Anthony Howard Minter

Microwave Interstellar Medium Emission in the Green Bank Galactic Plane Survey: Evidence for Spinning Dust

We observe significant dust-correlated emission outside of H II regions in the Green Bank Galactic Plane Survey (-4? < b < 4?) at 8.35 and 14.35?GHz. The rising spectral slope rules out synchrotron and free-free emission as majority constituents at 14 GHz, and the amplitude is at least 500 times higher than expected thermal dust emission. When combined with the Rhodes (2.326 GHz) and Wilkinson Microwave Anisotropy Probe (23-94 GHz) data, it is possible to fit dust-correlated emission at 2.3-94 GHz with only soft synchrotron, free-free, thermal dust, and an additional dust-correlated component similar to Draine & Lazarian spinning dust. The rising component generally dominates free-free and synchrotron for ? 14?GHz and is overwhelmed by thermal dust at ? 60?GHz. The current data fulfill most of the criteria laid out by Finkbeiner and coworkers for detection of spinning dust.

Heating of the Interstellar Diffuse Ionized Gas via the Dissipation of Turbulence

We have recently published observations that specify most of the turbulent and mean plasma characteristics for a region of the sky containing the interstellar diffuse ionized gas (DIG). These observations have provided virtually all of the information necessary to calculate the heating rate from dissipation of turbulence. We have calculated the turbulent dissipation heating rate employing two models for the interstellar turbulence. The first is a customary modeling as a superposition of magnetohydrodynamic waves. The second is a fluid-turbulence-like model based on the ideas of Higdon. This represents the first time that such calculations have been carried out with full and specific interstellar turbulence parameters. The wave model of interstellar turbulence encounters the severe difficulty that plausible estimates of heating by Landau damping exceed the radiative cooling capacity of the interstellar DIG by 3-4 orders of magnitude. Clearly interstellar turbulence does not behave like an ensemble of obliquely propagating fast magnetosonic waves. The heating rate due to two other wave dissipation mechanisms, ion-neutral collisional damping and the parametric decay instability, are comparable to the cooling capacity of the diffuse ionized medium. We find that the fluid-like turbulence model is an acceptable and realistic model of the turbulence in the interstellar medium once the effects of ion-neutral collisions are included in the model. This statement is contingent on an assumption that the dissipation of such turbulence because of Landau damping is several orders of magnitude less than that from an ensemble of obliquely propagating magnetosonic waves with the same energy density. Arguments as to why this may be the case are made in the paper. Rough parity between the turbulent heating rate and the radiative cooling rate in the DIG also depends on the hydrogen ionization fraction being in excess of 90% or on a model-dependent lower limit to the heating rate being approximately valid. We conclude that the dissipation of turbulence is capable of providing a substantial and perhaps major contribution to the energy budget of the diffuse ionized medium.

A Strongly Magnetized Pulsar within the Grasp of the Milky Way's Supermassive Black Hole

We acknowledge support by grants AYA 2012-39303, SGR2009-811, iLINK 2011-0303, AYA 2010-21097-C03-02, Prometeo 2009/103, AYA2010-17631, P08-TIC-4075, INAF 2010 PRIN grant, Chandra Awards GO2-13076X, G03-14060X, GO3-14099X and G03-14121X, and an EU Marie Curie IEF (FP7-PEOPLE-2012-IEF-331095).

The First Galactic Plane Survey at 8.35 and 14.35 GHz

We present the first images of the Galactic plane (GP; |b| < 5°, -15° < l < 255°) at 8.35 and 14.35 GHz. These observations used the National Radio Astronomy Observatory–NASA Green Bank Earth Station to survey the sky simultaneously at these frequencies. These are the first results from the GP survey observations, a program to monitor the sky at 8.35 and 14.35 GHz. The GP survey series is intended to detect short-lived radio sources. We present four independent observations of the Galactic plane, combined to provide a set of reference images of the Galactic plane. The first survey, GPA, covers 0.82 sr (6.5%) of the sky. A source list is presented for all sources brighter than 0.9 Jy at 8.35 GHz and also for all sources brighter than 2.5 Jy at 14.35 GHz. The FITS format images, residual images, source lists, and archive data are available over the Internet. Later papers will present the results of the variable source search.

G28.17+0.05: An Unusual Giant H I Cloud in the Inner Galaxy

New 21 cm H I observations have revealed a giant H I cloud in the Galactic plane that has unusual properties. It is quite well defined, about 150 pc in diameter at a distance of 5 kpc, and contains as much as 105 M☉ of atomic hydrogen. The outer parts of the cloud appear in H I emission above the H I background, while the central regions show H I self-absorption. Models that reproduce the observations have a core with a temperature 40 K and an outer envelope as much as an order of magnitude hotter. The cold core is elongated along the Galactic plane, whereas the overall outline of the cloud is approximately spherical. The warm and cold parts of the H I cloud have similar and relatively large line widths, ~7 km s-1. The cloud core is a source of weak, anomalously excited 1720 MHz OH emission, also with a relatively large line width, which delineates the region of H I self-absorption but is slightly blueshifted in velocity. The intensity of the 1720 MHz OH emission is correlated with NH derived from models of the cold core. There is 12CO emission associated with the cloud core. Most of the cloud mass is in molecules, and the total mass is greater than 2 × 105 M☉. In the cold core the H I mass fraction may be ~10%. The cloud has only a few sites of current star formation. There may be ~100 more objects like this in the inner Galaxy; every line of sight through the Galactic plane within 50° of the Galactic center probably intersects at least one. We suggest that G28.17+0.05 is a cloud being observed as it enters a spiral arm and that it is in the transition from the atomic to the molecular state.

The Effects of Thermal Heating via the Dissipation of Turbulence on Physical Conditions in the Galactic Diffuse Ionized Gas

The observed properties of the diffuse ionized gas (DIG) in our Galaxy are not easily reconcilable with simple photoionization models. Photoionization models, however, can reproduce the observed properties of H II regions. This suggests that there are different or additional physical processes at work in the DIG. We have developed a model of the DIG whereby it is ionized by a relatively soft ionizing spectrum (Teff ≤ 32,000 K) and is also heated by an additional thermal mechanism: the dissipation of turbulence. This model predicts the same electron temperature, [N II] λ6583/Hα ratio, [S II] λ6716/Hα ratio, and He I λ5876/Hα ratio as observed in the DIG. The model suggests that the observed [O III] emission from the diffuse interstellar medium (ISM) does not originate from the oxygen in the DIG. Without the turbulent thermal heating term, this model will not reproduce the observed properties of the DIG. The dissipation of turbulence may also be important in other phases of the ISM.

Neutral Hydrogen Absorption toward XTE J1810–197: The Distance to a Radio-emitting Magnetar

We have used the Green Bank Telescope (GBT) to measure H I absorption against the anomalous X-ray pulsar XTE J1810–197. Using a flat rotation curve, we find that XTE J1810–197 is located at a distance of 3.4 ± 0.6 kpc. The Galactic bar, however, does have an influence on Galactic rotation velocities that impacts the distance estimate of XTE J1810–197. When rotation curve models that include the Galactic bar are used, we find that XTE J1810–197 is located in the distance range of 3.1-4.3 kpc. From information gleaned from the literature, we find an upper limit on the distance to XTE J1810–197 of 4 kpc based on its infrared detection, its location just in front of an Infrared Dark Cloud (G10.74-0.13), and a measurement of the infrared absorption versus distance for this line of sight. Our best determination for the distance to XTE J1810–197 is thus 3.1-4 kpc. This distance, 3.1-4 kpc, is consistent with the distance to XTE J1810–197 of about 3.3 kpc derived from its dispersion measure, and estimates of 2-5 kpc obtained from fits to its X-ray spectra. We also used the GBT in an attempt to measure absorption in the four OH 18 cm lines against XTE J1810–197. We were unsuccessful in this, mainly because of its declining radio flux density. Analysis of H I 21 cm, OH 18 cm, and 12CO(2→ 1) emission toward XTE J1810–197 allows us to place a lower limit of NH 4.6 × 1021 cm−2 on the nonionized hydrogen column density to XTE J1810–197, consistent with estimates obtained from fits to its X-ray spectra.

TINY SCALE OPACITY FLUCTUATIONS FROM VLBA, MERLIN, AND VLA OBSERVATIONS OF H I ABSORPTION TOWARD 3C 138

The structure function of opacity fluctuations is a useful statistical tool to study tiny scale structures of neutral hydrogen. Here we present high-resolution observation of H I absorption toward 3C 138, and estimate the structure function of opacity fluctuations from the combined VLA, MERLIN, and VLBA data. The angular scales probed in this work are ~10-200 mas (about 5-100 AU). The structure function in this range is found to be well represented by a power law S τ(x) ~ x β with index β ~ 0.33 ± 0.07 corresponding to a power spectrum P τ(U) ~ U –2.33. This is slightly shallower than the earlier reported power-law index of ~2.5-3.0 at ~1000 AU to few pc scales. The amplitude of the derived structure function is a factor of ~20-60 times higher than the extrapolated amplitude from observation of Cas A at larger scales. On the other hand, extrapolating the AU scale structure function for 3C 138 predicts the observed structure function for Cas A at the pc scale correctly. These results clearly establish that the atomic gas has significantly more structures in AU scales than expected from earlier pc scale observations. Some plausible reasons are identified and discussed here to explain these results. The observational evidence of a shallower slope and the presence of rich small-scale structures may have implications for the current understanding of the interstellar turbulence.

SIMULTANEOUS MULTI-BAND RADIO AND X-RAY OBSERVATIONS OF THE GALACTIC CENTER MAGNETAR SGR 1745–2900

We report on multi-frequency, wideband radio observations of the Galactic Center magnetar (SGR 1745-2900) with the Green Bank Telescope for ~100 days immediately following its initial X-ray outburst in 2013 April. We made multiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us to examine the magnetars flux evolution, radio spectrum, and interstellar medium parameters (such as the dispersion measure (DM), the scattering timescale, and its index). During two epochs, we have simultaneous observations from the Chandra X-ray Observatory, which permitted the absolute alignment of the radio and X-ray profiles. As with the two other radio magnetars with published alignments, the radio profile lies within the broad peak of the X-ray profile, preceding the X-ray profile maximum by ~0.2 rotations. We also find that the radio spectral index γ is significantly negative between ~2 and 9 GHz; during the final ~30 days of our observations γ ˜ - 1.4, which is typical of canonical pulsars. The radio flux has not decreased during this outburst, whereas the long-term trends in the other radio magnetars show concomitant fading of the radio and X-ray fluxes. Finally, our wideband measurements of the DMs taken in adjacent frequency bands in tandem are stochastically inconsistent with one another. Based on recent theoretical predictions, we consider the possibility that the DM is frequency-dependent. Despite having several properties in common with the other radio magnetars, such as Lx.qui/Lrot < 1, an increase in the radio flux during the X-ray flux decay has not been observed thus far in other systems.

The Fine-Structure Lines of Hydrogen in H II Regions

The 2s1/2 state of hydrogen is metastable and overpopulated in H II regions. In addition, the 2p states may be pumped by ambient Lyα radiation. Fine-structure transitions between these states may be observable in H II regions at 1.1 GHz (2s1/2-2p1/2) and/or 9.9 GHz (2s1/2-2p3/2), although the details of absorption versus emission are determined by the relative populations of the 2s and 2p states. The n = 2 level populations are solved with a parameterization that allows for Lyα pumping of the 2p states. The Lyα pumping rate has long been considered uncertain, as it involves solution of the difficult Lyα transfer problem. The density of Lyα photons is set by their creation rate, easily determined from the recombination rate, and their removal rate. Here we suggest that the dominant removal mechanism of Lyα radiation in H II regions is absorption by dust. This circumvents the need to solve the Lyα transfer problem and provides an upper limit to the rate at which the 2p states are populated by Lyα photons. In virtually all cases of interest, the 2p states are predominantly populated by recombination, rather than Lyα pumping. We then solve the radiative transfer problem for the fine-structure lines in the presence of free-free radiation. In the likely absence of Lyα pumping, the 2s1/2 → 2p1/2 lines will appear in stimulated emission, and the 2s1/2 → 2p3/2 lines in absorption. Because the final 2p states are short lived, these lines are dominated by intrinsic line width (99.8 MHz). In addition, each fine-structure line is a multiplet of three blended hyperfine transitions. Searching for the 9.9 GHz lines in high emission measure H II regions offers the best prospects for detection. The lines are predicted to be weak; in the best cases, line-to-continuum ratios of several tenths of a percent might be expected with line strengths of tens to a hundred mK with the Green Bank Telescope. Predicted line strengths, at both 1.1 and 9.9 GHz, are given for a number of H II regions, high emission measure components, and planetary nebulae, based on somewhat uncertain emission measures, sizes, and structures. The extraordinary width of these lines and their blended structure will complicate detection.