SGR Main Properties

The Soft-Gamma Repeaters (SGRs) are X- and gamma-ray transient sources associated with compact stars, that unpredictably undergo periods of intense activity, lasting for days to months, during which they produce powerful super-Eddington outbursts. Typically, these periods of bursting activity are separated by relatively long intervals (years) of quiescence.

To date, only four SGRs are known and all have been located within supernova remnants (SNRs), suggesting that they should be young neutron stars (age $\leq 10^4$ y). Among these, only SGR0525-66, responsible for the energetic outburst of March 5, 1979 ([Mazets et al., 1979b,Cline et al., 1980]), is associated with the N49 remnant in the Large Magellanic Cloud, while the other SGRs lie within galactic SNRs (SGR1900+14, SGR1806-20 and SGR1627-41). There is also a likely fifth source, discovered in 1997, SGR1801-23 ([Cline et al., 2000]), but its position has not yet been determined accurately enough to investigate its possible association with a SNR.

When active, SGRs typically emit short ($\sim$0.1 s) very bright bursts recurring on different timescales of seconds to years, with typical energies of $\sim10^{41}$ ergs. Rarely, they undergo dramatic instabilities producing long and giant hard X- and soft $\gamma$-ray flares, reaching $\sim10^{44} - 10^{45}$ erg/s. Three SGRs have quiescent X-ray counterparts with luminosities of $\sim 10^{34}$ - $10^{35}$ erg/s. Recently, observations of the persistent soft X-ray counterparts have shown periodicities of 5-8 s and spin-down rates of $\sim 10^{-11} - 10^{-10}$ s/s ([Kouveliotou et al., 1998,Kouveliotou et al., 1999]). Basically, SGR bursts are different from Gamma-Ray Bursts (GRBs) as for many properties: first, they undergo repeated periods of intense activity; second, their spectra are softer than GRBs and they are well fitted by a thermal bremsstrahlung with $kT$ between 20 and 40 keV.

Before April 2001, only two giant flares had been detected, namely the 1979 March 5 from SGR0525-66 and the 1998 August 27 from SGR1900+14 ([Feroci et al., 1999a,Feroci et al., 2001b,Hurley et al., 1999c,Mazets et al., 1999]); the latter was responsible for the most intense radiation flux ever detected from a source located outside Solar System: actually, it affected the Earth's ionosphere ([Inan et al., 1999]).

The theoretical model known as the ``magnetar'' model ([Duncan & Thompson, 1992], Paczynski, 1992, [Thompson & Duncan, 1995]) considers a young neutron star with a very strong magnetic field ( $\sim 10^{14} - 10^{15}$ G), whose decay powers both the quiescent X-ray emission through heating of stellar interior and low-level seismic activity and periodically causes big crustquakes triggering short bursts (as well as giant flares). Differently from this model, accounting for both burst and persistent emission properties, models requiring accreting neutron stars with $B\sim 10^{11} - 10^{12}$ G can only account for the quiescent emission, while the hyper-Eddington bursts cannot be satisfactorily explained.

In particular, SGR1900+14 deserves our attention, since it produced two giant flares, that have been detected with the GRBM: August 1998 and April 2001. SGR1900+14 was discovered in 1979 following three bursts in two days ([Mazets et al., 1979a]). Localized by the IPN ([Hurley et al., 1999d]), it lies close to the SNR G42.8+0.6, implying a proper motion of at least 1000 Km/s. After its discovery, the source was detected again in 1992 ([Kouveliotou et al., 1993b]); after five years of quiescence, in May 1998 it entered an extremely active period, that reached its maximum with the giant hard X- and $\gamma$-ray flare of August 27, 1998.

Observations of the quiescent soft X-ray counterpart ([Vasisht et al., 1994]) have shown a 5.16-s periodicity and an average spin-down rate of $10^{-10}$ s/s ([Kouveliotou et al., 1999,Hurley et al., 1999e]). According to the magnetar model, this high spin-down rate is induced by the breaking of the magnetic field, which therefore must be very strong: B $\sim 10^{14} - 10^{15}$ G. Its spectrum is well fitted by a constant blackbody component (kT $\sim 0.5$ keV) plus a power law, that varies between the two different, i.e. quiescent and bursting, source states ([Woods et al., 1999]).