1. “Astrometry through beer goggles” Adam Deller Swinburne University
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 in M31: ~several MHz!! c.f. <1Jy brightest 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 ( 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 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. 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?
Frequency
Time

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