on young neutron stars as propellers and accretors ma bo, department of astronomy, nju, nanjing...

On Young Neutron Stars as Propellers and Accretors

Ma Bo,

Department of Astronomy, Nju, Nanjing

Citations:Alpar,M.A.,APJ554,1245,2000Illarionov and Sunyaev.1975Nature,440,772

SGRs and AXPs

Soft Gamma Repeaters were discovered as high-energy transient burst sources; some were later found also to be persistent X-ray pulsars, with periods of several seconds, that are spinning down rapidly. Anomalous X-ray Pulsars are identified through their persistent pulsations and rapid spin down; some have also been found to emit SGR-like bursts.

SGR1900+14-giant flare on1998.8.27

P-Mdot Diagram of AXP,SGR,DTN

Are they magenetars?

Gauss )PP(103.2B 2/1190

Large P and Pdot led to suggestion of magnetars:

For RXJ0720.4-3125,P=8.39s,Pdot=2.9E-11 s/s

Gauss 105B 140

Pdot Similarity

A non-accreting neutron star, energy dissipation (Alpar et al.1984)

pdisspation IE

The luminosities of RX J0720.4-3125, RX J185635-3754, and the other DTNs (Treves et al. 2000) give

-11013 s rad 10~10

The measured spin-down rates of AXPs and SGRs all fall inthis range, providing another similarity, in addition to the periods, between the DTNs, AXPs, and SGRs.

Propeller Spin-Down

The order of magnitude of this spin-down torque is (Illarionov and Sunyaev.1975 ， Alpar et al. 1984)

3A

2 r/IN

The magnetic dipole represents a “propeller”, rotating with a supersonic velocity in the accreting gas.

Accretion Starts

The propeller phase will last until accretion starts when a critical rotation period Pc is reached. (Illarionov & Sunyaev. 1975, A&A, 39, 185)

For RX J0720.4-3125,

，7/57/315

7/630eqc mM s 8.16PP

3/2eq

3/1ca )GM(rr

eq

2/1)GMI(

G109.7BG107.9 1211

The Asymptotic Spin-down Regime

Spin-down near equilibrium can be modeled as

This can explain the similarity of P for SGR AXP,DTN

eqeq

eq

)/t(exp])0([)t(

/)(

A debris disk around an isolated young neutron star(Nature,440,772)

yr10M/M

,M10M6

dd

sun5

d

Conclusions

Key Idea. The signature of a young neutron star depends on the presence and nature of the mass inflow of fallback material from the supernova explosion.

1 All neutron stars born with similar order of magnitude B~1E12 Gauss

2 Radio pulsars are formed if there is no mass inflow or not enough to protrude the light cylinder. Higher mass inflow rates do not allow radio pulsar activity, leading instead to DTNs, AXPs, and RQNSs.

Conclusion

3 The similarity in the rotation periods of these sources is not a coincidence but rather a consequence of the asymptotic approach to rotational equilibrium under a wide range of mass inflow rates.

4 With low Mdot the spin-down is too slow for the source to go through the propeller stage and reach the accretion stage before the circumstellar mass is depleted. These sources, observed as DTNs, are propellers for as long as 1E5~1E6 yr and make up the most numerous class. 6 The DTNs are the first observed examples of neutron stars in the propeller phase.

7 A nonaccreting neutron star under propeller spin-down has a luminosity provided by energy dissipation inside the star.

8 The RQNSs are the young neutron stars for

which Comptonization washes out beaming so that the rotation period is not observable.

Summary Picture

The End

Thank you!