GRB 170804A
GCN Circular 21410
Subject
GRB 170804A: Swift detection of a burst with optical counterpart
Date
2017-08-04T12:13:26Z (8 years ago)
From
Jamie A. Kennea at PSU/Swift-XRT <kennea@swift.psu.edu>
A. Cholden-Brown (PSU), D. N. Burrows (PSU),
P. A. Evans (U Leicester), J. A. Kennea (PSU), S. J. LaPorte (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester) and B. Sbarufatti (INAF-OAB/PSU) report on
behalf of the Swift Team:
At 12:01:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170804A (trigger=766194). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 6.433, -64.748 which is
RA(J2000) = 00h 25m 44s
Dec(J2000) = -64d 44' 52"
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 40 sec. The peak count rate
was ~1400 counts/sec (15-350 keV), at ~5 sec after the trigger.
The XRT began observing the field at 12:03:39.9 UT, 122.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 6.39106, -64.78341 which
is equivalent to:
RA(J2000) = 00h 25m 33.85s
Dec(J2000) = -64d 47' 00.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 142 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 (1.79 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.4
(+3.30/-2.37) x 10^21 cm^-2 (90% confidence).
The initial flux in the 2.5 s image was 5.37e-10 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 131 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
RA(J2000) = 00:25:34.45 = 6.39354
DEC(J2000) = -64:47:03.0 = -64.78416
with a 90%-confidence error radius of about 0.63 arc sec. This position is 6.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.01 with a 1-sigma error of about 0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02.
Burst Advocate for this burst is A. Cholden-Brown (aaronb AT swift.psu.edu).
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 21413
Subject
GRB 170804A: Enhanced Swift-XRT position
Date
2017-08-04T16:14:01Z (8 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester)
report on behalf of the Swift-XRT team.
Using 1208 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 170804A, 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 = 6.39233, -64.78429 which is equivalent
to:
RA (J2000): 00h 25m 34.16s
Dec (J2000): -64d 47' 03.4"
with an uncertainty of 1.8 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.
GCN Circular 21414
Subject
GRB 170804A: Swift-XRT refined Analysis
Date
2017-08-04T20:01:28Z (8 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
D.N. Burrows (PSU), K.L. Page (U. Leicester), B. Mingo (U. Leicester),
A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia
(ASDC), A. D'Ai (INAF-IASFPA), S. J. LaPorte (PSU) and A. Cholden-Brown
report on behalf of the Swift-XRT team:
We have analysed 6.5 ks of XRT data for GRB 170804A (Cholden-Brown et
al. GCN Circ. 21410), from 112 s to 19.6 ks after the BAT trigger. The
data comprise 95 s in Windowed Timing (WT) mode (the first 8 s were
taken while Swift was slewing) with the remainder in Photon Counting
(PC) mode. The enhanced XRT position for this burst was given by Evans
et al. (GCN Circ. 21413).
The light curve can be modelled with an initial power-law decay with an
index of alpha=2.90 (+0.34, -0.22), followed by a break at T+519 s to
an alpha of 1.13 (+0.20, -0.13).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.37 (+0.16, -0.13). The
best-fitting absorption column is consistent with the Galactic value
of 1.8 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.97 (+0.16, -0.14) and a best-fitting absorption
column consistent with the Galactic value. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum is 3.2 x 10^-11 (3.4 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.8 (+2.4, -0.0) x 10^20 cm^-2
Galactic foreground: 1.8 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.97 (+0.16, -0.14)
If the light curve continues to decay with a power-law decay index of
1.13, the count rate at T+24 hours will be 8.3 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.7 x
10^-14 (2.8 x 10^-14) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00766194.
This circular is an official product of the Swift-XRT team.
GCN Circular 21415
Subject
GRB 170804A: Swift/UVOT Detection
Date
2017-08-04T21:03:59Z (8 years ago)
From
Samantha Oates at MSSL <sro@mssl.ucl.ac.uk>
S. R. Oates (U. Warwick) and A. Cholden-Brown (PSU)
report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 170804A
132 s after the BAT trigger (Cholden-Brown et al., GCN Circ. 21410).
A fading source consistent with the XRT position
(Evans et al., GCN Circ. 21413) is detected in the initial UVOT exposures.
The preliminary UVOT position is:
RA (J2000) = 0:25:34.29 = 6.39288 (deg.)
Dec (J2000) = -64:47:03.1 = -64.78419 (deg.)
with an estimated uncertainty of 0.5 arc sec. (radius, 90% confidence).
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 132 281 147 18.9 +/- 0.1
white 871 1020 147 20.0 +/- 0.2
u_FC 289 539 246 18.7 +/- 0.1
v 796 1243 58 >18.7
b 545 565 19 18.9 +/- 0.4
uvw1 671 1292 78 >19.0
uvw2 596 1392 97 >19.2
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).
---------------------------------------------
Dr Samantha Oates
Leverhulme Early Career Fellow
Department of Physics
University of Warwick
Coventry
CV4, UK
Tel: +44 (024) 765 23383
Email: s.oates@warwick.ac.uk<mailto:s.oates@warwick.ac.uk>
-----------------------------------------------
GCN Circular 21416
Subject
GRB 170804A: Swift-BAT refined analysis
Date
2017-08-05T15:45:14Z (8 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
H. A. Krimm (NSF/USRA), S. D. Barthelmy (GSFC),
A. Cholden-Brown (PSU), J. R. Cummings (CPI),
A. Y. Lien (GSFC/UMBC), 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-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 170804A (trigger #766194)
(Cholden-Brown et al., GCN Circ. 21410). The BAT ground-calculated position is
RA, Dec = 6.404, -64.781 deg which is
RA(J2000) = 00h 25m 37.0s
Dec(J2000) = -64d 46' 50.9"
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 75%.
The mask-weighted light curve shows several over-lapping pulses that
start at ~T0 and end at ~T+60 s. T90 (15-350 keV) is 43.4 +- 9.8 sec
(estimated error including systematics).
The time-averaged spectrum from T+1.98 to T+60.90 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.67 +- 0.13. The fluence in the 15-150 keV band is 1.3 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+25.10 sec in the 15-150 keV band
is 0.9 +- 0.2 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/766194/BA/
GCN Circular 21417
Subject
GRB 170804A: LCO Observations
Date
2017-08-05T16:53:45Z (8 years ago)
From
Antonino Cucchiara at UVI <antonino.cucchiara@uvi.edu>
A. Cucchiara, D. Morris (U. of the Virgin Islands), and
N. Orange (OrangeWave), report on behalf of a larger
collaboration:
"On August 4.79 UT (T_0 +7.0h) we began observing the
center of the field of GRB 170804A (Cholden-Brown et al.
GCN 21410, Emery et al. GCN 21411) using the Las Cumbres
Observatory 1m telescope located in Siding Spring
Observatory.
Under poor sky conditions (seeing ~2.6 arcseconds) we
performed a series of 5x240s observations in SDSS r' and i���
bands for a total of 20 minutes on sky in each filter at
average airmass of 1.3.
In the full median stacked images we identified no optical
counterpart within the center of the Swift-XRT refined position
or at the UVOT optical counterpart location (Evans et al.
GCN 21413, Emery et al. GCN 21411) at the following
3-sigma limits:
r' > 21.4 mag
i' > 21.1 mag
These magnitudes are calibrated against nearby USNO-B1 sources,
converted in SDSS bands, and not corrected for Galactic extinction."