GRB 070721A, GRB 070721
GCN Circular 6676
Subject
GRB070721A: Swift/UVOT Detection of a Weak Afterglow
Date
2007-07-27T21:14:26Z (19 years ago)
From
Frank Marshall at GSFC <marshall@milkyway.gsfc.nasa.gov>
F. E. Marshall (NASA/GSFC), P. Schady (MSSL-UCL) and H. Ziaeepour (MSSL-UCL)
report on behalf of the Swift/UVOT team:
A comparison of the initial Swift/UVOT observations with those taken several days
after the BAT trigger (Ziaeepour et al. GCN Circ. 6639) reveals a weak, fading source
inside the refined XRT error circle (Evans et al. GCN Circ. 6644).
The source is only detected in the initial 100-s exposure with the white
filter starting 88 sec. after the trigger and the initial 400-s exposure with the V
filter starting 194 sec. after the trigger. The 2.4-sigma detection in
white corresponds to 21.4 mag., and the 3.4-sigma detection in V corresponds
to 20.2 mag. The best-fit position is RA, Dec = 3.16343,-28.55020, which is
equivalent to:
RA (J2000): 00 12 39.22
Dec (J2000): -28 33 00.7
with an uncertainty of 2.0" (radius, 90% confidence).
The source is slightly blended with 18th mag star noted by
Schady and Ziaeepour (GCN Circ. 6648), which is 4.7" away.
GCN Circular 6674
Subject
GRB 070721A: optical observations
Date
2007-07-27T17:14:46Z (19 years ago)
From
Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center <malesani@astro.ku.dk>
D. Malesani, J.P.U. Fynbo, P.M. Vreeswijk (DARK), I. Ilyn (AIP), report
on behalf of a larger collaboration:
We observed the field of GRB 070721A (Ziaeepour et al., GCN 6639) in two
different epochs, using the NOT and VLT telescopes. Observations were
carried out 0.78 and 1.98 days after the GRB. A bright star (R~17.3) is
close to the XRT position (Evans & Ziaeepour, GCN 6640). Inside the XRT
error circle, we detect a single source at the coordinates (J2000,
against USNO-B1):
RA = 00:12:39.13
Dec = -28:33:00.9
This source does not vary between the two epochs to within 0.2 mag, and
is pointlike under seeing conditions of 0.7". We measure R~22.9 using
VLT instrumental zeropoints (~0.1 mag error). No other sources are seen
inside the XRT error circle down to the limits reported below.
Mean UT T-T0 (d) t_exp (s) Instrument Rlim
--------------------------------------------------
Jul 22.20065 0.78320 4x600 NOT+StanCam 24.0
Jul 23.39637 1.97892 2x180 VLT+FORS2 25.5
T0 = trigger time = Jul 21.41745 UT.
This message can be cited.
GCN Circular 6648
Subject
Swift/UVOT Observations of GRB070721A
Date
2007-07-21T18:56:56Z (19 years ago)
From
Patricia Schady at MSSL/Swift <ps@mssl.ucl.ac.uk>
P. Schady (MSSL-UCL) and H. Ziaeepour (MSSL-UCL) report on behalf of the
Swift/UVOT team:
The Swift/UVOT observed the field of GRB 070721A starting 69 s after the
BAT trigger (Ziaeepour et al. GCN Circ.6639). We do not find any source,
in any of the UVOT observations inside the refined XRT error circle (Evans
et al., GCN Circ.6644). However, we note that there is an 18th mag star 4
arcseconds from the XRT refined position.
The 3-sigma upper limits for detecting a source inside the XRT error
circle are:
Filter T_mid(s) Exp(s) Mag (3-sig upper limit)
=================================================
White 1088 156 > 21.7
V 1194 453 > 20.9
B 1509 97 > 19.5
U 1359 117 > 20.0
UVW1 1334 117 > 19.6
UVM2 1309 117 > 19.2
UVW2 1530 97 > 19.4
where T_mid is the weighted mid time of the coadded exposure. The values
quoted above are not corrected for the expected Galactic extinction
corresponding to a reddening of E(B-V) = 0.01 mag in the direction of the
burst (Schlegel et al. 1998).
GCN Circular 6645
Subject
GRB 070721A : Faulkes Telescope South optical limit
Date
2007-07-21T15:25:42Z (19 years ago)
From
Andrea Melandri at Liverpool John Moores U <axm@astro.livjm.ac.uk>
A. Melandri, (Liverpool JMU), C. Guidorzi (Uni-Bicocca/INAF-OAB),
I.A. Steele, C.G. Mundell, D. Carter, R.J. Smith, C.J. Mottram,
D.F. Bersier, S. Kobayashi, M.J. Burgdorf, M.F. Bode (Liverpool JMU),
A. Gomboc (Ljubljana), P. O'Brien, E. Rol, N. Bannister (Leicester)
report
On 2007 Jul 21 at 13:47:14 UT the Faulkes Telescope South
observed the field of GRB070721A (trigger=285653, Ziaeepour
et al., GCN 6639).
No new source is detected inside the refined XRT error circle
(Evans et al., GCN 6644) and we can set an upper limit of
R > 20.0 between 3.9 and 4.8 hours after the trigger.
GCN Circular 6644
Subject
GRB 070721A: Swift/XRT refined analysis
Date
2007-07-21T14:31:11Z (19 years ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
P.A. Evans (U. Leicester) and H. Ziaeepour (UCL-MSSL) report on behalf of
the Swift/XRT team:
We have analyzed the first 1.3 ks of Swift XRT data for GRB 070721A
(Ziaeepour et al. GCN Circ. 6639), comprising 35 s in Windowed Timing (WT)
mode, and 1.3 ks in Photon Counting (PC) mode. The burst is clearly
detected. Using 799 s of overlapping XRT Photon Counting mode and UVOT
V-band data, 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 = 3.16348, -28.55017 which is equivalent to:
RA (J2000): 00 12 39.24
Dec (J2000): -28 33 00.6
with an uncertainty of 2.3 arcsec (radius, 90% confidence). This position
is 3.2" from the initial XRT position reported in GCN Circ. 6639, and 95"
from the BAT-refined position reported by Sakamoto et al. (GCN Circ.
6642).
The XRT light curve shows a fading behaviour, which is well fitted by a
broken power-law. The initial decay followed a slope of alpha=2.97
(+0.57/-0.42). At T0+281 (+112/-71) s the decay shallows to an alpha of
0.75 (+0.39/-0.49).
The Photon Counting mode spectrum is well fitted by an absorbed power-law,
with a column density of 1.61e20 cm^-2 (the Galactic value) and a gamma of
2.24 (+0.33/-0.26). This is consistent with the value derived for the BAT
spectrum (GCN Circ 6642). The observed (unabsorbed) 0.3-10 keV flux is
6.53e-12 (8.23e-12) erg cm^-2 s^-1.
If the source continues to decay at the same rate, we predict a count rate
of 2.7e-3 counts/s at T0+24 hours, which corresponds to an observed
(unabsorbed) flux of 1.87e-13 (2.36e-13) erg cm^-2 s^-1.
This circular is an official product of the Swift/XRT team.
GCN Circular 6642
Subject
GRB 070721A, Swift-BAT refined analysis
Date
2007-07-21T14:07:31Z (19 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
T. Sakamoto (GSFC/ORAU), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL),
A. Parsons (GSFC), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. Ukwatta (GWU), H. Ziaeepour (UCL-MSSL)
(i.e. the Swift-BAT team):
Using the data set from T-239 to T+352 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 070721A (trigger #285653)
(Ziaeepour, et al., GCN Circ. 6639). The BAT ground-calculated position is
RA, Dec = 3.144, -28.530 deg which is
RA(J2000) = 0h 12m 34.5s
Dec(J2000) = -28d 31' 47"
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 72%.
The mask-weighted lightcurve has a single peak starting at ~T-4 sec,
peaking at ~T+1 sec, and ending at ~T+8 sec. T90 (15-350 keV) is
3.4 +- 0.2 sec (estimated error including systematics).
The time-averaged spectrum from T-0.1 to T+3.7 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
2.46 +- 0.41. The fluence in the 15-150 keV band is 7.1 +- 1.8 x 10^-8 erg/cm2.
The 1-sec peak photon flux measured from T+0.12 sec in the 15-150 keV band
is 0.7 +- 0.1 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
We note that the fluence ratio in a simple power-law fit between the
25-50 keV band and the 50-100 keV band is 1.38. This fluence ratio is larger
than 1.32 which can be achieved in the Band function of alpha=-1.0, beta=-2.5,
and Epeak=30 keV. Thus, preliminary analysis shows that Epeak of the burst
is very likely around or below 30 keV. Therefore the burst can be classified
as an X-ray flash.
GCN Circular 6639
Subject
GRB 070721: Swift detection of a possible burst
Date
2007-07-21T10:59:02Z (19 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
H. Ziaeepour (UCL-MSSL), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
P. A. Evans (U Leicester), K. L. Page (U Leicester) and
E. Troja (INAF-IASFPA) report on behalf of the Swift Team:
At 10:00:56 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070721 (trigger=285653). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 3.176, -28.546 (J2000) with an uncertainty of 3 arcmin
(radius, 90% containment, including systematic uncertainty).
We have very little further information at this time (lightcurve, etc)
because there appears to be a problem with the messages.
The spacecraft slewed promptly and the XRT found a source
at location
RA(J2000) = 3.16355 degrees,
Dec(J2000) = -28.54928 degrees,
which is 40 arcseconds from the BAT position. We do not have
an error estimate, but the typical uncertainty for on-board centroids
is 6 arcseconds. The source flux was about 3.25e-10 cgs. At this point
we do not have enough data to be able to tell whether this source is
fading. We note that there is a bright 2MASS object at this position:
2MASS 00123943-2832566 (2.5 arcseconds from the XRT position).
We are waiting for processing of the downlinked Malindi data
for more details.
At this moment no UVOT data are available. We are waiting for the
next Malindi pass to download more data.
We note that while writing this Circular, Swift slewed to the next
trigger following this one. This is unusual and raises the
possibility that this burst is not real. We will not be certain until
the Malindi data can be analyzed.
Burst Advocate for this burst is H. Ziaeepour (hz AT mssl.ucl.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 OPS NOTE(24aug07): Per author's request, the trigger time was corrected
from 10:01:08 to 10:00:56. Acknowledgements to F. Marshall.]