GRB 180809A

F. Yatabe, S. Nakahira, T. Mihara, Y. Takao, M. Matsuoka (RIKEN),
M. Serino, T. Sakamoto,  S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU),
H. Negoro, M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.),
N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech),
S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA),
Y. Ueda, A. Tanimoto, T. Morita, S. Yamada (Kyoto U.),
Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.),
H. Tsunemi, T. Yoneyama (Osaka U.),
M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.),
T. Kawamuro (NAOJ),
K. Yamaoka (Nagoya U.),
M. Shidatsu (Ehime U.)
report on behalf of the MAXI team:
The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient
source at  11:38:25 UT on 9 August 2018.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (297.793 deg, -34.663 deg) = (19 51 10, -34 39 46) (J2000)
with a 90% C.L. statistical error of 0.09 deg and an additional systematic
uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 890 +- 57 mCrab
(4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy,we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (297.202, -33.964) deg = (19 48 48, -33 57 49) (J2000)
(R.A., Dec) = (297.086, -34.046) deg = (19 48 20, -34 02 46) (J2000)
(R.A., Dec) = (298.358, -35.535) deg = (19 53 25, -35 32 04) (J2000)
(R.A., Dec) = (298.475, -35.451) deg = (19 53 54, -35 27 03) (J2000)
There was no significant excess flux in the previous transit at UT 10:05
with an upper limit of 20 mCrab.

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a series of observations, tiled on the sky, of the
MAXI GRB 180809A. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00072

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the MAXI event is high: any X-ray source
considered to be a probable afterglow candidate will be reported via a GCN Circular
after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

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

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 180809A (Yatabe et al. GCN Circ. 23103
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 ) in a series of
observations tiled on the sky. The total exposure time is 777 s,
distributed over 6 tiles; the maximum exposure at a single sky location
was 898 s. The data were collected between T0+13.7 ks and T0+22.0 ks,
and are entirely in Photon Counting (PC) mode. 

An uncatalogued X-ray source is detected and is above the RASS limit,
and is therefore likely the GRB afterglow. Using 731 s of PC mode data
and 3 UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 297.90271, -34.78546 which is equivalent to:

RA (J2000): 19h 51m 36.65s
Dec(J2000): -34d 47' 07.6"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 9.1 arcmin from the MAXI position.	The light curve is
currently consistent with a constant source of mean count rate 2.2e-01
ct/sec. A power-law fit gives an index of 0.3 (+/-0.9). More data are
necessary to determine potential fading.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.4 (+0.7, -0.6). The
best-fitting absorption column is  8.4 (+5.4, -3.8) x 10^21 cm^-2, in
excess of the Galactic value of 1.2 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 3.2 x 10^-11 (8.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.4 (+5.4, -3.8) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 3.1 sigma
Photon index:	     2.4 (+0.7, -0.6)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020814.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00072.

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

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

We observed the field of GRB 180809A (Yatabe et al. GCN 23103
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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 2018-08-09 12:51:14 UT (73 min after the trigger) for 300
sec. At the position of the proposed X-ray afterglow (D'Elia et al.
GCN 23106
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 ), we do not detect any optical emission with a detection
limit of about 20 mag by comparing with the Gaia DR2 catalogue.

S. Ricciarini (U of Florence), A. Yoshida, T. Sakamoto, V. Pal'shin,
S. Sugita, Y. Kawakubo, A. Tezuka, S. Matsukawa, H. Onozawa,
T. Ito, H. Morita, Y. Sone (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (RIKEN), I. Takahashi (IPMU),
Y. Asaoka, S. Ozawa, S. Torii (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U),
W. Ishizaki (ICRR), M. L. Cherry (LSU),
A. V. Penacchioni, P. S. Marrocchesi (U of Siena)
and the CALET collaboration:

The GRB 180809A (MAXI/GSC detection: Yatabe et al., GCN Circ. 23103
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 ;
Fermi-GBM trigger #555507497) was detected in ground analysis
of the CALET Gamma-ray Burst Monitor (CGBM) data near the MAXI trigger
time at T0=11:38:25 UT on 9 August 2018.
The burst signal was seen by all CGBM detectors.

The burst light curve shows a weak pulse which starts
at T-15.1 sec and ends at T-4.2 sec.
The T90 and the T50 durations measured by the SGM data are
10.0 +- 1.6 sec and 5.0 +- 1.6 sec (40-1000 keV), respectively.

The ground processed light curve is available at

http://cgbm.calet.jp/cgbm_trigger/ground/1217849910/

The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at the Waseda University.

E. Bissaldi (Politecnico & INFN Bari)
reports on behalf of the Fermi GBM Team:


"At 11:38:12.08 UT on 9 August 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180809A (trigger 555507497 / 180809485),
which was also detected by MAXI/GSC (Yatabe et al. 2018, GCN 23103
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 ) and
Swift/XRT (D'Elia et al. 2018, GCN 23106
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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 60 degrees.

The GBM light curve consists of multiple emission episodes
with a duration (T90) of about 220 s (50-300 keV).
The time-averaged spectrum of the main emission episode
from T0-5 s to T0+15 s is adequately fit by a power law
function with an exponential high-energy cutoff.
The power law index is -1.37 +/- 0.06 and the cutoff energy,
parameterized as Epeak, is 620 +/- 230 keV.

The event fluence (10-1000 keV) in this time interval is
(6.8 +/- 0.4)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+3.2 s in the 10-1000 keV band is 4.0 +/- 0.3 ph/s/cm^2.


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

V. D'Elia (SSDC) reports on behalf of the Swift/XRT team:

Swift performed a second visit to the field of the  MAXI-detected burst GRB 180809A (Yatabe et al. GCN Circ. 23103
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 ). The XRT data comprise 3700 s of exposure time, acquired ~5 days after the GRB trigger and entirely in PC mode. 

No excess is found at the position of the candidate afterglow reported by D'Elia et al. (GCN Circ. 23106
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 ), down to a three sigma upper limit of ~4E-3 cts/s, corresponding to a 0.3-10 keV observed flux of ~1E-13 erg cm^-2 s^-1 (assuming a typical GRB spectrum). We thus conclude that the source reported by D'Elia et al. (GCN Circ. 23106
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 ) was indeed the X-ray counterpart of the MAXI GRB 180809A.

This circular is an official product of the Swift/XRT team