SKADS Virtual Telescope: Pulsar Survey IV: Globular Cluster Pulsars

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1 SKADS Virtual Telescope: Pulsar Survey IV: Globular Cluster Pulsars PI: S. Ransom Co-I: M. Kramer We propose to use the SKADS Virtual Telescope (SVT) to search Globular clusters for fast rotating pulsars in potentially extreme binary orbits for a large variety of science enabled by these discoveries, such as measurements of neutron star masses and cluster proper motions, or studies of the interstellar and intracluster medium. This survey is complemented by surveys along the in the Galactic Centre and in the Galactic centre and in the whole sky (see proposals by PI Smits). All of these are part of an effort that will cumulate in an Galactic Census of pulsars using the sensitivity of the full SKA, which will then also allow us to search for extra-galactic pulsars. The science case for surveys with the SVT and the full SKA are very similar, whereas the survey strategy as well as the instrumental requirements differ among these different types of surveys. We therefore propose these projects in separate proposals. Scientific Justification Since the discovery of the first pulsar in a globular cluster (GC; Lyne et al. 1987, Nature, 328, 399), it has been clear that these objects are among the most exotic and interesting of the pulsar population. GCs are rich breeding grounds for millisecond and/or binary pulsars (due to stellar interactions and exchange encounters), and they contain the majority of these systems known even though globular clusters account for only a tiny fraction of both the volume of the sky surveyed and the total mass of the Galaxy. GC pulsars provide a wide variety of exciting science results, while simultaneously raising many fundamental astrophysical questions 1. Ensembles of pulsars in individual clusters can produce unique science. There are currently 3 clusters with more than 10 known pulsars: M28 with 11, 47 Tucanae with 22, and Terzan 5 with 33. Cluster Dynamics: The projected positions of the pulsars with respect to the cluster centers as well as measurements of their period derivatives (which are often dominated by acceleration within the GC gravitational potential) provide a sensitive probe into the dynamics of the cluster and even constrain 1 There are at least 133 known GC pulsars, of which 129 are currently listed in P. Freire s catalog at pfreire/gcpsr.html For a recent review of GC pulsars, see Camilo & Rasio (2005). the mass-to-light ratio near the cluster center (Phinney 1992, Philos. Trans. Roy. Soc. London A.). These measurements can provide evidence for the presence (or absence) of black holes in the cores of the clusters (e.g. D Amico et al. 2002, ApJ, 570 L89). Statistical NS Mass Measurements: Using the ensemble of 32 Terzan 5 millisecond pulsar (MSP) timing solutions, the positions of the pulsars with respect to the cluster center allow a statistical measurement of the average NS mass ( M ) assuming standard cluster dynamics (see Heinke et al., 2005, ApJ, 625, 796). Inter-stellar and Intra-Cluster Medium: In 47 Tucanae, the pulsar positions, period-derivatives, and dispersion measures (DMs; the integrated electron column density along the line of sight to the pulsar) provided a definitive measurement of ionized gas within the cluster (Freire et al. 2001, ApJ, 557, L105). In Terzan 5, similar measurements provide a unique probe of interstellar medium electron density variations over pc scales and show that they are not inconsistent with Kolmogorov turbulence (Ransom 2006, SINS Proceedings, astro-ph/ ). Cluster Proper Motions: MSP timing provides very precise positions, which, over the course of 5 10 years, allow the measurement of the proper motion of the globular clusters themselves (e.g. Freire et al. 2003, MNRAS, 340, 1359; Jacoby et al. 2006, ApJ, 644, L113). Such measurements are very important in order to determine the Galactic orbits of clusters and to predict the effects of, for example, tidal stripping. Measuring cluster proper motions is exceedingly difficult in the optical (with HST, for instance), especially for the Galactic bulge clusters which are distant and plagued by extinction. Comparisons Between Clusters: Comparisons of the pulsar populations in Terzan 5 and 47 Tucanae tell us about the global properties of the MSP population and differences in the dynamics and properties of the clusters themselves (which affect MSP production and behavior). The spin-period distribution of the Terzan 5 pulsars is significantly

2 flatter than that of the 47 Tuc pulsars with more faster and more slower pulsars. In fact, Terzan 5 contains 5 of the 10 fastest known spinning pulsars, while 47 Tucanae contains only one within the top 10. Terzan 5 seems to have three times more MSPs of any particular luminosity than does 47 Tuc. Terzan 5 has only two known black-widow binaries compared with the five known in 47 Tuc. Likewise, Terzan 5 has six highly-eccentric binaries, compared to zero for 47 Tuc. Why these drastic differences? While ensemble studies of GC pulsars are very interesting, many of the pulsars are truly exotic objects worth studying individually: The Fastest MSPs: Ter5ad is the fastest MSP known (P = 1.396ms; Hessels et al. 2006, Science, 311, 1901) and finally beats the 23-yr-old record established by the first MSP discovered (Backer et al., 1982, Nature, 300, 615). Its discovery renewed hope in finding a sub-millisecond pulsar (a pulsar having spin period under 1 ms). The importance of such a discovery cannot be overstated: it would provide by far the most direct and most interesting constraint on the properties of matter at nuclear densities; hence, this is of major significance to physics in general (Lattimer & Prakash 2004, Science, 304, 536). As GCs are known to be excellent breeding grounds for MSPs, it seems likely that the first sub-msp (if they exist) would be found in a GC. Slow pulsars: A handful of slow pulsars have been found in GCs (e.g. PSR B in NGC 6342, NGC 6440A, NGC 6624B & C), and their very existence is a puzzle, as their ages ( yr) are much shorter than those of the GCs ( 10 9 yr). The pulsars must have formed relatively recently, yet the standard massive stellar progenitors should have gone supernova long ago. One possibility is that the apparently young pulsars formed via accretion induced collapse of a massive white dwarf, or that the neutron star was recently partially recycled by a binary companion. Thus the number and spatial distribution of slow pulsars in GCs is of considerable interest, both for neutron star formation mechanisms, as well as for binary dynamics. Main-Sequence MSP Systems Several pulsars have been recently discovered (including 47TucW, Ter5P, Ter5ad, and M28H) which appear to have bloated companion stars much like the prototype PSR J , a very interesting system in NGC 6397 (D Amico et al. 2001, ApJ, 561 L89). These pulsars are eclipsed for large fractions of their orbits and show irregular eclipses on some occasions. Timing positions show that they are usually associated with hard X-ray point sources, probably resulting from a shock between the MSP and stellar winds. In addition, they exhibit huge orbital period derivatives (and numerous higher order derivatives) likely due to tidal interactions. Will the orbital effects allow us to constrain the highly-uncertain tidal circularization theory? Will optical measurements of the orbital lightcurves allow precise NS mass measurements? Highly Eccentric Binaries: At least 12 GC pulsar systems are members of highly eccentric binaries, and most of those contain MSPs. Terzan 5 alone has at least 6 such systems. For comparison, only 1 eccentric MSP binary is known in the Galactic population. Given the angular reference that an ellipse provides, it is easy to measure orbital precession, which is dominated by general relativistic effects in the case of a compact companion and which provides the total system mass. Timing of several of these systems (especially Ter5I & J, Ransom et al. 2005, Science, 307, 892; and NGC 6440B) indicate massive neutron stars (>1.7M ) which constrain the equation-of-state of matter at nuclear densities. Such constraints are impossible to achieve in nuclear physics laboratories here on Earth. Planet systems? MSP-Black Hole systems?: There is already one confirmed globular cluster triple system (PSR B in M4) which contains an MSP, a white dwarf, and a planetary-mass component (e.g. Sigurdsson et al. 2003, Science, 301, 193). Ongoing timing observations show possible evidence for another MSP planet system (in NGC 6440), with one or more terrestrialmass planets. In addition, given the strong effects of mass segregation and the long lifetimes of MSPs, if any black holes exist in the GC system, it is not outrageous to think that we might uncover an MSP BH binary. Such a system would be an incredible testbed for strong-field gravity. While there are over 130 globular cluster pulsars currently known, an important point to understand is that we are certainly only seeing the tip of the iceberg, and there are likely to be many hundreds or even thousands of GC MSPs in the the Galactic GC system. Most GC surveys have come nowhere near the 2

3 predicted lower luminosity limit of the MSP population, with the possible exception of the Parkes surveys of 47 Tucanae, where the pulsars occasionally experience extremely strong flux amplification due to diffractive scintillation. With the great improvements in receivers, computational capability, and pulsar backends (i.e. rapid sampling of large bandwidths of many hundreds of MHz with hundreds or thousands of frequency channels) over recent years, our current limitation for finding additional GC pulsars is simply sensitivity. The SKA, and in the meantime, a 10% SKA, will simply revolutionize this field. 1 Which Globular Clusters for Early SKA Pulsar Searches? In order to maximize the early science during the staged SKA construction, we need a way to select the globular clusters where we are most likely to discover many new pulsars. One method involves examining a combination of the cluster density, total mass, and the distance (D). The first two parameters are linked to the stellar interaction rate in the core of the cluster (Γ c ; see Pooley et al. 2003, ApJ, 531, L131), which seems to be correlated with X-ray binary formation rates and possibly the present rate of MSP formation. Table 1 shows the 24 best GCs as ranked by Γ c D 2. In this proposal, we request observations of all of the clusters listed in Table 1. However, priority should be given to those clusters where the SVT-B and SVT- C will have the largest advantage over previous surveys. In general, clusters that have only been searched by Parkes (with G 0.73KJy 1 and T sys 25 30K) should be targeted first (and those clusters should be searched in rank order, with some additional weight given to clusters where there is at least one known pulsar, as those searches need only be conducted over a much smaller range of DMs). The next clusters searched should be those that have been observed by the GBT (with G 2KJy 1 and T sys 20 30K for most searches). A possible exception to these guidelines is Terzan 5, which given VLA observations of the total radio flux from the cluster (Fruchter & Goss, 2000, ApJ, 536, 865), contains up to 100 as yet unidentified MSPs. Observations with the SVT Most of the highest ranking GCs are in the Galactic bulge, with large columns of ionized gas along the lines of sight. In order to minimize the effects of scattering and dispersive smearing yet still collect enough of the typically steep-spectrum pulsar flux (average spectral index 1.6), observations towards bulge clusters should use a relatively wide bandwidth (up to 800 MHz) centered near 2 GHz. These parameters have proven to be near-optimal during the highlysuccessful GBT cluster surveys. Pulsars outside of the plane (i.e. those in high-latitude clusters) however, can be searched using lower frequencies ( 800 MHz). For the purposes of this proposal, we assume that the main search system for the bulge clusters includes only the SVT dishes, operating with 250 or 800MHz of BW (stage B and C, respectively) and centered at 2 GHz. However, we also compare a dish-only SVT-B and dishes and/or aperture array SVT-C systems operating with 100MHz of BW centered at 800MHz. Since the sensitivity of the SVT-A (G 1 KJy 1 and T sys =50K) is less than Parkes, we will not consider searching with it. In general, given that GC pulsar searching is currently almost completely sensitivity limited, new searches of any globular cluster only become interesting when sensitivities are improved over earlier surveys. Fortunately, there are several key clusters where this is true for SVT-B, and many clusters where it is true for SVT-C. Figure 1 shows the predicted sensitivities for 6 hr observations of high-galactic-latitude clusters with low DMs and low-galactic-latitude clusters (like the bulge clusters) with much higher DMs. A SKADS VT-B system will be a factor of times more sensitive than Parkes surveys of globular clusters, but not as sensitive as GBT surveys. During this stage of operations, deep surveys of the clusters in Table 1 that are visible only with Parkes will be prime targets for detecting new pulsars. When SVT-C comes on line, Figure 1 shows that it will be factors of 2 3 times more sensitive than the best GBT cluster surveys that can currently be conducted. Such improvements in sensitivity have the potential to uncover hundreds of new MSPs, as witnessed by the 60 new pulsars uncovered by the GBT with its factor of 3 5 improvement in sensitivity over earlier Parkes surveys (and even though the GBT cannot observe several key clusters). Besides raw sensitivity, one of the key aspects of GC pulsar observations is the size of the synthesized beam or beams that are actually searched. Mass segregation results in the vast majority of known GC MSPs residing near the centers of their parent clusters. In fact, 95% of GC pulsars with known timing positions are within 1 of their cluster centers and 85% are within 0.5. With the SVT array sizes and synthesized beam diameters shown in Table 2, the vast majority of pulsars will be visible in a single synthesized beam centered on each target cluster. This provides the huge advantage that we can conduct very sensitive (and likely productive) searches even in the early days of SVT stages B and C, when we will likely have access to limited correlator/phasing capabilities and/or a single pulsar backend. As the SVT continues to grow, correlator capabilities improve (i.e. to allow the simultaneous phasing 3

4 Table 1: The 24 best globular clusters in the Galaxy (from a total of more than 150), according to the Γ c D 2 metric (i.e. interaction rate divided by distance squared). This is indicated as a percentage of the predicted number of theoretically observable MSPs in the whole Galactic globular cluster system, assuming uniform sensitivity. If there are pulsars in the cluster, the number is reported as well as the average DM. Otherwise the DM is estimated from the distance to the cluster (D) and Galactic coordinates (l, b) using the Cordes and Lazio (2001, model of the electron distribution of the Galaxy. It is interesting to note that all of these clusters are located in the Southern hemisphere, except M15. ID Dec l b D Γ c D 2 N psr DM Best (kpc) (%) (cm 3 pc) Telescope 47 Tucanae Parkes M GBT NGC GBT NGC GBT Terzan GBT NGC GBT NGC GBT NGC Parkes M GBT Liller GBT NGC GBT NGC Parkes M GBT M Arecibo NGC Parkes NGC Parkes NGC GBT M GBT NGC GBT Terzan GBT M GBT NGC GBT NGC GBT NGC GBT of several independent beams), and additional pulsar backends become available, searches with a larger number of synthesized beams (for example, a hexagonal arrangement of 6 plus a center beam) will become feasible. These searches will find those pulsars that are slightly offset from the cores of their parent clusters. A final stage of searching, using the SVT-C dishes out to several kilometer distances from the array center, could be conducted using the raw visibilities from phased sub-arrays of antennas. A time series of synthesized images would be created where each independent pixel is searched for pulsars. These searches would identify many new MSPs which have been ejected or nearly-ejected from their parent clusters. In addition, because of their very large data rates and huge computational costs, they would pave the way for all-sky blind searches with the SVT and eventually the SKA itself. 4

5 Figure 1: The predicted sensitivities of the globular cluster surveys using the SVT-B and SVT-C as compared with the current state-of-the-art GBT/Spigot S-band system (which has uncovered over 60 new GC pulsars), the GBT/Spigot at 800MHz, and the center beam of the Parkes Multibeam system at 1.4GHz as used for its globular cluster surveys. Integration times of 6 hrs are assumed in all cases. The sensitivities become much worse for MSPs in the high-dm clusters when observing at 800MHz due to the multi-path propagation effects in the ISM. Table 2: Observing parameters for the SVT systems shown in Figure 1. Effective BW f ctr # Beam Diam Gain T rcvr Stage Type Diam (m) (MHz) (MHz) Chan (arcmin) (K Jy 1 ) (K) SVT-A 1 dishes SVT-B 2 dishes SVT-C 3 dishes SVT-C 3 dishes SVT-C 4 AA % of the 120 dishes within 180m 2 50% of the 500 dishes within 250m 3 50% of the 2000 dishes within 500m 4 100% of the aperture array (AA) within 1km 5