J. Takata

MULTI-WAVELENGTH EMISSIONS FROM THE MILLISECOND PULSAR BINARY PSR J1023+0038 DURING AN ACCRETION ACTIVE STATE

Recent observations strongly suggest that the millisecond pulsar binary PSR J1023+0038 has developed an accretion disk since 2013 June. We present a multi-wavelength analysis of PSR J1023+0038, which reveals that (1) its gamma-rays suddenly brightened within a few days in 2013 June/July and has remained at a high gamma-ray state for several months; (2) both UV and X-ray fluxes have increased by roughly an order of magnitude; and (3) the spectral energy distribution has changed significantly after the gamma-ray sudden flux change. Time variabilities associated with UV and X-rays are on the order of 100-500 s and 50-100 s, respectively. Our model suggests that a newly formed accretion disk, due to the sudden increase of the stellar wind, could explain the changes of all these observed features. The increase of UV is emitted from the disk, and a new component in gamma-rays is produced by inverse Compton scattering between the new UV component and pulsar wind. The increase of X-rays results from the enhancement of injection pulsar wind energy into the intra-binary shock due to the increase of the stellar wind. We also predict that the radio pulses may be blocked by the evaporated winds from the disk, and the pulsar is still powered by rotation.

Discovery of GeV γ-ray emission from PSR B1259-63/LS 2883

The binary system PSR B1259–63/LS 2883 consists of a 47.8 ms radio pulsar that orbits the companion Be star with a period of 3.4 years in a highly eccentric orbit. The system has been well sampled in radio, X-ray, and TeV γ-ray bands, and shows orbital phase-dependent variability in all observed frequencies. Here we report on the discovery of >100 MeV γ-rays from PSR B1259–63/LS 2883 through the 2010 periastron passage. Using data collected with the Large Area Telescope on board Fermi from 33 days before periastron to 75 days after periastron, PSR B1259–63/LS 2883 was detected at a significance of 13.6 standard deviations. The γ-ray light curve was highly variable over this period, with a changing photon index that correlates with the γ-ray flux. In particular, two major flares that occur after the periastron passage were observed. The onset of γ-ray emission occurs close to, but not at the same orbital phases as, the two disk passages that occur ~1 month before and ~1 month after the periastron passage. The fact that the GeV orbital light curve is different from that of the X-ray and TeV light curves strongly suggests that GeV γ-ray emission originates from a different component. We speculate that the observed GeV flares may be resulting from Doppler boosting effects.

Pulse Profiles, Spectra, and Polarization Characteristics of Nonthermal Emissions from the Crab-like Pulsars

We discuss nonthermal emission mechanisms of the Crab-like pulsars with both a two-dimensional electrodynamic study and a three-dimensional model. We investigate the emission process in the outer gap accelerator. In the two-dimensional electrodynamic study, we solve the Poisson equation of the accelerating electric field in the outer gap and the equation of motion of the primary particles with the synchrotron and the curvature radiation processes and the pair-creation process. We show a solved gap structure that produces a γ-ray spectrum consistent with EGRET observations. Based on the two-dimensional model, we construct a three-dimensional emission model to calculate the synchrotron and the inverse Compton processes of the secondary pairs produced outside the outer gap. We calculate the pulse profiles, the phase-resolved spectra, and the polarization characteristics in optical through γ-ray bands for comparison with the observation of the Crab pulsar and PSR B0540-69. For the Crab pulsar, we find that the outer gap geometry extending from near the stellar surface to near the light cylinder produces a complex morphology change of the pulse profiles as a function of the photon energy. This predicted morphology change is quite similar to that of the observations. The calculated phase-resolved spectra are consistent with the data from the optical to the γ-ray bands. We demonstrate that the 10%-20% of the polarization degree in the optical emissions from the Crab pulsar and the Vela pulsar is explained by the synchrotron emissions from the particle gyration motion. For PSR B0540-69, the observed pulse profile with a single broad pulse is reproduced for an emission region thicker and an inclination angle between the rotational axis and the magnetic axis smaller than the Crab pulsar.

A Revisit of the Phase-resolved X-Ray and Gamma-Ray Spectra of the Crab Pulsar

We use a modified outer-gap model to study the multifrequency phase-resolved spectra of the Crab pulsar. The emissions from both poles contribute to the light curve and the phase-resolved spectra. Using the synchrotron self-Compton mechanism and by considering the incomplete conversion of curvature photons into secondary pairs, the observed phase-averaged spectrum from 100 eV to 10 GeV can be explained very well. The predicted phase-resolved spectra can match the observed data reasonably well, too. We find that the emission from the north pole mainly contributes to leading wing 1. The emissions in the remaining phases are mainly dominated by the south pole. The widening of the azimuthal extension of the outer gap explains trailing wing 2. The complicated phase-resolved spectra for the phases between the two peaks, namely, trailing wing 1, the bridge, and leading wing 2, strongly suggest that there are at least two well-separated emission regions with multiple emission mechanisms—synchrotron radiation, inverse Compton scattering, and curvature radiation. Our best-fit results indicate that there may exist some asymmetry between the south and north poles. Our model predictions can be examined with GLAST.

Polarization of High-Energy Emission from the Crab Pulsar

We investigate the polarization of the high-energy emission from the Crab pulsar within the framework of the outer gap accelerator, following previous studies by Cheng and coworkers. A recent version of the outer gap, in which the gap extends from inside the null charge surface to the light cylinder, is used to examine the synchrotron radiation from the secondary and tertiary pairs that are produced outside the gap. We are able to simultaneously reproduce the light curve, the spectrum, and the polarization characteristics by taking into account the gyration of the particles. The polarization position angle curve and the degree of polarization are calculated and compared with the Crab optical data. We demonstrate that the radiation from inside the null charge surface produces the outer wing and off-pulse portions of the light curve and that the tertiary pairs contribute to the bridge emission. The emission from the secondary pairs explains the main features of the observed light curve and spectrum. On the other hand, the emissions both from inside the null charge surface and from the tertiary pairs are required in order to explain the optical polarization behavior of the Crab pulsar. The energy dependence of the polarization features is predicted by the model. The polarization position angle curve indicates that our viewing angle as measured from the pulsars rotational axis is greater than 90°.

Particle acceleration and non-thermal emission in the pulsar outer magnetospheric gap

A two-dimensional electrodynamic model is used to study particle acceleration and non-thermal emission mechanisms in the pulsar magnetosphere. We solve the distribution of the accelerating electric field with the emission process and the pair-creation process in the meridional plane, which includes the rotational and magnetic axes. By solving the evolutions of the Lorentz factor, and of the pitch angle, we calculate the spectrum in optical through γ-ray bands with the curvature radiation, synchrotron radiation and inverse-Compton process, not only for outgoing particles but also for ingoing particles, which have been ignored in previous studies. We apply this theory to the Vela pulsar. We find that the curvature radiation from the outgoing particles is the major emission process above 10 MeV bands. In soft γ-ray to hard X-ray bands, the synchrotron radiation from the incoming primary particles in the gap dominates in the spectrum. Below hard X-ray bands, the synchrotron emissions from both outgoing and ingoing particles contribute to the calculated spectrum. The calculated spectrum is consistent with the observed phase-averaged spectrum of the Vela pulsar. Taking into account the predicted dependence of the emission process and the emitting particles on the energy bands, we compute the expected pulse profile in X-ray and γ-ray bands with a three-dimensional geometrical model. We show that the observed five-peak pulse profile in the X-ray bands of the Vela pulsar is reproduced by the inward and outward emissions, and the observed double-peak pulse profile in γ-ray bands is explained by the outward emissions. We also apply the theory to PSR B1706−44 and PSR B1951+32, for which X-ray emission properties have not been constrained observationally very well, to predict the spectral features with the present outer-gap model.

Pulsar High Energy Emissions from Outer Gap Accelerator Closed by a Magnetic Pair-creation Process

We discuss outer gap closure mechanism in the trans-field direction with the magnetic pair-creation process near the stellar surface. The gap closure by the magnetic pair-creation is possible if some fraction of the pairs are produced with an outgoing momentum. By assuming that multiple magnetic field will affect the local field near the stellar surface, we show a specific magnetic field geometry near the stellar surface resulting in the outflow of the pairs. Together with the fact that the electric field is weak below null charge surface, the characteristic curvature photon energy emitted by incoming particles, which were accelerated in the outer gap, decreases drastically to ∼ 100MeV near the stellar surface. We estimate the height measured from the last-open field line, above which 100 MeV photons is converted into pairs by the magnetic pair-creation. We also show the resultant multiplicity due to the magnetic pair-creation process could acquire Me± ∼ 10− 10. In this model the fractional outer gap size is proportional to P. The predicted gamma-ray luminosity (Lγ) and the characteristic curvature photon energy (Ec) emitted from the outer gap are proportional to B P and BP respectively. This model also predicts that Lγ and Ec are related to the spin down power (Lsd) or the spin down age of pulsars (τ) as Lγ ∝ L 5/8 sd or Lγ ∝ τ , and Ec ∝ L 1/4 sd or Ec ∝ τ −1/2 respectively. Subject headings: pulsars: general– radiation mechanisms:non-thermal– -gamma rays:theory–magnetic fieldWe discuss outer gap closure mechanism in the trans-field direction with the magnetic pair-creation process near the stellar surface. The gap closure by the magnetic pair creation is possible if some fraction of the pairs are produced with an outgoing momentum. By assuming that multiple magnetic field will affect the local field near the stellar surface, we show a specific magnetic field geometry near the stellar surface resulting in the outflow of the pairs. Together with the fact that the electric field is weak below null charge surface, the characteristic curvature photon energy emitted by incoming particles, which were accelerated in the outer gap, decreases drastically to ~100 MeV near the stellar surface. We estimate the height measured from the last-open field line, above which 100 MeV photons are converted into pairs by the magnetic pair creation. We also show the resultant multiplicity due to the magnetic pair-creation process could acquire . In this model, the fractional outer gap size is proportional to P –1/2. The predicted γ-ray luminosity (L γ) and the characteristic curvature photon energy (Ec ) emitted from the outer gap are proportional to B 2 P –5/2 and B 3/4 P –1, respectively. This model also predicts that L γ and Ec are related to the spin-down power (L sd) or the spin-down age of pulsars (τ) as L γ ∝ L 5/8 sd or L γ ∝ τ–5/4 and Ec ∝ L 1/4 sd or Ec ∝ τ–1/2, respectively.

Discovery of an unidentified Fermi object as a black widow-like millisecond pulsar

The Fermi γ-ray Space Telescope has revolutionized our knowledge of the γ-ray pulsar population, leading to the discovery of almost 100 γ-ray pulsars and dozens of γ-ray millisecond pulsars (MSPs). Although the outer-gap model predicts different sites of emission for the radio and γ-ray pulsars, until now all of the known γ-ray MSPs have been visible in the radio. Here we report the discovery of a radio-quiet γ-ray-emitting MSP candidate by using Fermi, Chandra, Swift, and optical observations. The X-ray and γ-ray properties of the source are consistent with known γ-ray pulsars. We also found a 4.63 hr orbital period in optical and X-ray data. We suggest that the source is a black widow-like MSP with a ~0.1 M ☉ late-type companion star. Based on the profile of the optical and X-ray light curves, the companion star is believed to be heated by the pulsar while the X-ray emissions originate from pulsar magnetosphere and/or from intrabinary shock. No radio detection of the source has been reported yet, and although no γ-ray/radio pulsation has been found we estimate that the spin period of the MSP is ~3-5 ms based on the inferred γ-ray luminosity.

Gamma-ray spectral properties of mature pulsars: a two-layer model

We use a simple two-layer outer gap model, whose accelerator consists of a primary region and a screening region, to discuss the ?-ray spectrum of mature pulsars detected by Fermi. By solving the Poisson equation with an assumed simple step-function distribution for the charge density in these two regions, the distribution of the electric field and the curvature radiation process of the accelerated particles can be calculated. In our model, the properties of the phase-averaged spectrum can be completely specified by three gap parameters, i.e., the fractional gap size in the outer magnetosphere, the gap current in the primary region, and the gap size ratio between the primary region and the total gap size. We discuss how these parameters affect the spectral properties. We argue that although the radiation mechanism in the outer gap is a curvature radiation process, the observed gamma-ray spectrum can substantially deviate from the simple curvature spectrum because the overall spectrum consists of two components, i.e., a primary region and a screening region. In some pulsars, the radiation from the screening region is so strong that the photon index from 100?MeV to several GeV can be as flat as ~2. We show that the fitting fractional gap thickness of the canonical pulsars increases with the spin-down age. We find that the total gap current is about 50% of the Goldreich-Julian value and the thickness of the screening region is a few percent of the total gap thickness. We also find that the predicted ?-ray luminosity is less dependent on the spin-down power (L sd) for pulsars with L sd 1036?erg?s?1, while the ?-ray luminosity decreases with the spin-down power for pulsars with L sd 1036?erg?s?1. This relation may imply that the major gap closure mechanism is a photon-photon pair-creation process for pulsars with L sd 1036?erg?s?1, while it is a magnetic pair-creation process for pulsars with L sd 1036?erg?s?1.

X-Ray and Gamma-Ray Emissions from Rotation Powered Millisecond Pulsars

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