GRB 170318B

A. Cholden-Brown (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
J. A. Kennea (PSU), H. A. Krimm (CRESST/NSF/USRA),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 15:27:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170318B (trigger=743086).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 284.279, +6.269, which is 
   RA(J2000)  =  18h 57m 07s
   Dec(J2000) = +06d 16' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 3 sec.  The peak count rate
was ~1900 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 15:29:06.3 UT, 73.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 284.30606, 6.29833
which is equivalent to:
   RA(J2000)  = 18h 57m 13.45s
   Dec(J2000) = +06d 17' 54.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 141 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 does not constrain the column density. 

The initial flux in the 2.5 s image was 5.39e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 84 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 large, but uncertain extinction expected. 

Burst Advocate for this burst is A. Cholden-Brown (aaronb AT swift.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/too.html.)

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using  promptly downlinked XRT event data for GRB 170318B, we find an
enhanced XRT position of the afterglow: RA, Dec: 284.3062, 6.2994 which
is equivalent to:
   RA (J2000)  = 18 57 13.49
   Dec (J2000) = +06 17 57.9
with an uncertainty of 2.1 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/743086.

Position enhancement is is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

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

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

Using 839 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 170318B, 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 = 284.30611, +6.29924 which is equivalent
to:

RA (J2000): 18h 57m 13.47s
Dec (J2000): +06d 17' 57.3"

with an uncertainty of 1.9 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.

P. Veres and C. Meegan (both UAH)
report on behalf of the Fermi GBM Team:

"At 15:27:53.09 UT on 18 March 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170318B (trigger 511543678 / 170318644).
which was also detected by the Swift/BAT
(Cholden-Brown et al. 2008, GCN 20908
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 )
The GBM on-ground location is consistent with the Swift position.


The angle from the Fermi LAT boresight at the GBM trigger time is 93 degrees.


The GBM light curve shows  a single peak
with a duration (T90) of about 4 s (50-300 keV).
The time-averaged spectrum from T0-3.3 s to T0+1.8 s is
adequately fit by a simple power law function with index -1.40 +/- 0.10.


The event fluence (10-1000 keV) in this time interval is
(8.0 +/- 1.4)E-7 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-0.8 s in the 10-1000 keV band
is 1.9 +/- 0.2 ph/s/cm^2.


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

J.A. Kennea (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U.
Leicester), J.P. Osborne (U. Leicester), V. D'Elia (ASDC), A. D'Ai
(INAF-IASFPA), A. Melandri (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU),
D.N. Burrows (PSU) and	report on behalf of the Swift-XRT team: report
on behalf of the Swift-XRT team:

We have analysed 7.2 ks of XRT data for GRB 170318B, from 64 s to 41.1
ks after the  BAT trigger. The data comprise 95 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial rise, with a power-law
index of alpha=-0.5 (+0.4, -0.5), followed by a break at T+134 s  to an
alpha of 2.96 (+0.16, -0.15).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.6 (+/-0.4). The
best-fitting absorption column is  6.4 (+1.7, -1.4) x 10^22 cm^-2, in
excess of the Galactic value of 1.4 x 10^22 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.3 (+/-0.6) and a
best-fitting absorption column of 6.2 (+2.1, -1.7) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 8.0 x 10^-11 (3.5 x 10^-10) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.2 (+2.1, -1.7) x 10^22 cm^-2
Galactic foreground: 1.4 x 10^22 cm^-2
Excess significance: 4.6 sigma
Photon index:	     2.3 (+/-0.6)

If the light curve continues to decay with a power-law decay index of
2.96, the count rate at T+24 hours will be 4.5 x 10^-8 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x
10^-18 (1.6 x 10^-17) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00743086.

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

A. A. Breeveld (UCL-MSSL) and A. Cholden-Brown (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170318B
84 s after the BAT trigger (Cholden-Brown et al., GCN Circ. 20908
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 ).
No optical afterglow consistent with the XRT position (Goad et al. GCN 
Circ. 20910
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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            84          234          147         >20.4
u_FC               296          546          246         >19.8
white               84         6145          530         >21.0
v                  626         6557          432         >19.4
b                  552         5940          236         >20.0
u                  296         7172          659         >20.2
w1                 675         6967          432         >19.9
m2                5123         5323          197         >19.3
w2                 601         6352          216         >19.6

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 4.51 in the direction of the 
burst (Schlegel et al. 1998).

Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC),
William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J.
Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI),
Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz
(UCSC), Jes��s Gonz��lez (UNAM), Carlos Rom��n-Z����iga (UNAM), Harvey
Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and Vicki Toy
(UMD) report:

We observed the field of GRB 170318B (Cholden-Brown et al., GCN 20908
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 )
with the Reionization and Transients Infrared Camera (RATIR;
www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio
Astron��mico Nacional on Sierra San Pedro M��rtir from 2017/03 19.47 to
2017/03 19.51 UTC (19.76 to 20.76 hours after the BAT trigger),
obtaining a total of 0.71 hours exposure in the r and i bands and 0.30
hours exposure in the Z, Y, J, and H bands.

For a source within the Swift-XRT enhanced error circle (Goad et al.,
GCN 20910
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 ), in comparison with the USNO-B1 and 2MASS catalogs, we obtain
the following upper limits (3-sigma):

 r    > 25.64
 i    > 24.39
 Z    > 22.65
 Y    > 22.43
 J    > 21.38
 H    > 20.98

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
M��rtir.

H. A. Krimm (NSF/USRA), S. D. Barthelmy (GSFC), A. Cholden-Brown (PSU),
J. R. Cummings (CPI), N. Gehrels (GSFC), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU),
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 170318B (trigger #743086)
(Cholden-Brown, et al., GCN Circ. 20908
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 ).  The BAT ground-calculated 
position is
RA, Dec = 284.229, 6.335 deg which is
    RA(J2000)  =  18h 56m 54.9s
    Dec(J2000) = +06d 20' 04.5"
with an uncertainty of 3.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 67%.

The mask-weighted light curve shows an initial symmetric pulse of 
duration ~20 sec
from about T-15 sec to T+5 sec.  This is followed by low-level extended 
emission from
~T+30 to T+160 sec. T90 (15-350 keV) is 160.0 +- 35.8 sec (estimated 
error including systematics).

The time-averaged spectrum from T-15.33 to T+160.67 sec is best fit by a 
simple
power-law model.  The power law index of the time-averaged spectrum is
1.84 +- 0.59.  The fluence in the 15-150 keV band is 5.5 +- 1.9 x 10^-7 
erg/cm2.
The 1-sec peak photon flux measured from T-7.83 sec in the 15-150 keV band
is 0.1 +- 0.1 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/743086/BA/

K. P. Mooley (Hintze Fellow, Oxford) , T. D. Staley, R. P. Fender 
(Oxford), G. E. Anderson (Curtin), T. Cantwell (Manchester), D. 
Titterington, S. H. Carey, J. Hickish, Y. C. Perrott, N. Razavi-Ghods, 
P. Scott (Cambridge), K. Grainge, A. Scaife (Manchester)

The AMI Large Array robotically triggered on the Swift alert for GRB 
170318B (Cholden-Brown et al., GCN 20908
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 ) as part of the 4pisky program, 
and subsequent follow up observations were obtained up to 10 days 
post-burst. Our observations at 15 GHz on 2016 Mar 19.11, Mar 21.17 and 
Mar 23.24 (UT) do not reveal any radio source at the XRT location (Goad 
et al., GCN 20910
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 ), with 3sigma upper limits of 300 uJy, 150 uJy and 315 
uJy respectively.

We thank the AMI staff for scheduling these observations. The AMI-GRB 
database is a log of all GRB follow up observations with the AMI, and is 
available at http://4pisky.org/ami-grb/.