GRB 180727A
GCN Circular 23043
Subject
GRB 180727A: Swift detection of a burst
Date
2018-07-27T14:58:03Z (7 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
A. P. Beardmore (U Leicester), S. B. Cenko (GSFC),
P. A. Evans (U Leicester), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU),
R. L. C. Starling (U Leicester) and A. Tohuvavohu (PSU) report on
behalf of the Neil Gehrels Swift Observatory Team:
At 14:15:28 UT, the Swift Burst Alert Telescope triggered
and located GRB180727A (Trigger #850231). Due to a telemetry
gap, very little BAT information is immediately available.
The BAT light curve showed a single-peaked structure with
a duration of about 1.5 sec. The peak count rate
was ~5500 counts/sec (15-350 keV), at ~0 sec after the trigger.
The burst does not seem to show significant emission above 100 keV.
The full BAT information will be available after the
next ground downlink.
Using promptly downlinked data we find an uncatalogued X-ray source
located at RA, Dec 346.66536, -63.05052 which is equivalent to:
RA(J2000) = 23h 06m 39.69s
Dec(J2000) = -63d 03' 01.9"
with an uncertainty of 3.7 arcseconds (radius, 90% containment).
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.10 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.5
(+3.98/-2.45) x 10^21 cm^-2 (90% confidence).
UVOT took a finding chart exposure of 150 seconds with the white
filter. 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 19.2 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 18.0 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.02.
We note this GRB was also detected by the GBM on Fermi, and their
automated light curve analysis indicates it is likely to be a short GRB.
Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk).
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.)
GCN Circular 23044
Subject
GRB 180727A: Fermi GBM observation
Date
2018-07-27T19:30:47Z (7 years ago)
From
Peter Veres at UAH <veresp@gmail.com>
P. Veres (UAH) reports on behalf of the Fermi GBM Team:
"At 14:15:28.81 UT on 27 July 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180727A (trigger 554393733 / 180727594)
which was also detected by Swift (Beardmore et al., GCN 23043).
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 consists of a single pulse
with a duration (T90) of about 1.1 s (50-300 keV).
The time-averaged spectrum from T0-0.19 s to T0+0.83 s is
best fit by a power law function with an exponential
high-energy cutoff. The power law index is 0.14 +/- 0.28 and
the cutoff energy, parameterized as Epeak, is 69 +/- 4 keV.
The event fluence (10-1000 keV) in this time interval is
(3.32 +/- 0.18)E-7 erg/cm^2. The 64-msec peak photon flux measured
starting from T0+0.45 s in the 10-1000 keV band
is 7.2 +/- 1.2 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."
GCN Circular 23045
Subject
GRB 180727A: Swift-XRT refined Analysis
Date
2018-07-28T14:58:03Z (7 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
V. D'Elia (ASDC), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J.
LaPorte (PSU), Z. Liu (NAOC / U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A.
Melandri (INAF-OAB) and A.P. Beardmore report on behalf of the
Swift-XRT team:
We have analysed 5.9 ks of XRT data for GRB 180727A (Beardmore et al.
GCN Circ. 23043), from 58 s to 80.1 ks after the BAT trigger. The data
comprise 24 s in Windowed Timing (WT) mode (the first 7 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 503 s of PC mode data and 2 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 = 346.66632, -63.05183
which is equivalent to:
RA (J2000): 23h 06m 39.92s
Dec(J2000): -63d 03' 06.6"
with an uncertainty of 1.7 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay
index of alpha=2.4 (+0.3, -0.4).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+/-0.4). The
best-fitting absorption column is 2.1 (+1.8, -1.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 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 3.9 x 10^-11 (5.0 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 2.1 (+1.8, -1.4) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 2.2 sigma
Photon index: 1.9 (+/-0.4)
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00850231.
This circular is an official product of the Swift-XRT team.
GCN Circular 23047
Subject
GRB 180727A: Swift-BAT refined analysis
Date
2018-07-28T17:49:10Z (7 years ago)
From
Hans Krimm at NASA-GSFC/NSF <hans.krimm@nasa.gov>
A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester),
J. R. Cummings (CPI), H. A. Krimm (NSF/USRA), C. B. Markwardt (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+200 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180727A (trigger #850231)
(Beardmore et al., GCN Circ. 23043). The BAT ground-calculated position is
RA, Dec = 346.639, -63.066 deg which is
RA(J2000) = 23h 06m 33.3s
Dec(J2000) = -63d 03' 57.9"
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 85%.
The mask-weighted light curve shows a single peak of duration ~1 second,
peaking at T+0 sec. T90 (15-350 keV) is 1.1 +- 0.2 sec (estimated error
including systematics).
The time-averaged spectrum from T-0.1 to T+1.3 sec is best fit by a power law
with an exponential cutoff. This fit gives a photon index 0.59 +- 0.51,
and Epeak of 67.6 +- 14.8 keV (chi squared 43.23 for 56 d.o.f.). For this
model the total fluence in the 15-150 keV band is 2.7 +- 0.2 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T-0.05 sec in the 15-150 keV band is
3.6 +- 0.2 ph/cm2/sec. A fit to a simple power law gives a photon index
of 1.60 +- 0.11 (chi squared 57.46 for 57 d.o.f.). All the quoted errors
are at the 90% confidence level.
The duration and hardness of this burst are consistent with the general
BAT short GRB distribution.
The results of the batgrbpoduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/850231/BA/
GCN Circular 23062
Subject
GRB 180727A: Swift/UVOT Upper Limits
Date
2018-07-30T18:03:24Z (7 years ago)
From
Frank Marshall at Swift/UVOT <marshall@milkyway.gsfc.nasa.gov>
F. E. Marshall (NASA/GSFC) and A. Beardmore (U. Leicester)
report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 180727A
77 s after the BAT trigger (Beardmore et al., GCN Circ. 23043).
No optical afterglow consistent with the enhanced XRT position
(D'Elia et al., GCN Circ. 23045)
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) exposures and subsequent exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white_FC 77 227 147 >20.9
u_FC 290 303 12 >18.4
white 4280 5915 393 >21.7
v 4690 6312 379 >19.6
b 4075 5710 393 >21.2
u 5305 5505 197 >20.3
uvw1 5101 5301 197 >20.0
uvm2 4895 5095 197 >21.0
uvw2 4485 6122 393 >21.4
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).
GCN Circular 23072
Subject
GRB 180727A: Swift/BAT spectral lag result
Date
2018-08-03T04:01:20Z (7 years ago)
From
Takanori Sakamoto at AGU <tsakamoto@phys.aoyama.ac.jp>
J. P. Norris (BSU), Y. Kawakubo (AGU),
T. Sakamoto (AGU), S. D. Barthelmy (GSFC),
S. B. Cenko (GSFC), A. Y. Lien (GSFC/UMBC)
We report the spectral lag analysis for GRB 180727A
(Beardmore et al., GCN Circ. 23043; Lien et al., GCN Circ. 23047)
based on the BAT data. The lag analysis finds a lag of 80 +- 27 ms
for the 50-100 keV to 15-25 keV band. Despite the short T90 duration
(1.1 +- 0.2 s), the lag value is consistent with that of long GRBs.