Radio-quiet Gamma-ray Pulsars Research Articles

The Galactic population of canonical pulsars

Context. Current wisdom suggests that the observed population of neutron stars are manifestations of their birth scenarios and their thermal and magnetic field evolution. Neutron stars can be observed at various wavebands as pulsars, and radio pulsars represent by far the largest population of neutron stars. Aims. In this paper, we aim to constrain the observed population of the canonical neutron star period, its magnetic field, and its spatial distribution at birth in order to understand the radio and high-energy emission processes in a pulsar magnetosphere. For this purpose we design a population synthesis method, self-consistently taking into account the secular evolution of a force-free magnetosphere and the magnetic field decay. Methods. We generated a population of pulsars and evolved them from their birth to the present time, using the force-free approximation. We assumed a given initial distribution for the spin period, surface magnetic field, and spatial Galactic location. Radio emission properties were accounted for by the polar cap geometry, whereas the gamma-ray emission was assumed to be produced within the striped wind model. Results. We find that a decaying magnetic field gives better agreement with observations compared to a constant magnetic field model. Starting from an initial mean magnetic field strength of B = 2.5 × 108 T with a characteristic decay timescale of 4.6 × 105 yr, a neutron star birth rate of 1/70 yr and a mean initial spin period of 60 ms, we find that the force-free model satisfactorily reproduces the distribution of pulsars in the P−Ṗ diagram with simulated populations of radio-loud, radio-only, and radio quiet gamma-ray pulsars similar to the observed populations.

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.

Follow-up of 27 radio-quiet gamma-ray pulsars at 110–190 MHz using the international LOFAR station FR606

Context. The Fermi Large Area Telescope has detected over 260 gamma-ray pulsars. About one quarter of these are labeled as radio-quiet, that is they either have radio flux densities < 30 μJy at 1400 MHz, or they are not detected at all in the radio domain. In the population of nonrecycled gamma-ray pulsars, the fraction of radio-quiet pulsars is higher, about one half. Aims. Most radio observations of gamma-ray pulsars have been performed at frequencies between 300 MHz and 2 GHz. However, pulsar radio fluxes increase rapidly with decreasing frequency, and their radio beams often broaden at low frequencies. As a consequence, some of these pulsars might be detectable at low radio frequencies even when no radio flux is detected above 300 MHz. Our aim is to test this hypothesis with low-frequency radio observations. Methods. We have observed 27 Fermi-discovered gamma-ray pulsars with the international LOw Frequency ARray (LOFAR) station FR606 in single-station mode. We used the LOFAR high band antenna band (110−190 MHz), with an average observing time of 13 h per target. Part of the data had to be discarded due to radio frequency interference. On average, we kept 9 h of observation per target after the removal of affected datasets, resulting in a sensitivity for pulse-averaged flux on the order of 1−10 mJy. Results. We do not detect radio pulsations from any of the 27 sources, and we establish stringent upper limits on their low-frequency radio fluxes. These nondetections are compatible with the upper limits derived from radio observations at other frequencies. We also determine the pulsars’ geometry from the gamma-ray profiles to see for which pulsars the low-frequency radio beam is expected to cross Earth. Conclusions. This set of observations provides the most constraining upper limits on the flux density at 150 MHz for 27 radio-quiet gamma-ray pulsars. In spite of the beam-widening expected at low radio frequencies, most of our nondetections can be explained by an unfavorable viewing geometry; for the remaining observations, especially those of pulsars detected at higher frequencies, the nondetection is compatible with insufficient sensitivity.

Radio loud and radio quiet pulsars with hard emission

Two parameters such as magnetic fields near the light cylinder $B_{lc} $ and rates of rotational energy losses $\frac{dE}{dt} $ for 1795 radio pulsars without high energy emission (R), 52 radio loud gamma-ray pulsars ( $\gamma$ +R), 33 radio quiet gamma-ray pulsars ( $\gamma$ ) and 61 radio loud X-ray pulsars (X+R) are compared with each other in this work. Pulsars with registered X-ray and gamma-ray emission have the higher median values of $B_{lc} $ and $\frac{dE}{dt} $ as compared to the population of ordinary radio pulsars. There is a trend of increasing of luminosities in X-ray and gamma-ray ranges for higher values of magnetic field near the light cylinder. We hypothesize that non-thermal X-ray and gamma-ray emissions are generated near the light cylinder by the synchrotron mechanism.

Repeated State Change of Variable Gamma-Ray Pulsar PSR J2021+4026

Abstract PSR J2021+4026 is a radio-quiet gamma-ray pulsar and the first pulsar that shows state change of the gamma-ray emission and spindown rate. The state change of PSR J2021+4026 was first observed at 2011 October, at which the pulsar changes the state from high gamma-ray flux/low spindown rate state to low gamma-ray flux/high spindown rate state. In 2014 December, PSR J2021+4026 recovered the state before the 2011 state change over a timescale of a few months. We report that the long-term evolution of the gamma-ray flux and timing behavior suggests that PSR J2021+4026 changed the state near 2018 February 1 and entered a new low gamma-ray flux/high spindown rate state. At the 2018 state change, the averaged flux dropped from (1.29 ± 0.01) × 10−6 cts cm−2 s−1 to (1.12 ± 0.01) × 10−6 cts cm−2 s−1, which is behavior similar to that of the 2011 event. The spindown rate has increased by ∼3% in the new state since the 2018 state change. The shapes of pulse profile and spectrum in GeV bands also changed at the 2018 event, and they are consistent with behavior at the 2011 state change. Our results probably suggest that PSR J2021+4026 is switching between different states with a timescale of several years, like some radio pulsars (e.g., PSR B1828-11). PSR J2021+4026 will provide a unique opportunity to study the mechanism of the state switching.

Advancing astronomy, one screen saver at a time

Astronomy One of the major challenges of modern astronomy is finding important results in the overwhelming volume of acquired data. Clark et al. used the computers of tens of thousands of volunteers participating in the Einstein@Home project to search Fermi Large Area Telescope data. Their goal was to discover radio-quiet gamma-ray millisecond pulsars (MSPs). More than 10,000 donated CPU years in this survey led to the discovery of two isolated MSPs, including a rotation-powered MSP that has remained undetected in radio observations. Sci. Adv. 10.1126/sciadv.aao7228 (2018).

The Einstein@Home Gamma-ray Pulsar Survey. II. Source Selection, Spectral Analysis, and Multiwavelength Follow-up

Abstract We report on the analysis of 13 gamma-ray pulsars discovered in the Einstein@Home blind search survey using Fermi Large Area Telescope (LAT) Pass 8 data. The 13 new gamma-ray pulsars were discovered by searching 118 unassociated LAT sources from the third LAT source catalog (3FGL), selected using the Gaussian Mixture Model machine-learning algorithm on the basis of their gamma-ray emission properties being suggestive of pulsar magnetospheric emission. The new gamma-ray pulsars have pulse profiles and spectral properties similar to those of previously detected young gamma-ray pulsars. Follow-up radio observations have revealed faint radio pulsations from two of the newly discovered pulsars and enabled us to derive upper limits on the radio emission from the others, demonstrating that they are likely radio-quiet gamma-ray pulsars. We also present results from modeling the gamma-ray pulse profiles and radio profiles, if available, using different geometric emission models of pulsars. The high discovery rate of this survey, despite the increasing difficulty of blind pulsar searches in gamma rays, suggests that new systematic surveys such as presented in this article should be continued when new LAT source catalogs become available.

The Einstein@Home Survey for Gamma-ray Pulsars

AbstractSince the launch of the Fermi Gamma-ray Space Telescope in 2008, the onboard Large Area Telescope (LAT) has detected gamma-ray pulsations from more than 200 pulsars. A large fraction of these remain undetected in radio observations, and could only be found by directly searching the LAT data for pulsations. However, the sensitivity of such “blind” searches is limited by the sparse photon data and vast computational requirements. In this contribution we present the latest large-scale blind-search survey for gamma-ray pulsars, which ran on the distributed volunteer computing system, Einstein@Home, and discovered 19 new gamma-ray pulsars. We explain how recent improvements to search techniques and LAT data reconstruction have boosted the sensitivity of blind searches, and present highlights from the survey’s discoveries. These include: two glitching pulsars; the youngest known radio-quiet gamma-ray pulsar; and two isolated millisecond pulsars (MSPs), one of which is the only known radio-quiet rotationally powered MSP.

Detection of radio emission from the gamma-ray pulsar J1732−3131 at 327 MHz

Although originally discovered as a radio-quiet gamma-ray pulsar, J1732-3131 has exhibited intriguing detections at decameter wavelengths. We report an extensive follow-up of the pulsar at 327 MHz with the Ooty radio telescope. Using the previously observed radio characteristics, and with an effective integration time of 60 hrs, we present a detection of the pulsar at a confidence level of 99.82%. The 327 MHz mean flux density is estimated to be 0.5-0.8 mJy, which establishes the pulsar to be a steep spectrum source and one of the least-luminous pulsars known to date. We also phase-aligned the radio and gamma-ray profiles of the pulsar, and measured the phase-offset between the main peaks in the two profiles to be 0.24$\pm$0.06. We discuss the observed phase-offset in the context of various trends exhibited by the radio-loud gamma-ray pulsar population, and suggest that the gamma-ray emission from J1732-3131 is best explained by outer magnetosphere models. Details of our analysis leading to the pulsar detection, and measurements of various parameters and their implications relevant to the pulsar's emission mechanism are presented.

THE BRAKING INDEX OF A RADIO-QUIET GAMMA-RAY PULSAR

ABSTRACT We report the discovery and timing measurements of PSR J1208−6238, a young and highly magnetized gamma-ray pulsar, with a spin period of 440 ms. The pulsar was discovered in gamma-ray photon data from the Fermi Large Area Telescope (LAT) during a blind-search survey of unidentified LAT sources, running on the distributed volunteer computing system Einstein@Home. No radio pulsations were detected in dedicated follow-up searches with the Parkes radio telescope, with a flux density upper limit at 1369 MHz of 30 μJy. By timing this pulsar’s gamma-ray pulsations, we measure its braking index over five years of LAT observations to be n = 2.598 ± 0.001 ± 0.1, where the first uncertainty is statistical and the second estimates the bias due to timing noise. Assuming its braking index has been similar since birth, the pulsar has an estimated age of around 2700 years, making it the youngest pulsar to be found in a blind search of gamma-ray data and the youngest known radio-quiet gamma-ray pulsar. Despite its young age, the pulsar is not associated with any known supernova remnant or pulsar wind nebula. The pulsar’s inferred dipolar surface magnetic field strength is 3.8 × 1013 G, almost 90% of the quantum-critical level. We investigate some potential physical causes of the braking index deviating from the simple dipole model but find that LAT data covering a longer time interval will be necessary to distinguish between these.

RADIO-QUIET AND RADIO-LOUD PULSARS: SIMILAR IN GAMMA-RAYS BUT DIFFERENT IN X-RAYS

We present new Chandra and XMM-Newton observations of a sample of eight radio-quiet Gamma-ray pulsars detected by the Fermi Large Area Telescope. For all eight pulsars we identify the X-ray counterpart, based on the X-ray source localization and the best position obtained from Gamma-ray pulsar timing. For PSR J2030+4415 we found evidence for an about 10 arcsec-long pulsar wind nebula. Our new results consolidate the work from Marelli et al. 2011 and confirm that, on average, the Gamma-ray--to--X-ray flux ratios (Fgamma/Fx) of radio-quiet pulsars are higher than for the radio-loud ones. Furthermore, while the Fgamma/Fx distribution features a single peak for the radio-quiet pulsars, the distribution is more dispersed for the radio-loud ones, possibly showing two peaks. We discuss possible implications of these different distributions based on current models for pulsar X-ray emission.

Deep searches for decametre-wavelength pulsed emission from radio-quiet gamma-ray pulsars

We report the results of (a) extensive follow-up observations of the gamma-ray pulsar J1732-3131 that has been recently detected at decameter wavelengths, and (b) deep searches for counterparts of 9 other radio-quiet gamma-ray pulsars at 34 MHz, using the Gauribidanur radio telescope. No periodic signal from J1732-3131 could be detected above a detection threshold of $8\sigma$, even with an effective integration time of more than 40 hours. However, the average profile obtained by combining data from several epochs, at a dispersion measure of 15.44 pc/cc, is found to be consistent with that from the earlier detection of this pulsar at a confidence level of 99.2 %. We present this consistency between the two profiles as an evidence that J1732-3131 is a faint radio pulsar with an average flux density of 200--400 mJy at 34 MHz. Detection sensitivity of our deep searches, despite the extremely bright sky background at such low frequencies, is generally comparable to that of higher frequency searches for these pulsars, when scaled using reasonable assumptions about the underlying pulsar spectrum. We provide details of our deep searches, and put stringent upper limits on the decameter wavelength flux densities of several radio-quiet gamma-ray pulsars.

DISCOVERY OF X-RAY PULSATION FROM THE GEMINGA-LIKE PULSAR PSR J2021+4026

We report the discovery of X-ray periodicity of ~265.3 ms from a deep XMM-Newton observation of the radio-quiet gamma-ray pulsar, PSR J2021+4026, located at the edge of the supernova remnant G78.2+2.1 (gamma-Cygni).The detected frequency is consistent with the gamma-ray pulsation determined by the observation of Fermi Gamma-ray Space Telescope at the same epoch. The X-ray pulse profile resembles the modulation of hot spot on the surface of the neutron star. The phase-averaged spectral analysis also suggests that the majority of the observed X-rays have a thermal origin. This is the third member in the class of radio-quiet pulsars with the significant pulsations detected from both X-rays and gamma-ray regimes.

DISCOVERY OF NINE GAMMA-RAY PULSARS INFERMILARGE AREA TELESCOPE DATA USING A NEW BLIND SEARCH METHOD

We report the discovery of nine previously unknown gamma-ray pulsars in a blind search of data from the Fermi Large Area Telescope (LAT). The pulsars were found with a novel hierarchical search method originally developed for detecting continuous gravitational waves from rapidly rotating neutron stars. Designed to find isolated pulsars spinning at up to kHz frequencies, the new method is computationally efficient, and incorporates several advances, including a metric-based gridding of the search parameter space (frequency, frequency derivative and sky location) and the use of photon probability weights. The nine pulsars have spin frequencies between 3 and 12 Hz, and characteristic ages ranging from 17 kyr to 3 Myr. Two of them, PSRs J1803-2149 and J2111+4606, are young and energetic Galactic-plane pulsars (spin-down power above 6e35 erg/s and ages below 100 kyr). The seven remaining pulsars, PSRs J0106+4855, J0622+3749, J1620-4927, J1746-3239, J2028+3332, J2030+4415, J2139+4716, are older and less energetic; two of them are located at higher Galactic latitudes (|b| > 10 deg). PSR J0106+4855 has the largest characteristic age (3 Myr) and the smallest surface magnetic field (2e11 G) of all LAT blind-search pulsars. PSR J2139+4716 has the lowest spin-down power (3e33 erg/s) among all non-recycled gamma-ray pulsars ever found. Despite extensive multi-frequency observations, only PSR J0106+4855 has detectable pulsations in the radio band. The other eight pulsars belong to the increasing population of radio-quiet gamma-ray pulsars.

X-ray Vision

Astronomy![Figure][1] CREDIT: ESA In 2008, the Fermi Large Area Telescope detected a pulsar based solely on its gamma-ray pulsations. Now, using the XMM-Newton space-based telescope, Lin et al. and Caraveo et al. have revealed that this source, PSR J0007+7303, also pulsates in x-rays. Pulsars are magnetized, rapidly rotating neutron stars; like lighthouses, they appear to flash periodically as their beams of electromagnetic radiation cross our line of sight. For many decades, the vast majority of pulsars astronomers knew about were detected through their radio emission. PSR J0007+7303 was only the second radio-quiet pulsar to be detected in gamma rays; now it is also the second radio-quiet gamma-ray pulsar to show x-ray pulsations. These observations help build a more complete picture of this pulsar. The x-rays are believed to stem from both the heat of the neutron star's surface and from relativistic particles in the star's magnetosphere, which are also the origin of the gamma rays. In the case of PSR J0007+7303, the heat component appears to dominate; the x-ray pulsations possibly originate from a hot spot on the neutron star's surface, which is heated by return currents from the magnetosphere. Astrophys. J. 725 , L1; L6 (2010). [1]: pending:yes

X-RAY PULSATIONS FROM THE RADIO-QUIET GAMMA-RAY PULSAR IN CTA 1

Prompted by the Fermi-LAT discovery of a radio-quiet gamma-ray pulsar inside the CTA 1 supernova remnant, we obtained a 130 ks XMM-Newton observation to assess the timing behavior of this pulsar. Exploiting both the unprecedented photon harvest and the contemporary Fermi-LAT timing measurements, a 4.7σ single-peak pulsation is detected, making PSR J0007+7303 the second example, after Geminga, of a radio-quiet gamma-ray pulsar also seen to pulsate in X-rays. Phase-resolved spectroscopy shows that the off-pulse portion of the light curve is dominated by a power-law, non-thermal spectrum, while the X-ray peak emission appears to be mainly of thermal origin, probably from a polar cap heated by magnetospheric return currents, pointing to a hot spot varying throughout the pulsar rotation.

Statistical Properties of High‐Energy Radiation from Young Pulsars

We study the statistical properties of young pulsars with ages of <= 10(6) yr using the Monte Carlo method on the frame of a revised outer gap model. In our simulations, we first compare the simulated radio-loud pulsars with the observed ones in the ATNF Pulsar Catalogue, in which nine radio surveys are included. On the basis of consistency between the simulated and the observed young pulsars, we further simulate possible gamma-ray pulsars based on our outer gap model. In this model, geometric effects have been taken into account, and the high-energy radiation of a pulsar depends on period (P), magnetic field (B), and magnetic inclination angle (alpha). We take the gamma-ray beaming selection effects into account and study the statistical properties of gamma-ray pulsars with energy fluxes above the detectable threshold fluxes. For the EGRET detector, we predict that there may be similar to 8 radio-loud and similar to 24 radio-quiet gamma-ray pulsars if we take the neutron star birthrate to be 1.5 per century. For GLAST, we expect similar to 78 and similar to 740 radio-loud and radio-quiet gamma-ray pulsars. Most of the young gamma-ray pulsars are concentrated at the Galactic plane. We compared the energy fluxes of the simulated gamma-ray pulsars with those of low-latitude unidentified EGRET gamma-ray sources, indicating that the majority of these unidentified sources have pulsar origin.

Radio-Loud and Radio-Quiet Gamma-Ray Pulsars from the Galaxy and the Gould Belt

Radio-Loud and Radio-Quiet Gamma-Ray Pulsars from the Galaxy and the Gould Belt

Pulsar Slot Gaps and Unidentified Egret Sources

A new picture of pulsar high-energy emission is proposed that is different from both the traditional polar cap and outer gap models, but combines elements of each. The slot gap model is based on electron acceleration along the edge of the open field region from the neutron star surface to near the light cylinder. Along the last open field line, the pair formation front rises to very high altitude forming a slot gap, where the accelerating electric field is unscreened by pairs. The resulting radiation features both hollow cones from the lower-altitude pair cascades, seen at small viewing angles, as well as caustic emission on the trailing-edge field lines at high altitude, seen from both poles at large viewing angle. The combination of the small solid angle of slot gap emission (<< 1 sr) with a high probability of viewing the emission predicts that more gamma-ray pulsars could be detected at larger distances. In this picture, many of the positional coincidences of radio pulsars with unidentified EGRET sources become plausible as real associations, as the flux predicted by the slot gap model for many of the pulsars would provide the observed EGRET source flux. The expected probability of seeing radio-quiet gamma-ray pulsars in this model will also be discussed.

A Search for Radio‐quiet Gamma‐Ray Pulsars

Most Galactic point sources of gamma rays remain unidentified. The few (extrasolar) sources that have been identified are all young, rotation-powered pulsars, all but one of which were identified using radio ephemerides. The radio-quiet Geminga pulsar was identified only after pulsations were discovered in a coincident X-ray source. Observational evidence indicates that many of the unidentified Galactic sources are likely to be pulsars, and some theoretical models predict a potentially large population of radio-quiet gamma-ray pulsars. We present a new method for performing sensitive gamma-ray pulsar searches. We used this method to search several of the strongest EGRET sources for pulsations. This was a blind search for new pulsars, covering a frequency and a frequency-derivative phase space large enough to detect Crab-like pulsars as well as lower frequency, high magnetic field magnetars. No new pulsars were discovered, and we report upper limits constraining the characteristics of any signals contained in the data sets searched.