Pulsar Surface Research Articles

Identifying Subcomponents of Mean Pulse Profiles of PSR B1822–09 with Single Pulses

Using observational data obtained with the Five-hundred-meter Aperture Spherical radio Telescope, we succeeded in identifying subcomponents of different pulse profile components of PSR B1822−09 by the phase-to-phase cross correlations between intensity sequences of single pulses. The determined number of the subcomponents and edges of the subcomponents are consistent with the results of Gaussian fitting on the mean pulse profiles of PSR B1822−09. According to the results, the main pulse (MP) in both the radio burst mode (B mode) and the radio-quiet mode (Q mode), the precursor (PC), and the interpulse (IP) of PSR B1822−09 are all composed of three subcomponents. The correlation coefficients between intensity sequences at different pulse phases reveal a positive correlation between the subcomponent C8 of the MP and C3 of the IP in the Q mode, and a positive correlation between C5 of the PC and C2 of the IP during the switching process from the B mode to the Q mode. These complex correlations present a challenge for the existing scenarios that describe relations between different pulse profile components. We proposed a hypothesis that different subcomponents correspond to different subpatches on the pulsar surface, and emissions from a subpatch reveal responses of the subpatch to some variations like the voltage in the magnetosphere. Therefore, correlations between different subcomponents depend on the responses of the subpatches to the variations.

PSR B0943+10: Mode Switch, Polar Cap Geometry, and Orthogonally Polarized Radiation

As one of the paradigm examples to probe into pulsar magnetospheric dynamics, PSR B0943+10 (J0946+0951) manifests representatively, showing a mode switch, orthogonal polarization, and subpulse drifting, frequently studied below 600 MHz. Here, both integrated and single pulses are studied at a high frequency (1.25 GHz) with FAST. The mode switch is studied using a profile decomposition method. A phase space evolution for the pulsar’s mode switch shows a strange-attractor-like pattern. The radiative geometry is proposed by fitting polarization position angles with the rotating vector model. The pulsar pulse profile is then mapped to the sparking locations on the pulsar surface, and the differences between the main pulse’s and the precursor component’s radiative processes may explain the X-ray’s synchronization with radio mode switch. Detailed single pulse studies on B0943+10's orthogonally polarized radiation are presented, which may support certain models of radiative transfer of polarized emission. In particular, the difference in orthogonal polarization modes’ circular polarization might reflect the cyclotron absorption in pulsar magnetospheres. B0943+10's B and Q modes evolve differently with frequency and have different proportions of orthogonal modes, which indicates possible magnetospheric changes during mode switch. For Q mode pulse profile, the precursor and the main pulse components are orthogonally polarized, and are probably originated from different depths in the magnetosphere. The findings could impact significantly on the pulsar electrodynamics and the radiative mechanism related.

Modeling the Radial Distribution of Pulsars in the Galaxy

The Parkes 20 cm multibeam pulsar surveys have discovered nearly half of the known pulsars and revealed many distant pulsars with high dispersion measures. Using a sample of 1301 pulsars from these surveys, we have explored the spatial distribution and birth rate of normal pulsars. The pulsar distances used to calculate the pulsar surface density are estimated from the YMW16 electron-density model. When estimating the impact of the Galactic background radiation on our survey, we projected pulsars in the galaxy onto the Galactic plane, assuming that the flux density distribution of pulsars is uniform in all directions, and utilized the most up-to-date background temperature map. We also used an up-to-date version of the ATNF Pulsar Catalogue to model the distribution of pulsar flux densities at 1400 MHz. We derive an improved radial distribution for the pulsar surface density projected onto the Galactic plane, which has a maximum value at ∼4 kpc from the Galactic center. We also derive the local surface density and birth rate of pulsars, obtaining 47 ± 5 kpc−2 and ∼4.7 ± 0.5 kpc−2 Myr−1, respectively. For the total number of potentially detectable pulsars in the galaxy, we obtain (1.1 ± 0.2) × 104 and (1.1 ± 0.2)×105 before and after applying the Tauris & Manchester beaming correction model. The radial distribution function is used to estimate the proportion of pulsars in each spiral arm and the Galactic center.

Radio Pulsar B0950+08: Radiation in the Magnetosphere and Sparks above the Surface

We observed the nearby radio pulsar B0950+08, which has a 100% duty cycle, using the Five-hundred-meter Aperture Spherical Radio Telescope. We obtained the polarization profile for its entire rotation, which enabled us to investigate its magnetospheric radiation geometry and the sparking pattern of the polar cap. After we excluded part of the profile in which the linear polarization factor is low (≲30%) and potentially contaminated by jumps in position angle, the rest of the swing in polarization position angle fits a classical rotating vector model (RVM) well. The best-fit RVM indicates that the inclination angle, α, and the impact angle, β, of this pulsar, are 100.°5 and −33.°2, respectively, suggesting that the radio emission comes from two poles. We find that, in such RVM geometry, either the annular vacuum gap model or the core vacuum gap model would require that the radio emissions come from a high-altitude magnetosphere with heights from ∼0.25 R LC to ∼0.56 R LC, with R LC being the light cylinder radius. Both the main and interpulses’ sparking points are located away from the magnetic pole, which could relate to the physical conditions on the pulsar surface.

Cover Picture: Astron. Nachr. 2/2024

It has been proposed that the building blocks of pulsar matter could be ‘an’, strangeons analogy of atomic nucleons but with negative strangeness. A bare strangeon star should not have a smooth surface, but be covered with small hills (‘zits’), as indicated by red spots in this schematic illustration. These zits on the pulsar surface might be responsible for the magnetospheric activity and the mysterious coherent radio emission, as discussed in the contribution by Xu and Wang, this issue, p. e230153. Zits may also be responsible for the significantly non‐symmetrical sparking discovered in the 110‐min polarization observation of pulsar PSR B0950+08 targeted with China's Five‐hundred‐meter Aperture Spherical radio Telescope, FAST. Details can be found in the contribution by Wang et al., this issue, e240010. image

Non‐symmetrical sparking may hint “zits” on a pulsar surface

AbstractPulsar electrodynamics could be relevant to the physics of stellar surface, which remains poorly understood for more than half a centenary and is difficult to probe due to the absence of direct and clear observational evidence. Nevertheless, highly sensitive telescopes (e.g., China's Five‐hundred‐meter Aperture Spherical radio Telescope, FAST) may play an essential role to solve the problem since the predicted surface condition would have quite different characteristics in some models of pulsar structure, especially after the establishment of the standard model of particle physics. For instance, small hills (or “zits”) may exist on solid strangeon star surface with rigidity, preferential discharge, that is, gap sparking, may occur around the hills in the polar cap region. In this work, with the 110‐min polarization observation of PSR B095008 targeted by FAST, we report that the gap sparking is significantly non‐symmetrical to the meridian plane on which the rotational and magnetic axes lie. It is then speculated that this asymmetry could be the result of preferential sparking around zits which might rise randomly on pulsar surface. Some polarization features of both single pulses and the mean pulse, as well as the cross‐correlation function of different emission regions, have also been presented.

Repeating fast radio bursts reveal the secret of pulsar magnetospheric activity

AbstractThe puzzling mechanism of coherent radio emission remains unknown, but fortunately, repeating fast radio bursts (FRBs) provide a precious opportunity, with extremely bright subpulses created in a clear and vacuum‐like pulsar magnetosphere. FRBs are millisecond‐duration signals that are highly dispersed at distant galaxies but with uncertain physical origin(s). Coherent curvature radiation by bunches has already been proposed for repeating FRBs. The charged particles are created during central star's quakes, which can form bunches streaming out along curved magnetic field lines, so as to trigger FRBs. The nature of narrow‐band radiation with time‐frequency drifting can be a natural consequence that bunches could be observed at different times with different curvatures. Additionally, high linear‐polarization can be seen if the line of sight is confined to the beam angle, whereas the emission could be highly circular‐polarized if off‐beam. It is also discussed that pulsar surface may be full of small hills (i.e., zits) which would help producing bulk of energetic bunches for repeating FRBs as well as for rotation‐powered pulsars.

Single-pulse studies of three millisecond pulsars

Abstract Single-pulse studies are important to understand the pulsar emission mechanism and the noise floor in precision timing. We study total intensity and polarimetry properties of three bright millisecond pulsars – PSRs J1022+1001, J1713+0747, and B1855+09 – that have detectable single pulses at multiple frequencies. We report for the first time the detection of single pulses from PSRs J1022+1001 and J1713+0747 at 4.5 GHz. In addition, for those two pulsars, the fraction of linear polarization in the average profile is significantly reduced at 4.5 GHz, compared to 1.38 GHz, which could support the expected deviation from a dipolar field closer to the pulsar surface. There is a hint of orthogonal modes in the single pulses of PSR J1713+0747. More sensitive multifrequency observations may be useful to confirm these findings. The jitter noise contributions at 1.38 GHz, scaled to one hour, for PSRs J1022+1001, J1713+0747, and B1855+09 are ≈135, ≈45, and ≈60 ns, respectively and are consistent with previous studies. We also show that selective bright-pulse timing of PSR J1022+1001 yields improved root-mean-square residuals of ≈22 $\mu$s, which is a factor of ≈3 better than timing using single pulses alone.

A NuSTAR observation of the eclipsing binary system OAO 1657-415: The revival of the cyclotron line

Context. OAO 1657-415 is an accreting X-ray pulsar with a high-mass companion that has been observed by several telescopes over the years, in different orbital phases. Back in 1999, observations performed with Beppo-SAX lead to the detection of a cyclotron-resonant-scattering feature, which has not been found again with any other instrument. A recent NuSTAR X-ray observation performed during the brightest phase of the source allows us to perform sensitive searches for cyclotron-resonant-scattering features in the hard X-ray spectrum of the source. Aims. We aim to characterise the source by means of temporal and spectral X-ray analysis, and to confidently search for the presence of cyclotron-resonant-scattering features. Methods. The observation was divided into four time intervals in order to characterise each one. Several timing analysis tools were used to obtain the pulse of the neutron star, and the light curves folded into the time intervals. The NuSTAR spectrum in the energy range 3–79 keV was used, which was modelled with a power-law continuum emission model with a high-energy cutoff. Results. We identify the pulsations associated with the source in the full observation, and find these to be shifted due to the orbital Doppler effect. We show evidence that a cyclotron line at 35.6 ± 2.5 keV is present in the spectrum. We use this energy to estimate the dipolar magnetic field at the pulsar surface to be 4.0 ± 0.2 × 1012 G. We further estimate a lower limit on the distance to OAO 1657-415 of ≃1 kpc. We also find a possible positive correlation between the luminosity and the energy associated with the cyclotron line. Conclusions. We conclude that the cyclotron line at 35.6 ± 2.5 keV is the same as that detected by Beppo-SAX. Our detection has a significance of ∼ 3.4σ.

A search for variability of hard X-ray emission from the Vela pulsar wind nebula

Observations of hard X-ray emission from the Vela pulsar wind nebula (PWN) with the ISGRI camera aboard INTEGRAL gamma-ray observatory have been analysed with the aim to search for possible flux variability on scales from weeks to years, which could be caused by short-term evolution of pulsar wind structures similar to those governing sharp flares and flux depressions observed in the sub-GeV emission of the Crab PWN. No statistically significant flux depressions or flares have been found in none of the considered energy ranges: 20-50 keV, 50-100 keV, and 100-200 keV, however some hints of flux instability can be seen in the former two bands. If the variability of the pulsar wind termination surface or instabilities of turbulent magnetic field in the nebula predicted by a number of PWN models indeed influence the synchrotron spectrum of such objects, the variability of the 1-30 MeV emission from the Vela PWN could be checked with the next generation of gamma-ray facilities, like eASTROGAM or HERMES.

Coherent curvature radiation: maximum luminosity and high-energy emission

ABSTRACT High brightness temperature radio transients such as pulsars and fast radio bursts (FRBs) require the coherent radiation of particles. The antenna class of coherent radiation models require a large number of charged particles radiating in phase, therefore the particles must be spatially confined and have well-aligned velocities. Given these necessary conditions, we look at the magnetic field induced by the currents associated with coherently emitting accelerated particles and consider the interaction between the radiating particles and the induced magnetic field. We find a maximum luminosity of coherent curvature radiation that depends on source parameters such as surface magnetic field and neutron star spin period. We find that coherent radio emission across all luminosities can be explained by coherent curvature radiation and suggest it could be universally responsible for both FRBs and extreme galactic sources. Using the Crab Pulsar as an example, we constrain the emission parameters and origin of the most extreme nanoshots to within 60 km of the pulsar surface assuming coherent curvature radiation. In agreement with recent observations, we also predict simultaneous X-ray emission from small-scale particle gyration due to the induced field.

Clues from 4U 0142+61 on supernova fallback disc formation and precession

ABSTRACT The Nuclear Spectroscopic Telescope Array (NuSTAR) experiment detected a hard X-ray emission (10–70 keV) with a period of 8.68917 s and a pulse-phase modulation at 55 ks, or half this value, from the anomalous X-ray pulsar (AXP) 4U 0142+61. It is shown here that this evidence is naturally explained by the precession of a Keplerian supernova fallback disc surrounding this AXP. It is also found that the precession of discs formed around young neutron stars at distances larger than those considered in the past, may constitute almost neglected sources of gravitational waves with frequencies belonging to the sensitivity bands of the future space interferometers: Laser Interferometer Space Antenna (LISA), Advanced Laser Interferometer Antenna (ALIA), DECi-hertz Interferometer Gravitational wave Observatory (DECIGO), and Big Bang Observer (BBO). In this work, the gravitational wave emission from precessing fallback discs possibly formed around young pulsars such as Crab in a region extending beyond 8 × 107 m from the pulsar surface is estimated. It is also evaluated the role that infrared radiation emission from circumpulsar discs may play in contributing to inverse Compton scattering of TeV energy positrons and electrons. Extensive observational campaigns of disc formation around young and middle-aged pulsars may also contribute to solve the long-standing problem of a pulsar origin for the excess of positrons in cosmic rays observed near the Earth above 7 GeV. In the near future the James Webb Space Telescope, with unprecedented near- and mid-infrared observation capabilities, may provide direct evidence of a large sample of supernova fallback discs.

Instability of the pulsar surface in strong constant fields and generation of additional components due to combination scattering

Periodic structures on polar caps, arising due to instability of the pulsar surface in a strong magnetic field, can be responsible both for the appearance of additional components in pulsar radiation and the formation of sickle shaped hot spots arising as a possible result of Mandelstam–Raman scattering of the reflected radiation by returned positrons.

NICER Discovery of Millisecond X-Ray Pulsations and an Ultracompact Orbit in IGR J17494-3030

Abstract We report the detection of 376.05 Hz (2.66 ms) coherent X-ray pulsations in NICER observations of a transient outburst of the low-mass X-ray binary IGR J17494−3030 in 2020 October/November. The system is an accreting millisecond X-ray pulsar in a 75-minute ultracompact binary. The mass donor is most likely a ≃0.02 M ⊙ finite-entropy white dwarf composed of He or C/O. The fractional rms pulsed amplitude is 7.4%, and the soft (1–3 keV) X-ray pulse profile contains a significant second harmonic. The pulsed amplitude and pulse phase lag (relative to our mean timing model) are energy dependent, each having a local maximum at 4 and 1.5 keV, respectively. We also recovered the X-ray pulsations in archival 2012 XMM-Newton observations, allowing us to measure a long-term pulsar spin-down rate of Hz s−1 and to infer a pulsar surface dipole magnetic field strength of ≃109 G. We show that the mass transfer in the binary is likely nonconservative, and we discuss various scenarios for mass loss from the system.

Resonant Effects in a Photoproduction of Ultrarelativistic Electron-Positron Pairs on a Nucleus in the Field of the X-ray Pulsar

The resonant photoproduction of the electron-positron pairs on a nucleus near a surface of the X-ray pulsar was studied theoretically. The main feature of the processes, which are responsible for the formation of the electron-positron fluxes, is a capability to occur in a resonant way in the electromagnetic field of the X-ray pulsar. One of the properties of the resonant case is that the initial process of second order in the fine structure constant in an external field effectively reduces into two successive processes of the first order due to the fact that in the resonant conditions intermediate virtual electron (positron) becomes a real particle. It is shown that the resonances are possible only when the initial gamma quantum energy is more than the threshold energy, which significantly depends on the number of absorbed photons of an external electromagnetic field. Additionally, in the resonant conditions, the energies of the particles depend on the outgoing angle of a positron (channel A) or an electron (channel B). It is shown that the resonant differential cross section has an extremely large magnitude in units αZ2re2. A mechanism to explain the presence of anomalous fluxes of ultrarelativistic positrons near the surface of an X-ray pulsar was proposed.

Heating of the real polar cap of radio pulsars

ABSTRACT The heating of the real polar cap surface of radio pulsars by the bombardment of ultra-relativistic charges is studied. The real polar cap is a significantly smaller area within or close by the conventional polar cap, which is encircled by the last open field lines of the dipolar field $\vec{B}_\mathrm{ d}$. It is surrounded by those field lines of the small-scale local surface field $\vec{B}_\mathrm{ s}$ that join the last open field lines of $\vec{B}_\mathrm{ d}$ in a height of ∼105 cm above the cap. As the ratio of radii of the conventional and real polar cap Rdip/Rpc ∼ 10, flux conservation requires Bs/Bd ∼ 100. For rotational periods P ∼ 0.5 s, Bs ∼ 1014 G creates a strong electric potential gap that forms the inner accelerating region (IAR) in which charges gain kinetic energies ∼3 × 1014 eV. This sets an upper limit for the energy that backflowing charges can release as heat in the surface layers of the real polar cap. Within the IAR, which is flown through with a dense stream of extremely energetic charges, no stable atmosphere of hydrogen can survive. Therefore, we consider the polar cap as a solidified ‘naked’ surface consisting of fully ionized iron ions. We discuss the physical situation at the real polar cap, calculate its surface temperatures Ts as functions of Bs and P, and compare the results with X-ray observations of radio pulsars.

GeV–TeV Cosmic-Ray Leptons in the Solar System from the Bow Shock Wind Nebula of the Nearest Millisecond Pulsar J0437–4715

Abstract We consider the acceleration of leptons up to GeV–TeV energies in the bow shock wind nebula of PSR J0437–4715 and their subsequent diffusion through the interstellar magnetic fields. The leptons accelerated at the pulsar wind termination surface are injected into re-acceleration in colliding shock flows. Modeled spectra of synchrotron emission from the accelerated electrons and positrons are consistent with the far-ultraviolet and X-ray observations of the nebula carried out with the Hubble Space Telescope and Chandra X-ray Observatory. These observations are employed to constrain the absolute fluxes of relativistic leptons, which are escaping from the nebula and eventually reaching the solar system after energy-dependent diffusion through the local interstellar medium accompanied by synchrotron and Compton losses. It is shown that accelerated leptons from the nebula of PSR J0437–4715 can be responsible both for the enhancement of the positron fraction above a few GeV detected by PAMELA and AMS-02 spectrometers and for the TeV range lepton fluxes observed with H.E.S.S., VERITAS, Fermi, CALET, and DAMPE.

The radiation structure of PSR B2016+28 observed with FAST

With the largest dish Five-hundred-meter Aperture Spherical radio Telescope (FAST), both the mean and single pulses of PSR B2016$+$28, especially including the single-pulse structure, are investigated in detail in this study. The mean pulse profiles at different frequencies can be well fitted in a conal model, and the peak separation of intensity-dependent pulse profiles increases with intensity. The integrated pulses are obviously frequency dependent (pulse width decreases by $\sim\,20\%$ as frequency increases from 300 MHz to 750 MHz), but the structure of single pulses changes slightly (the corresponding correlation scale decreases by only $\sim\,1\%$). This disparity between mean and single pulses provides independent evidence for the existence of the RS-type vacuum inner gap, indicating a strong bond between particles on the pulsar surface. Diffused drifting sub-pulses are analyzed. The results show that the modulation period along pulse series ($P_3$) is positively correlated to the separation between two adjacent sub-pulses ($P_2$). This correlation may hint a rough surface on the pulsar, eventually resulting in the irregular drift of sparks. All the observational results may have significant implications in the dynamics of pulsar magnetosphere and are discussed extensively in this paper.

Not an oxymoron: some X-ray binary pulsars with enormous spin-up rates reveal weak magnetic fields

Three high-mass X-ray binaries have been discovered recently exhibiting enormous spinup rates. Conventional accretion theory predicts extremely high surface dipolar magnetic fields that we believe are unphysical. Instead, we propose quite the opposite scenario: some of these pulsars exhibit weak magnetic fields, so much so that their magnetospheres are crushed by the weight of inflowing matter. The enormous spinup rate is achieved before inflowing matter reaches the pulsar's surface as the penetrating inner disk transfers its excess angular momentum to the receding magnetosphere which, in turn, applies a powerful spinup torque to the pulsar. This mechanism also works in reverse: it spins a pulsar down when the magnetosphere expands beyond corotation and finds itself rotating faster than the accretion disk which then exerts a powerful retarding torque to the magnetic field and to the pulsar itself. The above scenaria cannot be accommodated within the context of neutron-star accretion processes occurring near spin equilibrium, thus they constitute a step toward a new theory of extreme (far from equilibrium) accretion phenomena.

Exploiting simultaneous multi-frequency observations to probe polar-cap processes

AbstractSub-pulse drifting has been regarded as one of the most insightful aspects of the pulsar radio emission. The phenomenon is generally explained with a system of emission sub-beams rotating around the magnetic axis, originating from a carousel of sparks near the pulsar surface (the carousel model). Since the observed radio emission at different frequencies is generated at different altitudes in the pulsar magnetosphere, corresponding sampling of the carousel on the polar cap differs slightly in magnetic latitude. When this aspect is considered, it is shown here that the carousel model predicts important observable effects in multi-frequency or wide-band observations. Also presented here are brief mentions of how this aspect can be exploited to probe the electrodynamics in the polar cap by estimating various physical quantities, and correctly interpret various carousel related phenomena, in addition to test the carousel model itself.