The late SWTCs have been conceived in order to improve the background estimate and the selection criteria themselves, so that they could be applied to an on-line quest, without producing an excess of wrong detections, or distributing strongly biased peak flux or fluence estimates of true GRBs.
The most relevant improvement introduced in the background estimate
of the ratemeter counts has been achieved by adopting two fitting
intervals around the bin to scan, before and after it, and
by performing a parabolic fit, instead of the naive average described in
eq. .
Let
th be the scanned bin and
and
the numbers of bins taken
before and after the
th bin, respectively (
=before,
=after);
and
are defined so that the
th bin is the first
bin of the
-bin background fitting interval before the scanned bin,
while the
th bin is the first bin of the
-bin
background fitting interval after the scanned bin.
Let
and
be the
th time bin and the counts for the
th bin, for the energy band
and for the detector unit
;
let
be the entire time interval used for the fit and
its corresponding index interval.
The background counts
are given by the 2nd order polynomial,
corresponding to the least square parabolic fit of the two above intervals;
to simplify the formulas in the expressions
, required
for calculating the 2nd order polynomial coefficients, we define
and we use the notation
, supposing that
and
are fixed.
The two different sets of values used for the on- and off-line quests
are reported below and will be discussed more deeply later on in this
chapter. In eqq.
some momenta are defined conveniently:
According to the least square method, in order to determine the best
fitting polynomial, we resolve the following set of equations
:
The solution to the set of eqq. gives the following
expressions for the coefficients of the parabolic polynomial fitting
the background (eqq.
)
:
In order to avoid biases of the GRB itself in the background fit,
the choice of a proper value for , suitable for long duration GRBs
in particular, becomes crucial. Actually, a small value for
would produce strong biases for all the bursts with time durations
greater than
s; on the other hand, a very big value for
could make the background fit worse, whenever the whole time interval,
lasting from the
th to the
th bins, would suffer
from higher degree polynomial background level variations.
Therefore, a proper choice, balancing the undesired effects of these
two extreme possibilities, is difficult and must be adapted to the needs
of the different quests.
Like in the case of the early SWTCs (see eq. ), we define
the net signal
and the
.
Let
,
,
,
,
,
,
,
, be different threshold parameters, whose meaning
will be explained below, in the description of the SWTCs.
At this point, all the ingredients required to express the late SWTCs are
available: the SWTCs are four, in addition to a further condition on the
hardness ratio
defined below.
Similarly to the early SWTCs (eq. ,
,
), the late SWTCs are expressed by the following
eqq.
,
,
,
:
The SWTC 1 (eq. ) is similar to the early SWTC 1
(eq.
), apart from the different values of the threshold
parameters.
The SWTC 2 (eq. ) is similar to the SWTC 1,
with the only difference that
the thresholds are no more the same for the two brightest units,
but it lowers those for the second brightest one:
in place of
and
in place of
.
The SWTC 3 (eq. ) lowers the single unit thresholds and,
at the same time, requires that the total sum of the net signals,
expressed in
, over the set of units matching the lower single
unit thresholds, must be greater than a proper threshold, one for each
energy band:
and
.
Finally, the SWTC 4 (eq. ) requires that in
at least one of the GRBM units 1 and 3, the ones co-aligned
with the WFCs (see fig.
), the counts
exceed the thresholds in both energy ranges in at least three
contiguous bins.