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SGRs and AXPs: Massive Rotating White Dwarfs versus Magnetares

Manuel Malheiro, Jorge Rueda, Remo Ruffini ICRA Sapienza Universitá di Roma, ICRANet, ITA

http://arxiv.org/abs/1102.0653

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I – SGRs and AXPs : Unusual X-ray Neutron Star Pulsars.

III – Magnetar x White Dwarf Physical Scales.

II – Recent Observations: SGR 0418+5729, Massive and Highly Magnetized White Dwarf Pulsars.

IV – SGRs and AXPs as fast rotating powered Magnetic White Dwarf Pulsars.

V – Outbursts mechanism: change of rotational energy from glitches.

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I- SGRs and AXPs: Unusual X-Ray Pulsars• Soft Gamma Repeaters (SGR) and Anomalus X-Ray Pulsars (AXP) are very slow rotating pulsars: long Periods P ~ (2 -11) s. Periods in a narrow range comparing to ordinary pulsarsP ~ (0.001 – 1) s .

• Spin-down rates dP/dt ~ (10-13 – 10-10 ) s larger than normal pulsar (10-15 – 10-14 ) s .

• Isolated stars and only 18 confirmed ( 9 SGRs and 9AXPs) comparing with more than 2000 pulsars known.

• Strong outburst of energies ~ (1041 - 1043) erg . SGRs, giant flares of even large energies ~ (1044 – 1047) erg.

• High X-ray luminosities Lx ~ (1032 – 1036) erg/s >> spin-down rotational energy rate (seen as neutron star pulsars).

• A reservoir of energy to explain the X-ray Luminosity : huge magnetic fields in the interior and at the surface of the star B ~ (1014 -1015) G .

• MAGNETARS - Magnetic-powered X-Ray pulsars (seen as neutron stars).

• Hot Stars – T ~ (0.4 – 0.6) KeV , larger than 106 K.

• Young Stars – spin-down ages (103-105) yr .

SGRs • Recurrent sources of soft gamma ray burst, short times t ~ 0.1 s and

high Luminosity Lx ~ (1037 – 1042) erg/s .Hard X-ray emission T ~ 100 KeV .

• Occasionaly active episodes producing many short X-Ray bursts. Lasting weeks or months, separated by quiescent phases of years or decades.

• Extremely rare: giant flares E ~ (1044-1047) erg (~ one at each 50 ys).

AXPs• Similar to SGR but weaker in intensity.• No recurrent gamma ray bursts

McGill catalogue of SGRs&AXPshttp://www.physics.mcgill.ca/~pulsar/magnetar/main.htmlRecent Review S. Meregheti, Astron. Astrophys. Rev.,15, 255(2008)

Issues of Magnetar Model• Inconclusive measurements of cyclotron resonance

lines to estimate the magnetic field strength. • No high-energy gamma emission of Magnetars

found recently by Fermi sattelite.• Not viable to relate magnetars to the energy of

supernova remnants, to formation of Black Holes or to Gamma Ray Burst (controversial).

• Very hot stars and almost the same temperature , not explained by neutron star thermal evolution.

Rotation-powered PulsarsNeutron Stars

N~1057 particlesM~1−2 Msol

R ~ 10-12 kmB ~ 108 .... 13 G, T ~ 106 .... 11 K

Spin fast - Periods (0.1 to 0.001) s

Spin down rate is small ~ 10-15 s/s

Efficiency η=Lx /(dErot/dt) ~10-2 -10-1

ρ ~1015g/cm3

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Neutron stars spinning fast emite pulsated radiation PULSAR

Crab Pulsar - PSR 0531+21( discovered in 1968 )P = 33 ms 30 rounds /s F. Pacini (1967/68) T. Gold (1968/69)

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Rotational periods of seven pulsars

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Rotational Energy Loss of Crab Pulsar

Energy radiated away by magnetic dipole of Crab Pulsar

Dipolar Magnetic Field

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SGRs andAXP (anomalous X-ray pulsars)

II – Recent Observations: SGR 0418+5729, Massive and Highly Magnetized White Dwarf

Pulsars.

• SGR 0418+5729 with Period P = 9.04 s with a low surface magnetic field B < 7.5 x 1012 G.

• Large spin down: an upper limit dP/dt < 6.0 x10-15s/s• Old star among SGRs: characteristic age t~ 24 Myr• Luminosity Lx = 6.2 x 1031 erg/s• Lx~ 200 dErot/dt• To explain the luminosity an interior toroidal magnetic field 100

times larger than the surface magnetic field is needed.• High Temperature T = 0.67 KeV similar to T of new SGRs.

N. Era, P. Esposito, R. Turolla, et al. Science, 330, 994 (2010)

Massive and Highly Magnetized White Dwarf Pulsars.

• Catalogue of more than 100 isolated massive white dwarfs and 25% are magnetic (M. Nalezyty and J. Madej 2004)

• four most massive white dwarfs M > 1.3 Msun are magnetic with B ~ 106 -109 G

• Magnetic white dwarfs have higher temperatures T~105 K • AE Aquarii : Hard X-Ray White Dwarf Pulsar found by

Suzaku (Y. Terada et al. 2008). Very fast and high B.• Similar properties of SGR 0418+5729: P = 33 s,

dP/dt = 5.64 10-14 s/s, Lx~1031 erg/s (< 4 Kev),B = 3 x108 G , T~ 0.5 KeVcharacteristic age t~ 9.4 Myr

III – Magnetar x White Dwarf Physical Scales.

• Paczynski (1990) – Massive fast rotating highly magnetized white dwarf for 1E 2259+586 (AXP) at SNR G109.1-1.0 (period of P = 6.98 s)

• Kepler frequency Ω κ = (G M/R3) ½ ~ (G ρ ) ½ ; Ω < ΩkP ~ 1 s , the average density ρ > 108 g/cm3 .

• Low density stars cannot spin fast .

• High density and massive white dwarfs: M= 1.4 Msun the maximal radius is R~1000Km = 102 RNS. • Momentum of Inertia INS ~ M R2 = 1045 g.cm2

• IWD = 104 INS = 1049 g.cm2

Observed Long Periods as a manifestation of the low white dwarfmass densities – cannot spin faster than two seconds

First red point – AXP unconfirmed - CXOU J171405.7-381031, P= 3.825 s

New scale of rotational energy losses

New scale of Dipolar Surface Magnetic Field

White Dwarf Magnetar

Soft Gamma Repeaters

Anomalus X-Ray Pulsars

Very Different Magnetic Field ScalesWhite Dwarfs Magnetars

White Dwarf Model – no more huge magnetic fields, all undercritical !!!

IV – SGRs and AXPs as fast rotating-powered Magnetic White Dwarf Pulsars.

White Dwarf Pulsar - dErot/dt ~10^2 to 10^5 Lx• Blue squares: SGR 1627-41 (P=2.6 s), 1E 1547.0-4950 (P=4.33s)• PSR 1627-41 (P=2.07 s) (the faster SGRs and AXPs)

White Dwarfs Neutron Star

Efficiency η = Lx /(dErot/dt) as a function of the star age

Magnetar Model – η is multiplied by 10^4 because small m. inertia I

Prediction of spin-downlower limit of SGR 0418+5729

• Maximal value of the Efficiency is one: Lx= dErot/dt

• Lower limit for Surface Dipolar Magnetic Field.

1.05 x 108 G < B < 7.47 x 108 G

V – Outbursts mechanism: change of rotational energy from glitches.

Delta E_rot = -2*pi^2*I*(Delta P/P)/P^2

Observed Glitches

• 1E 2259+586 - |Delta P/P| ~ 4 x 10-6

• 1E 1048.1-5937 - |Delta P/P| ~ 1.63 x 10-5

• RXS J17849.0-4000910 - |Delta P/P| ~ 1.7 x 10-6

• 1E 1841-045 - |Delta P/P| ~ 5.9 x 10-6

• Giant Flares large |Delta P/P| ~ 10-4 - 10-3