Pulsar ALFA Galactic Plane Survey Arecibo Project P2030 Progress Report

Transcription

1 Pulsar ALFA Galactic Plane Survey Arecibo Project P2030 Progress Report 1 August 2008 Jim Cordes 1 and Fernando Camilo 2 For the PALFA Consortium 3 ABSTRACT This report summarizes the PALFA Project, namely activity for projects P2030 (survey), P2177, P2283 (general timing) and P2391 (timing on the MSP J ). from mid-2007 to mid We discuss progress made toward the goals of our original survey proposal. In particular we analyze the yield of pulsars achieved so far, discuss the factors involved in determining the yield to date, and discuss how further processing of existing data and how the choice of observing parameters using the new spectrometers will increase the yield. We also describe the end to end data-analysis pipelines and follow-up timing analyses of discovered pulsars, relevant data products, databases and web-based tools for accessing them. Highlights: We have now discovered three millisecond pulsars, all three of which are objects of potential utility in the pulsar timing array for gravitational wave detection. The first, J , has now been timed for two years, demonstrating consistency with GR and providing a determination of the pulsar s mass as 1.68M to better than 1%. This object is in a highly-eccentric orbit that is itself a mystery, as discussed in the Science paper published this year (Champion et al. 2008). Another of our MSP discoveries is also a binary while the third MSP is isolated. A young, high-ė pulsar, J , is a likely counterpart to a TeV γ-ray source detected by HESS that most likely is a pulsar wind nebula. Timing of the young relativistic binary J will continue because the secular changes in pulse shape from geodetic precession allow us to sample the pulsar s beam shape. Timing will also allow us to determine the masses and test the orbital period decay against GR to reasonably high precision. Intermittent objects are being found and we are analyzing survey results to put limits on the rate of bursts from extragalactic sources. On the outreach side, ARCC students from UT Brownsville visited Arecibo and made presentations at the 2008 Jan AAS Meeeting; an ARCC Scholars Program has been started. Pulsar Yield: The PALFA survey has blindly detected 162 pulsars of which 46 are new (found in the periodicity analysis, the single-pulse analysis or both) and 11 were also found in the unpublished and uncataloged Parkes Deep Multibeam Survey (DMB) that found 14 pulsars in a portion of Arecibo s declination range. For comparison, there is a total of 201 pulsars in the ATNF catalog in our inner-galaxy survey region and 11 pulsars in our outer-galaxy survey region. Given that we have surveyed only about 1/3 of the directions in the inner-galaxy region, it is clear that we will rediscover all the known pulsars and we will find additional new pulsars. However, the yield-rate (e.g. per square degree) is not as high as we originally predicted. We have spent considerable time analyzing the situation and expect that the yield rate will go up considerably through a number of measures. Diagnosis of the pulsar yield: We conclude that 1. A significant number of new pulsars is contained in candidate signals in our pipeline outputs that have not yet been distinguished from the overall huge number of signal candidates that are mostly from RFI; we are confident that we will be able to winnow the large number of signal candidates into a short list of pulsar candidates with a high probability of being confirmed. 2. The shallower luminosity function identified in a recent analysis of the Parkes Multibeam survey analysis implies that we need to search more deeply to detect a larger fraction of the pulsars in the inner Galaxy. 3. Though we expect larger numbers of new pulsars to emerge from the PALFA survey, we emphasize that predicted numbers depend on the Galactic structure of the pulsar population (and of the electron density), which we will know well enough only after we have finished our search and its analysis. Prescription for the PALFA Survey: We also conclude that 1. Survey observations with the new PALFA spectrometers should be made to increase the overall product of bandwidth and integration time (BT) by a factor 6, particularly for Galactic longitudes 32 l 60 where a significant amount of volume is filled with pulsars inside the solar circle.

2 2 2. The pipelines should search to larger dispersion measures than currently for these low longitudes. 3. We intend to observe commensally with the extragalactic drift-scan survey, ALFALFA, in order to find millisecond pulsars, relativistic binaries and other objects in directions out of the Galactic plane. 4. We are looking at the feasibility of an out-of-plane survey using deeper integrations than those provided by the ALFALFA drift scans. Payoffs: The return will obviously include a larger number of pulsars of all types, but especially those of greatest interest (binaries, millisecond pulsars, GLAST targets, intermittent pulsars and high-velocity pulsars). We will also probe the pulsar population to sufficient distances that we can understand its Galactic structure, particularly whether there is any clustering toward spiral arms, as we would expect for young objects. Document: The main part of this document focuses primarily on events of the last year having to do with Consortium structure, notable results, a detailed discussion of the pulsar yield achieved so far and our expectation that this yield will go up substantially. The low yield was the primary emphasis of the 2007 Skeptical Review and we were asked to address it specifically in this 2008 report. Publications and websites are listed at the end of the main document and before appendices. Details of the survey, processing pipelines, and long-term data management are placed in appendices so that the document is self contained, overall. PALFA participants are listed after the appendices. Introduction: The Pulsar-ALFA (PALFA) Consortium is conducting a large-scale Galactic plane survey for pulsars and transients. Precursor work with ALFA began in August 2004 under project P1944, leading to our 2004 October proposal to NAIC for a multi-year survey, which became the P2030 project. An affiliated follow-up timing project using Arecibo (P2177, P2283) commenced in 2007 March and routinely acquires full-stokes arrival-times on some PALFA discoveries. A few specific PALFA pulsars have been timed elsewhere, including the GBT, Jodrell, Nancay, Parkes and Westerbork. Our goal is to complete a deep search of the Galactic plane, defined as latitudes within 5 degrees of the midplane for the two longitude sectors available to Arecibo. So far we have used the WAPP correlation spectrometers. In 2008 we will begin using the new PALFA spectrometers. We intend to extend the survey further out of the Galactic plane as a means for optimizing detection of millisecond pulsars (MSPs) and relativistic binary pulsars. The PALFA Consortium and the PALFA Executive Committee (PEC): The PALFA Consortium was formed in late 2002 in advance of the commissioning of ALFA in Since then the Consortium has conducted all aspects of the project including the Arecibo survey observations, data processing and management, timing observations at Arecibo and elsewhere, and multi-wavelength follow-up. Camilo and Cordes are the PIs on the Arecibo observing project (P2030), which has been active since early P2030 has received 500 hr per year that includes overhead and confirming observations. NAIC s Skeptical Review panel s latest review (2007 Sept) of all large ALFA projects recommended continuation of P2030. Other Consortium members have led companion projects (P2177, P2283) for long-term follow-up timing at Arecibo on PALFA discoveries. Others have led timing efforts on affiliated telescopes (the GBT, the Lovell telescope at Jodrell Bank, and the Nançay telescope in France). Data are centrally managed at Cornell, where they are archived, curated, partially processed and made available to the Consortium, discussed in the Appendix. As the Consortium has evolved, we have instituted the PEC in order to maintain corporate memory and to concentrate decision making among some of the Consortium members who are most active in the project. The current PEC membership is F. Camilo (Columbia), J. Cordes (Cornell), P. Freire (NAIC/Arecibo), D. Lorimer (U West Virginia), V. Kaspi (McGill; Chair), D. Nice (Bryn Mawr), S. Ransom (NRAO) and I. Stairs (UBC). The PEC has recently defined a two-tier membership scheme for the Consortium, Tier-1 comprising active members who are involved in a subset of the activities needed for the project or who play crucial roles in the project and are eligible for (rotating) membership in the PEC. Tier-2 includes less active members who play occasional, ad-hoc roles in follow-up on particular objects, such as multiwavelength observations. Our intention is to be as inclusive as possible while acknowledging the very significant, sustained efforts made by some Consortium members. Tier-2 members can propose to become more active and thus join Tier-1. Students PhD projects are protected through vetting of proposed activities through the PEC and the entire Consortium. Observing Actitivites of the Last Year: Prior to last year s report, we had made a total of 13k search pointings, about equally split between inner and outer Galaxy, 5.2k and 7.4k pointings, respectively, out of total planned pointings of 24k in each region (48k total). There has been little PALFA observing since our 1 Aug 2007 report. The telescope became active after the painting project only in late December 2007 and we have had mostly timing monitoring sessions in Since 1 Jan 2008, we have observed about 496 pointings in the inner Galaxy survey 1 Astronomy Department, Cornell University, Ithaca, NY 14853, cordes@astro.cornell.edu 2 Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY See end pages for list

3 area. We have confirmed two millisecond pulsars and a few longer period objects. Timing observations have also taken place at Green Bank (GBT), Jodrell Bank, Nancay, Parkes and Westerbork. Database Activities: Please see Appendix for a discussion of our database system. Raw data continue to be transferred to Cornell for archival using portable disk drives. Disk drives are then sent on to another processing site and then returned to Arecibo. In addition, using our Tracking Database, accessible through a password protected web application, 10 TB of data has been selected, restored from tape and securely transferred to remote processing sites via ftp so far. Remote processing sites upload their data products, in return, to a Common Database hosted at the Cornell Center for Advanced Computing from where candidate information and plots can be accessed remotely via a viewing program produced by Patrick Lazarus of McGill and rated according to likelihood of being a pulsar discovery. Information about the highest-rated candidates can then be retrieved and incorporated into a confirmation observing schedule. Development of PALFA Processing with Einstein@Home: Exploratory discussions have taken place for processing PALFA data on E@H clients in areas of parameter space that cannot be searched by Consortium computational facilities owing to throughput issues. In particular, the scheme now being implemented is to dedisperse raw data on a central server and send a single time series to an E@H client. There are tens of thousands of E@H clients around the world. The client will then search for circular binary pulsars with orbital periods less than one hour, a task that typically will take about 24 hr. This can be compared with a processing time 0.5 hr per time series in the acceleration search done in the PRESTO-based pipelines. Acceleration searches become insensitive to massive binaries for P orb < 1 hr. Given the merger rate of double neutron star binaries and the orbital lifetime at a given orbital period, there should be one binary in the Galaxy with an orbital period between 5 and 10 minutes. There would be larger numbers in going to longer periods. Raw data will be served from the Cornell Center for Advanced Computing, sent to the Albert Einstein Insitute (Hannover, Germany), and then disseminated and managed using the BOINC technology developed for SETI@Home and other grid computing projects. In addition to being an exciting research activity, this joint PALFA/E@H project is an excellent opportunity for further public outreach. Notable results from the PALFA Survey: We have discovered pulsars in all the target classes that will give the greatest long-term payoffs: MSPs, a relativistic binary pulsar, young pulsars with likely high-energy counterparts, a high-magnetic field object, and intermittent objects/rrats found with our single-pulse analysis. Our starting assumption is that intermittent objects will turn out to be highly-modulated pulsars, but that need not be the case. New astrophysical source classes are certainly a possibility and only larger source samples and follow-up observations will tell. Binary MSP J : This extraordinary binary millisecond pulsar (P = 2.15 ms, P b = 95d), the first MSP discovered in our survey, was found by the full-resolution pipeline running at McGill and an independent pipeline at Cornell. It is the most important discovery of our survey to date for many reasons. First, its DM is the largest for any known MSP, which demonstrates that high frequency resolution of the ALFA pulsar survey make it sensitive to MSPs in an unprecedented volume of our Galaxy. This has been confirmed by the two other MSP discoveries to date, which are also found at very high DMs. Second, this highly recycled pulsar is absolutely unusual in having a fairly massive companion (which is either a massive white dwarf (WD) or possibly a main-sequence star, as suggested by the spatial coincidence of a Sun-like star with the astrometric position of the pulsar) and an orbital eccentricity of These properties are at odds with the very small eccentricities common to all known Galactic MSP-WD systems, and defy our present understanding of the stellar evolution in binary systems; its possible origin is now under investigation (Champion et al. 2008). The large mass, the eccentric orbit, the fast spin period and the high timing precision that can be achieved at Green Bank and Arecibo allow the determination of three post-keplerian parameters: the rate of advance of periastron ( ω) and the range (r) and shape (s) of the Shapiro delay. The determination of three such parameters over-constrains the system, i.e., it allows a test of general relativity (alternatively, we can think of this as a verification that ω is relativistic). GR indeed passes the test, all these parameters give consistent estimates of the mass of the two components, as shown in Figure 1. Even more importantly, the mass of the MSP is 1.68 ± 0.01M. The precision of this measurement is unprecedented for any MSP, and it is confirmed by the measurement of multiple relativistic effects; furthermore, the mass is significantly higher than that of any neutron star with a precise mass measurement made until now! This shows beyond doubt that a) MSPs can accrete significant amounts of matter when they are being recycled, but more importantly b) has firm, long-term implications for the study of the equation of state for cold ultra-dense matter: such a large neutron star mass rules out some EOS models. Binary MSP J : This MSP (P = 13.1 ms, P orb = 1.95d) is in a circular orbit with a likely massive WD companion. It was discovered in the Cornell pipeline in 2007 Oct and immediately confirmed through observations with the GBT and Jodrell Bank, yielding a tentative timing solution. With DM = 164 pc cm 3, it is about 6.5 kpc distant using the NE2001 model. This object is also a good candidate for measuring the Shapiro delay, which allows us to determine the inclination angle. 3

4 4 ω r s Fig. 1. MAIN PANEL: Constraints on the masses of PSR J and its companion, based on Green Bank and Arecibo timing. The hashed area is excluded by knowledge of the mass function and by sin i 1, where i is the orbital inclination. The remaining constraints are derived from the rate of advance of periastron ( ω) and the range and shape (r, s) of the Shapiro delay, assuming they are of general-relativistic origin. The different constraints give a consistent measurement of the components masses, i.e., GR passes this test. The companion is significantly lighter than any NS measure to date (horizontal green bar), it is likely to be a massive WD or a MS star. The pulsar is significantly more massive than any other NS with a precise mass determination (vertical green bar). TOP: probability distribution function for the mass of the pulsar, the median is indicated with a vertical line, RIGHT: probability distribution function for the mass of the companion. Relativistic NS-NS Binary J : This 144-ms pulsar is in a 3.96-hr orbit (Lorimer et al. 2006). After the double pulsar (J ), it is the second-most relativistic binary system known. The pulsar is the youngest of any in a NS-NS binary (112 kyr), implying a birth rate 60 Myr 1 and a corresponding inspiral rate for NS-NS binaries of interest for gravitational wave detection and short-period GRBs. Follow-up at Arecibo, GBT, and Jodrell Bank show no pulsations from the companion (flux limit = 46 µjy), which imply that it is either unfavorably beamed, radio quiet, or not a NS. However, the mass determination from the measured apsidal advance ω = 7.6 deg yr 1 and gravitational redshift parameter γ shows the companion mass to be consistent with a NS (1.37±0.02M ) and larger than the pulsar mass, 1.25 ± 0.02M, as expected if it has undergone accretion while the pulsar has not (Kasian et al. 2007). Timing residuals are partly due to secular changes in pulse profile caused by geodetic precession, which was suggested in the discovery paper by comparing PALFA profiles with archival data from the Parkes multibeam survey in which the pulsar was present, but was classified as RFI. We expect to measure the orbital period derivative P b, determined by gravitational radiation, the rate of geodetic precession, and map the emission beam of the observed pulsar. The measurements of these effects are more difficult than for other binary pulsars where the recycled pulsar is timed; the young pulsar in J shows significant timing noise. High-energy Targets: We have discovered a high Ė, 82 ms Vela-like pulsar (J ) that appears associated with a TeV γ-ray source (Hessels et al. 2008). The high-energy emission is most likely due to a pulsar-wind nebula. A second pulsar, the isolated 68.7 ms J , has large Ė = erg s 1 and short spindown age, 82 kyr (Cordes et al. 2006). It has an unusually flat spectrum from 0.4 to 9 GHz ( spectral index < 0.3), somewhat reminiscent of the recently discovered flat-spectrum radio emission from two magnetars (Camilo et al. 2006), except that J has an ordinary G field. It resides in the error box of one unidentified EGRET source and is a top candidate for confirmation with GLAST.

5 5 Fig. 2. RIGHT: Single pulse from PALFA object G showing the characteristic dispersion sweep t ν 2 DM where the dispersion measure DM is the column density of free electrons to the pulsar. The line in the top spectrum is 21 cm hydrogen. LEFT: Single pulses from PALFA object J These consecutive, dedispersed pulses are the only three detected in a total of 1.2 hr. It appears to be a rotating radio transient (RRAT) with a period of 0.40 sec. Pulsars that are both radio and gamma-emitting objects are important for elucidating the beaming of the two kinds of radiation and also for probing the interaction of high-ė pulsars with their environment. Transient Events and Pulsar Intermittency: Pulsars have long been known to be intermittent through the appearance of bursts of pulses, pulse nulling and other modulations. Recent work has only underscored intermittency, including the discovery of rotating radio transients (RRATs) by a re-analysis of the Parkes Multibeam pulsar survey data using the single-pulse detection modules of the Cornell pipeline (McLaughlin et al. 2006, Nature, 439, 817). It is not yet clear to what extent RRATs represent a new physical class of radio pulsar as opposed to being merely an empirical extreme, where some objects are missed in standard periodicity searches but emit atypically strong pulses detectable in single-pulse analyses. We remain agnostic on this point because it is clear that the PALFA survey will yield good statistics on this empirical class. Of the previously known pulsars found blindly in our periodicity search analysis, we detect 63% in our single-pulse (SP) analysis, somewhat surprising if one assumes pulse amplitudes are fairly steady in the data streams that typically contain > 10 2 pulse periods. Obviously, pulse amplitudes are highly modulated, with RRATs being extreme cases. So far, we have found 8 objects through the Cornell singlepulse analysis that were missed in the periodicity analysis. One has been confirmed through reobservation (J ), one other has been reobserved several times without a redetection (J ), while in all cases, the characteristic differential arrival time t DM ν 2 DM is seen. An example is shown in Figure 2. While transients with an underlying period (like that in the figure) are probably pulsar like and, in the end, may be fairly ordinary pulsars in other respects, their extreme intermittency is a puzzle. Other SP detections may, of course, be from entirely different source classes, such as the event recently reported by Lorimer et al. (2007) that may have originated from a cosmological source. PALFA observations are sensitive to events like the Lorimer event and a paper now nearing completion (Deneva et al. 2008) places limits on event rates using PALFA data. Data products from our pipeline are suitable for assessing some aspects of intermittency by defining an intermittency ratio (McLaughlin & Cordes 2003) that is the ratio of S/N from the single-pulse analysis to S/N from the periodicity analysis: r (S/N) SP /(S/N) FFT. If r > 1, single pulse analysis yields higher S/N in the detection scheme while r < 1 implies the periodicity analysis is better. Figure 3 shows r for PALFA data, for Parkes Multibeam data, and for analytical results for theoretical pulse-amplitude distributions. The results indicate that short period pulsars, which provide larger N p in observations of a fixed duration (e.g. T = 268 s in most PALFA pointings), are likely to have r < 1 while long-period objects can have r 1. Truly intermittent objects with low pulse rates will invariably require the single-pulse detection analysis in order to be detected. Pulsar Detection Rate: So far we have observed 15k distinct sky positions with 7 beams per pointing (excluding test observations on known pulsars that we routinely make), about 1/3 of those needed to cover the inner and

6 6 Fig. 3. Intermittency ratio r plotted against N p, the number of pulses analyzed in a data set as defined in text. Ratios r > 1 signify that the single-pulse analysis provides larger S/N than a periodicity analysis. Some objects have only a lower bound on r, as do the RRAT objects. Filled red points are RRATs found from the Parkes multibeam survey, open red points are standard Parkes pulsars, black circles with arrows are single-pulse detections in our analysis without a corresponding periodicity detection, while black points are cases with both kinds of detection. Vertical bars in some cases indicate the range of r seen in multiple data sets. The dotted and dashed lines show r for various models of the pulse amplitude distribution. outer-galaxy regions at low latitudes. Figure 4 shows PALFA detections vs. DM and Galactic longitude along with known pulsars from the ATNF Pulsar Catalog (small dots). PALFA has detected 150 pulsars so far of which 46 are new objects. For comparison, there is a total of 200 pulsars in the ATNF pulsar catalog with periods greater than 10 ms. The distribution of PALFA detections implies that we reach the boundary of the free-electron disk in

7 7 Fig. 4. DM vs Galactic longitude. The topmost curve is the NE2001 model integrated to infinite (50 kpc) distance from the Sun. Regions corresponding to spiral-arm tangent points and other features are labeled as described in the NE2001 model (Cordes & Lazio 2002). The middle line shows DM for lines of sight that reach the solar circle at galactocentric radius a = 8.5 kpc, a distance d ss = 2a cos l. The lowest line is DM integrated to the tangent point, d t = a cos l for the inner Galaxy. Both d t and d ss vanish for l 90. Small (blue) points show pulsars from the ATNF pulsar catalog (Manchester et al. 2005). PALFA periodicity detections are shown as large (red) filled circles. RRAT detections are shown as open (green) squares. Fig. 5. LEFT: Plot of predicted vs. measured S/N from the periodicity search pipeline that yielded blind detections of known pulsars that had cataloged values of 1.4 GHz flux density. The largest S/N is for the millisecond pulsar B and the lowest predicted S/N is for a pulsar detected far off axis, for which the lack of sidelobes in the ALFA beam model causes the predicted S/N to be underestimated. RIGHT: Histogram of S/N for PALFA pulsar discoveries above a threshold S/N of 7. Given realistic spatial distributions in the Galaxy combined with the luminosity function, we expect the number of detections to increase in going to lower S/N. The decrease of the histogram signifies incompleteness that results from the fact that we have too many low S/N candidates (i.e. S/N <10) that include RFI as well as real pulsars and we have not yet separated and confirmed the best pulsar candidates. the 268-s integration time we use, at least for the most luminous pulsars. Other conclusions include: (1) For small longitudes, l 30, the density of detected pulsars falls off for DMs larger than that for the tangent point distance. For these longitudes, the actual pulsar density should remain high for these larger DMs, indicating that surveys of this part of the sky (primarily the Parkes multibeam survey) do not reach much past the tangent point. This makes

8 8 Fig. 6. Global period histogram from the Cornell pipeline that includes any signal above 8σ in the harmonic sum of the periodicity search; 60 Hz and a few of its harmonics have been excluded. The blue vertical lines indicate the periods of known pulsars that were blindly detected while the red lines are confirmed PALFA discoveries. The location of known and discovered pulsars near period bins with large counts indicates that pulsar detections are possible even in the presence of severe RFI. Period bins with large numbers of counts correspond to RFI signals that are seen in widely spaced pointings, allowing us to reject some candidate signals on the basis that they are widespread. However, some RFI is sporadic if not rare, making it less clear on a case by case basis how to discriminate pulsars from RFI. The period bin width is P = P 2 f, corresponding to 0.5 cycles of smearing over a 300 s observation, approximately the precision to which the period can be determined in the FFT + harmonic sum analysis. The forest of counts below about 2 ms is from encoded radar signals. sense given that the electron density model predicts very high DMs out to twice the tangent-point distance; (2) At longitudes of 30 to 90, where DM is not so high and selection effects are not as severe, there does seem to be a falloff in pulsar density beyond the solar circle, though the density is fairly high between the tangent-point line and the solar-circle line, particularly for longitudes 60 to 90 ; and (3) On the opposite side of the Galaxy (positive longitudes) where the PALFA survey samples the population, detections to date reach as far as the limiting DM and quite a few of the detections are near or beyond the solar circle. This suggests that the PALFA survey reaches as far as it should for sufficiently luminous pulsars. The eventual pulsar yield will depend on quantities we do not yet know well. For disk populations, a scale height description would generally be used. However, of order 50% of pulsars escape the Galaxy owing to their birth velocity kicks (Lyne & Lorimer 1994; Cordes & Chernoff 1998; Faucher-Giguère & Kaspi 2006). At low latitudes, the pulsar yield depends on the velocity distribution of the bound population, also not well known and certainly not well normalized with respect to the high-v objects. For canonical numbers (20% beaming fraction, 10 Myr radio lifetime, birth rate of 1.4 per 100 yr, galactocentric radial scale of 8 kpc, effective scale height of 0.5 kpc, and a

9 fiducial maximum detection distance D max = 5 kpc, on average), about one pulsar should be detected in 30 ALFA pointings (i.e beams): 6πR 2 ( ) ( ) ( ) gh ( ) 2 ( ) 5 kpc Rg H N pointing/psr 7f b N psr,g DmaxΩ (1) b 8 kpc 0.5 kpc f b This number corresponds to about 0.6 deg 2 per pulsar for nominal values of other parameters. For the inner Galaxy, we will survey about 430 deg 2, implying a total yield of 700 pulsars, subject to the caveat that not all directions in the inner Galaxy will be equally rich in pulsars, owing to the galactocentric radial scale of the pulsar population. In the ATNF pulsar catalog there are 200 previously known pulsars in the same Galactic longitude and latitude region, indicating that 500 new pulsars can be expected. While the eventual number could be lower, it could also be higher if we extend our reach (D max ) further than 5 kpc, as we advocate below. Understanding the Pulsar Yield: In the following we analyze various observational and astrophysical factors behind the current low yield of new pulsars but re-emphasize that the total yield (new + old) so far is not unreasonable. We conclude that a significant number of new pulsars is already contained in our pipeline output but we have not yet settled on a scheme for winnowing the list from the huge number of signal candidates that is efficient with respect to follow-up telescope confirmation. Several approaches are being explored by different Consortium groups that will be compared with respect to outcome (commonality of candidate pulsars) and used to select the most promising candidates for re-observation. We are confident that we will be able to do so. We also conclude, on astrophysical grounds, that observations with the new PALFA spectrometers should use a longer integration time so that we reach to greater distances at low Galactic longitudes. N psr,g Potential explanations for the low yield achieved to date include: 1. Receiver system and data acquisition problems, 2. Analysis pipeline errors, 3. Contamination by RFI and associated problems in identifying new pulsars, 4. Astrophysical reasons, such as Galactic structure of the pulsar population and of the electron density. The first two of these potential explanations can be dismissed because we detect known pulsars as expected and with S/N consistent with catalogued flux densities. Furthermore, we have re-discovered 11 of 14 pulsars found in the DMB (Deep Multibeam Survey) conducted by several of us (Camilo, Lorimer and McLaughlin, unpublished) using one-hour integrations with the Parkes telescope. That we have not yet covered all the sky region of the DMB survey suggests that PALFA will be complete with respect to DMB objects. Moreover, we detect the DMB pulsars at high S/N in on-axis detections using scans that are more than 10 times shorter. Some of the DMB objects are detected well off the boresight of some of the telescope beams in some pointings. The DMB investigators predicted that they would find about three times as many pulsars as they eventually found ( mclaughl/dmb/), which may bear an explanation in common with that for the PALFA yield, to some extent. The third and fourth explanations listed above both appear to be relevant for the low yield and we expect that the yield can be increased substantially, as we now discuss: Candidate Winnowing and RFI: Detection statistics on known pulsars show that the pipeline-achieved S/N from our periodicity analysis is consistent with that predicted from catalogued flux densities (left panel of Fig. 5). However, the histogram of S/N (right panel) indicates that detections of new pulsars tail off above our nominal threshold of 7σ. For any reasonable luminosity function and spatial distribution, the S/N histogram should increase monotonically in going to lower S/N. This reflects contamination from RFI that is manifested in the very large number of signal candidates above our nominal threshold. Figure 6 shows the histogram of raw signal candidates from the Cornell pipeline. Nearly every signal above threshold is shown, except for candidates found at 60 Hz or its harmonics. The figure demonstrates the forest among which we need to sift for high-quality candidates. Similar numbers result from the PRESTO pipelines. Candidates from both pipleines are examined visually, though cuts are made to lessen the number required for this. It is a research and development task in itself to develop automated filtering algorithms that can further decrease the number of candidates that need to be viewed. This R&D is taking place at several institutions in the Consortium, in particular McGill and Cornell. At Cornell, we have developed automated winnowing software that identifies known pulsars in the ATNF pulsar catalog by using location, period and DM tests. The remaining number of candidate signals is 2M. The number of candidate signals can be reduced from 2M down to thousands through additional simple filters, such as rejecting common candidates that appear in disparate sky positions and other reality checks. But getting from thousands down to a few hundred to 1000 is more challenging with respect to minimizing both false-positive and false-negative detections. We have applied additional filters to Cornell pipeline output based on a number of flags that use assessments of the RFI load on a given day or scan, the S/N, and the statistics on the number of harmonics seen. A stringent application of these filters yields a short pulsar candidate list that has a high proportion (10%) of pulsars (i.e. those already found in the PALFA survey in the quick look analysis or found by eye looking through data products; those from the uncataloged DMB survey; and a few pulsars in the ATNF catalog that leak through the D max 9

10 10 Fig. 7. Demonstration of RFI excision in the single-pulse analysis. Shown is the graphical output for a known pulsar showing before (LEFT) and after (RIGHT) excision of severe 12-s radar, which produces the dark vertical stripes. In both frames, each ALFA beam corresponds to one of the rows, which contain five panels from left to right: events above 5σ vs. trial DM channel and time (268 s total); histogram of events per DM channel, scatter plot of S/N vs. DM channel, mapping of DM channel to physical DM value, and histogram of S/N. The central beam is at the top, beam 6 at the bottom. The excision algorithm first identifies and removes the 12s radar interference using a Fourier method; then it removes isolated impulsive events that occur in more than three of the beams and that peak at low DM. The number of counts in the DM vs. S/N scatter plot (third panel in each row) is greatly reduced. catalog comparison at a high multiple of the spin period or at a discrepant dispersion measure.4 ) This approach misses pulsars we have found by visual inspection of output while at the same time finds good candidate objects that have not been identified in the visual inspection. Moreover, if we allow the filters to run on candidate lists that still include known pulsars in the ATNF catalog, we find that even some of them would not make it into the winnowed list owing, largely, to a low RFI score. A similar procedure, but with different heuristics, is being applied to the similarly large number of signal candidates from PRESTO pipelines and the results will be consolidated with those from the Cornell pipeline. After more experimentation and a less-severe cut, we will take a pulsar candidate list (generated through joint filtering of PRESTO-pipeline results) back to the telescope for confirmation shortly after this report is submitted (Aug-Sep 2008). The role of RFI in the single-pulse analysis is shown in Figure. 7. The figure displays output for the 7 ALFA beams before and after excision of RFI using the single-pulse candidate lists alone to create a mask in the DM-time plane for each beam. Most of the RFI is radar with a 12s period modulated by up to khz frequencies, which produces many candidates in our periodicity analysis. Work in the last year has included development of a similar frequency-time mask based on joint event statistics across the 7 ALFA beams that is applied before dedispersion. We are now applying this in our pipeline and consequently are getting far fewer signal candidates in our periodicity analysis. Astrophysical factors: Properties of the pulsar population that influence the rate of detection of new pulsars include: 1. Luminosity function: the pseudo-luminosity Lp = SD2 at 1.4 GHz ranges from approximately L1 = 0.1 to L2 = 104 mjy kpc2. Often the differential distribution is characterized as a power law, fl L x p, where x has been found to range between 1.6 and 1.8 in a recent study of the Parkes Multibeam survey (Lorimer et al. 2006, MNRAS, 372, 777). Many previous results in the literature identify a value of x = 2 (where the slope d log fl /d log L = x 1 usually is reported.) As we show, lower values of x favor a strategy that reaches to larger distances. 2. Period distribution: periods range from 1.4 ms to 8 s with most clustered around 0.7 s. 3. Spatial distribution: many pulsars are distributed in a disk with thickness of 0.5 kpc and radial scale 4 kpc but extending well past the solar circle. The asymmetry of spiral arms with respect to ` = 0 may account for some differences between the pulsar yields of the Parkes multibeam survey and the PALFA survey p Maximum detectable distance: Dmax corresponds to the volume sampled, Vmax Dmax, where Dmax = Lp /Smin, with Smin the minimum detectable flux density in a periodicity search. Smin is very strongly dependent on direction, distance and period. 4 Very strong pulsars can be detected at DM values far from the true value.

11 Fig. 8. LEFT: Maximum detection distance, D max, vs. spin period for the direction l, b = 35, 0 (at lower range of the declination coverage for Arecibo with b = 0) for our current PALFA observing parameters and for the Parkes Multibeam Survey. We have assumed a fixed intrinsic pulse duty cycle of 0.05 (independent of period). The four frames correspond to different values of the pseudo-luminosity L p, which is the period-averaged flux density D 2. The distribution of L p for pulsars is broad, covering > 5 orders of magnitude, because the emission is beamed and because the true beam luminosity is a strong function of spin parameters. The top and bottom boundaries of each shaded region are for full- and half-gain, respectively. The PALFA curves apply to the current observing parameters using the WAPP spectrometers (viz. 268 s integration and 100 MHz bandwith with 256 channels and 64 µs time sampling). Propagation effects, which limit D max at large distances, are calculated using the NE2001 electron density model (Cordes & Lazio 2002). For distances > 5 kpc, D max is limited by pulse broadening from scattering. RIGHT: Galactic plane showing spiral arms (blue) as in the NE2001 model, the solar circle (dashed line), solid lines of constant distance from the Sun at 1, 5, 10 and 15 kpc and lines showing the Galactic longitude coverage (red) for the PALFA survey. Small green points show pulsars in the ATNF catalog with b 5 and large red points show new PALFA discoveries. Note that most of the pulsars within 1 kpc of the Sun in the PALFA search range have been re-detected with PALFA, though this is not indicated in the figure. Distances are calculated using DM and the NE2001 model and are subject to large errors, particularly at the larger nominal distances. With current parameters the survey is complete out to 1 kpc, i.e. the entire luminosity function of steady pulsars is detectable out to 1 kpc. However, the survey samples only about 25% of the LF for pulsars at a distance of 5 kpc (but with considerable uncertainty due to characterization of the luminosity function and spatial distribution of pulsars). The spatial distribution of pulsars appears to peak at a galactocentric radius 4 kpc, which is equal to the galactocentric radius of the tangent point of the l = 32 limit of Arecibo s coverage of the Galactic plane. The asymmetry of spiral arms with respect to l = 0 may account for some differences between the pulsar yields of the Parkes multibeam survey and the PALFA survey. In Fig 8 theoretical values of D max are plotted against spin period P for four different pseudo-luminosities L p. The curves shown are for the PALFA survey using the current WAPP spectrometers and for the Parkes Multibeam survey. The upper and lower boundaries of the bands correspond to full, on-axis gain and 50% gain, respectively. The curves are direction dependent, but the direction used for the figure is representative of low latitude directions toward the inner Galaxy. From the figure and other considerations, we make the following observations about completeness of the PALFA survey to date: 1. Period coverage is complete for P > 10 ms because for periods longer than 10 ms, the D max curves flatten as pulses become much less affected by propagation effects (dispersion and scattering). 2. At large periods, the PALFA/WAPP data obtained so far reach a distance about 1.5 times further than the Parkes MB survey. 3. The luminosity function is sampled completely out to a distance of only 1 kpc for P > 10 ms and shorter period objects are less completely sampled. (This statement assumes that the luminosity function cuts off at L 1 rather than rolling off. Nonetheless, L 1 is a milestone type luminosity.) The figure shows that D max = 1 kpc for a luminosity L 1 = 0.1 mjy kpc 2 and P > 10 ms (and beamed toward us). Conversely, the fraction of the luminosity function sampled is increasingly smaller for pulsar samples distances greater than 1 kpc. For comparison, the Parkes Multibeam survey becomes incomplete beyond 0.7 kpc. (In both cases we are reading off from the curves in Figure 8 that correspond to 50% beam gain.) Also, the Parkes survey, with its wider channel bandwidths and longer sampling time, was less sensitive to the faster spin periods than is the PALFA survey.

12 12 4. A luminosity completeness fraction is area of the luminosity function above L p, which is F c (L p ) (L 1 /L p ) x 1 for L 1 L p L 2. Rewriting in terms of D and ηs min, where η allows us to scale the threshold flux density, we have F c (D) η (x 1) D 2(x 1), (2) where D is in kpc and we have used the fact that, for nominal S min (for current WAPPs and integration time of 268 s) at long P, the survey is complete out to 1 kpc. If our threshold is η = 2 times worse than nominal, we sample between 1/4 and 1/2 of the luminosity function. 5. While the luminosity function of pulsars further than 1 kpc is sampled increasingly incompletely with greater distance, the volume surveyed V s D 3 increases. The product F c V s is what really matters and scales as F c V s D 5 2x D to D 1.8 (3) for x = 2 and x = 1.6, respectively. If the volume is uniformly filled with pulsars (which it isn t of course), this scaling law indicates how detected pulsar numbers will grow with greater D. Eq. 3 shows that the growth of the detection rate depends strongly on x. The lowest x identified in analysis of the PMB survey indicates that reaching greater distances will enhance the pulsar yield substantially because F c V s V RFI: if our effective threshold is higher than we think because of RFI contamination, then η > 1 and the number of detections will be lower. For η = 2, for example, F c V max 2 (x 1) 0.5 to One of our conclusions is that we should increase D max by increasing the product of bandwidth with integration time, BT. We have η (BT ) 1/2, so an increase in BT yields an increase in detected numbers (BT ) (x 1)/2. 8. It is not practical to increase the completeness distance (that to which we reach luminosities down to L 1 ) to 10 kpc because that requires an increase of BT by a factor of Such completeness requires an SKA type instrument that provides a huge increase in telescope gain in addition to a modest increase in BT. However, we can increase the completeness distance to about 1.7 kpc through the increased B of the PALFA spectrometers and a doubling of the integration time. We propose that this be done where it is sensible to do so, i.e. for the longitude range 32 l 60 where the pulsar density is high out to distances of 15 kpc (c.f. Fig. 8). High luminosity pulsars will be detectable to well beyond 20 kpc and thus will probe the outer boundary of the pulsar population in those directions. Plans for 2008 Candidate Selection: We are still working on different schemes in the Consortium for winnowing candidates lists. We are planning a face-to-face meeting in the early Fall to discuss heuristics and algorithms for choosing candidates that we will use telescope time for confirmation. Confirmation time will come out of our P2030 survey time and we are cognizent of the need to have a well chosen pulsar candidate list in order to optimize telescope time usage. However, it is also true that there is an experimental aspect to finding the balance between false positives and false negatives. As described earlier under Understanding the Pulsar Yield, filtering of the Cornell pipeline s signal candidates using search analysis data products (not pulsar catalog information) yields a pulsar candidate list with a good fraction of pulsars (PALFA, DMB and aliased ATNF-catalog pulsars) and some very good candidate new pulsars. However, this particular scheme is too severe and we are still experimenting with other filtering heuristics and thresholds. The plan for the near future is to (a) compare pulsar candidate lists used by different winnowing methods to identify a short list of very good candidates that will be re-observed and (b) define a longer list of candidates of gray-area candidates that we will spot select for reobservation in order to empirically determine our success rate. Going forward, we will then optimize the use of telescope time for confirmation and survey time with respect to the rate of new detections, false positives and false negatives. The Common Database of signal candidates from all the pipelines is the enabling aspect of candidate selection. We are continuing to develop algorithms and scripts for using all the information in this database. New Spectrometer: The new spectrometer boards are being integrated into a PALFA spectrometer, both hardware and software wise. Over the next few months, observing functionality will be essentially the same as with the WAPPs (via CIMA). The PALFA spectrometer comprises polyphase filter banks that nominally are less susceptible to RFI than the WAPP correlators. However, the 300 MHz of processed bandwidth will contain more RFI than the band we have analyzed with the WAPPs. Only experience will tell how our data processing will have to evolve to handle the RFI. Availability of the new spectrometers has been slow because software has been needed to output raw data in PSRFITS format and observatory personnel have had limited time. J. Deneva (Cornell) will spend 6 months at Arecibo during the rampup period for the new spectrometers in Fall Longer Scans in Selected Parts of the Galactic Plane: As argued earlier, it appears sensible if not necessary to use longer integration times along with the wider bandwidth of the new PALFA spectrometers to reach more

13 deeply into the inner Galaxy for the lower longitudes ( 60. When the new spectrometers become available, we will take this approach. Commensal Drift Scan Observations with the ALFALFA Project: We are doing simulations of the MSP population to deterimine the yield of a commensal search. Feasibility Study for Longer-dwell Observations Out of the Galactic Plane: In addition to the commensal search during ALFALFA time, we are doing simulations to determine optimal parameters for a deeper search at latitudes out of the Galactic plane. The MSP population extends to higher latitudes, on average, than young, canonical pulsars and there is less longitude dependence because the population is old and has migrated far away from their birth sites. Consequently, a sensible strategy here is to search in LST ranges that are relatively undersubscribed. Finding MSPs in these ranges will also allow follow-up timing to be conducted with Arecibo. Multiple Passes on Pointing Directions: Our original proposal to NAIC discussed the pros and cons of multiple passes. We will begin making repeats of some sky positions because RFI was severe. Also, we need to assess how much better the new spectrometer will perform and whether it is optimal on that basis to reobserve all sky positions. More fundamentally, pulsar intermittency suggests that a multiple-pass strategy is needed to find rare objects that are potentially of the greatest interest in the overall yield. We can assess all these issues through at least repeated observations on a test area of the sky using the new spectrometers. Outreach: Our primary outreach activity involves the Arecibo Remote Command Center (ARCC) at the University of Texas, Brownsville, which involves students from the high school, undergraduate, and graduate levels in actual research at Arecibo. Using Arecibo s remote observing capabilities together with a dedicated center at UT Brownsville, student teams performed PALFA observations in early 2007 and will continue when observations resume. The observing itself is conducted with groups of three students. One of the three students has observing experience and s/he mentors the other two students. As students progress in their confidence and abilities, they earn the right to lead an observing run on their own. ARCC students also look through PALFA candidate files produced at Texas. As part of a three week summer academy, students learn the basics of Astronomy and Astrophysics together with signal processing and data analysis. This prepares them to evaluate the results of the PRESTO and Sigproc pipelines. As candidate viewing system has been developed in order for the students to look through the data both in the ARCC room and on their home computers ( Four of the ARCC students had the opportunity to visit the Arecibo observatory this year and perform followup observations for the PALFA survey on cite. A larger group of students presented the current results of the ARCC pulsar search, together with other research projects, at the January 2008 meeting of the American Astronomical Meeting. This year, we are launching the ARCC scholars program. Funded by an NSF PAARE grant, this project provides full four year undergraduate scholarships for up to ten students per year. The students must work on research projects associated to the ARCC program for the first two years of their undergraduate career, while maintaining good academic standing as a student in the physics department of UT Brownsville. We have five undergraduate students recruited for the fall term These students will be actively involved in the PALFA survey by leading observations, searching through candidates, and training the next generate of high school ARCC students. Also, in the year following this report, we will launch the Einstein@Home project for PALFA processing. This will involve tens of thousands of worldwide user clients covering a wide range of backgrounds, from high school students to scientists working in many different fields. Processing at Cornell: The Cornell pipeline does a standard periodicity search and single-pulse search without doing an acceleration search. As a consequence it runs faster than the other, PRESTO-based pipelines. It has been run on all data archived at the CAC and has provided signal candidates. During the last year we have discovered two MSPs with this pipeline along with several longer-period objects. As with the PRESTO pipelines, we are applying heuristics to the large signal-candidate list to yield a pulsar candidate list (beyond the very obvious candidates, like those mentioned above) worthy of confirmation at Arecibo. The code processes one beam on a single processor in 2 hr, corresponding to about 2.4 TB of raw data per week. 9k pointings have been processed out of the 13k search pointings made and of the 11k pointings that have been shipped to the CAC. The gap between the 9k processed and 11k shipped is due to the fact that in the early days, data were shipped such that only partial pointings were received at the CAC and the Cornell pipeline requires data from all seven beams simultaneously. The remaining pointings are mostly from the P1944 project which used shorter dwell times than in our standard observation. The backlog has been slow to clear because restoring data from tape back to disk at Arecibo has been limited by available personnel availability. Graduate student J. Deneva has spent time at Arecibo to rectify this backlog and is making good headway. Processing at McGill University: Approximately TB of raw data are processed per week. Methods for automatically identifying/filtering RFI at the database level are now being developed. This will reduce the number of candidates to examine by eye, at least on a first pass through the results. A total of 1500 pointings (excluding directed pointings at known pulsars) has been processed. Of the signal candidates, are human classified. 13