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M. Haverkorn

Publications and source records attributed to M. Haverkorn.

At least 91 records · Page 5Linked to original sources
arXiv2008

CMBPol Mission Concept Study: Foreground Science Knowledge and Prospects

We report on our knowledge of Galactic foregrounds, as well as on how a CMB satellite mission aiming at detecting a primordial B-mode signal (CMBPol) will contribute to improving it. We review the observational and analysis techniques used to constrain the structure of the Galactic magnetic field, whose presence is responsible for the polarization of Galactic emissions. Although our current understanding of the magnetized interstellar medium is somewhat limited, dramatic improvements in our knowledge of its properties are expected by the time CMBPol flies. Thanks to high resolution and high sensitivity instruments observing the whole sky at frequencies between 30 GHz and 850 GHz, CMBPol will not only improve this picture by observing the synchrotron emission from our galaxy, but also help constrain dust models. Polarized emission from interstellar dust indeed dominates over any other signal in CMBPol's highest frequency channels. Observations at these wavelengths, combined with ground-based studies of starlight polarization, will therefore enable us to improve our understanding of dust properties and of the mechanism(s) responsible for the alignment of dust grains with the Galactic magnetic field. CMBPol will also shed new light on observations that are presently not well understood. Morphological studies of anomalous dust and synchrotron emissions will indeed constrain their natures and properties, while searching for fluctuations in the emission from heliospheric dust will test our understanding of the circumheliospheric interstellar medium. Finally, acquiring more information on the properties of extra-Galactic sources will be necessary in order to maximize the cosmological constraints extracted from CMBPol's observations of CMB lensing. (abridged)

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arXiv2008

Science with ASKAP - the Australian Square Kilometre Array Pathfinder

[ABRIDGED VERSION] The 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 300 MHz 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. 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.

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arXiv2008

A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud

We present a study of the magnetic field of the Small Magellanic Cloud (SMC), carried out using radio Faraday rotation and optical starlight polarization data. Consistent negative rotation measures (RMs) across the SMC indicate that the line-of-sight magnetic field is directed uniformly away from us with a strength 0.19 +/- 0.06 microGauss. Applying the Chandrasekhar-Fermi method to starlight polarization data yields an ordered magnetic field in the plane of the sky of strength 1.6 +/- 0.4 microGauss oriented at a position angle 4 +/- 12 degs, measured counter-clockwise from the great circle on the sky joining the SMC to the Large Magellanic Cloud (LMC). We construct a three-dimensional magnetic field model of the SMC, under the assumption that the RMs and starlight polarization probe the same underlying large-scale field. The vector defining the overall orientation of the SMC magnetic field shows a potential alignment with the vector joining the center of the SMC to the center of the LMC, suggesting the possibility of a "pan-Magellanic'' magnetic field. A cosmic-ray driven dynamo is the most viable explanation of the observed field geometry, but has difficulties accounting for the observed uni-directional field lines. A study of Faraday rotation through the Magellanic Bridge is needed to further test the pan-Magellanic field hypothesis.

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arXiv2008

PGMS: to study the Galactic magnetism out of the Galactic plane

The Parkes Galactic Meridian Survey (PGMS) is a 5 deg X 90 deg strip to map the polarized synchrotron emission along a Galactic meridian from the Galactic plane down to the south Galactic pole. The survey is carried out at the Parkes radio telescope at a frequency of 2.3 GHz with 30 adjacent 8-MHz bands which enable Faraday Rotation studies. The scientific goal is twofold: (1) To probe the Galactic magnetism off the Galactic plane of which little is known so far. PGMS gives an insight into the Galactic magnetic field in the thick disc, halo, and disc-halo transition; (2) To study the synchrotron emission as foreground noise of the CMB Polarization, especially for the weak B-Mode which carries the signature of the primordial gravitational wave background left by the Inflation. PGMS observations have been recently concluded. In this contribution we present the survey along with first results.

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arXiv2008

Discovery of the radio and X-ray counterpart of TeV Gamma-ray source HESS J1731-347

We discover a faint shell-type radio and X-ray source, G353.6-0.7, associated with HESS J1731-347. G353.6-0.7 is likely an old supernova remnant (SNR), based on radio (0.8 GHz, 1.4 GHz and 5 GHz), infrared (8 $μ$m from the GLIMPSE Legacy Project and 21 $μ$m from the Midcourse Space Experiment), and X-ray (0.1 keV - 2.4 keV from the ROSAT survey and 5 - 20 keV from the INTEGRAL survey) data. The SNR, centered at ({\sl l}, {\sl b})=(353.55, -0.65) with a radius of $\sim$ 0.25$^{\circ}$, closely matches the outline of the recently discovered extended TeV source HESS J1731-347, which has no previously identified counterpart. A diffuse X-ray enhancement detected in the ROSAT all-sky survey is coincident with lower half shell of the SNR. Therefore the SNR is the best radio counterpart of both the HESS source and the diffuse X-ray enhancement. G353.6-0.7 has an age of $\sim$ 27000 yrs. Altogether, the new discovery provides the best case that an old SNR emits TeV $γ$-rays.

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arXiv2008

The outer scale of turbulence in the magneto-ionized Galactic interstellar medium

We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's spiral arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in spiral arms are present, and show power law behavior with a shallow spectrum. These results suggest that in the spiral arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and super bubble explosions are the main sources of energy on scales on the order of 100 parsecs.

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arXiv2007

Rotation Measures of Extragalactic Sources Behind the Southern Galactic Plane: New Insights into the Large-Scale Magnetic Field of the Inner Milky Way

We present new Faraday rotation measures (RMs) for 148 extragalactic radio sources behind the southern Galactic plane (253o < l < 356o, |b| < 1.5o), and use these data in combination with published data to probe the large-scale structure of the Milky Way's magnetic field. We show that the magnitudes of these RMs oscillate with longitude in a manner that correlates with the locations of the Galactic spiral arms. The observed pattern in RMs requries the presence of at least one large-scale magnetic reversal in the fourth Galactic quadrant, located between the Sagittarius- Carina and Scutum-Crux spiral arms. To quantitatively compare our measurements to other recent studies, we consider all available extragalactic and pulsar RMs in the region we have surveyed, and jointly fit these data to simple models in which the large-scale field follows the spiral arms. In the best-fitting model, the magnetic field in the fourth Galactic quadrant is directed clockwise in the Sagittarius-Carina spiral arm (as viewed from the North Galactic pole), but is oriented counter- clockwise in the Scutum-Crux arm. This contrasts with recent analyses of pulsar RMs alone, in which the fourth-quadrant field was presumed to be directed counter-clockwise in the Sagittarius- Carina arm. Also in contrast to recent pulsar RM studies, our joint modeling of pulsar and extragalactic RMs demonstrates that large numbers of large-scale magnetic field reversals are not required to account for observations.

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arXiv2007

Characteristics of supernova remnant G351.7+0.8 and a distance argument against its association with PSR J1721-3532

New images of the supernova remnant (SNR) G351.7+0.8 are presented based on 21cm HI-line emission and continuum emission data from the Southern Galactic Plane Survey (SGPS). SNR G351.7+0.8 has a flux density of 8.4+-0.7 Jy at 1420 MHz. Its spectral index is 0.52+-0.25 (S=v^{-alpha}) between 1420 MHz and 843 MHz, typical of adiabatically expanding shell-like remnants. HI observations show structures possibly associated with the SNR in the radial velocity range of -10 to -18 km/s, and suggest a distance of 13.2 kpc and a radius of 30.7 pc. The estimated Sedov age for G351.7+0.8 is less than 6.8 x 10^{4} yrs. A young radio pulsar PSR J1721-3532 lies close to SNR G351.7+0.8 on the sky. The new distance and age of G351.7+0.8 and recent proper-motion measurements of the pulsar strongly argue against an association between SNR G351.7+0.8 and PSR J1721-3532. There is an unidentified, faint X-ray point source 1RXS J172055.3-353937 which is close to G351.7+0.8. This may be a neutron star potentially associated with G351.7+0.8.

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arXiv2006

The WENSS and Dwingeloo surveys and the Galactic magnetic field

In this paper we use the diffuse Galactic synchrotron emission to study the properties of the Galactic magnetic field. We combined data from the low-frequency WENSS survey with single-dish observations carried out by Brouw and Spoelstra to (partially) fill in the central gap in the (u,v) plane that is missing from the interferometer data. The small bandwidth of the WENSS data meant that we could not determine rotation measures (RM) directly. Instead we interpreted the polarization angle gradients that we derived as gradients in RM, and to do this we found a new way to efficiently and reliably fit linear gradients to periodic data. RM are available for the single-dish observations, and we found that we have to rescale these single-dish RM to match the RM gradients that we derive from the combined WENSS/single-dish dataset with the single-dish only RM gradients. We tentatively show that the difference in beamsize between the datasets can be responsible for this. We interpret the scaled-up RM we find in terms of a simple toy model, and by combining our results with those by Haverkorn et al. (2004) we reconstruct the full 3D magnetic field vector along a number of lines-of-sight. For these lines-of-sight we derive the properties of the magnetic field component perpendicular to the Galactic plane.

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arXiv2006

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

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 64m single dish telescope. The survey spans a Galactic longitude of 253 deg < l < 358 deg and a latitude of |b| < 1 deg at a resolution of 100 arcsec and a sensitivity below 1 mJy/beam. 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 5.5 deg by 2 deg, 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.

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arXiv2005

Enhanced small-scale Faraday rotation in the Galactic spiral arms

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 magneto-ionized interstellar medium in the spiral arms on scales larger than ~ 0.5 deg, corresponding to ~ 17 pc in the nearest spiral arm probed. The non-zero slopes in interarm regions imply a much larger scale of RM fluctuations. We conclude that fluctuations in the magneto-ionic 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.

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arXiv2005

The magnetic field of the Large Magellanic Cloud revealed through Faraday rotation

We have measured the Faraday rotation toward a large sample of polarized radio sources behind the Large Magellanic Cloud (LMC), to determine the structure of this galaxy's magnetic field. The magnetic field of the LMC consists of a coherent axisymmetric spiral of field strength ~1 microgauss. Strong fluctuations in the magnetic field are also seen, on small (<0.5 parsecs) and large (~100 parsecs) scales. The significant bursts of recent star formation and supernova activity in the LMC argue against standard dynamo theory, adding to the growing evidence for rapid field amplification in galaxies.

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arXiv2005

The Southern Galactic Plane Survey: HI Observations and Analysis

We describe the HI component of the Southern Galactic Plane Survey (SGPS). The SGPS is a large-scale project to image at arcminute resolution the HI spectral line and 21 cm continuum emission in parts of the plane of the Milky Way. The survey covers Galactic longitudes 253 deg < l < 358 deg and latitudes |b| <1.5 deg (SGPS I), plus a first quadrant extension covering 5 deg < l < 20 deg and |b| <1.5 deg (SGPS II). The survey combines data from the Australia Telescope Compact Array and the Parkes Radio Telescope for sensitivity to angular scales ranging from 2 arcminutes to several degrees. The combined data cover 325 sq-deg and have an rms sensitivity of 1.6 K. Here we describe the HI observations and data reduction in detail, and present examples from the final data products. The complete dataset is publicly available through the Australia Telescope National Facility's HI Surveys archive. This dataset provides an unprecedented view of the neutral component of interstellar hydrogen in the inner Milky Way.

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arXiv2005

A Stellar Wind Bubble Coincident with the Anomalous X-ray Pulsar 1E 1048.1-5937: Are Magnetars Formed From Massive Progenitors?

We present 21-cm HI observations from the Southern Galactic Plane Survey of the field around the anomalous X-ray pulsar 1E 1048.1-5937, a source whose X-ray properties imply that it is a highly magnetized neutron star (a "magnetar"). These data reveal an expanding hydrogen shell, GSH 288.3-0.5-28, centered on 1E 1048.1-5937, with a diameter of 35x23 pc (for a distance of 2.7 kpc) and an expansion velocity of approx 7.5 km/s. We interpret GSH 288.3-0.5-28 as a wind bubble blown by a 30-40 M_sun star, but no such central star can be readily identified. We suggest that GSH 288.3-0.5-28 is the wind bubble blown by the massive progenitor of 1E 1048.1-5937, and consequently propose that magnetars originate from more massive progenitors than do radio pulsars. This may be evidence that the initial spin period of a neutron star is correlated with the mass of its progenitor, and implies that the magnetar birth rate is only a small fraction of that for radio pulsars.

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arXiv2004

Structure in the polarized Galactic synchrotron emission, in particular `depolarization canals'

The polarized component of the diffuse radio synchrotron emission of our Galaxy shows structure, which is apparently unrelated to the structure in total intensity, on many scales. The structure in the polarized emission can be due to several processes or mechanisms. Some of those are related to the observational setup, such as beam depolarization -- the vector combination and (partial) cancellation of polarization vectors within a synthesized beam --, or the insensitivity of a synthesis telescope to structure on large scales, also known as the 'missing short spacings problem'. Other causes for structure in the polarization maps are intrinsic to the radiative transfer of the emission in the warm ISM, which induces Faraday rotation and depolarization. We use data obtained with the Westerbork Synthesis Radio Telescope at 5 frequencies near 350 MHz to estimate the importance of the various mechanisms in producing structure in the linearly polarized emission. In the two regions studied here, which are both at positive latitudes in the second Galactic quadrant, the effect of 'missing short spacings' is not important. The properties of the narrow depolarization 'canals' that are observed in abundance lead us to conclude that they are mostly due to beam depolarization, and that they separate regions with different rotation measures. As beam depolarization only creates structure on the scale of the synthesized beam, most of the structure on larger scales must be due to depth depolarization. We do not discuss that aspect of the observations here, but in a companion paper we derive information about the properties of the ISM from the structure of the polarized emission.

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arXiv2004

Properties of the warm magnetized ISM, as inferred from WSRT polarimetric imaging

We describe a first attempt to derive properties of the regular and turbulent Galactic magnetic field from multi-frequency polarimetric observations of the diffuse Galactic synchrotron background. A single-cell-size model of the thin Galactic disk is constructed which includes random and regular magnetic fields and thermal and relativistic electrons. The disk is irradiated from behind with a uniform partially polarized background. Radiation from the background and from the thin disk is Faraday rotated and depolarized while propagating through the medium. The model parameters are estimated from a comparison with 350 MHz observations in two regions at intermediate latitudes done with the Westerbork Synthesis Radio Telescope. We obtain good consistency between the estimates for the random and regular magnetic field strengths and typical scales of structure in the two regions. The regular magnetic field strength found is a few microGauss, and the ratio of random to regular magnetic field strength is 0.7 +/- 0.5, for a typical scale of the random component of 15 +/- 10 pc. Furthermore, the regular magnetic field is directed almost perpendicular to the line of sight. This modeling is a potentially powerful method to estimate the structure of the Galactic magnetic field, especially when more polarimetric observations of the diffuse synchrotron background at intermediate latitudes become available.

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arXiv2004

Canals beyond Mars: Beam depolarization in radio continuum maps of the warm ISM

Multi-frequency radio polarimetric observations of the diffuse Galactic synchrotron background enable us to study the structure of the diffuse ionized gas via rotation measure maps. However, depolarization will introduce artifacts in the resulting rotation measure, most notably in the form of narrow, elongated ``depolarization canals''. We use numerical models of a non-emitting Faraday rotating medium to study the RM distribution needed to create depolarization canals by depolarization due to a finite beam width, and to estimate the influence of this depolarization mechanism on the determination of RM. We argue that the depolarization canals indeed can be caused by beam depolarization, which in turn is a natural consequence when observing a turbulent medium with limited resolution. Furthermore, we estimate that beam depolarization can induce an additional error of about 20% in RM determinations, and considerably less in regions that are not affected by depolarization canals.

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