GRB 160821B
GCN Circular 19833
Subject
GRB 160821B: Swift detection of a short burst
Date
2016-08-21T22:43:52Z (9 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
M. H. Siegel (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf
of the Swift Team:
At 22:29:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160821B (trigger=709357). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 279.987, +62.393, which is
RA(J2000) = 18h 39m 57s
Dec(J2000) = +62d 23' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve shows a single spike
structure with a duration of about 0.5 sec. The peak count rate
was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 22:30:19.1 UT, 66.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
279.9780, 62.3920 which is equivalent to:
RA(J2000) = 18h 39m 54.71s
Dec(J2000) = +62d 23' 31.3"
with an uncertainty of 2.5 arcseconds (radius, 90% containment). This
location is 15 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.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 5.75
x 10^20 cm^-2 (Willingale et al. 2013).
The initial flux in the 2.5 s image was 1.45e-09 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 76 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' covers 100% of
the XRT error region. The typical 3-sigma upper limit has
been about 19.6 mag. No correction has been made
for the expected extinction corresponding to E(B-V) of 0.05.
Burst Advocate for this burst is M. H. Siegel (siegel 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 19834
Subject
GRB 160821B: NOT optical afterglow candidate
Date
2016-08-22T02:30:07Z (9 years ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
D. Xu (NAOC/CAS), D. Malesani (DARK/NBI and DTU Space), A. de Ugarte
Postigo (IAA-CSIC, DARK/NBI), E. Gafton, I. Rivero Losada (NOT) report
on behalf of a larger collaboration:
We observed the field of the short GRB 160821B (Siegel et al., GCN
19833) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the
ALFOSC camera. Observations started at 23:02:07 UT on 2016-08-21 (i.e.,
0.548 hr after the burst), and 12 x 300s SDSS r-band frames were
obtained under variable cirrus.
We detected an uncatalogued point source at coordinates:
R.A. = 18:39:54.56
Dec. = +62:23:30.5
with an uncertainty radius of ~0.2 arcsec, which is within the XRT error
circle reported in Siegel et al. (GCN 19833). This source had
R=22.6+/-0.1 mag at 0.95 hr after the burst, calibrated against the star
of R = 17.6 at RA = 18:39:57.59, Dec = +62:22:31.7 in the USNO B1 catalogue.
We also note the presence of a bright, extended, nearby galaxy, 5.5"
away, with a magnitude R ~ 19.2, which is a candidate host galaxy of GRB
160821B.
Lacking variability, we cannot confirm whether the source we identified
is the afterglow of GRB 160821B. Further observations are encouraged.
GCN Circular 19837
Subject
GRB 160821B: Enhanced Swift-XRT position
Date
2016-08-22T07:39:48Z (9 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 1754 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 160821B, 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 = 279.97653, +62.39129 which is equivalent
to:
RA (J2000): 18h 39m 54.37s
Dec (J2000): +62d 23' 28.6"
with an uncertainty of 2.5 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 19838
Subject
Short GRB 160821B: MASTER early optical observations
Date
2016-08-22T09:08:42Z (9 years ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
R. Rebolo, M. Serra-Ricart, G. Israelian, N. Lodieu
The Instituto de Astrofisica de Canarias
V. Lipunov, N.Tyurina, E. Gorbovskoy, V.Kornilov,
P.Balanutsa, A.Kuznetsov, D.Kuvshinov
Lomonosov Moscow State University, Sternberg Astronomical Institute
K.Ivanov, O.Gres, N.M.Budnev, S.Yazev,
Irkutsk State University
D.Buckley, S. Potter, A.Kniazev, M.Kotze
South African Astronomical Observatory
A. Tlatov, V.Senik, D. Dormidontov, A.Parkhomenko
Kislovodsk Solar Station of the Pulkovo Observatory
V.Yurkov, Yu.Sergienko, A.Gabovich
Blagoveschensk Educational State University, Blagoveschensk
Hugo Levato, Carlos Saffe
Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE)
R.Podesta, C.Mallamaci, C.Lopez, F. Podesta
Observatorio Astronomico Felix Aguilar (OAFA)
MASTER-IAC twin robotic telescope (MASTER-Net:
http://observ.pereplet.ru, online view http://161.72.133.2/)
located in IAC (Tenerife, Spain) was pointed to the
GRB160821B (Siegel et al., GCN 19833) 34 sec after notice time
and 53 sec after trigger time at 2016-08-21 22:30:06 UT.
The alert coordinates altitude was 55 deg. at MASTER-IAC, the Sun
altitude was -35d, the Moon distance is 90 degrees.
On our first (10s exposure - two images in perpendicular polarizations)
set we not found optical transient within SWIFT error-box (Evans et al., GCN 19837)
and at NOT OT position (Xu et al., GCN 19834) brighter then 16.6.
The resuts of the first 5 minutes observations are:
Id Pro.type UT Exp.time Limit Filt. Tube.
(Start) (s) (5sigma) (polarizator)
285342 Alert 22:30:06 10 16.6 P- WEST
285343 Alert 22:30:06 10 16.6 P| EAST
285342 Alert 22:30:06 10 16.6 P- WEST
285345 Alert 22:30:43 20 17.1 P| EAST
285346 Alert 22:31:24 30 17.1 P- WEST
285347 Alert 22:31:24 30 16.6 P| EAST
285348 Alert 22:32:08 30 16.6 P- WEST
285349 Alert 22:32:08 30 16.6 P| EAST
285350 Alert 22:32:50 40 17.2 P- WEST
285351 Alert 22:32:50 40 17.0 P| EAST
285352 Alert 22:33:43 50 17.3 P- WEST
285353 Alert 22:33:43 50 17.2 P| EAST
285355 Alert 22:34:46 70 17.5 P- WEST
285354 Alert 22:34:46 70 0.0 P| EAST
The limit of the coadded west tube images is 18.7 with start time
22:30:06, finish time = 22:35:56 and mean time = 22:33:01 UT.
The message may be cited.
GCN Circular 19839
Subject
GRB 160821B: Swift/UVOT Upper Limits
Date
2016-08-22T12:46:33Z (9 years ago)
From
Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk>
A. A. Breeveld (UCL-MSSL) and M. H. Siegel (PSU) report on behalf of the
Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 160821B 76
s after the BAT trigger (Siegel et al., GCN Circ. 19833).
No optical afterglow consistent with the XRT position (Evans et al. GCN
Circ. 19837) or the NOT position (Xu et al. GCN Circ. 19834) 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) exposure and subsequent exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white_FC 76 226 147 >21.1
u_FC 288 501 209 >20.4
white 76 6306 344 >21.7
v 5080 5280 197 >19.1
b 5901 6101 197 >20.2
u 288 5895 406 >20.6
w1 5491 5691 197 >19.6
m2 5285 5485 197 >19.5
w2 4875 6439 322 >20.1
The magnitudes in the table are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.05 in the direction of the
burst (Schlegel et al. 1998).
GCN Circular 19841
Subject
GRB 160821B: Swift-XRT refined Analysis
Date
2016-08-22T14:18:55Z (9 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A.
Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB),
T.G.R. Roegiers (PSU) and M.H. Siegel report on behalf of the Swift-XRT
team:
We have analysed 9.5 ks of XRT data for GRB 160821B (Siegel et al. GCN
Circ. 19833), from 56 s to 35.9 ks after the BAT trigger. The data
comprise 198 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. 19837).
The late-time light curve (from T0+4.9 ks) can be modelled with a
power-law decay with a decay index of alpha=0.5 (+/-0.3).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.95 (+0.08, -0.07). The
best-fitting absorption column is 7.5 (+2.0, -1.7) x 10^20 cm^-2,
consistent with the Galactic value of 5.8 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 3.3 x 10^-11 (3.9 x
10^-11) erg cm^-2 count^-1.
A summary of the WT-mode spectrum is thus:
Total column: 7.5 (+2.0, -1.7) x 10^20 cm^-2
Galactic foreground: 5.8 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.95 (+0.08, -0.07)
If the light curve continues to decay with a power-law decay index of
0.5, the count rate at T+24 hours will be 3.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.4 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/00709357.
This circular is an official product of the Swift-XRT team.
GCN Circular 19843
Subject
GRB 160821B: Fermi GBM Detection
Date
2016-08-22T20:18:16Z (9 years ago)
From
Matthew Stanbro at UAH/Fermi <mcs0001@uah.edu>
M. Stanbro (UAH) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:
"At 22:29:13.33 UT on 21 August 2016, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 160821B (trigger 493511357 / 160821937), which
was also detected by the Swift/BAT (Siegel et al. 2016, GCN 19833)
The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight at the GBM trigger time is 61
degrees.
The GBM light curve consists of a few episodes
with a duration (T90) of about 1 s (50-300 keV).
The time-averaged spectrum from T0-0.13 s to T0+0.32 s is
best fit by a power law function with an exponential
high-energy cutoff. The power law index is -1.37 +/- 0.22 and
the cutoff energy, parameterized as Epeak, is 84 +/- 19 keV
The event fluence (10-1000 keV) in this time interval is
(1.68 +/- 0.19)E-06 erg/cm^2. The 64-millisec peak photon flux measured
starting from T0+0.00 s in the 10-1000 keV band
is 9.16 +/- 1.19 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."
GCN Circular 19844
Subject
GRB 160821B: Swift-BAT refined analysis
Date
2016-08-22T21:17:29Z (9 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
D. M. Palmer (LANL), 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), J. P. Norris (BSU),
T. Sakamoto (AGU), M. H. Siegel (PSU), M. Stamatikos (OSU),
T. N. Ukwatta (LANL), G. Younes (GWU) (i.e. the Swift-BAT team):
Using the data set from T-239 to T+690 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 160821B (trigger #709357)
(Siegel, et al., GCN Circ. 19833). The BAT ground-calculated position is
RA, Dec = 280.005, 62.387 deg which is
RA(J2000) = 18h 40m 01.2s
Dec(J2000) = +62d 23' 13.2"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 85%.
The mask-weighted light curve shows a FRED-like structure that starts
and peaks at ~T0, and ends at ~T+0.5 s. T90 (15-350 keV) is
0.48 +- 0.07 sec (estimated error including systematics).
The time-averaged spectrum from T+0.00 to T+0.54 sec is best fit by a
power law with an exponential cutoff. This fit gives a photon index of
0.11 +/- 0.88 and an Epeak of 46.3 +/- 6.4 keV (chi squared 58.76 for 56 d.o.f.).
For this model the total fluence in the 15-150 keV band is
1.0 +/- 0.1 x 10^-7 erg/cm2, and the 1-sec peak flux measured from
T-0.23 sec in the 15-150 keV band is 1.7 +/- 0.2 ph/cm2/sec.
A fit to a simple power law gives a photon index of 1.88 +- 0.15
(chi squared 75.26 for 57 d.o.f.). All the quoted errors are at the
90% confidence level.
The burst spectrum is on the softer end of the BAT short GRB distribution.
Using a 4-ms binned light curve, the lag analysis finds a lag of
8 +/- 4 ms for the 50-100 keV to 15-25 keV band, which is consistent with
those of a short GRB.
In addition to a short GRB, the BAT light curve and spectrum are also consistent
with those expected from an SGR. In particular, the double blackbody model
also produces an acceptable fit to the time-averaged spectrum
(chi squared 56.65 for 55 d.o.f.), with the blackbody temperatures
(kT1=4.05 and kT2=13.22 keV) consistent with those from an SGR. However,
the location of this burst (Galactic latitude of 25.13 deg and not in SMC or LMC),
and the fast decay of the XRT light curve makes it unlikely to be an SGR.
We therefore conclude that this burst is likely to be a short GRB.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/709357/BA/
GCN Circular 19846
Subject
GRB 160821B: WHT afterglow confirmation and redshift of candidate host
Date
2016-08-23T04:31:26Z (9 years ago)
From
Andrew Levan at U.of Leicester <A.J.Levan@warwick.ac.uk>
A.J. Levan (U. Warwick), K. Wiersema (U. Leicester), N.R. Tanvir (U. Leicester), D. Malesani (DARK/NBI, DTU Space), D. Xu (NAOC/CAS), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI) report for a larger collaboration:
We observed the location of GRB 160821B (Siegel et al. GCN 19833) with the William Herschel Telescope on La Palma, beginning at 22:57UT on 22 Aug 2016. Observations were obtained in the r and z bands. The candidate afterglow (Xu et al. GCN 19834) is present in our images, but has faded by approximately a factor of two between the two epochs taken approximately one day apart. This variability confirms the source as the afterglow of GRB 160821B.
In addition we obtained a spectrum of the candidate host galaxy (Xu et al. GCN 19834). Several prominent emission lines are visible, including Hbeta, [OIII] (4959/5007) and H-alpha, at a provisional redshift of z=0.16.
The offset of the afterglow from the candidate host galaxy is approximately 5.5���, corresponding to a physical offset of 15 kpc. While we cannot rule out a chance alignment of the afterglow with the host galaxy, we also note that the galaxy light does apparently extend to approximately the afterglow position.
We thank the staff of WHT, in particular Cecelia Farina, for their help with these observations.
GCN Circular 19847
Subject
GRB 160821B: GTC follow-up observation
Date
2016-08-23T04:45:23Z (9 years ago)
From
Soomin Jeong at IAA-CSIC <sjeong@iaa.es>
S. Jeong (SKKU/IAA-CSIC), I. H. Park (SKKU), Y. Hu (IAA-CSIC), R. Scarpa
(GTC) and A. J. Castro-Tirado (IAA-CSIC), on behalf of a larger
collaboration report:
We observed the field of the short GRB 160821B detected by Swift (Siegel et
al. GCNC
19833) and Fermi (Stanbro et al. GCNC 19843) using the 10.4m Gran
Telescopio Canarias (GTC)
equipped with OSIRIS. Observations started at 00:16:17.480 UT on Aug 22
(i.e.,
1.78 hr after the burst), in the riz-band filters.
We also detect the point-like source proposed by Xu et al. (GCNC 19834) as
a potential
afterglow candidate) within the XRT error circle reported by Siegel et al.
(GCNC 19833),
for which we measure a preliminary magnitude (pending final calibration) of
R = 22.40+/-0.06
at 1.83 hr after the burst.
Lacking variability, unless there is an initial plateau phase lasting
several hour in the
afterglow brightness, this would be consistent with a host galaxy detection.
Further GTC observations are planned.
We thanks D. Xu and D. Malesani for providing secondary reference stars in
the field.
GCN Circular 19854
Subject
GRB 160821B: VLA 5.0 GHz observations
Date
2016-08-24T20:22:19Z (9 years ago)
From
Wen-fai Fong at U of Arizona <wfong@email.arizona.edu>
W. Fong (University of Arizona), K. D. Alexander (Harvard), and T. Laskar
(NRAO/UC Berkeley) report:
"We observed the position of the short-duration GRB 160821B (Siegel et al.,
GCN 19833) with the Karl G. Jansky Very Large Array (VLA) beginning on 2016
Aug 22.088 UT (3.6 hr post-burst) at a mean frequency of 5.0 GHz. In 1 hour
of observations, we detect a faint radio source at the position:
RA(J2000) = 18:39:54.56
Dec(J2000) = 62:23:30.3
with an uncertainty of 0.3" in each coordinate. This position is consistent
with the position of the X-ray afterglow (Evans et al., GCN 19837), and
coincident with the position of the optical afterglow (Xu et al., GCN
19834; Levan et al., GCN 19846).
We measure a preliminary 5.0 GHz flux density of ~35 microJy. To assess
variability, we obtained an additional set of VLA observations starting at
26.5 hr after the burst. The source is no longer detected in these
observations to a 3-sigma limit of 18 microJy.
Due to the fading of the radio source, as well as the positional
coincidence with the X-ray and optical afterglows, we suggest this source
as the radio afterglow of GRB 160821B.
We thank the VLA staff for quickly executing these observations."
GCN Circular 19898
Subject
GRB 160821B: 15 GHz upper limits from AMI
Date
2016-09-06T15:24:11Z (9 years ago)
From
Kunal Mooley at Oxford U <kunal.mooley@physics.ox.ac.uk>
K. P. Mooley, T. D. Staley, R. P. Fender (Oxford), G. E. Anderson
(Curtin), T. Cantwell (Manchester), D. Titterington, S. H. Carey, J.
Hickish, Y. C. Perrott, N. Razavi-Ghods, C. Rumsey, P. Scott
(Cambridge), K. Grainge, A. Scaife (Manchester)
We observed the short GRB 160821B (Siegel et al., GCN 19833) with the
AMI Large Array as part of the 4pisky program. The observations at 15
GHz on 2016 Aug 22.85, Aug 23.82, Aug 25.83, and Sep 03.83 (UT) do not
reveal any radio source at the VLA location (Fong et al., GCN 19854),
with 3sigma upper limits of 138 uJy, 227 uJy, 129 uJy, and 171 uJy
respectively.
We note that, due to power outage, the AMI-LA did not trigger
robotically, and could only obtain the first observation 22 hours
post-burst. This is several hours after the VLA 6 GHz detection (~3.6
hours post-burst).
We thank the AMI staff for scheduling these observations. The AMI-GRB
database is a log of all GRB follow up observations with the AMI, and is
available at http://4pisky.org/ami-grb/.
GCN Circular 20222
Subject
GRB 160821B: HST detection of the optical and IR counterpart
Date
2016-12-01T02:36:37Z (9 years ago)
From
Eleonora Troja at GSFC <eleonora.troja@nasa.gov>
E. Troja (UMD/GSFC), N. Tanvir (U. Leicester), S. B. Cenko (NASA/GSFC),
A. Levan (U. Warwick), J. Barnes (U. Berkeley), A. Castro-Tirado (IAA-
CSIC), A. S. Fruchter (STScI), N. Gehrels (NASA/GSFC), J. Greiner (MPE),
N. Kawai (Tokyo Tech), R. Hounsell (UCSC), J. Hjorth (DARK/NBI), A. Lien
(NASA/GSFC), B. Metzger (Columbia), D. Perley (DARK/NBI), S. Rosswog
(U. Stockholm), T. Sakamoto (AGU), C. Thoene (IAA-CSIC), A. de Ugarte
Postigo (IAA-CSIC), and D. Watson (DARK/NBI) report:
We monitored the location of the short GRB 160821B (Siegel et al. GCN
19833; Xu et al. GCN 19834) with the Hubble Space Telescope under our
approved guest observer programs (GO14237 PI: Tanvir; GO14087 PI: Troja).
Observations were carried out with the Wide Field Camera (WFC3) in three
filters, F606W, F110W and F160W, at epochs 3.6, 10.4 and 23.2 days post-
burst. The GRB counterpart is clearly detected in all filters during the
first two epochs, and fades from a magnitude of F606W~25.8 (AB) in the
first epoch to become undetectable in the third epoch.
Assuming a redshift of z=0.162 from the nearby galaxy identified as the
likely host (Levan et al. GCN 19846), our observations rule out the
presence of an emerging supernova comparable to SN1998bw or to other SNe
associated to long GRBs. The observed fluxes constrain the contribution
of any r-process kilonova/macronova component to be at least a
factor ~5 fainter in the IR than that seen in GRB 130603B. The lack of
a bright supernova and the moderate-to-low ejecta mass implied by our
observations are consistent with this event being produced by the merger
of two neutron stars.
However, the current dataset cannot firmly exclude the presence of an
underlying, higher redshift host galaxy. Deeper HST observations aimed
at placing better constraints on the GRB redshift are on-going.
We thank the STScI staff, in particular Tricia Royle, for assistance
with rapidly scheduling our observations.