John H. Simonetti

An Imaging Survey of Northern Galactic Halpha Emission with Arcminute Resolution

We present preliminary results from a high-resolution, high-sensitivity imaging survey of the northern galactic Halpha emission. The survey is carried out using the Spectral Line Imaging Camera (SLIC) which incorporates a fast (f/1.2) lens attached to a cryogenic CCD in combination with a narrowband interference filter. The pixel size is 1.6 arcminutes and the diameter of each field is 10°. The fast optics, narrow bandpass (1.7 nm) filter, and high quantum-efficiency, low-noise CCD yield a high brightness sensitivity to Halpha emission on arcminute scales. This gives an equivalent sensitivity to emission measure structure below 1 pc cm^-6. Some faint features detected include a supershell connected with the star forming region W4 extending 7° above the galactic plane, and filaments possibly related to galactic loops II and III. In addition, we have carried out deep observations of fields in which anisotropies in the cosmic microwave background radiation have been detected. Our observations place stringent limits upon the contribution to the apparent microwave fluctuations from free-free emission in the galactic foreground.

Small-scale variations in the galactic magnetic field - The rotation measure structure function and birefringence in interstellar scintillations

The structure function of rotation measures of extragalactic sources and birefringence in interstellar scintillations are used to investigate variations in the interstellar magnetic field on length scales of about 0.01-100 pc and 10 to the 11th cm, respectively. Model structure functions are derived for the case of a power-law power spectrum of irregularities in the quantity (n(e)B), and an estimate for the structure function is computed for several regions of the sky using data on extragalactic sources. The results indicate an outer angular scale for rotation measure (RM) variations of not less than about 5 deg (a linear scale of about 9-90 pc at a distance of 0.1-1 kpc). There is also evidence for RM variations on angular scales as small as 1 arcmin, but it cannot be determined whether these are intrinsic to the source or caused by the interstellar medium. The effect of a random, Faraday-active medium on the diffraction of radio waves is derived, and an upper limit to the variations in n(e)B on a length scale of 10 to the 11th cm is obtained from available observations.

Design and Evaluation of an Active Antenna for a 29–47 MHz Radio Telescope Array

The eight-meter-wavelength transient array (ETA) is a new radio telescope consisting of 12 dual-polarized, 38 MHz-resonant dipole elements which are individually instrumented, digitized, and analyzed in an attempt to detect rare and as-yet undetected single dispersed pulses believed to be associated with certain types of astronomical explosions. This paper presents the design and demonstrated performance of ETAs dipole antennas. An inverted V-shaped design combined with a simple and inexpensive active balun yields sensitivity which is limited only by the external noise generated by the ubiquitous Galactic synchrotron emission over a range greater than the 27-49 MHz design range. The results confirm findings from a recent theoretical analysis, and the techniques described here may have applications in other problems requiring in situ evaluation of large low-frequency antennas

Detection in Hα of a Supershell Associated with W4

From H I observations, Normandeau, Taylor, & Dewdney have identified a possible Galactic chimney emanating from W4. We observed a 10° diameter field centered on this region in the Hα line using a CCD camera sensitive to faint extended emission. Our image shows an apparent shell of H II, which we interpret as the ionized inner wall of a superbubble produced by stellar winds from the very young star cluster OCl 352. An analysis of the ionization balance indicates that much of the Lyman continuum radiation from the star cluster is absorbed and does not escape from the disk. The shell appears to close 6° (or about 230 pc) above the star cluster, and at a Galactic latitude of 7°. The shell is quite elongated, with its major axis approximately perpendicular to the Galactic plane, as predicted for a superbubble formed in a stratified Galactic disk. The large size of the shell leads to an estimated age between 6.4 and 9.6 Myr, which exceeds that of OCl 352 (2.5 Myr). The reason for this discrepancy is unclear, although it is possible that an earlier epoch of stellar outflow has contributed to the growth of the W4 superbubble.

The Extreme Scattering Event toward PKS 1741–038: VLBI Images

We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the sources diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the sources diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order 1 AU, the level of scattering within an ESE lens may be a factor of 107 larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 μs, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 μm emission.

Detection of Radio Emission from Fireballs

We present the findings from the Prototype All-Sky Imager, a back end correlator of the first station of the Long Wavelength Array, which has recorded over 11,000 hr of all-sky images at frequencies between 25 and 75 MHz. In a search of this data for radio transients, we have found 49 long-duration (10 s of seconds) transients. Ten of these transients correlate both spatially and temporally with large meteors (fireballs), and their signatures suggest that fireballs emit a previously undiscovered low frequency, non-thermal pulse. This emission provides a new probe into the physics of meteors and identifies a new form of naturally occurring radio transient foreground.

Galactic foregrounds in Owens Valley Radio Observatory and UCSB South Pole 1994 cosmic microwave background anisotropy data

We study Galactic emission foreground contamination of the Owens Valley Radio Observatory and UCSB South Pole 1994 cosmic microwave background anisotropy data by cross-correlating with templates of infrared dust emission and new high-resolution Virginia Tech Spectral Line Survey (VTSS) and Southern Hα Sky Survey Atlas (SHASSA) Hα data. Hα data provide rough upper limits on the level of free-free emission in the data sets. The cross-correlation analysis does not contradict a two-component foreground emission hypothesis, with the two dust-correlated components being free-free emission and spinning-dust emission.

A Water Maser Flare in W49N: Amplification by a Rotating Foreground Cloud

We monitored the 22 GHz H2O maser emission toward W49N from December 1989 through May 1990. During this period we observed an outburst in a component at -66.25 km s-1. The flux density of the flaring component increased by a factor of 10 to a maximum of 4020 Jy over a period of 24 days and decreased over the following 34 days to 1400 Jy on the last day of monitoring. During the flux increase the line narrowed from about 1.1 to 0.8 km s-1; it subsequently rebroadened to 1.0 km s-1. Most interestingly, during the flaring behavior the line center shifted by approximately 0.5 km s-1 over the 58 day period. To explain the flare, particularly its shifting line center, we present a model of two interacting maser clouds. In the model, saturated maser radiation produced in a background cloud is amplified by an unsaturated, masing foreground cloud. Motion of the foreground cloud across the line of sight produces a flaring line, accompanied by line narrowing. We demonstrate that the observed flare in W49N may be explained by such a model where a rotating foreground cloud passes in front of a nonrotating background cloud. The differential amplification of the background clouds radiation produces the observed increase in flux density, line narrowing, and systematic shift in center velocity.

The Contribution Of Galactic Free-Free Emission to Anisotropies in the Cosmic Microwave Background Found by the Saskatoon Experiment

We made a sensitive, wide-field Hα image of the north celestial polar region. Using this image, we constrain the contribution of irregularities in interstellar free-free emission to the degree-scale anisotropies in the cosmic microwave background detected in recent observations at Saskatoon by the Princeton group. The analysis of the Hα image mimics the Saskatoon data analysis: the resulting signal is the strength of irregularities sampled with the Saskatoon beam (i.e., degree-scale) along the 85° declination circle. We found no such irregularities that could be attributed to Hα emission. The implied upper bound on the rms variation in free-free brightness temperature is less than 4.6 μK at 27.5 GHz. The observed cosmic microwave background anisotropies are much larger. Therefore, the contribution of irregularities in interstellar free-free emission to the observed anisotropies is negligible.

A Precision Test for an Extra Spatial Dimension Using Black-hole-Pulsar Binaries

We discuss the observable effects of enhanced black hole mass loss in a black hole-neutron star (BH-NS) binary, due to the presence of a warped extra spatial dimension of curvature radius L in the braneworld scenario. For some masses and orbital parameters in the expected ranges the binary components would outspiral—the opposite of the behavior due to energy loss from gravitational radiation alone. If the NS is a pulsar, observations of the rate of change of the orbital period with a precision obtained for the binary pulsar B1913+16 could easily detect the effect of mass loss. For M BH = 7 M ☉, M NS = 1.4 M ☉, eccentricity e = 0.1, and L = 10 μm, the critical orbital period dividing systems that inspiral from systems that outspiral is P ≈ 6.5 hr, which is within the range of expected orbital periods; this value drops to P ≈ 4.2 hr for M BH = 5 M ☉. Observations of a BH-pulsar system could set considerably better limits on L in these braneworld models than could be determined by torsion-balance gravity experiments in the foreseeable future.