GRB 180812A

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA),
V. D'Elia (ASDC), C. Gronwall (PSU), K. L. Page (U Leicester) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 08:22:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180812A (trigger=852903).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 245.886, +74.670, which is 
   RA(J2000)  =  16h 23m 33s
   Dec(J2000) = +74d 40' 11"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several peaks
with a total duration of about 25 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 08:24:17.6 UT, 107.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 245.8360, 74.6647 which
is equivalent to:
   RA(J2000)  = 16h 23m 20.64s
   Dec(J2000) = +74d 39' 53.0"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 51 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 (4.37 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.8
(+4.29/-3.49) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 112 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 A. Y. Lien (amy.y.lien AT 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.)

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 22s 
(and 52s after the GRB onset) to the
Swift trigger of GRB 180812A (Lien et al., GCNC 23129). The first image 
(10s exposure, r-band) was obtained at 08:23:25 UT. At the position of 
the Swift X-ray afterglow, a variable optical source with r about 17 mag 
is detected, which is fading in brightness in the late images. Therefore 
we propose this source to be the optical afterglow to GRB 180812A. 
Spectroscopic observations are encouraged.

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

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

Using 2046 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 180812A, 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 = 245.83465, +74.66472 which is equivalent
to:

RA (J2000): 16h 23m 20.32s
Dec (J2000): +74d 39' 53.0"

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.

Alan M. Watson (UNAM), Eleonora Troja (GSFC), Nat Butler (ASU), Rosa L.
Becerra (UNAM), William H. Lee (UNAM), Carlos Rom��n-Z����iga (UNAM), and
Alexander Kutyrev (GSFC) report:

We observed the field of GRB 180812A (Lien et al., GCN 23129
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 ) with the
COATLI 50-cm telescope and interim imager at the Observatorio
Astron��mico Nacional on the Sierra de San Pedro M��rtir
(http://coatli.astroscu.unam.mx) from 2018-08-12 08:13.5 to 11:53:16.7
(from 11.6 seconds after the alert or 43.1 seconds after the trigger to
3.5 hours after the trigger), obtaining a total of 2.46 hours of
exposure in the w filter.

We detect the optical counterpart reported by Carrasco et al. (GCN Circ.
23130
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 ). We see it rise from w = 17.4 to w = 17.2 at 160 seconds after
burst, and then fade rapidly.

We note that simultaneous observations with UVOT in the white filter do
not detect anything to a limit of about 19.8 (Lien et al., GCN 23129
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 ).
Our w filter is much redder than the UVOT white filter, which suggests
that this might be an intrinsically red afterglow or be attenuated by
intergalactic absorption in the blue.

These magnitudes are calibrated against the USNO-B1 catalog (adjusted to
an approximate AB system) and are not corrected for Galactic extinction
in the direction of the GRB.

We thank the COATLI technical team (Fernando ��ngeles, Oscar Chapa,
Salvador Cuevas, Alejandro Farah, Jorge Fuentes, Rosal��a Langarica,
Fernando Quir��s, and Carlos Tejada) and the staff of the Observatorio
Astron��mico Nacional.

[GCN OPS NOTE(12aug18): The GRB name in the Subject-line and the first paragraph
has been changed from "180612A" to "180812A".]

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

We have analysed 8.2 ks of XRT data for GRB 180812A (Lien et al. GCN
Circ. 23129
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 ), from 114 s to 24.0 ks after the  BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Goad et al. (GCN Circ. 23131
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.08 (+/-0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.74 (+0.16, -0.14). The
best-fitting absorption column is  6.8 (+4.3, -2.4) x 10^20 cm^-2,
consistent with the Galactic value of 4.4 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 3.8 x 10^-11 (4.2 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.8 (+4.3, -2.4) x 10^20 cm^-2
Galactic foreground: 4.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.74 (+0.16, -0.14)

If the light curve continues to decay with a power-law decay index of
1.08, the count rate at T+24 hours will be 2.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-13 (1.1 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/00852903.

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

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


"At 08:22:30.31 UT on 12 August 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180812A (trigger 555754955 / 180812349),
which was also detected by the Swift BAT and XRT instruments (Lien et al.
2008, GCN 23129
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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 46
degrees.

The GBM light curve shows several peaks with a duration (T90) of about 38 s
(50-300 keV). The time-averaged spectrum from T0-2.0 s to T0+34.8 s is
adequately fit by a power law function with an exponential high-energy
cutoff.
The power law index is -0.56 +/- 0.36 and the cutoff energy, parameterized
as Epeak, is 110 +/- 25 keV.

The event fluence (10-1000 keV) in this time interval is
(1.2 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+15.7 s in the 10-1000 keV band
is 2.5 +/- 0.2 ph/s/cm^2.

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

T. N. Ukwatta (LANL), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU) (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 180812A (trigger #852903)
(Lien et al., GCN Circ. 23129
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 ).  The BAT ground-calculated position is
RA, Dec = 245.839, 74.674 deg which is
  RA(J2000)  =  16h 23m 21.3s
  Dec(J2000) = +74d 40' 27.0"
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 26%.

The mask-weighted light curve shows a multi-peaked structure that starts
at ~T0 and ends at ~T+17 s. T90 (15-350 keV) is 16.51 +- 0.92 sec (estimated
error including systematics).

The time-averaged spectrum from T-0.30 to T+16.63 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.22 +- 0.27.  The fluence in the 15-150 keV band is 8.0 +- 1.4 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+15.62 sec in the 15-150 keV band
is 1.9 +- 0.4 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/852903/BA/

GRB 180812A: Swift/UVOT Detection

S. J. LaPorte (PSU) and A. Y. Lien (GSFC/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180812A
112 s after the BAT trigger (Lien et al., GCN Circ. 23129
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 ).
A fading source consistent with the XRT position
(Goad et al. GCN Circ. 23131
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 )
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  16:23:20.45 =  245.83522 (deg.)
    Dec (J2000) = 74:39:52.9 =  74.66471 (deg.)
with an estimated uncertainty of 0.50 arc sec. (radius, 90% confidence),
consistent with detections reported by BOOTES-5/JGT (Carrasco et al. GCN
Circ. 23130
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 ) and COATLI (Watson et al. GCN Circ. 23132
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 ).

Preliminary detections and 3-sigma upper limits using the UVOT photometric
system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white              140          289          431         19.81 +/- 0.17
white             1551         2092           77        >20.85
v                  600         6610          372        >18.9
b                  526         5994          352        >20.0
u                  270         5789          598        >19.8
w1                 650         6857          211        >19.5
m2                6616         6815          197        >20.2
w2                6206         6406          197        >20.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).