Gamma-ray Burst 090118
(All information courtesy of the instrument teams.)
Previous IAU Circulars
Results of Observations
- GCN Circular #8825
A. Copete, J. Grindlay (Harvard)
S. Barthelmy, C. Markwardt, N. Gehrels (NASA/GSFC)
We report the detection of a likely GRB candidate by the BAT Slew
Survey (BATSS), pending confirmation by Swift XRT, in the slew that
started at 13:54:02 UT and lasted 94 sec. The ground-calculated
position is RA, Dec = 49.865, +18.478 deg, which is
RA (J2000) = 03h 19m 28s
Dec(J2000) = +18d 28' 40"
with an uncertainty of 3.69 arcmin (90% confidence, including
systematics). The detection was triggered by simultaneous detections
of 6.9 sigma and 8.0 sigma from imaging in the overlapping energy
bands of 15-50 keV and 15-150 keV, respectively. The burst mask-tagged
lightcurve in the 15-150keV band shows a broad emission peak starting
at T+66 sec, lasting 19 sec, and peaking 3 times at T+71, T+74 and t+ T
+81 sec. T90 = 16sec and T50 = 8sec.
The time-averaged spectrum over the exposure time of the source is
best fit by a simple power law with photon index 1.35, and the fluence
in the 15-150 keV band is 4.0 x 10^-7 erg/cm^2.
A follow-up Swift ToO observation has been scheduled to begin at 20:52
UT, 6.97 hours after the burst. Optical/IR observers are encouraged to
image this field as soon as possible as it is an early-evening object.
- GCN Circular #8826
F. Olivares (MPE Garching), A. Rossi (Tautenburg Obs.), A. Kupcu Yoldas (ESO), J. Greiner, and A. Yoldas (both MPE Garching) report on behalf
of the GROND team:
We observed the field of the possible GRB 090118 (Swift BATSS trigger, Copete et al. GCN
#8825) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405)
mounted at the 2.2m MPI/ESO telescope at La Silla (Chile).
Observations started at 00:55 UT, about 11 hrs after the GRB.
Comparison to the Digital Sky Survey reveals no new sources within the Swift/BAT error circle
(Copete et al. GCN #8825). Co-added images of the first 8 minutes yield the following 5-sigma limits (in AB magnitudes):
r' > 23.2
i' > 22.5
The magnitudes are calibrated against SDSS.
- GCN Circular #8827
P. Schady (MSSL-UCL) reports on behalf of the Swift/UVOT team:
The Swift/UVOT began white-band observations of the field of the
likely GRB candidate detected in the BAT Slew Survey (BATSS), GRB
090118 (Copete et al., GCN 8825) at 2009-01-018 20:55:35 UT, 25.3 ks
after the burst. We do not detect any new source within the Swift BAT
error circle (Copete et al., GCN 8825) in a co-added white-band image
down to the following 5-sigma upper limit:
Filter T_start T_stop Exp Mag
(ks) (ks) (s) (5-sigma upper limit)
white 25.3 35.3 1292 > 21.1
The quoted upper limits have not been corrected for the expected
Galactic extinction along the line of sight of E_(B-V) = 0.13 mag. All
photometry is on the UVOT photometric system described in Poole et al.
(2008, MNRAS, 383, 627).
- GCN Circular #8828
GRB 090118: Swift-XRT observations
B.A. Rowlinson and K.L. Page (U. Leicester) report on behalf of the
Swift-XRT began a 6 ks follow-up ToO observation of the likely BAT
Slew Survey GRB 090118 (Copete et al., GCN Circ. 8825) 25.3 ks after the
start of the slew during which the burst was detected. No X-ray source
is detected within the 3.69 arcmin error circle, with a 3-sigma upper
of 5.8x10^-3 count s^-1.
However, there is a source just outside the 90% error circle, at a
position of RA, Dec =3D 49.8276, 18.4148, which is equivalent to:
RA (J2000) =3D 03 19 18.62
Dec (J2000) =3D +18 24 53.3
with an uncertainty of 7.1 arcsec (radius, 90% confidence). This is a
distance of 260.9 arcsec from the BATSS position.This source has a
count rate of (9.0 +/- 2.2)x10^-3 count s^-1 in the first 1.6 ks of data
and appears to be fading to a count rate of (5.0 +/- 1.4)x10^-3 count
s^-1 in the last 4.2 ks of data.
A spectrum extracted for this source can be fitted with a power-law of
spectral index Gamma =3D 5.9 (+1.8, -1.4) with an absorbing column density
of 4.6 (+3.1, -2.2) x 10^21 cm^-2, in excess of/consistent with the
Galactic column in this direction of 8.9x10^20 cm^-2.
This circular is an official product of the Swift-XRT team.
- GCN Circular #8829
Adria C. Updike and Dieter H. Hartmann (Clemson University) report:
Using the KPNO 4m telescope and the NEWFIRM instrument, we observed the
field of GRB 090118 (GCN 8825, Copete et al.) beginning 11.6 hours after
the burst in the J and H bands. Within the XRT error circle (Rowlinson &
Page, GCN 8828) we detect a faint source at coordinates
Variability has not been confirmed. Further observations are planned.
Time after burst Band Magnitude
11.6 hours J 21.0 +/- 0.5
12.5 hours H 20.2 +/- 0.4
A total of 40 minutes of exposures were taken in each band and the results
were stacked. Magnitudes are calibrated to the 2MASS point source catalog
and against a nearby star (RA 03:19:16.7, Dec +18:25:14, J = 14.05). No
correction has been made for galactic extinction.
- GCN Circular #8830
T. Sakamoto (UMBC/GSFC), D. Donato (ORAU/GSFC), N. Gehrels (GSFC),
T. Okajima (JHU/GSFC), T. N. Ukwatta (GWU/GSFC), Y. Urata (Saitama U),
C. Wallace (FGCU)
We observed the field of the possible GRB 090118 detected by the BAT Slew Survey
(Copete et al., GCN #8825) with the 14-inch Goddard Robotic Telescope (GRT)
located at the Goddard Geophysical and Astronomical Observatory
Thirteen set of 30 sec and 60 sec exposures were taken in the R filter
starting from Jan. 19 00:38:11 (UT) about 10.5 hr after the trigger. We
do not detect the optical afterglow both in the individual images and the
combined image within the BATSS error circle and also the X-ray source
found by the Swift/XRT (Rowlinson et al., GCN #8828).
The estimated three sigma upper limit of the combined image (total
exposure of 1170 sec) is ~17.6 mag using the USNO-B1 catalog.
- GCN Circular #8831
Adria C. Updike and Dieter H. Hartmann (Clemson University) report:
We observed the field of GRB 090118 (GCN 8825, Copete et al.) for a second
epoch beginning 36.7 hours after the burst. Our observations consisted of
80 min of J band exposures. In the stacked image, we still detect the
candidate (GCN 8829, Updike & Hartmann). We re-analyzed our first epoch
image relative to 17 2MASS field stars, as well as our second epoch image.
The brightness of the candidate source appears to be constant within
photometric uncertainties. While slow fading can't be ruled out, the
two-epoch results suggest that most likely the candidate source is
not the afterglow of GRB 090118. Third epoch observations are planned.
Time after burst Band Magnitude Exp time
11.6 hours J 21.2 +/- 0.6 40 min
36.7 hours J 21.4 +/- 0.6 80 min
The images have been placed on the following site.
- GCN Circular #8832
Y. Minowa, T.-S. Pyo, H. Terada, K. Aoki (Subaru, NAOJ), and
N. Kawai (Tokyo Tech) report on behalf of the Subaru GRB team:
We observed the field of GRB 090118 (GCN 8825, Copete et al.) with
IRCS on the 8.2 m Subaru Telescope in J band beginning 39.2 hours
after the burst (18 min exposure, mid-time 05:10 UT, Jan. 20).
The afterglow candidate reported by Updike et al. (GCN 8829, 8831)
was detected with J=21.3 mag, where the magnitude is estimated using
nearby 2MASS stars.
- GCN Circular #8840
B.A. Rowlinson and K.L. Page (U. Leicester) report of behalf of the
Swift-XRT completed a 10ks follow-up ToO obsevation of GRB 090118
(Copete et al. GCN Circ. 8825) approximately 250ks after the burst.
These observations confirm the previous Swift-XRT observations
(Rowlinson & Page GCN Circ. 8828) that a fading source has been detected
with a decay slope of alpha=0.61 +/- 0.3. This source is confirmed as
- GCN Circular #8850
A. Rossi (Tautenburg Obs.), T. Kruehler, J. Greiner and F. Olivares
(all MPE Garching), report on behalf of the GROND team:
We report on further analysis and new observations of the optical afterglow
candidate of GRB 090118 (Copete et al. 2009, GCN #8825).
We re-analyzed our first epoch images (Olivares et al. 2009, GCN Circ. 8826),
taken at about 11 hrs after the GRB. No source is seen in our NIR images at
the position of the X-ray afterglow (Rowlinson et al. 2008, GCN Circ. 8828),
giving upper limits (Vega system) of
J > 20.6
H > 19.6
K > 18.7
calibrated against 2MASS field stars.
The XRT position (Rowlinson et al. 2009, GCN Circ. 8828) is outside of the
field-of-view of the g'r'i'z' detectors, so no statement can be made on
the optical brightness during our first epoch.
We observed the field of GRB 090118 again with GROND on 2009-01-22 at a
mid-time of 01:30 UTC, about 100 hours after the burst. We clearly detect
the candidate mentioned by Updike and Hartmann (2009, GCN Circ. 8829) in
our optical filters, but not in the NIR:
g'= 23.46 +- 0.11 (AB)
r'= 23.27 +- 0.10 (AB)
i'= 22.95 +- 0.12 (AB)
z'= 22.25 +- 0.14 (AB)
J > 21.5 (Vega)
H > 20.5 (Vega)
K > 19.6 (Vega)
No correction for the foreground extinction of E(B-V)=0.135 mag
has been applied.
Comparing our g'r'i'z' values with the J and H detections of Updike & Hartmann
(2009a; GCN Circ. 8829) and Minowa et al. (2009; GCN Circ. 8832) about two
days earlier, we find that the spectral energy distribution is smooth, thus
supporting the non-fading nature of this object as indicated already by the
results of Updike & Hartmann (2009b; GCN Circ. 8831) and Minowa et al.
(2009; GCN Circ. 8832).
The detection in g'r'i'z' implies that the object, whatever its nature,
is at a redshift smaller than 3.5.
This excludes the possibility that this object is at high redshift and
exhibited a long plateau phase. The plateau phase would have to last more
than 3 days in the NIR range, while at X-rays a steady decline has been
seen (Rowlinson & Page 2009, GCN Circ. 8840). We conclude that this object
is very likely not the optical/NIR afterglow of GRB 090118. Instead, the
object's SED is consistent with a low-redshift galaxy.
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Jochen Greiner, last update: 24-Jan-2009