SGR1900+14: The interesting Case of 980901

The SGR1900+14 burst occurred on September 1, 1998, UT 17:06:27.06, was detected by both the GRBM and Konus/WIND. Owing to its fluence and duration, it has been classified as ``unusual'' burst; to some extent, it could be considered as an intermediate case between typical short duration bursts and powerful giant flares, lasting several dozen seconds. According to the SGR1900+14 bursting activity as monitored by Konus, a similar intense activity with multiple bursts has been detected on May 30, 1998, between 09:03 and 09:08 UT ([Aptekar, et al., 2001]), when, unfortunately, the BeppoSAX spacecraft was just passing over the SAGA, and the GRBM was consequently switched off.

As for the Sep 1 bursts, Konus was first triggered at 17:00:32.6, followed by these next bursts: 17:03:49.2, 17:06:25.1; since the Konus trigger-mode record lasts about 4 min, the latter could not have been acquired with HTR ([Aptekar, et al., 2001]); nevertheless, the GRBM did it! In order to better show the complex temporal structure of the overall burst, figg.  report the entire time profile (3), and the single profiles within three slices (3a), (3b), (3c), respectively, where ``3'' is the GRBM unit number.

Figure: GRBM3 Time Profiles of 980901, UT 17:06:27.06, from SGR1900+14: the overall profile (3) is split into three slices: (3a), (3b), (3c).

Figure: GRBM average spectrum of 980901, 17:06:27.06 UT, from SGR1900+14. The best fit with a black body law yields $kT = 16.8 \pm 1.3$ keV, $\chi^2 = 0.81$.

Furthermore, this burst was so bright, that it has been possible to extract its GRBM 240 channel spectrum, integrated over 128 s from 17:06:07.91 to 17:08:15.91 UT, i.e. from -19.15 s to 108.85 s with respect to the GRBM trigger time. The spectrum, properly grouped, has been fitted wih a simple black body law (fig. ), with a $\chi^2 = 0.81$ (19 dof); the black body temperature has come out to be $kT = 16.8 \pm 1.3$ keV (90% CL). The 50-100 keV fluence is $(0.7 \pm 0.1)\times 10^{-5} \rm erg\ \rm cm2$. These estimates seem to be quite consistent with the Konus values: $kT \sim 22 \pm 1$ keV (optically thin thermal bremsstrahlung), with a (10-200 keV) fluence of $\sim 2 \times 10^{-5} \rm erg\ \rm cm2$ ([Aptekar, et al., 2001]).