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Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters Sandro Mereghetti INAF - IASF Milano

This article provides a brief overview of the properties of Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs). It discusses recent results from XMM-Newton, including variability in AXP 1E1048 and spectral evolution in bursts from SGR 1806. The article also explores the relationship between AXP and SGR and presents models for their rotational energy and accretion processes.

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Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters Sandro Mereghetti INAF - IASF Milano

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  1. Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters Sandro Mereghetti INAF - IASF Milano

  2. OUTLINE Short review of AXP and SGR properties AXP / SGR relationship 2 recent results XMM-Newton > Variability in AXP 1E1048 INTEGRAL > Spectral evolution in bursts from SGR 1806

  3. NORMAL x-ray pulsars are rotating magnetized neutron stars 1) In binary systems powered by accretion from a companion star e.g. Vela X-1, Cen X-3 Periods from 60 ms to a few 1000 s 2) rotation powered radio pulsars e.g. Crab , Geminga, PSR 1957+20 Periods from 1.5 ms - a few seconds

  4. AXP in the context: Accreting pulsars Maximum X-ray Luminosity (erg/s) AXP are only found in the narrow period range 5-12 s Spin Period (s) Most accreting pulsars are in massive binaries

  5. AXP PROPERTIES No evidence for massive companion starslimits on ax sin i from timing limits on Fx/Fopt from optical/IR observations Period of a few seconds (6-12 s) Almost steady spin down Very soft X-ray spectrumkTBB < 0.5 keV ph > 3-4

  6. AXP have very soft X-ray spectra AXP

  7. AXP PROPERTIES 2 ( or 3 ? ) are in SNRs X-ray luminosity Lx = 1034 - 1036 erg s-1Lx > rotational energy loss for a neutron star

  8. ROTATION-POWERED PULSARS (Possenti et al. 2002) . Lx = Erot X-RAY LUMINOSITY SPIN - DOWN ENERGY LOSS

  9. Operational definition of AXP a spinning down pulsar, with a soft X-ray spectrum, apparently not powered by accretion from a (massive) companion star, and with luminosity larger than the rotational energy loss (assuming a neutron star)

  10. AXP census P dP/dt (s) (10-11 s/s) 4U 0142+61 8.7 0.2 - 1E 2259+586 7 0.05 CTB 109 1E 1048-5937 6.4 2-3 - 1E 1841-045 11.8 4 Kes 73 AX J1845-03 7 - G296+0.1, Var. RXS 1708-40 11 2 - CXO J0110-72 8 2 in SMC XTE J1810-197 5.5 1.8 - Var.

  11. MODELS ROTATIONAL ENERGY - isolated NS - isolated WD ? ? ACCRETION - very low mass companion - ISM / molecular clouds - disk around isolated NS • MAGNETIC ENERGY - field decay - enhanced thermal emission

  12. Isolated NS + accretion disk Thorne-Zytkow object(Van Paradijs et al. 1995, Ghosh et al. 1997) fall back after SN explosion(Chatterjiee et al. 2000, Alpar 2001) capture of SNR ejecta by fast moving NS(Marsden et al. 2000, 2001)

  13. Isolated NS with B > 1014 Gauss Originally developed for SGRs where evidence for high B is stronger due to large flares and bursts Extended to AXP due to similar P and dP/dt values Magnetar model (Thompson and Duncan) Emission powered by magnetic field decay and/or enhanced cooling

  14. Soft Gamma-ray Repeaters • Initially discovered as a peculiar class of Gamma-Ray Bursts • soft • repeating • About 5 currently known (1 in the LMC) • Not always active (long quiescent periods)

  15. SGRs vs. GRBs Durations Spectra Courtesy K. Hurley

  16. Energetics of SGRs Short Bursts: Peak Luminosity ~1038-1042 erg s-1 Total Energy ~1039-1042 erg Giant Flares: Peak Luminosity ~4 x 1044 erg s-1 Total Energy ~0.7-2 x 1044 erg

  17. Giant Flares 1998 August 27 from SGR 1900+14 1979 March 5 from SGR 0526-66 Feroci et al. 1999 Mazets et al. 1979, Cline et al. 1980

  18. Persistent X-ray emission from SGRs • Lx = 1035 -1036 erg /s (1-10 keV) • Pulsations with periods 5 - 10 s • secular spin-down at 10-11 s/s • power law (+ blackbody) spectra VERY SIMILAR TO AXPs !!

  19. AXP

  20. (Kaspi et al. 2003) SGR-like activity in the AXP 1E2259+586

  21. bursts have Lpeak: 1036-4 1038 erg/s Change in pulse morphology Glitch  = 4 10-6 (Kaspi et al. 2003)

  22. AXP=SGR ? Only observational selection effects introduced a distinction between these sources belonging to the same class of objects: in AXP the quiescent, pulsating emission was discovered first SGR were discovered through their bursts

  23. OUTLINE Short review of AXP and SGR properties AXP / SGR relationship 2 recent results XMM-Newton > Variability in AXP 1E1048 INTEGRAL > Spectral evolution in bursts from SGR 1806

  24. (Mereghetti et al. 2004) 2 EPIC XMM-Newton observation First evidence for significant variability in 1E 1048-59

  25. 2000 89 % 2003 53 % The pulsed fraction decreased while the flux increased Spectrum did not vary BB+PL kT=0.6 keV  = 3

  26. 4U 0142+61 SAX (Israel et al 1999) Power law photon index = 3.9 + Blackbody kT = 0.4 keV

  27. Most AXP require 2 component model PL + BB Phot.index kTBB RBB NH 1022 1E 1048-59 2.9 0.63 keV 0.4 d3 km 1.0 4U0142+61 3.9 0.40 keV 1.8 d1 km 1.1 1E 2259+58 3.6 0.41 keV 2.6 d4 km 0.9 RXS1708-40 2.6 0.46 keV 7.9 d8 km 1.4 AXJ1845-00 - 0.64 keV 3.9 d15 km 6 1E 1841-0045 3.0 - - 2

  28. Are the two spectral components related to distinct emitting regions and/or physical processes ?

  29. (Oezel, Psaltis, Kaspi 2001) small energy dependence of pulsed fraction requires ad hoc tuning of the BB and PL components

  30. Despite the large flux variation the spectral shape did not vary BB+PL in both observations kT = 0.6 keV phot. Index = 3 ... these are the typical parameters seen in this source

  31. 2000 89 % 2003 53 % BB and PL are not physically distinct components The pulsed fraction decreased while the flux increased

  32. OUTLINE Short review of AXP and SGR properties AXP / SGR relationship 2 recent results XMM-Newton > Variability in AXP 1E1048 INTEGRAL > Spectral evolution in bursts from SGR 1806

  33. SGR1806-20 entered a new period of activity in July 2003 • An INTEGRAL ToO observation started on 3 September 2003, while the source was still active • INTEGRAL continued to observe SGR 1806-20 (l = 9.99 deg, b = -0.24 deg) during the Galactic Center Deep Exposre (GCDE) until mid October • 24 bursts were detected by IBIS in real time by the INTEGRAL Burst Alert System (IBAS) and confirmed later by off-line analysis

  34. GCDE Number of bursts/day ToO INTEGRAL Julian Day 24 bursts from SGR 1806-20 have been detected with the INTEGRAL Burst Alert System.

  35. GCDE Bursts

  36. 3-20 keV JEM-X Yoff = -0.97º Zoff = -2.22º 15-40 keV 40-100 keV IBIS/ISGRI Fluence (15-100 keV) 2.5×10-8 erg cm-2 100-200 keV

  37. Spectral Analysis 15-100 keV IBIS/ISGRI spectra of the bursts with more than 500 net counts Optically Thin Thermal Bremsstrahlung model provides good fits (power-law, blackbody, Band GRB model are ruled out) kT ÷ 32-42 keV Conversion factor (15-100 keV, <kT> = 38 keV) 1 count s-1 = 1.5x10-10 erg cm-2 s-1

  38. INTEGRAL Log N- Log P (Peak Flux distribution) INTEGRAL Log N- Log S (Fluence distribution)

  39. THE SGR BURSTS OBSERVED BY IBIS ARE NORMAL IN MOST RESPECTS • Durations, energy spectra are typical • However, the fluences are very low,~1.5x10-8 erg/cm2 , 25-100 keV • These are the among the weakest bursts seen from this SGR; thanks to imaging, we are certain that the source is indeed SGR1806-20

  40. 6 sigma detection 2-3 mCrab source IBIS (20-40 keV) (INTEGRAL CP data ~ 1 Msec, courtesy Ada Paizis)

  41. The MAGNETAR model predictions Highly magnetized (B~1015 G), slowly rotating (P~ 5-8 s) neutron stars Bursts are triggered by a sudden shift in the magnetospheric footpoints driven by a fracture in the neutron star crust The radiation originates from the cooling of an optically thick pair-photon plasma e+e- plasma Thompson & Duncan (1995)

  42. Photon Index Time (seconds since trigger) • For typical (~0.1 s long) bursts: No signifcant spectral evolution predicted and in general NOT observed up to now (e.g. Fenimore et al. 1994, Kouveliotou et al. 1987) SGR 1900+14: an exception Two peculiar bursts of intermediate duration (~1 s) and and with hard (kT~100 keV) spectra Soft-to-hard evolution Woods et al. (1999)

  43. Spectral Evolution of weak bursts with INTEGRAL 15-40 keV 40-100 keV Götz et al., 2004, A&A submitted

  44. Hardness Ratio Counts/s Hardness-Intensity Anticorrelation with INTEGRAL (bursts with more than 200 net counts) Götz et al., 2004, A&A submitted

  45. Conclusions 1) XMM / EPIC detected the first significant variation in the flux and pulsed fraction of the AXP 1048the spectral invariance is a further evidence that the PL+BB spectral decomposition does not have a physical meaning 2) INTEGRAL / IBIS detected the first evidence for spectral evolution of fain SGR bursts as well as a hardness intensity anticorrelationthese properties are not (yet) foreseen in the magnetar model

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