Integrated Column Density Research Articles

Sensitivity limits of space-based interferometric gravitational wave observatories from the solar wind

Space-based interferometric gravitational wave instruments such as the ESA/NASA Laser Interferometer Space Antenna (LISA) observe gravitational waves by measuring changes in the light travel time between widely-separated spacecraft. One potential noise source for these instruments is interaction with the solar wind, in particular the free electrons in the interplanetary plasma. Variations in the integrated column density of free electrons along the laser links will lead to time-of-flight delays which directly compete with signals produced by gravitational waves. In this paper we present a simplified model of the solar plasma relevant for this problem, anchor key parameters of our model using data from the NASA \emph{Wind}/SWE instrument, and derive estimates for the effect in the LISA measurement. We find that under normal solar conditions, the gravitational-wave sensitivity limit from the free-electron effect is smaller than other noise sources that are expected to limit LISA's sensitivity.

BUDHiES IV: Deep 21-cm neutral Hydrogen, optical, and UV imaging data of Abell 963 and Abell 2192 at z ≃ 0.2

ABSTRACT In this paper, we present data from the Blind Ultra-Deep H i Environmental Survey (BUDHiES), which is a blind 21-cm H i spectral line imaging survey undertaken with the Westerbork Synthesis Radio Telescope. Two volumes were surveyed, each with a single pointing and covering a redshift range of 0.164 < z < 0.224. Within these two volumes, this survey targeted the clusters Abell 963 and Abell 2192, which are dynamically different and offer unique environments to study the process of galaxy evolution within clusters. With an integration time of 117 × 12 h on Abell 963 and 72 × 12 h on Abell 2192, a total of 166 galaxies were detected and imaged in H i. While the clusters themselves occupy only 4 per cent of the 73 400 Mpc3 surveyed by BUDHiES, most of the volume consists of large-scale structures in which the clusters are embedded, including foreground and background overdensities and voids. We present the data processing and source detection techniques and counterpart identification based on a wide-field optical imaging survey using the Isaac Newton Telescope and deep ultraviolet (UV) Galaxy Evolution Explorer (GALEX) imaging. Finally, we present H i and optical catalogues of the detected sources as well as atlases of their global H i properties, which include integrated column density maps, position–velocity diagrams, global H i profiles, and optical and UV images of the H i sources.

A repeating fast radio burst.

Fast radio bursts are millisecond-duration astronomical radio pulses of unknown physical origin that appear to come from extragalactic distances. Previous follow-up observations have failed to find additional bursts at the same dispersion measure (that is, the integrated column density of free electrons between source and telescope) and sky position as the original detections. The apparent non-repeating nature of these bursts has led to the suggestion that they originate in cataclysmic events. Here we report observations of ten additional bursts from the direction of the fast radio burst FRB 121102. These bursts have dispersion measures and sky positions consistent with the original burst. This unambiguously identifies FRB 121102 as repeating and demonstrates that its source survives the energetic events that cause the bursts. Additionally, the bursts from FRB 121102 show a wide range of spectral shapes that appear to be predominantly intrinsic to the source and which vary on timescales of minutes or less. Although there may be multiple physical origins for the population of fast radio bursts, these repeat bursts with high dispersion measure and variable spectra specifically seen from the direction of FRB 121102 support an origin in a young, highly magnetized, extragalactic neutron star.

Molecular formation along the atmospheric mass loss of HD 209458b and similar Hot Jupiters

The chemistry along the mass loss of Hot Jupiters is generally considered to be simple, consisting mainly of atoms, prevented from forming more complex species by the intense radiation field from their host stars. In order to probe the region where the temperature is low (T<2000K), we developed a 1D chemical and photochemical reaction model of the atmospheric mass loss of HD 209458b, involving 56 species, including carbon chain and oxygen-bearing ones, interacting through 566 reactions. The simulation results indicate that simple molecules like OH+, H2O+ and H3O+ are formed inside the region, considering that residual H2 survives in the exosphere, a possibility indicated by recent observational work. The molecules are formed and destroyed within a radial distance of less than 107km, but the estimated integrated column density of OH+, a potential tracer of H2, is high enough to allow detection, which, once achieved, would indicate a revision of chemical models of the upper atmosphere of Hot Jupiters. For low density Hot Jupiters receiving less intense XUV radiation from their host stars than HD 209458b, molecular species could conceivably be formed with a higher total column density.

Diagnostics of the molecular component of photon-dominated regions with mechanical heating

Context. Multitransition CO observations of galaxy centers have revealed that significant fractions of the dense circumnuclear gas have high kinetic temperatures, which are hard to explain by pure photon excitation, but may be caused by dissipation of turbulent energy. Aims. We aim to determine to what extent mechanical heating should be taken into account while modelling PDRs. To this end, the effect of dissipated turbulence on the thermal and chemical properties of PDRs is explored. Methods. Clouds are modelled as 1D semi-infinite slabs whose thermal and chemical equilibrium is solved for using the Leiden PDR-XDR code. Results. In a steady-state treatment, mechanical heating seems to play an important role in determining the kinetic temperature of the gas in molecular clouds. Particularly in high-energy environments such as starburst galaxies and galaxy centers, model gas temperatures are underestimated by at least a factor of two if mechanical heating is ignored. The models also show that CO, HCN and H2 O column densities increase as a function of mechanical heating. The HNC/HCN integrated column density ratio shows a decrease by a factor of at least two in high density regions with n \sim 105 cm-3, whereas that of HCN/HCO+ shows a strong dependence on mechanical heating for this same density range, with boosts of up to three orders of magnitude. Conclusions. The effects of mechanical heating cannot be ignored in studies of the molecular gas excitation whenever the ratio of the star formation rate to the gas density is close to, or exceeds, 7 \times 10-6 M yr-1 cm4.5 . If mechanical heating is not included, predicted column densities are underestimated, sometimes even by a few orders of magnitude. As a lower bound to its importance, we determined that it has non-negligible effects already when mechanical heating is as little as 1% of the UV heating in a PDR.

PSR J2030+3641: RADIO DISCOVERY AND GAMMA-RAY STUDY OF A MIDDLE-AGED PULSAR IN THE NOW IDENTIFIEDFERMI-LAT SOURCE 1FGL J2030.0+3641

In a radio search with the Green Bank Telescope of three unidentified low Galactic latitude Fermi-LAT sources, we have discovered the middle-aged pulsar J2030+3641, associated with 1FGL J2030.0+3641 (2FGL J2030.0+3640). Following the detection of gamma-ray pulsations using a radio ephemeris, we have obtained a phase-coherent timing solution based on gamma-ray and radio pulse arrival times that spans the entire Fermi mission. With a rotation period of 0.2 s, spin-down luminosity of 3e34 erg/s, and characteristic age of 0.5 Myr, PSR J2030+3641 is a middle-aged neutron star with spin parameters similar to those of the exceedingly gamma-ray-bright and radio-undetected Geminga. Its gamma-ray flux is 1% that of Geminga, primarily because of its much larger distance, as suggested by the large integrated column density of free electrons, DM=246 pc/cc. We fit the gamma-ray light curve, along with limited radio polarimetric constraints, to four geometrical models of magnetospheric emission, and while none of the fits have high significance some are encouraging and suggest that further refinements of these models may be worthwhile. We argue that not many more non-millisecond radio pulsars may be detected along the Galactic plane that are responsible for LAT sources, but that modified methods to search for gamma-ray pulsations should be productive -- PSR J2030+3641 would have been found blindly in gamma rays if only >0.8 GeV photons had been considered, owing to its relatively flat spectrum and location in a region of high soft background.

X-ray mass attenuation coefficients and imaginary components of the atomic form factor of zinc over the energy range of 7.2–15.2 keV

The x-ray mass attenuation coefficients of zinc are measured in a high-accuracy experiment between 7.2 and 15.2 keV with an absolute accuracy of 0.044% and 0.197%. This is the most accurate determination of any attenuation coefficient on a bending-magnet beamline and reduces the absolute uncertainty by a factor of 3 compared to earlier work by advances in integrated column density determination and the full-foil mapping technique described herein. We define a relative accuracy of $0.006%$, which is not the same as either the precision or the absolute accuracy. Relative accuracy is the appropriate parameter for standard implementation of analysis of near-edge spectra. Values of the imaginary components $f\ensuremath{''}$ of the x-ray form factor of zinc are derived. Observed differences between the measured mass attenuation coefficients and various theoretical calculations reach a maximum of about 5% at the absorption edge and up to 2% further than 1 keV away from the edge. The measurements invite improvements in the theoretical calculations of mass attenuation coefficients of zinc.

An improvement to the full-foil mapping technique for high accuracy measurement of X-ray mass attenuation coefficients

The limiting uncertainty in recent high accuracy measurements of the mass attenuation coefficient is the measurement of the integrated column density. An improvement in the design of the absorption foil holder is described which reduces the integrated column density uncertainty. The new design allows the edges of the foil to be more accurately mapped by the X-ray beam by reducing the largest source of uncertainty in the foil mapping: the uncertainty in the points along the foil edge. The method is shown to reduce the uncertainty in measurements of the mass attenuation coefficient of zinc foils. The reduced uncertainty in the full-foil mapping will allow the X-ray Extended Range Technique (XERT) to be applied to small non-metallic absorption foils more accurately.

Spatial variations of the sodium/potassium ratio in Mercury’s exosphere uncovered by high-resolution spectroscopy

High-resolution spectroscopy of Mercury has been obtained with two different instruments in 2006: the EMMI instrument at the 3.6-m NTT telescope of ESO La Silla Chile and the ESPADON spectrograph at the 3.6-m CFHT telescope on top of Mauna Kea (Hawaii). The disk of the planet has been scanned for spatial variation of the exospheric species. The large spectral range and high resolution allow simultaneous measurements of the integrated column density of Na and K. We measure Na/K ratio between 80 and 400 with values between 60 and 90 when the telescope was pointed towards the subsolar region of Mercury’s disk and much larger value when we looked to other part of the exosphere. Moreover, we observed that the Na and K exospheres display very different spatial distributions. Even if these two species are probably ejected with very similar mechanisms from the surface, their differences in mass and sensitivity to solar pressure acceleration imply very different behavior in Mercury’s exosphere.

Micrometry combined with profile mapping for the absolute measurement of Integrated Column Density (ICD) and for accurate X-ray mass attenuation coefficients using XERT

Absolute values of the column densities [ ρ t ] c of four gold foils were measured using micrometry combined with the 2D X-ray attenuation profile. The absolute calibration of [ ρ t ] c was made with a reference foil and the [ ρ t ] c of other foils were determined following the thickness transfer method. By this method, we obtain absolute calibration to 0.1% or better which was not possible using only the X-ray map of a single foil over its central region.

Image of Fomalhaut Dust Ring at 350 Microns: The Relative Column Density Map Shows Pericenter-Apocenter Asymmetry

We have imaged the circumstellar disk of Fomalhaut at 350 mm wavelength, using SHARC II (Submillimeter High Angular Resolution Camera II) at the Caltech Submillimeter Observatory. The spatial resolution of the raw images (9) has been enhanced by a factor of 3 using the HiRes deconvolution procedure. We find that at this wavelength and signal-to-noise ratio (approx.12), the observed morphology is that of a simple inclined ring (i approx. 70 deg), with little or no other apparent structure--this is the first observation that shows clearly the ring morphology of the disk. We have combined our 350 mm data with Spitzer Space Telescope images at 24, 70, and 160 mm in order to estimate the two-dimensional spatial variation of relative column density (tau map) using our DISKFIT procedure. The tau map is based on the following physical assumptions: (1) the wavelength variation of opacity is the same throughout the disk, (2) the radial variation of dust temperature is dictated by the energy balance of individual grains in the stellar radiation field, and (3) the vertical scale height of the disk follows a power-law radial variation. The results confirm the ringlike morphology but also show that the geometric center is displaced from the star by about 8 AU and that the ring has an apocentric enhancement of approximately 14% in integrated column density. If we interpret the displacement in terms of elliptical orbital motion due to gravitational perturbation by an unseen planet, then the implied forced eccentricity is ~0.06; dynamical modeling then predicts an apocentric density enhancement consistent with that inferred from the tau map.

Full-foil x-ray mapping of integrated column density applied to the absolute determination of mass attenuation coefficients

Recent measurements of mass attenuation coefficients have identified the determination of the thickness of the absorbing specimen as the major limitation to the accuracy of the measurement. We present a technique for determining the mass attenuation coefficient with high accuracy. The technique uses the integral of the density along a column extending through the thickness of the absorber, which we term the integrated column density. Attenuation measurements mapped across the entire absorber are used to determine a relative map of the integrated column density. These relative measurements are then placed on an absolute scale by comparison with the average integrated column density and are used to determine the mass attenuation coefficient. This approach correctly treats variations in the integrated column density across the foil. We illustrate the technique with an absolute measurement of the x-ray mass attenuation coefficient of molybdenum using a synchrotron beam of energy 41.568 keV ± 0.005 keV. We obtain cm2 g−1 ± 0.0032 cm2 g−1, accurate to 0.028%—over one order of magnitude more accurate than any previous work. The full-foil technique used to determine the mass attenuation coefficient is used to determine an integrated column density profile of a sample to a precision of around 0.05% of the thickness of the absorber. We demonstrate the sensitivity of the technique by observing a periodic thickness variation of order 0.1 µm occurring over a 5 mm length scale on a nominally 50 µm thick molybdenum foil.

Tracking ground state Ba+ions in an expanding laser–plasma plume using time-resolved vacuum ultraviolet photoionization imaging

We report results from a study of the integrated column density and expansion dynamics of ground-state-selected Ba+ions in a laser–plasma plume using a new experimental system—VPIF (vacuum-ultraviolet photoabsorption imaging facility). The ions are tracked by recording the attenuation of a pulsed and collimated vacuum ultraviolet beam, tuned to the 5p–6dinner-shell resonance of singly ionized barium, as the expanding plasma plume moves across it. The attenuated beam is allowed to fall on a CCD array where the spatial distribution of the absorption is recorded. Time-resolved ion velocity and integrated column density maps are readily extracted from the photoionization images.

Distribution and Kinematics of O vi in the Galactic Halo

Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of 100 extragalactic objects and two distant halo stars are analyzed to obtain measures of O VI λλ1031.93, 1037.62 absorption along paths through the Milky Way thick disk/halo. Strong O VI absorption over the velocity range from -100 to 100 km s-1 reveals a widespread but highly irregular distribution of O VI, implying the existence of substantial amounts of hot gas with T ~ 3 × 105 K in the Milky Way thick disk/halo. The integrated column density, log [N(O VI) cm-2], ranges from 13.85 to 14.78 with an average value of 14.38 and a standard deviation of 0.18. Large irregularities in the gas distribution are found to be similar over angular scales extending from 45°) range from -46 to 82 km s-1, with a high-latitude sample average of 0 km s-1 and a standard deviation of 21 km s-1. High positive velocity O VI absorbing wings extending from ~100 to ~250 km s-1 observed along 21 lines of sight may be tracing the flow of O VI into the halo. A combination of models involving the radiative cooling of hot fountain gas, the cooling of supernova bubbles in the halo, and the turbulent mixing of warm and hot halo gases is required to explain the presence of O VI and other highly ionized atoms found in the halo. The preferential venting of hot gas from local bubbles and superbubbles into the northern Galactic polar region may explain the enhancement of O VI in the north. If a fountain flow dominates, a mass flow rate of approximately 1.4 M⊙ yr-1 of cooling hot gas to each side of the Galactic plane with an average density of 10-3 cm-3 is required to explain the average value of log [N(O VI) sin |b|] observed in the southern Galactic hemisphere. Such a flow rate is comparable to that estimated for the Galactic intermediate-velocity clouds.

Sub-arcsecond atomic hydrogen absorption in the Seyfert galaxies NGC 7674 and NGC 7469

Neutral hydrogen has been studied on sub-arcsecond scales in two Seyfert galaxies, NGC 7469 and NGC 7674. The 1.4-GHz continuum emission from these sources agree with higher frequency measurements and shows linear, triple source in NGC 7674 and a compact source surrounded by a weak ring of radio emission in NGC 7469. In both of these sources we detect deep localized H I absorption in front of a single continuum component. In the case of NGC 7674 the H I absorption is only seen against the central component and has a peak optical depth of 0.21 ± 0.03, a linewidth of 114 ± 7 km s−1 and a heliocentric velocity of 8623 ± 5 km s−1. The integrated H I absorption against the central component corresponds to a column density of 5.0 ± 0.7 × 1021 atom cm−2. We interpret the lack of H I absorption against the east and west components (τ < 0.07 and 0.06) as evidence that the H I is localized within the central few hundred pc, similar to the H I observed against the nucleus of NGC 4151, and has an upper mass limit of ∼4 × 105 M⊙. We speculate that this H I may be associated with a dusty torus. Giant Meterwavelength Radio Telescope (GMRT) results show extended emission and give a rest velocity of 8699 ± 20 km s−1. The H I absorption against NGC 7469 shows a single deep feature against the strong central component. The line has a peak optical depth 0.25, a linewidth of 65 ± 6 km s−1 and a heliocentric velocity of 4843 ± 5 km s−1. The integrated column density is 3.6 ± 0.4 × 1021 atom cm−2. Unfortunately the other components in NGC 7469 are too weak to set significant limits to the H I optical depth, and hence, unlike NGC 7674, it is not possible to constrain the size of the absorbing gas.

IUE OBSERVATIONS OF HIGHLY IONIZED GAS TOWARD DISTANT STARS IN THE MILKY WAY

ABSTRACT Combined high-dispersion IUE spectra of interstellar Si IV, C, IV, and N V absorption along distnat lines toward early type stars in the Galactic disk and low halo are discussed. The highly ionized species have complex profiles and exhibit stronger absorption toward distant low latitude stars than toward high latitude stars. Absorption along low latitude directions is often broadened substantially (up to 100 km S6-1 in some cases) by Galactic rotation. Along these sight lines the Si IV, C IV, and N V profiles are more similar in shape to each other than to those of A1 III, which is a tracer of photoionized interstellar gas. The observed column density ratios of the high ions, <N(C IV)/N(Si IV)≥3.6±1.3 and <N(N IV)/N(N V)≥4.6±2.7, are similar in widely different regions within the Galaxy. The similarities in the high ion profile shape along individual sight lines and integrated column density ratios along differing sight lines suggest a common origin for these species in collisionally ionized gas associated with a global process such as a Galactic fountain.

Concentrations of hydrogen chloride and hydrogen fluoride measured during the MAP/GLOBUS campaign of September 1983

Within the context of the MAP/GLOBUS campaign of September 1983, several trace species have been observed by absorption spectroscopy at the two ground stations of the Jungfraujoch, Switzerland (3580 m altitude) and the Observatoire de Haute-Provence, France (1905 m altitude). The results obtained for HCl and for HF, expressed in terms of mean integrated columns above these sites are: Jungfraujoch: (2.1 ± 0.2) E15 mol cm −2 HCl (4.8 ± 0.2) E14 mol cm −2 HF Haute-Provence: (2.6 ± 0.2) E15 mol cm −2 HCl. Taking into account the difference in the altitude of the two stations, the reported HC1 results are in agreement to within their respective uncertainties. The integrated column density of HCl and HF above 11 km altitude, deduced from airplane observations on 9 September 1983, are: (1.65 ± 0.25) E15 mol cm −2 HCl above 11 km (3.7 ± 1.7) E14 mol cm −2 HF above 10 km supporting satisfactorily the ground measurements.

Diffused germanium dichroic mirror for optically pumped far-infrared lasers

A new type of dichroic reflector has been developed for the far-infrared region of the spectrum, based on n-type germanium with a diffused conduction layer. A 111-axis single-crystal plate of 40-Ω cm optical-grade germanium was implanted with 5×1014 As+ ions/cm2 by ion bombardment at 200 keV; the donor impurity ions were subsequently diffused into the substrate at a temperature of 850 °C (held for 20 h). The measured power-reflection spectrum displayed a shallow minimum at ∼70 cm−1, rising steeply below 50 cm−1 to reach 80% reflectivity at 10 cm−1. Computer modeling of the infrared reflectance of a thermally diffused conducting layer in n-type germanium—with the assumption of Conwell–Weisskopf mobility variation for the conduction electrons—gave an excellent fit to the observed reflection spectrum for an integrated column density of 1014 cm−2 and 10-μm half width, the latter being in good agreement with the value √Dt=11 μm calculated for thermal diffusion under the conditions indicated.

Spectral line profiles for a planetary corona

The Lyman and Balmer emission of a planetary corona depend on the exospheric temperature, the integrated column density of solar-illuminated hydrogen, and the region of phase space occupied by particles. Measurements of the intensity alone are incapable of defining the exosphere unambiguously. Line profiles, with high spectral resolution, can show whether a nonthermal component of the escaping hydrogen is present, and can indicate at what altitude satellite orbits of hydrogen atoms are depleted.