arXiv:1210.0977v1 [astro-ph.IM] 3 Oct 2012 h t fFbur 04 tta ie h ai param- basic the on time, place that At took 2004. observation February first of four 6th by The the Univer- written Technology. Swinburne was of from three proposal sity and This CSIRO at 2003. researchers late in mitted hspprw ecietehsoyo h rjc n its and project the In of history the status. rejection. current describe interference we frequency paper GW this radio the implement for and detect dy- timescale methods system to pulsar-based solar a are) produce of still namics, knowledge 2008). (and our improve were (Manchester background, goals project main (PPTA) The Array Timing sar in- the & (Hellings from sources formed GW 1983). background numerous Downs GW of a addition Lommen & coherent from Finn (e.g., or (Lom- events burst 2010) holes 2001), be black Backer could & binary men GWs supermassive Such individual to frequencies. from sensitive nano-Hertz pre- were with observations had GWs pulsar (1979) gravitational Detweiler that if shown Earth. or viously the accu- 2010) not pass al. (GWs) are signals waves et masses (Champion correlated planetary known system Partially rately solar the standards. if occur irregularities er- time or timing atomic instrumentation as observatory in such the cor- phenomenon in must terrestrial sets rors data a any pulsar to that more claimed respond or two They in sets. signal be- identical data signals unam- pulsar correlated be that different for could tween showed looking phenomena by work physical identified (Romani Their some biguously community of 1990). effects American Backer the & North Foster the 1989; from ob- came were pulsars indi- multiple pulsar each though treated Even vidually. projects previous 2004). these served, Ord (e.g., al. 2001) mechanism emission al. et pulse the et the Straten studying testing van for including (e.g., and relativity purposes of theory numerous general for have used observations These been pulsars. millisecond monitoring h rtrqetfralctdosrigtm a sub- was time observing allocated for request first The n20,R .Mnhse ntae h aksPul- Parkes the initiated Manchester N. R. 2004, In (PTA) array timing pulsar a for suggestions first The been has telescope Parkes the decades two past the For cec ihPre 0YasYug 1Ot o. 2011 Nov., 4 – Oct. 31 Young, L Years using 50 typeset @ Preprint Parkes with Science h usreiso ehns.I hspprw rvd nhistorical an provide we sc headings: paper Subject time this discoveries. wind, atomic In major solar system, the solar the mechanism. highlight our as emission in diverse pulsar systems as the planetary topics eve of on approximately masses observe results the currently scientific we numerous and to ongoing is project The SR srnm n pc cec,AsrlaTlsoeNat Telescope Australia Science, Space and Astronomy CSIRO is bevtosfrtePre usrTmn ra rjc wer project Array Timing Pulsar Parkes the for observations First 1. A 2. INTRODUCTION T E tl mltajv 5/2/11 v. emulateapj style X H PAST THE usr:general pulsars: cec ihPre 0YasYug 1Ot o. 2011 Nov., 4 – Oct. 31 Young, Years 50 @ Parkes with Science HTW’EDN N HTW’EDOING WE’RE WHAT AND DONE WE’VE WHAT H AKSPLA IIGARRAY: TIMING PULSAR PARKES THE ABSTRACT .Hobbs G. vrapro faot8y.PRJ8704 a rms had J1857+0943 PSR yr. 8 about residuals of timing J1939+2134 period and a J1857+0943 Observa- over PSRs Arecibo provided of publically had from observations They tory only were (1994). pulsars The al. et millisecond Kaspi unknown. of observations was available sets scientifically- needed produce data to precision pulsars required valuable timing span five data necessary total the least the However, and at observed. that be Backer & to indicated Foster understood; had were PTA (1990) viable a for eters rvddawdrbnwdh(pt 1 H) u the ob- but standard The MHz), 512 dedispersed. to MHz coherently 64 (up not were two WBC bandwidth data the wider provided whereas a data CPSR2 provided dedispersed coherently of (CPSR2). bands Caltech-Swinburne-Parkes- 2 “wide-band- the the Recorder in- and system, instruments (WBC) Backend filterbank cm correlator” 10/50 analogue 2005). dual-band an a al. cluded and et receiver et (Granet H-OH (Stavely-Smith receiver the in- receiver 1996), observations multibeam timing al. cm 20 pulsar the for cluded systems front- receiver main The end available. were system. instruments backend solar and the of observa- knowledge pulsar little our Very improve how could this. describing tions do available to no was made but literature papers 1996), been Tavella had few & Petit attempt A 1991; extensive timescale Petit did pulsar-based & concept. a so Guinot developing PTA and (e.g., on pulsar the written one been to of had directly use How- relate made galaxy. not only radio result 3C66B this the con- ever, binary within they supermassive system when possible hole GWs a black of and properties pulsars the both strained using background result a ical or likely. galaxies more merging was strings of cosmic cores black from the back- binary GW supermassive at a coalescing holes and that by (2003) showing formed Loeb were ground & (1992) Allen Wyithe & (2003), Caldwell Backer predictions & but frequency GWs, Jaffe logarithmic of from background as per cosmic density, presented a energy the in generally interval, the of on and model limit known, a inflationary were char- universe an with background early from GW a expected Upper of acteristics existence process. the red-noise on unexplained bounds an by dominated oa aiiy OBx7,Epn S 70 Australia 1710, NSW Epping 76, Box PO Facility, ional ttePre bevtr,vrosfotedsystems front-end various Observatory, Parkes the At astrophys- major first the provided (2004) al. et Jenet ytrewes h aahv led have data The weeks. three ry ls h neselrmdu and medium interstellar the ales, ∼ are u nFbur 2004. February in out carried e rvttoa ae,measuring waves, gravitational 1 µ vriwo h rjc and project the of overview n h iigo 13+14was J1939+2134 of timing the and s 2 Hobbs, G. serving receivers (the 20 cm multibeam and the dual- dual-band feed which requires two backend instruments band 10/50cm receivers) are shown in Figure 1 along for data capture and processing. For the low frequency with a recent backend instrument (the Parkes Digital Fil- band it is necessary to de-disperse the observations co- terbank). herently. Therefore CPSR2 was used for observations in the 50 cm band whereas the PDFB1 was used at the 2.1. 2005 and 2006 higher frequency. PDFB1 could only process a band- width of 256 MHz. A second digital filterbank system The PPTA project started to produce high-quality ob- (PDFB2), able to process the entire available bandwidth servations in March 2005. We selected around 20 mil- of ∼1024 MHz, was commissioned in March 2007. lisecond pulsars based on their pulse periods, known Any project related to pulsars, black holes, galaxies, stability and flux density. This number was based on clocks and planets has the potential to engage the pub- the analysis of Jenet et al. (2005) who demonstrated lic and school students. In December 2004 students from that an isotropic, stochastic GW background with an Kingswood High School (in New South Wales, Australia) expected amplitude could be detected with > 3σ con- were the first to take part in a PULSE@Parkes observing fidence if ∼20 pulsars were timed weekly over a period session (Hollow et al. 2008, Hobbs et al. 2009a). This of five years with an rms timing residual of 100ns. A outreach project allows high school students to carry map of the pulsar positions on the sky is given in Fig- out observations of pulsars using the Parkes telescope ure 2. The Figure includes 22 PPTA pulsars. Orig- remotely (Figure 3). Their sample of pulsars includes inally, a slightly larger sample of pulsars were cho- many of the PPTA pulsars providing new observations sen, but PSRs J0024−7204J, J1435−6100, J1623−2631, while also testing out the remote observing capabilities J1721−2457, J1757−5322, J2317+1439 did not provide at the Observatory that will soon become mainstream. adequate timing precision and were quickly dropped. More recently PSR J1732−5049 was also removed, but, 2.3. 2009 and 2010 as described below, two recently discovered pulsars have In order to develop algorithms to detect GW signals or now been added into the sample. provide an upper bound on their amplitude it is necessary Attempting to achieve the required pulse time of ar- to predict the size of the induced timing residuals that rival (ToA) precision for such a large sample of pulsars will be present for a specific pulsar. Sesana et al. (2008, was (and still is) very ambitious. Van Straten (2001) had 2009) described the expected signal strength for individ- showed that rms timing residuals of <100ns were possi- − ual sources and refined the calculations for a background ble for a single pulsar, PSR J0437 4715, but such timing induced by a large number of sources. Determining the precision had not been published for any other pulsar. size of the induced timing residuals is not trivial as the Obviously we would need new hardware to achieve the pulsar timing process requires the fitting and subsequent required level. Planning therefore begun on designing a removal of various analytic functional forms. This can new set of instrumentation. During June 2005 we de- reduce the size of any GW signal in the timing residuals. veloped a test system, the first Parkes Digital Filterbank Hobbs et al. (2009b) developed a set of routines that al- (PDFB1) that, along with CSPR2, provided the first ma- low the expected GW signal to be simulated within the jor data set for the PPTA project. Interference caused tempo2 software package. This allowed the effects of fit- by digital television transmissions led to Kesteven et al. ting to be accounted for when predicting the GW signal. (2005) developing adaptive filter algorithms that could The first analysis of the sensitivity of an actual PTA to be used to mitigate such interference in our observations. individual, continuous sources of GWs was presented by The first major result from the PPTA project was Yardley et al. (2010). This led to a sky-averaged con- published by Jenet et al. (2006). This provided the straint on the merger rate of nearby (z < 0.6) black hole most stringent upper bound on the existence of the GW 10 binaries with a chirp mass of 10 M⊙ of less than one background. This work, and most subsequent PPTA pa- merger every seven years. pers, have been based on a new pulsar software package, Champion et al. (2010) demonstrated progress towards tempo2 (Hobbs, Edwards & Manchester 2006; Edwards, another of the PPTA goals: looking for errors in the plan- Hobbs & Manchester 2006). Using this new software we etary ephemeris. We enhanced the tempo2 software to can now 1) process multiple pulsars simultaneously, 2) enable a signal to be fitted to multiple pulsars simultane- simplify new algorithmic development and 3) implement ously. This allowed errors in the masses of known solar all the relevant physical processes sufficiently accurately system planets to be identified and led to the most pre- for our project goals. cise published estimate for the mass of the Jovian system . × −4 2.2. of 9 547921(2) 10 M⊙. 2007 and 2008 It was becoming clear that the timing residuals for The timing residuals induced by a GW background, many of the pulsars in the PPTA sample are dominated irregularities in terrestrial time standards or errors in by an unknown red noise process that may be similar to the solar system ephemeris are all expected to have a the “timing noise” observed for the normal pulsars. An steep low-frequency (“red”) spectrum. The timing resid- analysis of 366 pulsars using data from the Jodrell Bank uals induced by dispersion measure variations are also Observatory allowed Hobbs, Lyne & Kramer (2010) to predicted to have a red noise spectrum that, for many compare various mechanisms for this noise and to pre- pulsars, will be larger than the signals that we hope to dict the expected amount of noise for millisecond pulsars. detect. You et al. (2007a,b) measured the size of the This was further developed by Lyne et al. (2010) who dispersion measure variations caused by the interstellar identified a process that led to variations in pulse shape medium and the solar wind. This work highlighted the that correlated with the spin-down rate. This opened up necessity for repeated observations using the 10/50cm the possibility that the timing noise in millisecond pul- The Parkes Pulsar Timing Array 3

Fig. 1.— The basic observing system at the Parkes telescope. The left figure shows the 10-50 cm dual band receiver. The central panel contains the 20 cm multibeam system and the right panel shows a digital filterbank backend system.

Fig. 2.— Sky-map of the pulsars included in the Parkes Pulsar Timing Array sample (large open circles), the European Pulsar Timing Array (open boxes), NANOGrav (solid boxes) and all known pulsars with P < 20 ms, P˙ < 10−18 that do not reside in a globular cluster (small dots). The solid line at a declination of +25◦ gives the approximate declination limit of the Parkes Observatory. sars could be measured and subsequently removed. Much national Pulsar Timing Array (IPTA) has been described of the low-frequency noise in our long data sets had pre- by Hobbs et al. (2010). viously been absorbed into arbitrary time offsets that During this time Keith et al. (2010) presented the occurred whenever the observing systems changed. We first results from the “High Time Resolution Universe implemented a new procedure to measure the time delay Pulsar Survey” being carried out at the Parkes Observa- between the topocentric reference point of the telescope tory. This survey is discovering new millisecond pulsars and the time tagging on the output data for as many of some of which are now being included in the IPTA pul- our backend systems as possible. This, and correlations sar sample. For instance, in September 2010 we added a of overlapping data sets, enabled the removal of all such newly discovered pulsar, PSR J1017−7156, to the PPTA arbitrary time offsets within the PPTA data set. sample. Searches for radio pulsars using gamma-ray Verbiest et al. (2009) provided new analytic calcula- source catalogues has also recently led to the discovery tions that predicted the likely GW background detection of a large number of millisecond pulsars. Regular ob- significance for a given set of observations. Their re- servations of a newly discovered pulsar from this survey, sults showed the necessity of observing even more than PSR J2241−5236 (Keith et al. 2011), began in February 20 pulsars in order to detect the GW background within 2010. five years. In order to produce data sets with a suffi- In June 2010, the coherent de-dispersion system ciently large number of pulsars, three PTA projects, the (CPSR2) was decommissioned and replaced by the PPTA, the European Pulsar Timing Array (EPTA) and ATNF-Parkes-Swinburne-Recorder (APSR) that enabled the North American Timing Array (NANOGrav) agreed coherent dedispersion across a bandwidth of 1GHz. By to collaborate and share data sets. The resulting Inter- October 2010 we were making use of a second coherent 4 Hobbs, G.

Fig. 3.— High school students from Caroline Chisholm College observing pulsars as part of the PULSE@Parkes outreach project. Photo credit: CSIRO. de-dispersion backend system, CASPSR, in parallel with developed the Parkes Observatory Pulsar Data Archive APSR. (Hobbs et al. 2011). This online archive includes as much recoverable data as possible from observations of 2.4. 2011 and current research pulsars at the Parkes Observatory and includes all the PPTA observations. Raw data files are automatically The PPTA upper bounds that had already been placed converted to the PSRFITS format (Hotan et al. 2004) on GW signals (both individual sources and the GW and are available for download 18 months after the data background) were used by Wen et al. (2011) to constrain collection. You et al. (2012) selected a small number of the coalescence rate of supermassive black-hole binaries, PPTA observations for PSR J1022+1001 when the line of but still no detection of the GWs had been made. In sight from the Earth to the pulsar went close to the Sun. order to attempt such a detection, Yardley et al. (2011) Using these observations they provided measurements of developed new algorithms that were able to detect the the solar wind electron density and its magnetic field GW background in simulated data. This work showed strength. that our observations were consistent with the hypoth- The first major data release will soon be made and a esis that no GW background is present with 76% con- full description of the PPTA project, its goals, the hard- fidence. The Yardley et al. algorithm is not effective ware used and its status will be reported in Manchester in the presence of significant low-frequency noise. This et al. (2012, in preparation). The preparation of this led to a new research paper, published by Coles et al. data set has required the development of a new method (2011), describing how pulsar data should be analysed in for removing the effects of dispersion measure variations the presence of red noise. We can use these new meth- (Keith et al. 2012, in preparation). The data have been ods for standard pulsar timing experiments as well as for used to form the first time scale based on the PPTA PTA-related research. sample of pulsars (Hobbs et al. 2012, in preparation). The first stage in any analysis of the PPTA observa- Furthermore we are developing new algorithms to limit tions relies on forming high-quality, calibrated pulse pro- the amplitude of the GW background and our expected files. Profiles from PDFB2 in the 20cm observing band new result (to be published by Shannon et al. 2012) will were published by Yan et al. (2011a,b) who identified significantly improve upon the limit (van Haasteren et many new features. Any time variability in the pulse al. 2011) obtained using EPTA data sets. profiles lead to an extra high frequency noise component. This was studied in detail by Oslowski et al. (2011) using 3. 25h of observations of PSR J0437−4715. They showed THE FUTURE that the ToA precision achievable will ultimately be lim- The development of a well-tested, optimal algorithm ited by the broad-band intensity modulation that is in- for detecting GW signals, a full understanding of all the trinsic to the pulsar’s stochastic radio signal. phenomena that can affect our data sets and a prediction The raw observation data files produced for the PPTA of the expected size of the GW signal are essential for the project are large; many terabytes of data now exist. In PPTA project. Not only will these developments allow order to provide public access to these data sets, we for the possibility of GW detection - the main, and most The Parkes Pulsar Timing Array 5 exciting goal of the PPTA project - but will also allow detected before the commissioning of the full SKA tele- us to predict when this momentous detection is likely scope. The number of pulsars observable and the phe- to occur. It is unlikely that the sample of pulsars will nomenal sensitivity will allow the SKA to produce data significantly change in the next few years, but we shall sets that not only will be used to detect GWs, but will doubtless include new discoveries in the PPTA sample. allow the GW properties to be studied in detail. It is also possible that new algorithms will be developed for the PPTA goals that make use of the orbital peri- 4. odicity of binary pulsars instead of a pulsar’s rotational CONCLUSION periodicity (see e.g., Kopeikin, 1997; Ilyasov, Kopeikin The first PPTA observing proposal, in 2003, contained & Rodin 1998). If this occurs we may need to include a seven Australian members. The current team consists selection of neutron-star–neutron-star binary systems in of 31 people across Australia, Germany, Poland, China the PPTA sample. and the U.S.A. The viability of using pulsar observations The PPTA data sets and algorithmic developments are to detect GWs has now been recognised by the wider being used in unexpected ways. Recent theoretical work GW community with the number of publications related has indicated that PTA observations are sensitive to the to PTAs increasing dramatically over the previous five gravitational wave memory effect (e.g., van Haasteren & years. Levin 2010 and references therein); an effect unlikely to Pulsar observations led to the first indirect detection of be detectable using other GW instruments. The tempo2 GWs using the PSR B1913+16 pulsar system (Hulse & software we have developed has led to many unexpected Taylor 1975). With the significant ongoing world-wide research projects. It is currently being used to determine effort, it is possible that PTAs will make the first di- how pulsars may help in the navigation of space probes rect detection of GWs. Pulsar projects provide the only in our solar system (e.g., Tian et al. 2012). viable means to study ultra-low frequency GWs and so It is likely that most future analyses will use IPTA PTAs are complementary to ground-based GW detector observations that combine the PPTA, EPTA and systems such as LIGO and VIRGO. GW detection is only NANOGrav data sets. Within a decade it is expected one of the PPTA goals. Using PPTA data, we have al- that the Square Kilometre Array pathfinder telescopes, ready developed the first major pulsar time scale as we ASKAP (Johnston et al. 2007) and Meerkat will be continue to search for unknown objects within our solar producing data to be included in the IPTA. The Chi- system. nese Five-hundred-metre Aperture Spherical Telescope The PPTA project was initiated with support from (FAST) will provide a collecting area that should rev- R. N. Manchester’s Australian Research Council (ARC) olutionise pulsar astronomy. Combined with a planned Federation Fellowship (FF0348478). We acknowledge 110 m diameter telescope at the Xinjiang Observatory, contributions to the project Worldwide from numerous the Chinese community will have the data sets for a pow- individuals and institutes. GH acknowledges support erful new PTA. One of only two key science projects for from the Australian Research Council #DP0878388. The the Phase 1 Square Kilometre Array (SKA) is related Parkes radio telescope is part of the Australia Telescope to high precision pulsar timing and searching for GWs. which is funded by the Commonwealth of Australia for It is therefore to be expected that GWs will have been operation as a National Facility managed by CSIRO.

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