GRB 170903A

C. B. Markwardt (NASA/GSFC), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), J. A. Kennea (PSU),
K. L. Page (U Leicester), J. L. Racusin (NASA/GSFC) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 12:49:07 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170903A (trigger=770528).  Swift did not slew due to an observing
constraint.  The BAT on-board calculated location is 
RA, Dec 254.512, +34.983, which is 
   RA(J2000)  =  16h 58m 03s
   Dec(J2000) = +34d 59' 01"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a few peaks
with a total duration of about 30 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~4 sec after the trigger. 

Due to an observing constraint, Swift will not slew until T0+49.5
minutes. There will be no XRT or UVOT data until this time. 

Burst Advocate for this burst is C. B. Markwardt (Craig.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.)

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

The XRT began observing the field of GRB 170903A at 13:41:04.6 UT,
3117.5 seconds after the BAT trigger. Using promptly downlinked data we
find an uncatalogued X-ray source with an enhanced position: RA, Dec
254.5260, 34.9796 which is equivalent to:
   RA(J2000)  = 16h 58m 06.23s
   Dec(J2000) = +34d 58' 46.6"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 43 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 (1.74 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 6.3
(+3.39/-2.89) x 10^21 cm^-2 (90% confidence).

H.X. Feng, D. Xu, Z.P. Zhu (NAOC), S.G. Ma, T.Z. Yang, J.H. Liu, P. 
Zong, H.B. Niu, J.Z. Liu,  A. Esamdin, L. Ma (all XAO), Y.D. Hu 
(IAA-CSIC), Y. Qing (Geneva Observatory) report:

We observed the field of GRB 170903A (Markwardt et al., GCN 21793
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 ) using 
the 1m telescope located at Nanshan, Xinjiang, China. We obtained 3x180s 
R-band frames, starting at 14:08:09 UT on 2017-09-03, i.e., 1.317 hr 
after the burst.

No optical source is detected at the XRT position (Campana et al., GCN 
21794
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 ) in the stacked image, down to a limiting magnitude of m(R)~21.3, 
calibrated with the nearby SDSS field.

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),
T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):
 
Using the data set from T-61 to T+242 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 170903A (trigger #770528)
(Markwardt, et al., GCN Circ. 21793
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 ).  The BAT ground-calculated position is
RA, Dec = 254.532, 34.985 deg, which is 
   RA(J2000)  =  16h 58m 07.7s 
   Dec(J2000) = +34d 59' 07.5" 
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 30%.
 
The mask-weighted light curve shows two slightly overlapping peaks starting
at ~T-5 sec, with a maximum at ~T+1 sec, and returning to baseline at ~T+70 sec.
T90 (15-350 keV) is 29.2 +- 4.4 sec (estimated error including systematics).
 
The time-averaged spectrum from T-10.00 to T+25.70 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.94 +- 0.12.  The fluence in the 15-150 keV band is 2.4 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+0.58 sec in the 15-150 keV band
is 3.9 +- 0.5 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/770528/BA/

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

Using 1379 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 170903A, 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 = 254.52613, +34.97887 which is equivalent
to:

RA (J2000): 16h 58m 6.27s
Dec (J2000): +34d 58' 43.9"

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.

A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), L. Izzo (HETH/IAA-CSIC)
C.C. Thoene (HETH/IAA-CSIC), D.A. Kann (HETH/IAA-CSIC), 
K.E. Heintz (DARK/NBI), N. Castro-Rodriguez (GRANTECAN, IAC, ULL), 
A. Marante (GRANTECAN) report on behalf of a larger collaboration:

We have observed the field of GRB 170903A (Markwardt et al., GCN 21793
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 ) 
with OSIRIS at the 10.4m GTC telescope. Observations started at 20:49 UT 
(8.0 hrs after the burst). Observations consisted of an acquisition image in 
i-band and 3x900s spectra with the R1000B grism, which covers the range 
between 3700 and 7880 AA.

The acquisition image shows a single source within the refined XRT error box
(Goad et al., GCN 21797
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 ), with an i-band magnitude (AB) of 23.17 +/- 0.15. 
The object appears unresolved at a seeing of 0.6���. Spectroscopy of it shows a 
weak trace covering the complete range above 4000 AA and a single, very 
prominent emission feature. Due to the strength of this feature and the 
non-detection of any other features in the spectral range, we identify it as the 
unresolved [OII] 3727/3729 doublet at a redshift of 0.886, which we propose 
as the redshift of the host galaxy and hence of the GRB.

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

We have analysed 7.0 ks of XRT data for GRB 170903A (Markwardt et al.
GCN Circ. 21793
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 ), from 3.4 ks to 32.1 ks after the  BAT trigger. The
data are entirely in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Goad et al. (GCN Circ. 21797
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.41 (+/-0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.22 (+0.19, -0.18). The
best-fitting absorption column is  5.6 (+1.1, -1.0) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 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 3.7 x 10^-11 (7.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.6 (+1.1, -1.0) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 9.3 sigma
Photon index:	     2.22 (+0.19, -0.18)

If the light curve continues to decay with a power-law decay index of
0.41, the count rate at T+24 hours will be 0.048 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-12 (3.5 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/00770528.

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

D. Kuroda, K. Yanagisawa, Y. Shimizu, H. Toda (OAO, NAOJ),
S. Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech)
report on behalf of MITSuME and OISTER collaboration:

We observed the field of GRB 170903A (Markwardt et al., GCNC 21793)
with the optical three color (g', Rc and Ic) CCD camera attached
to the MITSuME 50cm telescope of Okayama Astrophysical Observatory.

The observation started on 2017-09-03 12:49:43 UT (~35 sec after
the burst). We did not find any new point source within the
enhanced XRT error circle (Goad et al., GCNC 21797) in all
the three bands.

Three sigma upper limits of the OT are listed below.
We used SDSS-DR8 catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'    Rc     Ic
-----------------------------------------------------
0.03712    13:42:34    5280.0   >19.4  >19.5  >18.7
-----------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

E. Mazaeva (IKI),  E. Chornaya (UAFO), A. Pozanenko (IKI), A. Volnova 
(IKI), A. Matkin (UAFO), I. Molotov (KIAM)  report on behalf of  larger 
GRB follow-up collaboration:

We observed the field of  Swift GRB 170903A  (Markwardt et al., GCN 
21793
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 ) with SANTEL-650 (0.65m)  telescope  of UAFO/ISON-Ussuriysk 
observatory. We obtained several unfiltered images starting on  Sep. 03 
(UT) 13:20:15.  Within enhanced XRT position (Goad et al., GCN 21797
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 ) we 
do not detect any source.  Preliminary photometry of a combined image is 
following

Date       UT start  t-T0     Filter  Exp.      OT   Err.  UL
                     (mid, days)       (s)

2017-09-03 13:20:15  0.04847  CR      120*30    n/d  n/d   20.4


Photometry is based on nearby SDSS DR9 stars
SDSS_DR9_id         R(Lupton)
J165744.06+345751.4 16.01
J165803.01+345725.1 16.56
J165808.53+350159.7 15.58
J165749.08+345655.1 16.66
J165805.88+345230.1 16.24

A. A. Breeveld (UCL-MSSL) and C. B. Markwardt (NASA/GSFC) report on
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170903A
3484 s after the BAT trigger (Markwardt et al., GCN Circ. 21793
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 ).
No optical afterglow consistent with the XRT position (Goad et al. GCN
Circ. 21797
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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          3484         3634          147         >20.7
white             3484         4865          344         >21.6
v                 3640         5275          393         >19.7
b                 4460         4660          197         >20.5
u                 4255         4455          197         >21.0
w1                4050         9664          362         >20.7
m2                3845         5482          393         >20.4
w2                4871         5071          197         >21.8

The magnitudes in the table are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.02 in the direction of the
burst (Schlegel et al. 1998).

F. Berlato (MPE) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 12:49:07.79 UT on 3 September 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170903A (trigger 526135752 / 170903534), which
was also detected by the Swift/BAT (Markwardt et al., GCN 21793
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 ).

The GBM on-ground localization is consistent with the Swift location.

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

The GBM light curve shows two pulses with a duration (T90) of 
about 25 s (50-300 keV). The time-averaged spectrum from 
T0-1.5 s to T0+25.5 s is adequately fit by a Comptonized model, 
with index  -1.56 +/- 0.10 and Epeak 124 +/- 33 keV .
The event fluence (10-1000 keV) in the time interval is 
(4.34 +/- 0.54)E-06 erg/cm^2. The 1-sec peak photon flux (10-1000 keV) 
measured starting from T0-1.5 s is (5.80 +/- 0.36) ph/s/cm^2.

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

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 
170903A (Markwardt et al., GCN 21793
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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 2017 Sep 03.57, Sep 04.79, Sep 
06.82, Sep 12.66 (UT) do not reveal any radio source at the XRT location 
(Goad et al., GCN 21797
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 ), with 3sigma upper limits of 225 uJy, 120 uJy, 
126 uJy, 99 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/.