GRB 120811A
GCN Circular 13618
Subject
Swift Trigger 530581: GRB 120811A or a galactic transient
Date
2012-08-11T02:57:33Z (13 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
B.-B. Zhang (PSU), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC),
C. Gronwall (PSU), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA)
and D. M. Palmer (LANL) report on behalf of the Swift Team:
At 02:35:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB (120811A, trigger=530581). Swift did not slew to
the location. The BAT on-board calculated location is
RA, Dec 257.183, -22.736 which is
RA(J2000) = 17h 08m 44s
Dec(J2000) = -22d 44' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve shows a single peak
with a duration of about 30 sec. The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 sec after the trigger.
This source was matched to a source 4U 1708-23 (== X Oph X-2)
in the on-board catalog, with an offset from
the catalog position of 6 arcminutes. However, the latest
position available is from the 1972 UHURU catalog, which has
a large position uncertainty consistent with this offset.
At this time we cannot determine whether this is a GRB,
the catalogued 4U 1708-23, or some previously-unknown
Galactic transient.
Burst Advocate for this burst is B.-B. Zhang (bbzhang AT 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 13633
Subject
GRB 120811A: Swift-BAT refined analysis
Date
2012-08-12T12:26:25Z (13 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (GSFC/UMBC), M. Stamatikos (OSU),
J. Tueller (GSFC), T. N. Ukwatta (MSU), B.-B. Zhang (PSU)
(i.e. 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 120811A (trigger #530581)
(Zhang, et al., GCN Circ. 13618). The BAT ground-calculated position is
RA, Dec = 257.184, -22.735 deg, which is
RA(J2000) = 17h 08m 44.1s
Dec(J2000) = -22d 44' 06.7"
with an uncertainty of 2.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 98%.
The mask-weighted light curve shows two overlapping peaks starting at
~T-20 sec, peaking at ~T+2 sec, ending at ~T+25 sec with a flat section out to
~T+160 sec. T90 (15-350 keV) is 166 +- 52 sec (estimated error including
systematics).
The time-averaged spectrum from T-14.16 to T+169.06 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.95 +- 0.20. The fluence in the 15-150 keV band is 1.1 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+0.43 sec in the 15-150 keV band
is 1.0 +- 0.1 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/530581/BA/
While the BAT lightcurve and spectral results are consistant with
a GRB origin for this trigger, given that the galactic coordinates
are (lon,lat)=(0.6,10.3) deg, we cannot rule out a galactic source origin.
GCN Circular 13641
Subject
GRB 120811A: Swift/XRT-UVOT Obseravations
Date
2012-08-14T15:57:21Z (13 years ago)
From
Binbin Zhang at PSU <buz12@psu.edu>
B.-B. Zhang (PSU), S. R. Oates (UCL-MSSL), K. L. Page (U Leicester), P.A.
Evans (U. Leicester), D. N. Burrows (PSU), J. R. Cummings (GSFC/UMBC), J. A.
Kennea (PSU),H. A. Krimm (GSFC/USRA), E. Helder (PSU) and S. D. Barthelmy
(GSFC) report on behalf of the Swift-XRT and Swift-UVOT team:
We report further Swift XRT-UVOT observations of GRB 120811A (Zhang et al, GCN
13618).
We performed 9.1 ks follow-up XRT observations of the field at T0+1.6 hr,
T0+0.27 day, T0+0.94 days and T0+3.1 days (where T0 = 2012 Aug 11 at 02:35:18
UT ), in order to determine whether or not the X-ray source had faded. We find
a fading source with count rates in the 0.3-10 keV band as follows :
T-T0 Count Rate
(5.8+/-0.2)x10^3 s 0.05+/-0.01 count/s
(2.2+/-0.01)x10^4 s 0.02+/-0.01 count/s
(8.1+/-0.03)x10^4 s 0.02+/-0.001 count/s
(2.7+/-0.2)x10^5 s < 0.005 count/s
The data are entirely in Photon Counting (PC) mode. Using 1227 s of PC mode
data and 2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
257.16542, -22.71054 , which is equivalent to:
RA (J2000): 17h 08m 39.70s
Dec(J2000): -22d 42' 38.0"
with an uncertainty of 2.8 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay index of
alpha=0.31 (+0.13, -0.12).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+/-0.5). The best-fitting
absorption column is 7.4 (+4.2, -3.0) x 10^21 cm^-2, in excess of the Galactic
value of 1.5 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is
5.5 x 10^-11 (9.7 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 7.4 (+4.2, -3.0) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: 3.2 sigma
Photon index: 1.9 (+/-0.5)
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00530581.
The fact that the XRT source is fading suggests that Swift trigger 530581
(Zhang et al., GCN Circ. 13618) is most likely a GRB.
The Swift/UVOT began settled observations of the field of GRB 120811A 5627 s
after the BAT trigger (Zhang et al., GCN Circ. 13618). No optical afterglow
consistent with the XRT position 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 initial exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white 6243 6443 197 > 20.7
v 6654 6853 197 > 19.1
b 6038 7593 313 > 20.2
u 5832 7468 393 > 20.0
uvw1 5627 7263 393 > 19.9
uvw2 6449 6649 197 > 19.8
The magnitudes in the table are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.77 in the direction of the burst (Schlegel et
al. 1998).
This circular is an official product of the Swift-XRT and Swift-UVOT team.