Geminga Pulsar Research Articles

Diagnosing the Particle Transport Mechanism in the Pulsar Halo via X-Ray Observations

Pulsar halos (also termed “TeV halos”) are a new class of γ-ray sources in the Galaxy, which manifest as extended γ-ray emission around middle-aged pulsars, as discovered around the Geminga pulsar, the Monogem pulsar, and PSR J0622+3749 by the High-Altitude Water Cherenkov Observatory and the Large High-Altitude Air Shower Observatory. A consensus has been reached that the teraelectronvolt emission comes from the inverse Compton scattering of escaping electrons/positrons from the pulsar wind nebula of the soft background radiation field, while the particle transport mechanism in the halo is still in dispute. Currently, there are mainly three interpretations: the isotropic, suppressed diffusion model; the isotropic, unsuppressed diffusion model that considers the ballistic propagation of newly injected particles; and the anisotropic diffusion model. While the predicted γ-ray surface brightness profiles of all three models can be more or less consistent with the observations, the implications of the three models for cosmic-ray transport mechanisms and the properties of the interstellar magnetic field are quite different. In this study, we calculate the anticipated X-ray emission of pulsar halos under the three models. We show that the synchrotron radiation of these escaping electrons/positrons can produce a corresponding X-ray halo around the pulsar and that the expected surface brightness profiles are distinct in the three models. We suggest that sensitive X-ray detectors of a large field of view (such as eROSITA and the Einstein Probe) with a reasonably long exposure time are crucial to understanding the formation mechanism of pulsar halos and can serve as a probe of the properties of interstellar turbulence.

Gamma-ray spectra of the Crab, Vela and Geminga pulsars fitted with SED of the emission from their current sheet

We show that the spectral energy distribution (SED) of the tightly focused radiation generated by the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star fits the gamma-ray spectra of the Crab, Vela and Geminga pulsars over the entire range of photon energies so far detected by Fermi-LAT, MAGIC and HESS from them: over 102 MeV to 20 TeV. While emblematic of any emission that entails caustics, the SED introduced here radically differs from those of the disparate emission mechanisms currently invoked in the literature to fit the data in different sections of these spectra. We specify, moreover, the connection between the values of the fit parameters for the analysed spectra and the physical characteristics of the central neutron stars of the Crab, Vela and Geminga pulsars and their magnetospheres.

Modeling the Multiwavelength Radiation Properties in Pulsar Dissipative Magnetospheres

We explore the multiwavelength radiation properties of the light curves and energy spectra in the dissipative magnetospheres of pulsars. The dissipative magnetospheres are simulated by the pseudo-spectral method with the combined force-free and Aristotelian electrodynamics, which can produce self-consistent accelerating electric fields mainly distributed in the equatorial current sheet outside the light cylinder. The multiwavelength light curves and spectra are computed by using the multiple emission mechanisms of both the primary particles accelerated by the accelerating electric fields in the equatorial current sheet and the secondary pairs with an assumed distribution spectrum. We then compare the predicted multiwavelength light curves and spectra with the observed data from the Crab, Vela, and Geminga pulsars. Our modeling results can systematically reproduce the observed trends of the multiwavelength light curves and the spectra for these three pulsars well.

3HWC J0631+107/LHAASO J0631+1040: A TeV Halo Powered by the Pulsar J0631+1036?

PSR J0631+1036 is a middle-aged pulsar with properties similar to those of the nearby Geminga pulsar. It is bright in γ-rays, and has been noted as the only source possibly associated with the TeV source 3HWC J0631+107 (also the LHAASO J0631+1040). For understanding the nature of the TeV source, we analyze the GeV γ-ray data obtained with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope for the source region. We are able to remove the pulsar’s emission from the region from timing analysis, and find that the region is rather clean without possible GeV γ-ray emission present as the counterpart to the TeV source. By comparing this pulsar to Geminga and considering the spectral feature of the TeV source, we argue that it is likely the TeV halo powered by the pulsar.

Detection of extended γ-ray emission around the Geminga pulsar with H.E.S.S.

Geminga is an enigmatic radio-quiet γ-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy γ-ray emission around the pulsar was discovered by Milagro and later confirmed by HAWC, which are both water Cherenkov detector-based experiments. However, evidence for the Geminga pulsar wind nebula in gamma rays has long evaded detection by imaging atmospheric Cherenkov telescopes (IACTs) despite targeted observations. The detection of γ-ray emission on angular scales ≳2º poses a considerable challenge for the background estimation in IACT data analysis. With recent developments in understanding the complementary background estimation techniques of water Cherenkov and atmospheric Cherenkov instruments, the H.E.S.S. IACT array can now confirm the detection of highly extended γ-ray emission around the Geminga pulsar with a radius of at least 3º in the energy range 0.5–40 TeV. We find no indications for statistically significant asymmetries or energy-dependent morphology. A flux normalisation of (2.8 ± 0.7) × 10−12 cm−2 s−1 TeV−1 at 1 TeV is obtained within a 1º radius region around the pulsar. To investigate the particle transport within the halo of energetic leptons around the pulsar, we fitted an electron diffusion model to the data. The normalisation of the diffusion coefficient obtained of D0 = 7.6−1.2+1.5 × 1027 cm2 s−1, at an electron energy of 100 TeV, is compatible with values previously reported for the pulsar halo around Geminga, which is considerably below the Galactic average.

Geminga contribution to the cosmic-ray positron excess according to the gamma-ray observations

We attempt to interpret the cosmic-ray positron excess by injection from the nearby pulsar Geminga, assuming a two-zone diffusion scenario and an injection spectrum with a low energy cutoff. Since the high energy positrons and electrons from Geminga can induce γ rays via inverse Compton scattering, we take into account the extended γ-ray observations around Geminga from HAWC for ∼10 TeV and from Fermi-LAT for GeV. According to the extended γ-ray observation claimed by an analysis of Fermi-LAT data, we find that Geminga could explain the positron excess for a 30% energy conversion efficiency into positrons and electrons. However, based on the constraint on the extended γ rays given by another Fermi-LAT analysis, positrons from Geminga would be insufficient to account for the positron excess. Further robust analysis of Fermi-LAT data for the extended γ rays would be crucial to determine whether Geminga can explain the positron excess or not.

A three-component model for cosmic ray spectrum and dipole anisotropy

ABSTRACT Using a three-component, multiscale diffusion model, we show that the cosmic ray (CR) proton and helium spectra and the dipole anisotropy can be explained with reasonable parameters. The model includes a nearby source associated with the supernova remnant (SNR) that gave rise to the Geminga pulsar, a source at the Galactic Centre, and a component associated with the Galactic disc. The CR flux below TeV is dominated by the disc component. The centre source with a continuous injection of CRs starting about 18 Myr ago is needed to explain the anisotropy above 100 TeV. With the assumption of universal CR spectra injected by all SNRs, the nearby source can produce a TeV spectral bump observed at Earth via slow diffusion across the interstellar magnetic field, which needs to have an angle θ ≈ 5° between the field line and the line of sight towards the source, and have weak magnetic turbulence with the Alfvén Mach number MA ≈ 0.1. Considering the modulation of the Galactic-scale anisotropy by this magnetic field, in a quasi-local approach the field may be directed at a right ascension about −90° and a declination about −7.4° in the equatorial coordinate system.

Possible bump structure of cosmic ray electrons unveiled by AMS-02 data and its common origin along with the nuclei and positron

The local pulsar and its progenitor, SNR, can together accelerate the positron, electron and nuclei to very high energy. The famous excesses of positron(nuclei) above 20(200) GeV possibly come from such kind of local source. This hints that the primary electron should also hold “excess” above 200 GeV, synchronously accelerated along with the nuclei. The recent precise measurement of sharp dropoff at 284 GeV of positron by AMS-02 experiment takes chance to study this expected electron excess. In this work, the spatially-dependent propagation with a local source is used to reproduce the spectrum of positron, electron and proton. When considering the dropoff at 284 GeV of positron, a sharp bump structure for primary electron above 284 GeV is required to fit the total spectrum of positron and electron. Then we systematically study the common origin of the excesses of positron, electron and nuclei from Geminga pulsar and SNR. Those excesses can be reproduced under this unified single-source model. Lastly, we hope that the fine bump structure can be observed to support our model by AMS-02 experiment in future.

On the Radio Emission of the Geminga Pulsar and RBS 1223 at the Frequency of 111 MHz

On the Radio Emission of the Geminga Pulsar and RBS 1223 at the Frequency of 111 MHz

Test of the superdiffusion model in the interstellar medium around the Geminga pulsar

The TeV $\gamma$-ray halo around the Geminga pulsar is an important indicator of cosmic-ray (CR) propagation in the local zone of the Galaxy as it reveals the spatial distribution of the electrons and positrons escaping from the pulsar. Considering the intricate magnetic field in the interstellar medium (ISM), it is proposed that superdiffusion model could be more realistic to describe the CR propagation than the commonly used normal diffusion model. In this work, we test the superdiffusion model in the ISM around the Geminga pulsar by fitting to the surface brightness profile of the Geminga halo measured by HAWC. Our results show that the chi-square statistic monotonously increases as $\alpha$ decreases from 2 to 1, where $\alpha$ is the characteristic index of superdiffusion describing the degree of fractality of the ISM and $\alpha=2$ corresponds to the normal diffusion model. We find that model with $\alpha<1.32$ (or $<1.4$, depending on the data used in fit) is disfavored at 95\% confidence level. Superdiffusion model with $\alpha$ close to 2 can well explain the morphology of the Geminga halo, while it predicts much higher positron flux on the Earth than the normal diffusion model. This has important implication for the interpretation of the CR positron excess.

Energetic positron propagation from pulsars: an analytical two-zone diffusion model

We study the cosmic rays (CR) positrons propagation from the near Earth Geminga pulsar wind nebula on the basis of a analytical model of the two-zone spherically symmetric particle diffusion from a central source. We calculate the near Earth spectral distribution of positrons originating from the pulsar. The obtained spectra are compared with the results of another authors considering the problem of the positron excess revealed by PAMELA and AMS-02 experiments.

Detection of the Geminga pulsar with MAGIC hints at a power-law tail emission beyond 15 GeV

We report the detection of pulsed gamma-ray emission from the Geminga pulsar (PSR J0633+1746) between 15 GeV and 75 GeV. This is the first time a middle-aged pulsar has been detected up to these energies. Observations were carried out with the MAGIC telescopes between 2017 and 2019 using the low-energy threshold Sum-Trigger-II system. After quality selection cuts, ∼80 h of observational data were used for this analysis. To compare with the emission at lower energies below the sensitivity range of MAGIC, 11 years of Fermi-LAT data above 100 MeV were also analysed. From the two pulses per rotation seen by Fermi-LAT, only the second one, P2, is detected in the MAGIC energy range, with a significance of 6.3σ. The spectrum measured by MAGIC is well-represented by a simple power law of spectral index Γ = 5.62 ± 0.54, which smoothly extends the Fermi-LAT spectrum. A joint fit to MAGIC and Fermi-LAT data rules out the existence of a sub-exponential cut-off in the combined energy range at the 3.6σ significance level. The power-law tail emission detected by MAGIC is interpreted as the transition from curvature radiation to Inverse Compton Scattering of particles accelerated in the northern outer gap.

Evidences of low-diffusion bubbles around Galactic pulsars

Recently, a few-degrees extended $\gamma$-ray halo in the direction of Geminga pulsar has been detected by HAWC, Milagro and Fermi-LAT. These observations can be interpreted with positrons ($e^+$) and electrons ($e^-$) accelerated by Geminga pulsar wind nebula (PWN), released in a Galactic environment with a low diffusion coefficient ($D_0$), and inverse Compton scattering (ICS) with the interstellar radiation fields. We inspect here how the morphology of the ICS $\gamma$-ray flux depends on the energy, the pulsar age and distance, and the strength and extension of the low-diffusion bubble. In particular we show that $\gamma$-ray experiments with a peak of sensitivity at TeV energies are the most promising ones to detect ICS halos. We perform a study of the sensitivity of HAWC, HESS and the future CTA experiment finding that, with efficiencies of the order of a few %, the first two experiments should have already detected a few tens of ICS halos while the latter will increase the number of detections by a factor of 4. We then consider a sample of sources associated to PWNe and detected in the HESS Galactic plane survey and in the second HAWC catalog. We use the information available in these catalogs for the $\gamma$-ray spatial morphology and flux of these sources to inspect the value of $D_0$ around them and the $e^{\pm}$ injection spectrum. All sources are detected as extended with a $\gamma$-ray emission extended about $15-80$ pc. Assuming that most of the $e^{\pm}$ accelerated by these sources have been released in the interstellar medium, the diffusion coefficient is $2-30 \cdot 10^{26}$ cm$^2$/s at 1 TeV, i.e. two orders of magnitude smaller than the value considered to be the average in the Galaxy. These observations imply that Galactic PWNe have low-diffusion bubbles with a size of at least 80 pc.

Detection of a γ-ray halo around Geminga with the Fermi-LAT and implications for the positron flux

The HAWC Collaboration has discovered a γ-ray emission extended about 2 degrees around the Geminga and Monogem pulsar wind nebulae (PWNe) at γ-ray energies Eγ > 5 TeV. We analyze, for the first time, almost 10 years of γ-ray data obtained with the Fermi Large Area Telescope at Eγ > 8 GeV in the direction of Geminga and Monogem. Since these two pulsars are close the Galactic plane we run our analysis with 10 different interstellar emission models (IEMs) to study the systematics due to the modeling of this component. We detect a γ-ray halo around Geminga with a significance in the range 7.8 − 11.8σ depending on the IEM considered. This measurement is compatible with e+ and e− emitted by the PWN, which inverse-Compton scatter (ICS) with photon fields located within a distance of about 100 pc from the pulsar, where the diffusion coefficient is estimated to be around 1.1 ⋅ 1027 cm2/s at 100 GeV. We include in our analysis the proper motion of the Geminga pulsar which is relevant for γ rays produced for ICS in the Fermi-LAT energy range. We find that an efficiency of about 1% for the conversion of the spin-down energy of the pulsar into e+ and e− is required to be consistent with γ-ray data from Fermi-LAT and HAWC. The inferred contribution of Geminga to the e+ flux is at most 20% at the highest energy AMS-02 data. Our results are compatible with the interpretation that the cumulative emission from Galactic pulsars explains the positron excess.

Detection of a γ -ray halo around Geminga with the Fermi -LAT data and implications for the positron flux

The HAWC Collaboration has discovered a $\gamma$-ray emission extended about 2 degrees around the Geminga and Monogem pulsar wind nebulae (PWNe) at $\gamma$-ray energies $E_\gamma >5$ TeV. We analyze, for the first time, almost 10 years of $\gamma$-ray data obtained with the Fermi Large Area Telescope at $E_\gamma >$ 8 GeV in the direction of Geminga and Monogem. Since these two pulsars are close the Galactic plane we run our analysis with 10 different interstellar emission models (IEMs) to study the systematics due to the modeling of this component. We detect a $\gamma$-ray halo around Geminga with a significance in the range $7.8-11.8\sigma$ depending on the IEM considered. This measurement is compatible with $e^+$ and $e^-$ emitted by the PWN, which inverse-Compton scatter (ICS) with photon fields located within a distance of about 100 pc from the pulsar, where the diffusion coefficient is estimated to be around $1.1 \times 10^{27}$ cm$^2$/s at 100 GeV. We include in our analysis the proper motion of the Geminga pulsar which is relevant for $\gamma$ rays produced for ICS in the Fermi-LAT energy range. We find that an efficiency of about $1\%$ for the conversion of the spin-down energy of the pulsar into $e^+$ and $e^-$ is required to be consistent with $\gamma$-ray data from Fermi-LAT and HAWC. The inferred contribution of Geminga to the $e^+$ flux is at most $20\%$ at the highest energy AMS-02 data. Our results are compatible with the interpretation that the cumulative emission from Galactic pulsars explains the positron excess.

Possible origin of the slow-diffusion region around Geminga

ABSTRACT Geminga pulsar is surrounded by a multiTeV γ-ray halo radiated by the high-energy electrons and positrons accelerated by the central pulsar wind nebula (PWN). The angular profile of the γ-ray emission reported by High-Altitude Water Cherenkov Observatory indicates an anomalously slow diffusion for the cosmic-ray electrons and positrons in the halo region around Geminga. In the paper we study the possible mechanism for the origin of the slow diffusion. At first, we consider the self-generated Alfvén waves due to the streaming instability of the electrons and positrons released by Geminga. However, even considering a very optimistic scenario for the wave growth, we find this mechanism does not work to account for the extremely slow diffusion at the present day, if taking the proper motion of Geminga pulsar into account. The reason is straightforward as the PWN is too weak to generate enough high-energy electrons and positrons to stimulate strong turbulence at the late time. We then propose an assumption that the strong turbulence is generated by the shock wave of the parent supernova remnant (SNR) of Geminga. Geminga may still be inside the SNR, and we find that the SNR can provide enough energy to generate the slow-diffusion circumstance. The TeV haloes around PSR B0656+14, Vela X, and PSR J1826-1334 may also be explained under this assumption.

On the Gamma-Ray Nebula of Vela Pulsar. I. Very Slow Diffusion of Energetic Electrons within the TeV Nebula

Abstract High-energy particle transport in pulsar wind nebulae (PWNe) plays an essential role in explaining the characteristics revealed in multiwavelength observations. In this paper, the TeV–gamma-ray-emitting electrons in the Vela X PWN are approximated to be injected impulsively when the cocoon is formed due to the interaction between the SNR reverse shock and the PWN. By solving the diffusion-loss equation analytically, we reproduce the broadband spectral energy distribution and surface brightness profile simultaneously. The diffusion coefficient of TeV electrons and positrons, which is well constrained by the spectral and spatial properties of the TeV nebula, is thus determined to be 1 × 1026 cm2 s−1 for 10 TeV electrons and positrons. This coefficient is more than three orders of magnitude lower than that in the interstellar medium, in agreement with a constraint recently obtained from HAWC observations of a TeV nebula associated with the Geminga pulsar. These results suggest that slow diffusion of high-energy particles might be common in PWNe.

Constraining the Magnetic Field in the TeV Halo of Geminga with X-Ray Observations

Abstract Recently, the High Altitude Water Cherenkov (HAWC) collaboration reported the discovery of a TeV halo around the Geminga pulsar. The TeV emission is believed to originate from the inverse Compton scattering of pulsar-injected electrons/positrons off cosmic microwave background photons. During this time, these electrons should inevitably radiate X-ray photons via synchrotron radiation, providing a useful constraint on the magnetic field in the TeV halo. In this work, we analyze the data of XMM-Newton and Chandra, and obtain an upper limit for the diffuse X-ray flux in a 600″ region around the Geminga pulsar, which is at a level of . By numerically modeling both the X-ray and TeV observations assuming the isotropic diffusion of injected electrons/positrons, we find that the magnetic field inside the TeV halo is required to be &lt;1 μG, which is significantly weaker than the typical magnetic field in the interstellar medium. The weak magnetic field together with the small diffusion coefficient inferred from the HAWC observation implies that the Bohm limit of particle diffusion may probably have been achieved in the TeV halo. We also discuss alternative possibilities for the weak X-ray emission, such as the hadronic origin of the TeV emission or a specific magnetic field topology, in which a weak magnetic field and a very small diffusion coefficient might be avoided.

Constraints on the properties of the turbulent magnetic field around Geminga using HAWC measurements

We place constraints on the properties of the interstellar turbulence that surrounds Geminga pulsar, using the recent measurements from the HAWC Observatory in this region [1]. We propagate very-high-energy electrons in realizations of 3D isotropic Kolmogorov or Kraichnan turbulence, calculate their γ-ray emission, and compare with HAWC measurements. We show that the measurements can be well fitted for both models of the turbulence and for reasonable values of its strength, Brms, and coherence length, Lc. Our best fits are obtained for Brms ≃ 3 μG and Lc ≃ 1 pc. Furthermore, the absence of strong asymmetries in the observed emission favours Lc ≤ 5 pc.

Positron flux and γ-ray emission from Geminga pulsar and pulsar wind nebula

Nearby pulsars have been suggested as sources of $\sim$TeV $e^+/e^-$ Cosmic Ray (CR) excess on Earth. The High-Altitude Water Cherenkov Observatory (HAWC) detected extended TeV emission regions in the direction of two nearby middle-aged pulsars, Geminga and PSR B0656+14. By modeling the TeV emission as inverse Compton emission from electron-positron pairs diffusing in the interstellar medium (ISM), the HAWC collaboration derives a diffusion coefficient much smaller than the standard value in the vicinity of the two pulsars, which make them unlikely the origin of the positron excess. We propose that the observed $\gamma$-ray emission originate from the relic pulsar wind nebula. A two zone diffusion model with a slow diffusion in the nebula and a fast diffusion in the ISM can explain the HAWC surface brightness profile and the positron excess simultaneously. Inefficient diffusion in the $\gamma$-ray emission region surrounding a middle-aged pulsar maybe a common phenomenon that can be tested by future observation. The implied diffusion coefficient in the ISM is smaller than the one suggested by the standard CR propagation model, but it is fully consistent with the predictions of the spiral arm model.