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October 1999, Number 28
As many of you may have noticed, a July 1999 issue of the NAIC Arecibo Observatory Newsletter was not published. This was due to unforseen and unavoidable technical difficulties. The result is this "double" newsletter, something we (and, no doubt, you) would be happy to avoid in the future. We regret any inconvenience caused to our readers. н The Editors

Understanding Pulsar Weather Joanna Rankin, Physics Dept., University of Vermont, Burlington, VT While radio pulsars have proved so very important to astrophysics over the last three decades, results to date of the great efforts made to understand the physical mechanisms of their radio emissions can only be regarded as frustrating and dis-

INDEX
Understanding Pulsar Weather 1 Radio Astronomy Highlights ..... 4 Zeeman Splitting a New Result . 9 SETI Observations at Arecibo 11 Space & Atmospheric Sciences 12 New Space and Atmospheric Sciences Program ................................ 13 Planetary Radar ....................... 18 Secondary Adjustment ............. 20 New Era in Communications .. 20 Pulsar Astronomy Seminars .... 20 LAN Upgrade ........................... 21 AOVEF News ............................ 23 Summer Student Program ...... 25 Service Observing Specialist ... 28 Employee of the Year ............... 28 Comings and Goings ................ 29

Fig. 1: A short sequence of "drifting sub-pulses" from pulsar B0943+10. Pulse number (or revolution) is on the vertical axis, which is plotted against rotational phase or "longitude". The average profile is given in the bottom panel, and the pulse energy in the left-hand panel.

The NAIC is operated by Cornell University under a Cooperative Agreement with the National Science Foundation.


longitude (deg.)

appointing. Some progress has been made, but in baby steps rather than great leaps of insight. While physical models of pulsar radio emissions are the subject of hundreds of articles and several books, no such theory has been successfully applied in a detailed manner to the observations of even a single pulsar. This is bad! One wonders whether young graduate students should be discouraged from having anything to do with this benighted area! Somewhat by chance, however, study of one pulsar seems to be reopening these unpromising questions, providing images and insights, prompting development of new techniques, and very possibly offering routes of approach to understanding many of the "classical" phenomena that pulsars exhibit in their radio pulse sequences. The pulsar is an obscure object known as B0943+10, which is so weak that its pulse sequences have only so far been observed with the Arecibo instrument - though it was discovered (quite early in 1970) at 102 MHz using the Pushchino Observatory of the Lebedev Physical Institute in the then Soviet Union. Even at Arecibo, it is a poor prospect, and had it not been for the wonderful accident that Pushchino colleagues Svetlana Suleymanova and Vera Izvekova were able to join a 1992 pulsar polarimetry project organized by myself and then postdoc N. Rathnasree, we would probably have never put this star on the source list. Svetlana, however, has a "special relationship" with this "Russian" pulsar, and she simply would not take "no" for an answer! This was also a time of overpowering 430-MHz interference at Arecibo, but through some miracle the US Navy and other spectral villains kept their peace for the duration of this observation. The resulting 18-minute, 986-pulse sequence is certainly the highest quality observation ever recorded on this star and was well calibrated both in polarization and intensity. Its first 816 pulses are in its bright, regularly drifting "B"

Average profile 50

Fluctuation Spectra (longitude-resolved)

-10

0

10

150 100 50

0

0.1

0.2

0.3

0.4

frequency*P

Fig. 2: Fluctuation-power spectra (center panel) as a function of pulse longitude and frequency as well as the integral spectrum (bottom panel) of B0943+10. Note the primary and secondary features at about 0.46 and 0.07 cycles/period as well as the symmetrical "sidebands" around the former.

mode, whereas the last 170 have switched to the weaker, disorganized "Q" mode. This very sequence provided the first opportunity to study the pulsar's "profile modes" at 430 MHz, and it was the first-ever observation to actually "catch" a "B" н to н "Q" mode transition. Just at this level, the results were very interesting, and a report was published last year (Suleymanova et al. 1998 JAA 19, 1). During the course of this work, however, pursued during several visits to the Raman Research Institute in Bangalore, Avinash Deshpande and I discovered that B0943+10 has by far the most accurate and stable pattern of drifting subpulses of any known pulsar - considerably more so than the pulsar usually mentioned in this connection, B0809+74. The 1992 October sequence was so stable that we might even be tempted to call it "coherent". Moreover,

it just so happened that Deshpande had already developed some new strategies for studying pulsar fluctuations that suffer from aliasing. A typical "B" mode pulse sequence is shown in Fig. 1, where the intensity is plotted against both pulse number and longitude, with the former average (profile) at the bottom and the latter at the left. The characteristic diagonal "drift" bands and near-alternate-pulse modulation are quite evident. Note also the exceptionally strong individual pulses at an interval of about 40 periods. Computing the fluctuation-power spectra for each longitude, we obtain the result in Fig. 2. Note the primary feature at ~0.46 cycles/period (c/P), which reflects the approximately even-odd modulation pattern; it is unresolved in this 256-point Fourier transform as well as in longer ones н therefore its Q must be at least 500!

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Fig. 2 shows four clear modulation features, two more than have ever been seen before in the fluctuation spectra of any pulsar. Our problem is to understand their relationship and the emission configuration which they represent. First, we must understand whether these features reflect the actual modulation frequency, or whether they are aliases of faster fluctuations that cannot be adequately sampled at the slow rate at which the star's rotation brings the "window" of emission back into our view. Or, said differently, it cannot easily be decided whether the "drift" in Fig. 1 is to the right or to the left and whether the sub-pulses move just a little from pulse to pulse or a lot. To make a long story short, we have been able to resolve the aliasing by taking the approach of computing a fluctuation spectrum of the entire sequence with the unsampled regions between successive pulses interpolated with zeros. This involves a Fourier transforms of up to a million points, uses the circumstance that the fluctuations are continuously sampled within each pulse, and permits us to explore the harmonic structure of each feature. On this basis we find that the primary feature is in fact the firstorder alias of an actual 0.535 c/P phase modulation, and that the secondary feature (at ~0.07 c/P) is the second-order alias of its second harmonic at about 1.07 c/P. This explains the even-odd subpulse pattern, because 1/0.535 c/P = 1.87 P/c is the interval between sub-pulses at a given longitude. It also indicates that the sub-pulses "drift" from right to left (negatively) н that is, in the same direction as the star's rotation. We can also understand the pair of symmetrical features on either side of the principal one as indicative of an amplitude modulation on the phase modulation. If the "drifting" sub-pulses are produced by a system of sub-beams, rotating around the star's magnetic axis, then just such a pair of "sidebands" would result if some of the sub-beams are significantly stronger than others. The 0.535-c/P feature frequency divid-

ed by the 0.027-c/P "sideband" spacing unambiguously indicates that there are just 20 such sub-beams. The circulation time of a given sub-beam is then just 20 times the 1.87-P interval between adjacent sub-beams or some 37.3 periods. To test this hypothesis, we can fold the entire sequence at this interval, and, indeed, it shows just 20 clear emission elements, with some 2-3 times as bright as others! A good deal is known about the emission geometry of B0943+10, because it can be observed over a wide range of frequency and the observed widths of its profiles modeled. We find that the star's rotation and magnetic axes are closely aligned at an angle of 11.6ђ, and that our sightline makes an angle of 7.3ђ to its spin axis; therefore, the sightline cuts the

emission pattern poleward of the magnetic axis with a н4.3ђ impact angle. This geometrical information, together with the sense of the polarization-angle traverse, remarkably permits us to determine the absolute directions of rotation of both the pulsar and the sub-beam pattern. Simple-minded arguments suffice to show that the sub-beams rotate - that is, their tracks are circular. With these factors at hand, we have been able to map the sub-beam pattern of pulsar B0943+10 using what we call a "cartographic" transform. The observed sequence, in terms of pulse number and pulse longitude, is simply mapped back into the frame rotating around the magnetic axis of the star н that is, using a magnetic polar coordi-

Fig. 3: Map of the sub-beam pattern projected onto pulsar B0943+10's magnetic polar cap. The sub-beams rotate counter-clockwise, making a complete circuit every 41 seconds; whereas the pulsar rotates on its axis (shown as up in the map) every 1.1 seconds in a clockwise direction. The path of our sight line to the star is also indicated in the upper part of the map.

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nate system rotating at the sub-beam circulation rate. Such a map is shown in Fig. 3, with the star's rotational pole "up", the magnetic pole in the center of the diagram, and a portion of the sightline traverse indicated. Note the 20-fold sub-beam pattern. (Additional still and moving images of pulsar B0943+10's "weather" can be seen on the web at http://www.rri.res.in/~desh or http:// www.uvm.edu/~jmrankin under "What's New?".) Many aspects of this diagram are remarkable: The sub-beam pattern appears to represent a system of plasma columns which start at the star's polar cap, stream out through the radio emission region, and "up" toward the light-cylinder. At the stellar surface, the ring of plasma columns is no more than some 145 meters in radius н making their spacing less than 45 meters and size hardly 20 meters. Each of these plasma columns, however, carries some 2Ѕ1023 Watts, or about 10-3 of a Solar luminosity! These observations also permit us to explore phenomena in the polar-cap "gap" region, where physical conditions are some of the most "exotic" anywhere in the cosmos н with magnetic fields of the order of 1012 Gauss and an electric potential across this "gap" of some 1012 Volts! One theory, published 25 years ago by Malvin Ruderman & Peter Sutherland at Columbia University (1975 Ap. J. 196, 51) anticipated that electrical discharges н what they called "sparks" н would form in the polar-cap "gap" and that these "sparks" would circulated under the action of EЅB drift. Using this simple formalism, we are able to square our observed 41-sec circulation time with anticipated polar-cap conditions, as long as the radius of the plasma-column "feet" is about 75 m and the "gap" potential is somewhat lower than expected (about 3Ѕ1011 V). These results are reported in more detail in two papers (Deshpande & Rankin 1999, Ap. J., in press; and Deshpande & Rankin 1999, MNRAS, to be submitted).

More recently, we have confirmed the 20-sub-beam structure in two older Arecibo sequences, one at 430 MHz from 1972 and another at 111.5 MHz from 1992, and graduate student Ashish Asgekar has also done so using a 34MHz sequence recorded in 1998 at the Gauribidnur Array in India. Maps have also been constructed in full polarization, which show the sub-beam structure in terms of the linear and circular power in the pulsar's two polarization modes. The elliptically orthogonal nature of the modal polarization, long seen in various observations, is especially striking as displayed in the sub-beam maps. Furthermore, it has also proved possible to map the pulsar 's emission pattern through the transition from its regular "B" mode to its chaotic "Q" mode н implying that the circulation time remains nearly the same despite the dramatic change in observed emission-all the while providing insight into the nature of these "profile" mode changes. However insightful this one pulsar B0943+10 has proved to be, should we regard it as utterly unique? Yes & no! We know of no other pulsar which comes close to matching its "B" mode precision and stability. In this sense it is thus far unique, because this stability reflects the orderly sub-beam fluctuations which permit its overall configuration to be decoded without invoking other assumptions. However, other pulsars appear to have predictable circulation times without having as regular a "drift" pattern. We simply do not yet know what are the conditions which produce a B0943+10. "Null" pulses are well known to disrupt an otherwise regular "drift" pattern (as in pulsar B0809+74), and it is probably significant that no "null" pulse has ever been identified in B0943+10. Were the sub-beams unevenly spaced in magnetic azimuth or were their intensities not stable for several circulation times, the "drift" would appear quite irregular and any signature of a definite circulation time would be much more difficult to discern.

Pulsar B0943+10 then appears to have taught us a number of lessons: that "gap" discharges are not spacially continuous, but occur in columns, which can be stable over many circulations; that whatever the "action" that drives pulsar emission it moves and cannot therefore be a property either of the stellar surface or the fields; and that some pulsars are rather efficient in converting their stored rotational energy into radio emission. Consequently, we believe that these techniques hold considerable promise for shedding new light on the pulsar emission problem. We have been able to obtain preliminary maps of several other pulsars with geometrical configurations something like that of B0943+10 and plan to slowly work inward to consider pulsars where the sightline passes more centrally across the magnetic axis of the star. In the course of this project, we encounter many of the "classical" problems of pulsar radiation: core and conal beams, nested cones, modes, nulls, etc. Some of our results were presented in September at the IAU Colloquium #177 in Bonn, Germany.

Radio Astronomy Highlights Duncan Lorimer and Chris Salter HI in External Galaxies In a very productive visit to Arecibo, Chris Impey & Valorie Burkholder (Steward Observatory) measured the 21-cm HI emission of low surface brightness (LSB) galaxies taken from an optical sample selected on UKSTU plates. During the course of a ten-day run, they observed more than 100 galaxies with a detection rate of over 50%. The average HI mass detected was 109 MO, though they did make several 108 MO detections for nearby galaxies. These observers were able to reduce the data on site and left Arecibo with a list of over 50 new detections and measures of HI mass. They have learned that low surface brightness is most likely the result of low surface mass density, which in-

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2

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HI

/LB) (MO /LO) ее

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Fig. 4: The ratio of HI mass to blue luminosity plotted against central brightness in the B-band for HI detections from a LSB galaxy sample. (Courtesy Chris Impey)

hibits star formation. Therefore, galaxies with higher surface brightness should have lower gas mass fractions (indicating more efficient star formation) and lower HI mass to blue luminosity. Fig. 4 shows the new detections, combined with results on the same sample from an earlier Arecibo run; this is among the largest HI surveys of LSB galaxies ever carried out. There is a strong trend towards higher gas content for a lower surface mass density of stars. Unfortunately, the simple interpretation of this trend н LSB galaxies are young and therefore unevolved н ignores the red optical colors and high metallicities of many of these galaxies. This enigmatic population is still not understood. In May 1999, Chris Impey & Cathy Petry (Steward Observatory) obtained deep 21-cm HI observations toward the

quasar, Q1214+1804. The goal was to look for gas-rich galaxies at small impact parameters to this line of sight, and to subsequently relate the galaxies to 20 Ly- absorbers in the quasar spectrum. The redshift range 0 < Z < 0.2 was covered in two correlator settings. In addition to looking for galaxy counterparts to the diffuse hydrogen absorbers, the experiment provides an independent test of the blind HI surveys that define the HI mass function, work also done at Arecibo. Working over this broad redshift range exposed them to the full problems of interference and other artifacts. Fig. 5 shows the 1 HI mass detection limit as a function of Z for about 2 hours integration for tow of the configurations: low Z (top) and high Z (bottom). This represents about half of the low-Z data and a fifth of the high-Z data obtained.

1000 800 600 400 200 0 0.04 0.06 0.08 0.1
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Military Radar Lajas TARS

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Fig. 5: The 1- HI mass detection limit as a function of Z for about 2 hours On-Off data at low Z (top) and high Z (bottom). (Courtesy Chris Impey) RFI Notes From T.Ghosh: The degradation of detection limit at several z-ranges in this unprotected band is due to a number of military radars including a recently installed frequency-hopping system (`н' marked). There are also resonance modes at several frequencies in this range in the `L-wide'receiver. For more information, see http://www.naic.edu/~tghosh/smarg/smarg-vig1.html.

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Lajas TARS


The increasingly challenging astronomical environment at lower frequencies is clearly seen. The final pencil beam will reach a 5 limit at Z = 0.08 of about 3Ѕ108 MO, and will cover sufficient volume to detect 5-6 galaxies by their HI emission, in addition to any quasar absorbers that correspond to gas-rich galaxies. Galactic High Velocity Clouds (HVCs) have recently gained attention as being massive gas clouds distributed throughout the Local Group. The HVCs could be primordial objects raining on the Galaxy, either as remnants from the formation of the Local Group or as representatives from an intergalactic population of dark matter-dominated mini halos in which hydrogen has collected and remained stable on cosmological time scales (Blitz et al. 1999, Ap. J., 514, 818, and Braun & Burton 1999, A&A, 341, 437). These mini halos are generally predicted by semi-analytical simulations of galaxy and group formation in much larger numbers than can be accounted for by the known dwarf galaxy population. Assuming that the HVCs are dark matter dominated, with the fraction of hydrogen to total mass approximately equal to 0.1, it can be shown that they are gravitationally stable at typical distances of 1 Mpc from the Local Group's barycenter. Placed at those distances, the HVC HI masses are ~ 5Ѕ107 MO and typical diameters are 30 kpc. If this extragalactic interpretation of HVCs is true, similar clouds are expected in other galaxy groups. Martin Zwaan & Frank Briggs (Groningen) performed a targeted survey for these clouds in 5 nearby galaxy groups at distances between 25 and 40 Mpc, for which the Arecibo beam matches the typical cloud size, and most clouds should be detectable within a few minutes. A total of 300 pointings were observed on square grids centered on the group's barycenters. The pointings extended to radii of 2 Mpc and thus cover different environments within each group. The selected groups cover a range of compactness, group richness and total HI mass centered around the proper-

ties of the Local Group. The survey was sensitive to HI masses of 5Ѕ106 MO at the 5- level. After a first round of observations in April, all 4.5- peaks in the spectra were selected and re-observed in June with double the integration time. None of the peaks could be confirmed as being real HI detections. This null result places interesting upper limits on the space density of primordial gas clouds in galaxy groups. Phil Choi & Anthony Gonzalez (UC Santa Cruz) have observed 12 galaxy clusters in the redshift range of 0.12 < Z < 0.26 (1130-1270 MHz) using a driftscan observing mode. Mean integration times of 4 hr per cluster were achieved with the purpose of obtaining total cluster HI masses. All of the clusters in the sample are already well observed at both X-ray and optical wavelengths, so by combining that data with HI masses these observers will investigate the role that the hot intracluster medium (X-ray) and the cold galactic HI reservoir (radio) have on the star-formation history (optical) of the cluster. Despite working in a relatively unexplored region of the radio spectrum, clean observations with stable baselines were successfully obtained. In addition, a useful byproduct of these observations is that RFI in the frequency range of 1120-1280 MHz

was mapped out over the course of the week-long observing run. Molecular-Line Studies Jeremy Darling & Riccardo Giovanelli (Cornell) report the discovery of 11 OH megamasers (OHMs) and one OH absorber, along with upper limits on the OH luminosity of 54 other luminous infrared galaxies at Z > 0.1. The new megamasers show a wide range of spectral properties, but are consistent with the extant set of 55 objects, 8 of which have Z > 0.1. The new OH detections are the preliminary results of an in-progress Arecibo survey for OHMs which is expected to produce several dozen detections. The ultimate goal of the survey is to calibrate the luminosity function of OHMs to the low-Z galaxy merger rate (0.1 < Z < 0.2), and to use this measure to estimate the merger rate at higher Z using pointed and blind surveys. The survey will also provide an enhanced sample of OHMs for the study of their environments, engines, lifetimes, and structure. The selection criteria for candidate OHMs require IRAS detection at 60 Іm, and Z > 0.1. The redshift limit is imposed by the RFI environment, while IRAS detection guarantees that candidates are luminous infrared galaxies, which are galaxy mergers (Clements et al. 1999, MNRAS, 302, 391). OHMs

Fig. 6: OH megamaser detection for IRAS 21272+2514. This OH megamaser is the third most luminous known, (log LOH = 3.6). The arrows indicate the expected locations of the 1667- and 1665-MHz lines (left and right, respectively) based on the optical heliocentric velocity from the PSCz redshift survey (Saunders 1999, private communication). (Courtesy Jeremy Darling)

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require a high density of molecular gas, an energetic population-inversion mechanism (such as shocks, AGN or starbursts), and a radio-continuum source to stimulate emission. Mergers are quite efficient at shocking the ISM, funneling gas into the centers of the merging galaxies, fueling AGN, and producing bursts of star formation, so we expect mergers to be ideal environments for the production of OHMs. The detection rate is 1 in 6 to date, and a typical OHM detection is shown in Fig. 6. Murray Lewis (NAIC) has undertaken reobservation of OH/IR stars within the Arecibo sky in a program that has been both a learning experience in spectral-line use of the 305-m telescope, and a happy blend of science with student education. Its purpose is to obtain simultaneous 1612, 1665, and 1667 MHz quality spectra for about 400 OH/IR stars identified in earlier surveys of color-selected IRAS sources. Those observed by summer students in 1998 & 1999 provided instant visual gratification, as they are obvious, have a recognized classic norm, yet have diversity of morphology, intensity, and velocity range. These data are also well suited to learning reduction processes, with good feedback on both the software utilized and the observation's quality. Spectra are taken with an 0.39-MHz total bandwidth, giving a 0.14-kms-1 resolution at the OH lines. Moreover, the digital filters defining the narrower correlator bandwidths provide very smooth bandpasses. Our experience shows that observations of strong (>50 mJy) features need no comparison OFF-source spectra, nor is observation confined to nighttime by baseline ripples; good observations can be secured at midday. About 25% of Arecibo OH/IR stars have been reobserved to date, including all high-latitude stars. An extra stimulus is lent to this project though, by the new transient shell scenario. This expects the duration of the superwind supplying the dense shells hosting 1612-MHz masers to be rather

(a)
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19566+3423 24 April -1999
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Fig. 7: OH spectra of 19566+3423 at 1612, 1665 and 1667 MHz, a) in the late 1980' and b) on 24 April s, 1999. (Courtesy Murray Lewis)

brief, often < 1000 yr, while the duration of masers is even briefer. Thus, over 10 yr a few stars may exhibit intensity changes well beyond the factor of ~3 associated with the regular pulsation cycle. The program has already scored several successes in this regard. Thus, Ben Oppenheimer (REU student in 1998) found the peak intensity in 18455+0448 had dropped from 2000 to 200 mJy over 10 yr, while Lewis found that it then dropped to < 10 mJy in the following six months. Likewise, the OH masers in VY Her have decreased by a factor of >10 and disappeared. However, the most spectacular changes are those exhibited by 19566+3423, (see Fig. 7). Ten years ago this had a rather unusual 1612-MHz spectrum, prompting its early reobservation. While the factor of three increase in 1612-MHz intensity is dwarfed by the factor of 30 increase in 1665-MHz intensity, unprecedented changes in the velocity range of its emission have been observed. This has expanded from 16 to 42 kms-1 in the 1612-MHz line, and at 1667 MHz from 28 to 80 kms-1. It is presumably a supergiant or hypergiant star with a rapidly evolving circumstellar shell that may well have been lost in a sudden mass ejection event, rather than in a wind. Its

spectra are distantly reminiscent of IRC+10420, which has, however, always exhibited the same velocity range throughout the 24 yr since its discovery. The IRAS satellite provided positions for many thick dust shells. At Arecibo, about 400 of these were confirmed as OH/IR stars by detecting their 1612MHz masers, though up to 75% of candidates remained undetected. These OH/ IR star color "mimics" need explanation. Lewis has found that mimics are a mandatory feature of the transient-shell paradigm, in which a superwind only endures for ~500 yr after a He-shell flash, while the extra "flash" luminosity causes the star to expand, thereby increasing its period and mass-loss rate. The gas density in the circumstellar shell then rises past the threshold that allows dust to couple photon momentum to it, with an immediate increase in its expansion velocity. Since a newly accelerated shell quickly moves beyond its dust-shroud, its molecules are rapidly degraded by interstellar UV. Hence the mimics. The subsequent acquisition of masers by a mimic shows that its superwind has long climaxed, and that a protective dust-shroud from the current expansion is again extending the longev-

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tire 8-year data set, is 30 yr ago. They are developing an opershown as a solid curve. ation mode of the 168-MHz bandwidth 300 1612 MHz Since the amplitude of a Berkeley-Arecibo-Caltech Swift Pulsar propagation-induced peri- Instrument (BACSPIN) that can deliver 200 odicity would have to dedispersed time series of power in or100 scale as 2, it should be thogonally polarized channels (single or entirely invisible at 1130 subbands) and will soon have full Stokes 0 MHz given a ~4 Іs tim- parameter capability. Any observers in-20 0 20 40 LSR Velocity ( km/sec ) ing precision at that fre- terested in using BACSPIN are encourquency. Consequently, aged to contact Don Backer. Fig. 8: The 1612-MHz, OH-maser emission spectrum of 19586+3637, this result adds to the eviDuncan Lorimer, Fernando Camilo on 29 May 1999. (Courtesy Murray Lewis) dence in favor of a Moon(Jodrell Bank/Colombia) & Kiriaki Xilmass, 25.3-day orbit ouris have begun a timing campaign on companion to PSR B1257+12 and ity of molecules. 19586+3637, (alias 17 pulsars discovered over 25 years ago proves that this periodicity cannot be V1511 Cyg = IRC+40371), is a mimic by Hulse & Taylor during their survey induced by a propagation effect as posfrom Arecibo searches of 1988 and 1991 of the Galactic plane. The Hulse-Taylor tulated by Scherer et al. (and an earlier search at Nanчay). On survey is most remembered for its dis29 May 1999, Lewis found this star to Don Backer & Andrea Somer (UC covery of the binary pulsar, B1913+16, exhibit a 300-mJy 1612-MHz maser (see Berkeley) have begun to investigate "or- which Joe Taylor and collaborators have Fig. 8); a mimic that has recently become thogonal mode emission" from pulsars used as a Gravitational Laboratory for an OH/IR star. -- a phenomenon known for many the last 20 yr. Probably as a result of years, where it appears that two modes this, very little is known about the reof orthogonally or quasi-orthogonally maining pulsars. It is hoped that the new Pulsars polarized emission are competing in pul- series of measurements will yield some Alex Wolszczan, Ian Hoffman & Maciej sar magnetospheres. The net result is a surprises about the remaining pulsars Konacki (Penn State) and Kiriaki Xil- randomization of the integrated polariza- from this survey. ouris (NAIC/NRAO) have conducted tion position angle. In order to study Andy Fruchter (STScI), Kiriaki Xilmulti-frequency timing observations of the implications of this phenomenon on ouris, Duncan Lorimer, Jo Ann Eder & the planet pulsar, PSR B1257+12. Mea- the emission mechanism, Backer and surements were made on a daily basis Somer aim to measure the extent to Angel Vсzquez (NAIC) have confirmed between May 5 and June 6, 1999, at 430, which the two competing modes