GRB 061004
GCN Circular 5782
Subject
GRB 061004: Further VLT/NTT observations
Date
2006-11-02T13:34:24Z (19 years ago)
From
Pall Jakobsson at U Hertfordshire <P.Jakobsson@herts.ac.uk>
Pall Jakobsson (U. Hertfordshire), Johan P. U. Fynbo,
Brian L. Jensen , Jens Hjorth, Maximilian D. Stritzinger
(DARK, NBI), Andrew Levan (U. Hertfordshire), Nial Tanvir
(U. Leicester), Paul Vreeswijk, Cedric Ledoux (ESO),
Houri Ziaeepour (UCL-MSSL) and Mathew Smith (U. Portsmouth)
report on behalf of a larger collaboration:
We have performed follow-up observations of the GRB 061004
optical afterglow candidate (S1) reported in Jakobsson et al.
(GCN 5698). The S1 flux is roughly constant between the
three epochs of observations:
Tel/inst Date (UT) Delta-t Exp. time Magnitude
(days) (s)
--------------------------------------------------------------
VLT/FORS1 Oct 5.366 0.539 2*300 22.55 +/- 0.09
VLT/FORS1 Oct 7.344 2.517 2*300 + 120 22.82 +/- 0.07
NTT/EMMI Oct 15.352 10.525 3*500 22.73 +/- 0.20
In addition, S1 is 3.9" from the edge of the revised XRT error
circle (Racusin et al. GCN 5774). The most likely conclusion
is that S1 is not associated with the GRB.
There is no evidence for other sources inside the revised XRT
error circle. The limiting magnitude of our first epoch
observation is R > 25.1 (2 sigma). The corresponding upper limit
on beta_OX is 0.42, rendering the burst dark according to the
definition proposed by Jakobsson et al. (2004, ApJ, 617, L21).
Considering the brightness of S1, it is unlikely that it is a
normal galaxy if z ~ 3.3 (GCN 5698). The relatively low S/N of
the VLT spectrum makes it difficult to reach a firm conclusion
on the reality of absorption features (apart from the feature
around 5230 A). The lack of broad emission lines and the
marginal evidence for optical variability might indicate that
S1 is a blazar. An alternative scenario is that S1 is at a lower
redshift. The 5230 A feature could be the Balmer/4000 A break,
indicating a redshift of z ~ 0.3.
We thank Kim Page for providing us with the afterglow X-ray flux.
GCN Circular 5774
Subject
GRB 061004: Swift/XRT Astrometry Correction
Date
2006-10-31T23:38:37Z (19 years ago)
From
Judith Racusin at PSU <racusin@astro.psu.edu>
J. L. Racusin (PSU), D. N. Burrows (PSU), K. L. Page (U Leicester), H.
Ziaeepour (UCL-MSSL), and N. Gehrels (NASA/GSFC) report on behalf of the
Swift XRT team:
We have re-analyzed the full XRT data set of GRB 061004. XRT observed the
field of GRB 061004 for a total exposure time of 61 ks between October 4
and October 10, 2006 in Photon Counting mode.
Applying the new XRT boresight definition (Burrows et al., GCN 5750), we
find a boresight corrected position of RA(J2000): 06h 31m 10.68s,
Dec(J2000): -45d 54' 22.7", with an estimated uncertainty of 3.6
arcseconds (radius, 90% containment).
To further improve the accuracy of this position, we performed an
astrometry correction using 54 ks of the total exposure time (when the
satellite position was stable). In this data set we find 26 serendipitous
X-ray sources detected with the XIMAGE detect algorithm with S/N > 3, 5 of
which have near-by optical counterparts in the USNO-B1 catalog. We match
these sources to obtain a best fit mean frame shift, carefully accounting
for several instrumental factors including exposure map correction, and
additional hot pixel removal.
We calculate the statistical position errors using the empirical fits as
described in Moretti et al. (2006, A&A, 448, L9), assuming that the
astrometric correction removes the 3.5" systematic error normally applied
to XRT positions to account for errors in the star tracker attitude
solution.
The result of this analysis leads to a mean frame shift from the
previously reported position (Page et al., GCN 5695), of:
RA offset: +0.49s +/- 0.07s Dec offset: +1.3" +/- 1.0"
and a new XRT astrometry corrected position of:
RA(J2000): 06h 31m 10.94s
Dec(J2000): -45d 54' 23.3"
with an estimated uncertainty of 2.0 arcseconds (radius, 90% containment).
This position is 5.0 arcseconds from the original refined XRT position
given in Page et al. (GCN 5695), 2.8 arcseconds from the boresight
corrected XRT position, and 5.9 arcseconds from the faint VLT optical
object described in Jakobsson et al. (GCN 5698).
A figure comparing all of these positions is available at:
http://www.swift.psu.edu/images/grb061004_astrometry.gif
This Circular is an official product of the Swift XRT team.
GCN Circular 5698
Subject
GRB 061004: Optical afterglow candidate and redshift
Date
2006-10-05T21:08:41Z (19 years ago)
From
Pall Jakobsson at U Hertfordshire <palli@star.herts.ac.uk>
Pall Jakobsson (U. Hertfordshire), Johan P. U. Fynbo,
Brian L. Jensen , Jens Hjorth (DARK, NBI), Andrew Levan
(U. Hertfordshire), Nial Tanvir (U. Leicester), Paul Vreeswijk
and Cedric Ledoux (ESO) report on behalf of a larger collaboration:
Using FORS1 on the Very Large Telescope, we have imaged the
GRB 061004 field (Ziaeepour et al. GCN 5691) on Oct 5.369
(0.542 days post-burst). Inside the revised XRT circle (Page
et al. GCN 5695) we find an R ~ 23 mag point source, using
photometric zeropoints from the ESO webpages. At present
we cannot confirm that the source is fading. Its coordinates are:
R.A.(J2000) = 06:31:10.71
Dec(J2000) = -45:54:28.7
with a 0.3" error in each coordinate.
We obtained a 30 min spectrum (300V grating) of this source
immediately after imaging. The spectrum displays an absorption
feature around 5230 A, with the flux dropping substantially blueward
of this feature. Associating it with Ly-alpha gives a redshift of
z ~ 3.3 for this proposed afterglow candidate.
A finding chart can be found at:
http://www.dark-cosmology.dk/~brian_j/grb/grb061004.827/
We thank the Paranal staff, especially Chris Lidman and Rachel
Gilmour for excellent support.
GCN Circular 5697
Subject
GRB 061004: Las Campanas Optical/NIR Observations
Date
2006-10-05T17:05:04Z (19 years ago)
From
Edo Berger at Carnegie Obs <eberger@ociw.edu>
E. Berger (Carnegie), S. Gonzalez, and W. Krzeminski (LCO) report:
"We observed the XRT position (GCN 5695) of GRB 061004 (GCN 5694) using
the du Pont 100-inch telescope in K-band and the Swope 40-inch telescope
in r-band. A total of 1215 sec and 1200 sec were obtained in each filter,
respectively, starting on 2006 Oct 5.313 and Oct 5.338 UT. We do not
detect any sources within the XRT error circle to a 3-sigma limit of
K=18.9 mag and r=22.0 mag."
GCN Circular 5696
Subject
GRB061004: Swift/UVOT optical observations
Date
2006-10-05T11:38:35Z (19 years ago)
From
Samantha Oates at MSSL <sro@mssl.ucl.ac.uk>
S. R. Oates (UCL-MSSL), H. Ziaeepour (UCL-MSSL) report on behalf
of the Swift UVOT team
The Swift Ultra-Violet/Optical Telescope (UVOT) began observations of
GRB061004 at 19:51:14 UT, ~51 seconds after the initial Swift BAT trigger
(Ziaeepour et al, GCN 5691). No optical counterpart was detected at the
XRT position (Page et al, 6595) within the coadded images or with the V
and white finding charts.
The 3 sigma upper limits are provided for the coadded images and the
finding charts below:
Filter Start Stop Exposure 3SigmaUL
--------------------------------------------------------
WHITE (finding) 68 168 100 18.4
V (finding) 174 574 400 20.0
V 704 24752 1477 18.9
B 652 13187 928 19.9
U 625 18969 1225 20.5
UVW1 604 18895 2121 21.2
UVM2 580 17988 1357 20.4
UVW2 680 24092 1337 20.5
WHITE 665 6706 520 20.3
--------------------------------------------------------
The values quoted above are not corrected for the expected
Galactic extinction of E_{B-V}=0.049.
GCN Circular 5695
Subject
GRB 061004: Swift-XRT Team Refined Analysis
Date
2006-10-05T01:32:12Z (19 years ago)
From
Kim Page at U.of Leicester <kpa@star.le.ac.uk>
K.L. Page, A.P. Beardmore (U. Leicester) & H. Ziaeepour (UCL-MSSL) report
on behalf of the Swift-XRT Team:
We have analysed the first 2 orbits of Swift-XRT data obtained for GRB
061004 (GCN Circ. 5691; Ziaeepour et al.). Using 3.8 ks of Photon Counting
mode data, we derive a refined position of:
RA(J2000) = 06 31 10.48
Dec(J2000) = -45 54 24.6
with an estimated error radius of 5.5 arcsec (90% confidence, including
boresight uncertainties). This lies 0.7 arcsec from the original XRT
position given in GCN Circ. 5691 and 14.2 arcsec from the
ground-calculated BAT position in GCN Circ. 5694 (Krimm et al).
The X-ray light-curve shows an initial decay of alpha = 1.62 +/- 0.14,
breaking around 375 seconds after the burst to a flatter decay slope of
0.61 +/- 0.10.
The WT spectrum can be modelled using a power-law of Gamma = 1.53 +/-
0.48, absorbed by the Galactic column of 5.7e20 cm^-2. There is no
evidence for any excess NH. The mean observed (unabsorbed) flux over
0.3-10 keV for this spectrum (spanning a time of 68-86 seconds after the
trigger) is 2.83e-10 (3.07e-10) erg cm^-2 s^-1.
If the decay continues with a slope of alpha ~ 0.61, the count rate at 24
hours after the burst is predicted to be 0.012 count s^-1. Using a
conversion factor of 1 count s^-1 being approximately equivalent to
4.6e-11 erg cm^-2 s^-1 (observed flux), this corresponds to a flux of
2.8e-11 erg cm^-2 s^-1 (over 0.3-10 keV). The predicted unabsorbed flux at
24 hours is 3.2e-11 erg cm^-2 s^-1.
This circular is an official product of the Swift-XRT team.
GCN Circular 5694
Subject
GRB 061004, Swift BAT refined analysis
Date
2006-10-05T01:24:26Z (19 years ago)
From
Louis M Barbier at NASA/GSFC/Swift <lmb@cosmicra.gsfc.nasa.gov>
H. Krimm (GSFC/USRA), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (BYU-Idaho), C. Markwardt (GSFC/UMD), D. Palmer (LANL),
A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on behalf of the Swift-BAT team:
Using the data set from T-239.0 to T+595.8 sec from the recent telemetry
downlink,
we report further analysis of BAT GRB 061004 (trigger #232339)
(Ziaeepour, et
al., GCN Circ. 5691). The BAT ground-calculated position is RA,Dec =
97.795,
-45.903 deg {6h 31m 10.8s, -45d 54' 9.8"} (J2000) +- 0.8 arcmin, (radius,
sys+stat, 90% containment). The partial coding was 98%.
The light curve shows a single peak, extending out to T+8 seconds.
T90 (15-350 keV) is 6.2 +- 0.3 sec (estimated error including systematics).
The time-averaged spectrum from T+0.2 to T+8.1 is best fit by a simple
power-law
model. The power law index of the time-averaged spectrum is 1.81 +- 0.10.
The fluence in the 15-150 keV band is 5.7 +- 0.3 x 10-07 erg/cm2.
The 1-sec peak photon flux measured from T+2.67 sec in the 15-150 keV
band is 2.5 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90%
confidence level.
GCN Circular 5693
Subject
GRB 061004: Nearby 2MASS Galaxy
Date
2006-10-04T23:41:40Z (19 years ago)
From
Derek Fox at PSU <dfox@astro.psu.edu>
D.B. Fox (Penn State) reports:
"I note the presence of a 2MASS galaxy, 2MASX J06310985-4554013, at 23
arcsec distance from the XRT position for GRB 061004 (Ziaeepour et
al., GCN 5691). Estimated magnitudes of the host galaxy from the
2MASS catalog, via NED, are: J=15.17(24) mag, H=14.72(35) mag, and
Ks=13.60(22) mag (uncertainties in the least significant digits are
given in parentheses). The proximity of this galaxy to the X-ray
afterglow may be interesting in light of the relatively short 6-s
duration of GRB 061004."
GCN Circular 5691
Subject
GRB 061004: Swift detection of a burst
Date
2006-10-04T20:12:10Z (19 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
H. Ziaeepour (UCL-MSSL), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), S. R. Oates (UCL-MSSL), C. Pagani (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL), G. Sato (GSFC/ISAS),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester) and
D. E. Vanden Berk (PSU) report on behalf of the Swift Team:
At 19:50:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 061004 (trigger=232339). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA,Dec 97.748, -45.894 {06h 30m 60s, -45d 53' 36"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a peak
with a duration of about 6 sec. The peak count rate
was ~5000 counts/sec (15-350 keV), at ~3 sec after the trigger.
The XRT began observing the field at 19:51:30 UT, 60 seconds after the
BAT trigger. XRT found a bright, variable, uncatalogued X-ray source
located at RA(J2000) = 06h 31m 10.52s, Dec(J2000) = -45d 54' 24.0", with an
estimated uncertainty of 6 arcseconds (90% confidence radius).
This location is 124 arcseconds from the BAT on-board position, within
the BAT error circle. The initial flux in the 2.5s image was
4.6e-10 erg/cm2/s (0.2-10 keV).
UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 69 seconds after the BAT trigger. No 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 18.0 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 0.049