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GRB 110420B

GCN Circular 11944

Subject
GRB 110420B: Swift detection of a short hard burst
Date
2011-04-20T22:51:27Z (14 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
B. Sbarufatti (INAF-OAB/IASFPA), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), M. M. Chester (PSU), V. D'Elia (ASDC),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. A. Kennea (PSU), V. Mangano (INAF-IASFPA), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), M. Stamatikos (OSU/NASA/GSFC) and
E. Troja (NASA/GSFC/ORAU) report on behalf of the Swift Team:

At 22:42:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110420B (trigger=451794).  Swift did not immediately slew
because of an observing constraint. 
The BAT on-board calculated location is 
RA, Dec 320.049, -41.264, which is 
   RA(J2000)  =  21h 20m 12s
   Dec(J2000) = -41d 15' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single spike
with a duration of about 64 milli-sec.  The peak count rate
was ~6000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Due to an observing constraint, Swift will not slew until 
T0+40.4 minutes. There will be no XRT or UVOT data until
this time. 

Burst Advocate for this burst is B. Sbarufatti (boris.sbarufatti AT brera.inaf.it). 
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 11946

Subject
GRB 110420B: Swift-BAT refined analysis
Date
2011-04-21T01:20:10Z (14 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC), B. Sbarufatti (INAF-OAB/IASFPA),
M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (MSU)
(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 SHB GRB 110420B (trigger #451794)
(Sbarufatti, et al., GCN Circ. 11944).  The BAT ground-calculated position is
RA, Dec = 320.045, -41.277 deg, which is 
   RA(J2000)  =  21h 20m 10.9s 
   Dec(J2000) = -41d 16' 38.0" 
with an uncertainty of 2.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 35%.
 
The mask-weighted light curve shows a FRED-like pulse starting at ~T+0.00
and ending at ~T+0.08 sec.  T90 (15-350 keV) is 0.084 +- 0.021 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-0.004 to T+0.100 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index -1.52 +- 2.56, 
and Epeak of 79.5 +- 30.0 keV (chi squared 67.2 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 5.3 +- 1.1 x 10^-8 erg/cm2
and the 1-sec peak flux measured from T-0.45 sec in the 15-150 keV band is
0.7 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.11 +- 0.27 (chi squared 73.8 for 57 d.o.f.).  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/451794/BA/

GCN Circular 11947

Subject
GRB 110420B: Swift-XRT afterglow candidate
Date
2011-04-21T07:06:31Z (14 years ago)
From
Boris Sbarufatti at INAF-OAB/IASFPA <boris.sbarufatti@brera.inaf.it>
B. Sbarufatti (INAF-OAB/IASFPA), V. Mangano (INAF/IASFPA) report on behalf
of the Swift-XRT team:

We have analyzed 4.5 ks of XRT data for GRB 110420B (Sbarufatti  et al.
GCN Circ. 11944), from 2.6 ks to 14.4 ks after the  BAT trigger. The data
are entirely in Photon Counting (PC) mode. We find an afterglow candidate
with XRT position RA, Dec = 320.10472, -41.29713 which is equivalent to:

RA (J2000): 21 20 25.13
Dec(J2000): -41 17 49.7

with an uncertainty of 4.0 arcsec (radius, 90% confidence). We cannot
determine at the present time whether the source is fading. The source
lies 3.2 arcmin from the BAT onboard position and 2.95 arcmin from the BAT
refined position (Markwardt et al., GCN Circ. 11946). The observed rate is
4.5 x 10^-3 counts/s  (1.4 x 10^-13 erg cm^-2 s^-1), corresponding to a
3.8 sigma level detection.

More data are needed in order to confirm the nature of the source.

GCN Circular 11949

Subject
GRB 110420B, GROND observations
Date
2011-04-21T12:35:44Z (14 years ago)
From
Andrea Rossi at TLS Tautenburg <rossi@tls-tautenburg.de>
P. Afonso (MPE Garching), A. Rossi, S. Klose (TLS Tautenburg), and J. 
Greiner (MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 110420B (Swift trigger 451794; Sbarufatti et
al. GCN 11944) 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 on La Silla.

Observations started at 7:00 UT on April 21, 8.15 hours after the GRB 
trigger. They were obtained under challenging sky conditions, with a 
seeing > 1.5 arcsec, an average airmass of 1.7, bright moonlight, and 
passing cirrus.

In a first epoch, centered on the refined BAT error circle (Markwardt et 
al., GCN 11946), we do not detect any new object 
down to the DSS2 limit. The XRT error circle (Sbarufatti et al., GCN 
11947) was not covered by these observations in g'r'i'z'.

A subsequent epoch started about 30 min later and was centered on the XRT 
afterglow candidate (Sbarufatti et al., GCN 11947). Here we note the 
presence of a relatively bright object at the border of the XRT error 
circle at coordinates RA, DEC (J2000) 21:20:25.389, -41:17:51.79 (+/- 0.5 
arcsec), which could be a galaxy seen nearly face-on. This object is also 
visible in the DSS2-red image.  Unfortunately, due to the bad seeing we 
cannot decide with certainty at the moment if this object is a galaxy or a 
star. Its r'-band magnitude is r'(AB)  = 19.5 +/- 0.1. No other source is 
detected inside the XRT error circle down to r'(AB) = 23.2.

Magnitudes are derived by calibrating the images against GROND zeropoints 
and 2MASS field stars. We note that the Galactic reddening along the line 
of sight is E(B-V) = 0.03 mag (Schlegel et al. 1998).

GCN Circular 11950

Subject
GRB 110420B: Spectral lag indicates short hard burst
Date
2011-04-21T13:39:01Z (14 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
S.D. Barthelmy (GSFC) and J. Norris (BSU)
report:

For GRB 110420B (Trigger 451794, GCNs 11944 & 11946),
the spectral lag analysis of the Swift-BAT data for 40 msec
covering the spike, yields a lag of 0.6 +/-2.0 msec for the
25-50 to 100-300 keV bands and 2.4 +4/-2 msec for the
15-25 to 50-100 keV bands, both using a lightcurve binning of 2 msec.

GCN Circular 11952

Subject
GRB 110420B: Fermi GBM observation
Date
2011-04-21T13:55:31Z (14 years ago)
From
Lin Lin at UAH/NAOC <ll0005@uah.edu>
Lin Lin (UAH)
reports on behalf of the Fermi GBM Team:

"At 22:42:11.73 UT on 20 April 2011, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 110420B (trigger 325032133 / 110420946).

This burst was also detected by the Swift/BAT
(Sbarufatti et al. 2011, GCN 11944).
The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight is 123 degrees.

The GBM light curve shows a fast rise slow decay peak,
with a duration (T90) of about 0.048 s (50-300 keV).
The time-averaged spectrum from T0-0.024 s to T0+0.040 s is
well fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.12 +/- 0.31 and
the cutoff energy, parameterized as Epeak, is 296.8 +/- 58.7 keV.

The event fluence (10-1000 keV) in this time interval is
(2.65 +/- 0.36)E-7 erg/cm^2. The 0.008-sec peak photon flux measured
starting from T0-0.016 s in the 10-1000 keV band
is 44.1 +/- 6.1 ph/s/cm^2.

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

GCN Circular 11953

Subject
GRB 110420B: Swift-XRT afterglow candidate retraction
Date
2011-04-21T14:09:18Z (14 years ago)
From
Boris Sbarufatti at INAF-OAB/IASFPA <boris.sbarufatti@brera.inaf.it>
B. Sbarufatti (INAF-OAB/IASFPA) and V. Mangano (INAF/IASFPA) report on 
behalf of the Swift XRT team:

Further observations of the field of GRB 110420B (Sbarufatti  et al.
GCN Circ. 11944) have shown that the X-ray source reported by Sbarufatti 
et al. (GCN Circ. 11947) has not faded.  Moreover, the position is 
marginally consistent with the known X-ray source 1RXS J212024.5-411816. 
Therefore we conclude that the XRT source is not the burst afterglow.

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

GCN Circular 11955

Subject
GRB 110420B: Swift/UVOT Upper Limits
Date
2011-04-21T14:29:56Z (14 years ago)
From
Samantha Oates at MSSL <sro@mssl.ucl.ac.uk>
S. R. Oates (MSSL-UCL) & B. Sbarufatti (INAF-OAB/IASFPA) report on behalf of the Swift-UVOT team

The Swift/UVOT began settled observations of the field of GRB 110420B
2562 s after the BAT trigger (Sbarufatti et al., GCN Circ. 11944).
No optical afterglow is found within the BAT error circle.

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          2562         2711          147         >20.6
white             2562         4765          540         >20.5
v                 3541        16124         1322         >19.9
b                 2926        22368          621         >20.4
u                 2720        28151         1730         >20.0
w1                3950        28007         2844         >19.6
m2                3745        27103         2416         >19.5
w2                3336        15211         1279         >19.7
###############################################################

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).

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