- GCN Circular #9912
J. R. Cummings (NASA/GSFC/CRESST) and
C. B. Markwardt (NASA/GSFC/CRESST)
on behalf of the Swift-BAT team
At 15:35:36 Swift-BAT triggered on GRB 090915. This was
probably the same event as Fermi-GBM trigger 274721737. No
source was found onboard. A weak but significant source was
found in ground analysis, 15 degrees from the GBM ground-
calculated (2nd version) position as reported at
http://gcn.gsfc.nasa.gov/fermi_grbs.html. The GBM position
was also in the BAT FOV.
The BAT ground-calculated position is
RA, Dec = 237.990, +15.480 deg which is
RA(J2000) = 15h 51m 57.6s
Dec(J2000) = +15d 28' 48"
with an uncertainty of 3 arcmin, (radius, sys+stat, 90%
containment). The partial coding was 12%.
The mask-weighted light curve shows a single weak peak.
T90 (15-350 keV) is 8 +- 2 sec
(estimated error including systematics).
The time-averaged spectrum from T-3 to T+5 sec is best fit
by a power law. This fit gives a photon index 1.0 +- 0.2.
For this model the total fluence in the 15-150 keV band
is 5 +- 2 x 10-7 erg/cm2 and the 1-sec peak flux measured
from T+2 sec in the 15-150 keV band is 0.8 +- 0.3 ph/cm2/sec.
All the quoted errors are at the 90% confidence level.
A Swift TOO has been requested and approved. The narrow-
field instruments will observe the location approximately
13 hours after the burst.
- GCN Circular #9914
A. P. Beardmore, K. L. Page (U. Leicester) and T. Sakamoto (NASA/UMBC)
report on behalf of the Swift-XRT team :
The Swift-XRT started observing the field of GRB 090915 (Cummings and
Markwardt, GCN Circ. 9912) at 2009-09-16 03:59 UT, 12.4 hours after
the trigger. In 5ks of photon counting mode data, spanning two orbits,
we detect a source which fades from 0.0099 +/- 0.0022 count/s to
0.0033 +/- 0.0016 over a interval of 5.9ks.
Using 1678 s of XRT Photon Counting mode data and 1 UVOT image,
we find an astrometrically corrected X-ray position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue) for the source of RA, Dec (J2000) = 238.02021, 15.48775
which is equivalent to :
RA (J2000): 15 52 4.85
Dec (J2000): +15 29 15.9
with an uncertainty of 3.3 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
The spectrum of the data from 44.6 ks to 52.8 ks after the trigger can
be fit by an absorbed powerlaw with a photon index of 1.20 +0.97/-0.54
and a column density upper limit of 1.4 x 10^21 cm^-2, consistent with
the Galactic value of 3.2 x 10^20 cm^-2 in the direction of the
burst. The observed 0.3-10 keV flux for this spectrum is 4.9 +3.3/-4.9
x 10^-13 erg cm^-2 s^-1 which corresponds to an unabsorbed 0.3-10 keV
flux of 5.1 x 10^-13 erg cm^-2 s^-1.
This circular is an official product of the Swift-XRT team.
- GCN Circular #9918
E. A. Hoversten (PSU) and T. Sakamoto (NASA/UMBC) report on behalf of
the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 090915
44647 s after the BAT trigger (Cummings & Markwardt, GCN Circ. 9912).
A faint source is detected with 5.6-sigma confidence in the summed
UVOT observations at the position RA, Dec (J2000) = 238.02016,15.48772
which is equivalent to:
RA (J2000) = 15:52:04.84
Dec (J2000) = 15:29:15.8
with an estimated uncertainty of 0.8 arcsec (radius, 90% confidence).
This position is 0.2 arcsec from the UVOT-enhanced XRT position
(Beardmore, et al. GCN Circ. 9914) and the two positions agree within
the error bars. It is unclear at this point if the source is fading,
but follow-up Swift observations are being planned.
The observed magnitudes using the UVOT photometric system (Poole et
al. 2008, MNRAS, 383, 627) for the initial exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
=======================================================================
white 45492 52117 1647 22.15 +/- 0.24 (4.7 sigma)
v 46338 52837 1583 21.08 +/- 0.39 (2.8 sigma)
u 44647 51275 1647 21.40 +/- 0.43 (2.5 sigma)
The values quoted above are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.05 in the direction of the burst
(Schlegel et al. 1998).
- GCN Circular #9920
Adria C. Updike, Dieter H. Hartmann (Clemson University), Brian Murphy
(Butler University), and Kenneth Rumstay (Valdosta State University)
report:
We observed the field of GRB 090915 (Cummings & Markwardt GCN 9912,
Fermi-GBM trigger 274721737) using the 0.9m SARA telescope located at Kitt
Peak National Observatory beginning 44173 seconds after the burst and
continuing for about half an hour. Observations were carried out under
good weather conditions at high airmass.
In 21 minutes of stacked exposures, we marginally detect a source within
the XRT error circle (Beardmore, Page, & Sakamoto, GCN 9914) at R ~ 20.5
+/- 0.3 (as compared to field stars using the USNO B1.0 catalog). The
object is not found in the DSS catalog. Variability can not be
established from these single-epoch observations. It is likely that the
above source is the same identified by Hoversten & Sakamoto (GCN 9918).
To establish the decay, follow-up observations are encouraged.
This message may be cited.
- GCN Circular #9923
P. Afonso (MPE Garching), A. Updike (Clemson University), J. Greiner and R.
Filgas (MPE Garching) report on behalf of the GROND team:
GROND (Greiner et al. 2008, PASP 120, 405), mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile), observed the field of GRB 090915
(Cummings and Markwardt 2009, GCN #9912) simultaneously in the g'r'i'z'JHK
bands.
With 8 min total integration time in JHK and 7.7 min in g'r'i'z',
observations started on September 17, at 00:19 UTC (32.7 h = 117.8 ksec
after the burst) and were done at high air mass and under variable sky
conditions.
Inside the enhanced XRT error circle we do not detect the object reported
by Beardmore et al. (GCN #9914), Hoversten and Sakamoto (GCN #9918) and
Updike et al. (GCN #9920), down to the following limiting magnitudes (all
in the AB system):
g' > 23.7
r' > 23.2
i' > 22.7
z' > 21.3
J > 21.1
H > 20.2
K > 19.2
The upper limits were obtained using SDSS and 2MASS catalogs
field stars as reference. No correction has been made for the expected
extinction corresponding to E(B-V) = 0.05 mag in the direction of the burst
(Schlegel et al. 1998).
The non-detection of the source suggests this was indeed the
afterglow of GRB 090915.
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