FRB emission mechanisms. Maxim Lyutikov (Purdue, McGill)

Transcription

1 FRB emission mechanisms Maxim Lyutikov (Purdue, McGill)

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3 What FRBs are not msecs long (not AGNe) > 10 4 per day (not prompt GRBs) repetitive (not catastrophic events like NS-NS mergers) isotropic (extra-galactic); D ~ 1 Gpc - (cosmological) coherent emission, T b K (not just synchrotron) local DM ~ 100, constant over few yrs (not through early SN) Polarization - no clear idea (Petroff s talk) - If there is a road to Perytons-II, polarization is a suspect -Very unfortunately, there is no clear evidence for two or (18) different populations Chatterjee Tendulkar

4 Origin: only NS magnetospheres Instantaneous luminosity for the Repeater: L iso =4 D 2 ( F ) ergs 1 duration (assume intrinsic)-> size Equipartition B-field (lower limit): B eq = p p F D 8 c 3/2 = G (Lyutikov ) NS magnetospheres (Clean, relativistic) - Stellar-mass BHs (?) - accretion-supplied B-field is dirty. - WD magnetosphere(?) (Lyutikov, 2017) Oh, this murky business of pulsar/ns radio emission mechanisms 50 years of intense research, there are books published - please no cheesy order-of-magnitude scale this with that 4

5 Radio emission from NSs There are three types of coherent emission from NS: type-i, type-ii, type-iii Type I: log-normal dist., Crab precursor, polar caps, rotationally-driven Type II: GPs, power-law, Crab MP&IP, border between open/closed field lines, rotationally-driven Type III: magnetars, on close field lines, crustal shear-driven (reconnection, ~ Solar) 1810 Crab Serylak

6 FRBs from NS magnetospheres Model I: Giant pulses (Crab on steroids) - rotationally powered (type ii radio emission) Model II (hypothetical): magnetar flares - magnetically powered (type iii radio emission) 6

7 Pulsar radio emission: theory Let's never again talk about coherent curvature emission by bunches. Yes, there are wigglers (FELs) (c.f. Goldreich-Keely 1971) But: generally, plasma process in astrophysics are in kinetic regime (random phase), while in laboratory in reactive regime (finely tuned, all in phase) (Extra slide at the end of the talk, during the discussion part) 7

8 Plasma astrophysics primer: Plasma Maser. Plasma maser: non-equilibrium distribution f(p) (in momentum space, not physical space) results in coherent emission of plasma normal modes. Population inversion: there is more particles willing to emit a wave than willing to absorb it + k k? need > 0 for growth f(p k,p? k f(p? ) / (p? p? f<0 m eff,k 3 m e =(n +1/2)~ B Landau levels s = n<0 m eff,? s = n>0 m e / 1 k k v k s B / 8

9 A roadmap for radio emission theory: Guess (justify) the distribution functions (parallel, perp, curvature drift also might be important) Find dispersion relation for normal modes omega (k) Find resonant condition - are there any resonant particles? Find growth rate Find saturation level (quasilinear?) Or just do PICs - not yet, but we are getting close BTW, Hessels' talk: Narrow spectral features (and their drifts) - narrow range of velocities where resonant conditions are satisfied 9

10 We are not yet in a position to construct microscopic models of radio emission we are left only to discuss the macroscopic limitations (Still, Later on - my favorite microscopic model of radio-xgamma emission model for pulsars) 10

11 Macroscopic Model I (type ii emission): Rotationally powered super-giant Pulses very young SNRs, years free-free absorption in new SN shell DM through the shell Crab s GPs reach ~ 1% efficiency If F / L sd (no LOFAR) need ~ 10 4 higher peak power from 100 Mpc Few msec period, with Crab-like B-field - reasonable to expect Spin-down times ~ yrs DM 100s Rates within 100 Mpc are OK. ff 300 MHz t yrs Injection rate f inj / Ė 1 (observed f / Ė 3/2 - consistent with observed distribution of fast pulsar) 2 Lyutikov Very flat distribution of distances to a given brightness (type of Malmquist bias) 11

12 It was a good model, but not from ~ 1 Gpc Radio power cannot be larger than the spin-down (must be magnetospheric, hard to store much more energy in the magnetosphere) F = L sd 4 D 2 Need high B - short P (at least few msec) I NS SD = 2D 2 F P yrs min Constant DM: t >100 yrs The most powerful Crab s GP have eta = 10-2 For Crab: 6 yrs if all in GPs FRB could be even worth eta ~ msec spin at birth yrs< age < 600 yrs - NO. Repeating FRB at 1 Gpc + constant large DM excludes rotationally powered emission 12

13 It is truly amazing that radio power starts to contradict the limits on the total power 13

14 Macroscopic Model II: magnetar flares (type-iii) Solar type-iii radio emission in magnetars (Lyutikov 2006) Initial stage of a reconnection flare - jets of particles, hence coherent emission - like Crab flares (Lyutikov et al. 2016) Best case - observe radio burst associated with magnetar burst and flares. Constraining limits from SGR flare SGR flare was erg/s -> radio efficiency OK? But would give a GJy from 10 kpc - not seen in Parkes side-lobes (Tendulkar ) No radio from PSR J X-ray (radio efficiency < 10-8 ) 14

15 Glitches? There are no bright EM signals with glitches Magnetospheric changes are tiny, 10-6 Time scales - crustal shear ~ 100 msec (for ~ msec time scale the energy must be stored in the magnetosphere). Juste pour rire/just for laughs, let s make a glitch FRB model Imagine you are walking at 1 m/sec. And then, suddenly a glitch: on human reaction time of 100 msec, you change your speed by 10-6 to (1 m - micrometer)/sec. And then you convert all that energy into radio waves at 1 GHz. Boom - Mega-Jy signal 3 km away This is obviously ridiculous, but astrophysically, scaling this with that, it makes sense, right? Not for radio (could be OK for high energy) (Mechanical to radio vs EM to radio) 15

16 Best hope: other wavebands Radio: > erg/s Optical > erg/s for flat (nu Fnu ) X-rays> erg/s, GRB-like rates - not correct All sky X-rays monitors can see magnetar-type flare to ~ 40 Mpc, but targeted observation (eg. of the Repeating FRB) can detect with Chandra/NuStar magnetar-type flare to z ~1. Afterglows: jetted magnetar post- burst outflows (Keane 2016, Lyutikov 2006)?) Lyutikov

17 Counter-part strategy: optical Lyutikov & Lorimer 2016 Optical energetics >> radio (If ) Peak flux ~ 9m (but only for few msec - fast read-outs) m~ 15 image in 60 sec PTF, ASAS-SN, EVRYSCOPE (LSST) - PTF might have seen, as star-like points in single exposure. Optical would look at FRB every ~ 10 hours (10 sq. degrees field of view) fast readout is good Radio and optical - stare at the same patch But: For The Repeater, the optical power is in the erg/sec 17

18 Best observational strategy Simultaneous Radio - Optical Simultaneous pointed X-rays (The Repeater - e.g. NuStar) Multi-frequency radio (triggered LOFAR obs.) 18

19 My favorite pulsar radio emission mechanism for Type II radio emission (giant pulses) 19

20 Crab Normalized count rate Radio intensity (au) Counts/s 3 Counts (x10 2 Radio (Nancay telescope, 1.4 GHz) (a) Soft gamma-rays (Comptel, MeV) (e) Optical (SCam-3) (b) Gamma-rays (EGRET, >100 MeV) (f) X-rays (RXTE, 2-16 kev) (c) Gamma-rays (Fermi LAT, >100 MeV) (g) 1.61 Hard X-rays (INTEGRAL, kev) (d) VHE gamma-rays (MAGIC, >25 GeV) (h) Phase Phase Counts Counts Counts 3 Counts (x10 ) ) Aligned from radio to VHE gamma - common origin 20

21 What excites gyration: anomalous cyclotron resonance v> c/n n > 1 B-field - A particle can emit a cyclotron photon with no initial gyration k k v k = B / - Particle goes up in Landau levels and emits a photon - radio - Spontaneous down cyclotron boosted in UV-X-rays - IC - VHE gamma rays 21

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23 What excites gyration: anomalous cyclotron resonance v> c/n n > 1 B-field - A particle can emit a cyclotron photon with no initial gyration k k v k = B / - Particle goes up in Landau levels and emits a photon - radio - Spontaneous down cyclotron boosted in UV-X-rays - IC - VHE gamma rays Lyutikov 2012, also in prep. e Fe, ergês From ~100 MHz to ~ 2 TeV - 20 orders in energy eêhm e c 2 L 23

24 Conclusion Amazingly, radio power (+ constant DM and non-changing properties of The Repeater) puts energetic limits on the type of emission (rotationally vs magnetically powered; does not yet constrain particular instability that drives radio emission). Neither looks too promising for now - Mystery Perhaps there is some slack left in either: Highly non-stationary processes in pulsar outer magnetospheres Explosive reconnection events in magnetar magnetospheres - can they generate unstable distributions and radio bursts? PICs? Please no - FRB comes from here models 24

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26 A point on terminology: magnetar vs high B-field NS Let s not call rotationally powered effects from high-b NS a magnetar - just high-b NS Leave the magnetar term for B-field powered effects. 26

27 (Also: non-linearity parameter a ) EM non-linearity parameter a = If no B: Actually, a = ee m e c a ee m e c B apple 1 huge radiative losses (eg. induced Compton) 27

28 Coherent emission by bunches - critique 28