GRB 170906C

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 
170906C (Siegel et al., GCN 21826
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 ) as part of the 4pisky program. Our 
observation at 15 GHz on 2017 Sep 06.55 (UT) does not reveal any radio 
source at the XRT location (Evans et al., GCN 21831
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 ), with a 3sigma 
upper limit of 156 uJy. Further follow up observations have not been 
possible due to telescope maintenance.

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/.

A. Tsvetkova, S. Golenetskii, R.Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov, D. Svinkin, A. Lysenko, A. Kozlova and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration GRB 170906C (Swift-BAT trigger #770981:
Siegel et al., GCN 21826
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 ; Lien et al., GCN 21832
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 )
triggered Konus-Wind at T0=26780.985 s UT (07:26:20.985).

The burst light curve shows a single pulse
started at ~T0-40.8 s with a total duration of ~65.8 s.
The emission is seen up to ~4 MeV.

As observed by Konus-Wind, the burst
had a fluence of�� 1.60(-0.19,+0.28)x10^-5 erg/cm2,
and a 64-ms peak energy flux, measured from T0+0.354 s,
of 2.77(-0.73,+0.74)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 4 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with�� alpha = -1.06(-0.18,+0.19),
and Ep = 154(-19,+26) keV (chi2 = 64/72 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index:
beta < -2.56 (chi2 = 64/71 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB170906_T26780/

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.

L. Izzo, A. de Ugarte Postigo, D.A. Kann (IAA-CSIC/HETH) report on behalf of a larger collaboration:

We observed the field of GRB 170906C (Siegel et al., GCN 21826
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 ) with the T24 (Plane Wave CDK 0.61m) of the iTelescope.Net (http://www.itelescope.net) located at Auberry, CA. A set of 3 images of 300 s each were taken in the I filter, starting at 10:30:39 UT, ~ 2.9 hours after the GRB trigger.

We do not detect any source inside the XRT refined error region (Evans et al., GCN 21831
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 ) and estimate a 3-sigma upper limit of I > 19.0

Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), Carlos
Rom��n-Z����iga (UNAM), and William H. Lee (UNAM) report:

We observed the field of the GRB 170906C (Siegel et al., GCN Circ.
21826
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 ) with the COATLI 50-cm telescope and interim CCD imager (Watson et
al. 2016, Proc. SPIE, 9908, 50) at the Observatorio Astron��mico Nacional
on the Sierra de San Pedro M��rtir from 2017-09-06 08:17 to 12:11
obtaining a total of 2.93 hours of observation in the clear w filter.

We do not detect a source within the enhanced XRT error region reported
by Evans et al. (GCN Circ. 21831
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 ). Calibrating with respect to the
USNO-B1 catalog, we estimate a 3-sigma upper limit of:

w > 21.7

This magnitude is in the AB system and is not corrected for Galactic
extinction in the direction of the GRB.

We thank the COATLI technical team (Fernando ��ngeles, Oscar Chapa,
Salvador Cuevas, Alejandro Farah, Jorge Fuentes, Rosal��a Langarica,
Fernando Quir��s, and Carlos Tejada) and the staff of the Observatorio
Astron��mico Nacional.

M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170906C
1296 s after the BAT trigger (Siegel et al., GCN Circ. 21826
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 ).
No optical afterglow consistent with the XRT position
(Evans et al. GCN Circ. 21828
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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          1296         1446          147         >21.6
white             1296         2585          284         >21.4
v                 1578         2635          136         >19.4
b                 1504         2913          333         >19.9
u                 1479         2708          156         >20.6
w1                1454         2684          156         >19.9
m2                2467         2487           19         >17.8
w2                2591         2611           19         >19.0

The magnitudes in the table 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).

V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), D.N. Burrows (PSU), A.
Tohuvavohu (PSU), S. J. LaPorte (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri
(INAF-OAB) and M.H. Siegel report on behalf of the Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 170906C (Siegel et al. GCN
Circ. 21826
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 ), from 1.3 ks to 20.2 ks after the	BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Evans et al. (GCN Circ. 21831
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.65 (+0.07, -0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.69 (+0.21, -0.20). The
best-fitting absorption column is  9.4 (+1.4, -1.3) x 10^21 cm^-2, in
excess of the Galactic value of 3.3 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.4 x 10^-11 (1.2 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.4 (+1.4, -1.3) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^20 cm^-2
Excess significance: 11.3 sigma
Photon index:	     2.69 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
0.65, the count rate at T+24 hours will be 0.027 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.1 x
10^-13 (3.2 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/00770981.

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

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

Using the data set from T-60 to T+243 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 170906C (trigger #770981)
(Siegel, et al., GCN Circ. 21826
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 ).  The BAT ground-calculated position is
RA, Dec = 128.659, 70.805 deg, which is
   RA(J2000)  =  08h 34m 38.1s
   Dec(J2000) = +70d 48' 16.7"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 32%.

The mask-weighted light curve shows multiple overlapping peaks starting at
~T-40 sec, peaking at ~T+7 sec, and returning to baseline at ~T+100 sec.
T90 (15-350 keV) is 48.4 +- 4.7 sec (estimated error including systematics).

The time-averaged spectrum from T-35.21 to T+39.22 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.30 +- 0.22,
and Epeak of 104.8 +- 43.0 keV (chi squared 47.9 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 8.8 +- 0.3 x 10^-6 erg/cm2
and the 1-sec peak flux measured from T+6.18 sec in the 15-150 keV band is
7.2 +- 0.5 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.67 +- 0.05 (chi squared 56.9 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/770981/BA/

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

Using 1795 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 170906C, 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 = 128.65367, +70.81162 which is equivalent
to:

RA (J2000): 08h 34m 36.88s
Dec (J2000): +70d 48' 41.8"

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.

P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), D.N. Burrows
(PSU), J.A. Kennea (PSU), S. Campana (INAF-OAB) and G. Cusumano
(INAF-IASF PA) report on behalf of the Swift-XRT team:

The XRT began observing the field of GRB 170906C at 07:47:49.6 UT,
1292.1 seconds after the BAT trigger. Using promptly downlinked data we
find an uncatalogued X-ray source located at RA, Dec 128.65022,
70.81211 which is equivalent to:
   RA(J2000)  = 08h 34m 36.05s
   Dec(J2000) = +70d 48' 43.6"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 125 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 (3.30 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.1
(+0.39/-0.34) x 10^22 cm^-2 (90% confidence).

M. H. Siegel (PSU), S. D. Barthelmy (GSFC), P. A. Evans (U Leicester),
C. Gronwall (PSU), K. L. Page (U Leicester) and A. Tohuvavohu (PSU)
report on behalf of the Swift Team:

At 07:26:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170906C (trigger=770981).  Swift did not slew due to an
observing constraint.  The BAT on-board calculated location is 
RA, Dec 128.704, +70.782, which is 
   RA(J2000)  =  08h 34m 49s
   Dec(J2000) = +70d 46' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple overlapping peaks
with a total duration of about 30 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~6 sec after the trigger. 

Due to an observing constraint, Swift will not slew until T0+18.9
minutes. There will be no XRT or UVOT data until this time. 

Burst Advocate for this burst is M. H. Siegel (siegel 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.)