Normal Radio Pulsars Research Articles

Statistics and Evolution of Pulsars’ Parameters

The rotation periods, surface magnetic field strengths, as well as the spatial distribution of the several kinds of pulsars discovered sofar are analyzed statistically. It is revealed that the spatial distribution of the millisecond pulsars is more dispersive than that of the normal radio pulsars. And that the spatial distribution of the pulsars in low-mass X-ray binaries (LMXBs) is also more dispersive than that of the pulsars in high-mass X-ray binaries (HMXBs). The distribution of rotation periods of the isolated millisecond pulsars has a peak at 4.7ms, and the corresponding peak values for the normal radio pulsars and the millisecond pulsars in binaries are 0.6s and 3.5ms, respectively. The surface magnetic field strengths of the FERMI pulsars (the gamma-ray pulsars observed by the Large Area Telescope/Fermi Gamma-ray Space Telescope) and normal pulsars are all concentrated around 1012 Gs. It is found also that some young high-energy pulsars are associated with supernova remnants. In combination with the formation and evolution models of pulsars, we have made some remarks on the characteristics of these distributions.

THE FUNDAMENTAL PLANE FOR RADIO MAGNETARS

High magnetic fields are a distinguishing feature of neutron stars and the existence of sources (the soft gamma repeaters and the anomalous X-ray pulsars) hosting an ultra-magnetized neutron star (or magnetar) has been recognized in the past few decades. Magnetars are believed to be powered by magnetic energy and not by rotation, as with normal radio pulsars. Until recently, the radio quietness and magnetic fields typically above the quantum critical value (Bq~4.4x10^{13} G), were among the characterizing properties of magnetars. The recent discovery of radio pulsed emission from a few of them, and of a low dipolar magnetic field soft gamma repeater, weakened further the idea of a clean separation between normal pulsars and magnetars. In this Letter we show that radio emission from magnetars might be powered by rotational energy, similarly to what occurs in normal radio pulsars. The peculiar characteristics of magnetars radio emission should be traced in the complex magnetic geometry of these sources. Furthermore, we propose that magnetar radio activity or inactivity can be predicted from the knowledge of the star's rotational period, its time derivative and the quiescent X-ray luminosity.

New limits on the population of normal and millisecond pulsars in the Large and Small Magellanic Clouds

Abstract We model the potentially observable populations of normal and millisecond radio pulsars in the Large and Small Magellanic Clouds (LMC and SMC, respectively) where the known population currently stands at 19 normal radio pulsars. Taking into account the detection thresholds of previous surveys, and assuming optimal period and luminosity distributions based on studies of Galactic pulsars, we estimate that there are (1.79 ± 0.20) × 104 and (1.09 ± 0.16) × 104 normal pulsars in the LMC and SMC, respectively. When we attempt to correct for beaming effects, and the fraction of high-velocity pulsars which escape the clouds, we estimate birth rates in both the LMC and SMC to be comparable and in the range of 0.5–1 pulsars per century. Although higher than estimates for the rate of core-collapse supernovae in the clouds, these pulsar birth rates are consistent with historical supernova observations in the past 300 yr. A substantial population of active radio pulsars (of the order of a few hundred thousand) has escaped the LMC and SMC and populates the local intergalactic medium. For the millisecond pulsar (MSP) population, the lack of any detections from current surveys leads to respective upper limits (at the 95 per cent confidence level) of 15000 for the LMC and 23000 for the SMC. Several MSPs could be detected by a currently ongoing survey of the SMC with improved time and frequency resolution using the Parkes multibeam system. Giant-pulse emitting neutron stars could also be seen by this survey.

Population synthesis studies of isolated neutron stars with magnetic field decay

We perform population synthesis studies of different types of neutron stars (NSs) (thermally emitting isolated NSs, normal radio pulsars, magnetars) taking into account the magnetic field decay and using results from the most recent advances in NS cooling theory. For the first time, we confront our results with observations using simultaneously the log N–log S distribution for nearby isolated NSs, the log N–log L distribution for magnetars, and the distribution of radio pulsars in the diagram. For this purpose, we fix a baseline NS model (all microphysics input), and other relevant parameters to standard values (velocity distribution, mass spectrum, birth rates, etc.), allowing us to vary the initial magnetic field strength. We find that our theoretical model is consistent with all sets of data if the initial magnetic field distribution function follows a lognormal law with 〈log (B0/G)〉∼ 13.25 and ⁠. The typical scenario includes about 10 per cent of NSs born as magnetars, significant magnetic field decay during the first million years of a NS life (only about a factor of 2 for low-field NSs but more than an order of magnitude for magnetars), and a mass distribution function dominated by low-mass objects. This model explains satisfactorily all known populations. Evolutionary links between different subclasses may exist, although robust conclusions are not yet possible.

Timing observations of rotating radio transients

We present radio timing measurements of six rotating radio transient (RRAT) sources discovered in the Parkes Multibeam Pulsar Survey. These provide four new phase-connected timing solutions and two updated ones, making a total of seven of the original 11 reported RRATs now with high-precision rotational and astrometric parameters. Three of these seven RRATs have magnetic fields greater than 10^13 G, with spin-down properties similar to those of the magnetars and X-ray detected isolated neutron stars. Another two of these RRATs have long periods and large characteristic ages, and lie near the `death-line' for radio pulsar emission. The remaining two RRATs with timing solutions have properties typical of the bulk of the pulsar population. The new solutions offer insights into what might be responsible for the unusual emission properties. We demonstrate that the RRATs have significantly longer periods and higher magnetic fields than normal radio pulsars, and find no correlation with other spin-down parameters. These solutions also provide precise positions, which will facilitate follow-up studies at high energies, crucial for relating these sources with other neutron star populations.

Proper motions of thermally emitting isolated neutron stars measured with Chandra

The remarkable astrometric capabilities of the Chandra Observatory offer the possibility to measure proper motions of X-ray sources with an unprecedented accuracy in this wavelength range. We recently completed a proper motion survey of three of the seven thermally emitting radio-quiet isolated neutron stars (INSs) discovered in the ROSAT all-sky survey. These INSs (RX J0420.0−5022, RX J0806.4−4123 and RX J1308.6+2127) either lack an optical counterpart or have one so faint that ground based or space born optical observations push the current possibilities of the instrumentation to the limit. Pairs of ACIS observations were acquired 3 to 5 years apart to measure the displacement of the sources on the X-ray sky using as a reference the background of extragalactic or remote Galactic X-ray sources. We derive 2σ upper limits of 123 mas yr −1 and 86 mas yr −1 on the proper motion of RX J0420.0−5022 and RX J0806.4−4123, respectively. RX J1308.6+2127 exhibits a very significant displacement (∼9σ) yielding μ = 220 ± 25 mas yr −1 , the second fastest measured among all ROSAT-discovered INSs. The source is probably moving away rapidly from the Galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of ∼740 (d/700 pc) km s −1 might be the largest of all ROSAT INSs and its corresponding spatial velocity lies among the fastest recorded for neutron stars. RX J1308.6+2127 is thus a middle-aged (age ∼1 My) high velocity cooling neutron star. We investigate its possible origin in nearby OB associations or from a field OB star. In most cases, the flight time from birth place appears significantly shorter than the characteristic age derived from spin down rate. Overall, the distribution in transverse velocity of the ROSAT INSs is not statistically different from that of normal radio pulsars.

Simultaneous multifrequency single-pulse properties of AXP XTE J1810-197

We have used the 76-m Lovell, 94-m equivalent Westerbork Synthesis Radio Telescope (WSRT) and 100-m Effelsberg radio telescopes to investigate the simultaneous single-pulse properties of the radio emitting magnetar Anomalous X-ray Pulsar (AXP) XTE J1810-197 at frequencies of 1.4, 4.8 and 8.35 GHz during 2006 May and July. We study the magnetar's pulse-energy distributions which are found to be very peculiar as they are changing on time-scales of days and cannot be fit by a single statistical model. The magnetar exhibits strong spiky single giant-pulse-like subpulses, but they do not fit the definition of the giant pulse or giant micropulse phenomena. Measurements of the longitude-resolved modulation index reveal a high degree of intensity fluctuations on day-to-day time-scales and dramatic changes across pulse phase. We find the frequency evolution of the modulation index values differs significantly from what is observed in normal radio pulsars. We find that no regular drifting subpulse phenomenon is present at any of the observed frequencies at any observing epoch. However, we find a quasi-periodicity of the subpulses present in the majority of the observing sessions. A correlation analysis indicates a relationship between components from different frequencies. We discuss the results of our analysis in light of the emission properties of normal radio pulsars and a recently proposed model which takes radio emission from magnetars into consideration.

Origin and evolution of magnetars

Abstract We present a population synthesis study of the observed properties of the magnetars investigating the hypothesis that they are drawn from a population of progenitors that are more massive than those of the normal radio pulsars. We assume that the anomalous X-ray emission is caused by the decay of a toroidal or tangled up field that does not take part in the spin-down of the star. Our model assumes that the magnetic flux of the neutron star is distributed as a Gaussian in the logarithm about a mean value that is described by a power law , where Mp is the mass of the progenitor. We find that we can explain the observed properties of the magnetars for a model with Φ0 = 2 × 1025 G cm2 and γ= 5 if we suitably parametrize the time evolution of the anomalous X-ray luminosity as an exponentially decaying function of time. Our modelling suggests that magnetars arise from stars in the high-mass end (20 M⊙≤Mp≤ 45 M⊙) of this distribution. The lower mass progenitors are assumed to give rise to the radio pulsars. The high value of γ can be interpreted in one of two ways. It may indicate that the magnetic flux distribution on the main sequence is a strong function of mass and that this is reflected in the magnetic fluxes of the neutron stars that form from this mass range (the fossil field hypothesis). The recent evidence for magnetic fluxes similar to those of the magnetars in a high fraction (∼25 per cent) of massive O-type stars lends support to such a hypothesis. Another possibility is that the spin of the neutron star is a strong function of the progenitor mass, and it is only for stars that are more massive than ∼20 M⊙ that magnetar-type fields can be generated by the α−ω dynamo mechanism (the convective dynamo hypothesis). In either interpretation, it has to be assumed that all or a subset of stars in the mass range ∼20–45 M⊙, which on standard stellar evolution models lead to black holes via the formation of a fall-back disc, must give rise to magnetars. Unlike with the radio pulsars, the magnetars only weakly constrain the birth spin period, due to their rapid spin-down. Our model predicts a birthrate of ∼1.5–3 × 10−3 yr−1 for the magnetars.

A Search for New Galactic Magnetars in ArchivalChandraandXMM‐NewtonObservations

We present constraints on the number of Galactic magnetars, which we have established by searching for sources with periodic variability in 506 archival Chandra observations and 441 archival XMM-Newton observations of the Galactic plane (| b| < 5°). Our search revealed four sources with periodic variability on timescales of 200-5000 s, all of which are probably accreting white dwarfs. We identify 7 of 12 known Galactic magnetars, but find no new examples with periods between 5 and 20 s. We convert this nondetection into limits on the total number of Galactic magnetars by computing the fraction of the young Galactic stellar population that our survey covered. We find that easily detectable magnetars, modeled after persistent anomalous X-ray pulsars (e.g., with LX = 1035 ergs s−1 [0.5-10.0 keV] and Arms = 12% ), could have been identified in ≈5% of the Galactic spiral arms by mass. If we assume that three previously known examples randomly fall within our survey, then there are 59+ 92−32 in the Galaxy. Barely detectable magnetars (LX = 3 × 1033 ergs s−1 and Arms = 15% ) could have been identified throughout ≈0.4% of the spiral arms. The lack of new examples implies that <540 exist in the Galaxy (90% confidence). Similar constraints are found by considering the detectability of transient magnetars in outburst. For assumed lifetimes of 104 yr, the birth rate of magnetars is between 0.003 and 0.06 yr−1. Therefore, the birth rate of magnetars is at least 10% of that for normal radio pulsars, and could exceed that value, unless transient magnetars are active for ≳105 yr.

Soft gamma repeaters activity in time

AbstractIn this short note I discuss the hypothesis that bursting activity of magnetars evolves in time analogously to the glitching activity of normal radio pulsars (i.e. sources are more active at smaller ages), and that the increase of the burst rate follows one of the laws established for glitching radio pulsars. If the activity of soft gamma repeaters decreases in time in the way similar to the evolution of core‐quake glitches (∝t5/2), then it is more probable to find the youngest soft gamma repeaters, but the energy of giant flares from these sources should be smaller than observed 1044–1046 erg as the total energy stored in a magnetar's magnetic field is not enough to support thousands of bursts similar to the prototype 1979 March 5 flare. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The birthrate of magnetars

ABSTRACT Magnetars, neutron stars with ultra strong magnetic elds (B ˘10 14 10 15 G), mani-fest their exotic nature in the form of soft gamma-ray repeaters and anomalous X-raypulsars. This study estimates the birthrate of magnetars to be ˘0.22 per century witha galactic population comprising of ˘17 objects. A population synthesis was carriedout based on the ve anomalous X-ray pulsars detected in the ROSAT All-Sky Surveyby comparing their number to that of massive OB stars in a well de ned volume. Ad-ditionally, the group of seven X-ray dim isolated neutron stars detected in the samesurvey were found to have a birthrate of ˘2 per century with a galactic populationof ˘22,000 objects.Key words: pulsar: general | stars: neutron | stars: statistics | stars: magnetic elds | supernovae: general 1 INTRODUCTIONThe end products of massive stars with initial masses in therange 8M known SGRs (SGR 1806-20, SGR 1900+14, SGR 1627-41) <M initial <25M , neutron stars (NSs) consti-tute ˘0:0001% of the total stellar population of the MilkyWay in the form of active pulsars (Lyne & Graham-Smith2006). It is now understood that not all NSs turn out to ex-hibit radio pulsations. This missing link manifests itself in ahighly energetic class of NSs called magnetars (Zhang 2003).Duncan & Thompson (1992) proposed the existence of mag-netars with ultra strong magnetic elds 100 times strongerthan that observed for normal radio pulsars (B˘10

1E 1547.0-5408: A Radio-emitting Magnetar with a Rotation Period of 2 Seconds

The variable X-ray source 1E 1547.0-5408 was identified by Gelfand & Gaensler as a likely magnetar in G327.24-0.13, an apparent supernova remnant. No X-ray pulsations have been detected from it. Using the Parkes radio telescope, we discovered pulsations with period P = 2.069 s. Using the Australia Telescope Compact Array, we localized these to 1E 1547.0-5408. We measure = (2.318 ? 0.005) ? 10-11, which for a magnetic dipole rotating in vacuo gives a surface field strength of 2.2 ? 1014 G, a characteristic age of 1.4 kyr, and a spin-down luminosity of 1.0 ? 1035 ergs s-1. Together with its X-ray characteristics, these rotational parameters of 1E 1547.0-5408 prove that it is a magnetar, only the second known to emit radio waves. The distance is ?9 kpc, derived from the dispersion measure of 830 cm-3 pc. The pulse profile at a frequency of 1.4 GHz is extremely broad and asymmetric due to multipath propagation in the ISM, as a result of which only ?75% of the total flux at 1.4 GHz is pulsed. At higher frequencies the profile is more symmetric and has FWHM = 0.12P. Unlike in normal radio pulsars, but in common with the other known radio-emitting magnetar, XTE J1810-197, the spectrum over 1.4-6.6 GHz is flat or rising, and we observe large, sudden changes in the pulse shape. In a contemporaneous Swift X-ray observation, 1E 1547.0-5408 was detected with record high flux, fX(1-8 keV) ? 5 ? 10-12 ergs cm-2 s-1, 16 times the historic minimum. The pulsar was undetected in archival radio observations from 1998, implying a flux <0.2 times the present level. Together with the transient behavior of XTE J1810-197, these results suggest that radio emission is triggered by X-ray outbursts of usually quiescent magnetars.

Polarized radio emission from a magnetar

We present polarization observations of the radio emitting magnetar AXP J1810-197. Using simultaneous multi-frequency observations performed at 1.4, 4.9 and 8.4 GHz, we obtained polarization information for single pulses and the average pulse profile at several epochs. We find that in several respects this magnetar source shows similarities to the emission properties of normal radio pulsars while simultaneously showing striking differences. The emission is nearly 80-95% polarized, often with a low but significant degree of circular polarization at all frequencies which can be much greater in selected single pulses. The position angle swing has a low average slope of only 1 deg/deg, deviating significantly from an S-like swing as often seen in radio pulsars which is usually interpreted in terms of a rotating vector model and a dipolar magnetic field. The observed position angle is consistent at all frequencies while showing significant secular variations. On average the interpulse is less linearly polarized but shows a higher degree of circular polarization. Some epochs reveal the existence of non-orthogonal emission modes in the main pulse and systematic wiggles in the PA swing, while the interpulse shows a large variety of position angle values. We interprete many of the emission properties as propagation effects in a non-dipolar magnetic field configuration where emission from different multipole components is observed.

Rotational periods and other parameters of magnetars

The rotational periods P, period derivatives dP/dt, and magnetic fields B in the region where the emission of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) is generated are calculated using a model that associates the emission of these objects with the existence of drift waves at the periphery of the magnetosphere of a neutron star. The values obtained for these parameters are P = 11−737 ms, dP/dt = 3.7 × 10−16−5.5 × 10−12, and log B (G) = 2.63−6.25. We find a dependence between the X-ray luminosity of AXPs and SGRs, Lx, and the rate at which they lose rotational energy, dE/dt, which is similar to the Lx(dE/dt) dependence for radio pulsars with detected X-ray emission. Within the errors, AXPs/SGRs and radio pulsars with short periods (P < 0.1 s) display the same slopes for their log(dP/dt)-log P relations and for the dependence of the efficiency of their transformation of rotational energy into radiation on their periods. A dipole model is used to calculate the surface magnetic fields of the neutron stars in AXPs and SGRs, which turn out to be, on average, comparable to the surface fields of normal radio pulsars (〈log Bs (G)〉 = 11.90).

Nature of anomalous X-ray pulsars, soft gamma repeaters and radio pulsars with very long periods

A brief review of the known models for the description of anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) is given. A new model is proposed to explain the main properties of these objects on the basis of the concept of drift waves in the vicinity of the light cylinder of the neutron star with a surface magnetic field of about 1012 G. In the framework of this model, the rotation periods P, their derivatives d P/d t and the magnetic fields B in the regions of generation of emission observed in AXPs and SGRs are calculated. The intervals for these parameters are P = 11 − 737 ms, d P/d t = 3.7 × 10− 16 − 5.5 × 10− 12 and log B = 2.63 − 6.25. A modulation with periods P could be observed in the sources under consideration. The magnetic fields at the neutron star's surface calculated in the framework of the dipole model for AXPs and SGRs have the same order (⟨log B s⟩ = 11.90) as for normal radio pulsars. Pulsars of the types under consideration must have short periods (P ≈ 0.1 s) and a small ang...

A strongly magnetized strange star model for soft gamma-ray repeaters

Soft γ-ray repeaters (SGRs) have been currently argued as super-strongly magnetized neutron stars (magnetars) with magnetic fields with a few 10 14 G. Here we propose an alternate model, in which SGRs are young strange stars with superconducting cores and with poloidal magnetic fields of normal radio pulsars. For SGR 1806−20, its magnetic field is taken to be ∼3 × 10 13 G in order to match the stellar characteristic timescale and the age of the surrounding supernova remnant (SNR) associated with this burster. The movement of the magnetic flux tubes in these strange stars not only leads to crustal cracking of the stars, giving rise to deconfinement of crustal matter to strange matter, but also to movement of internal magnetic toruses together with the flux tubes. The former process will result in bursting and quiescent emission and the latter process will produce quasi-steady relativistic winds which will power the surrounding SNR. The observed persistent X-ray luminosity, bursting behavior and inferred wind luminosity of SGRs can be explained in this model. The possible relation between the sources of SGRs, anomalous X-ray pulsars and gamma-ray bursts is discussed.

RXTE observations of single pulses of PSR B0531+21

This article is the second in a series that analyzes about 1.87 million periods of the Crab pulsar, observed by the PCA detector aboard the RXTE X-ray observatory. At these energies, the pulsar displays none of the three phenomena that are often seen in normal radio pulsars -"pulse nulling" , "systematic sub-pulse drifting" and "mode changing" . The presence or absence of these three behaviors in the Crab pulsar at radio wavelengths, something that has not yet been rigorously established, might be important for a satisfactory understanding of the above three phenomena.

Magnetar Spin-Down.

We examine the effects of a relativistic wind on the spin-down of a neutron star and apply our results to the study of soft gamma repeaters (SGRs), which are thought to be neutron stars with magnetic fields greater than 1014 G. We derive a spin-down formula that includes torques from both dipole radiation and episodic or continuous particle winds. We find that if SGR 1806-20 puts out a continuous particle wind of 1037 ergs s-1, then the pulsar age is consistent with that of the supernova remnant, but the derived surface dipole magnetic field is only 3x1013 G, in the range of normal radio pulsars. If instead the particle wind flows are episodic with small duty cycle, then the observed period derivatives imply magnetar-strength fields, while still allowing characteristic ages within a factor of 2 of the estimated supernova remnant age. Close monitoring of the periods of SGRs will allow us to establish or place limits on the wind duty cycle and thus the magnetic field and age of the neutron star.

ROTATION INTRINSIC SPIN EFFECTS AND ITS ASTROPHYSICAL APPLICATION

On the basis of rotation-spin, torsion-spin and magnetic-spin effects, by which the spin precession arises the polarized magnetic field of neutron star to incline to the rotational axis. In the case of accretion layer structure of the recycled pulsar and the companion star magnetic influence considered, the magnetic inclinations of the recycled pulsars are different from that of the normal radio pulsars. The theoretical conclusions and consistent observational evidence is discussed.