GRB 070521

C. Guidorzi (Univ Bicocca&INAF-OAB), S. D. Barthelmy (GSFC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
C. B. Markwardt (GSFC/UMD), A. Moretti (INAF-OAB),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), P. Romano (Univ. Bicocca & INAF-OAB) and
G. Tagliaferri (INAF-OAB) report on behalf of the Swift Team:

At 06:51:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070521 (trigger=279935).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 242.669, +30.241 which is 
   RA(J2000)  =  16h 10m 41s
   Dec(J2000) = +30d 14' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with a duration of about 40 sec.  The peak count rate
was ~7000 counts/sec (15-350 keV), at ~20 sec after the trigger. 

The XRT began taking data at 06:52:27 UT, 77 seconds after the BAT
trigger.  The XRT on-board centroid algorithm did not find a source in the
image and no prompt position is available.  However, the downlinked X-ray
spectrum and lightcurve show that there is an X-ray object in the
field that appears to be fading. 
Using prompt downlinked data, we find a position RA, Dec 242.6606, 30.2579 which is
   RA(J2000)  = 16h10m 38.5s
   Dec(J2000) = +30d 15' 28.3''
with an uncertainty of 4.0 arcsec (radius, 90% containment). 


UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 81 seconds after the BAT trigger. No afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of
the BAT error circle. The typical 3-sigma upper limit has been about 18.5 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.03.

Richard J. Cool (Arizona), Daniel J. Eisenstein (Arizona), David W. Hogg
(NYU), Michael R. Blanton (NYU), David J. Schlegel (LBNL), J. Brinkmann
(APO),
Donald Q. Lamb (Chicago), Donald P. Schneider (PSU), and Daniel E. Vanden
Berk
(PSU) report:

The Sloan Digital Sky Survey (SDSS) imaged the field of burst GRB070521
prior to the burst.  As these data should be useful as a pre-burst comparison
and for calibrating photometry, we are supplying the images and photometry
measurements for this GRB field to the community.

Data from the SDSS, including 5 FITS images, 3 JPGS, and
3 files of photometry and astrometry, are being placed at
http://mizar.as.arizona.edu/~grb/public/GRB070521

We supply FITS images in each of the 5 SDSS bands of a 8'x8' region centered
on the GRB position (ra=242.669 (16:10:40.6), dec=30.2410 (30:14:27.6);
Swift-BAT TRIGGER 279935), as well as 3 gri color-composite JPGs (with
different stretches). The units in the FITS images are nanomaggies per pixel.
A pixel is 0.396 arcsec on a side. A nanomaggie is a flux-density unit equal
to 10^-9 of a magnitude 0 source or, to the extent that SDSS is an AB system,
3.631e-6 Jy.  The FITS images have WCS astrometric information.

In the file GRB070521_sdss.calstar.dat, we report photometry and astrometry
of 558 bright stars (r<20.5) within 15' of the burst location.  The
magnitudes
presented in this file are asinh magnitudes as are standard in the SDSS
(Lupton
1999, AJ, 118, 1406). Beware that some of these stars are not well-detected
in the u-band; use the errors and object flags to monitor data quality.

In the files GRB070521_sdss.objects_flux.dat and
GRB070521_sdss.objects_magnitudes.dat, we report photometry of 1833
objects detected within 6' of the GRB position.  We have removed saturated
objects and objects with model magnitudes fainter than 23.0 in the r-band.
The fluxes listed in GRB070521_sdss.objects_flux.dat are in nanomaggies
while the magnitudes listed in GRB070521_sdss.objects_magnitudes.dat are
asinh magnitudes.


All quantities reported are standard SDSS photometry, meaning that they are
very close to AB zeropoints and magnitudes are quoted in asinh magnitudes.
Photometric zeropoints are known to about 2% rms.  None of the photometry
is corrected for dust extinction.  The Schlegel, Finkbeiner, and Davis
(1998) predictions for this region are A_U=0.145 mag, A_g=0.107 mag, A_r =
0.078 mag, A_i=0.059 mag, and A_z=0.042 mag.

The file GRB070521_sdss.spectro.dat contains a list of the 5 objects with
SDSS spectroscopy within 6 arcminutes of the GRB position.  In addition to
the redshift and 1-sigma error for each object, this file also lists the
object spectroscopic classification.

SDSS astrometry is generally better than 0.1 arcsecond per coordinate.
Users requiring high precision astrometry should take note that the SDSS
astrometric system can differ from other systems such as those used in other
notices; we have not checked the offsets in this region.

More detailed information pertaining to our SDSS GRB releases can be found
in our initial data release paper (Cool et al. 2006, PASP 118, 733).  See the
SDSS DR4 documentation for more details: http://www.sdss.org/dr5.

These data have been reduced using a slightly different pipeline than that
used for SDSS public data releases.  We cannot guarantee that the values here
will exactly match those in the data release in which these data are
included.
In particular, we expect the photometric calibrations to differ by of order
0.01 mag.

This note may be cited, but please also cite the SDSS data release paper,
Adelman-McCarthy et al. (2006, ApJS, 162, 38), when using the data or
referring to the technical documentation.

E. O. Ofek, S. B. Cenko, and A. Rau (Caltech) report on behalf of a
larger collaboration:

The XRT error circle of GRB 070521 (Guidorzi et al. 2007; GCN 6431
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 ),
coincides with the outskirts of a nearby (z=0.0307) bright (i=14.6)
galaxy.

The XRT position is 30.7" from the galaxy center (19 kpc projected).

The SDSS coordinates of the galaxy are:
  	RA = 16:10:40.75
  	DEC= +30:15:18.5 (J2000.0)

For availbility of SDSS pre burst observations see Cool et al. (GCN 6432
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 ).

Klotz, A. (CESR-OMP), Boer M. (OHP), Atteia J.L. (LATT-OMP) report:

We imaged the field of GRB 070521 detected by SWIFT
(trigger 279935) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 23.8s after the GRB trigger
(8.5s after the notice). The elevation of the field decreased from
from 24 degrees above horizon and weather conditions
were highly variable (cloudy).

The date of trigger : t0 = 2007-05-21T06:51:10.656

The first image is trailed with a duration of 60.0s
from t0+23.8s to t0+83.8s
(see the description in Klotz et al., 2006, A&A 451, L39).
The limiting magnitude is R ~ 17.1.
We do not detect any OT at the XRT position mentioned
by Guidorzi et al. (GCNC 6431) but we have a faint
patch at coordinates 16h10m37.5s (+/-0.5s) +30d14'58"
(+/-5") J2000.0 near the limiting magnitude.

The second image is 30.0s exposure in tracking mode,
no OT is detected:
t0+89.4s to t0+119.4s : R > 17.5

We co-added a series of exposures:
t0+89.4s to t0+570.3s : R > 19.0

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

This message may be cited.

A. Melandri, I. A. Steele, C. G. Mundell, C. Guidorzi, D. Carter,
R. J. Smith, C. J. Mottram, D. F. Bersier, S. Kobayashi, M. F. Bode
(Liverpool JMU), A. Gomboc (Ljubljana), P. O'Brien, E. Rol,
N. Bannister (Leicester) report


The 2-m Faulkes Telescope North robotically followed up GRB070521
(Guidorzi et al., GCN 6431
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 , trigger=279935) beginning ~2.5 minutes
after the GRB trigger time (UT:06:51:10).

No new object has been detected inside the XRT error circle (Guidorzi
et al., GCN 6431
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 ) down to a limiting magnitude of R~19.3 (texp = 30
sec) and I~18.5 (texp = 10 sec), at a mean epoch of ~3 and ~6 minutes
after the burst, respectively (comparison with respect to the USNO B1
catalog).

Further observations are ongoing.

A. Rau, M. M. Kasliwal, and S. B. Cenko (Caltech) report on behalf of a
larger collabortation:

We obtained R-band photometry of GRB070521 with the Large Format
Camera mounted on the Palomar 200-inch telescope. Observations started 29
min post-burst and lasted 3x300s. Calibration was performed relative to
the SDSS (Cool et al.; GCN 6432
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 ).

We find no source in the XRT error circle (Guidorzi et al. 2007; GCN 6431
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 )
to a limiting magnitude of R>23.3.

M.Biskup, M.Cwiok, W.Dominik, G.Kasprowicz, A.Majcher, A.Majczyna,
K.Malek, L.Mankiewicz, M.Molak, K.Nawrocki, L.W.Piotrowski,
M.Sokolowski, J.Uzycki, G.Wrochna, F.Zarnecki
on behalf of "Pi of the Sky" collaboration http://grb.fuw.edu.pl

"Pi of the Sky" apparatus located at Las Campanas Observatory
imaged the region of GRB 070521 from 6:13 UT, i.e. 38 minutes
before the GRB, with 10s exposures (IR-cut filter only).
The most interesting exposure started at 6:51:10.9,
i.e. exactly in coincidence with the GRB trigger.
Variable thin clouds seriously disturbed the observations.
No OT was detected within the XRT error box.

Limits for 10s exposures:
6:50:44-6:50:54   >11.7
6:50:57-6:51:07   >11.9
6:51:10-6:51:20   >12.0 --- GRB trigger
6:51:24-6:51:34   >12.0
6:51:38-6:51:48   >12.2

Limits for 20 coadded images
6:42:51-6:47:17   >12.2
6:47:21-6:51:48   >12.6
6:51:52-6:56:18   >12.5
6:56:22-7:00:53   >12.7

D. Palmer (LANL), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
C. Guidorzi (Univ Bicocca&INAF-OAB), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU),
G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on behalf of the Swift-BAT team:
 
Using the data set from T-240 to T+377 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 070521 (trigger #279935)
(Guidorzi, et al., GCN Circ. 6431
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 ).  The BAT ground-calculated position is
RA, Dec = 242.659, 30.261 deg  which is 
   RA(J2000)  = 16h 10m 38.1s 
   Dec(J2000) = 30d 15' 37.9" 
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 47%.
 
The mask-weighted light curve shows five main peaks the first of which starts
at ~T-10 sec and the last ends at ~T+50 sec.  T90 (15-350 keV) is 37.9 +- 2 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-14.5 to T+49.7 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.10 +- 0.17, 
and Epeak of 195 +- 123 keV (chi squared 50 for 56 d.o.f.).  
For this model the total fluence in the 15-150 keV band is
8.0 +- 0.2 x 10^-6 erg/cm2 and the 1-sec peak flux measured from
T+30.48 sec in the 15-150 keV band is 6.7 +- 0.3 ph/cm2/sec.  
A fit to a simple power law gives a photon index of 1.38 +- 0.04
(chi squared 58 for 57 d.o.f.).  All the quoted errors are at the 90%
confidence level.

T. Hattori, K. Aoki (Subaru Telescope, NAOJ), and N. Kawai
(Tokyo Tech) report on behalf of the Subaru GRB team:

"We observed the field of GRB 070521 (Guidorzi et al., GCN 6431
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 ) 
with FOCAS on the Subaru Telescope starting at 07:31(UT) (40 min after
the trigger) with total exposures of 1920 sec (120s x 16) in z'-band 
and 360 sec (120s x 3) in i'-band.  

We detected a faint source inside the XRT error circle at

RA=16:10:38.55, DEC=30:15:27.7 

with a positional uncertainty of 0.5"
and an approximate magnitude of z' ~ 23.5.

We obtained 3600 sec long-slit spectra of this faint source 
covering a wavelength range from 6000 to 10000 Ang.  
We detected [O III] 5007, 4959 emission lines at z=0.553.
If this object is the host galaxy of GRB 070521, its location at a
relatively low redshift makes it a good candidate for searching the
supernova component.  Note, however, that the absence of a bright
afterglow may indicate that the GRB source is embedded in a region
with high extinction, or the host identification is spurious. 

The image may be viewed at 
http://www.hp.phys.titech.ac.jp/nkawai/grb/070521/.

We thank the Subaru Observatory for the support of this observation."

A. C. Updike, J. R. Puls, and D. H. Hartmann (Clemson University) report 
on behalf of the Clemson GRB Follow-Up Team:

We imaged the field of GRB 070521 (GCN 6431
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 , Guidorzi et al.) beginning 
14 minutes after the trigger (279935) with the SARA 0.9m at Kitt Peak 
under decent weather conditions.  In 50 minutes of stacked exposures in 
the R band, we detect no new sources in the XRT error circle down to a 
limiting magnitude of 20 +/- 0.4.  We do not detect the source noted by 
Hattori et al. (GCN 6444
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 ).  This magnitude is based on calibration to 5 
USNO B1.0 stars.

The SARA Homepage can be found at:
 http://saraobservatory.org

This message may be cited.

Pablo Saz Parkinson (UC Santa Cruz) on behalf of the Milagro
collaboration reports:

We have searched Milagro data for emission at GeV/TeV energies from GRB
070521 detected by Swift (GCN Circ 6431
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 , C. Guidorzi et al.), during the
main period of emission lasting 60s (GCN Circ 6440
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 , D. Palmer et al.).

No evidence for prompt GeV/TeV emission was found. TeV photons are
attenuated by pair production with infrared photons in intergalactic
space. We calculate an upper limit assuming two possible values for the
redshift: z=0.03 (GCN Circ 6433
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 , E. O. Ofek et al.), and z=0.55 (GCN Circ
6444
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 , T. Hattori et al.) using the extragalactic infrared background light
(EBL) absorption model of Primack et al. 2005 (AIP Conf. Proc. 745, p.
23).

A preliminary analysis, assuming a differential photon spectral
index of -2.4, gives upper limits on E^2dN/dE at 99% confidence of:

E^2 dN/dE at 250 GeV < 3.5 * 10^(-7) erg cm^(-2) (Assuming z=0.03)

and

E^2 dN/dE at 200 GeV < 5.5 * 10^(-6) erg cm^(-2) (Assuming z=0.55)

The energies quoted represent the approximate median energy of the
events that would be detected assuming a power law spectrum with
differential index -2.4 convolved with the absorption model.
These upper limits are preliminary and will be refined with further
analysis.

J. Greiner, C. Clemens, T. Kruehler, A. Kuepcue-Yoldas, N. Primak, 
G. Szokoly, A. Yoldas (all MPE Garching), S. Klose, U. Laux (Tautenburg Obs), 
and C.C. Thoene (DARK/NBI Copenhagen) report:

We have observed GRB 070521 (trigger 279935, Guidorzi et al. 2007, 
GCN #6431
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 ) with the 7-channel imager GROND, mounted at the 
2.2m Max-Planck Institute telescope at La Silla (ESO/Chile). Observations 
started at 06:59 UT, about 8 min. after the GRB trigger, and continued 
for 25 min until the telescope's elevation limit of 20 deg above horizon 
was reached. The observation consisted of a series of 60-second
exposures each in g', r', i, and z', and 10-second exposures in J, H, 
and Ks, respectively. The GRB field was observed through changing,
thick cloud coverage, therefore, only 50% of the images are useful and 
the flux in the remaining images is reduced.

No source is detected within the Swift XRT error circle (GCN #6431
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 ).
We derive the following, preliminary 2 sigma upper limits using
SDSS (Cool et al 2007, GCN #6432
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 ) and 2MASS photometry of stars within
the field: 
   g' > 22.0 
   r' > 21.7 
   i  > 20.7 
   z' > 20.3
   J  > 20.0 
   H  > 17.9 
   K  > 17.2

The object see by Subaru (Hattori et al, GCN #6444
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 ) is not seen
in any of our stacked images.

GROND is presently being commissioned.

S. B. Cenko (Caltech), P. A. Price (IfA/Hawaii), and E. Berger (OCIW)
report on behalf of a larger collaboration:

We have imaged the field of GRB070521 (Guidorzi et al.; GCN 6431
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 ) with the
Gemini Multi-Object Spectrograph (GMOS) and the Near-Infrared Imager
(NIRI) mounted on the Gemini North Telescope.  Observations were taken in
the g', i', z' (GMOS) and J, H, Ks (NIRI) filters, beginning at 7:36:54
UT on 21 May 2007.

We weakly detect the host candidate proposed by Hattori, Aoki, and Kawai
(GCN 6444
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 ) only in our i'- and Ks-band images.  In the table below we
report a summary of our observations.  Photometric calibration was
performed relative to SDSS (Cool et al.; GCN 6432
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 ) for the GMOS images
and the 2MASS point source catalog for our NIRI imaging.

Filter		t - t_b (hr)	Exposure (s)	Magnitude
-------------------------------------------------------------------------
g'		1.08		2 x 180		> 25.0
i'		1.21		2 x 180		~ 24.1
z'		1.33		4 x 180		> 23.0
J		1.92		15 x 30		> 21.5
H		2.20		15 x 30		> 21.0
Ks		2.50		15 x 30		~ 22.3

D. A. Perley, J. S. Bloom (UC Berkeley), J. X. Prochaska (UC Santa 
Cruz), S. Stanford (LLNL), M. Brodwin (JPL), and N. R. Butler (UCB) report:

Starting at 13:21:22 UT (2007-05-21), we imaged the field of GRB 070521 
(GCN 6431
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 ) in V and R band with the Keck I telescope (+ LRIS), in a 
series of five exposures of 300 seconds each.  The host galaxy candidate 
of Hattori et al. (GCN 6444
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 , "S1") is clearly detected, as is a second 
source ("S2") at the edge of the XRT error circle (Guidorzi et al., GCN 
6431
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 ).  We also detect a third, extended source ("S3") further to the 
south, not consistent with the preliminary XRT position reported by 
Guidorzi et al. but consistent with our preliminary re-analysis of the 
XRT data.

The positions of these sources (J2000) are:

S1:  16:10:38.562 +30:15:27.50
S2:  16:10:38.734 +30:15:31.38
S3:  16:10:38.754 +30:15:21.57

The astrometric uncertainty is about 0.3" in each coordinate.

Preliminary aperture magnitudes of these sources, calibrated with 
respect to the SDSS magnitudes from Cool et al. (GCN 6432
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 ) using the 
Lupton transform [1], are:

S1:  V = 25.46 +/- 0.12 , I = 23.88 +/- 0.12
S2:  V = 25.98 +/- 0.20 , I = 24.26 +/- 0.16
S3:  V = 26.63 +/- 0.39 , I = 24.39 +/- 0.17

These magnitudes are not corrected for Galactic extinction, which the 
NED extinction calculator [2] estimates to be E(B-V) = 0.027, or A_V = 
0.09, A_I = 0.05, in this direction.

An image of the field is located at:

http://lyra.berkeley.edu/~dperley/070521/070521keckI.png
http://lyra.berkeley.edu/~dperley/070521/070521keckI_clean.png



---
[1] http://www.sdss.org/dr5/algorithms/sdssUBVRITransform.html#Lupton2005
[2] http://nedwww.ipac.caltech.edu/forms/calculator.html

C. Guidorzi, P. Romano (Univ Bicocca&INAF-OAB), J. Hill (GSFC),
A. Beardmore, P.A. Evans (U. Leicester), A. Moretti (INAF-OAB),
report on behalf of the Swift team:

We have analysed the first five orbits of GRB 070521 (Guidorzi
et al., GCN Circ. 6431
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 ) with total observing times of 10.2 ks in
Photon Counting mode in the Swift XRT.
The XRT position, using the UVOT to astrometrically correct the XRT field,
assuming a fixed mapping between the XRT and UVOT instruments, is
RA(J2000)=242.6608 deg, Dec(J2000)=+30.2561 deg, corresponding to:

RA(J2000)  = 16h 10m 38.60s
Dec(J2000) = +30d 15' 21.8"

with an uncertainty of 1.7 arcsec (radius, 90% containment).
This was based on 3 intervals of overlapping XRT/UVOT V-band data
totalling 866 s. The UVOT astrometry is performed relative to USNOB1.

This is 6.6 arcsec from the initial X-ray position,
4.0 arcsec from the XRT position notice,
and 17 arcsec from the BAT refined position (Palmer et al., GCN Circ. 6440
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 ).

The XRT light curve exhibits an initial flaring behaviour superposed
to a power-law decay (index of 0.5 +/- 0.1) up to ~T+600 s.
A break in the power-law decay is observed at T+(6.8 +/- 0.9) ks,
after which it steepens with a power-law index of 1.7 +/- 0.1
up to T+34 ks (90% confidence intervals).

We extracted a spectrum of the PC data from T+4.7 ks to T+13 ks.
This can be fit with an absorbed power law with a photon index
of 2.11 +/- 0.16 and column density of (7+/-1)E21 cm^-2
significantly in excess of the Galactic value
(2.8E20 cm^-2; Dickey & Lockman, 1990).

The absorbed (unabsorbed) 0.3-10.0keV flux for that spectrum
is 1.6E-11 (3.2E-11) ergs cm^-2 s^-1.

Assuming the source continues to decay at the same rate, we predict
an XRT count rate of 7.9E-3 counts/s at T+24 hours, which corresponds
to an observed (unabsorbed) flux of 4.6E-13 (9.3E-13) ergs cm^-2 s^-1.

This circular is an official product of the Swift XRT team.

S.Maeno, R.Hara, H.Tanaka, E.Sonoda, M.Yamauchi
(University of Miyazaki)


We have observed the field of GRB 070521 (GCN 6431
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 ) with
the unfiltered CCD camera on the 30-cm telescope at University of Miyazaki.
The observation was started from 11:47:35 UT
on May 21(4.9 h after the trigger).
After co-adding a set of 32 images (11:47:35 - 12:46:04 UT) of 30 sec 
exposures, we have compared with the USNO A2.0 catalog.
Preliminary analysis shows there is no new source brighter
than 17.8mag.

F.E. Marshall (NASA/GSFC) and C. Guidorzi (U. Bicocca & INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 070521 starting 82 s after
the BAT trigger (Guidorzi et al., GCN Circ. 6431
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 ).  We do not find any source
in any of the UVOT observations inside the refined XRT error circle
(Guidorzi et al., GCN Circ. 6452
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 ). The 3-sigma upper limits for detecting 
a source in the first finding chart (FC) exposure and co-added frames are:

Filter            T_start(s)   T_stop(s)     Exp(s)     Mag (3-sigma UL)

White (FC)        82           182           98         >20.6
White             82           24605         1884       >22.3
V                 187          17181         1451       >20.4
B                 5304         23879         1967       >22.0
U                 5099         22968         1278       >21.7
UVW1              4894         18810         1111       >21.1
UWM2              4869         18085         1279       >20.8
UVW2              5713         12897         792        >21.1

The values quoted above are not corrected for the expected Galactic
extinction corresponding to a reddening of E(B-V) = 0.03 mag towards
the direction of the burst (Schlegel et al. 1998).

T. Minezaki (IoA, Tokyo) and P.A. Price (IfA, Hawaii) report on behalf
of a larger collaboration:

We observed the localisation of GRB070521 with the MAGNUM telescope +
MIPS dual-beam imager.  While our observations consist of both optical
and NIR images, we report here only the NIR imaging.  We do not find any
afterglow within the XRT error circle (GCN ##6431,6452) to the following
limits (based on flux calibration with a nearby 2MASS star):

Filter		t - t_GRB (min)		Limit (mag)
J		49.2			20.7
H		106.6			20.3
K		87.1			19.2

This message may be cited.

S. B. Cenko, E. O. Ofek (Caltech) and P. A. Price (IfA) report on behalf
of a larger collaboration:

We have re-imaged the field of GRB070521 (Guidorzi et al., GCN 6431
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 ) with
the Gemini Multi-Imaging Spectrograph mounted on the Gemini North
telescope.  We obtained 5 x 180 s in the Sloan i' filter at a mean epoch
of 10:23:38 UT 22 May.

The host candidate (S1) identified by Hattori et al. (GCN 6444
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 ; see also
Perley et al., GCN 6451
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 ) is well detected in our second epoch.  In
comparison with our first epoch of i' imaging (GCN 6450
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 ), we find
marginal evidence for fading of this source.  Specifically, we measure the
following magnitudes, with calibration performed relative to SDSS field
stars (Cool et al., GCN 6432
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 ):

t-tb (hr)		Magnitude
----------------------------------------------------------------
1.21			24.07 +\- 0.17
27.54			24.30 +\- 0.09

However, PSF-matched image subtraction using the ISIS software package
(Alard & Lupton, ApJ, 503, 1998) reveals no variability at the location of
this source.

We do not find any new sources inside the revised XRT error circle
(Guidorzi et al., GCN 6452
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 ) in either epoch of imaging, to limits of i' >
24.0 (epoch 1) and i' > 25.0 (epoch 2).

The source S3 is marginally detected in our second epoch near the
detection limit.  This would represent a decline from measurements
reported previously (I = 24.39; Perley et al., GCN 6451
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 ).  However, we
caution our photometry at this location is affected by a variable
background from the nearby bright galaxy and is therefore quite
uncertain.  We encourage further observations to establish the nature of
this object.

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks,
and T. Cline on behalf of the Konus-Wind team report:

The long GRB 070521 (Swift-BAT trigger #279935:
Guidorzi et al., GCN 6431
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 ; Palmer et al., GCN 6440
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 ) triggered
Konus-Wind at T0=24691.587 s UT (06:51:31.587).

The burst started at T-T0 ~-33 s and had a duration of ~55 s.
As observed by Konus-Wind the burst
had a fluence of 1.81(-0.31, +0.06)x10^-5 erg/cm2,
and a 64-ms peak flux measured from T0+5.312 s
4.12(-1.07, +0.78)x10^-6 erg/cm2/s
(both in the 20 keV - 1 MeV energy range).
The burst shows strong spectral evolution.

The time-integrated spectrum of the burst
(from T0 to T0+23.808 s: this interval comprises ~80%
of the burst total counts) can be fitted
(in the 20 keV - 1 MeV range)
by a power law with exponential cutoff model:
dN/dE ~ E^(-alpha)*exp(-E*(2-alpha)/Ep)
with alpha = 0.93 +/- 0.12
and Ep = 222(-21, +27) keV (chi2 = 43/56 dof).

All the quoted errors are at the 90% confidence level.

The Konus-Wind light curve of this GRB is available
at http://www.ioffe.rssi.ru/LEA/GRBs/GRB070521_T24691/

A. Pelangeon & J-L. Atteia (LATT-OMP) report:

We have used the spectral parameters obtained with the Konus
spectrum of GRB 070521 (Golenetskii et al., GCNC 6459)
to compute the spectral pseudo-redshift(**) of this burst
detected by Swift-BAT (Guidorzi et al., GCNC 6431).

We obtain: pz= 2.28 +/- 0.45

We note that this value cannot be made compatible with the spectroscopic
redhift z=0.553 found by Hattori et al. for source S1 (GCNC 6444).

We thank V. Pal'shin (Ioffe Inst.) for information on the spectral
parameters of the most intense part of this burst.


(**) cf. http://www.ast.obs-mip.fr/grb/pz

V. Rumyantsev (CrAO),  V. Biryukov (CrAO, SAI MSU),  A. Pozanenko (IKI) on
behalf of larger GRB follow up collaboration report:

We observed error box of GRB070521 (Guidorzi et al., GCN 6431
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 ) with
1-m Zeiss (Simeiz) telescope of CrAO in R-band on May 21 between (UT)
20:38 and 21:00. No object is found in a refined XRT error box
(Guidorzi et al., GCN 6452
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 ). Limiting magnitude of combined a image is
based on USNO A2.0:

Mid time (UT),  Exposure,   R_Lim (3sigma), Telescope,  Seeing

May 21.868    20x60         22.4             1-m Ziess   2.3"

This message may be cited.

[GCN OPS NOTE(02jun07): The binary attachment was removed as well as the
diagnostics from MIMEDefang reacting to that binary.]