Pulsar Emission Mechanism Research Articles

Decoding the Nature of Coherent Radio Emission in Pulsars I: Observational Constraints

Radio observations from normal pulsars indicate that the coherent radio emission is excited by curvature radiation from charge bunches. In this review, we provide a systematic description of the various observational constraints on the radio emission mechanism. We have discussed the presence of highly polarized time samples where the polarization position angle follow two orthogonal well-defined tracks across the profile that closely match the rotating vector model in an identical manner. The observations also show the presence of circular polarization, with both the right and left handed circular polarization seen across the profile. Other constraints on the emission mechanism are provided by the detailed measurements of the spectral index variation across the profile window, where the central part of the profile, corresponding to the core component, has a steeper spectrum than the surrounding cones. Finally, the detailed measurements of the subpulse drifting behaviour can be explained by considering the presence of non-dipolar field on the stellar surface and the formation of the partially screened gap (PSG) above the polar cap region. The PSG gives rise to a non-stationary plasma flow that has a multi-component nature, consisting of highly energetic primary particles, secondary pair plasma, and iron ions discharged from the surface, with large fragmentation resulting in dense plasma clouds and lower-density inter-cloud regions. The physical properties of the outflowing plasma and the observational constraints lead us to consider coherent curvature radiation as the most viable explanation for the emission mechanism in normal pulsars, where propagation effects due to adiabatic walking and refraction are largely inconsequential.

Processing system for coherent dedispersion of pulsar radio emission

Our study provides pulsar researchers with the possibility to utilize VLBI data for looking at pulsar radio emission with the extreme time resolution typical for broad band VLBI recorders. Pulsars emit micropulses and giant pulses shorter than a microsecond. The short pulse durations indicate high brightness temperatures. This constrains the physical nature of the pulsar radio emission mechanism, whose theory is not yet completely understood.Our paper describes a system for converting VLBI observation data using coherent dedispersion and compensation algorithms for two-bit signal sampling. Coherent dedispersion is the key to processing pulsar observations to obtain the highest temporal resolution. Correction for signal sampling makes it possible to eliminate parasitic effects that interfere with the analysis of pulsar diffraction patterns. A pipeline has been established that uses the developed converter and the Astro Space Center software correlator. It allows us to reprocess all Radioastron pulsar observations and conduct a search for giant pulses, which requires the highest temporal resolution possible.

Timing and Single-pulse Study of Pulsar J1909+0122 Discovered by CRAFTS

We report the discovery of PSR J1909+0122 by the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) as part of the Commensal Radio Astronomy FAST Survey. PSR J1909+0122 has a spin period of 1.257 s and a dispersion measure of 186.2 pc cm−3. The averaged pulse profile shows two distinct components. We performed a single-pulse study based on a one-hour observation at 1.25 GHz on 2021 August 23. We used a threshold of 5σ ep to measure the nulling fraction (NF) as 63% ± 1.5%. The longitude-resolved fluctuation spectra and fast Fourier transform spectra of the binary sequences revealed the quasi-periodicity of nulling with a period of 30 rotation periods. We examined the reliability of the periodicity by comparing it to random noise injection. The NF, , and modulation periodicity P M of PSR J1909+0122 were compared with other periodic nulling pulsars, showing that the source of J1909+0122 has the second largest NF in the population. Long-term timing observations over six months were used to derive the phase-connected ephemeris of this pulsar. The measured P and values disfavor dipolar geometry for polar gap models, and the prediction for a space-charge-limited flow model in the case of inverse Compton scattering is only just above the death line. In this work, PSR J1909+0122 has revealed possible correlations between nulling behavior and pulsar properties, which will help to shed light on the pulsar emission mechanism and its temporal evolution in future observations.

Testing the Circularity of PSR B0818-41's Carousel

The phenomenon of subpulse drifting is an important single-pulse phenomenon that can potentially provide important insights into the elusive radio emission mechanism in pulsars. We analyze the frequency behavior of the single pulses of B0818−41, observed from 300 to 500 MHz (Band 3 of the upgraded Giant Metrewave Radio Telescope, uGMRT), and compare it to the evolution of the average profile to place constraints on the geometry of the pulsar emission beam. We show that a circular carousel of discrete beamlets, where each beamlet is radially symmetric, is not consistent with the observed behavior, and we describe an alternative consistent range of possible elliptical carousel geometries. We also combine the uGMRT data with some archival Murchison Widefield Array observations and several other published profiles to characterize the profile evolution across a frequency range spanning ∼170 MHz to ∼1.4 GHz.

An Estimation of the Geometrical Structure of Polar Cap and Emission Property of Radio Pulsar: A Treatment from an Analytical Approach

Pulsars are believed to be one of the most interesting objects in the universe. The emission mechanism of pulsars is still a conundrum to physicists, as there is no completely acceptable theory that can establish a consensus between theory and observation. Pulsars possess a gigantic magnetic field, to the order of 1012 Gauss, and generate a very powerful radio beam from the magnetic pole. However, the powerful radio beam is generated by some complicated coherent plasma processes and acceleration in the pulsar magnetosphere. The location of the origin of the radio waves has been predicted to come out exclusively from the polar cap zone, whose boundary is defined by the footprint of the last open field line. However, in this paper, we mainly try to generate the shape of the polar cap structure from an analytical solution and discuss how it gets distorted for different geometrical parameters due to the presence of perturbation such as polar cap current flow. Also, apart from that, we try to emphasize understanding the variation of radio emission height and polarization angle with respect to different geometry-related parameters as well as with frequency.

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.

Investigating the high time-resolution statistics of pulsar radio signals using spectral self-noise

ABSTRACT While observations of the stationary component of pulsar radio signals have in many ways formed the basis of our understanding of radio pulsars, the statistical deviations of these signals contain information that has become increasingly relevant. Using high time–frequency resolution data from the MeerKAT telescope, we study the self-noise of the autocorrelation function of six radio pulsars. The self-noise of the autocorrelation function is used to investigate the statistics of the observed radio signals on nanosecond time-scales and for five pulsars it is found to deviate from the expected form for a Gaussian process. Comparing the measured distribution of the intensity fluctuations of the on-pulse window to simulated models, we find that a mixture model comprising a Gaussian process and a Bernoulli-sampled Gaussian process is able to produce the excess self-noise while also producing the observed distribution of intensities. The parameters of the mixture model describing the signals are estimated for three of the pulsars in our sample group. Studies of the statistics presented in this work provide observational information for constraining the numerous theories of pulsar radio emission mechanisms. The mixture model suggested in this work would produce excess timing residuals for high signal-to-noise ratio observations when compared to that expected for a Gaussian process. Additionally, the measure of spectral self-noise provides a means of separating Gaussian and non-Gaussian processes that provides a potential basis for the development of alternative pulsar detection algorithms.

Spectral analysis of the polarisation of drifting subpulses

Pulsars are among the most extreme objects in the Universe. Each one is born during a supernova – a dramatic explosion that happens when a bright star is many times bigger than our Sun and runs out of fuel. The heavy core of the star collapses under its own weight into something denser than the nucleus of an atom. Each pulsar has a magnetic field that is millions of times stronger than any that can be produced here on the Earth. Like a cosmic lighthouse, this magnetic field generates beams that sweep across the Galaxy with each rotation of the star (Gold, 1968); however, the underlying physics of the pulsar emission mechanism remains poorly understood after decades of study. Unlike the steady beam of a lighthouse, a pulsar's beam crackles and pops with lightning-like discharges of energy (Ruderman & Sutherland, 1975). By studying this highly dynamic signal, we learn about what generates the radio waves and what happens as they travel through the turbulent plasma in the pulsar's strong magnetic field. A relatively rare number of pulsars exhibit quasi-periodic variations in the polarization state of their emission (Primak et al., 2022). To provide new insight into the origin of this behaviour, I have developed a novel statistical method of studying quasi-periodic polarization fluctuations. I apply this technique to the first-discovered pulsar, and compare and contrast the results with previous work. The method can be applied to a wide range of radio pulsars and used to estimate the fraction of pulsars that exhibit quasi-periodic polarization fluctuations. In doing so, this experiment will provide new experimental constraints on theories of radio emission and propagation in the pulsar magnetosphere, and facilitate a deeper understanding of the physics of relativistic plasmas in strong magnetic fields.

Pulsar radio emission mechanism – II. On the origin of relativistic Langmuir solitons in pulsar plasma

ABSTRACT Observations suggest that coherent radio emission from pulsars is excited in a dense pulsar plasma by curvature radiation from charge bunches. Numerous studies propose that these charge bunches are relativistic charge solitons that are solutions of the non-linear Schrödinger equation (NLSE) with a group velocity dispersion (G), cubic non-linearity (q), and non-linear Landau damping (s). The formation of stable solitons crucially depends on the parameters G, q, and s as well as the particle distribution function (DF). In this work, we use realistic pulsar plasma parameters obtained from observational constraints to explore the parameter space of NLSE for two representative DFs of particles’ momenta: Lorentzian (long-tailed) and Gaussian (short-tailed). The choice of DFs critically affects the value of |s/q|, which, in turn, determines whether solitons can form. Numerical simulations show that well-formed solitons are obtained only for small values of |s/q| ≲ 0.1, while for moderate and higher values of |s/q| ≳ 0.5 soliton formation is suppressed. Small values for |s/q| ∼ 0.1 are readily obtained for long-tailed DF for a wide range of plasma temperatures. On the other hand, short-tailed DF provides these values only for some narrow range of plasma parameters. Thus, the presence of a prominent high-energy tail in the particle DF favours soliton formation for a wide range of plasma parameters. Besides pair plasma, we also include an iron ion component and find that they make a negligible contribution in either modifying the NLSE coefficients or contributing to charge separation.

Externally driven plasma models as candidates for pulsar radio emission

ABSTRACT Coherent radio emission from pulsars originates from excited plasma waves in an ultra-relativistic and strongly magnetized electron–positron pair plasma streaming along the open magnetic field lines of the pulsar. Traditional coherent radio emission models have relied on instabilities in this pair plasma. Recently, alternative models have been suggested. These models appeal to direct coupling of the external electromagnetic field to the superluminal O-mode (lt2 mode) during the time-dependent pair cascade process at the polar gap. The objective of this work is to provide generic constraints on plasma models based on lt2 mode using realistic pulsar parameters. We find that the very short time-scale associated with pair cascades does not allow lt2 mode to be excited at radio frequencies and the impulsive energy transfer can only increase the kinetic spread (‘temperature’) of the pair plasma particles. Moreover, under homogeneous plasma conditions, plasma waves on both branches of O mode (i.e. superluminal lt2 and subluminal lt1) cannot escape the plasma. In the strongly magnetized pair plasma, only the extraordinary mode (t mode) can escape freely. We show that any generic fictitious mechanisms do not result in the wave electric field of t mode to have predominant orientation either parallel or perpendicular to the magnetic field plane as observed. Such fictitious mechanisms will inevitably lead to depolarization of signals and cannot account for the highly polarized single pulses observed in pulsars. We suggest coherent curvature radiation as a promising candidate for pulsar radio emission mechanism.

Revisiting the subpulse drifting phenomenon in PSR J1822−2256: drift modes, sparks, and emission heights

ABSTRACT Subpulse drifting in pulsar radio emission is considered to be one of the most promising phenomena for uncovering the underlying physical processes. Here, we present a detailed study of such a phenomenon in observations of PSR J1822−2256, made using the upgraded Giant Meterwave Radio Telescope (uGMRT). Observations were made simultaneously using the band 3 (300–500 MHz) and band 4 (550–750 MHz) receivers of the uGMRT. The pulsar is known to exhibit subpulse drifting, mode changing, and nulling. Our observations reveal four distinct subpulse drifting modes of emission (A, B, C, and D) for this pulsar, with the drift periodicities of 17.9P1, 5.8P1, 8P1, and 14.1P1, respectively (where P1 is the pulsar rotation period), two of which exhibit some new features that were not reported in the previous studies. We also investigate the possible spark configuration, characterized by the number of sparks (n) in the carousel patterns of these four drift modes, and our analysis suggests two representative solutions for the number of sparks for a carousel rotation period, P4, which lies in the range of 13–16. The large frequency coverage of our data (300–750 MHz) is also leveraged to explore the frequency dependence of single-pulse characteristics of the pulsar emission, particularly the frequency-dependent subpulse behaviour and the emission heights for the observed drift modes. Our analysis suggests a clear modal dependence of inferred emission heights. We discuss the implications for the pulsar emission mechanism and its relation to the proposed spark configuration.

Hard X-ray polarimetry—an overview of the method, science drivers, and recent findings

The last decade has seen a leapfrog in the interest of X-ray polarimetry with a number of new polarization measurements in hard X-rays from AstroSat, POLAR, GAP, and PoGO+. The measurements provide some interesting insights into various astrophysical phenomena such as coronal geometry and disk–jet connection in black hole X-ray binaries, hard X-ray emission mechanism in pulsars, and gamma-ray bursts. They also highlight an increase in polarization with energy, which makes hard X-ray polarimetry extremely appealing. There are a number of confirmed hard X-ray polarimetry experiments which along with the existing instruments (AstroSat and INTEGRAL) make this field further exciting. Polarization experiments may also see significant progress in sensitivity with new developments in scintillator readouts, active pixel sensors, and cadmium zinc telluride detectors. In particular, the advent of hard X-ray focusing optics will enable the designing of compact focal plane polarimeters with a multifold enhancement in sensitivity. In this review, we will focus on the recent polarimetry findings and science potential of hard X-ray polarimetry along with possible improvements in the measurement techniques.

Radio Emission by Soliton Formation in Relativistically Hot Streaming Pulsar Pair Plasmas

Abstract A number of possible pulsar radio emission mechanisms are based on streaming instabilities in relativistically hot electron–positron pair plasmas. At saturation, the unstable waves can, in principle, form stable solitary waves, which could emit the observed intense radio signals. We searched for the proper plasma parameters that would lead to the formation of solitons, and investigated their properties and dynamics as well as the resulting oscillations of electrons and positrons that possibly lead to radio wave emission. We utilized a one-dimensional version of the relativistic particle-in-cell code ACRONYM initialized with an appropriately parameterized one-dimensional Maxwell–Jüttner particle distribution in velocity space to study the evolution of the resulting streaming instability in a pulsar pair plasma. We found that strong electrostatic superluminal L-mode solitons are formed for plasmas with normalized inverse temperatures ρ ≥ 1.66 or relative beam drift speeds with Lorentz factors γ > 40. The parameters of the solitons fulfill the conditions for wave emission. For appropriate pulsar parameters the resulting energy densities of superluminal solitons can reach 1.1 × 105 erg cm−3, while those of subluminal solitons reach only 1.2 × 104 erg cm−3. Estimated energy densities of up to 7 × 1012 erg cm−3 suffice to explain pulsar nanoshots.

Enhanced x-ray emission coinciding with giant radio pulses from the Crab Pulsar.

Giant radio pulses (GRPs) are sporadic bursts emitted by some pulsars that last a few microseconds and are hundreds to thousands of times brighter than regular pulses from these sources. The only GRP-associated emission outside of radio wavelengths is from the Crab Pulsar, where optical emission is enhanced by a few percentage points during GRPs. We observed the Crab Pulsar simultaneously at x-ray and radio wavelengths, finding enhancement of the x-ray emission by 3.8 ± 0.7% (a 5.4σ detection) coinciding with GRPs. This implies that the total emitted energy from GRPs is tens to hundreds of times higher than previously known. We discuss the implications for the pulsar emission mechanism and extragalactic fast radio bursts.

K-means Clustering-based Radio Neutron Star Pulsar Emission Mechanism

Aim: The aim of this paper is to work on K-means clustering-based radio neutron star pulsar emission mechanism. Background: The pulsars are a rare type of neutron stars that produce radio rays. Such types of rays are detectable on earth and have attracted scientists because of their concern with space-time, interstellar medium, and states of matter. During the rotation of pulsar rays, rays are emitted in the whole sky and after crossing the threshold value, the pattern of radio emission broadband is detected. As rotation speed of pulsar increases then accordingly the types of the patterns are produced periodically. Every pulsar emits different patterns that are a little bit different from each other which fully depends on its rotation. The detected signals are known as a candidates. It can be determined by the length of observation and it is average of all the rotations of the pulsar. Objective: The main objectives of this radio neutron star pulsar emission mechanism are: (a) Decision Tree Classifier (2) K-means Clustering (3) Neural Networks. Methods: The Pulsar Emission Data was broken down into two sets of data: Training Data and Testing Data. The Training Data is used to train the Decision Tree, the algorithm, K-means clustering and Neural Networks to allow it to identify, which attributes (Training Labels) are useful for identification of Neutron Pulsar Emissions. Results: The analysis used multiple machine learning algorithms. It was concluded that using neural networks is the best possible method to detect pulsar emissions from neutron stars. The best result achieved was 98% using Neural Networks. Conclusion: There are so many benefits of pulsar rays in different technologies. Earth can detect pulsar ray from low orbit. Earth can completely absorb X-ray in the atmosphere and due to this reason we can say that the wavelength is limited to those who do not have an atmosphere like space. According to our result, it can be concluded that the algorithm can be successfully used for detecting the pulsar signals.

State-switching Mechanism of Intermittent Pulsars

Abstract The emission feature of intermittent pulsars is significant for understanding the pulsar emission mechanism. Using the observational evidence of radio emissions turning on and off and the corresponding spin-down rates in these two states of an intermittent pulsar, we will examine the polar-cap potential drop, gap height, and curvature radii of a few intermittent pulsars within the regime of the pulsar polar-cap emission theory by applying the current loss and energy flux of particle flow to pulsar braking, which are generally associated with radio emission from the polar cap. It is seen that the polar-cap parameters of the intermittent pulsars are almost equal to their maximum values, which is the main prediction of the pulsar polar-cap theory with respect to the breaking of the radio emission. It is also noticed that the intermittent pulsars are distributed near the dipole death line in the diagram, which is consistent with their emission features and the calculated polar-cap parameters. To further confirm the state switching of the intermittent pulsar, the relationships among spin-down rate, gap height, potential drop, and activity duty cycles of PSR B1931+24 are discussed. It is found that the gap height has an anticorrelation with the activity duty cycle, which indicates that the intermittency of the radio emission has a close connection to the gap height, as indicated by the pulsar polar-cap emission theory.

A rotation-driven pulsar radio emission mechanism

ABSTRACT We propose and discuss an alternative pulsar radio emission mechanism that relies on rotation-driven plasma oscillations, rather than on a beam-driven instability, and suggest that it may be the generic radio emission mechanism for pulsars. We identify these oscillations as superluminal longitudinal waves in the pulsar plasma and point out that these waves can escape directly in the O mode. We argue that the frequency of the oscillations is ω0 ≈ ωp(2〈γ〉)1/2/γs, where γs is the Lorentz factor of bulk streaming motion and 〈γ〉 is the mean Lorentz factor in the rest frame of the plasma. The dependence of the plasma frequency ωp on radial distance implies a specific frequency-to-radius mapping, ω0∝r−3/2. Escape of the energy in these oscillations is possible if they are generated in overdense, field-aligned regions that we call fibres; the wave energy is initially refracted into underdense regions between the fibres, which act as ducts. Some implications of the model for the interpretation of pulsar radio emission are discussed.

Pulsar radio emission mechanisms: a critique

ABSTRACT We consider critically the three most widely favoured pulsar radio emission mechanisms: coherent curvature emission (CCE), beam-driven relativistic plasma emission (RPE), and anomalous Doppler emission (ADE). We assume that the pulsar plasma is 1D, streaming outwards with a bulk Lorentz factor γs ≫ 〈γ〉 − 1 ≳ 1, where 〈γ〉 is the intrinsic spread in the rest frame of the plasma. We argue that the formation of beams in a multicloud model is ineffective in the intrinsically relativistic case for plausible parameters because the overtaking takes too long. We argue that the default choice for the particle distribution in the rest frame is a Jüttner distribution and that relativistic streaming should be included by applying a Lorentz transformation to the rest-frame distribution, rather than the widely assumed relativistically streaming Gaussian distribution. We find that beam-driven wave growth is severely restricted by (a) the wave properties in pulsar plasma, (b) a separation condition between beam and background, and (c) the inhomogeneity of the plasma in the pulsar frame. The growth rate for the kinetic instability is much smaller and the bandwidth of the growing waves is much larger for a Jüttner distribution than for a relativistically streaming Gaussian distribution. No reactive instability occurs at all for a Jüttner distribution. We conclude that none of CCE, RPE, and ADE is tenable as the generic pulsar radio emission mechanism for ‘plausible’ assumptions about the pulsar plasma.

Pulsar radio emission mechanism − I. On the amplification of Langmuir waves in the linear regime

ABSTRACT Observations suggest that in normal period radio pulsars, coherent curvature radiation is excited within 10${{\ \rm per\ cent}}$ of the light cylinder. The coherence is attributed to Langmuir mode instability in a relativistically streaming one-dimensional plasma flow along the open magnetic field lines. In this work, we use a hot plasma treatment to solve the hydrodynamic dispersion relation of Langmuir mode for realistic pulsar parameters. The solution involves three scenarios of two-stream instability, viz. driven by high energy beams, due to longitudinal drift that leads to a separation of electron–positron distribution functions in the secondary plasma and due to cloud–cloud interaction causing spatial overlap of two successive secondary plasma clouds. We find that sufficient amplification can be obtained only for the latter two scenarios. Our analysis shows that longitudinal drift is characterized by high growth rates only for certain multipolar surface field geometry. For these configurations, very high growth rates are obtained starting from a few tens of km from the neutron star surface, which then falls monotonically with increasing distance. For cloud–cloud overlap, growth rates become high starting only after a few hundred km from the surface, which first increases and then decreases with increasing distance. A spatial window of up to around a 1000 km above the neutron star surface has been found where large amplitude Langmuir waves can be excited while the pair plasma is dense enough to account for high brightness temperature.

Second Braking Index of Intermittent Pulsar and Nulling Pulsar**This work was funded by Xin Jiang Autonomous Region Science and Technology Department Natural Science Fund Youth Project under No. 2017D01B02.

The braking index parameter of intermittent pulsar would provide important insight into the pulsar slowdown and emission mechanism. In previous work, we presented the first braking index of the intermittent pulsar B1931+24, J1841−0500, J1832+0029 and nulling pulsar B0823+26, by using their observed spin frequency first derivatives in “on” and “off” states. In this paper, to improve and justify our method for studying the braking mechanism of intermittent pulsars, we continue to formulate a method in which the second braking indices, , of these pulsars can also be given with only the observed spin frequency first derivatives and . This paper will discuss, according to the emission and spin-down property of intermittent pulsars, three different magnetospheric configuration changes in the polar cap region. We find our approach produces feasible values of the second braking index of intermittent pulsar. Our calculations of the second braking indices are in close agreement with predicted second braking indices m = n(2n − 1), which are given by simple spin-down law of pulsar spin evolution, resulting in indirect observational support to our model.