Rotating RAdio Transients (RRATs) ApJ, 2006, 646, L139 Nature, 2006, 439, 817 Astro-ph/0608311
Introduction 11 Rotating RAdio Transients (RRATs) (Mclaughlin et al 2006) Repeated, irregular radio bursts Bursts durations ~ 2 to 30 ms. intervals between bursts ~ 4 min to 3hr detectable radio emission <1s per day Concentrated at low Galactic latitudes, d~2-7kpc P~0.4-7s for 10 of 11 sources (5 of them P>4s), suggest they are rotating neutron stars
(Mclaughlin et al 2006)
proportion of RRATs with long period significantly exceed that of normal radio pulsars 3 of these sources have Pdot (astro-ph/0608311)
RRAT J1819-1458 Bursts every ~3min, the most active of the known RRATs Dispersion measure 196+/-3pc cm -3, d~3.6kpc Good fit to blackbody model Inferred blackbody radius 20km Surface temperature kt =120+/-40eV Slightly higher brightness observational selection effect (McLaughlin 2006) 0.5-8.0 kev, isotropic luminosity 3.6d33 erg /s
Discrepancy with magnetars Much colder and less luminous than magnetars Lacks the hard X-ray tail Estimated birth rate is higher than magnetars Only possible link, transient magnetar XTE J1810-197, while in quiescence, had a surface temperature kt =150-180eV Another link, X-ray luminosities > spin-down luminosities
Link to radio pulsar in X-ray properties Similar to radio sources with ages ~ 10 5 yr in spectra PSR J0538+2817 (30kyr) kt =160eV PSR B0656+14 (110kyr) kt =70eV X-ray emission suggests that this source is a normal radio pulsar Additional evidence, discovery from PSR B0656+14 of RRAT-like behaviour
GCRT J1745-3009 A similar bursting radio source GCRT J1745-3009 (Hyman et al 2005) flare ~ 1 Jy lasting ~ 10min each, and interval ~77min Located ~ 10 from GC, and just outside of the shelltype SNR G359.1-0.5 If it and SNR are related, it s age ~ 10 5 yr Investigation of the nature of RRATs can be used to understand pulsar formation, evolution, and radiation mechanisms
Several possible interpretations for GCRT J1745-3009 and RRATs A precessing pulsar (Zhu & Xu 2006) Binary neutron star (Turolla et al 2005) Transient white dwarf pulsar (Zhang & Gil 2005) Recently, Zhang et al (2006) present two possible interpretations: below the radio emission death line radio emission direction reversal
Reactivated dead pulsar model Death line is highly uncertain because it depends on magnetic field configuration, and the origin of gamma quanta, which are responsible for pair production Electron-positron pair production plays an essential role in radio emission Internal magnetic field evolution can produce the outbursts of radio emission The pair production condition is satisfied and NS become a radio pulsar when stronger multipole magnetic field emerge in the polar cap region
death-line
implications Large population may either be due to a higher birthrate of young pulsars of caused by a pileup effect at large periods Reconnection occurs at v A, growth time of internal field instability is Comparison with burst duration ~10ms, field readjustment occurs in the outer crust of NS, ρ 15 ~10-6
RRATs Michel & Dessler (1981,1983) argued that radio pulsars are surrounded by a supernova fallback disk with negligible accretion, while X-ray pulsars are surrounded by an accretion disk Debris disk may result from the captured interstellar medium (Popov et al 2000) Recent X-ray observations show that some young pulsars may have the jet, which suggests the existence of a disk (Blackman & Perna 2004) Wang et al (2006) report the discovery of mid-infrared emission from a cool disk around the isolated young X-ray pulsar 4U 0142+61
Most of neutron stars with a disk should have experienced the accretor and propeller phase (Illarionov & Sunyaev 1975) Disk wind may be strong enough to influence the structure of pulsar winds (Blackman & Perna 2004) Potential drop across magnetospheric gap ~ 10 12 V (Ruderman & Sutherland 1975) Number density at r A is greater than, wind or outflow from the disk for most NS with a debris disk can exist (Wang 1983) The density of outflow plasma
This density can be much larger than Goldreich- Julian density Deficiency of radio emission may result from the fact that redio waves with 75cm can be absorbed in the wind plasma (Illarionov & Sunyaev 1975)
The flow in the inner part of the accretion disk may have density fluctuations due to various instability Clumps may promote particle acceleration in the gap and lead to pulsar emission Assume typical clump separation is less than the disk height H in at R in, the duration of pulsar emission Its magnitude seems compatible with the burst durations measured so far
RRAT J1819-1458 X-ray counterpart was discovered (Reynolds et al 2006) Emission is consistent with a cooling NS of ~ 10 4-10 5 yr, at a distance <2kpc Contradiction: thermal, soft radiation is not expected from a propeller, in which nonthermal magnetospheric emission should dominate the mass inflow rate in the disk (Cannizzo et al 1990)
The maximum luminosity released by propeller process This is much smaller than the measure values 10 33 erg/s Imply that NS cooling could still dominate X-ray emission in this object
PSR B1931+24 Very long-term, quasi-periodic flaring behavior (Kramer et al 2006) Radio emission switches off in less than 10s after active phases of ~5-10d and remains undetectable for the next ~25-35d, when it switcher on again. Rotation spin-down 50% faster when it is on than when it is off Four objects that share similar properties were revealed by Parkes Multi-Beam Survey (Lyne 2006)
Pulsar + debris disk model Li (2006) suggest that 35d is the precession period of the debris disk Misalignment between Ω d and Ω P may lead to free precession of the disk (Katz 1973; Roberts 1974) Disk precession can also induced by the radiation or magnetic torques (Petterson 1977) Existence of a warped, precessing disk in X-ray binaries and AGN have extensive evidence The horizontal distance of the disk from the spin axis of NS always changes during the precession
When the disk penetrates inside LC, propeller process commences along with outflows from the disk Particle acceleration processes in magnetospheric gap are then quenches, and radio emission cuts off. Radiation switches on when disk moves outside LC When radiation switches on, both magnetic dipole radiation and pulsar wind brake NS, so spin-down faster than during the off phase This model suggests that PSR B1931+24 may appear as a RRAT during the off phase