Showing papers by "Australia Telescope National Facility published in 2006"

TEMPO2, a new pulsar-timing package - I. An overview

Abstract: Contemporary pulsar-timing experiments have reached a sensitivity level where systematic errors introduced by existing analysis procedures are limiting the achievable science. We have developed TEMPO2, a new pulsar-timing package that contains propagation and other relevant effects implemented at the 1-ns level of precision (a factor of ∼100 more precise than previously obtainable). In contrast with earlier timing packages, TEMPO2 is compliant with the general relativistic framework of the IAU 1991 and 2000 resolutions and hence uses the International Celestial Reference System, Barycentric Coordinate Time and up-to-date precession, nutation and polar motion models. TEMPO2 provides a generic and extensible set of tools to aid in the analysis and visualization of pulsar-timing data. We provide an overview of the timing model, its accuracy and differences relative to earlier work. We also present a new scheme for predictive use of the timing model that removes existing processing artefacts by properly modelling the frequency dependence of pulse phase.

Transient radio bursts from rotating neutron stars

Abstract: A previously unknown population of superdense neutron stars has been identified. Termed RRATs, for rotating radio transients, their radio emissions vary on very short timescales, with bursts of 2 to 30 milliseconds occurring at intervals of 4 minutes to 3 hours. Eleven such objects have been found so far but their ephemeral nature suggests that there are many more in our Galaxy that remain unseen. This discovery implies a several-fold increase in the estimated radio-pulsar population, currently about 100,000. The radio sky is relatively unexplored for transient signals1, although the potential of radio-transient searches is high. This was demonstrated recently by the discovery of a previously unknown type of source2,3, varying on timescales of minutes to hours. Here we report a search for radio sources that vary on much shorter timescales. We found eleven objects characterized by single, dispersed bursts having durations between 2 and 30 ms. The average time intervals between bursts range from 4 min to 3 h with radio emission typically detectable for 4 s, and the rate of change of the pulse period has been measured for three of them; for one source, we have inferred a high magnetic field strength of 5 × 1013 G. This suggests that the new population is related to other classes of isolated neutron stars observed at X-ray and γ-ray wavelengths4.

tempo2, a new pulsar timing package ¿ II. The timing model and precision estimates

Abstract: Tempo2 is a new software package for the analysis of pulsar pulse times of arrival. In this paper we describe in detail the timing model used by tempo2, and discuss limitations on the attainable precision. In addition to the intrinsic slow-down behaviour of the pulsar, tempo2 accounts for the effects of a binary orbital motion, the secular motion of the pulsar or binary system, interstellar, Solar system and ionospheric dispersion, observatory motion (including Earth rotation, precession, nutation, polar motion and orbital motion), tropospheric propagation delay, and gravitational time dilation due to binary companions and Solar system bodies. We believe the timing model is accurate in its description of predictable systematic timing effects to better than one nanosecond, except in the case of relativistic binary systems where further theoretical development is needed. The largest remaining sources of potential error are measurement error, interstellar scattering, Solar system ephemeris errors, atomic clock instability and gravitational waves.

The Parkes Multibeam Pulsar Survey - VI. Discovery and timing of 142 pulsars and a Galactic population analysis

Abstract: We present the discovery and follow-up observations of 142 pulsars found in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. These new discoveries bring the total number of pulsars found by the survey to 742. In addition to tabulating spin and astrometric parameters, along with pulse width and flux density information, we present orbital characteristics for 13 binary pulsars which form part of the new sample. Combining these results from another recent Parkes multibeam survey at high Galactic latitudes, we have a sample of 1008 normal pulsars which we use to carry out a determination of their Galactic distribution and birth rate. We infer a total Galactic population of 30 000 ± 1100 potentially detectable pulsars (i.e. those beaming towards us) having 1.4-GHz luminosities above 0.1 mJy kpc 2 . Adopting the Tauris & Manchester beaming model, this translates to a total of 155 000 ± 6000 active radio pulsars in the Galaxy above this luminosity limit. Using a pulsar current analysis, we derive the birth rate of this population to be 1.4 ± 0.2 pulsars per century. An important conclusion from our work is that the inferred radial density function of pulsars depends strongly on the assumed distribution of free electrons in the Galaxy. As a result, any analyses using the most recent electron model of Cordes & Lazio predict a dearth of pulsars in the inner Galaxy. We show that this model can also bias the inferred pulsar scaleheight with respect to the Galactic plane. Combining our results with other Parkes multibeam surveys we find that the population is best described by an exponential distribution with a scaleheight of 330 pc. Surveys underway at Parkes and Arecibo are expected to improve the knowledge of the radial distribution outside the solar circle, and to discover several hundred new pulsars in the inner Galaxy.

Transient pulsed radio emission from a magnetar

Abstract: Anomalous X-ray pulsars (AXPs) are slowly rotating neutron stars with very bright and highly variable X-ray emission that are believed to be powered by ultra-strong magnetic fields of >10(14) G, according to the 'magnetar' model. The radio pulsations that have been observed from more than 1,700 neutron stars with weaker magnetic fields have never been detected from any of the dozen known magnetars. The X-ray pulsar XTE J1810-197 was revealed (in 2003) as the first AXP with transient emission when its luminosity increased 100-fold from the quiescent level; a coincident radio source of unknown origin was detected one year later. Here we show that XTE J1810-197 emits bright, narrow, highly linearly polarized radio pulses, observed at every rotation, thereby establishing that magnetars can be radio pulsars. There is no evidence of radio emission before the 2003 X-ray outburst (unlike ordinary pulsars, which emit radio pulses all the time), and the flux varies from day to day. The flux at all radio frequencies is approximately equal--and at >20 GHz XTE J1810-197 is currently the brightest neutron star known. These observations link magnetars to ordinary radio pulsars, rule out alternative accretion models for AXPs, and provide a new window into the coronae of magnetars.

The VLA Galactic Plane Survey

Abstract: The VLA Galactic Plane Survey (VGPS) is a survey of H I and 21 cm continuum emission in the Galactic plane between longitude 18° and 67° with latitude coverage from |b| < 13 to |b| < 23. The survey area was observed with the Very Large Array in 990 pointings. Short-spacing information for the H I line emission was obtained by additional observations with the Green Bank Telescope. H I spectral line images are presented with a resolution of 1' × 1' × 1.56 km s-1 (FWHM) and an rms noise of 2 K per 0.824 km s-1 channel. Continuum images made from channels without H I line emission have 1' (FWHM) resolution. The VGPS images reveal structures of atomic hydrogen and 21 cm continuum as large as several degrees with unprecedented resolution in this part of the Galaxy. With the completion of the VGPS, it is now possible for the first time to assess the consistency between arcminute-resolution surveys of Galactic H I emission. VGPS images are compared with images from the Canadian Galactic Plane Survey (CGPS) and the Southern Galactic Plane Survey (SGPS). In general, the agreement between these surveys is impressive, considering the differences in instrumentation and image-processing techniques used for each survey. The differences between VGPS and CGPS images are small, 6 K (rms) in channels in which the mean H I brightness temperature in the field exceeds 80 K. A similar degree of consistency is found between the VGPS and SGPS. The agreement we find between arcminute-resolution surveys of the Galactic plane is a crucial step toward combining these surveys into a single uniform data set that covers 90% of the Galactic disk: the International Galactic Plane Survey. The VGPS data will be made available on the World Wide Web through the Canadian Astronomy Data Centre.

Pulsar rotation measures and the large-scale structure of the galactic magnetic field

Abstract: The large-scale magnetic field of our Galaxy can be probed in three dimensions using Faraday rotation of pulsar signals.Wereportonthedeterminationof223rotationmeasuresfrompolarizationobservationsofrelativelydistant southern pulsars made using the Parkes radio telescope. Combined with previously published observations, these data give clear evidence for large-scale counterclockwise fields (viewed from the north Galactic pole) in the spiral arms interior to the Sun and weaker evidence for a counterclockwise field in the Perseus arm. However, in interarm regions, including the solar neighborhood, we present evidence that suggests that large-scale fields are clockwise. Weproposethatthelarge-scaleGalacticmagneticfieldhasabisymmetricstructurewithreversalsontheboundaries of the spiral arms. Streaming motions associated with spiral density waves can directly generate such a structure from an initial, inwardly directed radial field. Large-scale fields increase toward the Galactic center, with a mean value of about 2 � G in the solar neighborhood and 4 � G at a galactocentric radius of 3 kpc. Subject headingg galaxies: magnetic fields — Galaxy: structure — ISM: magnetic fields — pulsars: general Online material: color figures

Upper bounds on the low-frequency stochastic gravitational wave background from pulsar timing observations: current limits and future prospects

Abstract: Using a statistically rigorous analysis method, we place limits on the existence of an isotropic stochastic gravitational wave background using pulsar timing observations. We consider backgrounds whose characteristic strain spectra may be described as a power-law dependence with frequency. Such backgrounds include an astrophysical background produced by coalescing supermassive black-hole binary systems and cosmological backgrounds due to relic gravitational waves and cosmic strings. Using the best available data, we obtain an upper limit on the energy density per unit logarithmic frequency interval of Ω h2 ≤ 1.9 × 10-8 for an astrophysical background that is 5 times more stringent than the earlier limit of 1.1 × 10-7 found by Kaspi and colleagues. We also provide limits on a background due to relic gravitational waves and cosmic strings of Ω h2 ≤ 2.0 × 10-8 and Ω h2 ≤ 1.9 × 10-8, respectively. All of the quoted upper limits correspond to a 0.1% false alarm rate together with a 95% detection rate. We discuss the physical implications of these results and highlight the future possibilities of the Parkes Pulsar Timing Array project. We find that our current results can (1) constrain the merger rate of supermassive binary black hole systems at high redshift, (2) rule out some relationships between the black hole mass and the galactic halo mass, (3) constrain the rate of expansion in the inflationary era, and (4) provide an upper bound on the dimensionless tension of a cosmic string background.

The Parkes multibeam pulsar survey: VI. Discovery and timing of 142 pulsars and a Galactic population analysis

Abstract: [ABRIDGED] We present the discovery and follow-up observations of 142 pulsars found in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. These new discoveries bring the total number of pulsars found by the survey to 742. In addition to tabulating spin and astrometric parameters, along with pulse width and flux density information, we present orbital characteristics for 13 binary pulsars which form part of the new sample. Combining these results from another recent Parkes multibeam survey at high Galactic latitudes, we have a sample of 1008 normal pulsars which we use to carry out a determination of their Galactic distribution and birth rate. We infer a total Galactic population of 30000 +/- 1100 potentially detectable pulsars (i.e. those beaming towards us) having 1.4-GHz luminosities above 0.1 mJy kpc squared. Using a pulsar current analysis, we derive the birth rate of this population to be 1.4 +/- 0.2 pulsars per century. An important conclusion from our work is that the inferred radial density function of pulsars depends strongly on the assumed distribution of free electrons in the Galaxy. As a result, any analyses using the most recent electron model of Cordes & Lazio predict a dearth of pulsars in the inner Galaxy. We show that this model can also bias the inferred pulsar scale height with respect to the Galactic plane. Combining our results with other Parkes multibeam surveys we find that the population is best described by an exponential distribution with a scale height of 330 pc.

The VLA Galactic Plane Survey

Abstract: The VLA Galactic Plane Survey (VGPS) is a survey of HI and 21-cm continuum emission in the Galactic plane between longitude 18 degrees 67 degr. with latitude coverage from |b| < 1.3 degr. to |b| < 2.3 degr. The survey area was observed with the Very Large Array (VLA) in 990 pointings. Short-spacing information for the HI line emission was obtained by additional observations with the Green Bank Telescope (GBT). HI spectral line images are presented with a resolution of 1 arcmin x 1 arcmin x 1.56 km/s (FWHM) and rms noise of 2 K per 0.824 km/s channel. Continuum images made from channels without HI line emission have 1 arcmin (FWHM) resolution. VGPS images are compared with images from the Canadian Galactic Plane Survey (CGPS) and the Southern Galactic Plane Survey (SGPS). In general, the agreement between these surveys is impressive, considering the differences in instrumentation and image processing techniques used for each survey. The differences between VGPS and CGPS images are small, < 6 K (rms) in channels where the mean HI brightness temperature in the field exceeds 80 K. A similar degree of consistency is found between the VGPS and SGPS. The agreement we find between arcminute resolution surveys of the Galactic plane is a crucial step towards combining these surveys into a single uniform dataset which covers 90% of the Galactic disk: the International Galactic Plane Survey (IGPS). The VGPS data will be made available on the World Wide Web through the Canadian Astronomy Data Centre (CADC).

Upper bounds on the low-frequency stochastic gravitational wave background from pulsar timing observations: current limits and future prospects

Abstract: Using a statistically rigorous analysis method, we place limits on the existence of an isotropic stochastic gravitational wave background using pulsar timing observations. We consider backgrounds whose characteristic strain spectra may be described as a power-law dependence with frequency. Such backgrounds include an astrophysical background produced by coalescing supermassive black-hole binary systems and cosmological backgrounds due to relic gravitational waves and cosmic strings. Using the best available data, we obtain an upper limit on the energy density per unit logarithmic frequency interval of \Omega^{\rm SMBH}_g(1/8yr) h^2 <= 1.9 x 10^{-8} for an astrophysical background which is five times more stringent than the earlier Kaspi et al. (1994) limit of 1.1 x 10^{-7}. We also provide limits on a background due to relic gravitational waves and cosmic strings of \Omega^{\rm relic}_g(1/8yr) h^2 <= 2.0 x 10^{-8} and \Omega^{\rm cs}_g(1/8yr) h^2 <= 1.9 x 10^{-8} respectively. All of the quoted upper limits correspond to a 0.1% false alarm rate together with a 95% detection rate. We discuss the physical implications of these results and highlight the future possibilities of the Parkes Pulsar Timing Array project. We find that our current results can 1) constrain the merger rate of supermassive binary black hole systems at high red shift, 2) rule out some relationships between the black hole mass and the galactic halo mass, 3) constrain the rate of expansion in the inflationary era and 4) provide an upper bound on the dimensionless tension of a cosmic string background.

The Survey for Ionization in Neutral Gas Galaxies. I. Description and initial results

Abstract: We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG), a census of star formation in H I selected galaxies. The survey consists of H alpha and R-band imaging of a sample of 468 galaxies selected from the H I Parkes All Sky Survey (HIPASS). The sample spans three decades in H I mass and is free of many of the biases that affect other star-forming galaxy samples. We present the criteria for sample selection, list the entire sample, discuss our observational techniques, and describe the data reduction and calibration methods. This paper focuses on 93 SINGG targets whose observations have been fully reduced and analyzed to date. The majority of these show a single emission line galaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs in a single field. All of the targets in this sample are detected in H alpha, indicating that dormant (non-star-forming) galaxies with M-H I greater than or similar to 3x10(7) M-circle dot are very rare. A database of the measured global properties of the ELGs is presented. The ELG sample spans 4 orders of magnitude in luminosity (H alpha and R band), and H alpha surface brightness, nearly 3 orders of magnitude in R surface brightness and nearly 2 orders of magnitude in H alpha equivalent width (EW). The surface brightness distribution of our sample is broader than that of the Sloan Digital Sky Survey (SDSS) spectroscopic sample, the EW distribution is broader than prism-selected samples, and the morphologies found include all common types of star-forming galaxies (e.g., irregular, spiral, blue compact dwarf, starbursts, merging and colliding systems, and even residual star formation in S0 and Sa spirals). Thus, SINGG presents a superior census of star formation in the local universe suitable for further studies ranging from the analysis of H II regions to determination of the local cosmic star formation rate density.

The subpulse modulation properties of pulsars at 21 cm

Abstract: We present the results of a systematic, unbiased search for subpulse modulation of 187 pulsars performed with the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands at an observing wavelength of 21 cm. Using new observations and archival WSRT data we have increased the list of pulsars that show the drifting subpulse phenomenon by 42, indicating that at least one in three pulsars exhibits this phenomenon. The real fraction of pulsars that show the drifting phenomenon is likely to be larger than 55%. The majority of the analysed pulsars show subpulse modulation (170), of which the majority were not previously known to show subpulse modulation and 30 show clear systematic drifting. The large number of new drifters we have found allows us, for the first time, to do meaningful statistics on the drifting phenomenon. We find that the drifting phenomenon is correlated with the pulsar age such that drifting is more likely to occur in older pulsars. Pulsars that drift more coherently seem to be older and have a lower modulation index. There is no significant correlation found between P3 and other pulsar parameters (such as the pulsar age), as has been reported in the past. There is no significant preference of drift direction and the drift direction is not found to be correlated with pulsar parameters. None of the four complexity parameters predicted by different emission models are shown to be inconsistent with the set of modulation indices of our sample of pulsars. Therefore none of the models can be ruled out based on our observations. We also present results on some interesting new individual sources like a pulsar that shows similar subpulse modulation in both the main- and interpulse and six pulsars with opposite drift senses in different components.

The Northern HIPASS catalogue - data presentation, completeness and reliability measures

Abstract: The Northern HIPASS catalogue (NHICAT) is the northern extension of the HIPASS catalogue, HICAT. This extension adds the sky area between the declination (Dec.) range of +2 degrees 300 km s(-1). Sources with -300 < nu(hel) < 300 km s(-1) were excluded to avoid contamination by Galactic emission. In total, the entire HIPASS survey has found 5317 galaxies identified purely by their HI content. The full galaxy catalogue is publicly available at http://hipass.aus-vo.org.

A CH3CN and HCO+ survey towards southern methanol masers associated with star formation

Abstract: We present the initial results of a 3-mm spectral-line survey towards 83 methanol maser-selected massive star-forming regions. Here, we report observations of the J = 5–4 and 6–5 rotational transitions of methyl cyanide (CH 3 CN) and the J = 1–0 transition of HCO + and H 13 CO + .CH 3 CN emission is detected in 58 sources (70 per cent of our sample). We estimate the temperature and column density for 37 of these using the rotational diagram (RD) method. The temperatures we derive range from 28–166 K, and are lower than previously reported temperatures, derived from higher J transitions. We find that CH 3 CN is brighter and more commonly detected towards ultracompact H ii (UCH ii) regions than towards isolated maser sources. Detection of CH 3 CN towards isolated maser sources strongly suggests that these objects are internally heated and that CH 3 CN is excited prior to the UCH ii phase of massive star formation.HCO + is detected towards 82 sources (99 per cent of our sample), many of which exhibit asymmetric line profiles compared to H 13 CO + . Skewed profiles are indicative of inward or outward motions, however, we find approximately equal numbers of red- and blue-skewed profiles among all classes. Column densities are derived from an analysis of the HCO + and H 13 CO + line profiles.80 sources have mid-infrared (mid-IR) counterparts: 68 seen in emission and 12 seen in absorption as ‘dark clouds’. Seven of the 12 dark clouds exhibit asymmetric HCO + profiles, six of which are skewed to the blue, indicating infalling motions. CH 3 CN is also common in dark clouds, where it has a 90 per cent detection rate.

Arecibo pulsar survey using ALFA. II. the young, highly relativistic binary pulsar J1906+0746

Abstract: 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)

The Parkes High-Latitude pulsar survey

Abstract: The Parkes High-Latitude pulsar survey covers a region of the sky enclosed by Galactic longitudes 220° < l < 260° and Galactic latitudes |b| < 60°. The observations have been performed using the 20-cm multibeam receiver on the Parkes 64-m radio telescope. A total of 6456 point-ings of 265 s each have been collected. The system adopted provided a sensitivity limit, for long-period pulsars with 5 per cent duty cycles, of ∼0.5 mJy. Data analysis resulted in the detection of 42 pulsars of which 18 were new discoveries. Four of these belong to the class of the millisecond - or recycled - pulsars; three of these four are in binary systems. The double pulsar system J0737-3039 is among those and has been presented elsewhere. Here, we discuss the other discoveries and provide timing parameters for the objects for which we have a phase-connected solution.

The Southern Galactic Plane Survey: Polarized Radio Continuum Observations and Analysis

Abstract: The Southern Galactic Plane Survey (SGPS) is a radio survey in the 21 cm H I line and in 1.4 GHz full-polarization continuum, observed with the Australia Telescope Compact Array and the Parkes 64 m single-dish telescope. The survey spans a Galactic longitude of 253° < l < 358° and a latitude of |b| < 15 at a resolution of 100'' and a sensitivity below 1 mJy beam-1. This paper presents interferometer only polarized continuum survey data and describes the data taking, analysis processes, and data products. The primary data products are the four Stokes parameters I, Q, U, and V in 25 overlapping fields of 55 by 3°, from which polarized intensity, polarization angle, and rotation measure are calculated. We describe the effects of missing short spacings, and discuss the importance of the polarized continuum data in the SGPS for studies of fluctuations and turbulence in the ionized interstellar medium and for studying the strength and structure of the Galactic magnetic field.

Class I methanol masers in the outflow of IRAS 16 547–4247

Abstract: The Australia Telescope Compact Array (ATCA) has been used to image class I methanol masers at 9.9, 25 (a series from J = 2 to 9), 84, 95 and 104 GHz located in the vicinity of IRAS 16 547-4247 (G343.12-0.06), a luminous young stellar object known to harbour a radio jet. The detected maser emission consists of a cluster of six spots spread over an area of 30 arcsec. Five spots were detected in only the 84- and 95-GHz transitions (for two spots the 84-GHz detection is marginal), while the sixth spot shows activity in all 12 observed transitions. We report the first interferometric observations of the rare 9.9- and 104-GHz masers. It is shown that the spectra contain a very narrow spike (< 0.03 km s -1) and the brightness temperature in these two transitions exceeds 5.3 × 10 7 and 2.0 × 10 4K, respectively. The three most southern maser spots show a clear association with the shocked gas traced by the H 2 2.12-μm emission associated with the radio jet and their velocities are close to that of the molecular core within which the jet is embedded. This fact supports the idea that the class I masers reside in the interface regions of outflows. Comparison with OH masers and infrared data reveals a potential discrepancy in the expected evolutionary state. The presence of the OH masers usually means that the source is evolved, but the infrared data suggest otherwise. The lack of any class II methanol maser emission at 6.7 GHz in the source raises an additional question, Is this source too young or too old to have a 6.7-GHz maser? We argue that both cases are possible and suggest that the evolutionary stage where the class I masers are active, may last longer and start earlier than when the class II masers are active. However, it is currently not possible to reveal the exact evolutionary status of IRAS 16 547-4247. © 2006 RAS.

The properties of extragalactic radio sources selected at 20 GHz

Abstract: We present some first results on the variability, polarization and general properties of radio sources selected at 20 GHz, the highest frequency at which a sensitive radio survey has been carried out over a large area of sky. Sources with flux densities above 100 mJy in the Australia

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Abstract: Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan's atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titan's atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

XMM–Newton observations of PSR B1259−63 near the 2004 periastron passage

Abstract: PSR B1259−63 is in a highly eccentric 3.4-yr orbit with a Be star and crosses the Be star disc twice per orbit, just prior to and just after periastron. Unpulsed radio, X-ray and gamma-ray emission observed from the binary system is thought to be due to the collision of pulsar wind with the wind of Be star. We present here the results of new XMM–Newton observations of the PSR B1259−63 system during the beginning of 2004 as the pulsar approached the disc of the Be star. We combine these results with the earlier unpublished X-ray data from BeppoSAX and XMM–Newton as well as with the ASCA data. The detailed X-ray light curve of the system shows that the pulsar passes (twice per orbit) through a well-defined Gaussian-profile disc with the half-opening angle (projected on the pulsar orbit plane) . The intersection of the disc middle plane with the pulsar orbital plane is inclined at θdisc≃ 70° to the major axis of the pulsar orbit. Comparing the X-ray light curve to the TeV light curve of the the system, we find that the increase of the TeV flux some 10–100 d after the periastron passage is unambiguously related to the disc passage. At the moment of entrance to the disc, the X-ray photon index hardens from Γ≃ 1.8 up to ≃1.2 before returning to the steeper value Γ≥ 1.5. Such behaviour is not easily accounted for by the model in which the X-ray emission is synchrotron emission from the shocked pulsar wind. We argue that the observed hardening of the X-ray spectrum is due to the inverse-Compton or bremsstrahlung emission from 10–100 MeV electrons responsible for the radio synchrotron emission.

Discovery of 14 Radio Pulsars in a Survey of the Magellanic Clouds

Abstract: A systematic survey of the Large and Small Magellanic Clouds for radio pulsars using the Parkes radio telescope and the 20 cm multibeam receiver has resulted in the discovery of 14 pulsars and the redetection of five of the eight previously known spin-powered pulsars believed to lie in the Magellanic Clouds. Of the 14 new discoveries, 12 are believed to lie within Clouds, three in the Small Cloud and nine in the Large Cloud, bringing the total number of known spin-powered pulsars in the Clouds to 20. Averaged over all positions within the survey area, the survey had a limiting flux density of about 0.12 mJy. Observed dispersion measures suggest that the mean free electron density in the Magellanic Clouds is similar to that in the disk of our Galaxy. The observed radio luminosities have little or no dependence on pulsar period or characteristic age and the differential luminosity function is consistent with a power-law slope of -1, as is observed for Galactic pulsars.

Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud

Abstract: We present new high-resolution (100'') neutral hydrogen (H I) self-absorption images of the Riegel-Crutcher cloud obtained with the Australia Telescope Compact Array and the Parkes Radio Telescope. The Riegel-Crutcher cloud lies in the direction of the Galactic center at a distance of 125 ± 25 pc. Our observations resolve the very large, nearby sheet of cold hydrogen into a spectacular network of dozens of hairlike filaments. Individual filaments are remarkably elongated, being up to 17 pc long with widths of less than ~0.1 pc. The strands are reasonably cold, with spin temperatures of ~40 K and in many places appearing to have optical depths larger than 1. Comparing the H I images with observations of stellar polarization, we show that the filaments are very well aligned with the ambient magnetic field. We argue that the structure of the cloud has been determined by its magnetic field. In order for the cloud to be magnetically dominated the magnetic field strength must be >30 μG.

Enhanced small-scale faraday rotation in the galactic spiral arms

Abstract: We present an analysis of the rotation measures (RMs) of polarized extragalactic point sources in the Southern Galactic Plane Survey. This work demonstrates that the statistics of fluctuations in RM differ for the spiral arms and the interarm regions. Structure functions of RM are flat in the spiral arms, while they increase in the interarms. This indicates that there are no correlated RM fluctuations in the magnetoionized interstellar medium in the spiral arms on scales larger than ~05, corresponding to ~17 pc in the nearest spiral arm probed. The nonzero slopes in interarm regions imply a much larger scale of RM fluctuations. We conclude that fluctuations in the magnetoionic medium in the Milky Way spiral arms are not dominated by the mainly supernova-driven turbulent cascade in the global ISM but are probably due to a different source, most likely H II regions.

A multiresolution analysis of the radio-FIR correlation in the Large Magellanic Cloud

Abstract: We investigate the local correlation between the 1.4-GHz radio continuum and 60-μm far-infrared (FIR) emission within the Large Magellanic Cloud (LMC) on spatial scales between 0.05 and 1.5 kpc. On scales below ∼1 kpc, the radio-FIR correlation is clearly better than the correlation of the cold gas tracers (CO and H i) with either the radio or the FIR emission. For the LMC as a whole, there is a tight correlation between the radio and FIR emission on spatial scales above ∼50 pc. By decomposing the radio emission into thermal and non-thermal components, however, we show that the scale on which the radio-FIR correlation breaks down depends on the thermal fraction of the radio emission; regions that show a strong correlation to very small scales are the same regions where the thermal fraction of the radio emission is high. Contrary to previous studies of the local radio-FIR correlation in the LMC, we show that the slope of the relation between the radio and FIR emission is non-linear. In bright star-forming regions, the radio emission increases faster than linearly with respect to the FIR emission (power-law slope of ∼1.2), whereas a flatter slope of ∼0.6-0.9 applies more generally across the LMC. Our results are consistent with a scenario in which the ultraviolet photons and cosmic rays in the LMC have a common origin in massive star formation, but the cosmic rays are able to diffuse away from their production sites. Our results do not provide direct evidence for coupling between the magnetic field and the local gas density, but we note that synchrotron emission may not be a good tracer of the magnetic field if cosmic rays can readily escape the LMC.

A search for distant radio galaxies from SUMSS and NVSS – III. Radio spectral energy distributions and the z–α correlation

Abstract: This is the third in a series of papers that present observations and results for a sample of 76 ultrasteep-spectrum radio sources designed to find galaxies at high redshift. Here we present multifrequency radio observations, from the Australia Telescope Compact Array, for a subset of 37 galaxies from the sample. Matched resolution observations at 2.3, 4.8 and 6.2 GHz are presented for all galaxies, with the z < 2 galaxies additionally observed at 8.6 and 18 GHz. New angular size constraints are reported for 19 sources based on high-resolution 4.8- and 6.2-GHz observations. Functional forms for the rest-frame spectral energy distributions are derived: 89 per cent of the sample is well characterized by a single power law, whilst the remaining 11 per cent show some flattening towards higher frequencies: not one source shows any evidence for high-frequency steepening. We discuss the implications of this result in light of the empirical correlation between redshift and spectral index seen in flux-limited samples of radio galaxies. Finally, a new physical mechanism to explain the redshift - spectral index correlation is posited: extremely steep-spectrum radio galaxies in the local universe usually reside at the centres of rich galaxy clusters. We argue that if a higher fractions of radio galaxies, as a function of redshift, are located in environments with densities similar to nearby rich clusters, then this could be a natural interpretation for the correlation. We briefly outline our plans to pursue this line of investigation.

TEMPO2: a New Pulsar Timing Package

Abstract: TEMPO2 is a new pulsar timing package that provides the precision necessary for modern millisecond timing projects and should supersede all existing pulsar timing packages such as TEMPO and PSRTIME. As TEMPO2 is the only program available that can analyse multiple pulsar datasets simultaneously it will become an integral part of pulsar timing array projects which aim to detect the signatures of gravitational radiation in pulsar timing residuals. In this paper we describe the basic functionality of TEMPO2.

Representations of spectral coordinates in FITS

Abstract: Greisen & Calabretta (2002) describe a generalized method for specifying the coordinates of FITS data samples. Following that general method, Calabretta & Greisen (2002) describe detailed conventions for dening celestial coordinates as they are projected onto a two-dimensional plane. The present paper extends the discussion to the spectral coordinates of wavelength, frequency, and velocity. World coordinate functions are dened for spectral axes sampled linearly in wavelength, frequency, or velocity, linearly in the logarithm of wavelength or frequency, as projected by ideal dispersing elements, and as specied by a lookup table.

Southern GEMS groups – I. Dynamical properties

Abstract: Here, we present an investigation of the properties of 16 nearby galaxy groups and their constituent galaxies. The groups are selected from the Group Evolution Multiwavelength Study (GEMS) and all have X-ray as well as wide-field neutral hydrogen (H I) observations. Group membership is determined using a friends-of-friends algorithm on the positions and velocities from the 6-degree Field Galaxy Survey and NASA/IPAC Extragalactic Database. For each group we derive their physical properties using this membership, including: velocity dispersions (σ v), virial masses (M V), total K-band luminosities [LK(Tot)] and early-type fractions (f early) and present these data for the individual groups. We find that the GEMS X-ray luminosity is proportional to the group velocity dispersions and virial masses: L X(r 500) ∝ σ 3.11±0.59 v and L X(r 500) ∝ M 1.13±0.27 V , consistent with the predictions of self-similarity between group and clusters. We also find that M V ∝ LK(Tot) 2.0±0.9 , i.e. mass grows faster than light and that the fraction of early-type galaxies in the groups is correlated with the group X-ray luminosities and velocity dispersions. We examine the brightest group galaxies (BGGs), finding that, while the luminosity of the BGG correlates with its total group luminosity, the fraction of group luminosity contained in the BGG decreases with increasing total group luminosity. This suggests that BGGs grow by mergers at early times in group evolution while the group continues to grow by accreting infalling galaxies. We form a composite galaxy group in order to examine the properties of the constituent galaxies and compare their properties with those of field galaxies. There are clear radial trends, with group galaxies becoming fainter, bluer and morphologically later types with increasing radius from the group centre, reaching field levels at radii >r 500(>0.7r 200). We divide the composite group-by-group X-ray luminosity and find that galaxies in high X-ray luminosity groups [log10 L X(r 500) 41.7 erg s −1 ] are redder with a higher giant-to-dwarf ratio and are more likely to be early-type galaxies than are those galaxies in low X-ray luminosity groups. We conclude that harassment and ram-pressure stripping processes are unlikely to cause these differences. The differences are more likely to be due to galaxy‐galaxy mergers and possibly some further mechanism such as strangulation. If mergers are the dominant mechanism then the properties of galaxies in the higher X-ray luminosity groups are a result of mergers at earlier epochs in smaller mass groups that have since merged to become the structures we observe today, while lower X-ray luminosity groups are still undergoing mergers today.