GRB 170906B

M. H. Siegel (PSU) and B. Sbarufatti (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170906B
81 s after the BAT trigger (Sbarufatti et al., GCN Circ. 21822
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 ).
No optical afterglow consistent with the XRT position
(Osborne et al. GCN Circ. 21823
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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            81          231          147         >20.2
u_FC               293          543          246         >19.3
white               81          930          243         >20.6
v                  623          815           39         >17.5
b                  548          741           39         >18.7
u                  293          716          265         >19.3
w1                 672          865           39         >18.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.42 in the direction of the burst
(Schlegel et al. 1998).

B. Sbarufatti (PSU) reports on behalf of the Swift-XRT team:

We have analysed 831 s of XRT data for GRB 170906B (Sbarufatti et al.
GCN Circ. 21822
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 ), from 63 s to 924 s after the	BAT trigger. The data
comprise 9 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. The enhanced
XRT position for this burst was given by Osborne et al. (GCN Circ.
21823
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.63 (+0.13, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.80 (+0.30, -0.27). The
best-fitting absorption column is  7.5 (+2.8, -2.2) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 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 5.0 x 10^-11 (7.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.5 (+2.8, -2.2) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^21 cm^-2
Excess significance: 4.3 sigma
Photon index:	     1.80 (+0.30, -0.27)

If the light curve continues to decay with a power-law decay index of
0.63, the count rate at T+24 hours will be 0.033 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-12 (2.6 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00770958.

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

H. A. Krimm (NSF/USRA), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), B. Sbarufatti (PSU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):

Using the data set from the recent telemetry downlink,
we report further analysis of BAT GRB 170906B (trigger #770958)
(Sbarufatti et al., GCN Circ. 21822
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 ).

Because this burst occurred only 755 s after the trigger time of GRB 170906A
(trigger #770957), data collection was interrupted, and as such we do not have
event-by-event data during the burst. Thus most of the usual BAT data products
that we usually report in the "refined analysis circular" are not available.

The BAT raw light curve shows roughly two overlapping pulses that start
at ~T-11 s and ends at ~T+7 s. The two peaks occur at ~T-5 s and ~T0,
respectively.

The results of the BAT analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/770958/BA/

R. Hamburg (UAH) and and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 00:55:40.85 UT on 06 September 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170906B (trigger 526352145 / 170906039).
which was also detected by Swift (Sbarufatti et al. 2017, GCN 21822
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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 53
degrees.

The GBM light curve shows a two-peaked emission
with a duration (T90) of about 12 s (50-300 keV).
The time-averaged spectrum from T0-0.0 s to T0+12.8 s is
well fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.11 +/- 0.13 and
the cutoff energy, parameterized as Epeak, is 59 +/- 4 keV.

The event fluence (10-1000 keV) in this time interval is
(2.7 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+5.1 s in the 10-1000 keV band
is 10.3 +/- 0.4 ph/s/cm^2.

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

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

Using 824 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 170906B, 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 = 232.20430, -28.25599 which is equivalent
to:

RA (J2000): 15h 28m 49.03s
Dec (J2000): -28d 15' 21.6"

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.

B. Sbarufatti (PSU), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. H. Siegel (PSU) and A. Tohuvavohu (PSU)
report on behalf of the Swift Team:

At 00:55:46 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170906B (trigger=770958). Swift slewed immediately to the burst. 
The BAT on-board calculated location is
RA, Dec 232.221, -28.251 which is
   RA(J2000) = 15h 28m 53s
   Dec(J2000) = -28d 15' 03.6"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve is not immediately available
for an unknown reason. 

The XRT began observing the field at 00:57:04.2 UT, 78.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 232.2037, -28.2563
which is equivalent to:
   RA(J2000)  = 15h 28m 48.90s
   Dec(J2000) = -28d 15' 22.8"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 58 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 (1.69 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 1.1
(+1.25/-0.71) x 10^22 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 80 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
0.42. 

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.)