Andrew W. Clegg

The Gaussian Plasma Lens in Astrophysics: Refraction

We present the geometrical optics for refraction of a distant background radio source by an interstellar plasma lens, with specific application to a lens with a Gaussian profile of free-electron column density. The refractive properties of the lens are specified completely by a dimensionless parameter α, which is a function of the wavelength of observation, the free-electron column density through the lens, the lens-observer distance, and the diameter of the lens transverse to the line of sight. A lens passing between the observer and a background source, due to the relative motions of the observer, lens, and source, produces modulations in the light curve of the background source. Because plasma lenses are diverging, the light curve displays a minimum in the background sources flux density, formed when the lens is on-axis, surrounded by enhancements above the nominal (unlensed) flux density. The exact form of the light curve depends only upon the parameter α and the relative angular sizes of the source and lens as seen by the observer. Other effects due to lensing include the following: (1) the formation of caustic surfaces, upon which the apparent brightness of the background source becomes very large; (2) the possible creation of multiple images of the background source; and (3) angular position wander of the background source. If caustics are formed, the separation of the outer caustics can be used to constrain α, while the separation of the inner caustics can constrain the size of the lens. We apply our analysis to two sources, which have undergone extreme scattering events: (1) 0954+658, a source for which we can identify multiple caustics in its light curve, and (2) 1741-038, for which polarization observations were obtained during and after the scattering event. We find general agreement between modeled and observed light curves at 2.25 GHz, but poor agreement at 8.1 GHz. The discrepancies between the modeled and observed light curves may result from some combination of substructure within the lens, an anisotropic lens shape, a lens which only grazes the source rather than passing completely over it, or unresolved substructure within the extragalactic sources. Our analysis also allows us to place constraints on the physical characteristics of the lens. The inferred properties of the lens responsible for the scattering event toward 0954+658 (1741-038) are that it was 0.38 AU (0.065 AU) in diameter with a peak column density of 0.24 pc cm-3 (10-4 pc cm-3), an electron density within the lens of 105 cm-3 (300 cm-3), and a mass of 6.5 × 10-14 M☉ (10-18 M☉). The angular position wander caused by the lens was 250 mas (0.4 mas) at 2.25 GHz. In the case of 1741-038, we can place an upper limit of only 100 mG on the magnetic field within the lens.

Rotation measures of low-latitude extragalactic sources and the magnetoionic structure of the Galaxy

Faraday rotation measurements of 56 extragalactic sources obtained predominantly in the range 45-93 deg, /b/ less than 5 deg are reported. Within the longitude range sampled in the present study, RM(l) = 1600 sin (l0 - l) rad/sq m, with a null in RM occurring at 62 deg. Under the assumption of a uniform circular geometry for the magnetic field lines, it is argued that the magnetoionic medium must exist to a Galactocentric radius of about 25 kpc to produce the observed magnitudes of the RMs, where it is assumed that ne and /B/ equal their local values of 0.03/cu cm and 2.1 micro-G out to Rm. The medium must exist to an even greater radius if ne and/or /B/ decrease with R, as is likely. Comparison of extragalactic and pulsar RMs along nearly coincident lines of sight is consistent with at least one field reversal exterior to the radius of the solar orbit about the Galactic center.

Variability of interstellar hydroxyl masers

The spectra of 10 interstellar OH maser complexes were monitored in the 1665 MHz transition for variability on short (minutes-hours) and long (days-years) time scales. Eight of the sources exhibit short time scale variability in the relative amplitudes of the spectral features above that expected due to radiometer noise, and all sources for which multiepoch observations were made show variability on long time scales. Amplitudes of the short time scale fluctuations are typically 5-10 percent of the peak intensity and are as large as 20 percent. Over long time scale, the fluctuations range from 0 to nearly 100 percent in amplitude. The OH emission spectrum from a main-line OH/IR star was also monitored. Long time scale variability is noted, distinct from that caused by pulsations of the stellar pumping source, while no short time scale variability (above 5 percent) is apparent. 46 refs.

Discontinuities in the dynamic spectrum of the pulsar 0823+26

The 1400 MHz dynamic spectrum of the pulsar 0823+26 exhibits sharp discontinuities which are characterized by changes in the diffractive scintillation pattern over transition times of ∼2 minutes. The discontinuities occur during periods in which the pulsars broad-band (40 MHz) light curve is undergoing strong oscillations with periods of ∼15 minutes. We believe these effects are due to coherent interference between multiple images created by strong refraction in the interstellar medium. In effect, we observe the discontinuities in the dynamic spectrum when the scintillation patterns of different images (each traveling different optical paths) alternately dominate the received intensity

Refraction from interstellar shocks

Interstellar shocks that are sufficiently thin (≲10 au) and sufficiently strong (ΔDM≳0.01 cm−3 pc) are capable of producing observable flux modulations and multiple images of background point radio sources. From the statistics of observed refraction events, inferences may be drawn concerining the relationship between small‐scale electron density structure and large‐scale phenomena like supernova explosions and strong stellar winds. In order for shock refraction events to be frequent, the effective cross section must be much larger than predicted for an ideal, spherical shock. The cross section is likely to be enhanced by turbulence that distorts the shock interface on scales of ∼1 au and by the prevalence of HI clouds and other non‐uniformities in the ISM that will be swept up or engulfed by the expanding blast wave.

VLA Polarization Observations of the Extragalactic Source 1741−038 during an Extreme Scattering Event

The extragalactic radio source 1741-038 underwent an extreme scattering event in 1992. Near the middle of the event and 1.5 yr after the end of the event we obtained Faraday rotation observations of this source. The results are consistent with no change in rotation measure during the scattering event. Many models of extreme scattering events suggest that the events are due to strong refraction by an intervening plasma irregularity in the Galactic interstellar medium. If the irregularity is moving with a typical interstellar velocity of 100 km s-1, and located 1 kpc distant, we constrain the mean parallel component of the magnetic field within the refractor to be 40 μG. While the general interstellar field is substantially less than this value (about 2 μG), the field strength within a shock-induced irregularity is expected to be greatly increased over typical interstellar values. The constraint on the mean parallel field within the refractor is much tighter (4 μG) if the refractor is associated with the North Polar Spur.

Turbulence in the coronal interstellar medium

Low frequency scintillation observations of the nearby pulsar 0950+08 show that C2n, the amplitude of the electron density fluctuation spectrum, is ≂10−4.5 m−20/3 along the line of sight, a value which is 5 to 10 times smaller that C2n measured towards other pulsars within a few kpc of Earth. Since the line of sight to PSR 0950+08 is contained mostly within the local X‐ray bubble we can conclude that the coronal phase of the interstellar medium is weakly scattering.