Lecture 4.Magnetars: SGRs and AXPsSergei Popov (SAI MSU)

MagnetarsdE/dt > dErot/dtBy definition: The energy of the magnetic field is released P-PdotDirect measurements of the field (Ibrahim et al.)Magnetic fields 10141015 G

Known magnetarsSGRs 0526-661627-411806-201900+14+candidatesAXPs CXO 010043.1-724U 0142+611E 1048.1-5937CXOU J164710.3-1 RXS J170849-40XTE J1810-1971E 1841-045AX J1844-02581E 2259+586

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MagnetarsStrong convection in a rapidly rotating (P ~ 1 ms) newborn neutron star generates a very strong magnetic field via dynamo actionMagnetars: neutron stars with surface field B > 10 BQED ~ 4 x1014 G (Duncan & Thomson 1992; Thomson & Duncan 1993)Rapid spin-down due to magneto-dipolar losses.

Magnetars on the Galaxy4 SGRs, 9 AXPs, plus candidates, plus radio pulsars with high magnetic fieldsYoung objects (about 104 year).Probably about 10% of all NSs.

Soft Gamma Repeaters: main propertiesEnergetic Giant Flares (GFs, L 1045-1047 erg/s) detected from 3 (4?) sourcesNo evidence for a binary companion, association with a SNR at least in one casePersistent X-ray emitters, L 1035 erg/sPulsations discovered both in GFs tails and persistent emission, P 5 -10 sHuge spindown rates, /P 10-10 ss-1

Soft Gamma Repeaters - IRare class of sources, 4 confirmed (+ 1): SGR 1900+14, SGR 1806-20, SGR 1627-41 in the Galaxy and SGR 0526-66 in the LMC Frequent bursts of soft -/hard X-rays: L ~ 1041 erg/s, duration < 1 sBursts from SGR 1806-20 (INTEGRAL/IBIS,,Gtz et al 2004)

Historical notes05 March 1979. The Konus experiment & Co. Venera-11,12 (Mazets et al., Vedrenne et al.)Events in the LMC. SGR 0520-66.Fluence: about 10-3 erg/cm2Mazets et al. 1979

N49 supernovaremnant in theLarge Magellaniccloud(e.g. G. Vedrenne et al. 1979)

Main types of activity of SGRsWeak bursts. L

Normal bursts of SGRs and AXPsTypical bursts of SGR 1806-29, SGR 1900+14 and of AXP 1E 2259+586 detected by RXTE (from the review by Woods, Thompson, astro-ph/0406133)(from Woods, Thompson 2004)

Intermediate SGR bursts Examples of intermediate bursts. The forth (bottom right) is sometimes defined as a giant burst (for example by Mazets et al.).

(from Woods, Thompson 2004)

Giant flare of the SGR 1900+14 (27 August 1998)Ulysses observations (figure from Hurley et al.)Initial spike 0.35 sP=5.16 sL>3 1044 erg/sETOTAL>1044 ergHurley et al. 1999

SGRs: periods and giant flares

0526-661627-411806-201900+14See the review inWoods, Thompsonastro-ph/0406133

Anomalous X-ray pulsarsIdentified as a separate group in 1995. (Mereghetti, Stella 1995 Van Paradijs et al.1995) Similar periods (5-10 sec) Constant spin down Absence of optical companions Relatively weak luminosity Constant luminosity

Anomalous X-ray Pulsars: main propertiesEight sources known (+ 1 transient): 1E 1048.1-5937, 1E 2259+586, 4U 0142+614, 1 RXS J170849-4009, 1E 1841-045, CXOU 010043-721134, AX J1845-0258, CXOU J164710-455216 (+ XTE J1810-197) + candidatesPersistent X-ray emitters, L 1034 -1035 erg/sPulsations with P 5 -10 sLarge spindown rates, /P 10-11 ss-1No evidence for a binary companion, association with a SNR in three cases

Known AXPs Sources Periods, s

CXO 010043.1-728.04U 0142+618.71E 1048.1-59376.4CXOU J164710.2-10.61RXS J170849-4011.0XTE J1810-1975.51E 1841-04511.8AX J1845-02587.01E 2259+5867.0

Are SGRs and AXPs brothers?Bursts of AXPs (from 5 now)Spectral propertiesQuiescent periods of SGRs (0525-66 since 1983)

Gavriil et al. 2002

A Tale of Two Populations ?R < ctrise 300 km: a compact objectPulsed X-ray emission: a neutron star

SGRs: bursting X/-ray sources

Single class ofobjectsAXPs: peculiar class of steady X-ray sources A Magnetar

Pulse profilesof SGRs and AXPs

SGRs and AXPs

Hard X-ray EmissionINTEGRAL revealed substantial emission in the 20 -100 keV band from SGRs and APXsHard power law tails with 1-3Hard emission pulse

SGRs and AXPs X-ray Spectra 0.5 10 keV emission is well represented by a blackbody plus a power lawAXP 1048-5937 (Lyutikov & Gavriil 2005)

SGRs and AXPs X-ray Spectra kTBB ~ 0.5 keV, does not change much in different sourcesPhoton index 1 4, AXPs tend to be softerSGRs and AXPs persistent emission is variable (months/years)Variability is mostly associated with the non-thermal component

Hardness vs Spin-down RateHarder X-ray spectrumLarger Spin-down rateCorrelation between spectral hardness and spin-down rate in SGRs and AXPs (Marsden & White 2001)Correlation holds also for different states within a single source (SGR 1806-20, Mereghetti et al 2005; 1 RXS J170849-4009, Rea et al 2005)

Theory of magnetarsThompson, Duncan ApJ 408, 194 (1993)Convection in a protoNS results in generation of strong magnetic fieldReconfiguration of the magnetic field structure(Figures from the web-page of Duncan)

Generation of the magnetic fieldThe mechanism of the magnetic field generation is still unknown.

Turbulent dynamo

- dynamo (Duncan,Thompson) 2 dynamo (Bonanno et al.) or their combinationIn any case, initial rotation of aprotoNS is the critical parameter.

Strong field via flux conservationThere are reasons to suspect that the magnetic fields of magnetars are not due to any kind of dynamo mechanism, but just due to flux conservation:Study of SNRs with magnetars (Vink and Kuiper 2006). If there was a rapidly rotating magnetar then a huge energy release is inevitable. No traces of such energy injections are found.

There are few examples of massive stars with field strong enough to produce a magnetars due to flux conservation (Ferrario and Wickramasinghe 2006)

Still, these suggestions can be criticized

Alternative theoryRemnant fallback discMereghetti, Stella 1995Van Paradijs et al.1995Alpar 2001Marsden et al. 2001Problems ..How to generate strong bursts?Discovery of a passive disc in one of AXPs (Wang et al. 2006). New burst of interest to this model.

Magnetic field estimatesDirect measurements of magnetic field (cyclotron lines)Spin downLong spin periods Ibrahim et al. 2002

SGR 1806-20 - I SGR 1806-20 displayed a gradual increase in the level of activity during 2003-2004 (Woods et al 2004; Mereghetti et al 2005) enhanced burst rate increased persistent luminosityThe 2004 December 27 EventBursts / day (IPN)20-60 keV flux (INTEGRAL IBIS) Mereghetti et al 2005

SGR 1806-20 - IIFour XMM-Newton observations (last on October 5 2004, Mereghetti et al 2005)Pulsations clearly detected in all observations ~ 5.5x10-10 s/s, higher than the historical valueBlackbody component in addition to an absorbed power law (kT ~ 0.79 keV)Harder spectra: ~ 1.5 vs. ~ 2The 2-10 keV luminosity almost doubled (LX ~ 1036 erg/s)

Twisted Magnetospheres IThe magnetic field inside a magnetar is wound upThe presence of a toroidal component induces a rotation of the surface layersThe crust tensile strength resists A gradual (quasi-plastic ?) deformation of the crustThe external field twists up (Thompson, Lyutikov & Kulkarni 2002)Thompson & Duncan 2001

A Growing Twist in SGR 1806-20 ?Evidence for spectral hardening AND enhanced spin-down-Pdot and -L correlationsGrowth of bursting activityPossible presence of proton cyclotron line only during bursts All these features are consistent with an increasingly twisted magnetosphere

Some conclusions and future developmentsTwisted magnetosphere model, within magnetar scenario, in general agreement with observationsResonant scattering of thermal, surface photons produces spectra with right propertiesMany issues need to be investigated furtherTwist of more general external fieldsDetailed models for magnetospheric currentsMore accurate treatment of cross section including QED effects and electron recoil (in progress)10-100 keV tails: up-scattering by (ultra)relativistic (e) particles ?Create an archive to fit model spectra to observations (in progress)

Hyperflare of SGR 1806-2027 December 2004 A giant flare from SGR 1806-20 was detected by many satellites: Swift, RHESSI, Konus-Wind, Coronas-F, Integral, HEND, 100 times brighter than any other!Palmer et al.astro-ph/0503030

IntegralRHESSICORONAS-F

27 Dec 2004: Giant flare of the SGR 1806-20Spike 0.2 sFluence 1 erg/cm2E(spike)=3.5 1046 ergL(spike)=1.8 1047 erg/sLong tail (400 s) P=7.65 sE(tail) 1.6 1044 ergDistance 15 kpc

Konus observationsMazets et al. 2005

The myth about Medusa

QPO in tails of giant flares of SGRs(Israel et al. 2005 astro-ph/0505255,Watts and Strohmayer 2005 astro-ph/0608463)

SGRs: monitoring and extraG[D. Frederiks et al. astro-ph/0609544]

What is special about magnetars?Westerlund 1Link with massive starsThere are reasons to suspect that magnetars are connected to massive stars.

Link to binary starsThere is a hypothesis that magnetars are formed in close binary systems (astro-ph/0505406).The question is still on the list.

Neutron stars and massive progenitors(astro-ph/0611589)AXP in Westerlund 1 most probably has a very massive progenitor >40 Msolar.

Are there magnetors in binaries?MagnetorAt the moment all known SGRs and AXPs are isolated objects.About 10% of NSs are expected to be in binaries.The fact that all known magnetars are isolated can be related to their origin, but this is unclear.If a magnetar appears in a very close binary system, then an analogue of a polar can be formed.The secondary star is inside the huge magnetosphere of a magnetar.This can lead to interesting observational manifestations.

Binaries with magnetars - magnetorsCan RCW 103 be a prototype?6.7 hour period (de Luca et al. 2006)RCW 103Possible explanations:Magnetar, spun-down by discDouble NS systemLow-mass companion + magnetar= magnetor