The GRBM HTR light curve for IPN

For the limits already discussed in section 1.3.2, the IPN needs light curves with high time resolutions (at least $\sim$ few $10^2$ ms): in the case of the GRBM, the light curve is rebinned to $31.25$ ms from the original $7.8125$ ms; an example of a GRBM light curve used for an IPN triangulation is shown in fig. . In this case, the burst position, previously measured by the WFC1 (GCN 1033, 1035), was refined with an IPN triangulation between Ulysses and the GRBM (GCN 1036), by crossing the IPN annulus with the WFC region: the intersection region area was $\sim 47$ square arcmin, from the initial $6\mbox{$^\prime$}$ radius circle of the refined WFC position (GCN 1035; see fig. ).

Figure: The Ulysses-BeppoSAX annulus intersects the WFC error circle of GRB010412 (courtesy of K. Hurley).

This burst shows a highly featured light curve, with a dramatic time variability on few ms timescales: these sharp temporal structures strongly constrain the cross-check between light curves of the same event from different experiments, limiting the width of the IPN annulus.

In Table  I list the GRBs that have been localized thanks to IPN localizations including the GRBM as one of the IPN nodes. Since the start of the on-line GRB quest, the IPN with GRBM localized 18 bursts: in some cases, the IPN position followed a previous one, obtained by the WFC/BeppoSAX (GRB000529, GRB000620, GRB010412 and GRB011121), by ASM/Rossi-XTE (GRB010324) and by HETE-II (GRB010612,GRB010629 and GRB010921); two bursts were also detected by BATSE/CGRO (GRB000429 and GRB000519).

Figure: GRB010923: the triangle spots the IPN position ( $\phi=223.7\rm ^{\circ}=-136.3\rm ^{\circ}$, $\theta=+9.3\rm ^{\circ}$) while the asterisk denotes the GRBM position centroid ( $\phi=213\rm ^{\circ}=-147\rm ^{\circ}$, $\theta=-5.0\rm ^{\circ}$) and the cross corresponds to the 2nd, wrong IPN solution ( $\phi=203.7\rm ^{\circ}=-156.3\rm ^{\circ}$, $\theta=-29.3\rm ^{\circ}$) : the GRBM position allowed to resolve the IPN ambiguity.

Eventually, the case of GRB010923 is particularly important for the localization capabilities of the GRBM: actually, the triangulation for this burst, made with Ulysses, Konus/WIND and GRBM/BeppoSAX data, gave two positions: then, the right one was determined thanks to the GRBM estimate (fig. ); in fig.  the IPN triangulation derived for this burst is shown: here the GRBM error circle is an approximation of the 90% CL region come out from GRBM localization shown in fig. .

Figure: GRB010923: the IPN triangulation redundancy is resolved by means of the GRBM error circle, as it includes only one of the two possible intersection regions (courtesy of K. Hurley).

Other two nice GRBs that were localized by means of IPN triangulations are GRB000630 (GCN 736) and GRB001204 (GCN 895): both were detected by four experiments (Ulysses, GRBM/BeppoSAX, Konus/WIND and NEAR), thus giving two independent annuli, whose ambiguity has been resolved by the forth spacecraft (fig. ). The localization of GRB000630 allowed the discovery of its optical afterglow (GCN 747); even if no afterglow candidate was found in the case of the short duration GRB001204 ( $T_{90} \simeq 0.4$ s, as estimated from the GRBM data), it has been possible to give upper limits on possible afterglows ([Hurley et al., 2001]).

Figure: Examples of localization of two IPN bursts: both GRB000630 (left panel) and GRB001204 (right panel) were detected by Ulysses, BeppoSAX/GRBM, Konus/WIND and NEAR. In both cases, the two IPN annuli, crossing each other, are shown. Only for the first an afterglow was found (optical band); GRB001204 was a short duration burst (courtesy of K. Hurley).

Table: 18 IPN GRBs with GRBM contribution (Apr. 2000 - Dec. 2001)

GRB Date	UT	R.A.	Decl.	Err.	Experiments	GCN
	($\clubsuit$)	($\rm ^{\circ}$)	($\rm ^{\circ}$)	( $\mbox{$^\prime$}$)	$^{(*)}$	#
19 Apr 2000	02:12:49	330.07	48.28	18	U+BS+KW+N	641
29 Apr 2000	10:07:22	98.34	-4.42	12	U+BS+BA+KW+N	657
16 May 2000	09:26:05	35.07	-14.83	30	U+BS+KW+N	671
19 May 2000	08:18:13	346.16	1.17	18	U+BS+BA+N	672
29 May 2000	08:43:12	2.28	-61.53	6	WFC$^{(\dagger)}$+BS+KW+N	678
15 Jun 2000	16:32:35	35.61	-17.87	12	U+BS+KW+N	710
20 Jun 2000	05:33:33	113.82	69.20	6	WFC$^{(\dagger)}$+U+BS+KW	723
30 Jun 2000	00:30:53	221.81	41.22	24	U+BS+KW+N	736
04 Oct 2000	14:51:27	240.09	-56.81	360$\times$20	U+BS+N	835
04 Dec 2000	08:01:09	40.29	12.90	12	U+BS+KW+N	895
12 Dec 2000	14:57:23	102.40	36.44	6	U+BS+KW+N	899
24 Mar 2001	11:37:32	107.78	20.08	15	ASM$^{(\star)}$+U+BS	1011
12 Apr 2001	21:46:29	290.91	13.62	6	WFC$^{(\dagger)}$+U+BS	1033,1035,1036
12 Jun 2001	02:33:13	270.82	-32.13	60	HETE$^{(\ddagger)}$+U+BS+KW	1065
29 Jun 2001	12:21:07	248.16	-18.72	15	HETE$^{(\ddagger)}$+U+BS	1076
21 Sep 2001	05:15:50	343.90	40.93	18	HETE$^{(\ddagger)}$+U+BS	1096, 1097
23 Sep 2001	09:24:30	302.07	18.03	18	U+BS+KW+GRBM$^{(\Box)}$	1104 + (a)
21 Nov 2001	18:47:11	173.63	-76.03	3	WFC$^{(\dagger)}$+U+BS	1148

$^{(*)}$

U=Ulysses, BS=BeppoSAX (GRBM), KW=Konus/WIND, N=NEAR, BA=BATSE.

$^{(\dagger)}$

Burst previously localized by the WFC/BeppoSAX.

$^{(\star)}$

Burst previously localized by the ASM/RXTE (GCN/RXTE, BACODINE).

$^{(\ddagger)}$

Burst previously localized by the HETE-II.

$^{(\Box)}$

In this case, the GRBM contributes twice: first, as a member of the IPN, and second, because its rough position allowed to solve the usual IPN ambiguity of two distant solutions (whenever the number of spacecrafts is $\leq 3$).

($\clubsuit$)

GRBM trigger time.

(a)

K. Hurley, private comm.