Faint WFC Bursts

One of the main goals of the automatic GRB quest on the GRBM data, was to limit the number of (typically faint) bursts within the field of view of the WFCs (with a detectable X-ray counterpart), that are missed by the GRBM on-board trigger logic: in such cases, the duty scientists at the SOC might not recognize the burst and, therefore, not check at the burst time in the WFC data, thus missing a prompt localization of a GRB. These cases mainly happen when the burst is not bright enough to overcome the on-board thresholds in at least two GRBM units, as required by the on-board trigger logic; nevertheless, its occurrence can be inferred from the analysis of the 1 s ratemeters. In other cases, the burst can be missed by the on-board trigger, because, in spite of its brightness, it occurred during GRBM dead times.

Figure: GRB010304. Top panel: 40-700 keV GRBM3 light curve; bottom panel: 2-28 keV WFC2 light curve (courtesy of J.J.M. in 't Zand). Example of WFC burst that did not trigger the GRBM on-board logic, but did the automatic GRB on-line quest (lkGRB[171]). The zero time corresponds to the S/W trigger time.

Since the automatic GRB quest has come out to be particularly suitable for discovering such bursts, the possibility of automatically alerting the BeppoSAX duty scientists, whenever a candidate WFC burst is found in the GRBM data, might be crucial. For the remaining cases, i.e. when the WFC burst triggers the GRBM on-board logic, as it often happens for the brightests (and, so far, long) ones, all the more reason for automatically detecting them; in any case, they are normally inspected by the BeppoSAX SOC people.

Since the on-line GRB quest has been monitoring the GRBM data (April 2000), 9 WFC bursts have been automatically detected by means of our SWTCs (table ); out of them, three were missed by the on-board trigger logic, because faint: GRB010304 (lkGRB[171], WFC2), GRB010501 (lkGRB[206], WFC1) and GRB010518 (lkGRB[218], WFC2).

The case of GRB010304 (fig. ) was particularly unlucky: in spite of its brightness, it has been localized only with a $\sim 1\rm ^{\circ}$ accuracy, due to the great spacecraft attitude uncertainty at the burst time; therefore, since no finer position could have been extracted, no GCN circular was issued.

Figure: GRB010501, UT 06:37:27 Top panel: 40-700 keV GRBM1 light curve; bottom panel: 2-28 keV WFC1 light curve (courtesy of J.J.M. in 't Zand). This faint burst did trigger the on-line quest, and was missed by the GRBM on-board logic, (lkGRB[206]). The zero time corresponds to the S/W trigger time.

Another interesting case of WFC burst that triggered the only on-line quest is given by GRB010501, UT 06:37:27: in fig. , showing the synchronized light curves of the GRBM unit 1 and of the WFC1, a peculiarity stands out immediately: actually, in the $\gamma$ band, the GRB seems to begin at least 15-20 s before the X-rays; the GRBM S/W trigger time coincides with the zero time. Unfortunately, though the automatic alert e-mail was sent as usual, it seems that this was not received by the SOC, thus missing the prompt discovery of the WFC X-ray counterpart, that has been found months later, during an off-line cross-check between WFC and GRBM data (J.J.M. in 't Zand, private comm.). On this subject, the following observation has to be pointed out: in the case of GRB010501, as well as for GRB000208, GRB010213 and GRB010304, the WFC team discovered them independently of the GRBM team during an off-line search, and only later, after cross-checking, both the GRBM and the WFC detections allowed their identification as WFC GRBs (except for GRB010304, which was recognized on line, although no GCN circular was issued, owing to the uncertainty on the spacecraft attitude at the burst time).

The last example of WFC burst missed by the GRBM on-board trigger logic is GRB010518 (fig. ): in this case, the automatic alert sent to the BeppoSAX SOC allowed the discovery of the X-ray counterpart in the WFC2 data. Nevertheless, the GCN circular (GCN 1062) delivering its coordinates, was issued only three days later, because of the difficulties in estimating the error circle, due to the faintness of the X-ray signal.

Figure: GRB010518, UT 06:43:09 Top panel: 40-700 keV GRBM3 light curve; bottom panel: 2-28 keV WFC2 light curve (courtesy of J.J.M. in 't Zand). Another example of faint burst that did trigger the on-line quest, and was missed by the GRBM on-board logic, (lkGRB[218], GCN 1062).

Unfortunately, other WFC GRBs, occurred after April 2000, have not been on-line detected, only because the automatic on-line quest was off at that epoch, due to problems already discussed previously: GRB001109, GRB010213, GRB010214, and GRB010220. Nevertheless, out of them, GRB010213 was the only burst, that did not trigger the on-board logic (it was during the GRBM dead time, due to a false trigger occurred 160 s earlier), and, for this reason, was missed by the SOC people (fig.). Indeed, this burst could have been on-line detected by the automatic quest, if only it had been automatically monitored on line.

Figure: GRB010213, UT 02:57:23 Top panel: 40-700 keV GRBM3 light curve; bottom panel: 2-28 keV WFC2 light curve (courtesy of J.J.M. in 't Zand). This bright burst did not trigger the GRBM on-board logic, because it occurred during the GRBM dead time; since the on-line quest was off, it was missed at the epoch and finally discovered during an off-line search.

Table: WFC GRBs (Apr. 2000 - Dec. 2001)

GRB Date	UT	R.A.	Decl.	Err.	On-line	On-b	Energy	lkGRB	Ref.
		($\rm ^{\circ}$)	($\rm ^{\circ}$)	( $\mbox{$^\prime$}$)	$^{(*)}$	trg	Band	#	#
24 Apr 2000	18:18:07	104.72	+49.87	3	N	N	X	-	GCN 644
28 May 2000	08:46:24	161.28	-33.98	2	Y	Y	X+$\gamma$	c	GCN 677
29 May 2000	08:43:12	2.35	-61.53	4	Y	Y	X+$\gamma$	c	GCN 676
08 Jun 2000	08:51:42	6.25	-69.03	4	N	N	X	-	GCN 696
15 Jun 2000	06:17:48	233.17	+73.85	2	-	N	X+$\gamma$	-	GCN 705
20 Jun 2000	05:33:33	113.82	+69.20	4	-	N	X+$\gamma$	-	GCN 718
11 Oct 2000	15:54:50	275.77	-50.90	2	Y	Y	X+$\gamma$	131	GCN 847
09 Nov 2000	09:23:17	277.53	+55.30	4	-	Y	X+$\gamma$	-	IAU 7519
13 Feb 2001	02:57:23	257.34	+39.26	1	-	N	X+$\gamma$	-	(a)
14 Feb 2001	08:48:11	265.25	+48.54	3	-	Y	X+$\gamma$	-	GCN 933
20 Feb 2001	22:51:07	39.25	+61.77	4	-	Y	X+$\gamma$	-	GCN 953
22 Feb 2001	07:23:30	223.05	+43.01	3	Y	Y	X+$\gamma$	164	GCN 960
04 Mar 2001	05:18:51	316.59	+53.21	60	Y	N	X+$\gamma$	171	(b)
12 Apr 2001	21:46:29	294.91	+13.62	6	Y	Y	X+$\gamma$	190	GCN 1035
01 May 2001	06:37:27	286.71	-70.18	6	Y	N	X+$\gamma$	206	(c)
18 May 2001	06:43:10	161.68	-57.79	5	Y	N	X+$\gamma$	218	GCN 1062
30 Oct 2001	06:28:02	310.91	+77.29	3	N	N	X	-	GCN 1119
21 Nov 2001	18:47:11	173.61	-76.03	2	Y	Y	X+$\gamma$	310	GCN 1149
11 Dec 2001	19:09:21	168.82	-21.93	2	N	N	X+$\gamma$	-	GCN 1189

(a)

Missed burst (J.J.M. in 't Zand, priv. comm.): on-line quest was not operating.

(b)

BeppoSAX attitude had $\sim 1\rm ^{\circ}$ uncertainty (no circular issued).

(c)

Missed burst (J.J.M. in 't Zand, priv. comm.).

$^{(*)}$

When ``-'' is reported, the on-line GRB quest was not operating.