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SOFT GAMMA REPEATERS

SOFT GAMMA REPEATERS AN OBSERVATIONAL REVIEW. SOFT GAMMA REPEATERS. Kevin Hurley UC Berkeley Space Sciences Laboratory. Kevin Hurley UC Berkeley Space Sciences Laboratory khurley@ssl.berkeley.edu. THE SOFT GAMMA REPEATERS ARE SPORADIC SOURCES OF BURSTS.

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SOFT GAMMA REPEATERS

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  1. SOFT GAMMA REPEATERS AN OBSERVATIONAL REVIEW SOFT GAMMA REPEATERS Kevin Hurley UC Berkeley Space Sciences Laboratory Kevin Hurley UC Berkeley Space Sciences Laboratory khurley@ssl.berkeley.edu

  2. THE SOFT GAMMA REPEATERS ARE SPORADIC SOURCES OF BURSTS • SGRs can remain dormant for many years; during these periods, no bursting behavior is observed • They become active and emit bursts at apparently random times • Two common types of bursts: • Short (100 ms, up to 1041 erg s-1) • Giant flares (several hundred seconds, periodic emission, up to 1046 erg s-1)

  3. BURSTING ACTIVITY OF 3 SGRs OVER 17 YEARS

  4. SINGLE, ~100 ms LONG BURST (MOST COMMON)

  5. TWO SGR GIANT FLARES SGR1806-20 DECEMBER 27, 2005 RHESSI 20-100 keV Eγ=8x1045 erg SGR1900+14 AUGUST 27 1998 ULYSSES 25-150 keV Eγ=4x1044 erg 7.56 s period 5.16 s period Three phases: 1) Fast rise (<1 ms) 2) Very intense initial spike, ~100 ms long 3) Periodic decay (~300 s)

  6. GIANT SGR FLARES ARE SPECTACULAR! • Occur perhaps every 30 years on a given SGR • Second only to supernovae in intensity • Intense (Eγ≳1046 erg at the source, 1 erg/cm2 at Earth) • Very hard energy spectra (up to >10 MeV) • Create transient radio nebulae • Cause dramatic ionospheric disturbances • Should be detectable in nearby galaxies

  7. SHORT BURST ENERGY SPECTRA, 2-150 keV: SUM OF TWO BLACKBODIES, kT=3.4 and 9.3 keV(SGR1900+14, Feroci et al. 2004) BeppoSAX PDS kT=9.3 keV BeppoSAX MECS kT=3.4 keV R=2 km at 10 kpc R= 14 km at 10 kpc

  8. GIANT FLARE ENERGY SPECTRUM: 175 keV BLACKBODY, THEN 10 keV BLACKBODY

  9. THE SGRs ARE QUIESCENT X AND γ-RAY SOURCES • Luminosities: 1034 – 1036 erg/s (>spin-down energy) • This quiescent component varies slowly, and exhibits pulsations (~10-20% pulsed fraction)

  10. QUIESCENT X-RAY SOURCE ASSOCIATED WITH SGR1806-20 ASCA, 2-10 keV INTEGRAL-IBIS, 18-60 keV 10-11 erg cm-2 s-1 10-10 erg cm-2 s-1

  11. QUIESCENT X-RAY FLUX LEVEL IS RELATED TO THE BURSTING ACTIVITY SGR1806-20 Woods et al. 2006 GIANT FLARE Bursts and quiescent emission are probably both related to magnetic stresses on the surface of the neutron star

  12. . . P and P-dot FROM QUIESCENT SOFT X-RAYS (2-10 keV)SGR1900 (P=5.16 s, P=10-10 s/s) SGR1806 (P=7.48 s, P~10-10 s/s) Hurley et al. 1999 Kouveliotou et al. 1998 10-10 s/s 8x10-11 s/s Woods et al. 1999 Woods et al. 2000

  13. SPINDOWN IS IRREGULAR, SOMETIMES RELATED TO BURSTING ACTIVITY, SOMETIMES NOT RELATED (Woods et al. 2002, 2006) SGR1900+14 Woods et al. 2006 GIANT FLARE This argues against accretion as the cause of the bursts

  14. BROADBAND QUIESCENT X-RAY SPECTRA Blackbody < 10 keV, Power Law > 20 keV SGRs AXPs Götz et al. 2006

  15. . High B Radio Pulsars MAGNETARS COMPARED TO OTHER NS: P-P DIAGRAM SGRs, AXPs Radio Pulsars Millisecond Radio Pulsars V. Kaspi 2006

  16. ESSENTIAL SGR PROPERTIES *initially thought to be an AXP

  17. HOSTS AND PROGENITORS • One or two SGRs are probably in supernova remnants • One SGR may have been ejected from its supernova remnant • Two SGRs are probably in massive star clusters • The SNR association implies a normal progenitor mass (~5-8 M) • The massive cluster association implies a massive progenitor (~50M)

  18. THE SGR-SNR CONNECTION • SGR0525-66 is almost certainly in the N49 SNR in the LMC • 1E1547-5408 lies within the radio SNR G327.24-0.13 • SGR0501-4516 may have been ejected from its supernova remnant

  19. MASSIVE CLUSTER-SGR ASSOCIATIONS SGR1900+14 • ~13 stars • 1-10 Myr old (Vrba et al. 2000) SGR1806-20 • ~12 stars • 3-5 Myr old • SGR progenitor mass ~48M (Fuchs et al. 1999; Bibby et al. 2008)

  20. COUNTERPARTS • Two SGRs exhibited transient radio nebulae after giant flares • One SGR has a persistent radio counterpart • One SGR has a variable NIR counterpart

  21. SGRs ARE TRANSIENT RADIO SOURCES AFTER GIANT FLARES: RADIO NEBULA CREATED BY GIANT FLARE FROM SGR1806-20 (Taylor et al. 2005) VLA THIS RADIO EMISSION COMES FROM AN EXPANDING CLOUD OF RELATIVISTIC ELECTRONS ACCELERATED IN THE MAGNETOSPHERE AND EXPELLED. BUT SGRs ARE NOT OBSERVABLE QUIESCENT RADIO SOURCES

  22. SGR1806-20 IS INVISIBLE IN THE OPTICAL (nH~6x1022 cm-2), BUT IT IS JUST BARELY VISIBLE IN THE INFRARED 1.5″ 10″ mKs=20 m K’ =22 Israel et al. 2005 Kosugi et al. 2005 This is the only optical or IR counterpart to an SGR so far

  23. IR FLUX IS NOT AN EXTRAPOLATION OF HIGH ENERGY QUIESCENT FLUX • But the IR flux varies with the quiescent flux and/or with bursting activity IR X-γ Israel et al. 2005

  24. ESSENTIAL SGR PROPERTIES

  25. HOW MANY SGRs ARE THERE? • 5 or 6 confirmed SGRs (depending on 1E1547) • Three unconfirmed SGRs: 1801-23, 1808-20, GRB050925 • Some short GRBs could be extragalactic giant magnetar flares • Muno et al. (2008) estimated <540 in the galaxy, based on Chandra, XMM data

  26. CONCLUSIONS • There is good evidence that the known SGRs are magnetars • There is growing evidence that the members of the magnetar family (AXPs and SGRs) are very similar to one another

  27. SGRs AND AXPs OBSERVATIONAL PROPERTIES COMPARED

  28. In 1992, Duncan and Thompson, and Paczyński, independently proposed that neutron stars with large magnetic fields could explain SGR bursts and giant flares • In 1995, Thompson and Duncan expanded their model to explain the AXPs • Duncan and Thompson called these neutron stars magnetars • Motivation: • High B  low opacity, so L>>LEddington is allowed • High B  neutron star magnetosphere can contain the energy of the radiating electrons in a giant flare • High B causes rapid spindown of newly born pulsar – in the case of SGR0525, the age of the SNR is 10,000 y, and the period is 8 s • High B is a reservoir of energy to power the quiescent emission and the bursts

  29. MAGNETARS • Definition: a neutron star in which the magnetic field, rather than rotation, provides the main source of free energy; the decaying field powers electromagnetic radiation (R. Duncan & C. Thompson, 1992; C. Thompson & R. Duncan, 1995, 1996) • Note that the definition does not specify the magnetic field strength • To explain SGRs and AXPs, however, B must be greater than the quantum critical value 4.4 x 1013 G, where the energy between electron Landau levels equals their rest mass • Some AXPs and SGRs require B~1015 Gauss, so these magnetars have the strongest cosmic magnetic fields that we know of in the universe

  30. ORIGIN OF THE MAGNETIC FIELD • Unknown, but there are two hypotheses: • Fossil field: massive (25 M) progenitor star’s field (104 G or more) is amplified during core collapse and frozen into highly conducting compact remnant (the neutron star). Initial period of neutron star is 4-10 ms, too slow for a dynamo to operate efficiently. • Dynamo amplification: field is generated by convective dynamo in the proto-neutron star. Initial period is 1-3 ms. • These hypotheses are not mutually exclusive

  31. MAKING A MAGNETAR WITH A DYNAMO(Duncan & Thompson 1992) • A neutron star undergoes vigorous convection in the first ~30 s after its formation • Coupled with rapid rotation (~1 ms period), this makes the neutron star a likely site for dynamo action • If the rotation period is less than the convective overturn time, magnetic field amplification is possible • In principle, B ~ 3 x 1017 G can be generated (magnetic field energy should not exceed the binding energy of a neutron star, so B<5 x 1018 G)

  32. Differential rotation and magnetic braking quickly slow the period down to the 5-10 s range • Magnetic diffusion and dissipation create hot spots on the neutron star surface, which cause the star to be a quiescent, periodic X-ray source • The strong magnetic field stresses the iron surface of the star, to which it is anchored • The surface undergoes localized cracking, shaking the field lines and creating Alfvèn waves, which accelerate electrons to ~100 keV; they radiate their energy in short (100 ms) bursts with energies 1040 – 1041 erg (magnitude 19.5 crustquake) • There is enough energy to power bursting activity for 104 y

  33. Thompson & Duncan 1995 B≈1015 G NEUTRON STAR

  34. Localized cracking can’t relieve all the stress, which continues to build • Over decades, the built-up stress ruptures the surface of the star profoundly – a magnitude 23.2 starquake • Magnetic field lines annihilate, filling the magnetosphere with MeV electrons • Initial spike in the giant flare is radiation from the entire magnetosphere (>1014 G required to contain electrons) • Periodic component comes from the surface of the neutron star

  35. THE STATISTICS OF SHORT SGR BURSTS ARE CONSISTENT WITH THE MAGNETAR MODEL • Burst durations • Distribution of the time between bursts • Number-Intensity relation for short bursts

  36. STATISTICS:DISTRIBUTION OF SHORT BURST DURATIONS (Gogus et al. 2001) LOGNORMAL LOGNORMAL

  37. STATISTICS:DISTRIBUTION OF THE TIME BETWEEN BURSTS SGR1900+14 Gogus et al. 1999 RXTE SGR1900 Gogus et al. 1999 LOGNORMAL

  38. STATISTICS:NUMBER-INTENSITY DISTRIBUTION Götz et al. 2006 POWER LAW

  39. STATISTICSDISTRIBUTIONS OF SGR PROPERTIES • Lognormal duration and waiting time distributions, and power law number-intensity distribution, are consistent with: • Self-organized criticality (Gogus et al. 2000) • system (neutron star crust) evolves to a critical state due to a driving force (magnetic stress) • slight perturbation can cause a chain reaction of any size, leading to a short burst of arbitrary size (but not a giant flare) • A set of independent relaxation systems (Palmer 1999) • Multiple, independent sites on the neutron star accumulate energy • Sudden releases of accumulated energy

  40. OUTLINE • History • SGRs • Bursts • X-and γ-ray time histories, giant flares, QPO’s • X-ray afterglows • energy spectra, lines • Quiescent emission in X- and γ-rays • AXPs • Interpretation of the data • Data at other wavelengths: radio, optical • Non-electromagnetic emissions: gravitational radiation • Magnetar locations: SNRs, massive clusters • Magnetar census • Terrestrial effects of giant flares • Extragalactic magnetars • The latest news (SGR0501, AXP 1E1547)

  41. AXPs • At least 4 AXPs have optical/IR counterparts • All have an IR excess with respect to an extrapolation of their X-ray blackbody spectra • One or two AXPs display transient, pulsed radio emission

  42. OUTLINE • History • SGRs • Bursts • X-and γ-ray time histories, giant flares, QPO’s • X-ray afterglows • energy spectra, lines • Quiescent emission in X- and γ-rays • AXPs • Interpretation of the data • Data at other wavelengths: radio, optical • Non-electromagnetic emissions: gravitational radiation • Magnetar locations: SNRs, massive clusters • Magnetar census • Terrestrial effects of giant flares • Extragalactic magnetars • The latest news (SGR0501, AXP 1E1547)

  43. Magnetars may be deformed during bursts and especially during giant flares • QPOs may be evidence of this deformation • It follows that they may be sources of gravitational radiation

  44. LIGO LIMITS ON GRAVITATIONAL RADIATION • Upper limit to GR from GRB070201, an SGR giant flare in M31 (Abbott et al. 2008) • Search for GR from SGR0501 bursts is in progress

  45. OUTLINE • History • SGRs • Bursts • X-and γ-ray time histories, giant flares, QPO’s • X-ray afterglows • energy spectra, lines • Quiescent emission in X- and γ-rays • AXPs • Interpretation of the data • Data at other wavelengths: radio, optical • Non-electromagnetic emissions: gravitational radiation • Magnetar locations: SNRs, massive clusters • Magnetar census • Terrestrial effects of giant flares • Extragalactic magnetars • The latest news (SGR0501, AXP 1E1547)

  46. BUT SGR1900+14 IS NOT ASSOCIATED WITH THE SNR G42.8+0.6! If the SGR originated in the SNR, a proper motion of 110 mas/yr is implied DeLuca et al. (2008) have set an upper limit to the proper motion using 5 years of Chandra data: < 70 mas/yr SGR1900

  47. AXPs • 3 or 4 AXPs are at the geometrical centers of SNRs • Implied proper motions are small, and these associations are considered to be likely • 1 AXP is in the cluster Westerlund 1

  48. OUTLINE • History • SGRs • Bursts • X-and γ-ray time histories, giant flares, QPO’s • X-ray afterglows • energy spectra, lines • Quiescent emission in X- and γ-rays • AXPs • Interpretation of the data • Data at other wavelengths: radio, optical • Non-electromagnetic emissions: gravitational radiation • Magnetar locations: SNRs, massive clusters • Magnetar census • Terrestrial effects of giant flares • Extragalactic magnetars • The latest news (SGR0501, AXP 1E1547)

  49. OUTLINE • History • SGRs • Bursts • X-and γ-ray time histories, giant flares, QPO’s • X-ray afterglows • energy spectra, lines • Quiescent emission in X- and γ-rays • AXPs • Interpretation of the data • Data at other wavelengths: radio, optical • Non-electromagnetic emissions: gravitational radiation • Magnetar locations: SNRs, massive clusters • Magnetar census • Terrestrial effects of giant flares • Extragalactic magnetars • The latest news (SGR0501, AXP 1E1547)

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