 1.5 arcsec).  This object and others at
similar brightness levels are clearly visible to the eye in the stacked
image.  We can place a firm lower limit on the magnitude of the host
galaxy at this level.

Previous magnitudes for the the magnitude of the host galaxy were
derived using data when the OT was bright and could have been affected
by the assumed decay model and photometric errors.

The true host of GRB 980326 is thus fainter than R=27.3 mag or that the
OT/GRB is not coincident with its host (e.g. the GRB was ejected from
the host galaxy or is of intergalactic origin).  If the former is true
then the host galaxy of GRB 980326 is by far the faintest host and this
faintness is unlikely due to dust extinction given the detection of
the optical transient in the B band (Groot et al. 1998)."

Andrew Fruchter (STScI), Paul Vreeswijk (University of Amsterdam) and
Peter Nugent (LBNL) report on behalf of a larger HST GRB collaboration:

We observed the field of GRB 980326 with HST/STIS on 31 December 2000,
just over 1000 days after the burst. The total exposure time was 7080
seconds in the 50CCD aperture. The images were reduced by the standard
HST pipeline, and drizzled to a combined image with a pixelscale of
0."0254.

We projected the location of the early afterglow from a KeckII/LRIS R
band image, taken by Filippenko, Leonard and Riess on March 27, 1998
(see GCN 33
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 ).  We estimate the 1-sigma error radius to be less than 1.5
drizzled HST pixels, which includes both the scatter in the
transformation (0.9 pixels, using 6 reference stars) and the
measurement error of the transient in the Keck image (0.8 pixels).

Within one pixel of the estimated position, there was evidence of a
small source.  We therefore convolved the image with a gaussian with a
FWHM equal to that of the PSF (3.5 drizzled pixels).  This procedure
uses the PSF as a matched filter to enhance the the signal-to-noise
ratio of unresolved sources.  In the convolved image an object is
found, again offset by one pixel from the expected position with a
significance of ~4.5 sigma.  We estimate a magnitude (in the broad STIS
50ccd filter) of V = 29.25 +/- 0.25.

Bloom et al., (1999, Nature, 401, 453) have suggested that the unusual
light curve of this GRB might be explained by an underlying supernova,
and on the basis of this estimated a redshift of about z~1.   A
supernova at this redshift (which in this model peaks around a
magnitude of 25) would be expected to be significantly fainter than
V=29 by now (as would the rapidly decaying GRB afterglow).  Therefore,
we may be detecting a faint host galaxy.  Deep HST images have in all
other cases detected a host underlying the GRB, when good astrometry
was available for the GRB.  If we are observing a host galaxy, then the
above magnitude could underestimate its brightness by as much as a few
tenths of a magnitude, due to extended emission missed in this
measurement.  A galaxy at a z~1 with this observed magnitude would lie
a rather remarkable 7 magnitudes below L*, the knee of the of the
galaxy luminosity function at that redshift.

Given the apparent point-source nature of the detected object, and the
lack of color information, we cannot exclude the possibility (which we
consider remote) that we are observing a light echo, from either the
GRB or the supernova.

Images of the field can be found at

http://www.stsci.edu/~fruchter/GRB/980326