A giant flare from the magnetar SGR 1806-20 -- a tsunami of gamma-rays

1. A giant flare from the magnetar SGR 1806-20-- a tsunami of gamma-rays Søren Brandt Danish National Space Center

2. Soft Gamma Repeaters (SGRs) and Magnetars • Magnetars are a class of neutron stars with extremely high magnetic fields, B >1015 Gauss • Two classes of objects are currently associated with magnetars: • Anomalous X-Ray Pulsars (AXPs) • Soft Gamma Repeaters (SGRs) • A handful of SGRs are known in the Galaxy and in the LMC • Soft gamma repeaters show erratic activity of short bursts of gamma rays, according to the magnetar model, associated with stress releases (”star-quakes”) in the solid neutron star crust • Before the advent of the Magnetar model the similarity of the log-normal statistical distributions of Earth-quakes and SGR bursts led to the suggestion of a star-quake model of SGRs • Three SGRs have shown giant flares (one each), releasing a significant fraction of the stored magnetic energy

3. Regular star-quake outbursts from SGR 1806-20typical fluence: 2-20x10-8 erg/cm2 (JEM-X observed more than 30) Observed peak: ~500 c/s Corrected peak: ~600 c/s Observed peak: ~600 c/s Corrected peak: ~10 000 c/s assuming 100 ms saturation

4. Giant flare from SGR 1806-20 • A giant flare was observed from SGR 1806-20 on December 27,2004 at 21:30:26 UT by all space based instruments sensitive to X- and gamma-rays (including some by reflections from the Moon only) • 106 degrees off INTEGRAL/JEM-X axis, way out of 10 degree FOV • 200 ms spike followed by 400 sec pulsed tail • Assuming 200 ms flat-top peak  peak trigger rate > 5 000 000 c/s • Dead time for accepted events is about 99.99 % • 7.57 +- 0.03 sec pulse period derived from 200 seconds of tail • Rough estimate of JEM-X effective area in 30-50 keV range: 0.1 cm2 • Estimated spike fluence*: ~1 erg/cm2 • Estimated spike energy release*: ~3x1046 erg at 15 kpc • corresponding to solar energy output for 250 000 years in < 200 ms!! • Estimated tail fluence: 0.3% of spike * references: Hurley et al., Palmer et al., Nature Vol 434, 2005

5. Accepted event rate in JEM-X (30-50 keV range)

6. SGR 1806-20 giant flare peakdead time corrections (~99.99%) imply 6x106 c/s in <200 ms peak (30-50 keV) ~99.99% dead time in peak

7. SGR 1806-20 pulsed tail

8. Excess count rate extends for more than 200 sec

9. Pulse profile for 7.57 s period based on 200 sec of tail

10. Giant flare pre-cursor at T-143 sec(background subtracted)

11. SGR 1806-20 raw spectrum

12. SGRs and short Gamma Ray Bursts • Brightest giant SGR flare ever observed (beating the famous March 5, 1979 event from LMC and the SGR1900+14 event in 1998) • In fact, this event is the brightest source (in terms of flux at the Earth) ever observed from outside the Solar system) • Are SGRs/Magnetars responsible for the class of short GRBs? • Small number statistics: 3 giant flares in 30 years • If a typical galaxy exhibits such a giant SGR flare event every 30 years, almost half of the BATSE short GRBs could be SGRs • The association of the recent GRB050509B, the first well localized short GRB with a galaxy at z~0.225 (GCN 3390) indicate that certainly not all short GRBs are giant SGR flares • On the other hand, some extra-galactic giant SGR flares should be observed, unless we have just witnessed an exceedingly rare event • The truth is out there............