13 th July 2005Poonam Chandra Introduction Giant flare from SGR 1806-20 What are SGRs? Comparison with other known SGRs Source of SGR giant flare Mechanisms for various SGR flare emissions Radio observations of SGR giant flare afterglow Radio emission from Afterglow Observations and results Distance estimations Comparison with other radio observations Short GRBs vs SGRs? Extragalactic SGRs- possible candidates for short GRBs! Plan of the talk
13 th July 2005Poonam Chandra SGR 1806-20 Giant flare on Dec 27, 2004 Detected by INTEGRAL, RHESSI, Wind Spacecraft, SWIFT, GMRT, VLA, ATCA etc. 80,000 counts/sec (RHESSI)
13 th July 2005Poonam Chandra SGR stands for S oft G amma-ray R epeater Gives repeated flares, whose energy fall in soft-gamma rays or hard X-rays in the electromagnetic spectrum 1806-20 RA Dec
13 th July 2005Poonam Chandra Precursor Spike Tail Pulsed tail emission Giant flare for 0.2 sec, tail for 382 sec, 1 sec precursor before 142 sec giant flare. 99.7% of the total energy Burst profile of Dec 27, 2004 giant flare Hurley et al (2005), Nature
13 th July 2005Poonam Chandra (peak is ~5 km overhead on this scale!) 15-25 keV 25-50 keV 50-100 keV 100-350 keV SGR 1806-20
13 th July 2005Poonam Chandra In 1/10 of a second as much energy as sun emits in 100,000 years continuously. 1000 times more bright than combining all the stars of Milky Way together. 100 times more energetic than any previous giant bursts.
13 th July 2005Poonam Chandra SGR 0526-66 SGR 1627-41 SGR 1900+14 SGR 1806-20 Other Soft Gamma Ray Repeaters Yellow points- cousins Anamalous Xray Pulsars also considered to be of same origin as SGRs
13 th July 2005Poonam Chandra (Marsdon & Higdon, 2001, Taylor & Cordes 1993) Sun
13 th July 2005Poonam Chandra 2005 SGR 1806-20 was 100 times larger in energy than any other previous busrt.
13 th July 2005Poonam Chandra The high energy of the giant burst implies rarity of the the such busrts. Since dN/dE E -1.6 Such giant flares happen once in a century
13 th July 2005Poonam Chandra WHAT COULD BE THE SOURCE OF SUCH A HUGE ENERGY? ``When you have eliminated all other possibilities, Sherlock Holmes instructed, whatever remains, however improbable, must be the answer to the puzzle.”
13 th July 2005Poonam Chandra Accretion due to binary Cannot explain the initial spike. Difficult to explain pure form of energy least contaminated by baryons. No binary associations found. Three candidates for SGRs Rotation energy of pulsar The maximum luminosity obtained is 10 33 ergs/s Very slow rotating objects, cannot explain such huge energy Powered by magnetic field (MAGNETAR)
13 th July 2005Poonam Chandra MAGNETAR Most accepted model A young neutron star with age <10,000 years. Extremely high magnetic field (~ 10 15 Gauss) SGRs are magnetars occasionally emitting energetic bursts in the early phase of their life times.
13 th July 2005Poonam Chandra Comparison of magnetic field strengths Earth0.6 Gauss Strong sunspots4000 Gauss Strongest lab mag. field5x10 5 Gauss Radio pulsar10 12 Gauss Magnetar10 15 Gauss
13 th July 2005Poonam Chandra Kouveliotou (Nature, 1998) found that SGR 1806-20 is oscillating with 7.56 sec period and slowing down at a speed of 1 sec/ 300 years
13 th July 2005Poonam Chandra The magnetic field required for SGR 1806-20 slow down rate is ~10 15 Gauss!!
13 th July 2005Poonam Chandra The characteristic age of the SGRs estimated are 10,000 years in contrast to 1 million years of age of neutron stars.
13 th July 2005Poonam Chandra Why high 10 15 G magnetic field? 1: Required for such high spin down observed. 2: Required to explain the energy of the explosion 3: To explain the trigger of SGR activity in 10 4 years. 4: Explain super eddington luminosity 5: Explains quiescent X-ray emission through mag. field decay. 6: Explains the initial spike, comparable to Alfven wave time crossing in magnetosphere of a neutron star (R*/t).
13 th July 2005Poonam Chandra HOW A MAGNETAR IS FORMED?
13 th July 2005Poonam Chandra Supernova explosion leaves neutron star as a remnant in the center.
13 th July 2005Poonam Chandra Giant Flares: Sudden Large-scale re-arrangement of the magnetic field Magnetar burst emission Thompson & Duncan 1996
13 th July 2005Poonam Chandra Global changes in the magnetic field geometry “Interchange instability” (Energy released (B ext 2 /8 )R 3 ) Flowers & Ruderman (1977)
13 th July 2005Poonam Chandra Small Bursts (SGR events): Cracking of crust leads to small displacements of magnetic field Thompson & Duncan 1996
13 th July 2005Poonam Chandra Inside twist => magnetic field lines outside the star also get twisted because they are anchored to the crust (cracking 5 km?). Thompson & Duncan 1996
13 th July 2005Poonam Chandra Tail emission: from the trapped fireball oscillating with the neutron star rotation period. Thompson & Duncan 1996
13 th July 2005Poonam Chandra Radio emission from SGR 1806-20 afterglow The ejected particles moving with very high speed hit the surrounding matter and generate synchrotron emission due to the relativistic electrons moving in a magnetic field. GMRT + VLA + ATCA
13 th July 2005Poonam Chandra Radio observations with the Giant Meterwave Radio Telescope 1420 MHz 610 MHz 50 MHz 150 MHz 235 MHz 325 MHz
13 th July 2005Poonam Chandra Advantages of GMRT 1: UV coverage is provided by the rotation of the earth. 2: Very high sensitivity at very low frequencies unlike WSRT and MOST. 3: Could resolve 12” away LBV 1806-20 source close by at low frequencies. Negative declination Positive declination
13 th July 2005Poonam Chandra GMRT observations of SGR 1806-20 From January 4, 2005 to February 24, 2005 Initially very frequently, almost everyday Snapshots, 40-60 minutes. Mostly in 240 and 610 MHz and in 1060 MHz at some occasions.
13 th July 2005Poonam Chandra SGR FoV of SGR before the giant burst
13 th July 2005Poonam Chandra GMRT map of SGR 1806-20 in 235 MHz band
13 th July 2005Poonam Chandra LBV source Fading SGR 1806-20 6 January 2005 16 January 2005
13 th July 2005Poonam Chandra Light curve from day 5 to day 50 Freq (GHz) CC 0.24-1.7 0.61-1.9 1.4-2.0-4.1-0.85 2.4-0.95-3.5-0.95 4.9-1.55-3.1-0.65 6.1-2.3 8.5-2.0-2.8-0.64
13 th July 2005Poonam Chandra Chromatic decay of the light curve between day 8 and day 18. Low frequencies decaying slower and high frequencies faster. Steepening of high frequencies between day 8 to 18. Flattening in some frequencies after 18 days. Features of the light curve
13 th July 2005Poonam Chandra Radio emission described by two components: 1: Rapidly decaying component. 2: Slowly decaying component
13 th July 2005Poonam Chandra Gaensler et al 2005, resolved source fainting with time
13 th July 2005Poonam Chandra Steepening from day 8.8 onwards. Chromatic decay is not apparent because of the lack of low frequency coverage in this paper, which does not include GMRT data. Gaensler et al 2005
13 th July 2005Poonam Chandra Measurement of parameters using Equipartition assumption If we assume that the total energy available for radio emission is equally divided between relativistic electrons and magnetic energy density i.e. equipartition between magnetic energy density and relativistic energy density
13 th July 2005Poonam Chandra Radio spectra of SGR 1806-20
13 th July 2005Poonam Chandra Implications and interpretations Hence Equipartition Magnetic field B min =13 mG and Equipartition energy density (when U B =U rel ) U min =10 43 erg U min
"name": "13 th July 2005Poonam Chandra Implications and interpretations Hence Equipartition Magnetic field B min =13 mG and Equipartition energy density (when U B =U rel ) U min =10 43 erg U min
13 th July 2005Poonam Chandra Distance estimation of SGR 1806-20 from the HI absorption spectra HI emission spectrum
13 th July 2005Poonam Chandra Source HI absorption spectrum
13 th July 2005Poonam Chandra In contrast to previously estimated distance of 15 kpc (Gaensler et al), the SGR 1806-20 lies between d=6.4 kpc - 9.8 kpc. Much closer.
13 th July 2005Poonam Chandra Association with the heavy mass cluster and Luminous Blue Variable? What kind of stars produce magnetars which forms SGRs?
13 th July 2005Poonam Chandra Can Extragalactic SGRs be the candidates for short Gamma Ray Bursts observed in other galaxies?
13 th July 2005Poonam Chandra What are Gamma Ray bursts (GRBs)? Most energetic events in the universe Long duration GRBs (t>2s) (Massive star explosions?) Short duration GRBs(t<2s) (NS-NS merger, SGRs?)
13 th July 2005Poonam Chandra Three ways to identify: 1: SGRs can be detected only if they are close by (because of lower energy scales), hence associated with bright galaxies. 2: SGRs should produce periodic tail following the giant bursts 3: SGRs having thermal Black body spectrum vs GRBs having powerlaw Hurley et al. 2005
13 th July 2005Poonam Chandra With BATSE sensitivity, it should have detected SGRs within 30 Mpc, i.e. 19 SGRs per year. Account for 40% of the total short GRBs. (With the given sensitivity, SWIFT can detect 53 SGRs per year.) However, no association with bright galaxies. Not found having thermal blackbody spectrum. Final probability reduces to 5% Hurley et al. 2005
13 th July 2005Poonam Chandra Inputs from our radio measurements: 1: Our revised distance estimate reduces the probability further. 2: U min /E gamma <1, whereas in GRB radio afterglow models, U min /E gamma =1 3: Decay rate of radio emission incompatible with GRB afterglow model. The extragalactic SGRs being short GRB candidates is highly improbable.
13 th July 2005Poonam Chandra Conclusions Most energetic burst observed, energy 100 times more than any previous burst. Energy powered by high magnetic field Not associated with the massive star cluster containing the LBV source. Rare event, probability once in a centuryChromatic decay in the radio flux densityUnlikely to be the major candidates for short duration GRBs.
13 th July 2005Poonam Chandra Acknowledgements Brian Cameron Alak Ray Shri Kulkarni Dail Frail M. Wieranga GMRT staff VLA staff
13 th July 2005Poonam Chandra SGR 1806-20 THANKS