Next: GRBM Efficiency with Direction
Up: GRBM Detection Efficiency of
Previous: GRBM Detection Efficiency of
  Contents
In order to estimate the number of GRBs that have been detected by BATSE,
and missed by the GRBM, the overall BATSE 4B catalog ([Paciesas et al., 1999]) has
been analysed, to select all the GRBs that should have been detected by the
GRBM. For this selection, some points have been considered: first of all,
the selection only regarded those GRBs occurred during the time intervals covered
by the GRBM 1 s ratemeters currently available, from which this GRBM catalog
has been extracted. A further selection concerned the bursts not Earth-blocked:
the visibility (sec.
) of each burst has been calculated
by using the BATSE direction and the GRBs have been split into four classes:
``surely visible'', ``surely occulted'', ``probably visible'', and
``probably occulted'', according to the visibility definition.
Only the ``surely visible'' GRBs have been chosen, i.e. those GRBs, whose
entire error circle is not Earth-blocked.
On the GRB sample so selected, a final selection, aimed to exclude all the
GRBs occurred in the nearby of the SAGA, where the off-line quest could not
work, has been applied.
At this stage, both the number of bursts detected with the GRBM by applying
the SWTCs and the number of BATSE missed GRBs by the GRBM are known;
typically, there are two possibilities for a burst to be missed by the off-line
quest within the GRBM data: first, the BATSE burst brightness is below the
GRBM sensitivity, so that no signal can be found at the burst time in the
GRBM data; second, the BATSE burst can be identified by visual inspection within
the GRBM ratemeters, but it nevertheless did not trigger any off-line quest.
The comparison between the number of detected bursts and the number of
expected bursts, according to the BATSE catalog, therefore allows to
estimate the GRBM off-line trigger efficiency, accounting for both the
GRBM intrinsic sensitivity and for the off-line quest algorithms used to
extract this catalog of GRBs.
The results of this procedure are summarized in
table
.
Table:
GRBM & BATSE GRBs: detected and expected.
Catalog |
# Expected |
# Detected |
# Missed |
 |
BATSE 4B |
409 |
283 |
126 |
% |
Kommers' |
53 |
18 |
35 |
% |
Stern's |
223 |
61 |
162 |
% |
Total |
685 |
362 |
323 |
% |
In table
the parameter
expresses the GRBM off-line quest efficiency, and it is calculated as the ratio
of the number of off-line detected bursts over the number of expected bursts.
This estimate has been calculated also in the cases of the two BATSE non-triggered
bursts taken into consideration previously.
From BATSE 4B to Stern's catalog, the efficiency
falls from an initial
69% down to
27%; if one accepts the
assumption that the non-triggered catalogs are affected by a negligible
contamination of non-burst events, the following observations can be made:
from BATSE 4B to Stern's, the minimum peak flux decreases down to
photons s
cm
([Stern et al., 2000b]), and correspondingly
the number of GRBs, whose peak fluxes are below the GRBM sensitivity, increases,
thus lowering the peak-flux-averaged GRBM efficiency. According to this value,
one may conclude that, on average, any GRB, with a peak flux greater the above
value of
photons s
cm
in the BATSE energy band
50-300 keV has a
% probability of being detected within the GRBM
data by means of the off-line quest algorithms, provided that it is not
hidden by the Earth.
Next: GRBM Efficiency with Direction
Up: GRBM Detection Efficiency of
Previous: GRBM Detection Efficiency of
  Contents
Cristiano Guidorzi
2003-07-31