Fermi Pulsars Research Articles

Prospects for the detection of very-high-energy pulsars with LHAASO and SWGO

ABSTRACT Pulsations from the Crab pulsar have been detected by the MAGIC telescopes at energies up to 1.5 TeV, and the pulsed emission from the Vela pulsar was detected by H.E.S.S., reaching tens of TeV. These discoveries, along with the proposed additional emission due to inverse Compton scattering at TeV energies, lead us to consider suitable candidates for detection with current and future extensive air show (EAS) experiments at very-high-energy (VHE; 0.1$-$100 TeV) ranges. Leveraging energy spectrum data from pulsars as observed by Fermi and Imaging Atmospheric Cherenkov Telescopes (IACTs) and considering the sensitivities of both LHAASO and SWGO, this study evaluates their detectability and estimates the time required for their significant detection. Our results indicate that LHAASO could detect the Crab’s pulsed signal within six years, while SWGO might detect Vela’s signal within one year. Observations of the most energetic Fermi pulsars with EAS experiments will provide insight into the nature of VHE pulsar emissions, helping us to clarify the primary characteristics of VHE pulsars.

Very-high-energy Emission from Pulsars

Abstract Air-Cherenkov telescopes have detected pulsations at energies above 50 GeV from a growing number of Fermi pulsars. These include the Crab, Vela, PSR B1706−44, and Geminga, with the first two having pulsed detections above 1 TeV. In some cases, there appears to be very-high-energy (VHE) emission that is an extension of the Fermi spectra to high energies, while in other cases, additional higher-energy spectral components that require a separate emission mechanism may be present. We present results of broadband spectral modeling using global magnetospheric fields and multiple emission mechanisms that include synchro-curvature (SC) and inverse Compton scattered (ICS) radiation from accelerated particles (primaries) and synchrotron self-Compton (SSC) emission from lower-energy pairs. Our models predict three distinct VHE components: SC from primaries whose high-energy tail can extend to 100 GeV, SSC from pairs that can extend to several TeV, and ICS from primary particles accelerated in the current sheet that scatter pair synchrotron radiation, which appears beyond 10 TeV. Our models suggest that H.E.S.S.-II and MAGIC have detected the high-energy tail of the primary SC component that produces the Fermi spectrum in Vela, Geminga, and PSR B1706−44. We argue that the ICS component peaking above 10 TeV from Vela has been seen by H.E.S.S. Detection of this emission component from the Crab and other pulsars is possible with the High Altitude Water Cherenkov Observatory and Cherenkov Telescope Array, and will directly measure the maximum particle energy in pulsars.

Constraining millisecond pulsar geometry using time-aligned radio and gamma-ray pulse profile

Context. Since the launch of the Fermi Gamma-Ray Space Telescope, several hundred gamma-ray pulsars have been discovered, some being radio-loud and some radio-quiet with time-aligned radio and gamma-ray light curves. In the second Fermi Pulsar Catalogue, 117 new gamma-ray pulsars have been reported based on three years of data collected by the Large Area Telescope on the Fermi satellite, providing a wealth of information such as the peak separation Δ of the gamma-ray pulsations and the radio lag δ between the gamma-ray and radio pulses. Aim. We selected several radio-loud millisecond gamma-ray pulsars with period P in the range 2–6 ms and showing a double peak in their gamma-ray profiles. We attempted to constrain the geometry of their magnetosphere, namely the magnetic axis and line-of-sight inclination angles for each of these systems. Method. We applied a force-free dipole magnetosphere from the stellar surface up to the striped wind region – well outside the light cylinder – to fit the observed pulse profiles in gamma-rays, consistently with their phase alignment with the radio profile. In deciding whether a fitted curve is reasonable or not, we employed a least-square method to compare the observed gamma-ray intensity with that found from our model, emphasising the amplitude of the gamma-ray peaks, their separation, and the phase lag between radio and gamma-ray peaks. Results. We obtained the best fits and reasonable parameters in agreement with observations for ten millisecond pulsars. Eventually, we constrained the geometry of each pulsar described by the magnetic inclination α and the light-of-sight inclination ζ. We found that both angles are larger than approximately 45°.

A Fundamental Plane for Gamma-Ray Pulsars

Abstract We show that the γ-ray pulsar observables, i.e., their total γ-ray luminosity, , spectral cutoff energy, , stellar surface magnetic field, B ⋆, and spin-down power , obey a relation of the form , which represents a 3D plane in their 4D logspace. Fitting the data of 88 pulsars of the second Fermi pulsar catalog, we show this relation to be , a pulsar fundamental plane (FP). We show that the observed FP is remarkably close to the theoretical relation obtained assuming that the pulsar γ-ray emission is due to curvature radiation by particles accelerated at the pulsar equatorial current sheet just outside the light cylinder. Interestingly, the FP seems incompatible with emission by synchrotron radiation. The corresponding scatter about the FP is ∼0.35 dex and can only partly be explained by the observational errors while the rest is probably due to the variation of the inclination and observer angles. We predict also that toward low for both young and millisecond pulsars implying that the observed death line of γ-ray pulsars is due to dropping below the Fermi band. Our results provide a comprehensive interpretation of the observations of γ-ray pulsars, setting requirements for successful theoretical modeling.

An image-based search for pulsars among Fermi unassociated LAT sources

We describe an image-based method that uses two radio criteria, compactness and spectral index, to identify promising pulsar candidates among Fermi Large Area Telescope (LAT) unassociated sources. These criteria are applied to those radio sources from the Giant Metrewave Radio Telescope all-sky survey at 150 MHz (TGSS ADR1) found within the error ellipses of unassociated sources from the 3FGL catalog and a preliminary source list based on 7 years of LAT data. After follow-up interferometric observations to identify extended or variable sources, a list of 16 compact, steep-spectrum candidates is generated. An ongoing search for pulsations in these candidates, in gamma rays and radio, has found six millisecond pulsars and one normal pulsar. A comparison of this method with existing selection criteria based on gamma-ray spectral and variability properties suggests that the pulsar discovery space using Fermi may be larger than previously thought. Radio imaging is a hitherto underutilized source selection method that can be used, as with other multi-wavelength techniques, in the search for Fermi pulsars.

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.

Fermi Gamma-Ray Pulsars: Understanding the High-energy Emission from Dissipative Magnetospheres

Abstract Based on the Fermi observational data, we reveal meaningful constraints for the dependence of the macroscopic conductivity (σ) of dissipative pulsar magnetosphere models on the corresponding spin-down rate, . Our models are refinements of the FIDO (Force-free Inside, Dissipative Outside) models, which have dissipative regions that are restricted on the equatorial current sheet outside of the the light-cylinder. Taking into account the observed cutoff energies of all of the Fermi pulsars and assuming that (a) the corresponding γ-ray pulsed emission is due to curvature radiation at the radiation-reaction-limit regime, and (b) this emission is produced at the equatorial current sheet near the light cylinder, we show that the Fermi data provide clear indications about the corresponding accelerating electric-field components. A direct comparison between the Fermi cutoff energies and the model ones reveals that σ increases with for high -values, while it saturates for low ones. This comparison indicates also that the corresponding gap width increases toward low -values. Assuming the Goldreich–Julian flux for the emitting particles, we calculate the total γ-ray luminosity (L γ ). A comparison between the dependence of the Fermi L γ -values and the model ones on indicates an increase of the emitting particle multiplicity with . Our modeling, guided by the Fermi data alone, enhances our understanding of the physical mechanisms behind the high-energy emission in pulsar magnetospheres.

HARD X-RAY EMISSION FROM SH 2‐104: A NuSTAR SEARCH FOR GAMMA-RAY COUNTERPARTS

ABSTRACT We present NuSTAR hard X-ray observations of Sh 2–104, a compact H ii region containing several young massive stellar clusters (YMSCs). We have detected distinct hard X-ray sources coincident with localized VERITAS TeV emission recently resolved from the giant gamma-ray complex MGRO J2019+37 in the Cygnus region. Fainter, diffuse X-rays coincident with the eastern YMSC in Sh2-104 likely result from the colliding winds of a component star. Just outside the radio shell of Sh 2–104 lies 3XMM J201744.7+365045 and a nearby nebula, NuSTAR J201744.3+364812, whose properties are most consistent with extragalactic objects. The combined XMM-Newton and NuSTAR spectrum of 3XMM J201744.7+365045 is well-fit to an absorbed power-law model with cm−2 and a photon index . Based on possible long-term flux variation and the lack of detected pulsations (≤43% modulation), this object is likely a background active galactic nucleus rather than a Galactic pulsar. The spectrum of the NuSTAR nebula shows evidence of an emission line at E = 5.6 keV, suggesting an optically obscured galaxy cluster at z = 0.19 ± 0.02 (d = 800 Mpc) and L X = 1.2 × 1044 erg s−1. Follow-up Chandra observations of Sh 2–104 will help identify the nature of the X-ray sources and their relation to MGRO J2019+37. We also show that the putative VERITAS excess south of Sh 2–104, is most likely associated with the newly discovered Fermi pulsar PSR J2017+3625 and not the H ii region.

GAMMA-RAY EMISSION IN DISSIPATIVE PULSAR MAGNETOSPHERES: FROM THEORY TOFERMIOBSERVATIONS

We compute the patterns of $\gamma$-ray emission due to curvature radiation in dissipative pulsar magnetospheres. Our ultimate goal is to construct macrophysical models that are able to reproduce the observed $\gamma$-ray light-curve phenomenology recently published in the Second Fermi Pulsar Catalog. We apply specific forms of Ohm's law on the open field lines using a broad range for the macroscopic conductivity values that result in solutions ranging, from near-vacuum to near Force-Free. Using these solutions, we generate model $\gamma$-ray light curves by calculating realistic trajectories and Lorentz factors of radiating particles, under the influence of both the accelerating electric fields and curvature radiation-reaction. We further constrain our models using the observed dependence of the phase-lags between the radio and $\gamma$-ray emission on the $\gamma$-ray peak-separation. We perform a statistical comparison of our model radio-lag vs peak-separation diagram and the one obtained for the Fermi standard pulsars. We find that for models of uniform conductivity over the entire open magnetic field line region, agreement with observations favors higher values of this parameter. We find, however, significant improvement in fitting the data with models that employ a hybrid form of conductivity; specifically, infinite conductivity interior to the light-cylinder and high but finite conductivity on the outside. In these models the $\gamma$-ray emission is produced in regions near the equatorial current sheet but modulated by the local physical properties. These models have radio-lags near the observed values and statistically best reproduce the observed light-curve phenomenology. Additionally, these models produce GeV photon cut-off energies.

MAGNETIC PAIR CREATION TRANSPARENCY IN GAMMA-RAY PULSARS

Magnetic pair creation $\gamma \to e^+e^-$ has been at the core of radio pulsar paradigms and central to polar cap models of gamma-ray pulsars for over three decades. The Fermi gamma-ray pulsar population now exceeds 140 sources and has defined an important part of Fermi's science legacy. Among the population characteristics well established is the common occurrence of exponential turnovers in their spectra in the 1--10 GeV range. These turnovers are too gradual to arise from magnetic pair creation in the strong magnetic fields of pulsar inner magnetospheres. By demanding insignificant photon attenuation precipitated by such single-photon pair creation, the energies of these turnovers for Fermi pulsars can be used to compute lower bounds for the typical altitude of GeV band emission. This paper explores such pair transparency constraints below the turnover energy, and updates earlier altitude bound determinations of that have been deployed in various Fermi pulsar papers. For low altitude emission locales, general relativistic influences are found to be important. Rotational aberration influences are also explored, and are found to be small at low altitudes, except near the magnetic pole. The altitude bounds are typically in the range of 2-7 stellar radii for the young Fermi pulsar population, and provide key information on the emission altitude in radio quiet pulsars that do not possess double-peaked pulse profiles. The bound for the Crab pulsar is at a much higher altitude, with the putative detection by MAGIC out to 350-400 GeV implying a lower bound of 310km to the emission region, i.e., approximately 20% of the light cylinder radius. These results are also extended to the super-critical field domain, where it is found that emission in magnetars originating below around 10 stellar radii will not appear in the Fermi-LAT band.

A SURVEY FOR Hα PULSAR BOW SHOCKS

We report on a survey for Hα bow shock emission around nearby γ-detected energetic pulsars. This survey adds three Balmer-dominated neutron star bow shocks to the six previously confirmed examples. In addition to the shock around Fermi pulsar PSR J1741−2054, we now report Hα structures around two additional γ-ray pulsars, PSR J2030+4415 and PSR J1509−5850. These are the first known examples of Hα nebulae with pre-ionization halos. With new measurements, we show that a simple analytic model can account for the angular size and flux of the bow shocks' apices. The latter, in particular, provides a new pulsar probe and indicates large moments of inertia and smaller distances than previously assumed in several cases. In particular, we show that the re-measured PSR J0437−4715 shock flux implies I = (1.7 ± 0.2) × 1045/(fH isin i) g cm2. We also derive a distance d ≈ 0.72 kpc for the γ-ray only pulsar PSR J2030+4415 and revised distances for PSRs J1959+2048 (1.4 kpc) and J2555+6535 (∼1 kpc), smaller than the conventional DM-estimated values. Finally, we report upper limits for 94 additional LAT pulsars. An estimate of the survey sensitivity indicates that for a warm neutral medium filling factor ϕWNM ∼ 0.3 there should be a total of approximately nine Hα bow shocks in our LAT-targeted survey; given that seven such objects are now known, a much larger ϕWNM seems problematic.

Beyond the 2nd Fermi Pulsar Catalog

AbstractOver thirteen times more γ‐ray pulsars have now been studied with the Large Area Telescope on NASA's Fermi satellite than the ten seen with the Compton Gamma‐Ray Observatory in the nineteen‐nineties. The large sample is diverse, allowing better understanding both of the pulsars themselves and of their roles in various cosmic processes. Here we explore the prospects for even more γ‐ray pulsars as Fermi enters the 2nd half of its nominal ten‐year mission. New pulsars will naturally tend to be fainter than the first ones discovered. Some of them will have unusual characteristics compared to the current population, which may help discriminate between models. We illustrate a vision of the future with a sample of six pulsars discovered after the 2nd Fermi Pulsar Catalog was written. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

VLT observations of the twoFermipulsars PSR J1357−6429 and PSR J1048−5832

Optical observations of pulsars are crucial to study the neutron star properties, from the structure and composition of the interior, to the properties and geometry of the magnetosphere. Historically, X and gamma-ray observations have paved the way to the pulsar optical identifications. The launch of Fermi opened new perspectives in the optical-to-gamma-ray studies of neutron stars, with the detection of more than 80 pulsars. Here, we aim to search for optical emission from two Fermi pulsars which are interesting targets on the basis of their spin-down age, energetics, and distance: PSR J1357-6429and PSR J1048-5832. The two pulsars and their pulsar wind nebulae (PWNe) are also detected in X-rays by Chandra and XMM. No deep optical observations of these two pulsars have been reported so far. We used multi-band optical images (V,R,I) taken with the VLT and available in the European Southern Observatory (ESO) archive to search for, or put tight constraints to, their optical emission. We re-assessed the positions of the two pulsars from the analyses of all the available Chandra observations and the comparison with the published radio coordinates. For PSR J1357-6429, this yielded a tentative proper motion mu=0.17+/-0.055 "/yr (70+/-15 deg position angle). We did not detect candidate counterparts to PSR J1357-6429 and PSR J1048-5832 down to V~27 and ~27.6, respectively, although for the former we found a possible evidence for a faint, unresolved object at the Chandra position. Our limits imply an efficiency in converting spin-down power into optical luminosity <7x10^{-7} and <6x10^{-6}, respectively, possibly close to that of the Vela pulsar.

Observation of the Fermi pulsar catalog at TeV energies with the Tibet air shower experiment

Using the Tibet-III air shower array, we search for steady TeV -rays from 18 pulsars in the Fermi Large Area Telescope pulsar catalog. Among them, we ob- serve 8 sources including the Crab instead of the expected

The contribution of Fermi gamma-ray pulsars to the local flux of cosmic-ray electrons and positrons

We analyze the contribution of gamma-ray pulsars from the first Fermi-Large Area Telescope (LAT) catalogue to the local flux of cosmic-ray electrons and positrons (e+e−). We present new distance estimates for all Fermi gamma-ray pulsars, based on the measured gamma-ray flux and pulse shape. We then estimate the contribution of gamma-ray pulsars to the local e+e− flux, in the context of a simple model for the pulsar e+e− emission. We find that 10 of the Fermi pulsars potentially contribute significantly to the measured e+e− flux in the energy range between 100 GeV and 1 TeV. Of the 10 pulsars, 2 are old EGRET gamma-ray pulsars, 2 pulsars were discovered with radio ephemerides, and 6 were discovered with the Fermi pulsar blind-search campaign. We argue that known radio pulsars fall in regions of parameter space where the e+e− contribution is predicted to be typically much smaller than from those regions where Fermi-LAT pulsars exist. However, comparing the Fermi gamma-ray flux sensitivity to the regions of pulsar parameter space where a significant e+e− contribution is predicted, we find that a few known radio pulsars that have not yet been detected by Fermi can also significantly contribute to the local e+e− flux ifa) they are closer than 2 kpc, and ifb) they have a characteristic age on the order of one mega-year.

GAMMA-RAY AND RADIO PROPERTIES OF SIX PULSARS DETECTED BY THEFERMILARGE AREA TELESCOPE

We report the detection of pulsed gamma-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850 and J1718-3825 using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their gamma-ray light curves are (at least given the current count statistics) single peaked. For two pulsars there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the gamma-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the gamma-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the gamma-ray light curves with high-energy beam models.

MILAGRO OBSERVATIONS OF MULTI-TeV EMISSION FROM GALACTIC SOURCES IN THE FERMI BRIGHT SOURCE LIST

We present the result of a search of the Milagro sky map for spatial correlations with sources from a subset of the recent Fermi Bright Source List (BSL). The BSL consists of the 205 most significant sources detected above 100 MeV by the Fermi Large Area Telescope. We select sources based on their categorization in the BSL, taking all confirmed or possible Galactic sources in the field of view of Milagro. Of the 34 Fermi sources selected, 14 are observed by Milagro at a significance of 3 standard deviations or more. We conduct this search with a new analysis which employs newly optimized gamma-hadron separation and utilizes the full eight-year Milagro data set. Milagro is sensitive to gamma rays with energy from 1 to 100 TeV with a peak sensitivity from 10 to 50 TeV depending on the source spectrum and declination. These results extend the observation of these sources far above the Fermi energy band. With the new analysis and additional data, multi-TeV emission is definitively observed associated with the Fermi pulsar, J2229.0+6114, in the Boomerang pulsar wind nebula (PWN). Furthermore, an extended region of multi-TeV emission is associated with the Fermi pulsar, J0634.0+1745, the Geminga pulsar.