GRB 110520A

D. Xu (WIS), S. Schulze, P. Jakobsson (U. Iceland), T. Pursimo (NOT),
report on behalf of a larger collaboration:

We continued to observe the field of GRB 110520A (De Pasquale et al.,
GCN 12020
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 ) with the Nordic Opitcal Telescope (NOT) equipped with
ALFOSC. We obtained 6x300 s R-band images, starting at 23:02:40.4 UT
on May 21 (i.e., 26.565 hrs after the burst).

There was nothing visible within the enhanced XRT error circle
(Osborne et al., GCN 12027
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 ). The optical source identified in our
previous imaging (Schulze et al., GCN 12025
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 ) is ~1.9" northeast away
of the center of the enhanced XRT error circle, which has a radius of
1.5" (90% C.L.). The source is still point-like and has R=23.7+/-0.1
mag, same as our previous measurement.

The optical source noted by Levan et al. (GCN 12026
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 ) was clearly
detected in our previous imaging. It was largely point-like and had
R=23.9+/-0.1 mag, calibrated with the #1464-0205280 star (R1=18.97,
R2=18.92) in the USNO B1 catalog. Now it has become extended (about
3.5" southeast away of the center of the enhanced XRT error circle)
and decayed to R=24.5+/-0.2 mag.

M. De Pasquale (MSSL-UCL) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 110520A
104 s after the BAT trigger (De Pasquale et al., GCN Circ. 12020
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 ).
No optical afterglow consistent with the enhanced XRT position (Osborne et 
al., GCN Circ. 12027
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 ) or at the position of the NOT and WHT sources 
(Schulze et  al., GCN Circ 12025
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 ; Levan et al. GCN Circ 12026
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 ) is detected 
in the initial and in the summed UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Poole et al. 2008, MNRAS, 383, 627) for the first finding chart (FC)
exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           104          253          147 	>21.1
u_FC               316          565          246 	>20.4
white              104        30973         3040 	>22.9
v                  645        23608         2083 	>21.0
b                  571        36751         2946 	>22.1
u                  316        42529         3178 	>21.8
w1                 694        41871         2965 	>21.8
m2                 842        40964         2262 	>21.8
w2                 621        19418         1911 	>22.0

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

T. N. Ukwatta (MSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), M. De Pasquale (UCL-MSSL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (GSFC/UMBC), M. Stamatikos (OSU), J. Tueller (GSFC)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 110520A (trigger #453747)
(De Pasquale, et al., GCN Circ. 12020
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 ).  The BAT ground-calculated position is
RA, Dec = 134.363, 56.418 deg which is 
   RA(J2000)  =  08h 57m 27.1s 
   Dec(J2000) = +56d 25' 04.9" 
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 69%.
 
The mask-weighted light curve shows a single FRED peak starting at ~T-2 sec,
peaking at ~T+4 sec, and ending at ~T+40 sec.  T90 (15-350 keV)
is 15.7 +- 1.8 sec (estimated error including systematics).
 
The time-averaged spectrum from T-1.2 to T+16.6 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.13 +- 0.10.  The fluence in the 15-150 keV band is 1.1 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+4.15 sec in the 15-150 keV band
is 1.1 +- 0.2 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/453747/BA/

J. Gorosabel (IAA-CSIC, Granada), R. Barrena (IAC, Tenerife), A.J. Castro-Tirado (IAA-CSIC, Granada), A. de Ugarte Postigo (DARK/NBI, Copenhagen), D. Malesani (DARK/NBI, Copenhagen), report on behalf of a larger collaboration:

"We have observed the field of GRB 110520A (De Pasquale et al., GCNC 12020) with the 82cm IAC80 telescope located at the Observatory of Teide, Tenerife. The observations were carried out in the I band on May 20.87397-20.94593 UT (0.495-2.222 hours after the GRB). No object is detected within the enhanced XRT error circle (Osborne et al. GCN 12027
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 ) down to I~22.1 (calibration based on USNO-B1). The objects noted by Schulze et al. (GCNC 12025) and Levan et al. (GCNC 12026) are not present in our co-added image."

A.P. Beardmore (U. Leicester) and M. De Pasquale (UCL-MSSL) report on
behalf of the Swift-XRT team:

We have analysed 11 ks of XRT data for GRB 110520A (De Pasquale  et al.
GCN Circ. 12020
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 ), from 84 s to 25.2 ks after the  BAT trigger. The data
comprise 74 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Osborne et
al. (GCN. Circ 12027).

The light curve initially follows a power-law decline with decay index
alpha=2.0, then shows a small flare from T+137 s to T+516 s, reaching a
peak count rate of 12 c/s at T+254 s, after which it decays with an
index of 1.42 (+0.12, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.77 (+/-0.17). The
best-fitting absorption column is  2.9 (+/-0.6) x 10^21 cm^-2, in
excess of the Galactic value of 2.7 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.0 x 10^-11 (7.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 2.7 x 10^20 cm^-2
Excess significance: 7.4 sigma
Photon index:	     1.77 (+/-0.17)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00453747.

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

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1918 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 110520A, 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 = 134.34074, +56.42735 which is equivalent
to:

RA (J2000): 08h 57m 21.78s
Dec (J2000): +56d 25' 38.5"

with an uncertainty of 1.5 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.

A.J. Levan (U. Warwick), N.R. Tanvir (U. Leicester), R. Canning (U.
Cambridge), F. Riddick (ING) report on behalf of a larger collaboration:

"We observed the location of GRB 110520A (GCN 12020
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 ) using the William
Herschel Telescope equipped with LIRIS. Observations began at 22:21
UT on 20 May 2011, 1.9 hours after the burst, and were conducted
in the zYJHK bands. We do not find any sources with the XRT
localization of GRB 110520A, down to preliminary limiting magnitudes
of z>22.1, J>20.8, H>20.1, K>19.5, calibrated against SDSS and 2MASS
observations of the field.

We do note two sources which lie just outside the XRT localization.
The object noted by Schulze et al. (GCN 12025
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 ) is very weakly
detected in our images, while an additional object at

RA(J2000) 08:57:22.02 
DEC(J2000) 56:25:35.31 
+/- 0.2"

is also present in all our observed bands, but is offset ~4" from
the XRT location.  At this stage we cannot ascertain if either of
these sources is a plausible afterglow for GRB 110520A."

S. Schulze (U. Iceland), D. Xu (WIS), D. Malesani (DARK/NBI), P.  
Jakobsson (U. Iceland),
S. Asadi, A. Barekat, L. Guaita, I. A. Gumus, S. Lindgren,  J.  
Lindkvist, K. Migotto, A. Razza,
T. Str�m, F. Taddia, G. Olofsson (Stockholm Observatory), report on  
behalf of a larger
collaboration:

We observed the field of GRB 110520A (De Pasquale et al., GCN 12020
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 )  
with the
Nordic Opitcal Telescope (NOT) equipped with ALFOSC. We obtained 3x600 s
R-band and 6x300 s I-band images. Observations started at 21:19:16.9 UT
on May 20 (i.e., 0.8414 hr after the burst).

An optical source is clearly detected in each of the stacked R-band  
and I-band images
at coordinates

RA(J2000)  = 08:57:21.84
Dec(J2000) = +56:25:40.2
Error-radius: ~0.1 arcsec

which is 2.7 arcsec away of the XRT position center reported at
http://www.swift.ac.uk/sper/index.php.

This source has R~23.7 mag at a median time 1.118 hrs post-burst,  
calibrated with the
#1464-0205280 star (R1=18.97, R2=18.92) in the USNO B1 catalog. At the  
moment
our data does not allow to test for variability of the source.

A. LaCluyze, M. Maturi, K. Ivarsen, J. Haislip, D. Reichart, J. Moore, 
A. Trotter, A. Foster, R.Egger, A. Oza, E. Speckhard, J. A. Crain, and 
M. Nysewanderreport:

Skynet observed the Swift/XRT localization of GRB 110520A (Swift 
trigger#453747) with the Dolomites Astronomical Observatory located at 
the Carlo Magno Hotel Spa Resort in Italy.  Due to local twilight, 
observations in SDSS g',r',i' and z' did not begin until ~25 minutes 
after the trigger.

We do not detect an optical transit within the Swift error circle in any 
filter.   Our earliest time photometry yields the following 3-sigma 
limiting magnitudes: g' 17.8 (mean time of 26.78 minutes after trigger), 
r' 19.7 (mean time of 31.32 minutes after trigger, two combined 
exposures), i' 18.5 (mean time of 37.10 minutes after trigger, two 
combined exposures), z' 17.8 (mean time of 41.95 minutes after trigger) 
based on comparison ~6 nearby SDSS sources.

This message may be cited.

R.J. Smith (Liverpool), A Melandri (INAF-OAB), N. Tanvir (Leicester), C.G. Mundell (Liverpool) report on behalf of a larger collaboration:

The 2-m Liverpool Telescope followed up GRB110520A (SWIFT trigger 453747; De Pasquale et al. GCN 12020
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 ) and obtained a 1200sec r' integration centred 1.36 hours after the SWIFT burst time.
We do not detect any optical transient to a 3 sigma detection limit of R > 23.5, based on comparison to nearby USNO-B catalogue stars.

No further observations are planned.

This message may be cited.

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

We imaged the field of GRB 110520A detected by SWIFT
(trigger 453747) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 45s after the GRB trigger
(15s after the notice). The elevation of the field decreased from
51 degrees above horizon and weather conditions
were good.

The date of trigger : t0 = 2011-05-20T20:28:48

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
We do not detect any OT in the XRT error box with a
limiting magnitude of:
t0+44.9s to t0+104.9s : R > 17.3

The second image is 30.0s exposure in tracking mode:
t0+119.6s to t0+149.6s : R > 18.5

We co-added a series of exposures:
t0+119.6s to t0+310.9s : 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.

V.Krushinski, I.Zalozhnich, A. Popov
Ural State University, Kourovka

E. Gorbovskoy, V. Lipunov, V.Kornilov, D.Kuvshinov, A.Belinski, N.Tyurina,
N.Shatskiy, P.Balanutsa, V.V.Chazov, P.V.Kortunov, A.Kuznetsov, 
D.Zimnukhov, M.Kornilov, A.Sankovich
Sternberg Astronomical Institute, Moscow State University

O. Gres, K.Ivanov, V.A.Poleshchuk, S.Yazev, N.M.Budnev, O.Chuvalaev
Irkutsk State University

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

V.Yurkov, Yu.Sergienko, I.Kudelina
Blagoveschensk Educational State University, Blagoveschensk

  MASTER II  robotic telescope (MASTER-Net: http://observ.pereplet.ru) 
located in Ural(Kaurovka) was pointed to the  GRB110520A 53 sec after 
GRB time and 23 sec after the trigger  at 2011-05-20 20:29:41.097 UT.
On our first (10s exposure) set we  haven`t found optical transient 
within SWIFT error-box (ra=08 57 31 dec=+56 23 59 r=0.050000).
The our unfilterd 5-sigma upper limit has been about 17.0 mag

  MASTER II  robotic telescope in Tunka(Siberia) also pointed to this GRB, 
28 sec after trigger time in two polarizations. But the Sun was too high
  (only 5 deg. below horizont) and upper limit is about 15.0 mag on the 
bosh  first 10 sec sets.

This message may be cited.

M. De Pasquale (UCL-MSSL), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
S. Campana (INAF-OAB), V. D'Elia (ASDC), P. A. Evans (U Leicester),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
C. B. Markwardt (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC), C. J. Saxton (UCL-MSSL),
M. H. Siegel (PSU), C. A. Swenson (PSU) and T. N. Ukwatta (MSU) report
on behalf of the Swift Team:

At 20:28:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110520A (trigger=453747).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 134.375, +56.402 which is 
   RA(J2000)  =  08h 57m 30s
   Dec(J2000) = +56d 24' 06"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a FRED-like pulse
with a duration of about 30 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 20:30:28.3 UT, 100.1 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 134.34208, 56.42720 which
is equivalent to:
   RA(J2000)  = 08h 57m 22.10s
   Dec(J2000) = +56d 25' 37.9"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 112 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.73 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2.5
(+1.48/-1.31) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 103 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT 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. 

Burst Advocate for this burst is M. De Pasquale (mdp AT mssl.ucl.ac.uk). 
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.)