Low Frequency VLBI

“Astrometry through beer goggles”

Adam DellerSwinburne University

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

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)

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. <1Jy brightest pulsars @2 GHz

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”

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

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

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

Previous low-freq VLBI

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

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

Tim

e

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!

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

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!

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

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)

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)

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