The Commensal Realtime ASKAP Fast Transient Survey (CRAFT[5]) will search the ASKAP data stream for fast (< 5sec) transient events, associated with the most extreme conditions in the Universe. The CRAFT search will run in parallel with all normal observing on ASKAP, giving a tremendous advance in the transient parameter space which can be searched. ASKAP offers high sensitivity, high resolution, and continous observations of a significant portion of the sky. However, to search across the wide field of view and at the data rates which are being provided by ASKAP is extremely challenging. Nevertheless, as reported here, we are on track to achieve the goals as laid out in the Survey Science Proposal. Using GPUs as a simple highly parallel compute-engine we can monitor the full field of view with a 5(sigma) sensitivity of ~Jy for a millisecond event covering the astronomically significant range of DMs. After that trigger detection we can download the beamformer data-buffer and image the sky at full sensitivity and spatial resolution with an arbitary frequency and time resolution

Dodson, R.

Harris, C.

Pal, S.

Wayth, Randall

English

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© 2010 Proceedings of Science. The author(s) and right holder(s) of such contributions grant(s) to all users a free, irrevocable, worldwide, right of access to, and a license to copy, use, distribute, transmit and display the work publicly and to make and distribute derivative works, in any digital medium for any responsible purpose, subject to proper attribution of authorship (community standards, will continue to provide the mechanism for enforcement of proper attribution and responsible use of the published work, as they do now), as well as the right to make small numbers of printed copies for their personal use.  PoS(ISKAF2010)064A GPU based Transient Dedisersion Search Enginefor CRAFTRichard Dodson∗,Christopher Harris, Sabyasachi Pal, and Randall Wayth,Int. Centre for Radio Ast. Research (ICRAR), Perth, AustraliaE-mail: richard.dodson@icrar.orgThe Commensal Realtime ASKAP Fast Transient Survey (CRAFT[5]) will search the ASKAPdata stream for fast (< 5sec) transient events, associated with the most extreme conditions inthe Universe. The CRAFT search will run in parallel with all normal observing on ASKAP,giving a tremendous advance in the transient parameter space which can be searched. ASKAPoffers high sensitivity, high resolution, and continous observations of a significant portion of thesky. However, to search across the wide field of view and at the data rates which are beingprovided by ASKAP is extremely challenging. Nevertheless, as reported here, we are on trackto achieve the goals as laid out in the Survey Science Proposal. Using GPUs as a simple highlyparallel compute-engine we can monitor the full field of view with a 5σ sensitivity of ∼Jy fora millisecond event covering the astronomically significant range of DMs. After that triggerdetection we can download the beamformer data-buffer and image the sky at full sensitivity andspatial resolution with an arbitary frequency and time resolution.ISKAF2010 Science Meeting - ISKAF2010June 10-14, 2010Assen, the Netherlands∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(ISKAF2010)064A GPU Dedisersion Search Engine for CRAFT Richard Dodson1. The CRAFT SurveyThe frontiers of physics are advanced by observations of the extremes, and short-timescaleDynamic Sources are associated with the most extreme energy density events in the Universe.They can be used as the ultimate laboratory; their emission must be generated by states of matterthat probe physical regimes that far exceed the range achievable in any terrestrial experiment. Theexistence alone of such high energy densities transforms our understanding of the behaviour ofmatter and space-time under these conditions. The Commensal Realtime ASKAP Fast Transient(CRAFT) Science Survey Project will capture these events using ASKAP. Both the survey and theinstrument will extend Radio Astronomy into high time resolution single-pulse observations thatwill continuously monitor a significant portion of the sky.The traditional operational mode of most radio telescope precluded the possibility of detectingrare sources with short lifetimes. Up till now the most sensitive radio telescopes, by design, havesmall fields of view, and positional accuracy comes from combining long observations, with anumber of antennas in an array, hiding all short timescale events. As expounded in a number ofplaces (e.g. [2]) there is the realisation that one of the unexplored regions of ‘parameter space’ thatthe SKA will allow us to explore is that of short timescale transient sources.A major new facility that will open this area is ASKAP, which is being built in Western Aus-tralia by CSIRO. The prototype is expected to be functioning by 2011, and the full array by 2013.ASKAP is formed from 36 antennas each with a 30 sq. deg. Field of View (FoV). Therefore itis ideal for the search for Highly Dynamic Sources with high sensitivity and with sufficient skycoverage to catch rare events.There are a number of existing pre-SKA surveys to hunt for such events, such as the ALFAtransient survey [4], the High Time Resolution Survey [7] on the Parkes-Multibeam and the Flys-eye survey [10] with the ATA dishes. These are indicated on Figure 1, which shows the resolution(in degrees) and sky-coverage (in degrees2) against sensitivity. To normalise (and simplify) thisfigure all surveys have been assumed to have a common bandwidth and system temperature of300MHz and 25K, so that the vertical axis is in Jys. The three mentioned surveys are essentiallysingle-dish surveys improved by a ‘multi-view’ scale factor. This is provided by the 13-beams inone dish for the Parkes-MB survey or the 42-dishes pointed in different directions for the Flys-Eyesurvey. These surveys therefore only probe the ‘explored’ zone between the green line (single dishresolution) and the red line (multi-view FoV) in Figure 1. The CRAFT survey on the other hand,being an interferometer with a wide FoV, has a good sensitivity to rare events combined with highpositional accuracy and spans 30 sq. deg. with a post-detection resolution of 10′′, as indicated bythe gray region.2. The TargetsPredictions have been made of a number of the types of events that would measure fundamentalastronomical parameters, and in the last few years there have been some detections of these classesof signals. These are well reported elsewhere, so here we only present a summary.2PoS(ISKAF2010)064A GPU Dedisersion Search Engine for CRAFT Richard DodsonFigure 1: The CRAFT survey will search a greater area of parameter space than other similar sur-veys. The parameter space searched by the Arecibo-ALFA, Parkes-MB and ATA-FlysEye surveysare indicated with orange boxes. These are bound between the green line, the single dish resolutionand the red line, the FoV with a multi-beam instrument. CRAFT has wide FoV and high resolutionas indicated by the gray area. This is an essential pre-requiste for the identification of a transientsource.The transient events that CRAFT is sensitive to include: pulsar Giant Pulses, which have beenobserved with brightness temperatures of 1037K[6]. We would be able to detect such a pulse acrossour Galaxy and beyond.Rotating RAdio Transient Sources (RRATS)[9] are similar to normal pulsars but with verylow duty cycles, having detectable emission for less than a second per day. To make a full surveyof RRATS populations we require single-pulse searches with wide FoV instruments. As such, theCRAFT survey is well matched to this task.Similarly, the Magnetar J1810-197 has been detected with peak single-pulse amplitudes ∼10Jy and pulse widths ∼0.15s[1]. Such sources could be detected across the Galaxy by CRAFT,potentially converting ASKAP into the premier Magnetar detector.CRAFT has been developed with the goal of discovering more examples of the mysterious‘Lorimer Burst’[8]. As such we have developed a system which will provide high time resolutionsingle-pulse observations that will continuously monitor a significant portion of the sky, which isessential to find more of this clearly rare class of sources. Several approaches are being tried. Wepresent in this paper the GPU-based Transient Dedispersion Search Engine which is able to detecttransients in real-time. This is followed up by the offline analysis of a data-buffer and thereforeprovide the dispersion, time and most importantly the position of the transient event. Full details3PoS(ISKAF2010)064A GPU Dedisersion Search Engine for CRAFT Richard Dodsoncan be found on the CRAFT webpages1.The ASKAP beamformer stores the data in memory in a circular data buffer and also formsthe autocorrelations which it provides to the CRAFT external hardware for an incoherent search.These data are sampled at every millisecond and every MHz and will be transmitted directly to thesystem as UDP data-packets. Once this system detects a transient event a trigger is passed backto the beamformer and the stored data is downloaded from memory for complete, full sensitivity,offline analysis.3. The CRAFT GPU Dedispersion Search EngineThe CRAFT first phase solution for the transient detection is to search in the summed totalpowers (per beam) from all antennas for event triggers. On the detection of an event, and thepassing of several simple sanity checks, the data buffer is downloaded for full, offline, imaging.The ASKAP beamformer will be used to provide the autocorrelations, for each 1MHz channel,for each beam, for each millisecond. This gives out 2-bytes words for the power in every one ofthe 304 1 MHz-channels and between 16–36 dual-polarisation beams every millisecond, and fromevery antenna. This quite modest data rate is easily handled by off-the-shelf systems. We wishto search from low Dispersion Measures (DM) of around ten pc/cm3, which excludes earth-originpulsed emissions, to values up to three thousand or so, which would in found in pulsed emissionwhich passed directly through the galactic centre. Based on these values a rough operations countgives the required performance as around 50 G-FLOPS. The GPU provides a highly parallel pro-cessor, which can repeat multiple operations on single datasets (i.e. multiple dedispersions), in aneasily programmed environment. They provide excellent compute-power per dollar and per watt,both significant considerations for the CRAFT project.The data transformation from Frequency-domain to the DM-domain is being performed onthe GPU. We are running CUDA on a NVIDA 9800-GT (a somewhat obsolete system) yet evenwith this, the approach we have developed would able to incoherently search all beams with fullbandwidth, and millisecond time resolution, in real time, with only two GPUs. The Figures 2a and2b show the output from the Frequency-Time domain to the DM-time domain Transform. The firstcase is the results from searching simulated ASKAP data at 1.4-GHz containing a very strong 1 msburst with a DM of 300 pc/cm3; a ‘Lorimer-like’ event. DMs between 10 and 1600 were searched.The performance is 18 times real-time, so two GPUs would be capable of searching the wholeASKAP FoV. The second case is for real data from Parkes observing the Vela pulsar which has aDM of 69 pc/cm3, with time sampling of 64 µsec. The variable flux burst every 89 ms is clearlydetected. The single GPU only manages half real time in this case, but a more modern card wouldeasily be able to keep pace.4. Towards the Final SystemThe dispersion transform is only a single, but important, stage in the CRAFT transient capturesystem. In addition we have developed plans for:1https://pm.atnf.csiro.au/askap/projects/show/sup-craft4PoS(ISKAF2010)064A GPU Dedisersion Search Engine for CRAFT Richard DodsonMilli!SecondsDM ChannelDM Search250 300 350 400 450 500 550 600 650 700050100150200250(a) Results from a DM-transform of fake‘Lorimer-ilke’ data matching the ASKAP con-figuration. 5 minutes of single beam data can beprocessed in 24 seconds, which would allow theprocessing of 18 beams/GPU in real time.(b) Results from searching data taken from the Parkes High Res-olution Timing Survey (1024 channels over 400-MHz with 64µsec resolution) on Vela. The regular single pulses are clearlydetectedFigure 2: Results from the GPU de-dispersion system.• Detection of the Event in the time-DM plane. The initial demonstrator has a simple peakdetection, but more sophisticated approaches will be developed.• Network connection for the data-flow. Collation of the data for the GPU is a significant task,but will be based around the UDP streams from the ASKAP beamformer. A packet formatis being defined which will allow multiple machines to collect the data they will need, whilstkeeping the number of packets to a minimum and keeping the data rates below networklimits.• The requirements for data-buffers, which are that they must store the raw voltages with suf-ficient time-spans to recover DMs upto several thousands at an affordable price. We have5PoS(ISKAF2010)064A GPU Dedisersion Search Engine for CRAFT Richard Dodsona strategy for the off-loading of the data and for the offline processing. Increasing the on-board beamformer DRAMS to 2GB matches most considerations. We plan to offload viathe same network as for the trigger, into an offline processing machine which will form thecorrelations with DiFX [3].5. In ConclusionWe have developed the first phase solution for the CRAFT survey. The GPU system is capablesearching the ASKAP beamformer autocorrelation, in real time, and providing the trigger for thedownload of the baseband data for full sensitivity and resolution analysis of ASKAP detections offast transient events.The disadvantages of this approach, which makes an incoherent sum across the frequencychannels, is that there is significant sensitivity loss. However to form a coherent sum would requireone search per synthesised beam rather than one search per primary beam, which increases thework load by a factor of 250,000 for a gain in sensitivity of a factor of 6. Coherent approaches arebeing investigated, but this GPU provides a simple solution at minimal cost which will provide thelarge fraction of the survey goals. Further development of this system could be expected to improvethe performance sufficiently for coherent beam searching, perhaps with a limited set of baselines,sky-coverage and parameters.References[1] Camilo etal, 2006, Nature, 442, 892, “Transient pulsed radio emission from a magnetar”[2] Cordes, 2009, SKA memo 97, “The SKA as a Radio Synoptic Survey Telescope: Widefield Surveysfor Transients, Pulsars and ETI”[3] Deller, etal, 2007, PASP, 119, 853, 318,“DiFX: A Software Correlator for Very Long BaselineInterferometry Using Multiprocessor Computing Environments”[4] Deneva, etal, 2009, ApJ, 703, 2259, “Arecibo Pulsar Survey Using ALFA: Probing Radio PulsarIntermittency and Transients”[5] Dodson etal, CRAFT Survey Proposal,https://pm.atnf.csiro.au/askap/attachments/1758/FASTproposal.pdf[6] Hankins etal, Nature, 2003, 422, 141, “Nanosecond radio bursts from strong plasma turbulence in theCrab pulsar”[7] Keith, etal, 2010, MNRAS, subm., “The High Time Resolution Universe Pulsar Survey I: Systemconfiguration and initial discoveries”[8] Lorimer et al, 2007, Science, 318,777, “A Bright Millisecond Radio Burst of Extragalactic Origin”[9] McLaughlin etal, 2006, Nature, 438, 817, “Transient radio bursts from rotating neutron stars”[10] Siemion, etal, 2009, AAS, 214, 601.04, “Results from the Allen Telescope Array: The ATA Fly’s EyeTransient Search”We wish to thank W. Van Straten for the Vela data used in Figure 2b.6

A GPU based Transient Dedisersion Search Engine for CRAFT

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A GPU based Transient Dedisersion Search Engine for CRAFT

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