GRB 180331A

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

At 04:14:55 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180331A (trigger=820347).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 66.017, +13.398 which is 
   RA(J2000) = 04h 24m 04s
   Dec(J2000) = +13d 23' 52"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 50 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~7 sec after the trigger.

The XRT began observing the field at 04:16:48.0 UT, 112.3 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 66.0242,
13.3979 which is equivalent to:
   RA(J2000)  = 04h 24m 05.81s
   Dec(J2000) = +13d 23' 52.5"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 25 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 (2.85 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 3
(+3.23/-2.75) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 115 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
0.60.

Burst Advocate for this burst is C. B. Markwardt (craig.b.markwardt 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.)

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

Using 1180 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 180331A, 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 = 66.02468, +13.39893 which is equivalent
to:

RA (J2000): 04h 24m 5.92s
Dec (J2000): +13d 23' 56.1"

with an uncertainty of 1.8 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.

N. P. M. Kuin (UCL-MSSL) and C. B. Markwardt (GSFC) report
on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB
180331A 116 s after the BAT trigger (Markwardt et al., GCN Circ.
22568
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 ).  No optical afterglow consistent with the XRT position
(Evans et al. GCN Circ. 22569
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 ) is detected in the initial UVOT
exposures.
Preliminary 3-sigma upper limits using the UVOT photometric
system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the
first finding chart (FC) exposure and subsequent exposures are:

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

white_FC           116          266          147         >20.2
u_FC               274          524          246         >19.9
white              116         1373          371         >20.4
v                  604         1250           78         >18.6
b                  530         1350           78         >19.4
u                  274          524          246         >19.9

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.60 in the direction of the burst
(Schlegel et al. 1998).

P.A. Evans (U. Leicester) and C. B. Markwardt (GSFC) report on behalf
of the Swift-XRT team:

We have analysed 4.2 ks of XRT data for GRB 180331A (Markwardt et al.
GCN Circ. 22568
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 ), from 115 s to 23.0 ks after the  BAT trigger. The
data comprise 70 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 22569
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.64 (+0.21, -0.19), followed by a break at T+368 s to
an alpha of 3.47 (+0.30, -0.25); it then breaks again at T+1.2 ks to a 
decay of alpha=1.2 (+0.5, -2.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.35 (+0.21, -0.20). The
best-fitting absorption column is  4.9 (+1.7, -1.5) x 10^21 cm^-2, in
excess of the Galactic value of 2.9 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 6.0 x 10^-11 (7.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.9 (+1.7, -1.5) x 10^21 cm^-2
Galactic foreground: 2.9 x 10^21 cm^-2
Excess significance: 2.3 sigma
Photon index:	     1.35 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
1.2, the count rate at T+24 hours will be 3.3e-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
2.0^-14 (2.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00820347.

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

T. Sakamoto (AGU), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), M. Stamatikos (OSU),
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 180331A (trigger #820347)
(Markwardt et al., GCN Circ. 22568
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 ).  The BAT ground-calculated position is
RA, Dec = 66.005, 13.386 deg which is
  RA(J2000)  =  04h 24m 01.1s
  Dec(J2000) = +13d 23' 09.2"
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 81%.

The mask-weighted light curve shows a weak pulse that starts at ~T0,
peaks at ~T+9 s, and ends at ~T+50 s. T90 (15-350 keV) is 48.0 +- 22.6 sec
(estimated error including systematics).

The time-averaged spectrum from T+0.58 to T+48.58 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.11 +- 1.04,
and Epeak of 66.1 +- 28.3 keV (chi squared 57.70 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 6.1 +- 1.0 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T+8.08 sec in the 15-150 keV band is
0.5 +- 0.1 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.46 +- 0.19 (chi squared 64.59 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/820347/BA/