Pulsar Models Research Articles

Morphological tests of the pulsar and dark matter interpretations of the WMAP haze

The WMAP haze is an excess in microwave emission coming from the center of the Milky Way galaxy. In the case of synchrotron emission models of the haze, we present tests for the source of radiating high-energy electrons/positrons. We explore several models in the case of a pulsar population or dark matter annihilation as the source. These morphological signatures of these models are small behind the WMAP Galactic mask, but are testable and constrain the source models. We show that detailed measurements of the morphology may distinguish between the pulsar and dark matter interpretations as well as differentiate among different pulsar models and dark matter profile models individually. Specifically, we find that a zero central density Galactic pulsar population model is in tension with the observed WMAP haze. The Planck Observatory's greater sensitivity and expected smaller Galactic mask should potentially provide a robust signature of the WMAP haze as either a pulsar population or the dark matter.

PROBING MILLISECOND PULSAR EMISSION GEOMETRY USING LIGHT CURVES FROM THEFERMI/LARGE AREA TELESCOPE

An interesting new high-energy pulsar sub-population is emerging following early discoveries of gamma-ray millisecond pulsars (MSPs) by the Fermi Large Area Telescope (LAT). We present results from 3D emission modeling, including the Special Relativistic effects of aberration and time-of-flight delays and also rotational sweepback of B-field lines, in the geometric context of polar cap (PC), outer gap (OG), and two-pole caustic (TPC) pulsar models. In contrast to the general belief that these very old, rapidly-rotating neutron stars (NSs) should have largely pair-starved magnetospheres due to the absence of significant pair production, we find that most of the light curves are best fit by TPC and OG models, which indicates the presence of narrow accelerating gaps limited by robust pair production -- even in these pulsars with very low spin-down luminosities. The gamma-ray pulse shapes and relative phase lags with respect to the radio pulses point to high-altitude emission being dominant for all geometries. We also find exclusive differentiation of the current gamma-ray MSP population into two MSP sub-classes: light curve shapes and lags across wavebands impose either pair-starved PC (PSPC) or TPC / OG-type geometries. In the first case, the radio pulse has a small lag with respect to the single gamma-ray pulse, while the (first) gamma-ray peak usually trails the radio by a large phase offset in the latter case. Finally, we find that the flux correction factor as a function of magnetic inclination and observer angles is typically of order unity for all models. Our calculation of light curves and flux correction factor for the case of MSPs is therefore complementary to the "ATLAS paper" of Watters et al. for younger pulsars.

The Galactic Pulsar Population

AbstractSimulations of the Galactic pulsar population are reviewed. These include snapshot and time-dependent models of normal pulsars as well as binary and milli-second pulsars.

Gamma-ray emission from pulsar/massive-star binaries

AbstractI present a review of the main phenomenological properties at high energies related to massive gamma-ray binaries and I discuss some aspects of pulsar models for these objects.

Pulsars versus dark matter interpretation of ATIC/PAMELA

In this paper, we study the flux of electrons and positrons injected by pulsars and by annihilating or decaying dark matter in the context of recent ATIC, PAMELA, Fermi, and HESS data. We review the flux from a single pulsar and derive the flux from a distribution of pulsars. We point out that the particle acceleration in the pulsar magnetosphere is insufficient to explain the observed excess of electrons and positrons with energy $E\ensuremath{\sim}1\text{ }\text{ }\mathrm{TeV}$ and one has to take into account an additional acceleration of electrons at the termination shock between the pulsar and its wind nebula. We show that at energies less than a few hundred GeV, the expected flux from a continuous distribution of pulsars provides a good approximation to the expected flux from pulsars in the Australia Telescope National Facility catalog. At higher energies, we demonstrate that the electron/positron flux measured at the Earth will be dominated by a few young nearby pulsars, and therefore the spectrum would contain bumplike features. We argue that the presence of such features at high energies would strongly suggest a pulsar origin of the anomalous contribution to electron and positron fluxes. The absence of features either points to a dark matter origin or constrains pulsar models in such a way that the fluctuations are suppressed. Also we derive that the features can be partially smeared due to spatial variation of the energy losses during propagation.

TESTING PULSAR RADIATION MODELS USING AN α-WEAK-DEPENDENT ALTITUDE RATIO

It is found that pulsar radiation altitude ratios between different radio frequencies are weak-dependent on the inclination angle α. This is proved via series expansion techniques and illustrated by using pulsar examples of PSR B0329+54, B1508+55, B2016+28, B1133+16, and B2319+60. It is emphasized that this α-weak-dependent radiation altitude ratio offers a good tool to test pulsar radiation models. We use the measured altitude ratios to constrain the parameter space for the Ruderman-Sutherland model and the inverse Compton scattering model. It is found that the Ruderman-Sutherland model is not compatible with the measured altitude ratios, while the results are compatible with the inverse Compton scattering model. The potential possible applications of this method in studying pulsar timing and in studying pulsar high energy radiation are also discussed.

Optical and infrared observations of the Crab Pulsar and its nearby knot

We study the spectral energy distribution (SED) of the Crab Pulsar and its nearby knot in the optical and in the infrared (IR) regime. We present high-quality UBVRIz, as well as adaptive optics JHK_sL' photometry, achieved under excellent conditions with the FORS1 and NAOS/CONICA instruments at the VLT. We combine these data with re-analyzed archival Spitzer Space Telescope data to construct a SED for the pulsar, and quantify the contamination from the knot. We have also gathered optical imaging data from 1988 to 2008 from several telescopes in order to examine the predicted secular decrease in luminosity. For the Crab Pulsar SED we find a spectral slope of alpha_nu = 0.27+-0.03 in the optical/near-IR regime, when we exclude the contribution from the knot. For the knot itself, we find a much redder slope of alpha_nu = -1.3 +- 0.1. Our best estimate of the average decrease in luminosity for the pulsar is 2.9+-1.6 mmag per year. We have demonstrated the importance of the nearby knot in precision measurements of the Crab Pulsar SED, in particular in the near-IR. We have scrutinized the evidence for the traditional view of a synchrotron self-absorption roll-over in the infrared, and find that these claims are unfounded. We also find evidence for a secular decrease in the optical light for the Crab Pulsar, in agreement with current pulsar spin-down models. However, although our measurements of the decrease significantly improve on previous investigations, the detection is still tentative. We finally point to future observations that can improve the situation significantly.

HIGH-ENERGY EMISSION FROM CTA 1

Observations indicate that there is a pulsar in CTA 1 and the pulsar powers a synchrotron pulsar wind nebula (PWN) embedded in a larger supernova remnant. We investigate high-energy photon emission from the CTA 1 in such a case. The emission is divided into pulsed and unpulsed components. For the pulsed emission, we calculate the gamma-ray spectrum in the vacuum outer gap model. For the unpulsed component, we calculate the spectra of nonthermal photons produced in the PWN on the frame of time-dependent pulsar wind model. Our results indicate that high-energy gamma rays are dominated by those produced in the pulsar (i.e., the pulsed component is dominant), but very high energy gamma rays are dominated by those produced in the PWN and will be detected by VERITAS, although the CTA 1 is a weaker TeV emitter.

LINEAR ACCELERATION EMISSION. II. POWER SPECTRUM

The theory of linear acceleration emission (LAE) is developed for a large amplitude electrostatic wave in which all particles become highly relativistic in much less than a wave period. An Airy-integral approximation is shown to apply near the phases where the electric field passes through zero and the Lorentz factors of all particles have their maxima. The emissivity is derived for an individual particle and is integrated over frequency and solid angle to find the power radiated per particle. The result is different from that implied by the generalized Larmor formula which, we argue, is not valid in this case. We also discuss a mathematical inconsistency that arises when one evaluates the power spectrum by integrating the emissivity over solid angle. The correct power spectrum increases as the 4/3rd power of the frequency at low frequencies, and falls off exponentially above a characteristic frequency. We discuss application of LAE to the emission of high-frequency photons in an oscillating model for pulsars. We conclude that it cannot account for gamma-ray emission, but can play a role in secondary pair creation.

LINEAR ACCELERATION EMISSION. I. MOTION IN A LARGE-AMPLITUDE ELECTROSTATIC WAVE

We consider the motion of a charge in a large amplitude electrostatic wave with a triangular wave form relevant to an oscillating model of a pulsar magnetosphere. The (one-dimensional) orbit of a particle in such a wave is found exactly in terms of Weierstrass functions. The result is used to discuss linear acceleration emission (at both low and high frequencies) in an oscillating model for pulsars. An explicit expression for the emissivity is derived in an accompanying paper (Melrose & Luo 2009), and used here to derive an expression for the absorption coefficient at low frequencies. We show that absorption can be negative, corresponding to maser emission. For the large amplitude required to trigger pair creation in an oscillating model, the rate of the maser growth is too small to be effective. Effective growth requires smaller amplitudes, such that the maximum Lorentz factor gained by acceleration in the wave is $\lapprox10$.

High-energy characteristics of the schizophrenic pulsar PSR J1846-0258 in Kes 75

<i>Aims. <i/>PSR J1846-0258 is a young rotation-powered pulsar with one of the highest surface magnetic field strengths, located in the centre of SN-remnant Kes-75. In June 2006 a magnetar-like outburst took place. Using multi-year RXTE and INTEGRAL observations covering the epoch of the outburst, we aim to study the temporal and spectral characteristics of PSR J1846-0258 over a broad ~3–300 keV energy range to derive constraints on theoretical scenarios aiming to explain this schizophrenic behaviour.<i>Methods. <i/>We explored all publically available RXTE observations of PSR J1846-0258 to generate accurate ephemerides over the period January 30, 2000–November 7, 2007. Phase-folding procedures yielded pulse profiles for RXTE PCA (~3–30 keV), RXTE HEXTE (~15–250 keV) and INTEGRAL ISGRI (~20–300 keV). The pulsed spectrum over the full ~3–300 keV energy range was derived, as well as the total spectrum (including the pulsar wind nebula) over the 20–300 keV band with the ISGRI. The timing, spatial, and spectral analyses were applied for epochs before, during, and after the magnetar-like outburst to study the evolution of the high-energy characteristics.<i>Results. <i/>ISGRI detected PSR J1846-0258/Kes-75 before outburst during 2003–2006 with a power-law-shape spectrum over the 20–300 keV energy range with photon index <i>Γ<i/> = and energy flux (20–300 keV) of erg/cm<sup>2<sup/> s. More than 90 days after the onset of the outburst, still during the decay phase, the same spectral shape was measured () with an indication for a 52% (2.3<i>σ<i/>) enhanced total emission, while one year after the outburst the hard X-ray non-thermal emission of PSR J1846-0258/Kes-75 was found to be back to its pre-outburst values. PCA monitoring of PSR J1846-0258 before the outburst yielded phase-coherent ephemerides confirming the earlier derived breaking index of the spindown. During the outburst, incoherent solutions have been derived. We show that the radiative outburst was triggered by a major spin-up glitch near MJD with a glitch size in the range . Using all pre-outburst observations of ISGRI and HEXTE for the first time pulse profiles have been obtained up to 150 keV with a broad single asymmetric pulse. The pulse shape did not vary with energy over the 2.9–150 keV energy range, nor did it change during the magnetar-like outburst. The time-averaged pre-outburst ~3–300 keV pulsed spectrum measured with the PCA, HEXTE, and ISGRI was fitted with a power-law model with . A fit with a curved power-law model gives an improved fit. Around 150 keV the pulsed fraction approaches 100%. For the first 32 days of the magnetar-like outburst, the 3–30 keV pulsed spectrum can be represented with two power laws, a soft component with index and a hard component with the pre-outburst value . Above ~9 keV, all spectra during outburst are consistent with the latter single power-law shape with index ~1.2. The 2–10 keV flux increased by a factor ~5 and the 10–30 keV flux increased with only 35%. After ~120 days the soft outburst and the enhancement of the hard non-thermal component both vanish.<i>Conclusions. <i/>The varying temporal and spectral characteristics of PSR J1846-0258 can be explained in a scenario of a young high-<i>B<i/>-field pulsar in which a major glitch triggered a sudden release of energy. Resonant cyclotron upscattering could subsequently generate the decaying/cooling soft pulsed component measured during outburst between 3 and 10 keV. The (variation in the) non-thermal hard X-ray component can be explained with synchrotron emission in a slot-gap or outer-gap pulsar model.

γ-RAY EMISSION FROM LS I +61 303: THE IMPACT OF BASIC SYSTEM UNCERTAINTIES

LS I +61 303 has been recently detected as a periodic γ-ray source by the Major Atmospheric Imaging Cerenkov (MAGIC) telescope. A distinctive orbital correlation of the γ-ray emission was found. This work shows that the range of uncertainties yet at hand in the orbital elements of the binary system LS I +61 303 as well as in the possible assumptions on the stellar wind of the optical companion play a non-negligible role in the computation of opacities to high-energy processes leading to γ-ray predictions. The geometry influence on the propagation and escape of γ-ray photons is explored. With this study at hand, we analyze the results of a pulsar wind zone model for the production of γ-rays and compare it with recent MAGIC observations.

Optical pulsations from the anomalous X-ray pulsar 1E 1048.1–5937

Abstract We present high-speed optical photometry of the anomalous X-ray pulsar 1E 1048.1-5937 obtained with ULTRACAM on the 8.2-m Very Large Telescope in 2007 June. We detect 1E 1048.1-5937 at a magnitude of i′= 25.3 ± 0.2, consistent with the values found by Wang et al. and hence confirming their conclusion that the source was approximately 1 mag brighter than in 2003–06 due to an on-going X-ray flare that started in 2007 March. The increased source brightness enabled us to detect optical pulsations with an identical period (6.458 s) to the X-ray pulsations. The root-mean-square (rms) pulsed fraction in our data is 21 ± 7 per cent, approximately the same as the 2–10 keV X-ray rms pulsed fraction. The optical and X-ray pulse profiles show similar morphologies and appear to be approximately in phase with each other, the latter lagging the former by only 0.06 ± 0.02 cycles. The optical pulsations in 1E 1048.1-5937 are very similar in nature to those observed in 4U 0142+61. The implications of our observations for models of anomalous X-ray pulsars are discussed.

Study on autonomous navigation based on pulsar timing model

The basic principle of pulsar timing model was introduced, and the general relativistic corrections were analyzed when pulse time of arrival (TOA) was transferred to coordinate TOA at the Solar System Barycentre. Based on the shifting, an iterative method of autonomous position determination for spacecraft was developed. Accordingly, the linear form of the position offset equation was evolved. Using the initial estimated value of spacecraft’s position as the input of pulsar timing equation, through calculation of the offset between measured or transferred and predicted TOA, the position offset can be solved by Least Squares. At last, the main error sources including modeling error and parameters error were discussed.

Electrodynamics of Magnetars. IV. Self‐Consistent Model of the Inner Accelerator with Implications for Pulsed Radio Emission

We consider the voltage structure in the open-field circuit and outer magnetosphere of a magnetar. The standard polar cap model for radio pulsars is modified significantly when the polar magnetic field exceeds 1.8 × 1014 G. Then a surface gap is effectively screened, because resonantly scattered X-rays can convert almost instantaneously to electron-positron pairs. We point out a second possible voltage distribution, in which backflowing charges spawn just enough pairs to compensate the gradient in the corotation charge density. We also examine the gap structure when the magnetic field is somewhat weaker and deduce a voltage 10-30 times larger over a range of surface temperatures. We examine carefully how the flow of charge back to the star above the gap depends on the magnitude of the current, on the curvature of the magnetic field lines, and on resonant drag. The rates of different channels of pair creation are determined self-consistently, including the nonresonant scattering of X-rays and collisions between gamma rays and X-rays. Both circuit solutions are subject to a two-stream instability due to a spatially extended return charge flow. The voltage is high enough to sustain the radio output of strongly magnetic pulsars, but not of the two radio magnetars with active magnetospheres, unless the gap is also present on the closed magnetic field lines. Several properties of the radio magnetars—their rapid variability, broad pulses, and unusually hard radio spectra—are consistent with a third possibility, that the current in the outer magnetosphere is strongly variable and a very high rate of pair creation is sustained by a turbulent cascade.

A Study on the Pulse Profiles of the HMXB 4U 1901+03

After a silence of some 32 years, the high-mass X-ray binary (HMXB) 4U1901+03 produced an outburst in Feb. 2003. With the observed data of RXTE (Rossi X-ray Timing Explorer) over a 5 month duration, we have made a systematic study of the pulse profile of this source, and obtained its time evolution and its correlation with the photon energy. It is found that the variations of the pulse profile and the pulse fraction with the accretion rate of the binary system exhibit a stepwise evolution, and that the pulse fraction reaches its peak at energies under ten KeV. The complex variation of the pulse profile indicates that the pulse profile can not be explained by a single geometrical or physical model, rather, it must be related with both the viewing angle and the radiation mechanism. The observed features are here discussed in terms of the standard radiation model of pulsars.

Constraining A General-Relativistic Frame-Dragging Model for Pulsed Radiation from a Population of Millisecond Pulsars in 47 Tucanae using GLAST LAT

Although only 22 millisecond pulsars (MSPs) are currently known to exist in the globular cluster (GC) 47 Tucanae, this cluster may harbor 30-60 MSPs, or even up to ~200. In this Letter, we model the pulsed curvature radiation (CR) gamma-ray flux expected from a population of MSPs in 47 Tucanae. These MSPs produce gamma-rays in their magnetospheres via accelerated electron primaries which are moving along curved magnetic field lines. A GC like 47 Tucanae containing a large number of MSPs provides the opportunity to study a randomized set of pulsar geometries. Geometry-averaged spectra make the testing of the underlying pulsar model more reliable, since in this case the relative flux uncertainty is reduced by one order of magnitude relative to the variation expected for individual pulsars (if the number of visible pulsars N=100). Our predicted spectra violate the EGRET upper limit at 1 GeV, constraining the product of the number of visible pulsars N and the average integral flux above 1 GeV per pulsar. GLAST/LAT should place even more stringent constraints on this product, and may also limit the maximum average accelerating potential by probing the CR spectral tail. For N=22-200, a GLAST/LAT non-detection will lead to the constraints that the average integral flux per pulsar should be lower by factors 0.03-0.003 than current model predictions.

A Morphological Approach to the Pulsed Emission from Soft Gamma Repeaters

We present a geometrical methodology to interpret the periodical light curves of soft gamma repeaters based on the magnetar model and the numerical arithmetic of the three-dimensional magnetosphere model for the young pulsars. The hot plasma released by the starquake is trapped in the magnetosphere, and photons are emitted tangentially to the local magnetic field lines. The variety of radiation morphologies in the burst tails and the persistent stages could be well explained by the trapped fireballs on different sites inside the closed field lines. Furthermore, our numerical results suggest that the pulse profile evolution of SGR 1806–20 during the 2004 December 27 giant flare is due to a lateral drift of the emitting region in the magnetosphere.

Pulse Phase‐Resolved Analysis of the High‐Mass X‐Ray Binary Centaurus X‐3 over Two Binary Orbits

We present a detailed analysis of observations of the high-mass X-ray binary Cen X-3, spanning two consecutive binary orbits performed with the RXTE satellite in early March 1997. During this time Cen X-3 had a luminosity of L2 − 10 keV ~ (4–5) × 1037 ergs s−1 and a pulse period of 4.814 s. The PCA and HEXTE light curves both show a clear reduction in count rate after midorbit for both binary revolutions. We therefore analyze two broadband spectra for each orbit, before and after midorbit. Consistent with earlier observations, these four joint PCA and HEXTE spectra can be well described using a phenomenological pulsar continuum model, including an iron emission line and a cyclotron resonance-scattering feature. While no strong spectral variations were detected, the second half of orbit 2 shows a tendency toward being softer and more strongly absorbed. In order to follow the orbital phase-dependent evolution of the spectrum in greater detail, we model spectra for shorter exposures, confirming that most spectral parameters show either a gradual or sudden change for the second half of the second orbit. A comparison with a simple wind model indicates the existence of an accretion wake in this system. We also present and discuss high-resolution pulse profiles for several different energy bands, as well as their hardness ratios. PCA and HEXTE spectra were created for 24 phase bins and fitted using the same model as in the phase-averaged case. Systematic pulse phase-dependent variations of several continuum and cyclotron line parameters were detected, most notably a significant increase of the cyclotron line energy during the early rise of the main peak, followed by a gradual decrease. We show that applying a simple dipole model for the magnetic field is not sufficient to describe our data.

A Pulsational Model for the Orthogonal Polarization Modes in Radio Pulsars

In an earlier paper, we introduced a model for pulsars in which non-radial oscillations of high spherical degree (\el) aligned to the magnetic axis of a spinning neutron star were able to reproduce subpulses like those observed in single-pulse measurements of pulsar intensity. The model did not address polarization, which is an integral part of pulsar emission. Observations show that many pulsars emit radio waves that appear to be the superposition of two linearly polarized emission modes with orthogonal polarization angles. In this paper, we extend our model to incorporate linear polarization. As before, we propose that pulsational displacements of stellar material modulate the pulsar emission, but now we apply this modulation to a linearly-polarized mode of emission, as might be produced by curvature radiation. We further introduce a second polarization mode, orthogonal to the first, that is modulated by pulsational velocities. We combine these modes in superposition to model the observed Stokes parameters in radio pulsars.