Wide-field, high sensitivity VLBI. surveying and astrometry with mas resolution. Adam Deller. VLBA Astrometry Symposium July 2009. High resolution interferometry. Traditionally, narrow fields for studying single compact objects (pulsars, AGN, masers) Astrometry is the current “killer app”
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Wide-field, high sensitivity VLBI
surveying and astrometry with mas resolution
Traditionally, narrow fields for studying single compact objects (pulsars, AGN, masers)
Astrometry is the current “killer app”
The VLBA is the currently the premier instrument for
precision VLBI astrometry
A typical VLBI image
Factor of 4 increase in continuum sensitivity through the bandwidth upgrade to 4 Gbps
Allows fainter astrometry targets
Additional benefit for the use of fainter, more nearby “in-beam” calibrators (and hence better astrometry)
Pradel et al. 2006
4 minute baseline sensitivity @ 4Gbps is 0.7 mJy, summing all bandwidth
Thus calibrators as faint as 5 mJy can be used as in-beam calibrators - and brighter calibrators can solve for even shorter term atmospheric/ionospheric variability
No comprehensive catalogue of the radio sky at high resolution exists (reasons later)
The nearest equivalent is the geodetic source list maintained at astrogeo.org, with ~4000 sources (bright enough for use as primary calibrators, but density <1/sq. deg.)
Thus every astrometry project must typically find in-beam calibrators with a
This typically involved selecting candidates from low resolution VLA surveys, testing compactness with higher resolution/frequency VLA observations, and finally VLBA follow-up… Tedious & slow!
Question: The VLA and VLBA primary beams are
the same size; so why are VLBA observations sotime-consuming that a VLA pre-filter is required?
Resolution is a curse: imaging the full VLBA primary beam (~0.25 sq. deg. @ 1.6 GHz) with 2x2 mas pixels (synthesized beam ~10 mas) requires a ~600 Gpixel image: 2.4 TB, which is almost entirely noise!!
Plus the correlated data for8 hours @ 4 Gbps totals~60 TB - infeasible
600 Gpixel !
Forming small images around multiple fields of interest is possible, however
Requires a “uv shift” to be performed, correcting the antenna-based delay difference for each desired phase centre
Can be done post-correlation, but the intermediate data volume is tremendous - 60 TB/8 hour VLBA track, as with imaging the full field (time/freq. resolution)
Such post-correlation shifting has been used to test wide-field VLBI imaging (e.g. Lenc et al., Middelberg et al.)
However, the most efficient implementation (minimizing I/O) is within the correlator, before data must be written to disk
Such a capability is in the final stages of being tested in DiFX, the software correlator integral to the upgraded VLBA
Lenc et. al.
Phase shift adds a negligible overhead to station-based cost of correlation
However, the baseline-based XMAC must be duplicated for each phase centre
Station-based processing for VLBA (10 stations) outweighs baseline-based by ~3:1
Therefore theoretical overhead of N fields is a (N-1)/3 slowdown to correlation speed
Alternative implementations exist where the rotation is done more analogously to post-correlator rotation, after subintegration
Zero station-based cost, greater baseline-based cost, but less frequently
Sacrifice time resolution (but still to an acceptable level) and computation reduced (factor of several lower overhead per field?)
The VLA FIRST survey* covered ~9,000 sq. deg. to an rms of ~150 Jy @ 5” resolution, detecting ~800,000 sources (20/pointing)
At 4 Gbps, VLBA sensitivity is comparable to the original VLA, and hence duplicating FIRST at VLBI resolution would take around the original VLA time (3000 hours)
Hugely useful for understanding nature of a source in general studies
800,000 uv datasets and images: 12 TB correlated data, 6.5 TB image data
Expect many non-detections; 30% hit rate (Porcas et al. 2004) still yields 240,000 VLBI images (optimistic? CDFS ~20-25%)
Provides an excellent grid of reference sources for astrometry (expect 1.5 detected sources > 5 mJy per pointing, total 60,000 calibrators)
The density of known calibrators is low: ~1 per 4 sq. deg. - only 1 per ~20 pointings!
~1min/pointing -> lengthy interpolation
How to calibrate phase with such infrequent “solid” calibrator scans?
Must bootstrap newly detected calibrators
Absolute accuracy of final positions depends on existing calibrators - I expect 1 -- 10 mas
Multiple VLBI fields/pointing has plenty of applications beyond selecting in-beam calibrators (either WAVRRS or targeted):
Globular cluster observations, with many astrometric targets in a single pointing
Star formation region studies (searching for compact radio emitters for astrometric analysis)
Discriminating AGN from starbursts in deep radio surveys (not really astrometry related)
Now: verification using the CDFS dataset (Middelberg et al.) already mentioned
Hot off the press: small shifts verified, bug affecting SNR with large shifts (probably precision related)
The combination of higher sensitivity and new correlator flexibility will allow much more efficient inbeam calibrators searches than previously possible
A wide survey to provide a database of ~50,000 VLBI calibrators is feasible
These capabilities will be available from ~ early 2010