GRB 170813A

K. L. Page (U Leicester), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report
on behalf of the Swift Team:

At 01:13:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170813A (trigger=767563).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 201.054, -5.469 which is 
   RA(J2000) = 13h 24m 13s
   Dec(J2000) = -05d 28' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 30 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~6 sec before the trigger. 

The XRT began observing the field at 01:15:30.5 UT, 134.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
201.0572, -5.4909 which is equivalent to:
   RA(J2000)  = 13h 24m 13.73s
   Dec(J2000) = -05d 29' 27.1"
with an uncertainty of 2.8 arcseconds (radius, 90% containment). This
location is 79 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 consistent with the Galactic value of 2.73
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 5.59e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 143 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
0.04. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 
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.)

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

Using 1507 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 170813A, 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 = 201.05759, -5.49079 which is equivalent
to:

RA (J2000): 13h 24m 13.82s
Dec (J2000): -05d 29' 26.9"

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.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea
(PSU), B. Sbarufatti (INAF-OAB/PSU), S.L. Gibson (U. Leicester) and K.
L. Page (U Leicester) report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 170813A (Page et al. GCN
Circ. 21468
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 ), from 124 s to 39.0 ks after the  BAT trigger. The data
comprise 26 s in Windowed Timing (WT) mode (the first 9 s were 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. 21469
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.35 (+0.20, -0.18), followed by a break at T+796 s to
an alpha of 0.9 (+0.5, -0.8).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+/-0.4). The
best-fitting absorption column is  2.3 (+1.7, -1.3) x 10^21 cm^-2, in
excess of the Galactic value of 2.7 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 4.0 x 10^-11 (5.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.3 (+1.7, -1.3) x 10^21 cm^-2
Galactic foreground: 2.7 x 10^20 cm^-2
Excess significance: 2.5 sigma
Photon index:	     1.8 (+/-0.4)

If the light curve continues to decay with a power-law decay index of
0.9, the count rate at T+24 hours will be 4.2 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-14 (2.2 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/00767563.

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

Nat Butler (ASU), Alan M. Watson (UNAM), 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 170813A (Page, et al., GCN 21468
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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 2017/08 13.15 to 2017/08 13.18 UTC (2.48 to
3.06 hours after the BAT trigger), obtaining a total of 0.36 hours exposure
in the r and i bands and 0.15 hours exposure in the Z, Y, J, and H bands.

For a source within the Swift-XRT error circle (Osborne, et al., GCN
21469
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 ), in comparison with the USNO-B1 and 2MASS catalogs, we obtain the
following upper limits (3-sigma):

  r > 22.46
  i > 22.08
  Z > 20.23
  Y > 19.82
  J > 19.38
  H > 19.13

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.

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), J. R. Cummings (CPI),
H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
K. L. Page (U Leicester), 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 170813A (trigger #767563)
(Page, et al., GCN Circ. 21468
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 ).  The BAT ground-calculated position is
RA, Dec = 201.057, -5.527 deg which is
 RA(J2000)  =  13h 24m 13.7s
 Dec(J2000) = -05d 31' 37.4"
with an uncertainty of 3.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 71%.

The mask-weighted light curve shows a initial ragged pulse with a fast rise starting
at about T-10 sec, peaking at T+3 sec and then decaying to near background by
T+40, followed by a second weaker pulse from T+40 to T+80 sec.
T90 (15-350 keV) is 75.5 +- 10.9 sec (estimated error including systematics).

The time-averaged spectrum from T-9.2 to T+78.9 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.17 +- 0.14.  The fluence in the 15-150 keV band is 1.9 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+2.64 sec in the 15-150 keV band
is 0.8 +- 0.2 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/767563/BA/

F. E. Marshall (NASA/GSFC) and K. L. Page (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170813A
144 s after the BAT trigger (Page et al., GCN Circ. 21468
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 ).
No optical afterglow consistent with the enhanced XRT position
(Osborne et al. GCN Circ. 21469
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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           144          293          147         >20.1
u_FC               302          551          246         >19.8
white              144         6769          727         >21.4
v                  632        11867         1140         >19.3
b                  557         6564          432         >20.7
u                  302         6359          659         >20.4
w1                 682         6154          413         >19.9
m2                4314        12580         1090         >20.5
w2                5339        10954         1145         >20.4

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

"At 01:13:25 UT on 13 August 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170813A (trigger 524279593/ 170813051),
which was also detected by the Swift/BAT (Page et al. 2017, GCN 21468
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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 41
degrees.

The GBM light curve shows multiple pulses
with a duration (T90) of about 112 s (50-300 keV).
The time-averaged spectrum from T0-2.0 s to T0+23.6 s is
adequately fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.52 +/- 0.11 and
the cutoff energy, parameterized as Epeak, is 547 +/- 85 keV.

The event fluence (10-1000 keV) in this time interval is
(8.09 +/- 0.51)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.7 s in the 10-1000 keV band
is 2.36 +/- 0.17 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."
-- 
Suraj Poolakkil
Fermi GBM Graduate Research Assistant
Dept. of Space Science
University of Alabama in Huntsville