GRB 110319B

B. Sbarufatti (INAF-OAB/IASFPA) reports on behalf of the Swift-XRT team:

Swift performed a second target of opportunity observation of
GRB 110319B on March 23, 2011.

The XRT began observing the field at 2011-03-23 17:57:33 UT, that is
T+340 ks ( 3.9 days) (Cummings, GCN Circ. 11813
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 ).
Using 4.9 ks of Photon Counting mode data we find that the source
reported previously (Sbarufatti, GCN Circ. 11814
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 ) has faded down to
a 3-sigma upper limit of 0.002 counts/s.
Assuming a power law decay starting from the first source detection
the upper limit is consistent with a decay index steeper than
alpha = 0.6.

The fading nature of the source confirms its afterglow origin.

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

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
C. B. Markwardt (GSFC), A. Melandri (INAF-OAB), T. Sakamoto (GSFC/UMBC),
M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (MSU)
(i.e. the Swift-BAT team):

Using the data set from T-48 to T+52 sec, we report further analysis of
GRB 110319B (Cummings, et al., GCN Circ. 11813
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 ).  The refined BAT position
is RA, Dec =  326.088, -56.774 deg, which is
    RA(J2000)  =  21h 44m 21.1s
    Dec(J2000) =  -56d 46m 26s
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
This position is 33 arcsec from the XRT position of the candidate
afterglow.  The partial coding was 100%.

The mask-weighted light curve shows a single FRED peak starting at ~T-3 sec,
peaking at ~T0, and ending at ~T+20 sec.  T90 (15-350 keV) is 14.5 +- 2 sec
(estimated error including systematics).

The time-averaged spectrum from T-3.5 to T+16.5 sec is best fit by a simple
power law.  This fit gives a photon index 1.39 +- 0.05.  For this
model the total fluence in the 15-150 keV band is 1.0 +- 0.1 x 10-6  erg/cm2
and the 1-sec peak flux measured from T0 sec in the 15-150 keV band is
1.25 +- 0.15 ph/cm2/sec.

F.E. Marshall (NASA/GSFC) and B. Sbarufatti (INAF-OAB/IASFPA)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 110319B
27685 s after the Fermi/GBM trigger (Cummings et al., GCN Circ. 11813
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 ).
No optical afterglow consistent with the XRT position
(Sbarufatti, GCN Circ. 11814
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 ) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Poole et al. 2008, MNRAS, 383, 627) for the initial exposures are:

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

white            28512        35117         1570         >21.8
v                29340        35964         1678         >20.0
u                27685        34323         1597         >20.8

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).

A. Updike (GSFC), J. Greiner, T. Kruehler, F. Schrey (all MPE Garching), 
A. Nicuesa Guelbenzu and S. Klose (both TLS Tautenburg) report on behalf 
of the GROND team.

We observed the field of GRB 110319B (GBM trigger 322256044 at 19:34:02 UT; 
Swift slew source, Cummings et al., GCN #11813
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 ) simultaneously in g'r'i'z'JHK 
with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO 
telescope at La Silla Observatory (Chile).

Observation started at 08:24 UT, which is 12.9 h after the trigger, and
were obtained under clear skies with an airmass of around 2.0 and seeing
of 1.6".

In stacked images with a total integration time of 25 min in g'r'i'z and
20 min in JHK (mid-time 9:12 UT), we do not detect a source in the 2.4" XRT 
error circle of the X-ray counterpart candidate (Sbarufatti et al., 
GCN #11814
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 ) to the following 3-sigma upper limits (AB system):

g' > 23.1 
r' > 23.2 
i' > 23.2 
z' > 23.2 
J  > 21.1 
H  > 20.5


These magnitudes are calibrated against GROND zeropoints and are not
corrected for the expected Galactic foreground extinction corresponding to
a reddening of E_(B-V)=0.03 mag in the direction of the burst (Schlegel et
al. 1998).

GROND is currently cooling down after hardware maintenance, and hence no 
proper K band data are available for this burst.

B. Sbarufatti (INAF-OAB/IASFPA) report on behalf of the Swift-XTR team

We have analysed 4.9 ks of XRT data for the Swift/BAT-detected burst:
GRB110319B (Cummings et al. et al. GCN Circ. 11813
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 ), from 27.7 ks to 35.9
ks after the  Swift/BAT trigger. The data are entirely in Photon
Counting (PC) mode.  An X-ray source is detected within the Swift/BAT
error circle. Using 2442 s of PC mode data and 2 UVOT images, we find
an enhanced XRT position (using the XRT-UVOT alignment and matching
UVOT field sources to the USNO-B1 catalogue): RA, Dec = 326.09490,
-56.78240 which is equivalent to:

RA (J2000): 21h 44m 22.77s
Dec(J2000): -56d 46' 56.5"

with an uncertainty of 2.4 arcsec (radius, 90% confidence). This
position is 53 arcsec from the Swift/BAT position.  The source has a
mean count rate of 9.4e-03 ct/sec; we cannot determine at the present
time whether it is fading.

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

J. R. Cummings (UMBC/CRESST/NASA-GSFC), D. M. Palmer (Los Alamos),
B. Sbarufatti (INAF-OAB/IASFPA), C. Gronwall (PSU)

At 19:34:02, during a Swift slew, Fermi GBM detected a burst.  The
IPN was alerted.  In a mosaic image of BAT slew event data a strong
source is found at RA, Dec 326.074, -56.773, which is:

RA (J2000)  21h 44m 17.8s
Dec (J2000) -56d 46m 24s

with an estimated uncertainty of 3 arcmin radius (90% confidence).
The burst was in the fully-coded field of view from the trigger
time to the end of the Swift slew.  This position is 60 degrees
(2.2 hours) from the Sun.

GRB 110319B was a long GRB with a T90 about 18 seconds. The
lightcurve as seen by BAT was a single FRED.

A Swift TOO is scheduled to begin at 03:10 March 20, 2011.

The BA for this burst is B. Sbarufatti (INAF-OAB/IASFPA).