GRB 060428A
GCN Circular 5018
Subject
GRB060428A: Swift XRT Team refined analysis
Date
2006-04-28T10:45:33Z (19 years ago)
From
Vanessa Mangano at INAF-IASFPA <vanessa@ifc.inaf.it>
V. Mangano (INAF-IASFPA), E. Troja (INAF-IASFPA),
D. N. Burrows (PSU) report on behalf of the Swift XRT Team:
We have analyzed the first orbit data of GRB060428A
(Mangano et al 2006, GCN 5014).
The XRT data set consists of 39 s exposure in Windowed
Timing (WT) mode followed by 1670 s exposure in Photon
Counting (PC) mode.
The refined position of the source is
RA (2000) = 08h 14m 10.98s
Dec (2000) = -37d 10' 10.3"
with an uncertainty of 3.6 arcsec (90% confinement).
This position is only 1.5 arcsec from the onboard
detected position (Mangano et al 2006, GCN 5014).
The 0.2-10 keV X-ray light curve shows an initial very
fast decay with slope -6.3 +/- 0.4, a break at 144 +/- 6 s
from the trigger and a flattening with a decay slope
of -0.11 +/- 0.03. A small flare with a gaussian profile
is present at 676 +/- 56 s after the trigger.
The WT and PC spectra were fitted separately with an
absorbed power law model and showed photon indices of
3.2 +/- 0.4 and 1.9 +/- 0.2, respectively (90% confidence level).
The WT spectrum, covering the time interval 77-116 s, mostly
accounts for the pre-break phase of the light curve.
Both spectra are highly absorbed, but do not show evidence of
an absorption column in excess with respect to the Galactic
value of 8.5 x 10^21 cm^-2.
The 0.2-10 keV unabsorbed fluxes of the two spectra are
2.9 x 10^-9 and 7.6 x 10^-11 erg cm^-2 s^-1, respectively.
If decaying at the present rate the source will be at the
flux level of 2.7 x 10^-11 erg cm^-2 s^-1 (corresponding to
a count rate level of 0.18 counts s^-1) after one day.
This Circular is an official product of the Swift XRT Team.
GCN Circular 5022
Subject
GRB 060428A: Refined analysis of the Swift-BAT burst
Date
2006-04-28T13:34:17Z (19 years ago)
From
Scott Barthelmy at NASA/GSFC <Scott@lheamail.gsfc.nasa.gov>
C. Markwardt (GSFC/UMD), L. Barbier (GSFC), S. Barthelmy (GSFC),
J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (BYU-Idaho), H. Krimm (GSFC/USRA), V. Mangano (INAF-IASFPA),
D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC),
G. Sato (ISAS), J. Tueller (GSFC)
on behalf of the Swift-BAT team:
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 060428A (trigger #207364)
(Mangano, et al., GCN 5014). The BAT ground-calculated position is RA,Dec =
123.535,-37.169 deg {8h 14m 8.5s, -37d 10' 8.6"} (J2000) +- 1.0 arcmin,
(radius, sys+stat, 90% containment). The partial coding was 65%.
The mask-weighted lightcurve shows a bright peak (starting at T-8 sec until
T+20 sec with the peak at T+5 sec) and a weaker peak (T+22 sec until T+42 sec).
T90 (15-350 keV) is 39.4 +- 2 sec (estimated error including systematics).
The time-averaged spectrum from T-6.8 to T+43.0 is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
2.04 +- 0.11. The fluence in the 15-150 keV band is
1.4 +- 0.1 x 10^-6 erg/cm2. The 1-sec peak photon flux measured
from T+4.41 sec in the 15-150 keV band is 2.4 +- 0.2 ph/cm2/sec.
All the quoted errors are at the 90% confidence level.
GCN Circular 5024
Subject
GRB060428A: Swift/UVOT observations.
Date
2006-04-28T15:30:29Z (19 years ago)
From
Massimiliano de Pasquale at MSSL-UCL <mdp@mssl.ucl.ac.uk>
M. De Pasquale (UCL-MSSL), V. Mangano (IASF Palermo) report on
behalf of the Swift/UVOT team:
The Swift/UVOT began taking data on the field of GRB060428A
at 03:23:48 UT on 2006-04-28, 60 s after the BAT trigger
(Mangano et al., GCN 5014), with the settling exposure. In the
refined XRT error circle (Magano et al., GCN 5018), we find
a weak (4 sigma detection) optical source in the V band at
about V=20, not present in USNO B1.0 catalogue. This source
however appears to be constant throughout the observation,
thus further observations will be needed to determine if it
is the optical afterglow of this burst.
No other afterglow candidate was detected at the refined XRT
position in summed images from any of the filters down to the
following three-sigma upper limits.
Filter T_range(s) Exp(s) 3sig_UL
V 60-7526 1657 20.2
B 548-7116 442 20.5
U 524-13380 436 20.5
UVW1 500-12666 1358 20.0
UVM2 476-11758 1425 19.9
UVW2 563-7322 472 19.5
These upper limits are uncorrected for the strong estimated
Galactic reddening of E_{B-V} = 1.25 mag (Schlegel et al. 1998).
GCN Circular 5033
Subject
GRB 060428A: PROMPT Observations
Date
2006-04-29T02:15:04Z (19 years ago)
From
Josh Haislip at U.North Carolina <haislip@physics.unc.edu>
J. Haislip, M. Nysewander, A. Foster, A. LaCluyze, D. Reichart, J. A.
Crain, K. Ivarsen, J. Kirschbrown, C. MacLeod, A. Trotter, and J. Carpenter
report on behalf of the UNC team of the FUN GRB Collaboration:
Skynet observed the localization of GRB 060428A (Mangano et al., GCN 5014)
with three of the 16-inch PROMPT telescopes at CTIO beginning 58 sec after
the burst (44 sec after notification) in Ug'r'i'z' (three simultaneously).
No new source is found to z' > 17.1 mag (3 sigma; 10 sec exposure) at 63
sec after the burst and z' > 19.1 (3 sigma; 4 x 40 sec exposures) at 5.3
min after the burst.
PROMPT is currently being built and commissioned.
GCN Circular 5111
Subject
GRB 060428a, SMARTS optical/IR afterglow detection
Date
2006-05-10T23:56:25Z (19 years ago)
From
Bethany Cobb at Yale U <cobb@astro.yale.edu>
B. E. Cobb (Yale), part of the larger SMARTS consortium, reports:
Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 060428a
(GCN 5014, Mangano et al.) with a mid-exposure time of
2006-04-29 00:17 UT, which is ~20.9 hours post-burst and
again at 2006-04-30 23:46 UT (~68.4 hours post-burst).
For each observation total summed exposure times amounted
to 36 minutes in I and 30 minutes in J.
In our first epoch images, a source is visible in both I and J
within the X-ray error region reported by Mangano et al. (GCN 5018).
This source may correspond to the V-band source detected by
De Pasquale & Mangano (GCN 5024) in the Swift UVOT imaging.
The coordinates are:
RA: 8:14:10.8
DEC: -37:10:11.4
The source is absent in our second epoch images.
Its transient nature (see magnitudes below)
suggests that the source is the afterglow of GRB 060428a.
time
post-burst I magnitude J magnitude
------------------------------------------------------
20.9 hours 21.14 +/- 0.16 18.98 +/- 0.14
68.4 hours >22.2+/-0.2 >19.6+/-0.2
These preliminary magnitudes were calibrated using several USNO-B1.0
stars in the I-band and several 2MASS standards in J.
The afterglow decay index is constrained by our observations to be
alpha >~ -0.8 from ~1 to 3 days post-burst. Because no optical
source was detected with magnitude <20 at only minutes post-burst
(see GCN 5033, Haislip et al. [z'>19.1 at 5.3 minutes] and GCN 5024, De
Pasquale & Mangano [V=20 at 1 minute]), our I-band detection at 20.9 hours
suggests that the early time decay rate of the afterglow must have been
significantly more shallow than -0.8. This may partially account for
the lack of detected variability of the UVOT source at early times.
Possibly, a light curve break occurred between our two sets of observations.