GRB 180828A
GCN Circular 23182
Subject
GRB 180828A: Swift detection of a GRB or possible Galactic Transient Swift J1754.9-2548
Date
2018-08-28T19:27:00Z (7 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
A. P. Beardmore (U Leicester), S. W. K Emery (UCL-MSSL),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), S. J. LaPorte (PSU),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:
At 18:57:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source which is either GRB 180828A or a previously-unknown
Galactic transient (trigger=856977).
Swift slewed immediately to the source location.
The BAT on-board calculated location is
RA, Dec 268.700, -25.800 which is
RA(J2000) = 17h 54m 48s
Dec(J2000) = -25d 47' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a complex
structure with a duration of about 15 sec. The peak count rate
was ~20,000 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 18:59:30.3 UT, 115.6 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 268.7184, -25.7991 which is equivalent to:
RA(J2000) = 17h 54m 52.42s
Dec(J2000) = -25d 47' 56.8"
with an uncertainty of 5.8 arcseconds (radius, 90% containment). No
event data are yet available to determine the column density using
X-ray spectroscopy.
UVOT took a finding chart exposure of 30 seconds with the White filter
starting 123 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.
Results from the list of sources generated on-board are not available at this
time. No correction has been made for the large, but uncertain, extinction
expected.
Due to its location in the Galactic Bulge (3.8 degrees from Sgr A*,
0.14 degrees below the plane), this trigger may be due to a previously
unknown X-ray transient. If this source is an X-ray transient, we
name it Swift J1754.9-2548. Although the initial GCN Notice
identified this as possibly related to Swift J1753.7-2544, the XRT
position of this transient is 16.5 arcmin away from Swift
J1753.7-2544, and therefore it is unrelated.
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 23183
Subject
GRB 180828A: Fermi GBM observation
Date
2018-08-29T04:19:38Z (7 years ago)
From
Oliver J Roberts at USRA/NASA <oliver.roberts@nasa.gov>
O.J. Roberts (USRA/NASA) and C. Meegan (UAH) report on behalf
of the Fermi GBM Team:
"At 18:57:26.58 UT on 28 August 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180828A (trigger 557175451 / 180828790),
which was also detected by Swift (Beardmore et al. 2018, GCN 23182).
The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight at the GBM trigger time using the
Swift-XRT position is 77 degrees.
The GBM light curve consists of multiple bright pulses with a duration
(T90) of 8.3 s (50-300 keV). The time-averaged spectrum from T0+1.5s to
T0+11 s is best fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.46 +/- 0.02 and the
cutoff energy, parameterized as Epeak, is 358 +/- 9 keV.
A Band function fits the interval equally well, with
Epeak =346 +/- 12 keV, alpha = -0.44 +/- 0.03 and
beta = -3.01 +/- 0.33.
The event fluence (10-1000 keV) over the T90 interval is
(3.39 +/- 0.5)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0 +8s in the 10-1000 keV band is 38 +/- 1 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."
GCN Circular 23184
Subject
GRB 180828A / Swift J1754.9-2548: Enhanced Swift-XRT position
Date
2018-08-29T07:03:44Z (7 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
P.A. Evans, M.R. Goad and A.P. Beardmore (U. Leicester) report
on behalf of the Swift-XRT team:
Using 973 s of XRT Photon Counting mode data and 1 UVOT
images, 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 = 268.71784, -25.79805 which is equivalent to:
RA (J2000): 17 54 52.28
Dec (J2000): -25 47 53.0
with an uncertainty of 2.0 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed athttp://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).
Preliminary analysis of the XRT light curve (http://www.swift.ac.uk/xrt_curves/00856977/)
suggests that this object is likely a GRB, not a Galactic transient. Analysis is ongoing.
This circular is an official product of the Swift-XRT team.
GCN Circular 23185
Subject
GRB 180828A: Swift-XRT refined Analysis
Date
2018-08-29T07:33:58Z (7 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
S. J. LaPorte (PSU), Z. Liu (NAOC / U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), V. D'Elia (ASDC), A. D'Ai
(INAF-IASFPA), A. Melandri (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti
(INAF-OAB/PSU) and A.P. Beardmore report on behalf of the Swift-XRT
team:
We have analysed 6.6 ks of XRT data for GRB 180828A (Beardmore et al.
GCN Circ. 23182), from 121 s to 28.1 ks after the BAT trigger. The
data comprise 1.8 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 2040 s of PC mode data and 4 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 =
268.71817, -25.79815 which is equivalent to:
RA (J2000): 17h 54m 52.36s
Dec(J2000): -25d 47' 53.3"
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=1.22 (+/-0.05).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.1 (+1.3, -1.1). The
best-fitting absorption column is 2.5 (+1.3, -1.0) x 10^23 cm^-2, in
excess of the Galactic value of 1.4 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 1.2 x 10^-10 (7.0 x 10^-10) erg
cm^-2 count^-1.
A summary of the WT-mode spectrum is thus:
Total column: 2.5 (+1.3, -1.0) x 10^23 cm^-2
Galactic foreground: 1.4 x 10^22 cm^-2
Excess significance: 4.0 sigma
Photon index: 2.1 (+1.3, -1.1)
If the light curve continues to decay with a power-law decay index of
1.22, the count rate at T+24 hours will be 2.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-13 (1.8 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00856977.
This circular is an official product of the Swift-XRT team.
GCN Circular 23188
Subject
GRB 180828A: Swift-BAT refined analysis
Date
2018-08-29T16:00:35Z (7 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), J. R. Cummings (CPI),
H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC),
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+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180828A (trigger #856977)
(Beardmore et al., GCN Circ. 23182). The BAT ground-calculated position is
RA, Dec = 268.704, -25.791 deg which is
RA(J2000) = 17h 54m 49.1s
Dec(J2000) = -25d 47' 28.3"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 19%.
The mask-weighted light curve shows a few overlapping pulses that starts at
~ T-7 s and ends at ~T+2 s, with some weak tail emission that lasts till
~ T+30 s. T90 (15-350 keV) is 14.0 +- 8.0 sec (estimated error including systematics).
The time-averaged spectrum from T-7.29 to T+28.71 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
0.89 +- 0.06. The fluence in the 15-150 keV band is 8.8 +- 0.3 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T-0.29 sec in the 15-150 keV band
is 17.8 +- 0.9 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
The BAT temporal and spectral characteristics are consistent with
those of a GRB.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/856977/BA/
GCN Circular 23189
Subject
GRB 180828A: RATIR Optical and NIR Observations
Date
2018-08-29T16:14:03Z (7 years ago)
From
Nat Butler at Az State U <natbutler@asu.edu>
Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier
Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI), Eleonora
Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), Jes��s
Gonz��lez (UNAM), Carlos Rom��n-Z����iga (UNAM), Harvey Moseley (GSFC), John
Capone (UMD), V. Zach Golkhou (U. Wash.), and Vicki Toy (UMD) report:
We observed the field of GRB 180828A (Beardmore, et al., GCN 23182) with
the Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on
the 1.5m Harold Johnson Telescope at the Observatorio Astron��mico Nacional
on Sierra San Pedro M��rtir from 2018/08 29.14 to 2018/08 29.28 UTC (8.29 to
11.70 hours after the BAT trigger), obtaining a total of 1.73 hours
exposure in the r and i bands and 0.79 hours exposure in the Z, Y, J, and H
bands.
We find one uncatalogued source within the Swift-XRT error circle
(Beardmore at at., GCN 23182) at RA, Dec = 17:54:52.5 -25:47:56.3 (J2000,
+/-0.5"). This source is outside the enhanced XRT error region (Evans et
al., GCN 23184), within which we find no new sources. In comparison with
the 2MASS catalog, we obtain the following detection and 3-sigma upper
limit:
J > 21.4
H = 20.8 +/- 0.3
These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB. We note that this field has high
extinction, with E[B-V]~16.
We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
M��rtir.
GCN Circular 23190
Subject
GRB 180828A: Swift/UVOT Upper Limits
Date
2018-08-29T17:21:47Z (7 years ago)
From
Sam Emery at MSSL-UCL <samuel.emery.15@ucl.ac.uk>
S.W.K. Emery (UCL-MSSL) and A. P. Beardmore (U. Leicester)
report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 180828A
124 s after the BAT trigger (Beardmore et al., GCN Circ. 23182).
No optical afterglow consistent with the XRT position
(LaPorte et al., GCN Circ. 23185) or the position of the source
detected outside the enhanced XRT error region by RATIR
(Butler et al., GCN Circ. 23189)
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 initial exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white 124 23102 1057 >21.8
v 16882 39624 1367 >20.3
b 5752 28839 1439 >21.7
u 5547 34577 1709 >21.1
uvw1 5342 33925 1082 >20.7
uvm2 5137 40314 1756 >21.0
uvw2 11215 38709 2087 >21.2
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 23.65 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 23191
Subject
GRB 180828A: AstroSat CZTI detection
Date
2018-08-29T21:32:04Z (7 years ago)
From
Vidushi Sharma at IUCAA <vidushi@iucaa.in>
T. Khanam, V. Sharma and D. Bhattacharya (IUCAA), V. Bhalerao (IIT-B), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
Analysis of AstroSat CZTI data showed the detection of a long GRB 180828A, which was also detected by Swift (Beardmore A. P. et al., GCN 23182) and Fermi-GBM (Roberts O. J. et al., GCN 23183).
The source was clearly detected in the 40-200 keV energy range. The light curve shows multiple peaks of emission with strongest peak at 18:57:35.500 UT. The measured peak count rate is 947 cts/s above the background in combined data of four quadrants, with a total of 3607 cts. The local mean background count rate was 557 cts/s. Using cumulative rates, we measure a T90 of 8.9 s.
It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range.
CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.
GCN Circular 23193
Subject
Konus-Wind observation of GRB 180828A
Date
2018-08-30T20:51:39Z (7 years ago)
From
Anna Kozlova at Ioffe Institute <ann_kozlova@mail.ioffe.ru>
A. Kozlova, S. Golenetskii, R. Aptekar, D. Frederiks,
M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline
on behalf of the Konus-Wind team, report:
The long-duration GRB 180828A (Swift-BAT detection:
Beardmore et al., GCN 23182; Markwardt et al., GCN 23188;
Fermi-GBM observation: Roberts & Meegan, GCN 23183;
AstroSat-CZTI detection: Khanam et al., GCN 23191)
triggered Konus-Wind at T0=68250.016 s UT (18:57:30.016).
The burst light curve shows a multipeaked structure
which starts at ~T0-6 s and has a duration of ~14.7 s.
The emission is seen up to ~10 MeV.
As observed by Konus-Wind, the burst
had a fluence of 4.41(-0.17,+0.17)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+7.456 s,
of 1.57(-0.24,+0.26)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst
(measured from T0 to T0+15.104 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.50(-0.12,+0.12)
and Ep = 341(-21,+24) keV (chi2 = 72/88 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 70/87 dof).
The spectrum near the maximum count rate
(measured from T0+6.912 to T0+15.104 s)
is best fit in the 20 keV - 10 MeV range
by the CPL model with alpha = -0.36(-0.23,+0.26)
and Ep = 395(-45,+59) keV (chi2 = 61/86 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.5
(chi2 = 61/85 dof).
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB180828_T68250/
All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.