GRB 180821A

Paul Vreeswijk (Radboud U. Nijmegen), Daniele Malesani, Jonatan
Selsing (DAWN/NBI, DARK/NBI), Nial Tanvir (U. Leicester), Giovanna
Pugliese (API/U. Amsterdam), Andrea Rossi (INAF-OAS) and Andrew Levan
(U. Warwick) report on behalf of the Stargate collaboration:

The Swift XRT error circle (Tohuvavohu et al., GCN 23158
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 ; Burrows et 
al., GCN 23164
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 ) of GRB 180821A (Krimm et al., GCN 23166
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 ) was observed
with the Very Large Telescope on the night of August 21/22, 2018,
using the telescope/instrument combinations Kueyen/X-shooter and
Antu/FORS2.

The FORS2 imaging observations started at 5:58 UT on August 22 (13.3
hours after the GRB trigger) and consisted of 8 exposures of 120
seconds in the z_Gunn filter. No object is detected in the enhanced
XRT error circle (Evans et al., GCN 23162
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 ), down to a 3-sigma limiting
magnitude of z(AB)=24.9 (see also Fernandez-Garcia, GCN 23159
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 ;
Guidorzi et al., GCN 23163
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 ; Kong, GCNs 23160
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  and 23165
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 ; Marshall et
al., GCN 23167
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 ). The z-band coverage of this field by the SkyMapper
Southern Sky Survey (see http://skymapper.anu.edu.au) was used in the
determination of the magnitude limit.

The X-shooter spectroscopic observations started at 6:55 UT on August
22 (14.3 hours after the GRB). A sequence of 4 spectra of 10 minutes
each was taken of the object at the Southwestern edge of the
preliminary XRT error circle, at coordinates R.A.: 00:31:09.0, decl.:
-38:37:39.5 (J2000). The reduced spectra show emission lines typical
of a star-forming galaxy, such as [OII] 3727, Hbeta, [OIII] 4959, 5007
and Halpha, at a redshift of z=0.58. Although the location of this
galaxy is consistent with the initial XRT afterglow localization, it
is about 6 arcseconds to the West and well outside of the enhanced XRT
error circle, and is therefore unrelated to the GRB.

We acknowledge the excellent assistance of the staff at Paranal
observatory, and in particular that of S. Brillant, C. Ledoux,
F. Gaete, E. Sedaghati and J. Velasquez.

F. E. Marshall (NASA/GSFC) and A. Tohuvavohu (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180821A
219 s after the BAT trigger (Tohuvavohu et al., GCN Circ. 23158
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 ).
No optical afterglow consistent with the enhanced XRT position
(Evans et al. GCN Circ. 23162
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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           219          369          147         >21.0
white              219          495          167         >21.1
v                  526          546           19         >17.7
v                 4604         6039          393         >20.0
b                  451          471           19         >19.3
b                 3988         5624          393         >21.3
u                  426          446           19         >18.6
u                 3783         5418          393         >20.5
w1                 402          421           19         >19.3
w1                5015        15906          241         >20.5
m2                4808        10398          812         >20.5
w2                4399         6034          393         >20.3

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).

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), M. Stamatikos (OSU), A. Tohuvavohu (PSU), 
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 180821A (trigger #854578)
(Tohuvavohu, et al., GCN Circ. 23158
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 ).  The BAT ground-calculated position is
RA, Dec = 7.739, -38.612 deg which is 
 RA(J2000)  =  00h 30m 57.4s 
 Dec(J2000) = -38d 36' 41.9" 
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 17%.

The mask-weighted light curve showed a broad peak with some structure, 
running from ~T+0 to T+70 sec. T90 (15-350 keV) is 64.00 +- 22.6 sec 
(estimated error including systematics).

The time-averaged spectrum from T+1.00 to T+65.00 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.79 +- 0.31.  The fluence in the 15-150 keV band is 1.4 +- 0.3 x 10^-6 erg/cm2.
Since the peak was so broad, the automated analysis software was unable to 
derive a significant peak photon flux at the highest point in the light curve, T+56.5 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/854578/BA/

A. K. H. Kong (NTHU, Taiwan) reports

We reanalyse the image obtained with the T30 telescope of
iTelescope.Net (Kong GCN 23160
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 ) together with the enhanced Swift XRT
position (Evans et al. GCN 23162
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 ). Within the new error circle, we do
not detect any source with a detection limit of ~20.5 mag. The result
is consistent with Guidorzi et al. (GCN 23163
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 ). We conclude that the
previous counterpart candidate (which is outside the enhanced X-ray
error circle) is not the optical afterglow.

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

We have analysed 8.8 ks of XRT data for GRB 180821A (Tohuvavohu et al.
GCN Circ. 23158
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 ), from 199 s to 46.5 ks after the  BAT trigger. The
data comprise 171 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 Evans
et al. (GCN Circ. 23162
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 ).

The late-time light curve (from T0+3.8 ks) can be modelled with a
power-law decay with a decay index of alpha=0.49 (+0.13, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 5.1 (+0.4, -0.3). The
best-fitting absorption column is  6.8 (+/-0.8) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.80 (+0.31, -0.29)
and a best-fitting absorption column of 3.4 (+1.0, -0.8) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 2.6 x 10^-11 (6.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.4 (+1.0, -0.8) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 6.2 sigma
Photon index:	     2.80 (+0.31, -0.29)

If the light curve continues to decay with a power-law decay index of
0.49, the count rate at T+24 hours will be 0.014 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x
10^-13 (9.4 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/00854578.

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

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:

We automatically observed Swift GRB 180821A (Tohuvavohu et al. GCN 
23158
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 ) with the LCO 2-m unit at Siding Springs on August 21, from 16:54 
to 17:07 UT (corresponding to 14 to 27 minutes from the GRB trigger 
time) with the SDSS i' filter. Within the enhanced Swift-XRT error 
circle (Evans et al. GCN 23162
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 ) we do not detect any source down to 
i'>21.0 mag from a 10x60s exposure at a mid time of 0.33 hours post GRB, 
as calibrated against nearby USNOB1 objects.

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

Using 2528 s of XRT Photon Counting mode data and 6 UVOT
images for GRB 180821A, 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 = 7.79011, -38.62713 which is equivalent
to:

RA (J2000): 00h 31m 9.63s
Dec (J2000): -38d 37' 37.7"

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.

A.K.H. Kong (NTHU, Taiwan) reports

We observed the field of GRB 180821A (Tohuvavohu et al. GCN 23158
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 )
with the T30 0.51m telescope of iTelescope.Net in Siding Spring,
Australia. The observation was done with a luminance filter (400-700
nm) beginning at 2018-08-21 17:26:22 UT (46 min after the BAT
trigger). At the position of the Swift X-ray afterglow, an optical
source is marginally detected with a magnitude of 20.4 +/- 0.2. We
propose this source is the optical afterglow to GRB 180821A. Follow-up
observations are encouraged.

E. Fernandez-Garcia, Y.-D. Hu, J. C. Tello, A. Ayala and A. J. 
Castro-Tirado (IAA-CSIC), I. Carrasco and C. Perez del Pulgar (Univ. de 
Malaga), M. D.
Caballero-Garcia and M. Jelinek (ASU-CAS, CZ), R. Cunniffe (Inst. of 
Physics, CZ) and R. Querel (NIWA), on behalf of a larger collaboration, 
report:

The 60cm BOOTES-3/YA robotic telescope at NIWA Lauder in Otago (New 
Zealand) automatically responded in 59s (and 193s after the GRB onset) 
to the Swift trigger of GRB 180821A (Tohuvavohu et al., GCNC 23158). The 
first images (clear filter) were obtained starting at 16:43:47 UT. At 
the position of the Swift/XR X-ray position, no optical afterglow is 
detected down to 19 mag on a 100s co-added image at 16:44:45 
(mid-exposure), in agreement with the non-detection by Swift/UVOT. 
Further analysis is ongoing.

We thank the staff at NIWA for its excellent support.

A. Tohuvavohu (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF/USRA), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU),
M. H. Siegel (PSU) and K. K. Simpson (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 16:40:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180821A (trigger=854578).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 7.763, -38.619 which is 
   RA(J2000) = 00h 31m 03s
   Dec(J2000) = -38d 37' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, the
immediately-available BAT lightcurve shows no obvious variation. 

The XRT began observing the field at 16:44:04.8 UT, 210.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 7.78833, -38.62660
which is equivalent to:
   RA(J2000)  = 00h 31m 09.20s
   Dec(J2000) = -38d 37' 35.8"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 76 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. 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 3.67e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 218 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
0.02. 

Burst Advocate for this burst is A. Tohuvavohu (aaronb AT swift.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.)