6 pages, 2 figures. To appear in the Proceedings of the 270. WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan. 21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper. Proceedings are available as MPE-Report 278
A solution to the sigma problem - that of finding a mechanism capable of converting Poynting energy flux to particle-borne energy flux in a pulsar wind - was proposed several years ago by Coroniti and Michel who considered a particular prescription for magnetic reconnection in a striped wind. This prescription was later shown to be ineffective. In this paper, we discuss the basic microphysics of the reconnection process and conclude that a more rapid prescription is permissible. Assuming dissipation to set in at some distance outside the light-cylinder, we compute the resulting radiation signature and find that the synchrotron emission of heated particles appears periodic, in general showing both a pulse and an interpulse. The predicted spacing of these agrees well with observation in the case of the Crab and Vela pulsars. Using parameters appropriate for the Crab pulsar - magnetization parameter at the light cylinder sigma_L = 6 x 10^4, Lorentz factor Gamma=250 - reasonable agreement is found with the observed total pulsed luminosity. This suggest that the high-energy pulses from young pulsars originate not in the co-rotating magnetosphere within the light cylinder (as in all other models) but from the radially directed wind well outside it.
The radiation of a pulsar wind is computed assuming that at roughly 10 to 100 light cylinder radii from the star, magnetic energy is dissipated into particle energy. The synchrotron emission of heated particles appears periodic, with, in general, both a pulse and an interpulse. The predicted spacing agrees well with the Crab and Vela pulse profiles.Using parameters appropriate for the Crab pulsar (magnetisation parameter at the light cylinder $σ=6\times10^4$, Lorentz factor $Γ=250$) agreement is found with the observed total pulsed luminosity. This suggests that the high-energy pulses from you…
We present numerical simulations of the interaction between a collimated, bipolar ``pulse'' ejected from a star and a continuous wind ejected from a stellar companion. We explore the characteristics of the predicted H$α$ intensity maps by varying selected input parameters. We find that the asymmetry (in size and strength) between the two lobes of the proto-planetary nebula OH 231.8+4.2 is reproduced in this scenario if the wind ejected by the companion star has a strong latitude dependence.
The general properties of the non-thermal non-pulsed X-ray emission of rotation powered pulsars are investigated in the context of a pulsar wind nebula model. An examination of the observed X-ray emission from a sample of 23 pulsars from ASCA in the energy range between 2-10 keV reveals that the relation of X-ray luminosity, $L_x$, to the pulsar spin down power, $\dot{E}$, is steeper for the non-pulsed component than for the pulsed component. Specifically, $L_{x}^{npul} \propto \dot{E}^{1.4 \pm 0.1}$ for the non-pulsed component, whereas $L_{x}^{pul} \propto \dot{E}^{1.2 \pm 0.08}$ for the pul…
The non-thermal, non-pulsed X-ray emission of MSPs is investigated. As in young pulsars, MSPs emit a relativistic wind, which interacting with the interstellar medium and/or a binary companion can significantly contribute to the non-pulsed emission of these pulsars. An application and extension of a simple model developed for young pulsars is applied to the old recycled MSP B1957+20. It is found that the pulsar wind can, indeed, contribute to both the resolved and unresolved X-ray emission. For other MSP in the Galactic field where the spectral index of the non-pulsed component has been measur…
The Stokes parameters of the pulsed synchrotron radiation produced in the striped pulsar wind model are computed and compared with optical observations of the Crab pulsar. We assume the main contribution to the wind emissivity comes from a thin transition layer where the dominant toroidal magnetic field reverses its polarity. The radial component of the field is neglected, but a small meridional component is added. The resulting radiation is linearly polarized (Stokes V=0). In the off-pulse region, the electric vector lies in the direction of the projection on the sky of the rotation axis of t…
PSR B1259-63 is the only known binary system with a radio pulsar from which the non-pulsed radio and X-ray emission was detected. The companion star in this system is a Be star SS 2883. A rapidly rotating radio pulsar is expected to produce a wind of relativistic particles. Be stars are known to produce highly asymmetric mass loss. Due to the interaction of the pulsar wind and the Be star wind the system of two shocks between the pulsar and the Be star forms. In this paper we show that the observed non-pulsed radio emission from the system is a result of the synchrotron emission of the relativ…
Gamma-ray bursts, if they are generated in the process of interaction between relativistic strongly magnetized winds and an ambient medium, may be accompanied by very short pulses of low-frequency radio emission. The bulk of this emission is expected to be at the frequencies of $\sim 0.1-1$ MHz and cannot be observed. However, the high-frequency tail of the low-frequency radio emission may reach a few ten MHz and be detected, especially if the strength of the magnetic field of the wind is extremely high.
Recent observations of the polarisation of the optical pulses from the Crab pulsar motivated detailed comparative studies of the emission predicted by the polar cap, the outer gap and the two-pole caustics models. In this work, we study the polarisation properties of the synchrotron emission emanating from the striped wind model. We use an explicit asymptotic solution for the large-scale field structure related to the oblique split monopole and valid for the case of an ultra-relativistic plasma. This is combined with a crude model for the emissivity of the striped wind and of the magnetic fiel…
The double pulsar system J0737-3039 is not only a test bed for General Relativity and theories of gravity, but also provides a unique laboratory for probing the relativistic winds of neutron stars. Recent X-ray observations have revealed a point source at the position of the J0737-3039 system, but have failed to detect pulsations or orbital modulation. Here we report on Chandra X-ray Observatory High Resolution Camera observations of the double pulsar. We detect deeply modulated, double-peaked X-ray pulses at the period of PSR J0737-3039A, similar in appearance to the observed radio pulses. Th…
We review current theoretical ideas on pulsar winds and their surrounding nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the striped wind, together with models of magnetic dissipation are discussed. It is shown that the observational signature of this dissipation is likely to be point-like, rather than extended, and that pulsed emission may be produced. The possible pulse shapes and polarisation properties are described. Particle acceleration at the termination shock of the wind is discussed, and it is argued that two distinct mechanisms must be operating, with the…
We have conducted a search for giant pulses from four millisecond pulsars using the 100m Green Bank Telescope. Coherently dedispersed time-series from PSR J0218+4232 were found to contain giant pulses of very short intrinsic duration whose energies follow power-law statistics. The giant pulses are in phase with the two minima of the radio integrated pulse profile but are phase aligned with the peaks of the X-ray profile. Historically, individual pulses more than 10-20 times the mean pulse energy have been deemed to be ``giant pulses''. As only 4 of the 155 pulses had energies greater than 10 t…
We have carried out a survey for `giant pulses' in 5 millisecond pulsars. We detect individual pulses from the high Edot pulsar PSR B1821-24 with energies exceeding 50x the mean pulse energy. These giant pulses are concentrated in a narrow phase window coincident with the power-law non-thermal pulse seen in hard X-rays. This is the third example of the giant pulse phenomenon. It supports the idea that large B fields in the outer magnetosphere are critical to the formation of such pulses and further suggests a direct connection between giant pulses and high energy emission.
The giant outburst from SGR 1806-20 was observed on 2004 December 27 by many spacecraft. This extremely rare event exhibits a striking similarity to the two giant outbursts thus far observed, on 1979 March 5 from SGR 0526-66 and 1998 August 27 from SGR 1900+14. All the three outbursts start with a short giant radiation pulse followed by a weaker tail. The tail pulsates with the period of neutron star rotation of $\sim$5--8 s, to decay finally in a few minutes. The enormous intensity of the initial pulse proved to be far above the saturation level of the gamma-ray detectors, with the result tha…
We consider the effects of rapid pair creation by an intense pulse of gamma-rays propagating ahead of a relativistic shock. Side-scattered photons colliding with the main gamma-ray beam amplify the density of scattering charges. The acceleration rate of the pair-loaded medium is calculated, and its limiting bulk Lorentz factor related to the spectrum and compactness of the photon source. One obtains, as a result, a definite prediction for the relative inertia in baryons and pairs. The deceleration of a relativistic shock in the moving medium, and the resulting synchrotron emissivity, are compa…
Consider binary system with a millisecond pulsar ejecting relativistic particles and an optical star emitting soft photons with the energy $ω\simeq 1-10$ eV. These low-energy photons are scattered by the relativistic electrons and positrons of the pulsar's wind. The scattered photons forms a wide spectrum from hard X-ray band up to gamma band $ε\simeq 1-1000$GeV >.When the pulsar wind is isotropic the luminosity of gamma radiation $L_γ=L_γ(ψ)$ depends heavily on the angle $ψ$ between the directions to the optical star and to the observer from the pulsar. During the orbital motion this angle va…
Seven giant radio pulses were recorded from the millisecond pulsar PSR B1937+21 during $\approx$ 8.1 minutes observation by the Ooty Radio Telescope (ORT) at 326.5 MHz. Although sparse, these observations support most of the giant pulse behavior reported at higher radio frequencies (430 to 2380 MHz). Within the main componentof the integrated profile, they are emitted only in a narrow ($\la$ 47 $μ$s) window of pulse phase, close to its peak. This has important implications for doing super-high precision timing of PSR B1937+21 at low radio frequencies.
Formation of giant radio pulses is attributed to propagation effects in the plasma of pulsar magnetosphere. Induced scattering of radio waves by the plasma particles is found to lead to an efficient redistribution of the radio emission in frequency. With the steep spectrum of pulsar radiation, intensity transfer between the widely spaced frequencies may imply significant narrow-band amplification of the radiation. This may give rise to giant pulses. It is demonstrated that the statistics of giant pulse intensities observed can be reproduced if one take into account pulse-to-pulse fluctuations …
I review the constraints placed on relativistic pulsar winds by comparing optical and X-ray images of the inner Crab Nebula on the one hand with two-dimensional MHD simulations on the other. The various proposals in the literature for achieving the low magnetisation required at the inner edge of the Nebula, are then discussed, emphasising that of dissipation in the striped-wind picture. The possibility of direct observation of the wind is examined. Based on the predicted orientation of the polarisation vector, I outline a new argument suggesting that the off-pulse component of the optical emis…
We report here on multifrequency radio observations of the pulsed emission from PSR B1259-63 around the time of the closest approach to its B2e companion star. There was a general increase in the pulsar's dispersion measure and scatter broadening, and a decrease in the flux density towards periastron although changes in these parameters were seen on timescales as short as minutes. The pulsed emission disappeared 16 days prior to periastron and remained undetectable until 16 days after periastron. The observations are used to determine the parameters of the wind from the Be star. We show that a…
We report on Chandra ACIS and XMM-Newton MOS/PN imaging observations of two pulsar wind nebulae (K3/PSR J1420-6048 and G313.3+0.1=`the rabbit') associated with the Galactic unidentified gamma-ray source GeV J1417-6100. With the excellent ACIS imaging, the very energetic pulsar PSR J1420-6048 is separated from its surrounding nebula. This nebula has surprisingly little compact structure, although a faint arc is seen near the pulsar. Similarly, two point sources are resolved in the rabbit nebula. The large XMM-Newton collecting area provides useful spectral constraints on the rabbit and the asso…
BeppoSAX observations of the single pulse burst GRB960720 have allowed a detailed study of its X-ray properties: pulse width in different energy bands, spectral evolution from 2 to 700 keV, etc. We show that the early (0-5s) X-ray emission and the gamma-rays are well explained by internal shocks in a relativistic wind while the late (5-20s) X-ray emission could come from the reverse shock generated in the wind when it interacts with the external medium. The results for a medium of uniform density are compared to the observations.
Photospheric granulation may excite transverse kink pulses in anchored vertical magnetic flux tubes. The pulses propagate upwards along the tubes with the kink speed, while oscillating wakes are formed behind the wave front in a stratified atmosphere. The wakes oscillate at the kink cut-off frequency of stratified medium and gradually decay in time. When two or more consecutive kink pulses with different polarizations propagate in the same thin tube, then the wakes corresponding to different pulses may superimpose. The superposition sets up helical motions of magnetic flux tubes in the photosp…
Giant pulses are short, intense outbursts of radio emission with a power-law intensity distribution that have been observed from the Crab Pulsar and PSR B1937+21. We have undertaken a systematic study of giant pulses from PSR B1937+21 using the Arecibo telescope at 430, 1420, and 2380 MHz. At 430 MHz, interstellar scattering broadens giant pulses to durations of $\sim50 μ$secs, but at higher frequencies the pulses are very short, typically lasting only $\sim1$-$2 μ$secs. At each frequency, giant pulses are emitted only in narrow ($\lsim10 μ$s) windows of pulse phase located $\sim 55$-$70 μ$sec…
We report on Australia Telescope Compact Array observations of the ~10^5 yr old pulsar PSR B0906-49. In an image containing only off-pulse emission, we find a weak, slightly extended source coincident with the pulsar's position, which we argue is best interpreted as a pulsar wind nebula (PWN). A trail of emission extending behind the pulsar aligns with the major axis of the PWN, and implies that the pulsar is moving north-west with projected velocity ~60 km/s, consistent with its scintillation speed. The consequent density we infer for the pulsar's environment is >2 cm^(-3), so that the PWN ar…
We employed a sample compiled by Norris et al. (2005, ApJ, 625, 324) to study the dependence of the pulse temporal properties on energy in long-lag, wide-pulse gamma-ray bursts. Our analysis shows that the pulse peak time, rise time scale and decay time scale are power law functions of energy, which is a preliminary report on the relationships between the three quantities and energy. The power law indexes associated with the pulse width, rise time scale and decay time scale are correlated and the correlation between the indexes associated with the pulse width and the decay time scale is more o…
