GRB 180720C

C. Perez del Pulgar and A. Castellon (Univ. de Malaga), E. 
Fernandez-Garcia, Y. Hu, I. Carrasco
and A. J. Castro-Tirado (IAA-CSIC), on behalf of a larger collaboration, 
report:

The 60cm BOOTES-2/TELMA robotic telescope at IHSM La Mayora (UMA-CSIC) 
in Algarrobo Costa (Spain) automatically
responded in 53s (and 132s after the GRB onset) to the Swift trigger of 
GRB 180720C (LaPorte et al., GCNC 22982).
The first image (10s exposure, unfiltered) was obtained at 22:26:09 UT. 
At the position of the Swift X-ray
afterglow (Evans et al. GCNC 22987), no optical afterglow is detected 
down 18 mag in this crowded field in
the direction of the Galactic Center (i.e. high extinction towards the 
source is very likely present).
This is consistent with the limits reported by Guidorzi et al. (GCNC 
22991), Oates et al. (GCNC 22992) and
Gorbovskoy et al. (GCNC 23009).

The message may be cited.

E. Gorbovskoy, V.Lipunov, N.Tiurina, V. Lipunov, V.Kornilov, A.Kuznetsov, 
V.Chazov, I.  Gorbunov, D.Zimnukhov, D.Kuvshinov, P.Balanutsa, 
V.Vladimirov, D.Vlasenko
Lomonosov Moscow State University,SAI

D. Buckley,
South African Astronomical Observatory (SAAO)

R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres
The Instituto de Astrofisica de Canarias (IAC)

A. Tlatov, V.Senik, D. Dormidontov
Kislovodsk Solar Station of the Pulkovo Observatory

R. Podesta, F. Podesta, C. Lopez, C.Francile
Observatorio Astronomico Felix Aguilar (OAFA)

H.Levato,
Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE)

O. Gres, N.M.Budnev , Yu.Ishmuhametova
Irkutsk State University (ISU)

A. Gabovich, V. Yurkov, Yu. Sergienko
Blagoveschensk Educational State University (BSPU)

MASTER-Kislovodsk robotic telescope (MASTER-Net: 
http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, 
vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station 
of Pulkovo observatory) was pointed to the  GRB180720C 21 sec after notice 
time and 102 sec after trigger time at 2018-07-20 22:25:39 UT.


MASTER-IAC robotic telescope (MASTER-Net: http://observ.pereplet.ru, 
Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in 
Spain (IAC Teide Observatory) was pointed to the  GRB180720C 258 sec after 
notice time and 1668 sec after trigger time at 2018-07-20 22:56:57 UT.

MASTER-SAAO robotic telescope (MASTER-Net: http://observ.pereplet.ru, 
Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in 
South Africa (South African Astronomical Observatory) was pointed to the 
GRB180720C 30 sec after notice time and 111 sec after trigger time at 
2018-07-20 22:25:48 UT.

MASTER-OAFA robotic telescope (MASTER-Net: http://observ.pereplet.ru, 
Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in 
Argentina (OAFA observatory of San Juan National University) was pointed 
to the  GRB180720C 21 sec after notice time and 102 sec after trigger time 
at 2018-07-20 22:25:39 UT.


We have a number of images on this observatories. The observations made 
very  close to Galactic center (~ 1 degree).  The upper limit on 
observaroties is about 18 mag.

The message may be cited.

A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), J.A.
Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), P.A.
Evans (U. Leicester), S.L. Gibson (U. Leicester), Z. Liu (NAOC / U.
Leicester) and S.J. LaPorte report on behalf of the Swift-XRT team:

We have analysed 7.2 ks of XRT data for GRB 180720C (LaPorte et al. GCN
Circ. 22982
Loading...
 
 ), from 105 s to 69.3 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. 22987
Loading...
 
 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.59 (+/-0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.1 (+0.6, -0.5). The
best-fitting absorption column is  2.8 (+1.2, -1.0) x 10^22 cm^-2, in
excess of the Galactic value of 6.8 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 6.7 x 10^-11 (1.7 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.8 (+1.2, -1.0) x 10^22 cm^-2
Galactic foreground: 6.8 x 10^21 cm^-2
Excess significance: 3.7 sigma
Photon index:	     2.1 (+0.6, -0.5)

If the light curve continues to decay with a power-law decay index of
0.59, the count rate at T+24 hours will be 6.3 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.2 x
10^-13 (1.1 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/00848932.

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

J. R. Cummings (CPI), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF/USRA), S. J. LaPorte (PSU),
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-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180720C (trigger #848932)
(LaPorte et al., GCN Circ. 22982
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 265.655, -26.634 deg which is
  RA(J2000)  =  17h 42m 37.1s
  Dec(J2000) = -26d 38' 02.5"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 98%.

The mask-weighted light curve shows a main single-pulse structure that
starts at ~T-20 s, peaks at ~T0, and ends at ~T+20 s. In addition, there are
some weak emission prior to this main pulse, starting at ~T-150 s.
T90 (15-350 keV) is 124.2 +- 32.7 sec (estimated error including systematics).

The time-averaged spectrum from T-150.52 to T+23.34 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.39 +- 0.13.  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-0.27 sec in the 15-150 keV band
is 1.1 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

The temporal and spectral characteristics of this burst are
consistent with those of a long GRB.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/848932/BA/

S. R. Oates (U. Warwick) and S. J. LaPorte (PSU) report on behalf
of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180720C
100 s after the BAT trigger (LaPorte et al., GCN Circ. 22982
Loading...
 
 ).
No optical afterglow consistent with the XRT position (Evans et al., GCN
Circ. 22987
Loading...
 
 ) 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)         100         250          147           >20.3
white              100         5490         541           >21.0    
u (FC)             312         425          111           >19.2
u                  312         11199       1193           >20.6
v                  4263        5901         393           >19.1
b                  3646        11902       1072           >20.8
uvw1               4674        10285       1133           >20.3
uvm2               4469        6106         393           >19.7
uvw2               4058        5696         393           >19.8

The magnitudes in the table are not corrected for the Galactic extinction
due to the significant reddening of E(B-V) = 2.11 in the direction of the burst
(Schlegel et al. 1998).

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

The 2-m robotic Liverpool Telescope automatically began observing Swift 
GRB 180720C (LaPorte et al. GCN 22982
Loading...
 
 ) on July 20, 22:28:18 UT (261 
seconds after the GRB trigger time) with the RINGO3 polarimeter and the 
IO:O camera in the SDSS-R filter. Within the enhanced Swift-XRT error 
circle (Evans et al. GCN 22987
Loading...
 
 ) we do not find any uncatalogued object 
down to r'>20.4 mag at a mid time of 47 minutes post GRB with a 3x10s 
exposure, as calibrated against nearby Pan-STARRS objects.

O.J. Roberts (USRA/NASA) and C. Meegan (UAH) report on behalf
of the Fermi GBM Team:

"At 22:23:53.15 UT on 20 July 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180720C (trigger 553818238 / 180720933),
which was also detected by Swift (S.J. LaPorte et al. 2018, GCN 22982
Loading...
 
 ).
The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time using 
the Swift-XRT position is 111 degrees.

The GBM light curve consists of broad, single peak with a 
duration (T90) of about 23 s (50-300 keV). The time-averaged spectrum 
from T0-6.1 to T0+17.4 s is adequately fit by a power law function 
with an exponential high-energy cutoff. The power law index is 
-1.12 +/- 0.08 and the cutoff energy, parameterized as Epeak, 
is 583 +/- 161 keV.

The event fluence (10-1000 keV) over this time interval is
(2.83 +/- 0.05)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.3 s in the 10-1000 keV band is 2.8 +/- 0.3 ph/s/cm^2.


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

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

Using 2824 s of XRT Photon Counting mode data and 7 UVOT
images for GRB 180720C, 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 = 265.63557, -26.62903 which is equivalent
to:

RA (J2000): 17h 42m 32.54s
Dec (J2000): -26d 37' 44.5"

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.

S. J. LaPorte (PSU), P. A. Evans (U Leicester), J.D. Gropp (PSU),
J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (George Washington University),
B. Sbarufatti (PSU) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 22:23:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB 180720C or Galactic transient (trigger=848932). 
Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 265.623, -26.584 which is 
   RA(J2000) = 17h 42m 30s
   Dec(J2000) = -26d 35' 01"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 30 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 22:25:34.8 UT, 97.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 265.63486, -26.63041 which is
equivalent to:
   RA(J2000)  = 17h 42m 32.37s
   Dec(J2000) = -26d 37' 49.5"
with an uncertainty of 5.7 arcseconds (radius, 90% containment). This
location is 171 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.  We cannot determine whether the source is
fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (6.76 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.7
(+3.01/-2.29) x 10^22 cm^-2 (90% confidence). 

We note that the detected source is at the edge of the reduced field of view available with the promptly down linked data. Because of that the accuracy of the position could be lower than stated. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 100 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 none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. Because of the density of
catalogued stars, further analysis is required to report an upper limit for any
afterglow in the region. No correction has been made for the large, but
uncertain, extinction expected. 

While the BAT and XRT data are consistent with a GRB, because of the
proximity to the Galactic Center, at present we cannot rule out a
Galactic transient nature for the source. 

Burst Advocate for this burst is S. J. LaPorte (extragsam AT gmail.com). 
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.)