GRB 150821A
GCN Circular 18186
Subject
GRB 150821A: Swift detection of a burst with an optical afterglow
Date
2015-08-21T09:59:12Z (10 years ago)
From
Kim Page at U.of Leicester <klp5@leicester.ac.uk>
E. Troja (NASA/GSFC/UMCP), M. G. Bernardini (INAF-OAB),
S. B. Cenko (GSFC), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC),
P. A. Evans (U Leicester), A. Melandri (INAF-OAB) and
K. L. Page (U Leicester) report on behalf of the Swift Team:
At 09:44:00 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150821A (trigger=652847). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 341.906, -57.875 which is
RA(J2000) = 22h 47m 37s
Dec(J2000) = -57d 52' 30"
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 at least 100 sec. The peak count rate
was ~2840 counts/sec (15-350 keV), at ~35 sec after the trigger.
The XRT began observing the field at 09:48:04.2 UT, 243.3 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 341.9129, -57.8941 which is equivalent to:
RA(J2000) = +22h 47m 39.10s
Dec(J2000) = -57d 53' 38.8"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
location is 70 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy.
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 251 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
RA(J2000) = 22:47:39.18 = 341.91326
DEC(J2000) = -57:53:38.1 = -57.89393
with a 90%-confidence error radius of about 0.61 arc sec. This position is 0.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.48 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02.
Burst Advocate for this burst is E. Troja (eleonora.troja AT nasa.gov).
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 18187
Subject
GRB 150821A: VLT/X-shooter redshift
Date
2015-08-21T11:30:04Z (10 years ago)
From
Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst <malesani@dark-cosmology.dk>
V. D'Elia (ASI/ASDC and INAF-OAR), T. Kruehler (ESO), K. Wiersema (U.
Leicester), N. R. Tanvir (U. Leicester), J. Japelj (U. Ljubljana), G.
Pugliese (API/UvA), D. Malesani (DARK/NBI), B. Milvang-Jensen
(DARK/NBI), J. P. U. Fynbo (DARK/NBI), report on behalf of a larger
collaboration:
We observed the optical counterpart of GRB 150821A (Troja et al., GCN
18186) with the ESO VLT equipped with the X-shooter spectrograph. Four
spectra with individual exposure time of 600 s were secured covering the
wavelength range 3,000-25,000 AA. Observations were carried out in
robotic "rapid response mode" (RRM) and spectroscopy began 12.4 min
after the BAT trigger (7.7 min after the XRT position, at which point
the VLT was triggered). The afterglow is well detected in our
acquisition image, with a magnitude of R ~ 16 (11.1 min after the GRB),
calibrated to one nearby USNO star.
We detect a number of absorption features at a common redshift of z =
0.755, including Mg II, Mg I, Fe II, Zn II. We also detect
fine-structure lines at the same redshift (e.g. FeII* 2396 / 2389, and
Ni II** 2166 / 2217 / 2223), confirming that the absorption system is
local to the GRB and not intervening.
We thank the kind assistence of the observing staff at Paranal, in
particular Claudia Cid and Marcela Espinoza.
GCN Circular 18190
Subject
GRB 150821A: Fermi GBM detection
Date
2015-08-21T15:43:49Z (10 years ago)
From
Oliver Roberts at UCD/Fermi <oliver.roberts@ucd.ie>
O.J. Roberts (UCD) reports on behalf of the Fermi GBM team:
"At 09:44:20.33 UT on August 21st 2015, the Fermi Gamma-Ray
Burst Monitor triggered and located GRB 150821A (trigger
461843064/150821406), which was also detected by Swift
(Troja et al. 2015, GCN 18186).
The trigger resulted in an Autonomous Repoint Request (ARR)
due to the high peak flux of the GRB. This ARR was accepted
and the spacecraft slewed to the GBM in-flight location. Both
the GBM in-flight and on-ground locations are consistent with
the Swift position.
The angle of the burst direction to the Fermi LAT boresight
is 57 degrees.
The GBM light curve consists of a FRED-like burst, with two
unresolved peaks and a duration of about 103 s (50-300 keV).
The time-averaged spectrum from T0-2.0s to T0+101.4s is well
fit by a Band function with Epeak = 322 +/- 18 keV,
Alpha =-1.21 +/- 0.01 and Beta = -2.27 +/- 0.10.
The event fluence (10-1000 keV) in this time interval is
(6.87 +/- 0.08)E-05 erg/cm^2. The 1.024 s peak photon flux
measured starting from T0+17.1s in the 10-1000 keV band
is 11.1 +/- 0.3 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."
GCN Circular 18191
Subject
GRB 150821A: Enhanced Swift-XRT position
Date
2015-08-21T16:45:25Z (10 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester)
report on behalf of the Swift-XRT team.
Using 910 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 150821A, 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 = 341.91332, -57.89384 which is equivalent
to:
RA (J2000): 22h 47m 39.20s
Dec (J2000): -57d 53' 37.8"
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 18192
Subject
GRB 150821A: Swift/UVOT Detection
Date
2015-08-21T19:28:53Z (10 years ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
N. P. M. Kuin (UCL-MSSL) and E. Troja (NASA/GSFC/UMCP) report on behalf
of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 150821A
251 s after the BAT trigger (Troja et al., GCN Circ. 18186). We use the
UVOT position as reported there which is consistent with the enhanced
XRT position (Goad et al. GCN Circ. 18191).
Preliminary detections and 3-sigma upper limits using the UVOT photometric
system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early
exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white 251 401 147 16.59 +/- 0.03
v 560 580 20 17.21 +/- 0.18
b 484 504 19 17.48 +/- 0.12
u 460 479 19 16.70 +/- 0.11
w1 435 629 39 17.62 +/- 0.18
m2 410 7287 372 >19.8
w2 535 1904 156 19.85 +/- 0.28
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 18194
Subject
GRB 150821A: Swift-XRT refined Analysis
Date
2015-08-21T23:59:03Z (10 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo
(INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R.
Roegiers (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),
C. Pagani (U. Leicester) and E. Troja report on behalf of the Swift-XRT
team:
We have analysed 7.7 ks of XRT data for GRB 150821A (Troja et al. GCN
Circ. 18186), from 233 s to 40.6 ks after the BAT trigger. The data
comprise 1.1 ks 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 Goad et al.
(GCN Circ. 18191).
The late-time light curve (from T0+5.9 ks) can be modelled with a
power-law decay with a decay index of alpha=1.26 (+0.15, -0.13).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.85 (+/-0.03). The
best-fitting absorption column is 1.66 (+0.09, -0.08) x 10^22 cm^-2,
at a redshift of 0.755, in addition to the Galactic value of 1.8 x
10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon
index of 2.21 (+/-0.15) and a best-fitting absorption column of 2.2
(+0.4, -0.3) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10
keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11
(7.1 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.8 x 10^20 cm^-2
Intrinsic column: 2.2 (+0.4, -0.3) x 10^22 cm^-2 at z=0.755
Photon index: 2.21 (+/-0.15)
If the light curve continues to decay with a power-law decay index of
1.26, the count rate at T+24 hours will be 8.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.1 x
10^-13 (6.0 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/00652847.
This circular is an official product of the Swift-XRT team.
GCN Circular 18195
Subject
GRB 150821A: GROND afterglow observations
Date
2015-08-22T04:11:28Z (10 years ago)
From
Thomas Kruehler at MPE Garching <kruehler@mpe.mpg.de>
T. Kruehler (ESO), F. Schrey, W. Bornemann, A. Rau and J. Greiner
(all MPE) report on behalf of a larger collaboration:
We observed the field of GRB 150821A (Swift trigger 652847;
Troja et al., GCN 18186) simultaneously in g'r'i'z'JHK
with GROND (Greiner et al. 2008, PASP 120, 405) mounted
at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile).
Observations started at 02:02 UT on 2015-08-22, 16.3 hours after the
GRB trigger. For the optical afterglow (Troja et al., GCN 18186,
D'Elia et al. GCN 18187, Kuin & Troja, GCN 18192) we derive magnitudes of:
g = 23.2 +- 0.1
r = 22.6 +- 0.1
i = 22.3 +- 0.1
z = 22.0 +- 0.2
at a midtime of 17.0 hours after the trigger. This photometry is in the
AB system and calibrated against GROND zeropoints. Because GROND is
presently being recommissioned, our measurements are preliminary.
GCN Circular 18196
Subject
GRB 150821A, Swift-BAT refined analysis
Date
2015-08-22T15:32:50Z (10 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
M. Stamatikos (OSU), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU),
E. Troja (NASA/GSFC/UMCP), 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 150821A (trigger #652847)
(Troja, et al., GCN Circ. 18186). The BAT ground-calculated position is
RA, Dec = 341.913, -57.883 deg which is
RA(J2000) = 22h 47m 39.0s
Dec(J2000) = -57d -52' -58.7"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 1.5%.
Due to the low partial coding fraction of this burst, the light curves and
burst durations are created using the special DETECTION mask aperture.
However, the spectral analysis are done using the default FLUX aperture
for more accurate results, and thus the spectral analysis is only available
after T0+174.24 s, when the burst came into the BAT calibrated field of view.
Note that the interval used for the spectral analysis included < 1% of the
total fluence.
The mask-weighted light curve starts at ~T0+20 s and contains several
overlapping pulses. The main structure ends at ~T0+70 s, with a long-lasting
tail until ~T0+330 s. T90 (15-350 keV) is 172.1 +- 99.0 sec (estimated error
including systematics).
The time-averaged spectrum from T+174.2 to T+330.7 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
2.10 +- 0.37. The fluence in the 15-150 keV band is 7.9 +- 1.9 x 10^-7 erg/cm2.
The 1-s peak spectral analysis is unavailable because the 1-s peak duration
happens before the burst came into the BAT calibrated field of view.
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/652847/BA/
GCN Circular 18197
Subject
GRB 150821A: LCOGT Sutherland observations
Date
2015-08-23T11:56:57Z (10 years ago)
From
Cristiano Guidorzi at Ferrara U,Italy <guidorzi@fe.infn.it>
C. Guidorzi (U. Ferrara), S. Kobayashi (LJMU), A. Gomboc (U. Ljubljana),
C. Mundell (U. Bath) on behalf of a larger collaboration report:
The 1-m LCOGT telescopes in Sutherland (South Africa) began observing
Swift GRB 150821A (Troja et al. GCN 18186) on August 21, 19:39 UT (~9.9
hours after the burst trigger) with SDSS r and i filters.
At the position of the optical afterglow (Troja et al. GCN 18186; D'Elia
et al. GCN 18187; Kuin et al. GCN 18192; Kruehler et al. GCN 18195) we
do not detect any source down to the following limits:
Mid Time Exposure Filter Magnitude
(hrs) (s)
---------------------------------------------------
10.4 2880 r' > 20.4
10.6 2880 i' > 21.0
---------------------------------------------------
Calibration is done against R2 and I values of USNOB-1 nearby stars.
GCN Circular 18199
Subject
GRB 150821A: LCOGT Siding Springs early observations
Date
2015-08-24T15:29:32Z (10 years ago)
From
Cristiano Guidorzi at Ferrara U,Italy <guidorzi@fe.infn.it>
C. Guidorzi (U. Ferrara), S. Kobayashi (LJMU), A. Gomboc (U. Ljubljana),
C. Mundell (U. Bath) on behalf of a larger collaboration report:
The 1-m LCOGT telescopes in Siding Springs (Australia) began observing
Swift GRB 150821A (Troja et al. GCN 18186) on August 21, 10:22 UT (38
minutes after the burst trigger) with SDSS r and i filters under
initially poor sky conditions.
We detect the optical afterglow (Troja et al. GCN 18186; D'Elia et al.
GCN 18187; Kuin et al. GCN 18192; Kruehler et al. GCN 18195) with the
following magnitudes:
Mid Time Exposure Filter Magnitude
(min) (s)
-----------------------------------------------------
135 1680 r' 19.1 +- 0.4
233 480 r' 19.7 +- 0.3
398 720 r' 20.3 +- 0.2
-----------------------------------------------------
Calibration is done against R2 and I values of USNOB-1 nearby stars.
GCN Circular 18200
Subject
Konus-Wind observation of GRB 150821A
Date
2015-08-24T15:32:49Z (10 years ago)
From
Anna Kozlova at Ioffe Institute <ann_kozlova@mail.ioffe.ru>
S. Golenetskii, R. Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik,
M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, A. Kozlova,
and T. Cline on behalf of the Konus-Wind team, report:
The long-duration GRB 150821A (Swift-BAT trigger #652847:
Troja et al., GCN 18186; Stamatikos et al., GCN 18196;
Fermi GBM detection: Roberts, GCN 18190)
triggered Konus-Wind at T0=35074.166 s UT (09:44:34.166).
The burst light curve shows a single multi-peaked pulse
which started at ~T0-1.4 s and had a total duration of ~161.6 s.
The emission is seen up to ~10 MeV.
As observed by Konus-Wind, the burst
had a fluence of 9.16(-2.22,+2.38)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+4.768 s,
of 3.68(-1.02,+1.27)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst
(measured from T0 to T0+164.096 s)
is best fit in the 20 keV - 18 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.47 (-0.09,+0.15),
the high energy photon index beta = -2.21 (-7.79,+0.29),
the peak energy 350 (-125,+202) keV
(chi2 = 129/97 dof)
The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 18 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.07 (-0.10,+0.15),
the high energy photon index beta = -3.13 (-6.87,+0.80),
the peak energy 327 (-66,+58) keV
(chi2 = 91/91 dof)
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB150821_T35074/
The background variations in the soft KW band are due to solar activity.
Assuming the redshift z=0.755 (D'Elia et al., GCN 18187)
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.27, and Omega_Lambda = 0.73,
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is ~1.4x10^53 erg,
the peak luminosity L_iso is ~1.0x10^52 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i, is ~614 keV.
All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.
GCN Circular 18224
Subject
GRB 150821A: Flattening in the UVOT light curve
Date
2015-09-01T15:33:59Z (10 years ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
Paul Kuin, Massimiliano de Pasquale (UCL/MSSL), Daniele
Malesani (DARK/NBI) and Peter Brown (TAMU) report on
behalf of the Swift UVOT team:
The light curve from GRB 150821A as observed by Swift/UVOT has
been monitored continuously since its discovery, up to about
300 ks in all seven filters, and in u, v and white since.
The GRB brightness decreased as a straight power law with
index -1.32 (+0.06/-0.05) until about 10 ks. At 100 ks
there was a clear flattening in the light curve, and around
250 ks a slight brightening may have taken place, but the
brightness could also be flat within our observational errors.
Data taken at 835 ks are consistent with a flat or slightly
decreasing brightness.
The summed 5 ks image in the white band from 300 - 800 ks
shows a nearby extended object to the north-west with
white = 22.01 +/- 0.17 mag, slightly brighter than the GRB at
about 10" distance which currently is at white=23.2 +/- 0.7
magnitudes, and is located about 5" from the edge of the
nearby object.
At this time it is unclear whether the observed flattening is
due to the host galaxy (hosts at this distance have typically
a size of 1" only) or are the emergence of a supernova
contribution to the afterglow. At the redshift of z = 0.755
(D'Elia et al., GCN Circ. 18187), the observed brightness
would imply a SN significantly brighter than those typically
associated with GRBs.
Further ground observations are encouraged.