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GCN Circular 9499

Subject
Discovery of a new soft gamma repeater source, SGR 0418+5729
Date
2009-06-09T04:24:27Z (15 years ago)
From
Alexander van der Horst at NASA/MSFC <Alexander.J.VanDerHorst@nasa.gov>
A. J. van der Horst, V. Connaughton, C. Kouveliotou, M. S. Briggs, and
W. S. Paciesas report for the Fermi/GBM team,

V. Pal'shin, S. Golenetskii, R. Aptekar, D. Frederiks, E. Mazets,
P. Oleynik, D. Svinkin, M. Ulanov, and K. Hurley report for the Konus-RF 
team,

J. R. Cummings, D. Palmer, and N. Gehrels report for the Swift-BAT team

"The Fermi Gamma-Ray Burst Monitor (GBM) has triggered on two SGR-like
bursts on June 5 at 20:40:48.88 (trigger 265927250 / 090605862) and
21:01:35.06 UT (trigger 265928497 / 090605876). Their on-ground calculated
locations, using the Fermi GBM trigger data, are:
- RA, Dec (J2000) = 70.0, +55.6 (equivalent to 4h40m, +55d35'), with a
1-sigma statistical uncertainty of 4.8 degrees, and
- RA, Dec (J2000) = 60.5, +55.4 (equivalent to 4h02m, +55d22'), with a
1-sigma statistical uncertainty of 6.0 degrees, respectively.
For both triggers there is additionally a systematic error which is
currently estimated to be 2 to 3 degrees.
The angles from the Fermi LAT boresight are 81 and 66 degrees, 
respectively.

The time-averaged GBM spectra of both triggers are best fit by a power law
with a high-energy exponential cutoff. The indices are -0.26 +/- 0.39 and
-0.38 +/- 0.76, and the cutoff energies, parametrized as Epeak, are
36.0 +/- 2.4 and 22.9 +/- 3.3, for the first and second trigger, 
respectively.
The first event was a single pulse of 0.03 seconds, while the second
trigger was double-peaked with a duration of 0.09 seconds.

The first burst was also seen by Konus-RF which was triggered at
2009-06-05 74448.88972 s UT (20:40:48.890).
We have triangulated this burst to a Konus-RF - GBM annulus centered at
RA, Dec (J2000) = 84.314 (05h 37m 15s), -6.669 (-6d 40' 07"), with a
radius of 66.544 +/- 4.812 deg (3 sigma). This annulus is consistent with
the two on-ground calculated GBM locations. The nearby, confirmed SGR
source, SGR 0501+4518 (RA, Dec (J2000) = 75.265, +45.272) is ~12 degrees
away from the centerline of this annulus and is, therefore, excluded as the
origin of these two new emissions.

Swift BAT triggered on the first GBM event (BAT trigger #354187). No
source was found onboard. After the alert through the IPN network,
the event data were analyzed on the ground, and a low-significance
source was found at a position consistent with the GBM position and
IPN annulus. The position is RA, Dec (J2000) = 64.606, +57.489,
equivalent to
RA(J2000) = 04h 18m 25s
Dec(J2000) = +57d 29' 16''
with an uncertainty of 4 arcmin (radius, statistical+systematic, 90%
containment). As seen in BAT, the burst T90 was 3.5 +/- 1.0 ms.
There was no emission seen above 100 keV.

We have also used the BAT light curve to obtain a BAT-GBM annulus
centered at RA, Dec (J2000) = 129.334 (08h 37m 20s), +0.546 (+0d 32' 45"),
with a radius of 76.480 +/- 1.452  deg  (3 sigma).  The annulus contains
the Swift localization and is consistent with both GBM locations.
The second GBM event was seen very weakly in BAT, but did not cause a
rate trigger, so no further data are available.

A map of the GBM, BAT locations, and the Konus-RF - GBM and BAT-GBM
annuli can be obtained at:
http://gammaray.nsstc.nasa.gov/gbm/science/magnetars.

The Galactic coordinates of the Swift location are L, B = 148.00, 5.07 deg,
placing the source on the galactic plane. We tentatively propose this as 
a new
soft gamma repeater source, SGR 0418+5729."

[GCN OPS NOTE(09jun09): Per author JC's request, the  BAT Trigger Number
in 4th paragraph was changed fro 351188 to 351187.]
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