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Discovery of a new soft gamma repeater source, SGR 0418+5729

ATel #2077; 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, N. Gehrels report for the Swift-BAT team
on 10 Jun 2009; 17:06 UT
Credential Certification: Alexander van der Horst (Alexander.J.VanDerHorst@nasa.gov)

Subjects: Gamma Ray, Soft Gamma-ray Repeater

Referred to by ATel #: 2096, 2121, 2127, 2151, 2152, 2159, 2244

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 #354188). 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.