A giant flare from the magnetar SGR 1806-20

-- a tsunami of gamma-rays

Søren Brandt Danish National Space Center

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

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

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

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

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

SGR 1806-20 pulsed tail

Excess count rate extends for more than 200 sec

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

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

SGR 1806-20 raw spectrum

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............