Cross-check with other GRB Catalogs

One of the main differences with respect to the on-line quest is represented by the automatic cross-checks with other GRB catalogs, derived from other missions and/or experiments: whenever a candidate burst is automatically found, the quest algorithms perform a search for simultaneous events throughout the other GRB catalogs. When a burst is found to be in common with others, it is classified as common burst candidate, and some useful informations derived, like the burst direction, are referred to the BeppoSAX local frame of reference, to check whether the two bursts, i.e. the GRBM candidate and the simultaneous one from other catalogs, are really the same burst.

The catalogs used for the cross-check can be divided into two classes: the first includes all the bursts, that have been localized with high accuracy (typically $\sim$ few arcmin.): this mainly includes WFC GRBs (table ), IPN bursts (table ), and other bursts from the ASM/Rossi-XTE and HETE-II. The second class includes the catalogs, whose GRB directions are affected by intermediate uncertainties ($\sim$ few degrees): first of all, the BATSE 4B catalog ; then, two catalogs including BATSE non-triggered bursts have been taken into account as well: the Kommers' catalog ([Kommers et al., 1999]) and the Stern's catalog ([Stern et al., 2000b]).

Since these two BATSE non-triggered bursts catalogs mainly include faint events, whose burst nature could be not so well established in some cases, they have been often used with caution.

Obviously, the search for bursts in common with BATSE has been limited to the time interval, during which both the GRBM and BATSE were operating, i.e. from July 1996 (when the first scientific observations started) to June 2000 (when the CGRO was driven into the ocean).

To better give an idea of the pieces of information automatically yielded, whenever a burst candidate is found to be simultaneous with bursts from other catalogs, below we report the case of GRB961228, occurred at UT 00:29:58, that triggered both the GRBM and BATSE (trigger # 5729; see fig. ).

[4]

------------------------------------------------------------------
Sun R.A. and Dec.:  276.0   -23.3
Sun LS phi  :  24.8
Sun LS theta: -13.8
 
LSra:  36.3  298.9  216.3  118.9
LSde:  -7.5  -44.4    7.5   44.4
 
lkGRB[0]         #LSs: 4        Good HRR: 6
Trigger Time (UTC):    88198.2          28 Dec 1996 00:29:58
Sun visib.: 2
Earth Phi: 275.1        Theta: -10.5
LS E-elev:  -54.5    18.8   102.2    28.9
LS Ecos  :  0.979   0.088  -0.979  -0.088
......onboard trigger delay: 0  (obs.: 23)
*********BATSE burst delay: -26
     GRB E-elev:   45.5
     GRB visib.: 2
     GRB phi:  42.4     theta:  70.6
 
Nsig(trg):    6.4    4.0    2.9    2.1    3.5    3.3    2.5    1.8
Nsig(pfl):   11.1    8.9    7.1    6.8    8.2    7.2    5.7    4.7
Bkg lev. :    846   1229    918    842    988   1029    950   1039
ChiSq R. :  1.269  1.203  0.948  1.043  1.079  1.146  1.024  1.062
 
Peak fl. :    323    311    216    198    259    231    177    152
Error    :   44.9   52.7   45.4   43.4   47.3   47.9   45.6   47.3
 
Fluence  :   4357   3993   2700   2551   3035   2878   2047   1819
Error    :  208.3  230.8  198.8  181.5  220.4  210.1  200.4  198.3
 
Dur  (s) :    26.00    20.00    20.00    18.00
Abundance:       23       20       20       18
H. Ratio :    0.696    0.721    0.758    0.713
HR Ratio :    0.966    0.919    0.977    0.951    1.011    1.063
HR W-ave :    0.716  +/-  0.037
GRBM-AC angle (deg):   1.6
------------------------------------------------------------------

First of all, this burst triggered the on-board logic, then the delay between the S/W trigger time and the BATSE trigger time is given in the line with several asterisks: in this case, BATSE was triggered $\sim$ 26 s before the GRBM. The burst geometry, according to the BATSE estimate of the arrival direction, is then reported in the next lines: in particular, the parameter called ``GRB E-elev'' expresses the elevation angle above the Earth limb of the BATSE direction (in this case: $\sim +45\rm ^{\circ}$). The GRB visibility is calculated, according to the definition given in the previous section, with only one remark: while for WFC or IPN bursts the error box can be neglected for our aims, when dealing with BATSE bursts it cannot be neglected any more. Since, in such cases, we deal with point-like sources with error regions, instead of extended sources like in the case of the Sun discussed before, then we have to little adjust the visibility definition for BATSE bursts, as follows:

$$\mbox{BATSE GRB Visibility} \left\{ \begin{array}{ll} \mb... ...ally} hidden, and GRB {\bf is} hidden}\\ \end{array} \right.$$ (32)

Figure: GRB961228, UT 00:29:58, is an example of a GRBM-BATSE common burst. Upper panel: GRBM 40-700 keV light curve (summed counts of all units); lower panel: BATSE $> 55$ keV light curve.

Thus, when the whole error circle is not Earth-blocked, the burst is said to be ``surely visible''; on the other side, when this is entirely Earth-blocked, the burst is said ``surely occulted''. In the remaining cases, i.e. when the error circle is partially occulted, the burst is called ``probably occulted'' and ``probably visible'', when the burst position is Earth-blocked or not, respectively.

Figure: GRB980923, UT 08:22:58, is an example of a GRBM-Stern's BATSE common burst. Upper panel: GRBM 40-700 keV light curve (unit 3); lower panel: BATSE $55-300$ keV light curve as taken from Stern's catalog (event identificator: 11079d).