Young Radio Pulsars Research Articles

Connecting the young pulsars in Milky Way globular clusters with white dwarf mergers and the M81 fast radio burst

ABSTRACT The detections of four apparently young radio pulsars in the Milky Way globular clusters are difficult to reconcile with standard neutron star formation scenarios associated with massive star evolution. Here, we discuss formation of these young pulsars through white dwarf mergers in dynamically old clusters that have undergone core collapse. Based on observed properties of magnetic white dwarfs, we argue neutron stars formed via white dwarf merger are born with spin periods of roughly $10{\!-\!}100\,$ ms and magnetic fields of roughly $10^{11}{\!-\!}10^{13}\,$ G. As these neutron stars spin down via magnetic dipole radiation, they naturally reproduce the four observed young pulsars in the Milky Way clusters. Rates inferred from N-body cluster simulations as well as the binarity, host cluster properties, and cluster offsets observed for these young pulsars hint further at a white dwarf merger origin. These young pulsars may be descendants of neutron stars capable of powering fast radio bursts analogous to the bursts observed recently in a globular cluster in M81.

Pulsar polarization: a broad-band population view with the Parkes Ultra-Wideband receiver

ABSTRACTThe radio polarization properties of the pulsar population are only superficially captured by the conventional picture of pulsar radio emission. We study the broadband polarization of 271 young radio pulsars, focusing particularly on circular polarization, using high-quality observations made with the Ultra-Wideband Low receiver on Murriyang, the Parkes radio telescope. We seek to encapsulate polarization behaviour on a population scale by defining broad categories for frequency- and phase-dependent polarization evolution, studying the co-occurrences of these categorizations and comparing them with average polarization measurements and spin-down energy ($\dot{E}$). This work shows that deviations of the linear polarization position angle from the rotating vector model are linked to the presence of circular polarization features and to frequency evolution of the polarization. Polarization fraction, circular polarization contribution, and profile complexity all evolve with $\dot{E}$ across the population, with the profiles of high-$\dot{E}$ pulsars being simple and highly linearly polarized. The relationship between polarization fraction and circular contribution is also seen to evolve such that highly polarized profiles show less variation in circular contribution with frequency than less strongly polarized profiles. This evolution is seen both across the population and across frequency for individual sources. Understanding pulsar radio polarization requires detailed study of individual sources and collective understanding of population-level trends. For the former, we provide visualizations of their phase- and frequency-resolved polarization parameters. For the latter, we have highlighted the importance of including the impact of circular polarization and of $\dot{E}$.

Young radio-loud gamma-ray pulsar light curve fitting

Context. Since the launch of the Fermi gamma-ray telescope, several hundred radio-loud gamma-ray pulsars have been detected, many belonging to millisecond pulsars but some belonging to the young pulsar population with spin periods longer than 30 ms. Aims. Observing simultaneously pulsed radio and gamma-ray emission from these stars helps to constrain the geometry and radiation mechanisms within their magnetosphere and to localize the multiple photon production sites. In this paper we fit the time-aligned gamma-ray light curves of young radio-loud gamma-ray pulsars. We assume a dipole force-free magnetosphere where radio photons emanate from high altitudes above the polar caps and gamma rays originate from outside the light cylinder, within the striped wind current sheet. Methods. We computed a full atlas of radio and gamma-ray pulse profiles depending on the magnetic axis obliquity and line-of-sight inclination with respect to the neutron star rotation axis. By applying a χ2 fitting technique, we were able to pin down accurately the magnetosphere geometry. Further constraints were obtained from radio polarization measurement following the rotating vector model, including aberration and retardation effects. Results. We find a good agreement between our model and the time-aligned single- or double-peaked gamma-ray pulsar observations. We deduce the magnetic inclination angle and the observer line of sight with respect to the rotation axis within a small error bar. The distinction between radio-loud or radio-quiet gamma-ray pulsars or only radio pulsars can entirely be related to the geometry of the associated emitting regions. Conclusions. The high-altitude polar cap model combined with the striped wind represents a minimalistic approach able to reproduce a wealth of gamma-ray pulse profiles for young radio pulsars. Based on self-consistent force-free simulations, it gives a full geometrical picture of the emission properties without resorting to detailed knowledge of the individual particle dynamics and energetics.

Statistical tests of young radio pulsars with/without supernova remnants: implying two origins of neutron stars

ABSTRACT The properties of the young pulsars and their relations to the supernova remnants (SNRs) have been the interesting topics. At present, 383 SNRs in the Milky Way Galaxy have been published, which are associated with 64 radio pulsars and 46 pulsars with high-energy emissions. However, we noticed that 630 young radio pulsars with the spin periods of less than half a second have been not yet observed the SNRs surrounding or nearby them, which arises a question of that could the two types of young radio pulsars with/without SNRs hold the distinctive characteristics? Here, we employ the statistical tests on the two groups of young radio pulsars with (52) and without (630) SNRs to reveal if they share the different origins. Kolmogorov–Smirnov (K–S) and Mann–Whitney–Wilcoxon (M–W–W) tests indicate that the two samples have the different distributions with parameters of spin period (P), derivative of spin period ($\dot{P}$), surface magnetic field strength (B), and energy loss rate ($\dot{E}$). Meanwhile, the cumulative number ratio between the pulsars with and without SNRs at the different spin-down ages decreases significantly after $\rm 10\!-\!20\, kyr$. So we propose that the existence of the two types of supernovae (SNe), corresponding to their SNR lifetimes, which can be roughly ascribed to the low- and high-energy SNe. Furthermore, the low-energy SNe may be formed from the $\rm 8\!-\!12\, M_{\odot }$ progenitor, e.g. possibly experiencing the electron capture, while the main-sequence stars of $\rm 12\!-\!25\, M_{\odot }$ may produce the high-energy SNe probably by the iron core collapse.

30 glitches in 18 radio pulsars

Pulsar timing is a classic technology of detecting irregularities in pulsar rotation. We carried out this method for 18 young radio pulsars, with long-term timing observations obtained between 2007 and 2015 using the Parkes 64-m radio telescope. As a result, 30 glitches were identified, ranging from 0.75 × 10−9 to 8.6 × 10−6 in the relative glitch sizes Δν/ν, where ν = 1/P is the pulse frequency. These glitches are composed of 26 new glitches and four published glitches with new exponential recoveries. All pulsars exhibit normal glitches, and six pulsars were observed to undergo a glitch event for the first time. We discuss the properties and implications for neutron-star physics of these glitches, and show that they are in agreement with previous work, except that the cumulative probability distributions of the mean waiting times for PSRs J0537–6910, J1341–6220 and J1740–3015 are not in consonance with the Poisson model.

Magnetic field decay in young radio pulsars

The role of magnetic field decay in normal radio pulsars is still debated. In this paper, we present results which demonstrate that an episode of magnetic field decay in hot young neutron stars can explain anomalous values of braking indices recently measured for more than a dozen of sources. It is enough to have few tens of per cent of such hot neutron stars in the total population to explain observables. Relatively rapid decay operates at ages ≲ few ×100 kyrs with a characteristic timescale of a similar value. We speculate that this decay can be related to electron scattering off phonons in neutron star crusts. This type of decay saturates as a neutron star cools down. Later on, a much slower decay due to crustal impurities dominates. Finally, we demonstrate that this result is in agreement with our early studies.

Braking indices of young radio pulsars: theoretical perspective

ABSTRACT Recently, Parthsarathy et al. analysed long-term timing observations of 85 young radio pulsars. They found that 15 objects have absolute values of braking indices ranging ∼10–3000, far from the classical value n = 3. They also noted a mild correlation between measured value of n and characteristic age of a radio pulsar. In this article, we systematically analyse possible physical origin of large braking indices. We find that a small fraction of these measurements could be caused by gravitational acceleration from an unseen ultra-wide companion of a pulsar or by precession. Remaining braking indices cannot be explained neither by pulsar obliquity angle evolution, nor by complex high-order multipole structure of the poloidal magnetic field. The most plausible explanation is a decay of the poloidal dipole magnetic field which operates on a time-scale ∼104−105 yr in some young objects, but has significantly longer time-scale in other radio pulsars. This decay can explain both amplitude of measured n and some correlation between n and characteristic age. The decay can be caused by either enhanced crystal impurities in the crust of some isolated radio pulsars or, more likely, by enhanced resistivity related to electron scattering off phonons due to slow cooling of low-mass neutron stars. If this effect is indeed the main cause of the rapid magnetic field decay manifesting as large braking indices, we predict that pulsars with large braking indices are hotter in comparison to those with n ≈ 3.

Multiwavelength observations of PSR J2032+4127 during the 2017 periastron passage

ABSTRACT PSR J2032+4127 is only the second known gamma-ray binary where it is confirmed that a young radio pulsar is in orbit around a Be-star. The interaction of the pulsar wind with the mass outflow from the companion leads to broad-band emission from radio up to TeV energies. In this paper we present results of optical monitoring of the 2017 periastron passage with the Nordic Optical Telescope. These observations are complemented by X-ray (Swift/XRT, NuSTAR) and GeV (Fermi/LAT) monitoring. Joint analysis of the evolution of the parameters of the H α line and the broad-band (X-ray to TeV) spectral shape allows us to propose a model linking the observed emission to the interaction of the pulsar and Be-star winds under the assumption of the inclined disc geometry. Our model allows the observed flux and spectral evolution of the system to be explained in a self-consistent way.

Timing of young radio pulsars – II. Braking indices and their interpretation

ABSTRACT In Paper I of this series, we detected a significant value of the braking index (n) for 19 young, high-$\dot{E}$ radio pulsars using ∼10 yr of timing observations from the 64-m Parkes radio telescope. Here, we investigate this result in more detail using a Bayesian pulsar timing framework to model timing noise and to perform selection to distinguish between models containing exponential glitch recovery and braking index signatures. We show that consistent values of n are maintained with the addition of substantial archival data, even in the presence of glitches. We provide strong arguments that our measurements are unlikely due to exponential recovery signals from unseen glitches even though glitches play a key role in the evolution of a pulsar’s spin frequency. We conclude that, at least over decadal time-scales, the value of n can be significantly larger than the canonical 3 and discuss the implications for the evolution of pulsars.

Timing of young radio pulsars – I. Timing noise, periodic modulation, and proper motion

ABSTRACT The smooth spin-down of young pulsars is perturbed by two non-deterministic phenomenon, glitches, and timing noise. Although the timing noise provides insights into nuclear and plasma physics at extreme densities, it acts as a barrier to high-precision pulsar timing experiments. An improved methodology based on the Bayesian inference is developed to simultaneously model the stochastic and deterministic parameters for a sample of 85 high-$\dot{E}$ radio pulsars observed for ∼10 yr with the 64-m Parkes radio telescope. Timing noise is known to be a red process and we develop a parametrization based on the red-noise amplitude (Ared) and spectral index (β). We measure the median Ared to be $-10.4^{+1.8}_{-1.7}$ yr3/2 and β to be $-5.2^{+3.0}_{-3.8}$ and show that the strength of timing noise scales proportionally to $\nu ^{1}|\dot{\nu }|^{-0.6\pm 0.1}$, where ν is the spin frequency of the pulsar and $\dot{\nu }$ is its spin-down rate. Finally, we measure significant braking indices for 19 pulsars and proper motions for 2 pulsars, and discuss the presence of periodic modulation in the arrival times of 5 pulsars.

Targeted search for young radio pulsars in the SMC: discovery of two new pulsars

ABSTRACT We report the first rotation powered pulsars discovered in the Small Magellanic Cloud (SMC) in more than a decade. PSR J0043–73 and PSR J0052–72 were discovered during a Parkes Multi-Beam (PMB) survey of MCSNR J0127–7332, and five new, optically selected, supernova remnant (SNR) candidates identified by the XMM–Newton survey. In addition to the candidates, we adjusted the PMB rotation to include additional nine SNRs and pulsar wind nebulae. We searched for young pulsars (1–200 ms) employing a Fourier analysis with PRESTO, as well as a search for longer period pulsars (200 ms–360 s) with a fast folding algorithm. Our targeted survey had a limiting flux density of 0.039 mJy for periods greater than 50 ms. Although not the main target of this search it was also sensitive to millisecond pulsars. PSR J0043–73 has a period and dispersion measure of 937.429 37 (26) ms and 115.1 (3.4) pc cm−3, respectively, and PSR J0052–72 has a period of 191.444 328 (46) ms and a DM of 158.6 (1.6) pc cm−3.

Fast radio bursts: Superpulsars, magnetars, or something else?

We briefly review main observational properties of fast radio bursts (FRBs) and discuss two most popular hypothesis for the explanation of these enigmatic intense millisecond radio flashes. FRBs most probably originate on extragalactic distances, and their rate on the sky is about a few thousand per day with fluences above [Formula: see text]1[Formula: see text]Jy[Formula: see text]ms (or with fluxes larger than few tenths of Jy). Two leading scenarios describing these events include strong flares of magnetars and supergiant pulses of young radio pulsars with large rotational energy losses, correspondingly. At the moment, it is impossible to choose between these models. However, new telescopes can help to solve the puzzle of FRBs in near future.

PSR J1755−2550: a young radio pulsar with a massive, compact companion

Radio pulsars found in binary systems with short orbital periods are usually fast spinning as a consequence of recycling via mass transfer from their companion stars; this process is also thought to decrease the magnetic field of the neutron star being recycled. Here, we report on timing observations of the recently discovered binary PSR J1755$-$2550 and find that this pulsar is an exception: with a characteristic age of 2.1 Myr, it is relatively young; furthermore, with a spin period of 315 ms and a surface magnetic field strength at its poles of 0.88$\times$10$^{12}$ G the pulsar shows no sign of having been recycled. Based on its timing and orbital characteristics, the pulsar either has a massive white dwarf (WD) or a neutron star (NS) companion. To distinguish between these two cases, we searched radio observations for a potential recycled pulsar companion and analysed archival optical data for a potential WD companion. Neither work returned conclusive detections. We apply population synthesis modelling and find that both solutions are roughly equally probable. Our population synthesis also predicts a minimum mass of 0.90 M$_{\odot}$ for the companion star to PSR J1755$-$2550 and we simulate the systemic runaway velocities for the resulting WDNS systems which may merge and possibly produce Ca-rich supernovae. Whether PSR J1755$-$2550 hosts a WD or a NS companion star, it is certainly a member of a rare subpopulation of binary radio pulsars.

Radio-interferometric and near-IR observations of PSR J1357—6429

PSR J1357—6429 is a young radio pulsar detected in X-rays and γ-rays. We performed radio-interferometric and high spatial resolution near-infrared observations of the pulsar. Based on the obtained radio data, we estimated the most accurate pulsar position, RA = 13:57:02.525(14) and Dec = 64:29:29.89(15). Using the new and archival radio data, we obtained the 90 per cent pulsar proper motion upper limit μ < 106 mas yr-1, which corresponds to the pulsar transverse velocity vtr < 1300 km s-1. The near-infrared imaging revealed a point source which we propose as a pulsar near-infrared counterpart candidate. It is confidently detected in the J and Ks bands, with J = 23.51±0.24 and Ks = 21.82±0.25. The respective dereddened near-infrared fluxes are compatible with the long-wavelength extrapolation of the pulsar X-ray spectrum. If the candidate is the true counterpart, by this property PSR J1357—6429 would be similar to the nearby middle-age pulsar PSR B0656+14. In this case, both pulsars demonstrate an unusually high near-infrared efficiency relative to the X-ray efficiency as compared to other pulsars detected in both ranges.

The search for isolated BH candidates based on kinematics of pulsars - their former companions in disrupted binaries

AbstractWe propose searching for isolated stellar-mass black hole (BH) candidates based on the fact that more than 50% of radio pulsars have originated in binary systems, where the other component could have evolved into a BH prior to the second supernova event of the system, which caused its disruption. We selected isolated, relatively young radio pulsars with known parallaxes and proper motions and traced their trajectories back to their presumed birth locations. These locations were then analyzed for possible BH candidates based on the available positional, photometric, and spectral data. We present the first results for 2 pulsars, J0139+5814 and J0922+0638. Seven BH candidates were selected for further analysis.

Near-infrared observations of PSR J1357−6429

PSR J1357$-$6429 is a young radio pulsar that was detected in X-rays and $\gamma$-rays. We present the high spatial resolution near-infrared imaging of the pulsar field in $J$, $H$ and $K_s$ bands obtained with the VLT/NaCo using the Adaptive Optic system. We found a faint source at the most precise pulsar radio position which we propose as the pulsar near-infrared counterpart candidate. It is confidently detected in the $J$ and $K_s$ bands, with $J$ = 23.51$\pm$0.24 and $K_s$ = 21.82$\pm$0.25. There is a hint of the source in the $H$ band with an upper limit $H$ $>$ 22.8. The dereddened source fluxes are compatible with the extrapolation of the pulsar X-ray spectrum towards the near-infrared. If the candidate is the true counterpart, by this property PSR J1357$-$6429 would be similar to the nearby middle-age pulsar PSR B0656+14. In this case, both pulsars demonstrate an unusually high near-infrared efficiency relative to the X-ray efficiency as compared to other pulsars detected in both ranges.

Low braking index of PSR J1734−3333: an interaction between fall-back disc and magnetic field?

Abstract Recent timing observation reported that the radio pulsar PSR J1734−3333 with a rotating period P = 1.17 s is slowing down with a period derivative $\dot{P}=2.28\times 10^{-12}\ \rm s\,s^{-1}$. Its derived braking index n = 0.9 ± 0.2 is the lowest value among young radio pulsars with the measured braking indices. In this Letter, we attempt to investigate the influence of the braking torque caused by the interaction between the fall-back disc and the strong magnetic field of the pulsar on the spin evolution of PSR J1734−3333. Analytical result show that this braking torque is obviously far more than that by magnetic dipole radiation for pulsars with spin period of &amp;gt;0.1 s, and play an important role during the spin-down of the pulsars. Our simulated results indicate that, for some typical neutron star parameters, the braking index and the period derivative approximately in agreement with the measured value of PSR J1734−3333 if the material inflow rate in the fall-back disc is 2 × 1017 g s− 1. In addition, our scenario can account for the measured braking indices of four young pulsars. However, our predicted X-ray luminosity are one to two order of magnitude higher than the observation. We proposed that this discrepancy may originate from the instability of fall-back disc.

On the primary beam deceleration in the pulsar wind

We investigate the motion of the primary beam outside the light cylinder in the pulsar wind. Inside the light cylinder both primary and secondary plasma move along dipole magnetic field lines where their energies can be arbitrary. But at larger distances the theory predicts quasi-radial motion with the velocity exactly corresponding to the drift velocity which cannot be the same for primary and secondary plasma. Hence, the deceleration of the primary beam is to take place simultaneously resulting in the acceleration of the secondary plasma. We investigate this process in the three-fluid MHD approximation and demonstrate that for most pulsars the energy of the beam remains practically unchanged. Only for young radio pulsars (Crab, Vela) essential deceleration up to the energy of the secondary plasma takes place outside the fast magnetosonic surface $r_{\rm F} \sim (10$-$100) R_{\rm L}$, the energy of secondary plasma itself increasing insufficiently.

Hall drift and the braking indices of young pulsars

Braking index measurements of young radio pulsars are all smaller than the value expected for spin down by magnetic dipole braking. We investigate magnetic field evolution in the neutron star crust due to Hall drift as an explanation for observed braking indices. Using numerical simulations and a semi-analytic model, we show that a $\approx 10^{14}\ {\rm G}$ quadrupolar toroidal field in the neutron star crust at birth leads to growth of the dipole moment at a rate large enough to agree with measured braking indices. A key factor is the density at which the crust yields to magnetic stresses that build up during the evolution, which sets a characteristic minimum Hall timescale. The observed braking indices of pulsars with inferred dipole fields of $\lesssim 10^{13}\ {\rm G}$ can be explained in this picture, although with a significant octupole component needed in some cases. For the stronger field pulsars, those with $B_d\gtrsim 10^{13}\ {\rm G}$, we find that the magnetic stresses in the crust exceed the maximum shear stress before the pulsar reaches its current age, likely quenching the Hall effect. This may have implications for the magnetar activity seen in the high magnetic field radio pulsar PSR~J1846-0258. Observations of braking indices may therefore be a new piece of evidence that neutron stars contain subsurface toroidal fields that are significantly stronger than the dipole field, and may indicate that the Hall effect is important in a wider range of neutron stars than previously thought.

X-RAY MEASUREMENT OF THE SPIN-DOWN OF CALVERA: A RADIO- AND GAMMA-RAY-QUIET PULSAR

We measure spin-down of the 59 ms X-ray pulsar Calvera by comparing the XMM-Newton discovery data from 2009 with new Chandra timing observations taken in 2013. Its period derivative is P-dot = (3.19+/-0.08)e-15, which corresponds to spin-down luminosity E-dot = 6.1e35 erg/s, characteristic age tau_c = P/2P-dot = 2.9e5 yr, and surface dipole magnetic field strength B_s = 4.4e11 G. These values rule out a mildly recycled pulsar, but Calvera could be an orphaned central compact object (anti-magnetar), with a magnetic field that was initially buried by supernova debris and is now reemerging and approaching normal strength. We also performed unsuccessful searches for high-energy gamma-rays from Calvera in both imaging and timing of >100 MeV Fermi photons. Even though the distance to Calvera is uncertain by an order of magnitude, an upper limit of d < 2 kpc inferred from X-ray spectra implies a gamma-ray luminosity limit of < 3.3e32 erg/s, which is less than that of any pulsar of comparable E-dot. Calvera shares some properties with PSR J1740+1000, a young radio pulsar that we show by virtue of its lack of proper motion was born outside of the Galactic disk. As an energetic, high-Galactic-latitude pulsar, Calvera is unique in being undetected in both radio and gamma-rays to faint limits, which should place interesting constraints on models for particle acceleration and beam patterns in pulsar magnetospheres.