Pulsar luminosity distribution in 47-Tuc

1. Pulsar luminosity distribution in 47-Tuc Tim Connors, Vacation student University of Sydney

2. Galactic Pulsar Distribution • Luminosity distribution of galactic pulsars is dN = L-1d logL. Same everywhere -> reasonable? • Every lower decade of flux has 10 times more pulsars -> same integrated flux1 • To a point anyway - where is the low luminosity cutoff?

3. 47 Tuc • Have ~22 known pulsars in 47 Tuc, of which we know 14 positions from timing • The unlocated pulsars are ~10 times fainter than the located pulsars - however, all are only visible during scintillation • Up to 200 pulsars below current limit predicted to lie in cluster1

4. 47 Tuc • Spacial distribution of unresolved pulsars same as those resolved • These should contribute about the same amount of flux as the identified pulsars • In all, we expect 4mJy of integrated flux, since the known pulsars integrate to 2mJy

5. Project Setup • We have quite a few days of observing, at different configurations • Wide (6km), for a high resolution image • Several (375m, 750m) arrays to be sensitive to extended component

6. Surrounding field • -0.3 - +1 mJy • The field is typical of 20cm data • Got to make sure potential interferers are removed from UV data before imaging

7. Image from 6km arrays 1408 MHz 8x10” beam 28mJy 3, 6, 12, 24s 0-4mJy

8. Combined 375m/750m image (Grand sum) 1408 MHz 63x68” beam 35mJy 2.5, 5, 10, 20s Scintillation changes sources between days, so we get the ‘average’

9. Grand sum, with point sources removed Subtract point sources from UV data Should be left with extended or possibly variable sources

10. Don’t discard the 6th antenna! b • 27th Dec, 2000 • 50mJy x 3,4,5,8s • Ok, so the pulsars scintillate. How bright are they today? • Then model the pulsars that we can see a

11. Pulsar and calibrator fluxes Each point above 3s that corresponded to a located pulsar, plus the two bright constant sources (a = 643mJy, b = 226mJy) near the centre were scaled so the brightest source (a) was the same as in the high resolution mosaic (ie, it was used as a calibrator)

12. 27th Dec, pulsars, a and b removed • 27th Dec, 2000 • 50mJy • 1.7, 3.4, 6.8s • Removed 2 constant sources (a, b), and 3s pulsars • Peak is 380mJy

13. Grand sum, a and b removed • 2 sources too close to remove automatically finally removed from grand sum. • a also served as a calibrator - fix flux given by 6th antenna • Peak is 510mJy

14. Grand sum, pulsars, a and b removed • Which enabled us to subtract the pulsars visible on any day • Then imaged the whole thing • Peak is 230mJy

15. Now what? • Now we just have to work out what it all means! • Did we get a significant flux there in the centre? There are a lot of hills around - RMS is still bigger than theoretical • What proportion of flux at end is due to known, but unlocated pulsars, and what is due to an unknown quantity of unresolved pulsars? (Should be safe from sources that aren’t pulsars, because they don’t scintillate2)

16. Now what? • We don’t even see the 2mJy expected from the known pulsars, let alone 2mJy once we have subtracted the ~0.5 - 1mJy from the unresolved pulsars. Why don’t we see these - has the Parkes group overestimated the mean fluxes of the 22 pulsars, or is our array not compact enough, or something entirely else? • Unfinished project, but only 2 weeks left!

17. References [1] F Camilo, D.R. Lorimer, P. Freire, A.G. Lyne, R.N. Manchester, Observations of 20 millisecond pulsars in 47 Tucanae at 20cm, 2000, ApJ, 535, 975 [2] D. McConnell, J.G. Ables, Radio sources near the core of globular cluster 47 Tucanae, 2000, MNRAS, 311, 841 [3] A.G. Lyne, R.N. Manchester, J.H. Taylor, The galactic population of pulsars, 1985, MNRAS, 213, 613 Acknowledgments ATCA and ATNF for providing the vacation scholarship program and the invaluable observing experience Dave McConnell for being an excellent supervisor