High-energy Pulsars Research Articles

Searching for X-ray counterparts of unassociated Fermi-LAT sources and rotation-powered pulsars with SRG/eROSITA

Context. The latest source catalog of the Fermi-LAT telescope contains more than 7000 γ-ray sources at giga-electronvolt energies, with the two dominant source classes thought to be blazars and rotation-powered pulsars. Despite continuous follow-up efforts, around 2600 sources have no known multiwavelength association. Aims. Our target is the identification of possible (young and recycled) pulsar candidates in the sample of unassociated γ-ray sources via their characteristic X-ray and γ-ray emission. To achieve this, we cross-matched the Fermi-LAT catalog with the catalog of X-ray sources in the western Galactic hemisphere from the first four all-sky surveys of eROSITA on the Spektrum-Roentgen-Gamma (SRG) mission. We complement this by identifying X-ray counterparts of known pulsars detected at γ-ray and radio energies in the eROSITA data. Methods. We used a Bayesian cross-matching scheme to construct a probabilistic catalog of possible pulsar-type X-ray counterparts to Fermi-LAT sources. Our method combines the overlap of X-ray and γ-ray source positions with a probabilistic classification (into pulsar and blazar candidates) of each source based on its γ-ray properties and a prediction on the X-ray flux of pulsar- or blazar-type counterparts. Finally, an optical and infrared counterpart search was performed to exclude coronally emitting stars and active galactic nuclei from our catalog. Results. We provide a catalog of our prior γ-ray-based classifications of all 2600 unassociated sources in the Fermi-LAT catalog, with around equal numbers of pulsar and blazar candidates. Our final list of candidate X-ray counterparts to suspected new high-energy pulsars, cleaned for spurious detections and sources with obvious non-pulsar counterparts, contains around 900 X-ray sources, the vast majority of which lie in the 95% γ-ray error ellipse. We predict between 30 and 40 new pulsars among our top 200 candidates, with around equal numbers of young and recycled pulsars. This candidate list may serve as input to future follow-up campaigns, looking directly for pulsations or for the orbital modulation of possible binary companions, where it may allow for a drastic reduction in the number of candidate locations to search. We furthermore detect the X-ray counterparts of 15 known rotation-powered pulsars, which were not seen in X-rays before.

Synchro-curvature emitting regions in high-energy pulsar models

ABSTRACT The detected high-energy pulsars’ population is growing in number, and thus, having agile and physically relevant codes to analyse it consistently is important. Here, we update our existing synchro-curvature radiation model by including a better treatment of the particle injection, particularly where the large pitch angle particles dominate the spectra, and by implementing a fast and accurate minimization technique. The latter allows a large improvement in computational cost, needed to test model enhancements, and to apply the model to a larger pulsar population. We successfully fit the sample of pulsars with X-ray and gamma-ray data. Our results indicate that, for every emitting particle, the spatial extent of their trajectory where the pitch angle is large and most of the detected X-ray radiation is produced is a small fraction of the light cylinder. We also confirm with this new approach that synchrotron radiation is not negligible for most of the gamma-ray pulsars detected. In addition, with the results obtained, we argue that J0357+3205 and J2055+2539 are MeV pulsar candidates and are suggested for exhaustive observations in this energy band.

The Multiple Outputs of Energetic Pulsars

AbstractThis talk commented on our progress in understanding high Ė pulsar output, with the photon power dominated by GeV radiation and the total power dominated by the e±/B wind. We are increasingly appreciating the anisotropy in these outflows, with high energy pulsar beaming probed by the distribution of γ-ray pulse profiles and wind anisotropy mapped by synchrotron images of PWNe. Possible hemispheric asymmetry and the prospects for additional probes of pulsar spindown, particularly from compact binaries in the black widow class, are briefly mentioned.

Prospects for the detection of high-energy (E > 25 GeV) Fermi pulsars with the Cherenkov Telescope Array

Around 160 gamma-ray pulsars were discovered by the Fermi Large Area Telescope (LAT) since 2008. The most energetic of them, 12 objects with emission above 25 GeV, are suitable candidates for the detection with the current and future Imaging Atmospheric Cherenkov Telescopes above few tens of GeV. We perform an analysis of the Fermi-LAT data of these high-energy pulsars in order to determine if such objects can be detected with the Cherenkov Telescope Array (CTA). Our goal is to forecast the significance of their point source detection with CTA. We analyze 5 years of the Fermi-LAT data fitting the spectra of each pulsar at energies E>10 GeV with a power-law function. Assuming no spectral cut-off, we extrapolate the resulting spectra to the very high energy range (VHE, E>0.1 TeV) and simulate CTA observations of all 12 pulsars with the ctools software package. Using different analysis tools, individual CTA sensitivity curves are independently calculated for each pulsar and cross-checked with the ctools results. Our simulations result in significant CTA detections of up to 8 pulsars in 50 h. Observations of the most energetic Fermi pulsars with CTA will shed light on the nature of the high-energy emission of pulsars, clarifying whether the VHE emission detected in the Crab pulsar spectrum is present also in other gamma-ray pulsars.

THREE-DIMENSIONAL NON-VACUUM PULSAR OUTER-GAP MODEL: LOCALIZED ACCELERATION ELECTRIC FIELD IN THE HIGHER ALTITUDES

We investigate the particle accelerator that arises in a rotating neutron-star magnetosphere. Solving the Poisson equation for the electro-static potential, the Boltzmann equations for relativistic electrons and positrons, and the radiative transfer equation simultaneously, we demonstrate that the electric field is substantially screened along the magnetic field lines by the pairs that are created and separated within the accelerator. As a result, the magnetic-field-aligned electric field is localized in the higher altitudes near the light cylinder and efficiently accelerates the positrons created in the lower altitudes outwards but not the electrons inwards. The resulting photon flux becomes predominantly outwards, leading to typical double-peak light curves, which are commonly observed from many high-energy pulsars.

Pulsar high energy emission due to inverse Compton scattering

We discuss growing evidence that pulsar high energy is emission is generated via Inverse Compton mechanism. We reproduce the broadband spectrum of Crab pulsar, from UV to very high energy gamma-rays - nearly ten decades in energy, within the framework of the cyclotron-self-Compton model. Emission is produced by two counter-streaming beams within the outer gaps, at distances above ∼ 20 NS radii. The outward moving beam produces UV-X-ray photons via Doppler-booster cyclotron emission, and GeV photons by Compton scattering the cyclotron photons produced by the inward going beam. The scattering occurs in the deep Klein-Nishina regime, whereby the IC component provides a direct measurement of particle distribution within the magnetosphere. The required plasma multiplicity is high, ∼106 – 107, but is consistent with the average particle flux injected into the pulsar wind nebula.

Prospects for observations of pulsars and pulsar wind nebulae with CTA

The last few years have seen a revolution in very-high γ-ray astronomy (VHE; E>100GeV) driven largely by a new generation of Cherenkov telescopes (namely the H.E.S.S. telescope array, the MAGIC and MAGIC-II large telescopes and the VERITAS telescope array). The Cherenkov Telescope Array (CTA) project foresees a factor of 5 to 10 improvement in sensitivity above 0.1TeV, extending the accessible energy range to higher energies up to 100TeV, in the Galactic cut-off regime, and down to a few tens GeV, covering the VHE photon spectrum with good energy and angular resolution. As a result of the fast development of the VHE field, the number of pulsar wind nebulae (PWNe) detected has increased from one PWN in the early ’90s to more than two dozen firm candidates today. Also, the low energy threshold achieved and good sensitivity at TeV energies has resulted in the detection of pulsed emission from the Crab Pulsar (or its close environment) opening new and exiting expectations about the pulsed spectra of the high energy pulsars powering PWNe. Here we discuss the physics goals we aim to achieve with CTA on pulsar and PWNe physics evaluating the response of the instrument for different configurations.

THE γ-RAY SPECTRUM OF GEMINGA AND THE INVERSE COMPTON MODEL OF PULSAR HIGH-ENERGY EMISSION

We reanalyze the Fermi spectra of the Geminga and Vela pulsars. We find that the spectrum of Geminga above the break is exceptionally well approximated by a simple power law without the exponential cut-off, making Geminga's spectrum similar to that of Crab. Vela's broadband gamma-ray spectrum is equally well fit with both the exponential cut-off and the double power law shapes. In the broadband double power-law fits, for a typical Fermi spectrum of a bright \gamma-ray pulsar, most of the errors accumulate due to the arbitrary parametrization of the spectral roll-off. In addition, a power law with an exponential cut-off gives an acceptable fit for the underlying double power-law spectrum for a very broad range of parameters, making such fitting procedures insensitive to the underlying Fermi photon spectrum. Our results have important implications for the mechanism of pulsar high energy emission. A number of observed properties of \gamma-ray pulsars, i.e., the broken power law spectra without exponential cut-offs and stretching in case of Crab beyond the maximal curvature limit, spectral breaks close to or exceeding the maximal breaks due to curvature emission, a Crab patterns of relative intensities of the leading and trailing pulses repeated in the X-ray and \gamma-ray regions, all point to the inverse Compton origin of the high energy emission from majority of pulsars.

Statistics and Evolution of Pulsars’ Parameters

The rotation periods, surface magnetic field strengths, as well as the spatial distribution of the several kinds of pulsars discovered sofar are analyzed statistically. It is revealed that the spatial distribution of the millisecond pulsars is more dispersive than that of the normal radio pulsars. And that the spatial distribution of the pulsars in low-mass X-ray binaries (LMXBs) is also more dispersive than that of the pulsars in high-mass X-ray binaries (HMXBs). The distribution of rotation periods of the isolated millisecond pulsars has a peak at 4.7ms, and the corresponding peak values for the normal radio pulsars and the millisecond pulsars in binaries are 0.6s and 3.5ms, respectively. The surface magnetic field strengths of the FERMI pulsars (the gamma-ray pulsars observed by the Large Area Telescope/Fermi Gamma-ray Space Telescope) and normal pulsars are all concentrated around 1012 Gs. It is found also that some young high-energy pulsars are associated with supernova remnants. In combination with the formation and evolution models of pulsars, we have made some remarks on the characteristics of these distributions.

Detection of the Small Magellanic Cloud in gamma-rays with Fermi/LAT

The flux of gamma rays with energies >100MeV is dominated by diffuse emission from CRs illuminating the ISM of our Galaxy through the processes of Bremsstrahlung, pion production and decay, and inverse-Compton scattering. The study of this diffuse emission provides insight into the origin and transport of CRs. We searched for gamma-ray emission from the SMC in order to derive constraints on the CR population and transport in an external system with properties different from the Milky Way. We analysed the first 17 months of continuous all-sky observations by the Large Area Telescope of the Fermi mission to determine the spatial distribution, flux and spectrum of the gamma-ray emission from the SMC. We also used past radio synchrotron observations of the SMC to study the population of CR electrons specifically. We obtained the first detection of the SMC in high-energy gamma rays, with an integrated >100MeV flux of (3.7 +/-0.7) x10e-8 ph/cm2/s, with additional systematic uncertainty of <16%. The emission is steady and from an extended source ~3{\deg} in size. It is not clearly correlated with the distribution of massive stars or neutral gas, nor with known pulsars or SNRs, but a certain correlation with supergiant shells is observed. The observed flux implies an upper limit on the average CR nuclei density in the SMC of ~15% of the value measured locally in the Milky Way. The population of high-energy pulsars of the SMC may account for a substantial fraction of the gamma-ray flux, which would make the inferred CR nuclei density even lower. The average density of CR electrons derived from radio synchrotron observations is consistent with the same reduction factor but the uncertainties are large. From our current knowledge of the SMC, such a low CR density does not seem to be due to a lower rate of CR injection and rather indicates a smaller CR confinement volume characteristic size.

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.

Outer-Gap versus Slot-Gap Models for Pulsar High-Energy Emissions: The Case of the Crab Pulsar

We analytically examine the capabilities of rotation-powered pulsars as the sources of gamma rays and show that their phase-averaged gamma-ray flux is proportional to the product of the spin-down flux and the gap transfield thickness cubed irrespective of the emission models. Applying the scheme to the Crab pulsar, we demonstrate that the outer-gap model reproduces the observed GeV fluxes and that the slot-gap model reproduces at most 20% of the observed fluxes because of the small transfield thickness. An implication for the relationship between the gamma-ray and the spin-down fluxes is discussed.

The Polarization of High-Energy Pulsar Radiation in the Striped Wind Model

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 that 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 the pulsar. This property is unique to the wind model and in good agreement with the data. Other properties such as a reduced degree of polarization and a characteristic sweep of the polarization angle within the pulses are also reproduced. These properties are qualitatively unaffected by variations of the wind Lorentz factor, the electron injection power-law index, and the inclination of the line of sight.

OPTIMA An Optical Pulsar Timing Analyser

AbstractOPTIMA is a small and mobile highspeed photometer which is primarily intended for the observation of young high energy pulsars at optical wavelengths. The detector system consists of a fiber fed photon counter based on avalanche photodiodes, a GPS timing receiver and an integrating CCD camera to ensure the correct positioning of the targeted object. In January 1999, OPTIMA proved its functionality by measuring the lightcurve of the Crab pulsar.

EGRET observations of high energy gamma ray pulsars

Observations with the EGRET instrument on CGRO have so far revealed gamma ray emission at E>100 MeV from 6, possibly 7 pulsars: 5 (6) radio pulsars and the radio quiet pulsar Geminga. The observed gamma ray lightcurves show mostly two peaked emission patterns; only PSR B1706-44 has a more complex lightcurve. The gamma ray photon number spectra between 30 MeV and several GeV can be described with power law distributions with indices from −1.2 to −2.2 although a moderate bend in some spectra is not excluded. Above a few GeV several pulsars show a marked spectral break. Gamma ray pulsars are characterised by their high rotational energy loss and their relative proximity (on a galactic scale) to Earth. In terms of derived parameters, gamma ray pulsars have a young age ( τ ∝ P / P < 10 6 years), strong magnetic fields (B ∝ P P ̇ ≈ 10 12 G) , and high voltages available over the pulsar's open field lines (Φ ∝ P ̇ /P 3 ≈ 3 × 10 14 − 4×10 16 V) . Their efficiency to convert rotational energy into high energy radiation increases systematically with age and the observed gamma ray spectra in the EGRET energy range become correspondingly harder. For several of the the brighter pulsars, phase resolved spectra have been derived showing significant spectral changes over the pulsar's rotation. Multiwavelength beaming patterns, i.e. lightcurves observed throughout the electromagnetic spectrum, reinforce this picture that young pulsars have an extremely rich phenomenology in shaping their emission. While the gamma ray lightcurves of most pulsars are quite similar, emission at lower energies appears unrelated in all cases except the Crab pulsar. The underlying physics describing the high energy radiation processes in the pulsar's magnetosphere and close to the neutron star's surface is still in a state of development. Competing models have achieved to explain many individual features of pulsars but no comprehensive, self consistent treatment of particles, fields, and radiation is yet available. Progress in understanding the emission processes is critical for questions related to the detection and population statistics of more high energy pulsars, and the total contribution of high energy pulsars to the galactic gamma ray emission.

High Energy Pulsars: Pulses and Populations

AbstractRecent X-ray and γ-ray observations have detected a number of isolated spin-powered pulsars. Studies of the pulse profiles of these objects are providing a useful guide to the site of the high energy emission. The γ-ray pulses, which are closely tied to the primary population of radiating particles, seem to be an especially useful discriminant between pulsar models. With an understanding of how the pulsar luminosity and beaming factors evolve with spin parameters, pulsar population syntheses can also be used to check the predictions of high energy emission models. In turn, comparison with γ-ray sky surveys constrains the properties of the young pulsar population.

Pulsed high-energy gamma rays from PSR 1055-52

The Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory has detected a high-energy gamma-ray source at a position coincident with that of the radio pulsar PSR 1055-52. Analysis of the EGRET data at the radio pulsar period of 197 ms has revealed pulsed gamma-radiation at energies above 300 MeV, making PSR 1055-52 the fifth detected high-energy gamma-ray pulsar. The pulsed radiation from PSR 1055-52 has a very hard photon spectral index of -1.18 +/- 0.16 and a high efficiency for converting its rotational energy into gamma-rays. No unpulsed emission was observed.

Pulsed high-energy γ-rays from the radio pulsar PSRI706–44

OF the more than 500 known radio pulsars, only three have been detected as γ-ray sources. Using the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory satellite, we have detected pulsed γ-radiation, above 100 MeV in energy, from a fourth radio pulsar, PSR1706–44. Within the pulse period of 102 ms, the γ-emission forms a single broad peak, in contrast to the two narrow peaks seen in the other high-energy γ-ray pulsars. The emission mechanism in all cases is probably the same, the differences arising from the geometry of the magnetic and rotation axes and the line of sight. In these pulsars, γ-ray emission accounts for as much as 1% of the total neutron star spin-down energy, much more than emerges at optical or radio frequencies, so study of this emission is important in understanding pulsar emission and evolution.