GRB 100305A

E. Troja (NASA/GSFC/ORAU), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), S. Campana (INAF-OAB),
J. R. Cummings (NASA/UMBC), A. de Ugarte Postigo (INAF-OAB),
P. A. Evans (U Leicester), J. M. Gelbord (PSU),
O. Godet (U Leicester), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), H. A. Krimm (CRESST/GSFC/USRA),
R. Margutti (Univ Bicocca&OAB), C. Pagani (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 09:05:38 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100305A (trigger=414905).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 168.365, +42.376 which is 
   RA(J2000) = 11h 13m 28s
   Dec(J2000) = +42d 22' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a total duration of about 20 sec, starting at ~T+50 
relative to the start of the image trigger interval.  The peak count rate
was ~500 counts/sec (15-350 keV), at ~65 sec after the trigger. 

The XRT began observing the field at 09:07:57.6 UT, 139.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 168.36762, 42.40281 which
is equivalent to:
   RA(J2000)  = 11h 13m 28.23s
   Dec(J2000) = +42d 24' 10.1"
with an uncertainty of 4.4 arcseconds (radius, 90% containment). This
location is 96 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
2.14e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 4.86e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 122 seconds with the White filter
starting 144 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 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
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
0.01. 

Burst Advocate for this burst is E. Troja (eleonora.troja 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.)

S. B. Cenko (UC Berkeley) reports on behalf of a larger collaboration:

We have imaged the field of GRB100305A (Troja et al., GCN 10470
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 ) with the
automated Palomar 60-inch telescope.  Images were obtained in the Sloan
g', r', i', and z' filters beginning at 09:09 UT on 5 March 2010 (~ 3.5
minutes after the burst).

In coadded images, we find no sources inside the XRT error circle to the
following limits (calculated with respect several point sources in the
SDSS DR7 catalog):

Filter		Mid-point	t_burst		Exposure	Magnitude
-------------------------------------------------------------------------
r'		09:25		19.5 min	720.0		> 22.0
z'		09:27		21.7 min	420.0		> 20.5
i'		09:28		23.3 min	660.0		> 22.0

The lack of an optical afterglow candidate, together with the long
duration and lack of X-ray absorption in excess of the Galactic value
(Troja et al., GCN 10470
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 ), make GRB100305A a candidate high redshift
event.  We encourage additional follow-up at longer wavelengths.

A. N. Morgan, J. S. Bloom, and S. B. Cenko (UC Berkeley) report:

We observed the field of GRB 100305A (Troja et al., GCN 10470
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 ) with
the 1.3m PAIRITEL located at Mt. Hopkins, Arizona. Observations began
at 2010-03-05 09h29m30s UT, ~0.4 hours after the Swift trigger. �In
initial mosaics (effective exposure time of ~795.6 seconds) taken
simultaneously in the J, H, and Ks filters, we do not detect any
sources within the XRT error circle.

The preliminary photometry yields:

post_burst
t_mid(min) �exp(s) �filt � �U. Limit (3 sig)
34.6 � � � �795.6  �J � � � > 19.2
34.6 � � � �795.6  �H � � � > 18.0
34.6 � � � �795.6 � Ks � � �> 17.3

Observations are ongoing. �All magnitudes given in the Vega system,
calibrated to 2MASS. �No correction for Galactic extinction has been
made to the above reported values.

A. Cucchiara (PSU),  report on behalf of a larger collaboration:

"On March 5.42 UT we observed the field of GRB 100305A (Troja et
al., GCN 10470
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 ) with the Gemini-N telescope and the GMOS instrument.

In our 300s r'-band and 900s i'-band images we clearly detect
a source inside the XRT error circle at the following position:

RA (J2000) = 11:13:28.30
Dec (J2000) = 42:24:10.50

with an uncertainty of 0.5" in each coordinate.

Using 2 USNO-B1 stars we calibrated our photometry and obtained

r' = 23.95 +- 0.30 mag
i' = 23.71 +- 0.50 mag

At the moment no evidence of fading can be addressed.
Further analysis are in progress.

We thank the Gemini staff for performing this observation,
in particular Andrew Stephens."

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

Using 2448 s of XRT Photon Counting mode data and 6 UVOT
images for GRB 100305A, 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 = 168.36691, +42.40393 which is equivalent
to:

RA (J2000): 11h 13m 28.06s
Dec (J2000): +42d 24' 14.1"

with an uncertainty of 1.7 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. Henden (AAVSO), J. Gross (SRO), B. Denny (DC-3), D. Terrell (SwRI), and
W. Cooney (SRO) report:

We obtained imagery of the GRB 100305A burst reported by Troja et al.
(GCN 10470
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 ) using the Sonoita Research Observatory (SRO) 35cm telescope
in southern Arizona, utilizing an automatic VOevent trigger.  The
exposures began about 3 minutes after the burst and continued
for approximately two hours.  20 Rc-band, 10 V-band, and a set of
BVRI exposures for field calibration were acquired.
Inspection of the enhanced Swift-XRT position given by Osborne, et al.
(GCN 10474
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 ) does not show any optical afterglow to the following limits,
based on using star USNOB 1323-0268523 at 11:13:38.45 +42:23:45.7
as having magnitudes R=15.98 and V=16.67):

UTmid     delT    exp   filt  lim
(hrs)     (min)  (# x s)
09.1544  003.63  01x060  Rc  19.1
09.2379  008.64  10x060  Rc  20.5
09.4821  023.29  05x180  Rc  21.3
09.8411  044.83  05x300  Rc  21.6
10.8564  105.75  10x300  V   21.1

The AAVSO thanks the Curry Foundation for its support of the AAVSO
International High Energy Network.

A. de Ugarte Postigo, R. Margutti, B. Sbarufatti (INAF-OAB) and E. Troja
(NASA/GSFC/ORAU) report on behalf of the Swift-XRT team:

We have analysed 2.5 ks of the Swift-XRT data for GRB 100305A (Troja
et al. GCN Circ. 10470
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 ), from 145 s to 6.0 ks after the  BAT trigger. The
data comprise 84 s in Windowed Timing (WT) mode with the remainder
in Photon Counting (PC) mode.

The best position of the X-ray afterglow is the UVOT enhanced XRT
position given in Osborne et al. (GCN Circ. 10474
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 ).

Analysis of the light curve shows an initial steep power-law decay with
an index of alpha=3.5 (+/-0.4), followed by a break at T+300 s to an
alpha of 0.84 (+0.11, -0.58).

A spectrum formed from the WT mode data can be fitted with an
absorbed power-law with a photon spectral index of 1.96 (+0.21,-0.20).
The best-fitting absorption column is  1.7 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.14 (+0.24,-0.20)
and a best-fitting absorption column of 1.7 (+0.60, -0.30) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.8 x 10^-11 (5.8 x 10^-11) erg
cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
0.84, the count rate at T+24 hours will be 0.015 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.8 x
10^-13 (8.7 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/00414905.

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

M. H. Siegel (PSU) and E. Troja (NASA/GSFC/ORAU) report on behalf
of the Swift/UVOT team.

The Swift/UVOT began settled observations of the field of
GRB 100305A 144s after the BAT trigger (Troja et al.,
GCN Circ. 10470
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 ).  Data summed from the first and second orbits
do not reveal a source at the enhanced position of the X-ray
afterglow (Osborne et al., GCN Circ. 10474
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 ) or the candidate
afterglow reported by Cucchiara et al. (GCN Circ. 10473
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 ).

The 3-sigma upper limits for the finding chart (fc) and
summed exposures are reported below:

FILTER     T_start(s)  T_stop    Exposure     Mag/3UL
========================================================
white fc      144        266        120       > 20.60
white        4636       4836        196       > 21.10
v            3612       5247        393       > 19.85
b            4432       6048        373       > 20.68
u            4227       5863        393       > 20.45
uvw1         4022       5656        393       > 20.46
uvm2         3817       5451        393       > 20.2
uvw2         4842       5042        196       > 20.17
=======================================================

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.01 (Schlegel et al.,
1998, ApJS, 500, 525). The photometry is on the UVOT photometric
system described in Poole et al. (2008, MNRAS, 383, 627).

A. Cucchiara (PSU), report on behalf of a larger collaboration:

"We continued our observations of GRB 100305A with the Gemini-N
telescope mounted with the GMOS camera.
A sequence of z'-band images has been taken for a total exposure
time of 15 min.

No clear object is visible in the position of the optical afterglow
previously reported (GCN 10473
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 ) at the following 3-sigma upper limit:

z' > 23.42 mag

We also report a corrected error measure in our photometry.
The correct magnitude estimates at T0 + 2h are:

r' = 23.95 +- 0.30 mag
i' = 23.71 +- 0.06 mag

We apologize for any inconvenience this may have caused."

H. A. Krimm (GSFC/USRA), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), 
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), A. M. Parsons (GSFC),
T. Sakamoto (GSFC/UMBC), G. Sato (ISAS), M. Stamatikos (GSFC/ORAU),
E. Troja (NASA/GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 100305A (trigger #414905)
(Troja, et al., GCN Circ. 10470
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 ).  The BAT ground-calculated position is
RA, Dec = 168.373, 42.381 deg which is
    RA(J2000)  =  11h 13m 29.5s
    Dec(J2000) = +42d 22' 50.7"
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 22%.

The mask-weighted light curve shows a broad structure of two overlapping peaks
beginning at T+0 and continuing to T+80 sec with the highest peak at T+60 sec.
There is soft, low-level emission extending to ~T+200 sec. T90 (15-350 keV) is 
69.7 +- 8.7 sec (estimated error including systematics).

The time-averaged spectrum from T-9.2 to T+70.3 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.27 +- 0.23.  The fluence in the 15-150 keV band is 1.5 +- 0.2 x 10^-6 
erg/cm^2. The 1-sec peak photon flux measured from T+62.72 sec in the 15-150 keV 
band is 0.9 +- 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/414905/BA/

Adria C. Updike (Clemson University), Peter A. Milne (Steward
Observatory), G. Grant Williams (MMTO), and Dieter H. Hartmann (Clemson
University) report:

We observed the field of GRB 100305A (Troja et al., GCN 10470
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 ) with the
0.6m Super-LOTIS telescope located at Kitt Peak National Observatory
beginning 106 seconds after the trigger.  Observations were
carried out in the R band for approximately 45 minutes under good
conditions.  We do not detect any sources within the XRT enhanced error
circle (Osborne et al., GCN 10474
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 ) or the source detected by Gemini-N
(Cucchiara, GCN 10473
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 ).

Band  Exptime  co-adds  time    UL
-------------------------------------
R      10        5      106    > 17.5
R      20        5      202    > 19.2
R      60       30      389    > 20.5


Exptime is given in seconds; time is seconds after trigger of first
observation.  All limits given relative to USNO B1.0 field stars.

D. Kuroda (OAO, NAOJ),  H. Hanayama, T. Miyaji, J. Watanabe(IAO, NAOJ),
K. Yanagisawa (OAO, NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech) report on behalf of the MITSuME collaboration:

We observed the field of GRB 100305A (Troja et al. GCNC 10470)
with the optical three color (g', Rc and Ic) CCD camera attached
to the Murikabushi 1m telescope of Ishigakijima Astronomical
Observatory.

The observation started on 2010-03-05 13:03:55 UT (~4h after the
burst). We did not find any new point source in the XRT error
circle (Osborne et al., GCNC 10474) in all the three bands.

Three sigma upper limits of the OT are listed below. We used
SDSS catalog for flux calibration.

T0+[day]   MID-UT   T-EXP[sec]    g'     Rc     Ic
------------------------------------------------------
0.18816    13:36:35  3300.0      >22.1  >21.6  >20.4
------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]