GRB 180818B

E. Mazaeva (IKI), A. Pozanenko (IKI),  A. Kusakin (FAPHI), I. Reva 
(FAPHI),  A. Volnova (IKI), M. Krugov (FAPHI) report on behalf of larger 
GRB follow-up collaboration:

We observed the field of GRB 180818B  (Marshall et al., GCN 23149
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 ; Veres 
et al., GCN 23153
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 ) with Zeiss-1000 1-m telescope of Tien Shan 
Astronomical Observatory starting on August 08 (UT) 22:20:38. We 
obtained several images in R-filter.  Within enhanced XRT Swift-XRT 
position (Beardmore et al., GCN 23151
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 )  we do not detect any object.  In 
  particular we do not detect the afterglow candidate reported in  GCN 
23161
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  (Hu et al.) Preliminary photometry of the field is following.

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

2018-08-18  22:20:38 0.42710  R      36*60  n/d  n/d    20.4

The photometry is based on several nearby USNO-B1.0 stars.

Y.-D. Hu, A. Sota, V. Casanova and A.J. Castro-Tirado (IAA-CSIC 
Granada), on behalf of a larger collaboration, report:

We observed the field of the Swift/Fermi burst GRB 180818B (Marshall et 
al. GCNC 23149, Veres et al. GCNC 23153), with the 150-cm telescope of 
the Observatorio de Sierra Nevada (OSN) near Granada, Spain.

Two I-band observations were taken on Aug 19 and 21 (starting at 03:40 
and 04:19 UT respectively, i.e. ~15.2 h and ~63.8 h after trigger) with 
total exposure times of 2100s and 1200s respectively. Within the 
enhanced Swift/XRT position (Beardmore et al. GCNC 23151), an optical 
source was detected on the first epoch with an I-band magnitude of 21.6, 
which faded by at least 0.5 mag on the 2nd epoch observation. We propose 
this source to be the optical afterglow to GRB 180818B.

The above mentioned magnitude is calibrated against the USNO-B1 catalog 
and is not corrected for Galactic extinction in the direction of the 
GRB. Further observations are encouraged.

A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows
(PSU), A. Tohuvavohu (PSU), A.P. Beardmore (U. Leicester), P.A. Evans
(U. Leicester), S.L. Gibson (U. Leicester), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB) and F.E. Marshall report on behalf of the Swift-XRT
team:

We have analysed 8.1 ks of XRT data for GRB 180818B (Marshall et al.
GCN Circ. 23149
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 ), from 210 s to 52.8 ks after the  BAT trigger. The
data comprise 205 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 23151
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 ).

The late-time light curve (from T0+4.5 ks) can be modelled with a
power-law decay with a decay index of alpha=0.86 (+/-0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.47 (+0.12, -0.11). The
best-fitting absorption column is  2.89 (+0.25, -0.23) x 10^21 cm^-2,
in excess of the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.77 (+0.07, -0.06)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.8 x 10^-11 (4.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.27 (+0.11, -0.00) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.77 (+0.07, -0.06)

If the light curve continues to decay with a power-law decay index of
0.86, the count rate at T+24 hours will be 0.030 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.3 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/00853882.

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

J. R. Cummings (CPI), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), F. E. Marshall (NASA/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-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180818B (trigger #853882)
(Marshall et al., GCN Circ. 23149
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 ).  The BAT ground-calculated position is
RA, Dec = 104.227, 39.310 deg which is
  RA(J2000)  =  06h 56m 54.5s
  Dec(J2000) = +39d 18' 35.1"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 89%.

The mask-weighted light curve shows two pulses. The first pulse starts at
at ~T-100 s, peaks at ~T-80 s, and ends at ~T-50 s. The second pulse starts
at ~T+10 s, peaks at ~T+30 s, and ends at ~T+60 s. T90 (15-350 keV) is
134.4 +- 9.8 sec (estimated error including systematics). However, note that
the burst came into the BAT FOV at ~T-128 s, and thus there might be additional
burst emission beforehand.

The time-averaged spectrum from T-93.58 to T+62.74 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.77 +- 0.12.  The fluence in the 15-150 keV band is 2.3 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T-82.11 sec in the 15-150 keV band
is 1.2 +- 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/853882/BA/

P. Veres (UAH), C. Meegan (UAH) and A. von Kienlin (MPE)
report on behalf of the Fermi GBM Team:

"At 12:28:57.24 UT on 18 August 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180818B (trigger 556288142 / 180818520).
which was also detected by the Swift/BAT
(Marshall et al., GCN 23149
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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 89 degrees.

The GBM light curve consists of two separate emission episodes
with a duration (T90) of about 169 s (50-300 keV).
The time-averaged spectrum from T0-16 s to T0+125 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is 1.8 +/- 1.32 and
the cutoff energy, parameterized as Epeak, is 78 +/- 9 keV.

The event fluence (10-1000 keV) in this time interval is
(4.7 +/- 0.7)E-7 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-2.4 s in the 10-1000 keV band
is 2.1 +/- 0.2 ph/s/cm^2.

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

N. P. M. Kuin (UCL-MSSL) and F. E. Marshall (NASA/GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180818B
212 s after the BAT trigger (Marshall et al., GCN Circ. 23149
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 ).
No optical afterglow consistent with the XRT position
(Beardmore et al. GCN Circ. 23151
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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           212          362          147         >19.9
white              212        11082         1482         >21.1
v                  369        12690         1148         >19.8
b                  467         6919          471         >19.9
u                  442         6714          469         >19.6
w1                 418         6509          468         >19.9
m2                 393         6304          471         >20.5
w2                 517        11988         1160         >20.3

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.09 in the direction of the burst
(Schlegel et al. 1998).

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

Using 1716 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 180818B, 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 = 104.22470, +39.31571 which is equivalent
to:

RA (J2000): 06h 56m 53.93s
Dec (J2000): +39d 18' 56.6"

with an uncertainty of 1.7 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.

F. E. Marshall (NASA/GSFC), V. D'Elia (ASDC),
S. W. K Emery (UCL-MSSL), J. A. Kennea (PSU),
M. J. Moss (George Washington University), K. L. Page (U Leicester)
and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 12:30:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180818B (trigger=853882).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 104.235, +39.329 which is 
   RA(J2000) = 06h 56m 56s
   Dec(J2000) = +39d 19' 43"
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 130 sec, starting ~100 sec before
the trigger time during the previous spacecraft slew.  The peak count 
rate was ~1000 counts/sec (15-350 keV), at ~-90 sec after the trigger. 

The XRT began observing the field at 12:33:42.4 UT, 203.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 104.22468, 39.31662 which
is equivalent to:
   RA(J2000)  = 06h 56m 53.92s
   Dec(J2000) = +39d 18' 59.8"
with an uncertainty of 4.4 arcseconds (radius, 90% containment). This
location is 53 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

The initial flux in the 2.5 s image was 1.30e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 212 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
0.09. 

Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.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.)