General Properties

This burst occurred on May 16, 1998, at 11:23:37 UT (off-line trigger time), and was detected by all GRBM units, in both energy bands, owing to its extraordinary brightness (fig. ).

Figure: GRBM light curves of GRB980516 (from top to bottom: GRBM1, ... GRBM4, AC1,..., AC4.

Unfortunately, no HTR data are available for this burst, because it occurred just 48 s after the passage over the SAGA; hence, the on-board logic was not ready yet to be triggered. The burst temporal structure consists of a sort of precursor, starting at the trigger time, taken as zero-time (fig. ), lasting $\sim$10-15 s, and followed by the huge pulse, whose rise starts at $t\sim 20$ s. An approximate duration estimate for the whole burst is $\sim$80 s, although it has $T_{90} = 16.0 \pm 2.2$ s (see catalog, app. A).

Figure: Zoomed background subtracted light curves of the GRBM 1+2 units; in both energy bands (top: 40-700 keV; bottom: $>$100 keV) the onset is clearly visible at $t\sim 0$ s.

Below the main features of the burst are reported in the automatic format table, after proper dead time corrections and background fit refinements. The total counts over all units are really impressive: $673,000$ and $493,000$ for the 40-700 keV and $>$ 100 keV energy bands, respectively, with a 40-700 keV total peak count rate $\sim$ 109,000 cts/s!

[4]

------------------------------------------------------------------ 
Sun R.A. and Dec.:   52.6    19.0
Sun LS phi  :  21.1
Sun LS theta: -18.9
 
LSra: 157.1   64.5  337.1  244.5
LSde: -21.8   -6.4   21.8    6.4
 
lkGRB[0]         #LSs: 4        Good HRR: 6
Trigger Time (UTC):    41137.7          16 May 1998 11:23:37
Sun visib.: 2
Earth Phi: 194.4        Theta:   1.7
LS E-elev:    9.2    99.1    38.1   -51.8
LS Ecos  :  0.249  -0.968  -0.249   0.968
 
Nsig(trg):    9.5   10.5    2.1    3.0    7.0    7.0    0.9    2.2
Nsig(pfl): 1484.7 1681.2  323.9  396.3  991.4 1155.2  230.9  265.3
Bkg lev. :    721    808    911    850    963    924   1043   1175
ChiSq R. :  1.193  2.272  1.197  1.028  1.259  1.649  1.090  1.182
 
Peak fl. :  39866  47790   9776  11554  30766  35114   7456   9094
Error    :  202.0  221.2  103.7  111.6  178.7  190.4   92.5  101.7
 
Fluence  : 244814 300301  59190  68443 184368 214181  42790  51890
Error    :    521    576    295    307    466    496    273    296
 
Dur  (s) :    58.00    61.00    22.00    29.00
Abundance:       48       49       22       29
H. Ratio :    0.753    0.713    0.723    0.758
HR Ratio :    1.056    1.041    0.993    0.986    0.941    0.954
HR W-ave :    0.732  +/-  0.002
------------------------------------------------------------------

One might ask why BATSE was not triggered by such an impressive burst, given that its sensitivity is better than GRBM's. By looking into the DISCLA data from BATSE, that are continuous ratemeter data and, therefore, are the analogous of the GRBM 1 s ratemeter data, one discovers that the burst occurred during a data gap, lasting about 27 min, probably connected with the CGRO passage over the SAGA. This explains why BATSE did not see this burst.