Vela Pulsar Project Early Results

Transcription

1 Vela Pulsar Project Early Results Steve Olney (VK2XV) NRARAO 13th April 2015 Some encouraging results are described and possible future developments Introduction main GUI overview. As you can see there is a lot of information crowded onto the screen. For over two years now (starting from the end of 2012) I have been pursuing the gaol of trying to detect the Vela Pulsar signal with a small antenna arbitrarily defined as an antenna with a physical capture area of less than 5 m2. During that time, upon advice of Joe Taylor (K1JT), I have gone from lower observational frequencies to higher frequencies. The current attempt is conducted at an observational frequency of 400 MHz. NOTE: The following results are presented as general interest only and do NOT constitute any claim of being valid. The general methodology used can be found in any good reference book on pulsar astronomy. I use Handbook of Pulsar Astronomy by Lorimer & Kramer as my Vela Pulsar Project 'bible'. The ONLY unique aspect to the method used is a bespoke selection of system parameters tailored specifically to the properties of the Vela Pulsar signal. This is done because I have a theory that this particular configuration increases the chance of successful detection. For the sake of convenience I term this configuration the FFT Sweetspot. The theory behind this configuration has been presented elsewhere and won't be discussed here. In the near future details of this bespoke configuration will be re-presented if the project achieves success. Figure 1: Main GUI Screen This main GUI screen is comprised of a number of sections. Topleft is a section containing general information (Figure 2), most of which is updated in real-time. Suffice to say at this point is that the FFT Sweetspot configuration, in my opinion, turns the usual negative effect of dispersion into a positive factor. This theory remains to be proven by practical results. Here presented is information about efforts to prove my theory with practical results. Summary of Results So Far The project development has progressed to the point where some real data can be acquired and analysed. Initially to test my FFT Sweetspot theory, simulations in software were done. The results of those simulations led to the selection of the Vela Pulsar as the target pulsar, as well an observational frequency of 400 MHz and an observational bandwidth of 5 MHz in accordance with the principles of the FFT Sweetspot theory. Details of that principle will re-presented in a note later on as mentioned above. Figure 2: General Information Next came the development of software for data acquisition and analysis closely followed by a start on the parallel development of hardware. The next section contains Vela positional information (Figure 3). This is also updated in real-time. The next section across the screen contains a visual representation of Vela's position in the sky (Figure 4). Note that it is not a true altitude-azimuth plot, but simply represents that path traced while looking at the south celestial pole. Specifically it tells me visually when Vela is above or below the horizon and when it is traversing the antenna beamwidth. Software Data Acquisition and Analysis The software is written in C# in a Windows environment (apologies to all those UNIX aficionados out there) using Microsoft Visual C# Express I find this a great software development environment for my purposes. Figure 1 shows the 1

2 Figure 3: Vela Positional Information Figure 4: Vela Visibility and Period Data Below the visibility section in Figure 4 is a small section ( Vela Pulsar Period Data ) containing the predicted frequency by two different methods. The linear fit value in green is used during the offline analysis stage to mark the predicted frequency. The next section across to the right on the GUI is time control (Figure 5). Here the display can be set to update in real-time, or in time lapse mode (increments every second by user selectable steps of months, days, hours or minutes). Lastly the time can be set manually to within 1 second to any time so desired. A cool feature of this is that I can, via a drop down menu, specify that a range of parameters are written to a file while stepping through a time range. For example, by writing predicted frequency to the file I can produce data that can be loaded into a spreadsheet allowing the plotting of the frequency versus time. This plot can be very interesting as it shows the effects on the instantaneous observed pulse frequency caused by the Vela Pulsar's intrinsic spin down plus the doppler shift caused by the earth spinning as well as its orbital trajectory around the Sun. The section on the bottom right (Figure 6) is the data acquisition control panel which contains controls for sample rate, data run 2

3 length, filename tags, and various other factors. There is a Record Now button as well as a button to assert the automatic recording scheduler which runs unattended commencing from the hour angle specified. Figure 7: DFT Dynamic Track Search Window Hardware RF Chain and Data Acquisition Working backwards through the hardware chain from the data acquisition computer (Windows 7 Pro OS) which runs the previously described software, we have an external USB soundcard interface. This external soundcard ( the blue object in Figure 8) is a cheap ebay purchase and was selected because it has a relatively large case which can be disassembled important because it needed to be modified. Figure 5: Time Mode Panel The remaining section on the GUI residing at the bottom left is an antenna positioning control. At the moment the coding for this function is done, but untested, as this may or may not be a requirement of the future. Figure 8: Soundcard Interface and Rubidium Vapour Frequency Source Figure 6: Data Acquisition Control Panel The modifications done were the removal of the low value chip input coupling capacitor replacing it with a 4.7uF value to lower the LF corner frequency to 5 Hz and the replacement of the onboard 12 MHz quartz crystal with an interface allowing connection to the external Rubidium Vapour Frequency Source as shown in Figure 8. This gives an accuracy and stability of the sampling clock of the order of 1 part in From the menu bar across the top of the main GUI a number of modal windows can be launched. A number of them are test bed functions, while some of them are analysis tools. Included in this group are test file generators, experimental processing modules and the like. Also accessed from that menu are the daily working modules used to apply RFI excision and to do offline analysis of the data. The soundcard is digitising (at 1500 sps) the output of a 45 MHz IF amplifier whose output is full wave detected and lowpass filtered. Included just before detection is a 5 MHz bandwidth filter operating at the IF frequency. The amplifier/detector is pictured in Figure 9 which shows a heatsink and fan which is necessary because the ERA-5 amplifiers dissipate a fair amount of heat. As an example Figure 7 shows the DFT Dynamic Track Search module. This is the module used to analyse the data and produce a display of the result along with some statistical analysis. Analyses can be done at various levels of bin zoom and number. The graph is annotated with predicted Vela pulse frequency, the maximum peak frequency and some statistics and a panel containing file data parameters. The input to the 45 MHz IF amplifier comes from a tap into a Yaesu FRG-9600 connected to the first IF output of the tuner. 3

4 within about 1 part in No discernible sidebands were detectable indicating that the USB soundcard interface was not dropping samples. To determine what antenna might be needed in terms of directivity and sidelobe pattern a rough RFI survey was done. It was during this survey that some unexpected results were obtained. Result #1 The RFI survey was conducted over a number of days using an existing 6M dipole mounted about 3.5 m above ground level as shown in Figure 11. Antenna simulation software gives this antenna a directivity of about MHz. Figure 9: 45 MHz IF Amplifier and Detector The input to the FRG-9600 comes from a MiniCircuits ZKL-2R7 amplifier whose output is padded by a 3 db attenuator to enhance stability. Figure 11: 6M Dipole RFI Survey Antenna On examining the results it was found that on some days (9 days out of 28) a significant peak was evident right at the predicted Vela frequency. This was unexpected. Upon summing the spectrum data from those 9 days incoherently ( 9 x 4 hours = 36 hours of data) this peak was enhanced and is shown in Figure 13. Figure 10: Operating Position As stated, when presented in a previous post, this is an interesting observation, but in no way constitutes a positive result for at least two reasons. Firstly, the result was arrived by looking at results and then going back and cherry-picking the input data to make the results look better. Secondly, the result is not conclusively statistically significant enough. A connection is then made to a LNA (0.6 db NF, 20 db gain) through a 9 db attenuator pad. The LNA is the kitmanlaw2008 product tested by Whitman Reeve and Christian Monstein in the July-August 2013 SARA Journal. From the input of that LNA there is a 25 m run of RG-58 to the roof where a plastic lunch box contains a LNA BPF LNA configuration which finally connects to the antenna. It was postulated at the time that perhaps, because the 6M dipole is linearly polarised, the bad days could be the result of cross polarisation of the antenna with incoming linearly polarised signal while the good days were more aligned. Note that the Vela pulsar signal polarisation swings almost 90 degrees in position angle during the ON phase of the pulse. NOTE: The configuration for Result #1 below had the LNA BPF LNA chain located inside the house at the operating position. Currently there is 12V SLA battery powering the LNAs in that same plastic lunch box. Result #2 Initial Activities and Early Results Being not convinced, but certainly encouraged, I decided to change the antenna and re-locate the LNA->BPF->LNA chain to a position in close proximity to that antenna. A regime of software testing was carried out to verify the algorithms by firstly using artificially generated files to stress test the code. Another test was to use a second, separate Rubidium Vapour Frequency Source to gate noise at 10 times a second into the RF input and checking that the spectrum spike occurs exactly at that frequency (10 Hz). Note that no calibration is needed as the sampling clock frequency is known to The antenna used for this second data run was an existing quadrifilar antenna used for amateur satellite reception (436 MHz). This antenna has some directionality (perhaps 8 dbi). It is a circularly polarised antenna and so there should be no good days and bad days. All days should middling. What the quadrifilar antenna directivity and axial ratio 400 MHz is unknown. 4

5 Note that there is no cherry-picking of data in the second run as all 15 consecutive days data were included in the summation. While it took 60 hours of data summation to get the result in Figure 14, the expected advantage of the second antenna configuration was likely to have been largely neutralised by the high levels of RFI during the second data run which was not evident during the first data run. Comments So, once again we are left with a result where there appears to be a signal which behaves like a Vela Pulsar signal (tracks exactly as the expected daily drift in topocentric frequency) which holds over a time span of some 43 days. Figure 12: Quadrifilar Antenna Repeating the process over 15 days and incoherently summing the spectrum data (15 x 4 hours = 60 hours of data) showed a peak again at the predicted frequency as shown in Figure 14. I can think of no mechanism which could cause such an artefact or any signal that could mimic a Vela Pulsar signal so well. None of the clocks in the hardware is synchronised to the Vela Pulsar pulse period. Also, there is no place in the software where the processing is synchronised with the pulse period. Extensive testing with data which are known NOT to contain Vela signals (i.e., data collected when Vela is below the horizon) shows no such peak. Nevertheless it seems an extraordinary result and so needs further observations to establish whether it is a real result or not. I am in the process of building an antenna which is specifically designed for reception of the Vela Pulsar signal (as opposed to the current two which were just 'lying around'). This antenna should show a marked improvement if these results are valid. Figure 13: Result #1: Summing the 9 Best Days out of 28 Days Conclusions Not only that, but the peak was at a frequency about 27 ppm lower in frequency as calculations show the Vela pulse frequency should have moved over the intervening 29 days between the two data runs. A suite of software and accompanying hardware has been developed for the express purpose of detecting a signal from the Vela Pulsar. This configuration is built according to principles of my FFT Sweetspot method. Early results are encouraging but remain unverified. The construction and successful commissioning of an antenna specifically designed for the purpose of detecting a signal from the Vela Pulsar should show an improvement on the results obtained so far. If an improvement is observed within reasonable bounds as predicted the results presented here could be considered as real. Nevertheless they cannot be viewed as verified as their statistics are not significant enough. If an improvement is not observed within reasonable bounds then these results can be discarded, but remain as a good lesson in the need for good statistics to establish validity. Figure 14: Result #2: Summing of all 15 days of Data (No Cherrypicking) Considering the data was plagued by high levels of RFI, especially from day 12 onwards (and day 7 was severely chopped around by an electrical storm), this is an unexpected result. 5