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Identifying the Source of Perytons at the Parkes Radio Telescope

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Identifying the Source of Perytons at the Parkes Radio Telescope Mon. Not. R. Astron. Soc. 000, 000{000 (0000) Printed 10 April 2015 (MN LATEX style file v2.2) Identifying the source of perytons at the Parkes radio telescope E. Petroff1;2;3?, E. F. Keane1;4;3, E. D. Barr1;3, J. E. Reynolds2, J. Sarkissian2, P. G. Edwards2, J. Stevens2, C. Brem2, A. Jameson1, S. Burke-Spolaor5, S. Johnston2, N. D. R. Bhat6;3, P. Chandra7, S. Kudale7, S. Bhandari1;3 1 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Mail H30, PO Box 218, VIC 3122, Australia. 2 CSIRO Astronomy & Space Science, Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia 3 ARC Centre of Excellence for All-sky Astrophysics (CAASTRO). 4 SKA Organisation, Jodrell Bank Observatory, Cheshire, SK11 9DL, UK 5 National Radio Astronomy Observatory, 1003 Lopezville Rd., Socorro, NM 87801, USA 6 International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102, Australia 7 National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune University Campus, Ganeshkhind, Pune 411 007, India 10 April 2015 ABSTRACT \Perytons" are millisecond-duration transients of terrestrial origin, whose frequency-swept emission mimics the dispersion of an astrophysical pulse that has propagated through tenuous cold plasma. In fact, their similarity to FRB 010724 had previously cast a shadow over the interpretation of \fast radio bursts," which otherwise appear to be of extragalactic origin. Until now, the physical origin of the dispersion- mimicking perytons had remained a mystery. We have identified strong out-of-band emission at 2.3{2.5 GHz associated with several peryton events. Subsequent tests re- vealed that a peryton can be generated at 1.4 GHz when a microwave oven door is opened prematurely and the telescope is at an appropriate relative angle. Radio emis- sion escaping from microwave ovens during the magnetron shut-down phase neatly explain all of the observed properties of the peryton signals. Now that the peryton source has been identified, we furthermore demonstrate that the microwaves on site could not have caused FRB 010724. This and other distinct observational differences show that FRBs are excellent candidates for genuine extragalactic transients. Key words: surveys | methods: data analysis | site testing 1 INTRODUCTION detectability | is routinely used to screen out local interfer- ence detections in pulsar searches (Kocz et al. 2012; Keane \Peryton" is the moniker given to a group of radio signals et al. 2010). which have been reported at the Parkes and Bleien Radio arXiv:1504.02165v1 [astro-ph.IM] 9 Apr 2015 Observatories at observing frequencies ∼ 1:4 GHz (Burke- Perytons' most striking feature, which sets them apart Spolaor et al. 2011; Kocz et al. 2012; Bagchi et al. 2012; from `standard' interference signals, is that they are swept Saint-Hilaire et al. 2014). The signals are seen over a wide in frequency. The frequency dependent detection of the sig- field-of-view suggesting that they are in the near field rather nal is sufficiently similar to the quadratic form of a bona than boresight astronomical sources (Kulkarni et al. 2014). fide astrophysical signal which has traversed the interstel- They are transient, lasting ∼ 250 ms across the band, and lar medium, that the origin of the first fast radio burst, the 25 perytons reported in the literature occured only dur- FRB 010724 (Lorimer et al. 2007), was called into ques- ing office hours and predominantly on weekdays. These char- tion by Burke-Spolaor et al. (2011). This was mainly based acteristics suggest that the perytons are a form of human- upon the apparent clustering of peryton dispersion measures −3 generated radio frequency interference (RFI). In fact one (DMs) around ∼ 400pc cm , which is within ∼ 10% of of the perytons' defining characteristics | their wide-field FRB 010724's DM. Ongoing searches are actively searching for FRBs and perytons and are capable of making rapidly identifying de- ? Email: epetroff@astro.swin.edu.au tections. In this paper we report on three new peryton dis- c 0000 RAS 2 E. Petroff coveries from a single week in January 2015 made with the these observations were searched for perytons in the months Parkes radio telescope. In addition to the rapid identifica- of January−March 2015 in this work. Three events were tion within the Parkes observing band, the RFI environment discovered, all occurring in the week of 19 January on the over a wider frequency range was monitored with dedicated 19th (Monday), 22nd (Wednesday), and 23rd (Thursday) equipment at both the Parkes Observatory and the Australia of January, 2015 in a rotating radio transient search, the Telescope Compact Array (located 400km north of Parkes). PULSE@Parkes outreach project (Hobbs et al. 2009), and For one event, the Giant Metrewave Radio Telescope, in In- SUPERB, respectively. For the event on the 23rd of Jan- dia, was being used to observe the same field as Parkes. uary, simultaneous coverage with the Giant Metrewave Ra- Below, in x 2, we describe the observing setup and details of dio Telescope (GMRT) was also available, which was shad- the on-site RFI monitors. In x 3 we present the results of the owing Parkes as part of the SUPERB project's effort to lo- analysis of our observations, and our successful recreation of calise FRBs. peryton signals. x 4 discusses, in more depth, the identified The peryton search for SUPERB and other BPSR data sources of the signals and we compare the perytons to the is performed after the Parkes data has been transferred to known FRB population in x 5. We present our conclusions the gSTAR supercomputer facility at Swinburne University in x 6. of Technology. The peryton search is performed by sum- ming the frequency-time data of all 13-beams from BPSR and searching these summed data for single pulses using heimdall for pulses with a signal-to-noise ratio (S/N) > 10 2 OBSERVATIONS −3 and DM > 10 pc cm . This method ensures that dispersed As part of the SUrvey for Pulsars and Extragalactic Radio pulses occurring in a majority of beams are efficiently de- Bursts (SUPERB1, Keane et al. in prep.), at Parkes, real- tected even if they may be too weak in individual beams to time pulsar and transient searches are performed. The live be detected in signle-beam searches. For the perytons iden- transient searching system developed for SUPERB, which tified in January 2015, once the date and UTC time were uses the heimdall2 single pulse search software package, established the Parkes and ATCA RFI monitor data were is now routinely used by several projects. The survey data checked around the times of the perytons for the presence are taken using the Berkeley Parkes Swinburne Recorder of signals that might be correlated with the appearance of a (BPSR) which is used to produce Stokes I data from 1024- peryton at 1.4 GHz. The same search technique was applied channel filterbanks covering a total bandwidth of 400 MHz to search for perytons in the HTRU intermediate and high centred at 1382 MHz with a time resolution of 64 s and latitude surveys of Keith et al. (2010). The HTRU interme- 2-bit digitisation. For each pointing 13 such data streams diate latitude survey was conducted between 2008 and 2010 are recorded, one for each beam of the multi-beam re- and the high latitude component was conducted between ceiver (Staveley-Smith et al. 1996). 2009 and 2014. The HTRU survey concluded in February, The survey has been running since April 2014 to search 2014 and as such no RFI monitor data is available for events for pulsars and fast radio bursts. In December 2014 an RFI detected in these data nor for any peryton detected before monitoring system was installed on the Parkes site identi- those reported here. cal to ones which had been in operation at the Australia Telescope Compact Array (ATCA) since November 2014. The RFI monitor itself is a Rhode & Schwarz EB500 Mon- itoring Receiver capable of detecting signals across a wide 3 RESULTS range of frequencies from 402 MHz to 3 GHz. The frequency 3.1 Three perytons and time resolution of the monitoring system are limited to 2 MHz and 10 s, respectively. The antenna is mounted on The properties of the three perytons discovered in January a rotator, which sweeps out 360 degrees in Azimuth every 2015 are noted in Table 1 and Figure 1 shows the time- 12 minutes, then returns to an azimuth of 0 for another 8 frequency structure. These events are typical perytons in minutes before repeating the cycle. A spectrum is produced that they are bright and detectable in all beams of the every 10 seconds, which is obtained by stepping in 20 MHz multi-beam receiver. They are also apparently dispersed or steps across the full band. So each 10 sec spectrum has only `chirped' in frequency, but not strictly obeying the quadratic 0.1 sec of data at any given frequency. The installation of cold plasma dispersion law; signals from pulsars and FRBs the monitor gives an unprecedented view of the RFI `envi- are observed to obey this law precisely (Hassall et al. 2012; ronment' at the telescope at any given time and this setup Thornton et al. 2013). They have a typical peryton spec- is ideal for identifying very strong signals of RFI which may trum, being broadband, but brighter at higher frequencies. corrupt observations with the main dish at Parkes. Conversely, an off-axis detection of an astronomical source In January−March 2015 319.2 hours (13.3 days) of 13- (i.e. one effectively at infinity) would be suppressed at the beam BPSR data were recorded for the SUPERB survey highest frequencies, but the near-field beam pattern is rad- alone to search for pulsars and fast radio bursts.
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