Examples of non-triggered Bursts

Figure: Time profiles of NTB 010518 in both energy bands of unit 3, that detected it: this burst did not trigger the on-board logic, because too faint; nevertheless, it triggered the SWTC 4, making it possible to discover the faint X-ray counterpart in the WFC 2 (GCN 1062).

In figg. , and some examples of NTBs detected with late SWTCs are shown. In order to give an idea of the various brightnesses in the different units and energy ranges, the background level and the +2$\sigma$ level over the background are plotted, as they are automatically calculated by the GRB quest: the NTB980511 (fig. ) and NTB 961116 (fig. ) have been caught only with the off-line quest (at that time the on-line quest was not operating yet), while the NTB010518 (fig. ) was detected within a few hours after its occurrence.

Figure: Time profiles of NTB 980511, also detected by BATSE (# 6753); this GRB triggered SWTC 3, thanks to the counts in the units nn. 2 and 3, the two brightest ones (this agrees with the BATSE direction, $\phi=44$$^\circ$, $\theta=-15$$^\circ$). A small data gap is visible $\sim 30$ s before the GRB. The background fits and the +2$\sigma$ levels are overplotted, as well. The two intervals around the burst, used for fitting, are highlighted.

The case of GRB010518 (fig. ) is particularly important: it happened on May 18, 2001, UT 06:43:10, lasting $\sim 25$ s in the 40-700 keV range and $\sim 30$ s in the WFC range (2-28 keV): this burst, in the FOV of WFC 2, was too faint to trigger the GRBM on-board logic, but it triggered SWTC 4 of the on-line quest, which automatically alerted the duty scientists at the BeppoSAX-SOC: indeed, the WFC 2 images localized a diffuse, unknown faint faiding source, whose position was distributed with an error radius of $\sim 5$$^\prime$ a few days later (GCN 1062) and not earlier, because of the long time needed to image such a faint burst (see fig. ).

The NTB980511 (fig. ) occurred at UT 01:55:26 and did not trigger the on-board logic, because too faint; it nevertheless triggered the SWTC 3; BATSE, that detected it as well (trig. 6753), gave an estimate of the arrival direction, which is consistent with the pointing directions of the two brightest GRBM units, 2 and 3: in the BeppoSAX local frame of reference, $\phi=44$$^\circ$, $\theta=-15$$^\circ$(statistical error radius of $\sim 5$$^\circ$).

Eventually, the NTB961116 (fig. ), occurred at UT 20:33:09, triggered the SWTC 2 in the units 1 and 2, and it was a short duration burst: very unfortunately, since a spike triggered the on-board logic just 100 s earlier, the HTR counts stopped exactly 2 s before the burst, thus depriving us of the knowledge of its ms temporal structure.

Figure: Time profiles of the short NTB 961116; this GRB triggered SWTC 2 in the units nn. 1 and 2. The background fits and the +2$\sigma$ levels are overplotted, as well. The two intervals around the burst, used for fitting, are highlighted. The presence of a spike at $t=-100$ s little biases the fit in the cases of GRBM1 and GRBM4; this spike triggered the on-board logic ``just too early'', preventing the GRBM from acquiring HTR data of the NTB.