NAIC/AO No.28.Pm
Total Page：16
File Type：pdf, Size：1020Kb
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. – The Editors Understanding Pulsar Weather Joanna Rankin, Physics Dept., Univer- sity 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 Fig. 1: A short sequence of “drifting sub-pulses” from pulsar B0943+10. Pulse number (or revolution) is on Service Observing Specialist ... 28 the vertical axis, which is plotted against rotational phase or “longitude”. The average profile is given in Employee of the Year ............... 28 the bottom panel, and the pulse energy in the left-hand panel. Comings and Goings ................ 29 The NAIC is operated by Cornell University under a Cooperative Agreement with the National Science Foundation. appointing. Some progress has been Average profile Fluctuation Spectra made, but in baby steps rather than great 50 (longitude-resolved) 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 ap- plied in a detailed manner to the obser- vations of even a single pulsar. This is bad! One wonders whether young grad- uate students should be discouraged from having anything to do with this benight- 010 ed area! longitude (deg.) Somewhat by chance, however, study of one pulsar seems to be reopening these unpromising questions, providing imag- -10 es and insights, prompting development of new techniques, and very possibly of- fering routes of approach to understand- 150 ing many of the “classical” phenomena that pulsars exhibit in their radio pulse 100 sequences. 50 The pulsar is an obscure object known as B0943+10, which is so weak 0 0.1 0.2 0.3 0.4 that its pulse sequences have only so far frequency*P been observed with the Arecibo instru- ment − though it was discovered (quite early in 1970) at 102 MHz using the 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 Pushchino Observatory of the Lebedev 0.07 cycles/period as well as the symmetrical “sidebands” around the former. Physical Institute in the then Soviet Union. Even at Arecibo, it is a poor pros- mode, whereas the last 170 have it just so happened that Deshpande had pect, and had it not been for the wonder- switched to the weaker, disorganized already developed some new strategies ful accident that Pushchino colleagues “Q” mode. This very sequence provid- for studying pulsar fluctuations that suf- Svetlana Suleymanova and Vera Izvek- ed the first opportunity to study the pul- fer from aliasing. ova were able to join a 1992 pulsar po- sar’s “profile modes” at 430 MHz, and A typical “B” mode pulse sequence larimetry project organized by myself it was the first-ever observation to actu- is shown in Fig. 1, where the intensity is and then postdoc N. Rathnasree, we ally “catch” a “B” – to – “Q” mode tran- plotted against both pulse number and would probably have never put this star sition. Just at this level, the results were longitude, with the former average (pro- on the source list. Svetlana, however, very interesting, and a report was pub- file) at the bottom and the latter at the has a “special relationship” with this lished last year (Suleymanova et al. 1998 left. The characteristic diagonal “drift” “Russian” pulsar, and she simply would JAA 19, 1). not take “no” for an answer! This was bands and near-alternate-pulse modula- also a time of overpowering 430-MHz During the course of this work, how- tion are quite evident. Note also the ex- interference at Arecibo, but through ever, pursued during several visits to the ceptionally strong individual pulses at an some miracle the US Navy and other Raman Research Institute in Bangalore, interval of about 40 periods. Comput- spectral villains kept their peace for the Avinash Deshpande and I discovered ing the fluctuation-power spectra for duration of this observation. that B0943+10 has by far the most ac- each longitude, we obtain the result in curate and stable pattern of drifting sub- Fig. 2. Note the primary feature at ~0.46 The resulting 18-minute, 986-pulse pulses of any known pulsar − cycles/period (c/P), which reflects the ap- sequence is certainly the highest quality considerably more so than the pulsar proximately even-odd modulation pat- observation ever recorded on this star usually mentioned in this connection, tern; it is unresolved in this 256-point and was well calibrated both in polar- B0809+74. The 1992 October sequence Fourier transform as well as in longer ization and intensity. Its first 816 pulses was so stable that we might even be ones – therefore its Q must be at least are in its bright, regularly drifting “B” tempted to call it “coherent”. Moreover, 500! October 1999, Number 28 2 NAIC/AO Newsletter Fig. 2 shows four clear modulation ed by the 0.027-c/P “sideband” spacing emission pattern poleward of the mag- features, two more than have ever been unambiguously indicates that there are netic axis with a –4.3° impact angle. seen before in the fluctuation spectra of just 20 such sub-beams. The circulation This geometrical information, together any pulsar. Our problem is to understand time of a given sub-beam is then just 20 with the sense of the polarization-angle their relationship and the emission con- times the 1.87-P interval between adja- traverse, remarkably permits us to de- figuration which they represent. First, cent sub-beams or some 37.3 periods. To termine the absolute directions of rota- we must understand whether these fea- test this hypothesis, we can fold the en- tion of both the pulsar and the sub-beam tures reflect the actual modulation fre- tire sequence at this interval, and, indeed, pattern. Simple-minded arguments suf- quency, or whether they are aliases of it shows just 20 clear emission elements, fice to show that the sub-beams rotate − faster fluctuations that cannot be ade- with some 2-3 times as bright as others! that is, their tracks are circular. quately sampled at the slow rate at which A good deal is known about the emis- With these factors at hand, we have the star’s rotation brings the “window” sion geometry of B0943+10, because it been able to map the sub-beam pattern of emission back into our view. Or, said can be observed over a wide range of of pulsar B0943+10 using what we call differently, it cannot easily be decided frequency and the observed widths of its a “cartographic” transform. The ob- whether the “drift” in Fig. 1 is to the right profiles modeled. We find that the star’s served sequence, in terms of pulse num- or to the left and whether the sub-pulses rotation and magnetic axes are closely ber and pulse longitude, is simply move just a little from pulse to pulse or aligned at an angle of 11.6°, and that our mapped back into the frame rotating a lot. sightline makes an angle of 7.3° to its around the magnetic axis of the star – To make a long story short, we have spin axis; therefore, the sightline cuts the that is, using a magnetic polar coordi- been able to resolve the aliasing by tak- ing the approach of computing a fluctu- ation spectrum of the entire sequence with the unsampled regions between suc- cessive 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 first- order alias of an actual 0.535 c/P phase modulation, and that the secondary fea- ture (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 sub- pulse 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) – that is, in the same direc- tion 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 ampli- tude modulation on the phase modula- tion. If the “drifting” sub-pulses are produced by a system of sub-beams, ro- tating around the star’s magnetic axis, then just such a pair of “sidebands” Fig. 3: Map of the sub-beam pattern projected onto pulsar B0943+10’s magnetic polar cap.