SGR Events detected with GRBM

Since the end of 1996 the GRBM detected two giant flares from SGR1900+14 and 22 short typical SGR bursts from the followig sources: 11 from SGR1900+14, 9 from SGR1627-41, and 2 from SGR1806-20. Out of this 22 short bursts, only 4 have triggered the off-line GRB quest (table ); the remaining 18 have not been discovered automatically, owing to the spectral softness of SGR bursts, below the HR threshold (eq. ); on the other side, this proves the effectiveness of the GRB search algorithms in distinguishing the classical GRBs from SGRs (only 4 out of 22 bursts + 2 giant flares have been mistaken for GRBs by the automatic off-line quest). The other 18 SGR bursts show no AC ($>$ 100 keV) significant signal in any of the four GRBM units; as their duration is $<$ 1 s, except for some of them, it has been possible to recognize them only through the analysis of HTR data. Due to this, the search for SGR events was limited to the set of GRBM on-board triggered events: 15 out of the remaining 18 SGR bursts detected with the GRBM resulted from a cross-check between the list of GRBM on-board triggers and a recent list of SGR events detected by Konus/WIND ([Aptekar, et al., 2001]); the remaining 3, all from SGR1900+14, show HTR light curves that are typical of SGRs, so we classified them as true SGR1900+14 bursts; all occurred when the source was extremely active.

Finally, in addition to these 22 bursts, other two SGR burst candidates have been detected: 980831, UT 22:52:40.71, attributed to SGR1900+14, and 980618, UT 03:44:00.04, attributed to SGR1627-41; since they have not been detected by Konus/WIND, they need confirmation. Table  reports all these SGR events as seen with the GRBM; the two SGR burst candidates have been classified as ``uncertain''.

Table: SGR Events detected with GRBM.

SGR	Date	U.T.	$\phi$	$\theta$	$T_{90}$	Comments
name			($\rm ^{\circ}$)	($\rm ^{\circ}$)	(s)
1900+14	30 May 1998	23:27:35.28	216.5	-3.8	$0.16 \pm 0.06$
1900+14	27 Aug 1998	10:22:15.69	240.8	+47.5	$184 \pm 4$	Giant Flare
1900+14	27 Aug 1998	11:06:57.39	240.8	+47.5	$0.52 \pm 0.09$	No Konus det.
1900+14	29 Aug 1998	10:16:32.30	152.9	-24.5	$3.01 \pm 0.35$	Intermediate Fl.
1900+14	29 Aug 1998	18:18:43.52	152.9	-24.5	$1.6 \pm 0.2$
1900+14	31 Aug 1998	16:34:40.36	260.6	+41.8	$0.37 \pm 0.06$	No Konus det.
1900+14	31 Aug 1998	22:52:40.71	260.6	+41.8	$0.38 \pm 0.12$	Uncertain
1900+14	01 Sep 1998	17:06:27.06	114.7	+36.4	$24.6 \pm 3.9$	Unusual
1900+14	02 Sep 1998	07:23:50.13	114.7	+36.4	$1.3 \pm 0.1$	No Konus det.
1900+14	15 Sep 1998	18:17:40.98	355.6	+89.8	$0.20 \pm 0.07$	NFI Observation
1900+14	10 Jan 1999	08:39:01.51	11.1	-8.1	$0.77 \pm 0.07$
1900+14	05 Apr 1999	08:21:00.70	65.2	+40.2	$0.45 \pm 0.20$	Ulysses det.; hard
1900+14	18 Apr 2001	07:55:11.50	276.7	+9.0	$27.3 \pm 0.4$	Giant Flare; WFC1 det.!
1900+14	02 Jul 2001	03:34:07.16	207.8	+19.1	$3.2 \pm 0.5$	HETE-II det.
1627-41	17 Jun 1998	19:58:29.95	221.6	+6.8	$0.12 \pm 0.02$
1627-41	17 Jun 1998	20:06:28.39	221.6	+6.8	$0.34 \pm 0.09$
1627-41	17 Jun 1998	21:04:38.75	221.6	+6.8	$0.16 \pm 0.06$
1627-41	17 Jun 1998	21:37:18.67	221.6	+6.8	$0.10 \pm 0.10$
1627-41	17 Jun 1998	21:57:08.42	221.6	+6.8	$0.16 \pm 0.06$
1627-41	18 Jun 1998	03:28:28.03	221.6	+6.8	$0.70 \pm 0.04$
1627-41	18 Jun 1998	03:44:00.04	221.6	+6.8	$0.04 \pm 0.03$	Uncertain
1627-41	22 Jun 1998	13:29:54.94	140.8	-18.0	$0.12 \pm 0.02$
1627-41	22 Jun 1998	14:11:24.42	140.8	-18.0	$0.34 \pm 0.12$
1627-41	25 Jun 1998	10:56:18.79	133.8	+21.0	$0.09 \pm 0.02$
1806-20	22 Nov 1996	02:17:39.61	9.1	+36.7	$0.34 \pm 0.14$
1806-20	01 Jun 1999	10:00:05.29	173.6	+34.4	$0.24 \pm 0.09$

The BeppoSAX local direction of the SGR sources is expressed by the local angles $\phi$ and $\theta$; both fluence (counts) and duration ($T_{90}$) have been estimated from the 40-700 keV data, owing to the spectral softness of SGR bursts. The last column reports a comment for some bursts, e.g. when the burst has not been detected by Konus, when other experiments detected it in addition to Konus; in particular, the SGR1900+14 burst occurred on April 05, 1999 was detected by Ulysses as well: the Ulysses-BeppoSAX annulus indeed included the SGR1900+14 position, making it the best candidate ([Preger et al., 1999b]). This burst has the peculiarity of an unusual spectral hardness: actually, it triggered the off-line trigger conditions, thanks to its average $HR \sim 0.64 \pm 0.07$, which is very high for usual SGR bursts ($< 0.3$), but, on the other side, too low for short duration GRBs, whose average HR has been found to be $0.870 \pm 0.007$ (sec. ). However, other bursts from SGR1900+14 look particularly hard, like in the case of the burst occurred on January 10, 1999, ( $HR \sim 0.54 \pm 0.10$, confirmed by BATSE and Ulysses data ([Woods et al., 1999b]).

Other bursts from SGR1900+14 have been detected by other experiments as well: this is the case of the burst occurred on July 2, 2001, first detected by HETE-II (Ricker et al., GCN 1078), and then confirmed by the GRBM detection (Montanari et al. , GCN 1081).

The SGR1900+14 burst occurred on August 29, 1998, i.e. two days after the impressive giant flare of August 27, has been classified as ``intermediate'' burst in table , owing to its unusual properties; this burst was also detected by BATSE and RXTE ([Ibrahim et al., 2001]).

One SGR1900+14 burst, occurred on September 15, 1998, was in the field of view of the NFI/BeppoSAX, while the April 2001 giant flare ([Guidorzi et al., 2001c], Guidorzi et al. , GCN 1041) was in the WFC1 field of view; the extraordinary brightness of this flare switched off the WFC by rate protection.

Figure: GRBM Light Curves of some SGR1900+14 bursts (40-700 keV).

Figure: GRBM Light Curves of other SGR1900+14 bursts (40-700 keV).

In figg. ,  the time profiles of the short SGR1900+14 bursts are shown; the two giant flares are treated later on. The energy band is 40-700 keV in all figures, since only in this range HTR profiles are acquired; the numbers among brackets refer to the GRBM unit numbers used for the light curves: e.g., ``(1+2+3+4)'' means that the light curve shown is the sum of all GRBM units curves, and so on.

Figure: GRBM Light Curves of some SGR1627-41 bursts (40-700 keV).

Figure: GRBM Light Curves of other SGR1627-41 bursts (40-700 keV).

Similarly, figg. ,  show the time histories of the bursts from SGR1627-41. In the case of SGR1806-20, fig.  shows both bursts detected with the GRBM; the light curve of 961122 belongs to the GRBM unit 1, instead of unit 4, that is the brightest, because only the HTR data of unit 1 are available within the current GRBM archive.

Figure: GRBM Light Curves of two bursts from SGR1806-20 (40-700 keV).