Shami Chatterjee

Science with ASKAP: the Australian square-kilometre-array pathfinder

AbstractThe future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries. The SKA will be 50 times more sensitive than any existing radio facility. A majority of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from 300xa0MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is aimed squarely in this frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phase-array feed systems on parabolic reflectors. This large field-of-view makes ASKAP an unprecedented synoptic telescope poised to achieve substantial advances in SKA key science. The central core of ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of the sites selected by the international community as a potential location for the SKA. Following an introductory description of ASKAP, this document contains 7 chapters describing specific science programmes for ASKAP. In summary, the goals of these programmes are as follows: nThe detection of a million galaxies in atomic hydrogen emission across 75% of the sky out to a redshift of 0.2 to understand galaxy formation and gas evolution in the nearby Universe.The detection of synchrotron radiation from 60 million galaxies to determine the evolution, formation and population of galaxies across cosmic time and enabling key cosmological tests.The detection of polarized radiation from over 500,000 galaxies, allowing a grid of rotation measures at 10′ to explore the evolution of magnetic fields in galaxies over cosmic time.The understanding of the evolution of the interstellar medium of our own Galaxy and the processes that drive its chemical and physical evolution.The high-resolution imaging of intense, energetic phenomena by enlarging the Australian and global Very Long Baseline networks.The discovery and timing of a thousand new radio pulsars.The characterization of the radio transient sky through detection and monitoring of transient sources such as gamma ray bursts, radio supernovae and intra-day variables.The combination of location, technological innovation and scientific program will ensure that ASKAP will be a world-leading radio astronomy facility, closely aligned with the scientific and technical direction of the SKA. A brief summary chapter emphasizes the point, and considers discovery space.

The Vertical Structure of Warm Ionised Gas in the Milky Way

We present a new joint analysis of pulsar dispersion measures and diffuse Hα emission in the Milky Way, which we use to derive the density, pressure and filling factor of the thick disk component of the warm ionised medium (WIM) as a function of height above the Galactic disk. By excluding sightlines at low Galactic latitude that are contaminated by Hii regions and spiral arms, we find that the exponential scale-height of free electrons in the diffuse WIM is 1830–250+120 pc, a factor of two larger than has been derived in previous studies. The corresponding inconsistent scale heights for dispersion measure and emission measure imply that the vertical profiles of mass and pressure in the WIM are decoupled, and that the filling factor of WIM clouds is a geometric response to the competing environmental influences of thermal and non-thermal processes. Extrapolating the properties of the thick-disk WIM to mid-plane, we infer a volume-averaged electron density 0.014 ± 0.001 cm–3, produced by clouds of typical electron density 0.34 ± 0.06 cm–3 with a volume filling factor 0.04 ± 0.01. As one moves off the plane, the filling factor increases to a maximum of ~30% at a height of ≈1–1.5 kpc, before then declining to accommodate the increasing presence of hot, coronal gas. Since models for the WIM with a ≈1 kpc scale-height have been widely used to estimate distances to radio pulsars, our revised parameters suggest that the distances to many high-latitude pulsars have been substantially underestimated.

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.

Discovery of Pulsations and a Possible Spectral Feature in the X-ray Emission from Rotating Radio Transient J1819-1458

PSR J1819-1458 is a rotating radio transient (RRAT) source with an inferred surface dipole magnetic field strength of 5 × 1013 G and a 4.26 s spin period. We present XMM-Newton observations of the X-ray counterpart of this source, CXOU J181939.1-145804, in which we identify pulsations and a possible spectral feature. The X-ray pulsations are at the period predicted by the radio ephemeris, providing an unambiguous identification with the radio source and confirmation of its neutron star nature. The X-ray pulse has a 0.3-5 keV pulsed fraction of 34% and is aligned with the expected phase of the radio pulse. The X-ray spectrum is fit well by an absorbed blackbody with kT = 0.14 keV with the addition of an absorption feature at 1 keV, with total absorbed flux of 1.5 × 10-13 ergs cm-2 s-1 (0.3-5 keV). This absorption feature is well modeled by a Gaussian or resonant cyclotron scattering model, but its significance is dependent on the choice of continuum model. We find no evidence for any X-ray bursts or aperiodic variability on timescales of 6 ms to the duration of the observation and can place the most stringent limit to date of ≤3 × 10-9 ergs cm-2 s-1 on the absorbed 0.3-5 keV flux of any bursts.

A Precise Proper Motion for the Crab Pulsar, and the Difficulty of Testing Spin-Kick Alignment for Young Neutron Stars

We present a detailed analysis of archival Hubble Space Telescope data that we use to measure the proper motion of the Crab pulsar, with the primary goal of comparing the direction of its proper motion with the projected axis of its pulsar wind nebula (the projected spin axis of the pulsar). We demonstrate that our measurement, using 47 observations spanning >10?yr, is robust and has an uncertainty of only ?0.4?mas?yr?1 on each component of the proper motion. However, we then consider the various uncertainties that arise from the need to correct the proper motion that we measure to the local standard of rest at the position of the pulsar and find -->?? = ? 11.8 ? 0.4 ? 0.5 mas yr ?1 and -->?? = + 4.4 ? 0.4 ? 0.5 mas yr ?1 relative to the pulsars standard of rest, where the two uncertainties are from the measurement and the reference frame, respectively. Comparing this proper motion to the symmetry axis of the pulsar wind nebula, we must consider the unknown velocity of the pulsars progenitor (assumed to be ~10?km?s?1), and hence add an additional uncertainty of ?2?mas?yr?1 to each component of the proper motion. This implies a projected misalignment with the nebular axis of -->14?? 2?? 9?, consistent with a broad range of values including perfect alignment. We use our proper motion to derive an independent estimate for the site of the supernova explosion with an accuracy that is 2-3 times better than previous estimates. We conclude that the precision of individual measurements which compare the direction of motion of a neutron star to a fixed axis will often be limited by fundamental uncertainties regarding reference frames and progenitor properties.

VLBA measurement of the transverse velocity of the magnetar XTE J1810-197

We have obtained observations of the magnetar XTE J1810-197 with the Very Long Baseline Array (VLBA) at two epochs separated by 106 days, at wavelengths of 6 and 3.6 cm. Comparison of the positions yields a proper-motion value of 13.5 ± 1.0 mas yr-1 at an equatorial position angle of 209.4° ± 2.4° (east of north). This value is consistent with a lower significance proper-motion value derived from infrared observations of the source over the past 3 years, also reported here. Given its distance of 3.5 ± 0.5 kpc, the implied transverse velocity corrected to the local standard of rest is 212 ± 35 km s-1 (1 σ). The measured velocity is slightly below the average for normal young neutron stars, indicating that the mechanism(s) of magnetar birth need not lead to high neutron star velocities. We also use Australia Telescope Compact Array, Very Large Array, and these VLBA observations to set limits on any diffuse emission associated with the source on a variety of spatial scales, concluding that the radio emission from XTE J1810-197 is >96% pulsed.

Proper Motions of PSRs B1757–24 and B1951+32: Implications for Ages and Associations

Over the last decade, considerable effort has been made to measure the proper motions of the pulsars B1757–24 and B1951+32 in order to establish or refute associations with nearby supernova remnants and to understand better the complicated geometries of their surrounding nebulae. We present proper motion measurements of both pulsars with the Very Large Array, increasing the time baselines of the measurements from 3.9 yr to 6.5 yr and from 12.0 yr to 14.5 yr, respectively, compared to previous observations. We confirm the nondetection of proper motion of PSR B1757–24, and our measurement of (μα,μδ) = (− 11 ± 9, − 1 ± 15) mas yr−1 confirms that the association of PSR B1757–24 with SNR G5.4–1.2 is unlikely for the pulsar characteristic age of 15.5 kyr, although an association cannot be excluded for a significantly larger age. For PSR B1951+32, we measure a proper motion of (μα,μδ) = (− 28.8 ± 0.9, − 14.7 ± 0.9) mas yr−1, reducing the uncertainty in the proper motion by a factor of 2 compared to previous results. After correcting to the local standard of rest, the proper motion indicates a kinetic age of ~51 kyr for the pulsar, assuming it was born near the geometric center of the supernova remnant. The radio-bright arc of emission along the pulsar proper motion vector shows time-variable structure, but moves with the pulsar at an approximately constant separation ~2.5, lending weight to its interpretation as a shock structure driven by the pulsar.

CONSTRAINING THE PROPER MOTIONS OF TWO MAGNETARS

We attempt to measure the proper motions of two magnetars—the soft gamma-ray repeater SGR 1900+14 and the anomalous X-ray pulsar 1E 2259+586—using two epochs of Chandra observations separated by ~5 yr. We perform extensive tests using these data, archival data, and simulations to verify the accuracy of our measurements and understand their limitations. We find 90% upper limits on the proper motions of 54 mas yr–1 (SGR 1900+14) and 65 mas yr–1 (1E 2259+586), with the limits largely determined by the accuracy with which we could register the two epochs of data and by the inherent uncertainties on two-point proper motions. We translate the proper motions limits into limits on the transverse velocity using distances, and find v ⊥ < 1300 km s–1 (SGR 1900+14, for a distance of 5 kpc) and v ⊥ < 930 km s–1 (1E 2259+586, for a distance of 3 kpc) at 90% confidence; the range of possible distances for these objects makes a wide range of velocities possible, but it seems that the magnetars do not have uniformly high space velocities greater than 3000 km s–1. Unfortunately, our proper motions also cannot significantly constrain the previously proposed origins of these objects in nearby supernova remnants or star clusters, limited as much by our ignorance of ages as by our proper motions.

A 20-yr radio light curve for the young supernova remnant G1.9+0.3

The radio source G1.9+0.3 has recently been identified as the youngest known Galactic supernova remnant, with a putative age of� 100 years. We present a radio light curve for G1.9+0.3 based on 25 epochs of observation with the Molonglo Observatory Synthesis Telescope, spanning 20 years from 1988 to 2007. These observations are all at the same frequency (843 MHz) and comparable resolutions (43 00 � 91 00 or 43 00 � 95 00 ) and cover one fifth of the estimated lifetime of the supernova remnant. We find that the flux densit y has increased at a rate of 1:22� 0:24 0:16 per cent yr 1 over the last two decades, suggesting that G1.9+0.3 is undergoing a

VLBI Neutron Star Astrometry: Techniques and Initial Results

We outline some new techniques being used and present initial results from a pulsar astrometry program at the NRAO Very Long Baseline Array (VLBA). We find a proper motion of 89.0±0.4 mas/yr and a preliminary parallax of 1.05±0.25 mas for B0919+06.