Gravitational Waves from Magnetars Sandro Mereghetti INAF – IASF Milano

See review: Mereghetti 2008, A&A Rev. 15, 225 What is a Magnetar ? Isolated neutron stars where the main source of energy is the magnetic field [ most observed NS have B = 109 - 1012 G and are powered by accretion, rotational energy, residual internal heat ] In Magnetars external field: B = 1014 - 1015 G internal field: B > 1015 G See review: Mereghetti 2008, A&A Rev. 15, 225 [ arXiv:0804.0250 ]

Magnetars emit: “Persistent” X-rays short bursts of soft gamma-rays Lx~1035-36 erg/s ~1-200 keV pulsed at few seconds, spin-down short bursts of soft gamma-rays Lx ~1039-42 erg/s kT~30-40 keV durations ~0.1-1 sec Giant Flares Lx > 1044 erg/s very rare events (only three observed ) Hurley et al. 1999

Period – Period derivative plot for Magnetars (Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters ) NOTE : vertical bars indicate Pdot variability range

GW from rotating neutron stars Rotational deformation Konno et al. 2000, Normal pulsars are very weak GW emitters (ellipticity too small) Magnetars have larger deformations, but (for most of their life) rotate too slowly Magnetic deformation

Some possibilities for GW emission from magnetars 1) Magnetar formation the high magnetic field of magnetars is thought to result from a highly efficient dynamo requiring P<3 ms at birth What is the magnetars birth rate ?

Magnetars birthrate ~ a few every 104 years large uncertainties: small statistics (~10 persistent sources) uncertain lifetimes (~104 yrs ?), number and duty cycle of transient magnetars Keane & Kramer, 2008, MNRAS Birthrate of radio PSR and core collapse SN (1-3 / century) already in reasonable agreement  no much room for other populations of NS Magnetars ~ 0.1-0.3 / century i.e. up to ~10% of radio PSRs See also: Gill & Heyl 2007, MNRAS 381,52 (~0.22 / century + transients) Muno et al. 2008, ApJ 680, 639 (~0.3 – 6 / century )

Some possibilities for GW emission from magnetars 1) Magnetar formation the high magnetic field of magnetars is thought to result from a highly efficient dynamo requiring P<3 ms at birth What is the magnetars birth rate ? too small for Galactic events, but OK if detectable up to the Virgo cluster (~2000 galaxies at 20 Mpc) What is the GW luminosity of a magnetar formation event ? depends on intensity and geometry of internal and external field

Some possibilities for GW emission from magnetars 1) Magnetar formation (Stella et al. 2005, Dall’Osso et al 2008) the high magnetic field of magnetars is thought to result from a highly efficient dynamo requiring P<3 ms at birth ~1 / year expected in Virgo Cluster 2) Giant Flares (Andersson & Kokkotas 1998, de Freitas Pacheco 1998, Ioka 2001) extremely energetic events related to giant crustal fractures (“starquakes”) fast oscillations observed during giant flares – seismic oscillations

3 Giant Flares from SGRs 1979 March 5 - SGR 0626-66 Initial spike: 1.6 1044 erg Pulsating tail: 4 1044 erg 1998 August 27 - SGR 1900+14 Initial spike: > 7 1043 erg Pulsating tail: 5 1043 erg 2004 December 27 – SGR 1806-20 Initial spike: 4 1046 erg Pulsating tail: 1044 erg (Hurley et al. 2005, Palmer et al. 2005, Mereghetti et al. 2005, Terasawa et al. 2005, Boggs et al. 2007, Frederiks et al. 2007)

Implications of December 2004 Giant Flare E ~ 5 1046 ergs emitted in the Dec 2004 GF from SGR 1806-20 [~100 times more energetic than the other two observed GFs ] This sets a lower limit on the internal magnetic energy, depending on the number of such events in a magnetar’s lifetime (Stella et al. 2005): 1 such event in ~30 yrs from 5 SGRs  recurrence time ~15 yrs/magnetar ~70 events like Dec 2004 GF in ~104 yrs magnetar lifetime  total energy release ~4 1048 ergs (independent of beaming) B > 1015.7 G [ up to ~1016 G including also neutrino luminosity] CAVEATs: recurrence time is probably longer (including AXP and transients ) uncertainties in GF flux (and spectrum) measurement due to its exceptional brightness distance of SGR 1806 could be smaller than the assumed 15 kpc

2.8 light seconds Peak affected by instrument saturation Mereghetti et al. 2005, ApJ 624, L105 Initial giant pulse backscattered by the Moon SGR 1806-20 Giant Flare 2004 Dec 2004

SGR 1806-20 distance No direct methods ! Based on possible associations with other objects Distance of G10.0-0.3 (which is not a SNR, but a wind nebula powered by the LBV star) is well determined 15.1 [+1.8 -1.3] kpc. d of SGR assumes that it is a member of the cluster of massive stars to which also the LBV belongs. ~ 15 kpc (Corbel & Eikenberry 2004) (Bibby et al. 2008)

Magnetars formation Outer dipole field B~1014-15 G (Duncan & Thompson 1992) Very strong internal B-fields in a newborn differentially rotating fast-spinning neutron star For initial spin periods of Pi∼1–2 ms, differential rotation can store ∼1052 (Pi /1 ms)2 ergs, that can be converted into a magnetic field of up to 3x1017 (Pi /1ms)-1 G. Bd Bt Outer dipole field B~1014-15 G Inner toroidal field B > 1015 G

Toroidal inner field: prolate shape with ellipticity -6 Toroidal inner field: prolate shape with ellipticity -6.4x10-4 (Bt /1016.3 G)2 Poloidal inner B-field : Oblate star Toroidal inner B-field : Prolate star

Outer dipole field Inner toroidal field Symmetry axis of deformed star, in general, not (exactly) coaligned with the spin axis Viscous damping of free precession leads to an orthogonal rotator within ~1 day from birth Condition for fast orthogonalization for different initial periods (1, 2.6 ms) and initial angles (1, 2 degrees) (Dall’Osso, Shore & Stella 2008, MNRAS ) Inner toroidal field Outer dipole field

Advanced LIGO/Virgo expected S/N for newborn magnetar in Virgo Cluster (Dall’Osso, Shore & Stella 2008, arXiv:0811.4311)

(Dall’Osso, Shore & Stella 2008, arXiv:0811.4311) Dependence on initial spin period Pi = 0.97 ms Pi = 1.13 ms Pi = 2 ms Pi = 2.58 ms (Dall’Osso, Shore & Stella 2008, arXiv:0811.4311) Competition between dipole emission and GW energy losses The maximum S/N depends only on initial period

There is no evidence that the SNRs associated to Magnetars are more energetic than “standard” SNRs Vink & Kuiper 2006, MNRAS 370, L14

long orthogonalization timescale E SNR > 1051 ergs long orthogonalization timescale Outer dipole field Inner toroidal field

(Dall’Osso, Shore & Stella 2008, arXiv:0811.4311) Pi = 1 ms Pi = 1.13 ms S/N = 9 S/N = 8 S/N = 7 Pi = 2 ms Pi = 2.6 ms

Conclusions Gravitational waves can be revealed from newly born magnetars in the Virgo Cluster with advanced GW detectors …but this requires a favorable combination of initial spin period, dipole strength of external field, and internal magnetic energy Expected rate close to 1 per year [depending on fraction of magnetars satisfying above constraints] GW from Galactic magnetars possibly associated to oscillations after Giant Flares for upper limits see Baggio et al . 2005, Abbot et al. 2007, 2008

EXTRA SLIDES

Radio (unpulsed) Transient emission after (Giant) Flares Seen in two SGRs, but possibly present in all bright flares SGR 1900+14 VLA 8.46 GHz Frail et al. 1999, Nature

Radio emission after SGR 1806 giant flare Gelfand 2007 200 mJy 7 days after GF Gaensler et al. 2005 Cameron et al. 2005 Granot et al. 2006 Gelfand 2007

SGR 1806-20 Giant Flare of Dec 27, 2004 UHECR  High E neutrons can travel several kpc before decaying and arrive to earth (or produce n that maintain directionality) n flux depends on barion load of fireball (Ioka et al. 2005) Upper limits on n Auger (Anchordoqui et al. 2007, ICRC) AMANDA (Zornoza et al. 2006, Achterberg et al. 2006) RICE (Besson et al. 2007)

Discovery of QPO (Quasi Periodic Oscillations) in the tail of the Dec 27 Giant Flare Israel et al. 2005, ApJ 628, L53

High Frequency Oscillations in Giant Flares 0526-66 – 43 Hz (Barat etal.1983) 1806-20 – 90 Hz, ~18 Hz, ~30 Hz (Israel et al. 2005) 626 Hz (Watt & Strohmayer 2006) 1900+14 – 84 Hz, 28 Hz, 53 Hz, 155 Hz (Strohmayer & Watt 2005) Large scale NS crust fractures trigger global seismic oscillations (analogous to earthquakes) Torsional modes of NS crust  potentially important diagnostic for NS Caveats: Magnetic field coupling between crust and core  magnetic stresses would significantly reduce the mode amplitude in less than 1 sec and redistribute the energy within the liquid core (Levin 2006) or lead to global MHD oscillations (Glampedakis et al. 2006) Other possibilities? B confined in the crust or QPO originate in magnetosphere