GRB 210731A
GCN Circular 30584
Subject
GRB 210731A: continued GROND observations
Date
2021-08-02T15:06:50Z (4 years ago)
From
Ana Nicuesa at TLS Tautenburg <ana@tls-tautenburg.de>
A. Nicuesa Guelbenzu, S. Klose, S. Schmidl (all TLS Tautenburg), A. Rau
(MPE Garching), and D. A. Kann (HETH/IAA-CSIC) report:
Using GROND mounted at the 2.2m MPG telescope at ESO La Silla Observatory
(Chile), we continued to observe the bright afterglow of GRB 210731A
(Swift trigger 1062336; Troja et al., GCN 30568).
A comparison of our first-epoch optical data (Nicuesa Guelbenzu et al.,
GCN 30574) with those from de Wet et al. (GCN 30570) indicated that the
optical afterglow underwent a relatively long-lasting plateau phase (see
also Zhen & Filippenko, GCN 30582). Our 2nd epoch GROND observations now
reveal that this plateau phase has finished and the afterglow has entered
a declining phase.
At a mean time of 1.22 days post burst we measure (AB magnitudes)
r' = 20.74 +/- 0.02 mag,
calibrated against the Pan-STARRS catalog. This is in good agreement with
the VLT X-shooter result at 1.19 d (Kann et al., GCN 30583).
We thank Sam Kim for excellent support and for performing the
observations.
GCN Circular 30583
Subject
GRB 210731A: VLT/X-shooter redshift
Date
2021-08-02T09:53:24Z (4 years ago)
From
Andrew Levan at U.of Leicester <a.levan@astro.ru.nl>
D.A. Kann (HETH/IAA-CSIC), L. Izzo (DARK/NBI), A.J. Levan (Radboud), D.B. Malesani (DTU Space), S. de Wet (UCT) report for the STARGATE collaboration:
"We observed the afterglow of GRB 210731A (Troja et al. GCN 30568, de Wet et al. GCN 30570) with the ESO VLT UT3 equipped with the X-shooter spectrograph on 1 Aug 2021. A total of 4800 s (4x1200 s) of spectra were obtained starting at 02:45 UT (approximately 28.5 hours after the burst).
The afterglow is well detected in the acquisition imaging with r=20.7+/- 0.1, and a continuum is visible across the X-shooter spectral range. The spectra contain several prominent absorption features of Fe II, Al III, Mg II and Mg I, at a common redshift z=1.2525. This is consistent with the detection of the afterglow in all the UVOT filters (Kuin & Troja GCN 30572), and we suggest it to be the redshift of GRB 210731A.
We thank the Paranal staff for the excellent assistance in securing these observations."
GCN Circular 30582
Subject
GRB 210731A: KAIT Optical Detection
Date
2021-08-02T09:19:57Z (4 years ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
WeiKang Zheng and Alexei V. Filippenko (UC Berkeley) report on
behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, responded to the Swift GRB 210731A (Troja et al.,
GCN 30568) starting at ~9.04 hours after the Swift trigger.
A total of 30x60s images were obtained in the clear (roughly R)
filter. The optical afterglow (de Wet et al., GCN 30571; Kuin et al.,
GCN 30572; Guelbenzu et al., GCN 30574; Lipunov et al., GCN 30578;)
was clearly detected in each single image. We measured its brightness
of 18.6 +/- 0.1 mag at 9.26 hours after burst, calibrated to the
Pan-STARRS1 catalog.
GCN Circular 30580
Subject
GRB 210731A: Swift-BAT refined analysis
Date
2021-08-01T21:12:47Z (4 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
M. Stamatikos (OSU), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
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+409 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 210731A (trigger #1062336)
(Troja et al., GCN Circ. 30568). The BAT ground-calculated position is
RA, Dec = 300.311, -28.056 deg which is
RA(J2000) = 20h 01m 14.5s
Dec(J2000) = -28d 03' 20.0"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 39%.
The mask-weighted light curve shows a single-pulse structure that
starts at ~T-1 s, peaks at ~T+11 s, and ends at ~T+30 s.
T90 (15-350 keV) is 22.51 +- 2.79 sec (estimated error
including systematics).
The time-averaged spectrum from T-0.78 to T+25.50 sec is best fit
by a power law with an exponential cutoff. This fit gives a photon
index -0.25 +- 0.59, and Epeak of 106.5 +- 26.8 keV (chi squared
50.28 for 56 d.o.f.). For this model the total fluence in the 15-150 keV
band is 2.2 +- 0.2 x 10^-6 erg/cm2 and the 1-sec peak flux measured
from T+11.28 sec in the 15-150 keV band is 1.6 +- 0.3 ph/cm2/sec.
A fit to a simple power law gives a photon index of 1.01 +- 0.11
(chi squared 67.96 for 57 d.o.f.). 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/1062336/BA/
GCN Circular 30578
Subject
GRB 210731A: MASTER optical afterglow detection
Date
2021-08-01T18:45:53Z (4 years ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
V. Lipunov, E. Gorbovskoy, N.Tyurina, K.Minkina, I.Gorbunov,
P.Balanutsa,V.Vladimirov, A.Kuznetsov, D.Cheryasov (Lomonosov Moscow State
University,SAI, Physics Department),
O.A. Gres, N.M. Budnev (Irkutsk State University, API),
R. Podesta, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar
OAFA),
R. Rebolo, M. Serra, N. Lodiu (The Instituto de Astrofisica de Canarias),
D. Buckley (South African Astronomical Observatory),
R. Podesta, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar
OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
Three MASTER robotic telescopes (Global 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) in Spain (IAC, Canary) and South Africa (SAAO observatory) automatically
observed Swift GRB 210801A (Troja et al., GCN 30568) started 60-70s after
trigger (Lipunov ) up to 2021-08-01 09:16:46 (Lipunov et al., GCN 30567).
MASTER-OAFA auto-detection system ( Lipunov et al., "MASTER Global Robotic
Net", Advances in Astronomy, 2010, 30L )
discovered OT source at (RA, Dec) = 20h 01m 13.07s , -28d 03m 39s.6
(err = +-0".9).
The OT magnitude in 'clear' filter is 18.3m (mlim = 19.8) at plato ( see
detection de Wet et al., GCN 30570).
The OT is seen in dozeens images. There is no minor planet at this place.
We have reference image on 2017-08-22.12663 UT with 'clear' filter 19.7m.
GCN Circular 30576
Subject
GRB 210731A: Swift-XRT refined Analysis
Date
2021-08-01T17:24:27Z (4 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T.
Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), D.N. Burrows (PSU) and E. Troja report on behalf of the
Swift-XRT team:
We have analysed 7.9 ks of XRT data for GRB 210731A (Troja et al. GCN
Circ. 30568), from 191 s to 60.1 ks after the BAT trigger. The data
comprise 62 s in Windowed Timing (WT) mode (the first 8 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 3002 s of PC mode data and 8 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 = 300.30466, -28.06096
which is equivalent to:
RA (J2000): 20h 01m 13.12s
Dec(J2000): -28d 03' 39.4"
with an uncertainty of 2.2 arcsec (radius, 90% confidence).
The light curve can be modelled with an initial power-law decay with an
index of alpha=3.6 (+/-0.5), followed by a break at T+424 s to an alpha
of 0.86 (+0.18, -0.19).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 3.07 (+0.29, -0.21). The
best-fitting absorption column is 1.36 (+0.47, -0.21) x 10^21 cm^-2,
consistent with the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.97 (+0.26,
-0.19) and a best-fitting absorption column of 1.35 (+0.77, -0.20) x
10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 3.4 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.35 (+0.77, -0.20) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.97 (+0.26, -0.19)
If the light curve continues to decay with a power-law decay index of
0.86, the count rate at T+24 hours will be 0.022 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.3 x
10^-13 (9.3 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/01062336.
This circular is an official product of the Swift-XRT team.
GCN Circular 30574
Subject
GRB 210731A: GROND observations
Date
2021-08-01T16:26:36Z (4 years ago)
From
Ana Nicuesa at TLS Tautenburg <ana@tls-tautenburg.de>
A. Nicuesa Guelbenzu, S. Klose, S. Schmidl (all TLS Tautenburg)
and A. Rau (MPE Garching) report:
We observed the field of GRB 210731A (Swift trigger 1062336; Troja et
al., GCN 30568) simultaneously in g'r'i'z'JHK with GROND mounted at
the 2.2m MPG telescope at ESO La Silla Observatory (Chile).
Observations started on 01-AUG-2021 at 02:33 UT, about 4.2 hours after
the GRB trigger. They were performed at an average seeing of 1.0
arcsec and at an average airmass of 1.1.
At a meantime of 2:46 UT, for the GRB afterglow (de Wet et al,
GCN 30570; Kuin et al., GCN 30572) we measure the following AB
magnitudes:
g' = 18.71 +/- 0.01 mag,
r' = 18.44 +/- 0.01 mag,
i' = 18.19 +/- 0.01 mag,
z' = 18.01 +/- 0.01 mag,
J = 17.66 +/- 0.02 mag,
H = 17.38 +/- 0.02 mag, and
K = 17.14 +/- 0.15 mag.
Optical magnitudes are calibrated against the Pan-STARRS catalog,
NIR magnitudes against 2MASS stars in the field.
After correcting for a Galactic foreground reddening of E(B-V)=0.09
mag (Schlegel et al. 1998), the SED is a power-law with no evidence
for extinction by dust in the GRB host galaxy.
We thank Sam Kim for excellent support and for performing the
observations.
GCN Circular 30573
Subject
GRB 210731A: Fermi GBM Observation
Date
2021-08-01T15:26:20Z (4 years ago)
From
Oliver J Roberts at USRA/NASA <oliver.roberts@nasa.gov>
S. Lesage (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 22:21:07 UT on 31 July 2021, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 210731A (trigger 649462872/210731931)
which was also detected by Swift BAT (E. Troja et al. 2021, GCN 30568).
The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 102 degrees.
The GBM light curve consists of a single emission episode with a duration (T90)
of about 26 s (10-1000 keV). The time-averaged spectrum
from T0+0.003 to T0+19.968 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.1 +/- 0.1 and the cutoff energy,
parameterized as Epeak, is 175 +/- 11 keV.
The event fluence (10-1000 keV) in this time interval is
(4.9 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+10.3 s in the 10-1000 keV band is 2.9 +/- 0.3 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
GCN Circular 30572
Subject
GRB 210731A: Swift/UVOT Detection of an optical afterglow
Date
2021-08-01T14:44:28Z (4 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 210731A
210 s after the BAT trigger (Troja et al., GCN Circ. 30568).
An optical afterglow consistent with the XRT position (D'Ai et al.
GCN Circ. 30569) and discovered by Meerlicht (de Wet ert al, GCN Circ.
30570) is also detected in the initial UVOT exposures. Interestingly, the
initial rise to peak is slow.
Preliminary 3-sigma magnitudes 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 210 273 62 20.2 +/- 0.38
b 17547 18204 587 19.02 +/- 0.08
u 29863 30504 624 18.66 +/- 0.09
v 11787 12145 348 18.60 +/- 0.17
uvw1 28956 29857 886 19.33 +/- 0.14
uvw2 34767 35667 886 20.41 +/- 0.27
uvm2 23308 23846 530 19.40 +/- 0.21
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.092 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 30570
Subject
GRB 210731A: MeerLICHT optical afterglow detection
Date
2021-08-01T09:28:36Z (4 years ago)
From
Simon de Wet at UCT <dwtsim002@myuct.ac.za>
S. de Wet (UCT), A.J. Levan (Radboud), P.J. Groot (Radboud/UCT/SAAO)
P.M. Vreeswijk (Radboud) report on behalf of the MeerLICHT consortium:
Following the detection of GRB210731A by Swift and its X-ray
counterpart (Troja et al., GCN30568), the 0.6m MeerLICHT telescope,
located at Sutherland, South Africa obtained a repeating series of
60s images in the q,u,q,g,q,r,q,i,q,z bands. Observations began at
2021-07-31, 22:25:54 UT, 5 minutes after the Swift detection, and
3 minutes after the distribution of the BAT alert, and continued
until 2021-08-01, 02:40:21 UT.
We find a new transient candidate in our first q-band exposure (318
seconds post-trigger) with magnitude q = 19.50 +/- 0.09 at coordinates:
RA (J2000) = 20:01:13.19 (300.30494d)
Dec (J2000) = -28:03:40.10 (-28.06114d)
calibrated against Gaia DR2. This position is 0.3" from the updated
XRT position, and which we regard as the optical afterglow to
GRB210731A. We note that no credible afterglow candidate was reported
by UVOT at ~210s post burst (Troja et al., GCN30568).
We detect the afterglow in further observations in the q,g,r and i
bands but not in u and z. Our q-band light curve shows a complex
shape with a steep rise and drop of ~0.7 mag within the first 2000s,
then a gradual rise of 0.7 mag for a further 3000s and a plateau
until ~10000s post-trigger. The final q-band magnitude of the
source was q=18.95 +/- 0.07 at 2021-08-01, 01:06:47 UT. Our first
g,r and i source detections and u and z non-detections are, at:
g = 18.93 +/- 0.06 at 22:31:29 UT
r = 18.63 +/- 0.06 at 22:34:59 UT
i = 18.34 +/- 0.09 at 22:38:33 UT
u > 19.40 at 22:28:06 UT
z > 18.57 at 22:41:58 UT
Reference images of the field show no underlying host galaxy down
to the 5-sigma limiting magnitude, at the position of the afterglow,
at: q > 20.95. There is also no source identified at this location
in archival PanSTARRS images of the field.
MeerLICHT is built and run by a consortium consisting of Radboud
University, University of Cape Town, the South African Astronomical
Observatory, the University of Oxford, the University of Manchester
and the University of Amsterdam.
GCN Circular 30569
Subject
GRB210731A: Enhanced Swift-XRT position
Date
2021-08-01T09:24:41Z (4 years ago)
From
Antonino D'Ai at IASF-PA <antonino.dai@inaf.it>
Antonino D'Ai (INAF-iASFPA), P.A. Evans and M.R. Goad (U. Leicester) report
on behalf of the Swift-XRT team:
Using 963 s of XRT Photon Counting mode data and 4 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 =
300.30500, -28.06105 which is equivalent to:
RA (J2000): 20 01 13.20
Dec (J2000): -28 03 39.8
with an uncertainty of 2.6 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 is an official product of the Swift-XRT team.
GCN Circular 30568
Subject
GRB 210731A: Swift detection of a burst
Date
2021-07-31T22:34:42Z (4 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
E. Troja (NASA/GSFC/UMCP), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), B. Sbarufatti (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:
At 22:21:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210731A (trigger=1062336). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 300.314, -28.042 which is
RA(J2000) = 20h 01m 15s
Dec(J2000) = -28d 02' 30"
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 40 sec. The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 22:24:29.4 UT, 200.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 300.3036, -28.0595 which is equivalent to:
RA(J2000) = 20h 01m 12.86s
Dec(J2000) = -28d 03' 34.2"
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.
The initial flux in the 2.5 s image was 7.96e-10 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 63 seconds with the White filter
starting 210 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
0.092.
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/)