GRB 180721A

B. Sbarufatti (PSU), F. E. Marshall (NASA/GSFC) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 11:06:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180721A (trigger=849018).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 347.695, +4.880 which is 
   RA(J2000) = 23h 10m 47s
   Dec(J2000) = +04d 52' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 60 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 11:07:45.3 UT, 56.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
347.7100, 4.8553 which is equivalent to:
   RA(J2000)  = 23h 10m 50.41s
   Dec(J2000) = +04d 51' 19.1"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 103 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 (6.47 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.7
(+3.22/-2.74) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.38e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
119 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 19.2 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.07. 

Burst Advocate for this burst is B. Sbarufatti (bxs60 AT 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.)

C. Guidorzi, R. Martone (U. Ferrara), S. Kobayashi (LJMU), C.G. Mundell 
(U. Bath), A. Gomboc (U. Nova Gorica), I.A. Steele (LJMU), A. Cucchiara, 
D. Morris (U. of Virgin Islands) on behalf of a large collaboration report:

The LCO 2-m unit at Haleakala Observatory (Hawaii) automatically began 
observing Swift GRB 180721A (Sbarufatti et al. GCN 22994
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 ) on July 21, 
11:21:40 UT (14.9 minutes after the GRB trigger time) with the SDSS i' 
filter. Within the Swift-XRT error circle we do not detect any source 
down to i'>20.2 mag at a mid time of 20.4 minutes post GRB with a 5x120s 
exposure, as calibrated against nearby SDSS objects.

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

Using 1486 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 180721A, 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 = 347.71167, +4.85678 which is equivalent
to:

RA (J2000): 23h 10m 50.80s
Dec (J2000): +04d 51' 24.4"

with an uncertainty of 2.8 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.

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), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester) and B.
Sbarufatti report on behalf of the Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 180721A (Sbarufatti et al.
GCN Circ. 22994
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 ), from 63 s to 17.7 ks after the  BAT trigger. The data
comprise 17 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Goad et al. (GCN Circ. 22997
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.9 (+0.8, -0.5), followed by a break at T+203 s to an
alpha of 0.84 (+/-0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.14 (+0.24, -0.23). The
best-fitting absorption column is  1.4 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 6.5 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 3.2 x 10^-11 (4.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 6.5 x 10^20 cm^-2
Excess significance: 2.1 sigma
Photon index:	     2.14 (+0.24, -0.23)

If the light curve continues to decay with a power-law decay index of
0.84, the count rate at T+24 hours will be 3.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.6 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00849018.

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 T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180721A (trigger #849018)
(Sbarufatti et al., GCN Circ. 22994
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 ).  The BAT ground-calculated position is
RA, Dec = 347.708, 4.871 deg which is
  RA(J2000)  =  23h 10m 49.9s
  Dec(J2000) = +04d 52' 16.1"
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The mask-weighted light curve shows a single-pulse structure that starts
at ~T-1 s, peaks at ~T+2 s, and ends at ~T+10 s. The main pulse is followed
by some weak emission that last till ~T+50 s. T90 (15-350 keV) is 47.6 +- 3.0 sec
(estimated error including systematics).

The time-averaged spectrum from T-0.39 to T+48.92 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.25 +- 0.22.  The fluence in the 15-150 keV band is 5.9 +- 0.9 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+1.82 sec in the 15-150 keV band
is 0.9 +- 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/849018/BA/

N. P. M. Kuin (UCL-MSSL) and B. Sbarufatti (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180721A
120 s after the BAT trigger (Sbarufatti et al., GCN Circ. 22994
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 ).
No optical afterglow consistent with the XRT position (Goad et al, GCN
Circ. 22997
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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

u_FC               120          370          246         >20.1
v                  425         1693          156         >19.1
b                  375         1642          175         >19.9
u                  120         1617          529         >20.1
w1                 474         1742          156         >19.6
m2                1697         1717           19         >18.3
w2                 401         1668          156         >20.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.07 in the direction of the burst
(Schlegel et al. 1998).

I. Carrasco (Univ. de Malaga), Y. Hu, E. Fernandez-Garcia, J. C. Tello 
and A. J. Castro-Tirado (IAA-CSIC),
D. Hiriart and W. H. Lee (UNAM), S. Jeong and I. H. Park (SKKU), M. D. 
Caballero-Garcia (ASU-CAS, CZ) and
R. Cunniffe (Inst. of Physics, CZ) on behalf of a larger collaboration, 
report:

The 60cm BOOTES-5/JGT robotic telescope at Observatorio Astronomico 
Nacional in San Pedro Martir (Mexico)
automatically responded in 21s (and 53s after the GRB onset) to the 
Swift trigger of GRB 180721A (Sbarufatti
et al., GCNC 22994). The first image (10s exposure, r-band) was obtained 
at 11:07:10 UT. At the position of
the Swift X-ray afterglow (Goad et al. GCNC 22997), no optical afterglow 
is detected down 18.1 mag. Furthermore,
on the co-add of the images obtained until 11:33:52 UT no afterglow is 
detected down to 21 mag, consistent with
the limit reported by Guidorzi et al. (GCNC 22995).

We thank the staff at Observatorio Astronomico Nacional in San Pedro 
Martir for its excellent support.

M. Oeda, R. Itoh, K. L. Murata, Y. Tachibana, S. Harita, K. Morita,
K. Shiraishi, K. Iida, R. Adachi, S. Niwano,
Y. Yatsu, and N. Kawai (Tokyo Tech)
report on behalf of the MITSuME collaboration:

We searched for the optical counterpart of GRB 180721A
(B. Sbarufatti et al., GCN Circular #22994
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 )
with the optical three color (g', Rc, and Ic) CCD cameras attached to
the MITSuME 50cm telescope of Akeno Observatory, Yamanashi, Japan.

The observation started on 2018-07-21 13:44:45.05 UT.
We did not find any point source at the enhanced position of the
XRT error circle (M.R. Goad et al., GCN Circular #22997
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 ) in all three bands.
We obtained following 5-sigma upper limits for the magnitudes.

T0+[min]    MID-UT      T-EXP[sec]    g'       Rc        Ic
-------------------------------------------------------------------------------------------------------
157.95     14:27:28  4,740   >16.9   >17.3   >16.9
-------------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the burst
T-EXP: Total Exposure time
We used UCAC-4 catalog for flux calibration.

WeiKang Zheng and Alexei V. Filippenko (UC Berkeley) report on
behalf of the KAIT GRB team:

The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, responded to Swift GRB 180721A (Sbarufatti et al.,
GCN 22994
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 ) starting at 11:25:23 UT, ~19 minutes after the burst.
Observations were performed with an automatic sequence in the
clear (roughly R), V, and I filters, and the exposure time was 20 s
per image. We do not detect any optical afterglow candidate within
the enhanced XRT position error circle (Goad et al., GCN 22997
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 )
neither in single image, nor in the co-add images.
The typical limiting magnitude of our single clear image is about
19.5 mag calibrated to the Pan-STARRS1 catalog.