Low Frequency VLBI

1. Low Frequency VLBI “Astrometry through beer goggles” Adam Deller Swinburne University

2. Outline • What you get (resolution, FOV, sensitivity) • Previous low frequency VLBI • What you can do with low frequency VLBI: • Localise transients • ISM studies • Nearby astrometry • Why it’s hard (ionosphere) and what can be done to counter this

3. Disclaimer • I’m not a low-frequency expert! • This is (mainly) not VLBI in the current, disconnected/narrow field sense • Simply (connected) long baselines • Focus on E-LOFAR since this is the only truly long-baseline instrument (LWA and VLA+VLBA shown for comparison)

4. Sensitivity • LWA ~1 mJy in 1 hour at 20 MHz • E-LOFAR ~0.1 mJy in 1 hour at 200 MHz • VLBA + phased VLA ~10 mJy in 12 hours at 74 MHz • By way of comparison: EVLA, E-MERLIN, EVN, upgraded VLBA all heading towards micro-Jy sensitivity in single pointings • PSR 1937+21 in M31: ~several mJy @30 MHz!! c.f. <1Jy brightest pulsars @2 GHz

5. Resolution • LWA (400km baselines) >2” • VLA+VLBA (8000 km baselines) >100 mas • E-LOFAR (1000+km baselines) >200 mas • Cf. Existing low frequency surveys: VLSS (74 MHz) 80”, WENSS (327 MHz) 54” • Cf. other widefield surveys: FIRST 5”, NVSS 45”, ATLAS 6” • Optical surveys ~1”

6. Station FOV • LWA: ~2.5-10° FWHM (80-20 MHz) x8 • VLA + VLBA: Arcminute(s) (phased VLA) x1 • E-LOFAR: ~1-10° FWHM (300-30 MHz) x8+ • Full FOV unlikely to be available to E-LOFAR due to time/bandwidth smearing

7. Reasons for low-freq VLBI • “The love of the game”: very little is known about the low-frequency sky at high resolution • E.g. accurate fluxes for compact sources • Solar observations (LOFAR) • High-Z AGN/starburst discriminant • Studying the ISM via pulsars • Interferometric scintillation measurements • Angular broadening • Astrometry (?!?!) Transients, pulsar kinematics

8. Previous low-freq VLBI • Lenc et al. 2008 looked for VLBI counterparts to NVSS/WENSS sources using the VLBA + some EVN at 327 MHz, piggyback on targeted observation • Detected 27 WENSS sources (expect 270 if all unresolved) from 2x13 sq. deg. fields • Sensitivity several mJy/beam

9. Previous low-freq VLBI

10. Previous low-freq VLBI • The Lenc et al. imaging took months!!! • All other previous low-freq VLBI surveys combined yield a similar number of published images (Altschuler et al. 1995; Lazio & Cordes 1998; Chuprikov et al. 1999; Cai et al. 2002) • Thus both the techniques and understanding of the source population (starburst/AGN/Galactic) are fledgling

11. Time Frequency ISM studies with VLBI • Angular broadening for turbulence measurement: imaging pulsars • Interferometric measurements of scintillation: Brisken et al. (Arecibo, GBT, phased Westerbork, Jodrell Bank) • Measurement of the transfer function of the ISM for coherent de-scattering?

12. Astrometry • Will never be as good as higher frequency observations - obviously!! • Single epoch accuracies ~mas at best? • Refractive scintillation may be short-term limit? • Three reasons why this is still absolutely crucial for low frequency instruments: • Often we just want ~arcseconds eg transients • Many sources are nearby, large motions • If this is the only possible , we must!

13. Astrometry • Spatial localisation of transients • LOFAR transient follow up: TBB vs lower freq and wait for dispersed signal to arrive? • Faint steep spectrum objects: • Kinematics of pulsars, other neutron stars • Might be tough to do parallax unless very nearby but proper motion should be ok • Because of wide fields we may be able to observe *many* times/year c.f. currently

14. Extrapolating from 1600 MHz • 7 observations with the LBA (1400 km baseline, 5 antennas) phase referenced, 1 out-of-beam cal, primitive ionosphere corr. • 40 mJy pulsar, “Thermal” SNR errors 0.25 mas, “systematic” iono errors 0.3 mas • Scale by 10 and 10^2 for resolution and ionosphere to 140 MHz: 2.5,30 mas, but sensitivity/calibration will be much better!

15. Challenges for low-freq VLBI • Calibration, calibration, calibration… • Ionosphere is biggest problem, but most VLBI sources are weak so dynamic range/confusion is also a big deal • Compared to shorter baselines we have less calibrators and bigger fluctuations • Multiple calibrators and good a priori ionosphere (field calibration, Lazio) crucial

16. Multi-calibrator approach • The LOFAR ionosphere model must be refined further within the long-baseline FOV to get accurate astrometry • With more than one calibrator, can refine calibration across the beam with increasing accuracy • Bright calibrators = rapid corrections • Can “2 selfcal” on target (Brisken)

17. Multi-calibrator approach • Expect ~3 good calibrators per square degree (>10 mJy flux, <200 mas in extent, Lenc et al. 2008) • What FOV will E-LOFAR have - will 3 calibrators per degree be enough? • Maybe lower frequency will be better astrometrically!!! (More calibrators, brighter)

18. Conclusions • VLBI is possible at low frequencies! • VLBI is useful at low frequencies! • Pulsars are one of the prime targets for low-freq VLBI, both as targets and to illuminate the ISM • “Integrated” VLBI will dominate the science • Maximising FOV and sensitivity is crucial in order to get the best possible calibration