GRB 010921

K. Hurley and T. Cline, on behalf of the Ulysses and HETE
GRB teams;

F. Frontera, E. Montanari, C. Guidorzi, and M. Feroci, on
behalf of the BeppoSAX GRBM team;

G. Ricker, D. Lamb, and S. Woosley on behalf of the HETE Science Team;

R. Vanderspek, G. Crew, J. Doty, G. Monnelly, J. Villasenor, N.
Butler, T. Cline, J.G. Jernigan, A. Levine, F. Martel, E. Morgan, G.
Prigozhin, J. Braga, R. Manchanda, and G. Pizzichini, on behalf of
the HETE Operations and HETE Optical-SXC Teams;

N. Kawai, M. Matsuoka, Y. Shirasaki, T. Tamagawa, K. Torii, T.
Sakamoto, A. Yoshida, E. Fenimore, M. Galassi, T. Donaghy, and  C. Graziani, on behalf of the HETE WXM Team;

J-L Atteia, M. Boer, J-F Olive, and J-P Dezalay, on behalf
of the HETE FREGATE Team;

report:

Ulysses and BeppoSAX observed GRB010921 (=H1761, GCN 1096
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 ).  Triangulation
gives an annulus centered at RA(2000), Decl.(2000)=
232.299 deg., +67.605 deg., with radius 60.003 +/- 0.156 deg
(3 sigma).  This annulus intersects the HETE-II WXM error
box (GCN 1096
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 ) at

RA(2000)	Dec(2000)

343.596         +40.669
343.763         +41.069
344.027         +40.816 
344.195         +41.215

to form an error box whose area is approximately 250 sq. arcmin.
Some improvement to this error box is possible.

A. Mahabal, S. G. Djorgovski, and R. Brunner of the Caltech DPOSS
group, with D. W. Fox and P. A. Price of the Caltech-NRAO-CARA GRB
collaboration, report:

"We have made Digital Palomar Observatory Sky Survey (DPOSS) images of
the GRB 010921 error box (GCN #1097
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 ) publicly available through the
React Group webpages at:

    http://www.srl.caltech.edu/~react/GRB010921.html

The images may also be accessed directly as:   

    http://www.srl.caltech.edu/~react/fits/GRB010921j.fits
    http://www.srl.caltech.edu/~react/fits/GRB010921f.fits
    http://www.srl.caltech.edu/~react/fits/GRB010921n.fits

The images are 1801x1801 pixels, centered at (J2000) RA 22:55:34.86,
Dec +40:56:32.1, and covering 30'x30'.  The images are from the second
Palomar Sky Survey and were digitized at STScI; they have subsequently
been corrected for vignetting effects and have corrected astrometric
coefficients in the image headers.  Note that the image bandpasses are
green (j), red (f), and near-infrared (n), corresponding,
respectively, to the IIIa-J, IIIa-F, and IV-N emulsions used in the
survey.  Going forward, we hope to make DPOSS images of GRB fields in
the northern hemisphere available on a regular basis.

Please contact Ashish Mahabal <aam@astro.caltech.edu> or one of the
other authors of the Circular if you have any questions."

D. W. Fox, R. Burruss, E. Berger, J. S. Bloom, S. G. Djorgovski, and
S. R. Kulkarni (Caltech), with M. Haynes (Cornell), report on behalf
of the larger Caltech-NRAO-CARA GRB collaboration:

"R. Burrus obtained Sloan r' images of the joint HETE/IPN localization
for GRB010921 (HETE trigger #1761; GCNs #1096
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  and #1097
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 ) with the Hale
200-inch telescope at Palomar Observatory beginning 22 hours after the
trigger (22 September 03:18 UT).  The instrument used was the Large
Format Camera (LFC) and the observations were 4 min in duration (3
pointings). Visual comparison with the Digital Palomar Observatory Sky
Survey F plate reveals no obvious new source to the depth of the DPOSS
image (R ~ 20.5 mag)."

This message may be cited.

A. Henden (USRA/USNO) reports on behalf of the USNO GRB team:

We have acquired UBVRcIc all-sky photometry for
an 11x11 arcmin field that covers the center of the
error box for GRB010921 (GCN 1096
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 ,1097
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 ) with the USNOFS 1.0-m telescope
on one photometric night.  Stars brighter than
V=13.5 are saturated and should be used with care.
We have placed the photometric data on our anonymous ftp site:
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb010921.dat
The current photometry has a potential external zero-point
error of about two percent.  The astrometry in this file
is based on linear plate solutions with respect to USNO-A2.0.
The internal errors are less than 100mas.
Further calibration of this field will be performed if
an optical afterglow is identified.

A. Henden (USRA/USNO) reports on behalf of the USNO GRB team:

We have acquired single R-band images with the NOFS 1.0m
telescope from 010922 0654-0807 UT, forming a mosaic that
covers nearly all of the HETE/IPN error box (GCN 1097
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 ).

Inspecting the DPOSS images posted by Fox, et al. (GCN 1098
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 ),
this region shows a few galaxies, but also a fair amount
of nebulosity and bright stars.  It is located at galactic
longitude 100.5 and latitude -16.8, so not very far out of
the galactic plane.  At the same time, the star colors 
(Henden GCN 1100
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 ) indicate that there is not a great deal
of reddening in the field.

In agreement with Fox, et al. (GCN 1099
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 ), we find no obvious
afterglow candidate down to the DPOSS plate limit.  There are
several probable variable stars and a couple of faint high proper
motion objects.

P. A. Price, E. Berger, D. W. Fox, D. A. Frail, S. R. Kulkarni,
J. S. Bloom, R. S. Burrus, S. G. Djorgovski, M. Haynes, and
A. Mahabal report on behalf of the Caltech-NRAO-CARA GRB
collaboration:

Following our observations of 2001 Sep 22.14 UT (t_GRB + 22 hr;
GCN #1099
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 ), we re-observed the error box of GRB 010921 (HETE
trigger #1761) with the Large Format Camera (LFC) on the Palomar
200-inch telescope on 2001 Sep 27.35 UT.  Both observations covered
the entire 250 square arcminute error box (GCN ##1096, 1097)
with three pointings in Sloan r'.  In preliminary reduction
of the images, we identified a number of variable sources, and
so further observations were made of the error box on 2001 Oct
17.15 UT with LFC.  We now identify one source within the error
box that exhibited a power-law decay, with index alpha ~ 1.6,
superposed on a bright (R ~ 21.7 mag) galaxy.  The coordinates
of this source are

	RA: 22:55:59.9, Dec: +40:55:53 (J2000)

with an estimated error of 0.6 arcseconds.

Radio observations made with the VLA on 2001 Oct 17.15 reveal
a radio source at the location of the optical transient with
a nu^1/3 spectrum between 4.86 and 22.5 GHz, as has been seen
in previous GRB afterglows.

At the above coordinates, we also find a source detected with
the USNOFS 1.0-m telescope (GCN #1100
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 ), which when corrected for
Galactic extinction has a ~ nu^-2.3 spectrum over BVRI, possibly
indicating extinction in the source frame.

While all properties of this transient source appear to be
consistent with those of a GRB afterglow, we can not with this
information completely exclude the possibility that this object
is a low luminosity AGN.  Nevertheless, we believe that this
object is most likely the afterglow of GRB 010921, detected
and localised by HETE.

This message may be cited.

GRB010921: Spectroscopy of the Host Galaxy

S.G. Djorgovski, A. Mahabal, P.A. Price, J.S. Bloom, D.E. Reichart, E. Berger, 
D.W. Fox, S.R. Kulkarni, D.A. Frail, R. Sari, T. Galama, F. Harrison, and 
S. Yost report on behalf of the Caltech-NRAO-CARA GRB collaboration: 

"We obtained a spectrum of the host galaxy of the proposed afterglow of 
GRB010921 (Price et al., GCN 1107
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 ) using the Double Spectrograph at the
Palomar 200-inch telescope, on 17 October 2001 UT.  Preliminary reductions
of the spectrum indicate a redshift of z = 0.450 +- 0.005, on the basis of 
4 emission lines, [O II] 3727, H-beta, [O III] 5007, and H-alpha.  The 
spectrum is typical for an actively star-forming galaxy, with no sign of 
an active nucleus.  Further analysis is in progress.

Assuming an H_0 = 65 km/s/Mpc, Omega_m = 0.3, Omega_lambda = 0.7 cosmology,
we derive the luminosity distance of 8.30e+27 cm.  Using the fluence of 
1.0e-6 erg/cm2 (Ricker et al. GCN #1096
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 ), the total energy release in the 
restframe range 12-123 keV (redshifted bandpass of the HETE band at 8-85 keV) 
is 6.0e+50 erg.  Though no spectral information was reported, we estimate that 
the isotropic equivalent energy release in the 20-2000 keV range (see Bloom
et al. 2001, AJ, 121, 2879) was E_iso(gamma) = (2.65 +- 1.45)e+51 erg.

To conform with the constant energy of E_gamma = 5e+50 erg (Frail et al.
2001, ApJ Letters, accepted; astro-ph/0102282), the time of the jet break 
would be t_jet ~ 130 days after the GRB.  However, a possible supernova 
associated with this event may be detectable now."

This note can be cited.

S. Klose, B. Stecklum (Thueringer Landessternwarte Tautenburg),  
   on behalf of a larger collaboration, 

report: 

The improved error box of GRB 010921 (Hurley et al., GCN #1097
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 ) was
imaged in the I band using the Tautenburg Schmidt telescope on 
Sep. 22, 18:45 UT - 23:13 UT (38 frames, 2 min exposure time each;
field of view about 35 x 35 arcmin).

The source reported by Price et al. (GCN #1107
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 ) is clearly detected. 
Based on the standard stars provided by Henden (GCN #1100
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 ) we measure 
a magnitude of I=18.93 +/- 0.05.

This message is quotable.

H.S. Park, G. Williams, S. Barthelmy, D. Hartmann, K. Hurley 
report on behalf of the LOTIS collaboration:

LOTIS observed the GRB 010921 optical afterglow area reported 
in GCNC #1107 on 09/21/2001 at 06:08:31 (52 min after the burst)
during a routine sky patrol. LOTIS uses 4 11 cm aperture
telephoto lenses to view an 8.8 x 8.8 deg field of view in
the R, V and clear bands. We searched for an optical counterpart 
in the region of GRB 010921 and we found no source. The 10 sigma 
limits are:

	clear filter	15.3 +/- 0.15
	V filter	14.2 +/- 0.15
	R filter	shutter failed to open

Further analys is in progress.

This message may be cited.

P.A. Price, S.R. Kulkarni, D.W. Fox, E. Berger, J.S. Bloom,
S.G. Djorgovski, D.A. Frail, T.J. Galama, F.A. Harrison,
A. Mahabal, D.E. Reichart, R. Sari and S.A. Yost of the
Caltech-NRAO-CARA GRB collaboration report:

We re-observed the optical transient associated with
GRB 010921 (GCN ##1107,1108) with the Palomar 200-inch
telescope in Sloan g'r'i'z' on 2001 Oct 19.1 UT.  Magnitudes
of the counterpart were measured relative to stars in the
field calibrated with the Palomar 60-inch telescope and
the USNOFS 1.0-metre telescope (GCN #1100
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 ).  We find that,
at this epoch, the counterpart has a spectral flux distribution
that peaks between the r' and i' bands, and falls away on
the red and blue sides as ~ nu^3.5 and ~ nu^-2.1 respectively.

This spectral flux distribution finds a natural explanation
in terms of a supernova underlying the GRB afterglow (Bloom
et al. 1999; Reichart 1999; Galama et al. 2000).  If this
interpretation is correct, the source should continue to
fade beyond R ~ 22 mag over a timescale of a few weeks.

We strongly encourage further monitoring of this source,
especially in the blue and near infrared.

This message may be cited.

References:
Bloom, J.S. et al., 1999, Nature, 401, 453.
Reichart, D.E., 1999, ApJ, 521, L111.
Galama, T.J. et al., 2000, ApJ, 536, 185.

D. Q. Lamb, B. C. Lee, D. L. Tucker, D. E. Vanden Berk, P. Newman,
J. Krzesinski and A. N. Kleinman, on behalf of the SDSS GRB team,
report:

We observed the HETE error box for GRB 010921 (GCN 1096
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 ) using the SDSS
0.5-m "Photometic Telescope" at APO under partly cloudy conditions on
2001 September 22 beginning at UT 02:36 and on 2001 September 23
beginning at UT 05:23.  Images of the entire improved HETE error box
(GCN 1097
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 ) were acquired in the SDSS u, g , r, i, and z filters on both
nights.

The optical transient (OT) reported by Price et al. (GCN 1107
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 ; see also
GCN 1108
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  and GCN 1113
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 ) was r'=19.6 +/-0.3 on September 22 and
significantly fainter on September 23 (we place an upper limit on r' of
roughly 20.0).  The OT is marginally visible (at perhaps the 1 to 2
sigma level) in g and i on September 22 and in g and r on September 23.
We place upper limits on u, g, i, and z of roughly 20.5, 18.5, 17.5,
and 15.0 on September 22, with the completeness limit of the i and z
filters heavily degraded by clouds, and 19.5, 20.0, 18.5, and 18.5 on
September 23, again impacted by clouds.  These upper limits are
consistent with a ~ nu^{-2.3} spectrum over u, g, r, i, and z (GCN
1107
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 ).  Images can be found at http://sdss.fnal.gov:8000/grb/.

This message is quotable.

H.S. Park, G. Williams, S. Barthelmy, T. Cline, D. Hartmann, 
K. Hurley, W. Pereira
report on behalf of the LOTIS collaboration:

Super-LOTIS observed the HETE/IPN error box of GRB 010921
(GCNC #1096, #1097) on 09/22/2001 starting at 03:05 UTC, 
21.8 hr after the burst. Super-LOTIS is a 0.6 m telescope 
located at Kitt Peak. These observations were made with a 
clear filter and an effective integration time is 1000 s. 
We detect the afterglow reported by Price et al. (GCNC #1107) 
at 2 epochs. The magnitude of the afterglow is calibrated 
against R band secondary standards reported in GCNC #1100 
by Henden.

	UTC		Ropen
	9/22/01 03:05	19.4 +/- 0.2
	9/22/01 06:25	19.9 +/- 0.2
	9/23/01 03:05	> 21.2 +/- 0.3
	9/23/01 06:27	> 21.2 +/- 0.3

Further analysis is in progress.

This message can be cited.

GRB 010921: HST Detection of the Afterglow and Host Galaxy

J. S. Bloom, S. R. Kulkarni, F. A. Harrison, T. J. Galama, P. A. Price,
S. G. Djorgovski, D. W. Fox, E. Berger, and D. A. Frail report on behalf
the larger Caltech-NRAO-CARA GRB Collaboration:

"We observed the field of GRB 010921 (Ricker et al., GCN #1096
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 ; Hurley et
al., GCN #1097
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 ; Price et al., GCN #1107
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 ) on 2001 October 26-27 UT using
the WFPC F450W, F555W, F606W, F814W, F850LP filters on the Hubble Space
Telescope for a total of 1200 s integration in each bandpass.  The
observations were conducted as part of the HST-GO Proposal #8867 (S.
Kulkarni, P.I.).

An astrometric comparison of the HST images with the Price et al.
discovery image from Palomar reveals that the transient position is
consistent (to better than 1 sigma) with a red compact source O.41 arcsec
to the East of the center of a galaxy. The Palomar localization is
inconsistent (at the 3-sigma level) with the center of the galaxy (note:
no correction has yet been made to the Palomar transient position in the
Palomar image for the contribution of the galaxy to the centroid position;
correcting for this effect would tend to push the Palomar transient
position further from the galaxy center).  Given that the red color of the
compact source is consistent with the Price et al. spectral slope and that
the localization is consistent, we identify this source as the afterglow
of GRB 010921 and the galaxy (z=0.45; Djorgovski et al. #1108) as the
host."

An image of the host and the optical transient may be found at:

   http://www.astro.caltech.edu/~jsb/GRB/grb010921-host-reg.ps

The 1 and 3-sigma uncertainty contours of the OT position measured from
the Palomar discovery images is shown.

Links to false-color images of the WFPC field and close-up images of the
OT-host system may be found at:

   http://www.astro.caltech.edu/~jsb/grb010921

Analysis of the broadband afterglow spectra is underway and more HST
observations are scheduled.

This message may be cited.

Julie McEnery on behalf of the Milagro collaboration reports:

Milagro observed GRB010921 at GeV/TeV energies during the burst
duration (12 s) reported by the HETE WXM (GCN 1096
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 ). No evidence for
GeV/TeV emission was found.  A preliminary analysis (assuming a
differential photon spectral index of -2.4) gives an upper limit on
the fluence at the 99% confidence level of:

J(E > 2 TeV) < 2 * 10^(-7) erg cm^(-2) 

The spectrum of the host galaxy of the proposed afterglow of GRB010921
implies a redhift of 0.45 (GCN 1108
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 ). We expect that TeV photons will
be attenuated by pair production with infrared photons in
intergalactic space so we also calculate an upper limit on the fluence,
assuming a spectrum truncated at 150 GeV. In this case we 
find an upper limit on the fluence at the 99% confidence level of:

J(100-150 GeV) < 4 * 10^(-5) erg cm^(-2) 

These upper limits are preliminary and will be refined with further
analysis. These limits do not incorporate systematic uncertainties 
which may be of order 50%.

D. Q. Lamb, D. G. York, R. J. McMillan and D. Johnston report:

We have observed the improved HETE error circle for GRB020127 (= H1902)
(GCN 1229
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 ) using the ARC 3.5-m telescope at APO under excellent seeing
conditions but a very bright moon on 2002 January 28 beginning at UT
01:38 (4.43 hours after the GRB).  A series of images covering an area
somewhat larger than the entire improved HETE error circle was acquired
in SDSS r' and i' filters.

Visual inspection of the series of r' and i' images reveals no new
object brighter than the DSS limiting magnitude for this field (R ~
19.5).

This message is quotable.

[GCN OPS NOTE (28Jan02):  See errata notice in GCN# 1231.]

P.A. Price, B.P. Schmidt (RSAA, ANU) and S.R. Kulkarni (Caltech)
report on behalf of the larger REACT GRB collaboration:

As a part of our AO-9 HST GRB program, we observed the afterglow
(Price et al GCN #1107
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 ) of GRB 010921 (Ricker et al. GCN #1096
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 ).
We obtained WFPC2 observations through multiple filters, designed
to detect or constrain underlying supernovae for low-redshift
GRBs.  The observations were obtained on 2001 Oct 26, Nov 6,
Nov 25 and 2002 Jan 4.  We have drizzled, registered and subtracted
the images to obtain host-subtracted fluxes at each of the first
three epochs, assuming the flux of the GRB in the final epoch
image is negligible.  The light-curve is available from

            http://www.mso.anu.edu.au/~pap/grb010921/

We identify a break in the light-curve at approximately 35 days
which may be due to collimation of the ejecta.  If so, then
we calculate a jet-corrected gamma-ray energy release of
6.7 x 10^50 erg, consistent with the clustering of gamma-ray
energy releases found by Frail et al. (2001, ApJ, 562, L55).

SN 1998bw at z = 0.451 should peak in the F702W and F814W bands,
at approximately 4 uJy.  No evidence for such a component in the
light-curve is seen in the F555W, F702W, F814W and F850LP bands.
These measurements rule out the existence of a SN with a luminosity
greater than approximately 20% that of SN 1998bw underlying the GRB.
We note, however, that we cannot rule out the existence of a more
typical SN Ib/c, such as SN 1994I.  We strongly recommend that
observers planning a search for a SN underneath a GRB afterglow do
not assume that the SN will be as luminous as SN 1998bw.

A more detailed analysis of the data is in progress.

This message may be cited.

[GCN ED NOTE (06Mar02):  A type was corrected: "6.7 x 10^51" to "6.7 x 10^50".]