J. L. Han

Arecibo pulsar survey using alfa. I. Survey strategy and first discoveries

We report results from the initial stage of a long-term pulsar survey of the Galactic plane using the Arecibo L-band Feed Array (ALFA), a seven-beam receiver operating at 1.4 GHz with 0.3 GHz bandwidth, and fast-dump digital spectrometers. The search targets low Galactic latitudes, |b| 5°, in the accessible longitude ranges 32° l 77° and 168° l 214°. The instrumentation, data processing, initial survey observations, sensitivity, and database management are described. Data discussed here were collected over a 100 MHz passband centered on 1.42 GHz using a spectrometer that recorded 256 channels every 64 μs. Analysis of the data with their full time and frequency resolutions is ongoing. Here we report the results of a preliminary, low-resolution analysis for which the data were decimated to speed up the processing. We have detected 29 previously known pulsars and discovered 11 new ones. One of these, PSR J1928+1746, with a period of 69 ms and a relatively low characteristic age of 82 kyr, is a plausible candidate for association with the unidentified EGRET source 3EG J1928+1733. Another, PSR J1906+07, is a nonrecycled pulsar in a relativistic binary with an orbital period of 3.98 hr. In parallel with the periodicity analysis, we also search the data for isolated dispersed pulses. This technique has resulted in the discovery of PSR J0628+09, an extremely sporadic radio emitter with a spin period of 1.2 s. Simulations we have carried out indicate that ~1000 new pulsars will be found in our ALFA survey. In addition to providing a large sample for use in population analyses and for probing the magnetoionic interstellar medium, the survey maximizes the chances of finding rapidly spinning millisecond pulsars and pulsars in compact binary systems. Our search algorithms exploit the multiple data streams from ALFA to discriminate between radio frequency interference and celestial signals, including pulsars and possibly new classes of transient radio sources.

Pulsar rotation measures and the magnetic structure of our Galaxy

We have obtained 63 rotation measures (RMs) from polarization observations of southern pulsars, of which 54 are new measurements and three are varied from previous values. The new pulsar RM data at high Galactic latitudes are mostly consistent with the antisymmetric RM distribution found previously. For the Galactic disc, evidence for a field reversal near the Perseus arm, and possibly another beyond it, is presented. Inside the Solar Circle, in addition to the two known field reversals in or near the Carina–Sagittarius arm and the Crux–Scutum arm, a further reversal in the Norma arm is tentatively identified. These reversals, together with the pitch angle derived from pulsar RM and stellar polarization distributions, are consistent with bisymmetric spiral models for the large-scale magnetic field structure in the disc of our Galaxy. However, discrimination between models is complicated by the presence of smaller scale irregularities in the magnetic field, as well as uncertainties in the theoretical modelling.

An Eccentric Binary Millisecond Pulsar in the Galactic Plane

Binary pulsar systems are superb probes of stellar and binary evolution and the physics of extreme environments. In a survey with the Arecibo telescope, we have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 milliseconds in a highly eccentric (e = 0.44) 95-day orbit around a solar mass (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{M}_{{\odot}}\) \end{document}) companion. Infrared observations identify a possible main-sequence companion star. Conventional binary stellar evolution models predict neither large orbital eccentricities nor main-sequence companions around millisecond pulsars. Alternative formation scenarios involve recycling a neutron star in a globular cluster, then ejecting it into the Galactic disk, or membership in a hierarchical triple system. A relativistic analysis of timing observations of the pulsar finds its mass to be 1.74 ± 0.04 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{M}_{{\odot}}\) \end{document}, an unusually high value.

The spiral structure of our Milky Way Galaxy

Context. The spiral structure of our Milky Way Galaxy is not yet known. HII regions and giant molecular clouds are the most prominent spiral tracers. Models with 2−4 arms have been proposed to outline the structure of our Galaxy. Aims. Recently, new data of spiral tracers covering a larger region of the Galactic disk have been published. We wish to outline the spiral structure of the Milky way using all tracer data. Methods. We collected the spiral tracer data of our Milky Way from the literature, namely, HII regions and giant molecular clouds (GMCs). With weighting factors based on the excitation parameters of HII regions or the masses of GMCs, we fitted the distribution of these tracers with models of two, three, four spiral-arms or polynomial spiral arms. The distances of tracers, if not available from stellar or direct measurements, were estimated kinetically from the standard rotation curve of Brand & Blitz (1993, A&A, 275, 67) with R0 = 8.5 kpc, and Θ0 = 220 km s −1 or the newly fitted rotation curves with R0 = 8.0 kpc and Θ0 = 220 km s −1 or R0 = 8.4 kpc and Θ0 = 254 km s −1 . Results. We found that the two-arm logarithmic model cannot fit the data in many regions. The three- and the four-arm logarithmic models are able to connect most tracers. However, at least two observed tangential directions cannot be matched by the three- or four-arm model. We composed a polynomial spiral arm model, which can not only fit the tracer distribution but also match observed tangential directions. Using new rotation curves with R0 = 8.0 kpc and Θ0 = 220 km s −1 and R0 = 8.4 kpc and Θ0 = 254 km s −1 for the estimation of kinematic distances, we found that the distribution of HII regions and GMCs can fit the models well, although the results do not change significantly compared to the parameters with the standard R0 and Θ0.

GALAXY CLUSTERS IDENTIFIED FROM THE SDSS DR6 AND THEIR PROPERTIES

Clusters of galaxies in most previous catalogs have redshifts z � 0.3. Using the photometric redshifts of galaxies from the Sloan Digital Sky Survey Data Release 6 (SDSS DR6), we identify 39,668 clusters in the redshift range 0.05 2 � 10 14 M� ) clusters of z � 0.42. The false detection rate is � 5%. We obtain the richness, the summed luminosity, and the gross galaxy number within the determined radius for identified clusters. They are tightly related to the X-ray luminosity and temperature of clusters. Cluster mass is rel ated to the richness and summed luminosity with M200 / R 1.90� 0.04 and M200 / L 1.64� 0.03 r , respectively. In addition, 685 new candidates of X-ray clusters are found by cross-identification of our clusters with the so urce list of the ROSAT X-ray survey. Subject headings:galaxies: clusters: general — galaxies: distances and reds hifts

Arecibo Pulsar Survey Using ALFA. II. The Young, Highly Relativistic Binary Pulsar J1906+0746

We report the discovery of PSR J1906+0746, a young 144 ms pulsar in a highly relativistic 3.98 hr orbit with an eccentricity of 0.085 and expected gravitational wave coalescence time of � 300 Myr. The new pulsar was found during precursor survey observations with the Arecibo 1.4 GHz feed array system and retrospectively detected in the Parkes Multibeam plane pulsar survey data. From radio follow-up observations with Arecibo, Jodrell Bank, GreenBank,andParkes,wehavemeasuredthespin-downandbinaryparametersofthepulsaranditsbasicspectral and polarization properties. We also present evidence for pulse profile evolution, which is likely due to geodetic precession, a relativistic effect caused by the misalignment of the pulsar spin and total angular momentum vectors. Our measurements show that PSR J1906+0746 is a young object with a characteristic age of 112 kyr. From the measured rate of orbital periastron advance (7N57 � 0N03 yr � 1 ), we infer a total system mass of 2:61 � 0:02 M� . While these parameters suggest that the PSR J1906+0746 binary system might be a younger version of the double pulsar system, intensive searches for radio pulses from the companion have so far been unsuccessful. It is therefore not known whether the companion is another neutron star or a massive white dwarf. Regardless of the nature of the companion, a simple calculation suggests that the Galactic birthrate of binaries similar to PSR J1906+0746is � 60Myr � 1 .ThisimpliesthatPSRJ1906+0746willmakeasignificantcontributiontothecomputed cosmic inspiral rate of compact binary systems. Subject headingg pulsars: general — pulsars: individual (PSR J1906+0746)

Neutron Star Kicks in Isolated and Binary Pulsars: Observational Constraints and Implications for Kick Mechanisms

We study observational constraints on neutron star (NS) kicks for isolated pulsars and for NSs in binary systems. We are particularly interested in the evidence of kick-spin alignment/misalignment and its dependence on the NS initial spin period. For several young pulsars, X-ray observations of compact nebulae showed that pulsar proper motion is aligned with the spin direction as defined by the symmetry axis of the nebula. We also critically examine the measurements of the proper motion and the projected spin axis from a large sample of pulsars with well-calibrated polarization data. We find that among the two dozen pulsars for which reliable measurements are available, there is a significant correlation between the spin axis and the proper motion. For various NS binaries, including double NS systems, binaries with massive main-sequence star companions, and binaries with massive white-dwarf companions, we obtain constraints on the kick magnitudes and directions from the observed orbital characteristics of the system. We find that the kick velocity is misaligned with the NS spin axis in a number of systems, and the NS spin period (when available) in these systems is generally longer than several hundred milliseconds. These constraints, together with the spin-kick alignment observed in many isolated pulsars, suggest that the kick timescale is hundreds of milliseconds to 1 s, so that spin-kick alignment or misalignment can be obtained depending on the initial spin period of the NS. We discuss the implication of our result for various NS kick mechanisms.

The Spatial Energy Spectrum of Magnetic Fields in Our Galaxy

Interstellar magnetic fields exist over a broad range of spatial scales, extending from large Galactic scales (~10 kpc) down to very small dissipative scales (1 pc). In this paper, we use a set of 490 pulsars distributed over roughly one-third of the Galactic disk out to a radius R 10 kpc (assuming R☉ = 8.5 kpc) and combine their observed rotation and dispersion measures with their estimated distances to derive the spatial energy spectrum of the Galactic interstellar magnetic field over the scale range 0.5-15 kpc. We obtain a nearly flat spectrum, with a one-dimensional power-law index α = -0.37 ± 0.10 for EB(k) = Ckα and an rms field strength of approximately 6 μG over the relevant scales. Our study complements the derivation of the magnetic energy spectrum over the scale range 0.03-100 pc by Minter & Spangler, showing that the magnetic spectrum becomes flatter at larger scales. This observational result is discussed in the framework of current theoretical and numerical models.

The observed spiral structure of the Milky Way

Context. The spiral structure of the Milky Way is not yet well determined. The keys to understanding this structure are to increase the number of reliable spiral tracers and to determine their distances as accurately as possible. HII regions, giant mol ecular clouds (GMCs), and 6.7-GHz methanol masers are closely related to high mass star formation, and hence they are excellent spiral tracers. The distances for many of them have been determined in literature with trigonometric, photometric and/or kinematic methods. Aims. We update the catalogs of Galactic HII regions, GMCs, and 6.7-GHz methanol masers, and then outline the spiral structure of the Milky Way. Methods. We collected data for more than 2500 known HII regions, 1300 GMCs, and 900 6.7-GHz methanol masers. If the photometric or trigonometric distance was not yet available, we determined the kinematic distance using a Galaxy rotation curve with the current IAU standard, R0 = 8.5 kpc and �0 = 220 km s −1 , and the most recent updated values of R0 = 8.3 kpc and �0 = 239 km s −1 , after we modified the velocities of tracers with the adopted s olar motions. With the weight factors based on the excitation parameters of HII regions or the masses of GMCs, we get the distributions of these spiral tracers. Results. The distribution of tracers shows at least four segments of arms in the first Galactic quadrant, and three segments in the f ourth quadrant. The Perseus Arm and the Local Arm are also delineated by many bright HII regions. The arm segments traced by massive star forming regions and GMCs are able to match the HI arms in the outer Galaxy. We found that the models of three-arm and fourarm logarithmic spirals are able to connect most spiral tracers. The four-arm model provides a better match to the observed tangential directions of spiral arms, and is consistent with the two-fo ld symmetry of the Galaxy structure. A model of polynomial-logarithmic spirals not only delineates the tracer distribution but als o matches the observed tangential directions.

Circular polarization in pulsar integrated profiles

We present a systematic study of the circular polarization in pulsar integrated profiles, based on published polarization data. For core components, we find no significant correlation between the sense change of circular polarization and the sense of linear position-angle variation. Circular polarization is not restricted to core components and, in some cases, reversals of circular polarization sense are observed across the conal emission. In conal double profiles, the sense of circular polarization is found to be correlated with the sense of position-angle variation. Pulsars with a high degree of linear polarization often have one hand of circular polarization across the whole profile. For most pulsars, the sign of circular polarization is the same at 50-cm and 20-cm wavelengths, and the degree of polarization is similar, albeit with a wide scatter. However, at least two cases of frequency-dependent sign reversals are known. This diverse behaviour may require more than one mechanism to generate circular polarization.