GRB 180924A

M. H. Siegel (PSU) and T. N. Ukwatta (LANL)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180924A
83 s after the BAT trigger (Ukwatta et al., GCN Circ. 23261
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 ).
No optical afterglow consistent with the XRT position
(Evans et al. GCN Circ. 23262
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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            83          233          147         >20.8
u_FC               241          491          246         >19.7
white               83          542          167         >20.9
v                  572          592           19         >17.1
b                  497          517           19         >18.4
u                  241          491          246         >19.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.03 in the direction of the burst
(Schlegel et al. 1998).

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

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

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)

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-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 Swift  GRB 180924A (Ukwatta  et al. GCN 23261
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 , Evans et al. GCN 23262
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 , Stamatikos et al. GCN 23264
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 , Melandri et al. GCN 23266
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 )
57128 sec after notice time and 57128 sec after trigger time at 2018-09-25 07:14:30 UT.

There is no new astrophysical sources detected brighter then 19.3m (unfiltered)
inside Swift XRT error-box (03h 16m 47.87s -58d 31' 57.3", r=1.7arcsec Evans et al. GCN 23262
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 )

The message may be cited.

====================================================================

The galacitc latitude b =  -49.7 deg.
The observations made on zenit distance = 27 deg.
The moon (99.8 % bright part) was 40 deg. above the horizon. The distance 
between  moon and  object is 66
The sun  altitude  was -35.5 deg.

A. Melandri (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti
(INAF-OAB/PSU), D.N. Burrows (PSU), Z. Liu (NAOC / U. Leicester), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC) and T.N. Ukwatta report on behalf of the
Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 180924A (Ukwatta et al. GCN
Circ. 23261
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 ), from 82 s to 52.3 ks after the  BAT trigger. The data
comprise 141 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 23262
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.23 (+0.11, -0.09), followed by a break at T+485 s to
an alpha of 0.70 (+0.06, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.96 (+0.14, -0.13). The
best-fitting absorption column is  1.0 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.0 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.88 (+0.20, -0.19)
and a best-fitting absorption column of 6.9 (+5.2, -4.5) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.9 (+5.2, -4.5) x 10^20 cm^-2
Galactic foreground: 2.0 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     1.88 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
0.70, the count rate at T+24 hours will be 0.017 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.8 x
10^-13 (6.6 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/00863186.

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

M. Stamatikos (OSU), 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), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+304 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180924A (trigger #863186)
(Ukwata, et al., GCN Circ. 23261
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 ).  The BAT ground-calculated position is
RA, Dec = 49.207, -58.524 deg, which is 
   RA(J2000)  =  03h 16m 49.7s 
   Dec(J2000) = -58d 31' 27.6" 
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
 
The mask-weighted light curve shows a couple peaks starting at ~T-90 sec,
peaking at ~T=50 and ~T0 sec, and returning to baseline ~T+30 sec.
T90 (15-350 keV) is 95.1 +- 10.9 sec (estimated error including systematics).
 
The time-averaged spectrum from T-85.15 to T+26.35 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.94 +- 0.14.  The fluence in the 15-150 keV band is 1.4 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+2.73 sec in the 15-150 keV band
is 0.6 +- 0.1 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/863186/BA/

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

Using 1363 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 180924A, 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 = 49.19945, -58.53258 which is equivalent
to:

RA (J2000): 03h 16m 47.87s
Dec (J2000): -58d 31' 57.3"

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.

T. N. Ukwatta (LANL), A. P. Beardmore (U Leicester),
A. A. Breeveld (UCL-MSSL), P. A. Evans (U Leicester),
C. Gronwall (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), M. H. Siegel (PSU)
and R. L. C. Starling (U Leicester) report on behalf of the Neil
Gehrels Swift Observatory Team:

At 15:22:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180924A (trigger=863186).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 49.161, -58.505 which is 
   RA(J2000) = 03h 16m 39s
   Dec(J2000) = -58d 30' 19"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 130 sec.  The peak count rate
was ~1492 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 15:23:39.1 UT, 76.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
49.2002, -58.5321 which is equivalent to:
   RA(J2000)  = 03h 16m 48.05s
   Dec(J2000) = -58d 31' 55.6"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 122 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.03
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.35e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 82 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
0.03. 

This source lies within the current (Sector 2) field-of-view of TESS camera 3. 

Burst Advocate for this burst is T. N. Ukwatta (tilan.ukwatta 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.)