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GRB 091020

GCN Circular 10048

Subject
GRB 091020: Swift detection of a burst with an optical afterglow
Date
2009-10-20T21:47:36Z (16 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
J. L. Racusin (NASA/GSFC/ORAU), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
N. P. M. Kuin (MSSL), W.B Landsman (GSFC), V. Mangano (INAF-IASFPA),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), A. Rowlinson (U Leicester),
M. H. Siegel (PSU), E. Troja (NASA/GSFC/ORAU) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 21:36:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091020 (trigger=373458).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 175.715, +50.972 which is 
   RA(J2000) = 11h 42m 52s
   Dec(J2000) = +50d 58' 19"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a bright peak
with a second much smaller peak at T+30 s, for a total duration 
of about 45 sec.  The peak count rate was ~5000 counts/sec (15-350 keV),
at ~2 sec after the trigger. 

The XRT began observing the field at 21:38:05.5 UT, 80.8 seconds after
the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located
at RA, Dec 175.7284, +50.9788 which is equivalent to:
   RA(J2000)  = 11h 42m 54.81s
   Dec(J2000) = +50d 58' 43.6"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 39 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 3.01e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 89 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	11:42:55.18 = 175.72993
  DEC(J2000) = +50:58:41.9  = 50.97831
with a 90%-confidence error radius of about 0.74 arc sec. This position is 3.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
15.73 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is J. L. Racusin (judith.racusin AT nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

GCN Circular 10050

Subject
GRB 091020: Enhanced Swift-XRT position
Date
2009-10-21T00:30:43Z (16 years ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1088 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 091020, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 175.73005, +50.97839 which is equivalent
to:

RA (J2000): 11h 42m 55.21s
Dec (J2000): +50d 58' 42.2"

with an uncertainty of 1.8 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

GCN Circular 10051

Subject
GRB 091020, Swift-BAT refined analysis
Date
2009-10-21T01:28:59Z (16 years ago)
From
Hans Krimm at NASA-GSFC <hans.krimm@nasa.gov>
D. M. Palmer (LANL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), A. M. Parsons (GSFC),
J. L. Racusin (NASA/GSFC/ORAU), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):

Using the data set from T-61 to T+242 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 091020 (trigger #373458)
(Racusin, et al., GCN Circ. 10048).  The BAT ground-calculated position is
RA, Dec = 175.727, 50.977 deg which is
    RA(J2000)  =  11h 42m 54.4s
    Dec(J2000) = +50d 58' 36.6"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 43%.

The mask-weighted light curve began with a small hump at T-10 sec, rising
sharply to a peak at T+2 sec and decaying exponentially out to T+50 sec,
with a much smaller peak superimposed at T+30 sec. T90 (15-350 keV) is
34.6 +- 2.7 sec (estimated error including systematics).

The time-averaged spectrum from T-8.7 to T+38.3 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.53 +- 0.07.  The fluence in the 15-150 keV band is
3.7 +- 0.1 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from
T+1.14 sec in the 15-150 keV band is 4.2 +- 0.3 ph/cm2/sec.  All the
quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/373458/BA/

GCN Circular 10052

Subject
GRB 091020: MASTER optical transient polarimetry
Date
2009-10-21T05:15:37Z (16 years ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <gcncirc@observ.inetcomm.ru>
E. Gorbovskoy,D.Kuvshinov, V. Lipunov, V.Kornilov, D.Kuvshinov,
A.Belinski,  A. Krylov, N.Shatskiy, N.Tyurina, P.Balanutsa, V.V.Chazov,

Sternberg Astronomical Institute, Moscow State University

A. Tlatov, A.V. Parkhomenko, D. Dormidontov
Kislovodsk Solar Station of the Pulkovo Observatory

V.Krushinski, I.Zalognikh, T.Kopytova, A. Popov
Ural State University, Kourovka

S.Yazev, K.Ivanov, N.M.Budnev
Irkutsk State University

V.Yurkov
Blagoveschensk Educational State University, Blagoveschensk

MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru, 2x400 
mm, 8 square degrees, 2x16Mpx Apogee CCD) located
at Kislovodsk  was pointed to the  GRB 091020 (Racusin et al., GCN 10048) 
by internet   3422 s after the GRB time. The 
large delay  was caused with   technical problems on site.

We see bright optical counterpart (~17.5 mag) on the firrst images at UVOT 
position.

We have 130  images with 180 sec synchronous exposition each and 19.5 
magnitude limit during ~4 hours in two polarization.

The power low decay with alpha ~= 1.3 during  2.5 hours is observed. 
This is the preliminary result.
The reduction is continued.



The message may be cited.

mailto: lipunov@sai.msu.ru

GCN Circular 10053

Subject
GRB 091020: NOT spectroscopic redshift
Date
2009-10-21T08:24:22Z (16 years ago)
From
Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst <malesani@astro.ku.dk>
Dong Xu, Johan P.U. Fynbo (DARK/NBI), Nial R. Tanvir (Univ. Leicester), 
Jens Hjorth, Giorgos Leloudas, Daniele Malesani (DARK/NBI), Pall 
Jakobsson (Univ. Iceland), Paul A. Wilson (NOT and Univ. Oslo), Johannes 
Andersen (NBI and NOT), report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 091020 (Racusin et al., GCN 
10048; Gorbovskoy et al., GCN 10052) with the Nordic Optical Telescope 
equipped with ALFOSC. We detect the afterglow with a magnitude R ~ 20, 
based on a nearby USNO star, with mean time Oct 21.220 UT (0.315 days 
after the GRB).

We secured low-resolution spectroscopy (30 min exposure), covering the 
wavelength range 3500-9100 AA, starting on Oct. 21.231 UT. Wavelength 
calibration was carried out using archival frames.

In a preliminary analysis we detect several absorption features, which 
we interpret as FeII 2344, 2374, 2382, Al III 1854, 1862, Mg II 2798 (Mg 
II 2803 is blended with the telluric A band), all at the common z = 
1.71. There is flux in the spectrum down to ~3800 AA, setting a firm 
upper limit z < 2.13 to the GRB redshift.

GCN Circular 10054

Subject
GRB091020 Swift/UVOT observations
Date
2009-10-21T11:18:58Z (16 years ago)
From
Samantha Oates at MSSL <sro@mssl.ucl.ac.uk>
S. R. Oates (MSSL-UCL) and J. L. Racusin (NASA/GSFC/ORAU) report on  
behalf of the Swift/UVOT team:

The Swift/UVOT began observerving the field of GRB 091020 89 s
after the BAT trigger (Racusin et al., GCN Circ. 10048). We detect the
optical afterglow in all filters at the refined UVOT position
RA, Dec 175.730, +50.97833, which is

     RA  (J2000)  11:42:55.20
     Dec (J2000)  50:58:42.0

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence).
This position is consistent with the enhanced XRT position (Beardmore
et al., GCN 10050). The detection in the white, v, b and u filters and
the weak detection in the uvw1 filter is consistent with a redshift
z~1.7, which is consistent with redshift reported by NOT (Xu, et al.,  
GCN 10053).

Preliminary magnitudes and 3 sigma upper limits are reported below.

Filter    T_start(s) T_stop(s)  Exposure(s)   Mag  Err
------------------------------------------------------------------------
white       89        239        147          15.73 +/- 0.01
white       874       1024       147          17.27 +/- 0.03
v           799       819        19           16.40 +/- 0.15
b           1153      1173       19           17.27 +/- 0.15
u           707       727        19           17.06 +/- 0.16
uvw1        683       6929       490          20.29 +/- 0.35
uvm2        657       6723       490          > 20.29
uvw2        608       6314       303          > 20.27
-------------------------------------------------------------------------

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.02 mag (Schlegel et al.,
1998, ApJS, 500, 525).  The photometry is on the UVOT photometric system
described in Poole et al. (2008, MNRAS, 383,627).

GCN Circular 10056

Subject
GRB 091020: Swift-XRT refined analysis
Date
2009-10-21T14:08:47Z (16 years ago)
From
Andy Beardmore at U Leicester <apb@star.le.ac.uk>
A. P. Beardmore, P. A. Evans (U. Leicester) and J. L. Racusin
(NASA/GSFC/ORAU) report on behalf of the Swift-XRT team:

We have analysed 13 ks of Swift-XRT data for GRB 091020 (Racusin et
al., GCN Circ. 10048), from 87 s to 29.6 ks after the BAT trigger
(#373458). The data comprise 350 s in Windowed Timing (WT) mode with
the remainder in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Beardmore et al. (GCN. Circ 10050).

The light curve shows an initial steep decay with an index of 3.7 +/-0.6,
breaking at T+130 s to a shallower decay of slope 0.90 +/- 0.03, with a
small flare superimposed at T+190s. A further break to a decay slope of
1.39 +/- 0.1 is apparent at T+6.6 ks.

The WT mode spectrum, from 87 s to 434 s after the trigger, can be fit
by an absorbed power-law with a photon index of 1.99 +/- 0.07. The
best-fitting absorption column is (6.08 +1.04 -0.99) x 10^21 cm^-2, at
a redshift of 1.71 (Xu et al., GCN Circ 10053), in addition to the
Galactic value of 1.4 x 10^20 cm^-2 (Kalberla et al. 2005). The PC
mode spectrum from 436 s to 7.3 ks (with an exposure time of 3.5 ks)
has a photon index of (2.15 +0.12 -0.11) and a best-fitting intrinsic
absorption column of (5.1 +1.6 -1.5) x 10^21 cm^-2. The counts to
observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from
this spectrum is 3.4 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay at the same rate, the count rate
at T+24 hours will be 0.015 count s^-1, which corresponds to an
observed (unabsorbed) 0.3-10 keV flux of 4.8 x 10^-13 (6.3 x 10^-13)
erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00373458.

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

GCN Circular 10058

Subject
GRB 091020: Lick observations
Date
2009-10-21T14:24:31Z (16 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley (UC Berkeley) reports:

I observed the location of GRB 091020 (Racusin et al, GCN 10048) 
starting at UT 2009-10-21 12:30 using the Nickel 40-inch telescope at 
Lick Observatory.   A series of nine exposures in R-band were acquired, 
followed by five exposures in I-band and an additional sequence in 
R-band during morning twilight.  All exposures except for the first were 
of 180 seconds duration.

The afterglow is marginally detected in individual frames and 
well-detected in a combined stack.  Calibrating relative to five nearby 
SDSS standard stars transformed to R-band using the transformation 
equation of Lupton (2005), the estimated magnitude of the afterglow in 
the first R-band stack is:

R = 20.91 +/- 0.13 mag  (t_mid = 15.17 hr)

Additional follow-up is planned if weather permits.

GCN Circular 10059

Subject
Konus-Wind observations of GRB 091020 (correction to GCN 10057)
Date
2009-10-21T14:24:40Z (16 years ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Dmitry Frederiks reports:

I made a mistake in the subject line of GCN 10057:
it should be "Konus-Wind observations of GRB 091020"
instead of "Konus-Wind observations of GRB 091010".

I apologize for the confusion.

GCN Circular 10060

Subject
GRB 091020: Additional Lick observations
Date
2009-10-22T14:22:00Z (16 years ago)
From
Daniel Perley at U.C. Berkeley <dperley@astro.berkeley.edu>
D. A. Perley (UC Berkeley) reports:

Further to observations the previous night (GCN 10058), I re-observed 
the afterglow of GRB 091020 between UT 2009-10-22 12:23 and 13:25 using 
the Nickel 40-inch telescope at Lick Observatory in a series of 
seventeen 180 second exposures (51 minutes total integration) in R-band.

The afterglow is detected in in the combined stack of all frames. 
Calibrating relative to the same SDSS standard stars as in GCN 10058, 
the estimated magnitude of the afterglow at this time is:

R = 21.81 +/- 0.23 mag  (t_mid = 39.29 hr)

The implied average decay index over the 24 hours since the previous 
observation is alpha=0.87.

GCN Circular 10076

Subject
GRB 091020: Early TLS Multicolor detections, red afterglow
Date
2009-10-25T05:58:15Z (16 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
D. A. Kann, U. Laux, M. Roeder & H. Meusinger (TLS Tautenburg) report:

We observed the afterglow of the Swift GRB 091020 (Racusin et al., GCN 
10048) with the 1.34m Schmidt telescope of the Thueringer Landessternwarte 
Tautenburg, Germany, as soon as it had risen to an airmass of

2.2, starting 0.167 days after the GRB. We obtained 3 x 300 sec images in 
Z, Ic, Rc, V and B each. We detect the afterglow in all single images.

As a comparison star, we use the SDSS star at:

RA (J2000) =    11:42:49.2 (175.7050129)
Dec. (J2000) = +51:00:52.42 (+51.01456163)

For this star, using the transformation equations of Lupton (2005) (taking 
the mean of the two transformation equations each), and transforming the 
z' value to Vega mags following Fukugita (1995), we derive the

magnitudes:

B =  19.109
V =  17.740
Rc = 16.759
Ic = 15.927
Z =  15.563

We derive the following afterglow magnitudes:

dt		Filter	mag	dmag
____________________________________
0.166947	Z	18.17	0.13
0.170987	Z	18.10	0.13
0.175038	Z	18.16	0.13
0.179181	Ic	18.45	0.06
0.183221	Ic	18.30	0.05
0.187260	Ic	18.32	0.05
0.191404	Rc	19.43	0.04
0.195444	Rc	19.40	0.04
0.199483	Rc	19.40	0.04
0.203627	V	19.90	0.10
0.207666	V	20.00	0.10
0.211706	V	20.06	0.10
0.215849	B	20.78	0.10
0.219889	B	20.71	0.08
0.223928	B	20.72	0.08

Using further reported magnitudes (Oates et al., GCN 10054; Gorbovskoy et 
al., GCN 10052, Xu et al., GCN 10053, Perley, GCN 10058, GCN 10060), we 
find that the Rc band light curve is very well described by a

single power-law decay with alpha = 1.12 +/- 0.06, B and V light curves 
agree with an achromatic light curve evolution. We find a red afterglow 
with spectral slope beta ~ 2, and note that the Ic - Z color is

very blue, this may be due to a filter mismatch between the SDSS z' filter 
and our Z filter.

This message may be cited.

GCN Circular 10080

Subject
GRB 091020 : Xinglong TNT upper limit
Date
2009-10-26T12:31:47Z (16 years ago)
From
L.P. Xin at NAOC <xlp@bao.ac.cn>
L.P. Xin, X.F. Wang, J. Wang, Y.L. Qiu, J.Y. Wei, 
W.K. Zheng, J.S. Deng, and C. Wu, J.Y. Hu 
on behalf of EAFON report:

We observed the optical afterglow of GRB 091020 
(e.g. Racusin et al., GCN 10048; Gorbovskoy et al., 
GCN 10052; Perley GCN 10060) with Xinglong TNT 
telescope from  Oct. 22, 20:27:52(UT),1.952 days 
after the burst. The optical afterglow (Gorbovskoy 
et al., GCN 10052; Perley GCN 10060) was not detected 
in our combined image with a duration of 7*600 sec.
The upper limit is estimated to be R~21.4 mag 
from  USNO-B1.0 R2 magnitude, at the mean time of 
1.974 days since the trigger.

This message may be cited.

For more information about Xinglong GRBs Follow-up 
observations, please visit the web-site:
http://www.xinglong-naoc.org/grb/

GCN Circular 10088

Subject
Radio afterglow detection from GRB 091020
Date
2009-10-26T23:00:25Z (16 years ago)
From
Dale A. Frail at NRAO <dfrail@nrao.edu>
Dale A. Frail (NRAO), Poonam Chandra (RMC), and Tanay Singh
report on behalf of a larger collaboration:

"We used the Very Large Array (VLA) to observe the field of view
towards the Swift burst GRB 091020A (GCN 10048, 10050, 10051) at a
frequency of 8.46 GHz. Data has been taken at 6 different epochs:
October 21.66, 22.54, 22.77, 22.99, 24.60 and 26.71 UT. The average
flux density of the source during the first 4 epochs was 108 +/- 25
uJy. The source has since increased in flux, rising to 230 +/- 42 uJy
on Oct 24.60 UT and 436 +/- 42 uJy on Oct. 26.71 UT. Our current
best-fit position is (J2000) R.A.=11:42:55.268, Dec.=+50:58:41.36 with
a conservative error of +/-0.6". Further observations are planned.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."

GCN Circular 10090

Subject
GRB 091020: Late TLS detection
Date
2009-10-27T02:31:21Z (16 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
D. A. Kann, U. Laux, M. Roeder & H. Meusinger (TLS Tautenburg) report:

We observed again the afterglow of the Swift GRB 091020 (Racusin et al., 
GCN 10048) with the 1.34m Schmidt telescope of the Thueringer 
Landessternwarte Tautenburg, Germany, on the 26th of October, 5.2 days 
after the GRB. We obtained 11 x 600 sec images in Rc before clouds shut us 
down again. At the position of the afterglow, we detect a faint source in 
the stacked image.

We derive the following afterglow magnitudes using the same comparison 
star as Kann et al., GCN 10076:

dt		Filter	mag	dmag
____________________________________
5.213357	Rc	23.62	0.31


Kann et al., GCN 10076, found a single power-law decay with alpha = 1.12 
+/- 0.06. Compared to the extrapolation of this fit, there is no 
significant evidence for a break yet.

No further observations are possible due to an instrument change.

This message may be cited.

GCN Circular 10095

Subject
GRB 091020: Fermi GBM observation
Date
2009-10-28T14:27:17Z (16 years ago)
From
Vandiver Chaplin at UAH/Fermi-GBM <chapliv@email.uah.edu>
V.Chaplin (UAH) reports on behalf of the Fermi GBM Team:

"At 21:36:43.82 UT on 20 October 2009, the Fermi
Gamma-Ray Burst Monitor triggered on GRB 091020
(trigger 277767405 / 091020900), which was also detected by
the Swift-BAT (Racusin et al., GCN 10048).
The angle from the Fermi LAT boresight of the Swift-XRT position
(Beardmore et al., GCN 10050) is 118 degrees.

The GBM light curve consists of a single pulse
with a duration (T90) of about 37 s (8-1000 keV).
The time-averaged spectrum from T0-7.7 s to T0+21 s is
best fit by a Band function with Epeak = 47.9 +/- 7.1 keV,
alpha = 0.2 +/- 0.4, and beta = -1.7 +/- .02.

The event fluence (8-1000 keV) in this time interval is
(1.0 +/- 0.2)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.5 s in the 8-1000 keV band
is 7.4 +/- 0.5 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

GCN Circular 10231

Subject
GRB 091020: MASTER-Net optical polarization observations
Date
2009-12-02T19:15:38Z (16 years ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <gcncirc@observ.inetcomm.ru>
E. Gorbovskoy, V. Lipunov, V.Kornilov, A.Belinski, N.Shatskiy, N.Tyurina, 
D.Kuvshinov, P.Balanutsa, V.V.Chazov, P.V.Kortunov, A.Kuznetsov, 
D.Zemnukhov, M. Kornilov
Sternberg Astronomical Institute, Moscow State University

V.Krushinski, I.Zalozhnih, T.Kopytova
Ural State University, Kourovka

A. Tlatov, A.V. Parhomenko, D. Dormidontov
Kislovodsk Solar Station of the Pulkovo Observatory

K.Ivanov, S.Yazev, N.M.Budnev, E.Konstantinov, V.Lenok
Irkutsk State University

S.Sergienko, V.Yurkov
Blagoveschensk Educational State University, Blagoveschensk

MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru, 2x400
mm, 2x 4 square degrees, 2x16Mpx Apogee CCD) located
at Kislovodsk  was pointed to the  GRB 091020 (Racusin et al., GCN 10048)
by internet   3422 s after the GRB time. The
large delay  was caused with   technical problems on site (Gorbovskoy et 
al., GCN 10052).

The first 7 images was taken in R-band. The exposure each images is 180s.

West-telescope
GRB Time         OT Mag         Error
1.02097639      17.93           0.22
1.07643278      17.43           0.09
1.13188834      17.78           0.14
1.18734417      18.03           0.19
1.24280056      17.87           0.15
1.29825584      17.58           0.10
1.35371584      17.94           0.12

The result of the unfiltred polarization combined images is:

West-telescope
GRB Time         OT Mag         Error
(Hours)

1.604558        18.65           0.10
1.983004        18.91           0.08
2.285582        19.04           0.08
2.562864        19.28           0.11
2.84015         19.36           0.09
3.117435        19.32           0.08
3.394721        19.35           0.06
3.672004        19.23           0.10
3.949285        20.07           0.16
4.226602        20.67           0.28
4.503883        20.07           0.22



East-telescope

GRB Time          GRB           Error
(Hours)

1.604718        18.60           0.06
1.983232        18.46           0.06
2.285826        19.14           0.09
2.563109        19.11           0.09
2.84039         19.56           0.10
3.117671        19.38           0.09
3.394954        19.70           0.17
3.672234        19.53           0.11
3.949516        19.92           0.16
4.226796        20.03           0.19
4.50408         19.99           0.16

Using all our polarizated magnitudes we
find that the  light curve is very well described by a
single power-law decay with alpha = 1.2 +/- 0.1, that is in good agreement
with the following observations (Kann et al., GCN 10076)


The light curve of our observations is available at
http://observ.pereplet.ru/images/GRB091020/grb.html  .
The message may be cited.

mailto: lipunov@sai.msu.ru

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