GRB 060923A
GCN Circular 5673
Subject
GRB 060923A: Probable host galaxy in the R-band
Date
2006-10-01T17:32:56Z (19 years ago)
From
Andrew Levan at U.of Leicester <anl@star.le.ac.uk>
A.J. Levan (Hertfordshire), N.R. Tanvir (Leicester), J. Gorosabel (IAA-CSIC)
report for a larger collaboration:
We observed the location of GRB 060923A (Tanvir et al. GCN Circ 5587; Fox
et al. GCN Circ 5597) in the R-band using the VLT and FORS2. At the
location of the afterglow we find a faint, extended source (angular size
~2") with R~25.5. We suggest this is likely to be the host galaxy of GRB
060923A.
If this galaxy is the host of GRB 060923A then its detection in the R-band
implies an upper limit for the redshift of z~5. Therefore we believe that
extinction is the most likely cause for the extremely red J-K colour for
the afterglow at early times.
We that the staff of the VLT for their excellent support.
GCN Circular 5671
Subject
GRB 060923A: VLA K-band Observations
Date
2006-10-01T00:58:58Z (19 years ago)
From
Poonam Chandra at U Virginia/NRAO <pc8s@virginia.edu>
P. Chandra (UVA/NRAO) and D. A. Frail (NRAO) report on behalf of
the Caltech-NRAO-Carnegie GRB Collaboration:
"We used the Very Large Array to observe the field of view toward
GRB060923A (GCN 5583) at a frequency of 22.5 GHz on 2006 September
30 starting at 19.65 UT. The peak radio brightness at the position of the
afterglow of Fox et al. (GCN 5597) is -200 uJy � 260 uJy.
The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."
GCN Circular 5624
Subject
GRB 060923A: Schedule of Spitzer Space Telescope Observations
Date
2006-09-27T01:31:24Z (19 years ago)
From
Derek Fox at PSU <dfox@astro.psu.edu>
D.B. Fox (Penn State) reports on behalf of a larger collaboration:
"We have activated a sequence of Spitzer Space Telescope IRAC imaging
observations of the afterglow of GRB 060923A (Stamatikos et al., GCN
5583). Projected start times for these observations are:
Epoch 1: 10:45 UT on 27 Sep 2006
Epoch 2: 04:00 UT on 29 Sep 2006 (est.)
Epoch 3: 15:15 UT on 3 Oct 2006 (est.)
Observations for each epoch will last approximately 80 minutes.
Observers interested in coordination with Epochs 2 and 3 should
contact the author by email in advance to confirm the final schedule."
GCN Circular 5620
Subject
GRB 060923A: Radio Observations
Date
2006-09-26T22:23:15Z (19 years ago)
From
Dale A. Frail at NRAO <dfrail@nrao.edu>
Dale A. Frail (NRAO) and P. Chandra (NRAO/UVA) report on behalf of a
larger collaboration:
"We used the Very Large Array to observe the field of view toward
GRB060923A (GCN 5583) at a frequency of 8.46 GHz on 2006 October 26.06 UT.
The peak radio brightness at the position of the afterglow of Tanvir
et al. (GCN 5587) and Fox et al. (GCN 5597) is -30 uJy +/- 55 uJy.
Further observations are planned.
The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."
GCN Circular 5605
Subject
GRB 060923A: Gemini Second Epoch and Afterglow Confirmation
Date
2006-09-24T22:34:47Z (19 years ago)
From
Derek Fox at PSU <dfox@astro.psu.edu>
D.B. Fox (Penn State) reports on behalf of a larger collaboration:
"We have imaged the Swift XRT localization region for GRB060923A
(Stamatikos et al., GCN 5583) with Gemini-North + NIRI on a second
occasion. The candidate afterglow of Tanvir et al. (GCN 5587) and Fox
et al. (GCN 5597) is no longer detected, and we estimate its
brightness as:
K-band: Ks>21.5 mag at 05:05 UT, 24 Sep (mean epoch)
using the same photometric calibration as for the previous night (GCN
5597). Fading of the candidate by >~1 mag between our two epochs is
separately confirmed in an image subtraction analysis.
This confirms the afterglow nature of the source, and thus the highly
reddened, high-redshift and/or highly-extinguished nature of the
afterglow of GRB 060923A."
GCN Circular 5599
Subject
GRB 060923A: Swift XRT refined analysis
Date
2006-09-23T19:14:02Z (19 years ago)
From
Maria Laura Conciatore at ASDC <conciatore@asdc.asi.it>
M.L. Conciatore, M. Capalbi, M. Perri, D.N. Burrows (PSU) and M.
Stamatikos (NASA/ORAU) report on behalf of the Swift XRT team:
We have analysed the first five orbits of Swift XRT data from GRB
060923A (Stamatikos et al., GCN 5583). A ~2.5ks PC mode image provides
a refined X-ray position of:
RA(J2000) = 16h 58m 28.2s
Dec(J2000) = +12d 21' 40.0"
with an uncertainty of 6.0 arcseconds radius (90% containment).This is
3.4 arcseconds from the XRT position given by Stamatikos et al.(GCN
5583) and 74.8 arcsec from the refined (ground-calculated) BAT
position (Tueller et al., GCN 5589). This position is 1.14 arcseconds
from the detected source reported by Tanvir et al. (GCN 5587), and
1.32 arcseconds from the one reported by Fox et al. (GCN 5597) as a
candidate counterpart afterglow.
X-ray light curve shows an initial power-law decay with a decay index
of 2.7+/-0.3. Then, after a slow rebrightening starting at ~650s, from
T+4200 the source decays with a slope of 1.23 +/-0.1.
An absorbed power-law fit to the X-ray spectrum from T+100s to T+960s
gave a photon index of 2.1+/-0.2 and a column density of (1.2+/0.1)e21
cm**-2. We note the galactic hydrogen column density in the direction
of the burst is 5.1e20 cm**-2. The 0.2-10.0 keV observed flux for this
spectrum is 4.1e-11 ergs cm**-2 s**-1, which corresponds to an unabsorbed
flux of 4.9e-11 ergs cm**-2 s**-1.
Assuming that the X-ray emission continues to decline at the same
rate, we predict a 0.2-10 keV XRT count rate of 0.006 count/s at
T+24hr, which corresponds to an observed 0.2-10 keV flux of 2.4e-13
erg/cm**2/s.
This circular is an official product of the Swift XRT Team.
[GCN OPS NOTE(24sep06): Per author's request, the author list was added.]
GCN Circular 5597
Subject
GRB 060923A: Keck and Gemini Observations
Date
2006-09-23T18:17:24Z (19 years ago)
From
Derek Fox at PSU <dfox@astro.psu.edu>
D.B. Fox (Penn State), A. Rau (Caltech), & E.O. Ofek (Caltech) report
on behalf of a larger collaboration:
"We have imaged the Swift XRT localization region for GRB060923A
(Trigger 230662; Stamatikos et al., GCN 5583) with Keck-I + LRIS
(I-band) and Gemini-North + NIRI (J- and K-band). We identify the
candidate afterglow of Tanvir et al. (GCN 5587) in our K-band images
as the only point source consistent with the XRT localization, with a
brightness estimated as follows:
K-band: Ks~20.6 mag at 07:04 UT
We estimate its brightness (without correcting for Galactic
extinction) by reference to the 2MASS star at R.A. 16:58:31.463,
Dec. +12:22:12.88 (J2000), which has J=15.175 mag, H=14.585 mag, and
Ks=14.457 mag in that catalog.
The position of the source by reference to NOMAD catalog sources in
the vicinity is:
R.A. 16:58:28.15, Dec. +12:21:38.9 (J2000)
with an estimated uncertainty of less than 0.5" in each coordinate.
The source is undetected in our earlier J-band and I-band images. Our
upper limits on the brightness of the source in these bands are:
I-band: I>23.5 mag at 06:11 UT
J-band: J>23.7 mag at 06:42 UT
The I-band photometry is referenced to the SDSS imaging of this field
(Cool et al., GCN 5585) while the J-band zero-point is derived from
the 2MASS star referenced above. Magnitudes have not been corrected
for Galactic extinction.
If this source is the afterglow of GRB060923A, the red colors are
suggestive of a highly extinguished or high-redshift burst. In
particular, correcting for the Schlegel, Finkbeiner & Davis (1998)
estimate of E(B-V)=0.058 mag Galactic extinction, and assuming a
temporal decay of t^(-1) and an intrinsic afterglow spectrum of F_nu ~
nu^(-1), these limits indicate suppression of the afterglow flux
(relative to K-band) by a factor of >18 at 0.9 micron (I-band) and by
a factor of >14 at 1.23 micron (J-band).