K. S. Cheng

High-energy radiation from rapidly spinning pulsars with thick outer gaps

We propose a self-consistent mechanism to estimate the size of the acceleration region in the outer magnetosphere of pulsars (outer gap) and calculate the high-energy radiation produced by the synchrocurvature mechanism from the outer gap. We find that a power-law energy distribution of the accelerated particles can be obtained if the outer gap is thick enough that E inside the gap can be approximately proportional to (?r/c)1/2B(r). We apply our model to explain X-rays and ?-rays from Geminga and PSR B1055-52, whose outer gaps may occupy ~70% of the outer magnetosphere region. If the radius of curvature near the light cylinder of the medium outer gap is larger than the dipolar structure, then perhaps this model may also apply to PSR B1951+32, PSR B1706-44, and others.

Multicomponent X-Ray Emissions from Regions near or on the Pulsar Surface

We present a model of X-ray emission from rotation-powered pulsars, which in general consist of one nonthermal component, two hard thermal components, and one soft thermal component. The nonthermal X-rays come from synchrotron radiation of e ± pairs created in the strong magnetic field near the neutron star surface by curvature photons emitted by charged particles on their way from the outer gap to the neutron star surface. The first hard thermal X-ray component results from polar-cap heating by the return current in the polar gap. The second hard thermal X-ray component results from polar-cap heating by the return particles from the outer gap. Because of cyclotron resonance scattering, most of the hard thermal X-rays will be effectively reflected back to the stellar surface and eventually reemitted as soft thermal X-rays. However, some of the hard thermal X-rays can still escape along the open magnetic field lines, where the e+/e− pair density is low. Furthermore, the characteristic blackbody temperatures of the two hard X-ray components emitted from the polar-cap area inside the polar gap and the polar-cap area defined by the footprints of the outer-gap magnetic field lines are strongly affected by the surface magnetic field, which can be much larger than the dipolar field. In fact, the strong surface magnetic field can explain why the effective blackbody radiation area is nearly 2 orders of magnitude larger than that deduced from the dipolar field for young pulsars (2 orders of magnitude less for old pulsars). Our model indicates how several possible X-ray components may be observed, depending on the magnetic inclination angle and viewing angle. Using the expected X-ray luminosity and spectra, we explain the observed X-ray spectra from pulsars such as Geminga, PSR B1055-52, PSR B0656+14, and PSR B1929+10.

Conversion of Neutron Stars to Strange Stars as a Possible Origin of gamma -Ray Bursts.

This paper argues that conversion of neutron stars to strange stars as an origin of cosmological stars in the binaries with low-mass companions. Our model may provide an explanation why the binary millisecond pulsars seem to have same low magnetic fields.

AFTERGLOW EMISSION FROM HIGHLY COLLIMATED JETS WITH FLAT ELECTRON SPECTRA: APPLICATION TO THE GRB 010222 CASE?

We derive light curves of the afterglow emission from highly collimated jets if the power-law index (p )o f the electron energy distribution is above 1 but below 2. We find the following: (1) Below the characteristic synchrotron frequency, the light-curve index depends generally on p. (2) As long as the jet expansion is spherical, the light-curve index above the characteristic frequency increases slowly as the spectral index of the emission increases. (3) Once the jet enters the spreading phase, the high-frequency emission flux decays as proportional to rather than proportional to . All these results differ from those in the case of . We compare ( p6)/4 p tt p 1 2 our analytical results with the observations on the GRB 010222 afterglow and conclude that the jet model may be unable to explain the observed data. Thus, a more promising explanation for this afterglow seems to be the expansion of a relativistic fireball or a mildly collimated jet in a dense medium. Subject headings: gamma rays: bursts — relativity — shock waves

Gamma-Ray Luminosity and Death Lines of Pulsars with Outer Gaps

We reexamine the outer-gap size by taking the geometry of the dipole magnetic field into account. Furthermore, we also consider that instead of taking the gap size at half of the light cylinder radius to represent the entire outer gap, it is more appropriate to average the entire outer-gap size over the distance. When these two factors are considered, the derived outer-gap size f (P, B, [r] (alpha)) is a function not only of the period P and magnetic field B of the neutron star but also of the average radial distance to the neutron star, [r], which depends on the magnetic inclination angle alpha. We use this new outer-gap model to study the gamma-ray luminosity of pulsars, which is given by L-gamma = f(3) (P, B, [r] (alpha))L-sd, where L-sd is the pulsar spin-down power, and to study the death lines of gamma-ray emission of the pulsars. Our model can predict the gamma-ray luminosity of an individual pulsar if its P, B, and alpha are known. Since different pulsars have different alpha, this explains why some gamma-ray pulsars have very similar P and B but very different gamma-ray luminosities. In determining the death line of gamma-ray pulsars, we have used a new criterion based on a concrete physical property, i.e., that the fractional size of the outer gap at the null-charge surface for a given pulsar cannot be larger than unity. In an estimate of the fractional size of the outer gap, two possible X-ray fields are considered: (1) X-rays produced by neutron star cooling and polar-cap heating, and (2) X-rays produced by the bombardment of relativistic particles from the outer gap onto the stellar surface ( the outer gap is called a self-sustained outer gap""). Since it is very difficult to measure alpha in general, we use a Monte Carlo method to simulate the properties of gamma-ray pulsars in our Galaxy. We find that this new outer-gap model predicts many more weak gamma-ray pulsars, which have a typical age between 0.3 and 3 Myr, than does the old model. For all simulated gamma-ray pulsars with self-sustained outer gaps, the gamma-ray luminosity L-gamma satisfies L-gamma proportional to L-sd(delta), where the value of delta depends on the sensitivity of the gamma-ray detector. For EGRET, delta similar to 0.38, whereas delta similar to 0.46 for GLAST. For gamma-ray pulsars with L(sd)less than or similar toL(sd)(crit), delta similar to 1, and L-sd(crit) = 1.5 x 10(34)P(1/3) ergs s(-1) is determined by f([r] similar to r(L)) = 1. These results are roughly consistent with the observed luminosity of gamma-ray pulsars. These predictions are very different from those of the previous outer-gap model, which predicts a very flat relation between L-gamma and L-sd.

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.

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.

Annihilation Emission from the Galactic Black Hole

Both diffuse high-energy gamma rays and an extended electron-positron annihilation line emission have been observed in the Galactic Center (GC) region. Although X-ray observations indicate that the Galactic black hole Sgr A* is inactive now, we suggest that Sgr A* can become active when a captured star is tidally disrupted and matter is accreted into the black hole. As a consequence the Galactic black hole could be a powerful source of relativistic protons. We are able to explain the current observed diffuse gamma rays and the very detailed 511 keV annihilation line of secondary positrons by p-p collisions of such protons, with appropriate injection times and energy. Relativistic protons could have been injected into the ambient material if the black hole captured a 50 M☉ star at several tens times 106 yr ago. An alternative possibility is that the black hole continues to capture stars with ~1 M☉ every 105 yr. Secondary positrons produced by p-p collisions at energies 30 MeV are cooled down to thermal energies by Coulomb collisions and are annihilated in the warm neutral and ionized phases of the interstellar medium with temperatures about several eV, because the annihilation cross section reaches its maximum at these temperatures. It takes about 10 million years for the positrons to cool down to thermal temperatures so that they can diffuse into a very large extended region around the GC. A much more recent star capture may also be able to account for recent TeV observations within 10 pc of the GC, as well as for the unidentified GeV gamma-ray sources found by EGRET at GC. The spectral difference between the GeV and TeV flux could be explained naturally in this model as well.

PROBING PRE-GALACTIC METAL ENRICHMENT WITH HIGH-REDSHIFT GAMMA-RAY BURSTS

We explore high-redshift gamma-ray bursts (GRBs) as promising tools to probe pre-galactic metal enrichment. We utilize the bright afterglow of a Population III (Pop III) GRB exploding in a primordial dwarf galaxy as a luminous background source, and calculate the strength of metal absorption lines that are imprinted by the first heavy elements in the intergalactic medium (IGM). To derive the GRB absorption line diagnostics, we use an existing highly-resolved simulation of the formation of a first galaxy which is characterized by the onset of atomic hydrogen cooling in a halo with virial temperature & 10 4 K. We explore the unusual circumburst environment inside the systems that hosted Pop III stars, modeling the density evolution with the self-similar solution for a champagne flow. For minihalos close to the cooling threshold, the circumburst density is roughly proportional to (1 + z) with values of about a few cm −3 . In more massive halos, corresponding to the first galaxies, the density may be larger, n & 100cm −3 . The resulting afterglow fluxes are weakly dependent on redshift at a fixed observed time, and may be detectable with the James Webb Space Telescope (JWST) and Very Large Array (VLA) in the near-IR and radio wavebands, respectively, out to redshift z & 20. We predict that the maximum of the afterglow emission shifts from near-IR to millimeter bands with peak fluxes from mJy to Jy at different observed times. The metal absorption line signature is expected to be detectable in the near future. GRBs are ideal tools for probing the metal enrichment in the early IGM, due to their high luminosities and featureless power-law spectra. The metals in the first galaxies produced by the first supernova (SN) explosions are likely to reside in low-ionization stages (C II, O I, Si II and Fe II). We show that if the afterglow can be observed sufficiently early, analysis of the metal lines may distinguish whether the first heavy elements were produced in a pair-instability supernova (PISN), or a core-collapse (Type II) SN, thus constraining the initial mass function of the first stars. Subject headings: cosmology: observations – cosmology: theory – galaxies: high-redshift – gamma rays: bursts – quasars: absorption lines

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°.