GRB 200901B

B. Sbarufatti (PSU), P. A. Evans (U Leicester),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 07:09:26 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200901B (trigger=994165).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 217.879, -38.716 which is 
   RA(J2000) = 14h 31m 31s
   Dec(J2000) = -38d 42' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, 
the available BAT light curve shows no significant structure. 

The XRT began observing the field at 07:12:16.1 UT, 169.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 217.8572, -38.7117 which is equivalent to:
   RA(J2000)  = 14h 31m 25.73s
   Dec(J2000) = -38d 42' 42.1"
with an uncertainty of 5.5 arcseconds (radius, 90% containment). This
location is 63 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 1.11e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
175 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 19.2 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.09. 

Burst Advocate for this burst is B. Sbarufatti (bxs60 AT psu.edu). 
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/)

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

Using 903 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 200901B, 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 = 217.85911, -38.71147 which is equivalent
to:

RA (J2000): 14h 31m 26.19s
Dec (J2000): -38d 42' 41.3"

with an uncertainty of 4.2 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. A. Breeveld (UCL-MSSL) and B. Sbarufatti (PSU) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 200901B 176 s after the BAT trigger 
(Sbarufatti et al., GCN Circ. 28337
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 ).
A source consistent with the XRT position (Sbarufatti et al. GCN Circ. 28337
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 )is detected in the 
initial UVOT exposures in the u filter only, but many exposures are smeared, so the magnitudes may 
not be reliable.

Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 
2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

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

v                  482          801           58        >18.1
b                  431          750           58        >19.1
u                  176          426          245         19.3 �� 0.3
w1                 531          851           58        >18.1
m2                 506          826           58        >17.9
w2                 457          776           58        >18.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of 
E(B-V) = 0.09 in the direction of the burst (Schlegel et al. 1998).

M. Stamatikos (OSU), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), B. Sbarufatti (PSU),
T. N. Ukwatta (LANL) (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 200901B (trigger #994165)
(Sbarufatti et al., GCN Circ. 28337
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 ).  The BAT ground-calculated position is
RA, Dec = 217.860, -38.704 deg which is
   RA(J2000)  =  14h 31m 26.5s
   Dec(J2000) = -38d 42' 15.2"
with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 82%.

The mask-weighted light curve shows some weak emission that
starts at ~T0 and ends at ~T+900 s. The available event data end
at T+963 s, and the burst went out of the BAT FOV soon afterwards
(~T+1000 s). T90 (15-350 keV) is 705.94 +- 148.71 sec (estimated error
including systematics).

The time-averaged spectrum from T+32.14 to T+901.32 sec is best fit by a
power law
with an exponential cutoff.  This fit gives a photon index 0.95 +- 0.63,
and Epeak of 46.7 +- 8.6 keV (chi squared 61.80 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 4.3 +- 0.4 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+68.62 sec in the 15-150 keV band is
0.4 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.95 +- 0.13 (chi squared 71.14 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/994165/BA/

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) ,
M. Capalbi (INAF-IASFPA), A. Tohuvavohu (U. Toronto), D.N. Burrows
(PSU) and B. Sbarufatti report on behalf of the Swift-XRT team:

We have analysed 4.6 ks of XRT data for GRB 200901B (Sbarufatti et al.
GCN Circ. 28337
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 ), from 11.3 ks to 35.0 ks after the  BAT trigger. The
data are entirely in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Osborne et al. (GCN Circ. 28339
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 ).

The light curve can be modelled with a series of power-law decays. The
light curve initially rises, with an index alpha=-0.6 (+0.7, -0.9). At
T+239 s it breaks to an alpha of 0.54 (+0.08, -0.07) before breaking
again at T+950 s to a final decay with index alpha=2.61 (+1.29, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.15 (+/-0.05). The
best-fitting absorption column is  1.66 (+0.27, -0.26) x 10^21 cm^-2,
in excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.8 x 10^-11 (6.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.66 (+0.27, -0.26) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 2.9 sigma
Photon index:	     1.15 (+/-0.05)

If the light curve continues to decay with a power-law decay index of
2.61, the count rate at T+24 hours will be 8.7 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.1 x
10^-15 (5.5 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00994165.

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