T. Joseph W. Lazio

Finding Radio Pulsars in and beyond the Galactic Center

Radio wave scattering is enhanced dramatically for Galactic center sources in a region with radius 15. Using scattering from Sgr A* and other sources, we show that pulse broadening for pulsars in the Galactic center is at least 6.3ν-4 s (ν = radio frequency in GHz) and is most likely 50-200 times larger because the relevant scattering screen appears to be within the Galactic center region itself. Pulsars beyond—but viewed through—the Galactic center suffer even greater pulse broadening and are angularly broadened by up to ~2. Periodicity searches at radio frequencies are likely to find only long-period pulsars, and then only if optimized by using frequencies 7 GHz and by testing for small numbers of harmonics in the power spectrum. The optimal frequency is ν ~ 7.3 GHz[Δ0.1 P(α1/2)]-1/4, where Δ0.1 is the distance of the scattering region from Sgr A* in units of 0.1 kpc, P is the period (in seconds), and α is the spectral index. A search for compact sources using aperture synthesis may be far more successful than searches for periodicities because the angular broadening is not so large as to desensitize the survey. We estimate that the number of detectable pulsars in the Galactic center may range from ≤1 to 100, with the larger values resulting from recent, vigorous starbursts. Such pulsars provide unique opportunities for probing the ionized gas, gravitational potential, and stellar population near Sgr A*.

Scintillation-induced Intermittency in SETI

We use scattering theory, simulations, and empirical constraints on interstellar scintillations to discuss the intermittency of radio signals from extraterrestrial intelligence (ETI). The number of ETI sources in the Galaxy has a direct influence on the expected dynamic range of fluxes in a survey, through inverse square-law effects and, equally importantly, by the number of independent statistical trials made on modulations caused by interstellar scintillations. We demonstrate that scintillations are very likely to allow initial detections of narrowband signals, while making redetections extremely improbable, a result that follows from the skewed, exponential distribution of the modulation. This conclusion holds for relatively distant sources but does not apply to radio SETI toward nearby stars (100 pc). Recent SETI has found nonrepeating, narrowband events that are largely unexplained. We consider three models in order to assess these events and to analyze large surveys in general: (model I) radiometer noise fluctuations; (model II) a population of constant Galactic sources that undergo deep fading and amplification due to interstellar scintillation, consistent with ETI transmissions; and (model III) real, transient signals (or hardware errors) of either terrestrial or extraterrestrial origin. We derive likelihood and Bayesian tests of the models for individual events and globally on entire surveys. Applying them to The Planetary Society/Harvard META data, we find that models II and III are both highly preferred to model I, but that models II and III are about equally likely. In the context of model II, the likelihood analysis indicates that candidate events above threshold (~32 σ) are combinations of large amplitude noise fluctuations and scintillation gains, making it highly probable that events seen once will only very rarely be seen again. Ruling out model II in favor of model III is difficult—to do so, many more reobservations (e.g., thousands) are needed than were conducted in META (hundreds) or the reobservation threshold must be much lower than was used in META. We cannot, therefore, rule out the possibility that META events are real, intrinsically steady ETI signals. Our formalism can be used to analyze any SETI program. We estimate the number of reobservations required to rule out model II in favor of model III, taking into account that reobservations made promptly sample the same scintillation gain as in the original detection, while delayed reobservations sample a decorrelated scintillation modulation. The required number is a strong function of the thresholds used in the original survey and in reobservations. We assess optimal methods for applying statistical tests in future SETI programs that use multiple site and multiple beam observations as well as single site observations. We recommend that results be recorded on many more events than have been made to date. In particular, we suggest that surveys use thresholds that are far below the false-alarm threshold that is usually set to yield a small number of noise-induced detections in a massive survey. Instead, large numbers of events should be recorded in order to (1) demonstrate that background noise conforms to the distribution expected for it; and (2) investigate departures from the expected noise distribution as due to interference or to celestial signals. In this way, celestial signals can be investigated at levels much smaller than the false-alarm threshold. The threshold level for archiving candidate intensities and their corresponding sky positions is best defined in terms of the recording and computational technology that is available at a cost commensurate with other survey costs.

The Radial Extent and Warp of the Ionized Galactic Disk. II. A Likelihood Analysis of Radio-Wave Scattering toward the Anticenter

We use radio-wave scattering data for extragalactic sources and pulsars to constrain the distribution of ionized gas in the outer Galaxy. Like previous models, our model for the H II disk includes parameters for the radial scale length and scale height of the ionized gas. In addition, we have used the known H I distribution in the outer Galaxy in constructing our model, and we allow the H II disk to warp and flare. We also include the Perseus arm in our model. We use a likelihood analysis of 18 anticenter sources with measured scattering observables: 11 extragalactic sources and 7 pulsars. We find that the strength of scattering in the Perseus arm is no more than 60% of the level contributed by spiral arms in the inner Galaxy and is equivalent to a scattering diameter of 1.5 mas at 1 GHz. Our analysis favors an unwarped, nonflaring disk with a scale height of 1 kpc, though this may reflect the nonuniform and coarse coverage of the anticenter provided by the available data. One extragalactic source has a size a factor of 2 smaller than predicted by our model, possibly indicating the existence of holes in the scattering material. The lack of a warp in the scattering material indicates that VLBI observations near 1 GHz with an orbiting station having baseline lengths of a few Earth diameters will not be affected by interstellar scattering at moderate Galactic latitudes, | b | ≈ 15°. The radial scale length is 15-20 kpc, but the data cannot distinguish between a gradual decrease in the electron density and a truncated distribution. We favor a truncated one because we associate the scattering with massive star formation, which is also truncated near 20 kpc. A radial extent of 20 kpc is also comparable to the radial extent of Hα emission observed for nearby spiral galaxies. We find that the distribution of electron density turbulence must decrease more rapidly with Galactocentric distance than does the distribution of hydrogen. Alternate ionizing and turbulent agents—the intergalactic ionizing flux and the passage of satellite galaxies through the disk—are unlikely to contribute significant amounts to scattering in the anticenter. We cannot exclude the possibility that a largely ionized but quiescent disk, similar to that inferred for some Lyα absorbers, extends to 100 kpc.

The Radial Extent and Warp of the Ionized Galactic Disk. I. A VLBA Survey of Extragalactic Sources toward the Anticenter

We report multifrequency Very Long Baseline Array observations of 12 active galactic nuclei seen toward the Galactic anticenter. All of the sources are at |b| < 10°, and seven have |b| < 05. Our VLBA observations can detect an enhancement in the angular broadening of these sources due to an extended H II disk, if the orientation of the H II disk in the outer Galaxy is similar to that of the H I disk. Such an extended H II disk is suggested by the C IV absorption in a quasars spectrum, the appearance of H I disks of nearby spiral galaxies, and models of Lyα cloud absorbers and the Galactic fountain. We detect 11 of the 12 sources at one or more frequencies; nine of the sources are compact and suitable for an angular broadening analysis. A preliminary analysis of the observed angular diameters suggests that the H II disk does not display considerable warping or flaring and does not extend to large Galactocentric distances (R 100 kpc). A companion paper (Lazio & Cordes) combines these observations with those in the literature and presents a more comprehensive analysis.

Images of HCO(+) (1-0) emission in a molecular cloud near 1E 1740.7 - 2942

We have observed the hard X-ray source 1E 1740.7 - 2942 in the HCO(+) (1-0) line using the Owens Valley millimeter interferometer. Previous single-dish observations have found HCO(+) emission coincident with the location of the radio continuum hot spots of the radio source associated with 1E 1740.7 - 2942. Our higher resolution observations show a 15 sec offset between the HCO(+) emission and the location of the radio hot spots. We propose that the lack of emission results from a large ionization rate, exceeding 10-15/s, in the neighborhood of 1E 1740.7 - 2942.

The Low-Frequency Array (LOFAR): Opening a New Window on the Universe

Decametric wavelength imaging has been largely neglected in the quest for higher angular resolution because ionospheric structure limited interferometric imaging to short « 5 km) baselines. The long wavelength (LW, 2-20 m or 15-150 MHz) portion of the electromagnetic spectrum thus remains poorly explored. The NRL-NRAO 74 MHz Very Large Array has demonstrated that self-calibration techniques can remove ionospheric distortions over arbitrarily long baselines. This has inspired the Low Frequency Array (LOFAR)-a fully electronic, broad-band (15150 MHz)antenna array which will provide an improvement of 2-3 orders of magnitude in resolution and sensitivity over the state of the art.

A VLA search for the Geminga Pulsar at 74 and 326 MHz

We have used the VLA to image the location of the gamma-ray pulsar Geminga at 74 and 326 MHz. Upper limits to the pulse-averaged flux density, taking diffractive scintillation (DISS) into account, are S S

The Galactic Center at 327 MHz

Figure 1 presents a wide-field, high dynamic-range, 327 MHz VLA2 image of the Galactic center (GC). This image was constructed from archival VLA data using new 3-D image restoration techniques which resolve the problem of noncoplanar baselines encountered at long wavelengths. In a recent paper (LaRosa et al. 2000) we presented a catalog of over a hundred sources from this image, 23 extended sources and 78 small-diameter sources. The catalog contains flux densities, positions, sizes, and, where possible, a 20/90 cm spectral index. We also present subimages of all the extended sources. We refer the reader to LaRosa et al. (2000) for the details. In this note we will concentrate on observations of the nonthermal filaments and briefly describe a new model for their formation. The origin and evolution of the nonthermal filaments (NTFs) observed in the GC is an outstanding problem. All of the 7 classifiedNTFs are visible on Figure 1: Four of these are labeled threads, the other three ate the Snake,the Pelican,and the Sgr C filament. The wide-field imaging at 327 MHz lead to the discovery of the Pelican (Lang et al. 1999). This filament has the distinction of being the farthest NTF in projection from Sgr A and the only NTF that is parallel to the Galactic plane. One critical issue for understanding the activity and overall structure of the GC is whether these filamentary sources trace a pervasive, large-scale magnetic field or are local independent structures (e.g., Yusef-Zadeh 1989; Morris 1994, 1996; Uchida & Gusten 1995; Yusef-Zadeh, Wardle & Parastaran 1997; Shore & LaRosa 1999; Lang et al. 1999; Lang, Morris & Echevarria 1999; LaRosa et al. 2000).

A VLA Survey for Radio Pulsars in the Galactic Center

A radio pulsar in the Galactic center will suffer 350 seconds of pulse broadening. An imaging search for compact sources is far less desensitized by angular broadening than a periodicity search is affected by pulse broadening. We have conducted an imaging survey of the GC using the VLA and have detected approximately 200 sources. Six compact, steep-spectrum sources have been identified; additional compact sources with no spectral information have also been identified. Additional observations are in progress.