New VLBA capabilities with DiFX

1. New VLBA capabilities with DiFX Wide-field imaging, multi-field imaging and more • Adam Deller • NRAO / UC Berkeley

2. Outline • The DiFX software correlator and its usage with the VLBA • New capabilities offered by DiFX compared to the VLBA hardware correlator: • Broad compatibility • Spectral/temporal resolution • Pulsar analysis • Commensal science • Wide-field / multi-field capabilities

3. The DiFX software correlator • A C++ program running on commodity computer hardware (rack-mounted, multi-core servers) • Development commenced in 2005, adopted by Australian Long Baseline Array in 2006, NRAO testing from 2008 and complete switch by December 2009 • Supported by numerous libraries and applications for job configuration, FITS file building etc; ~10 active developers (NRAO, MPIfR, ATNF/Curtin, Haystack)

4. The DiFX software correlator

5. The DiFX software correlator • Performance is good; hardware capable of supporting 10 stations x 512 Mbps would cost ~$12,000 in 2011 • Low barriers to getting started has encouraged many adopters • Many contributors to code • This combined with ease of coding in C++ c.f. FPGAs has contributed to the rapid development of new features like the ones focused on today

6. Unique DiFX capabilities • Compatibility, expandability • Initial reason for adoption - needed something capable of expansion to 4 Gbps system • incremental nature is extremely useful (hardware purchased in 4 stages, minimizing overall cost through Moore’s Law) • Handles all input/output VLBI formats • Flexibility in parameter setting • Time, frequency resolution in particular

7. Unique DiFX capabilities • Much more flexible pulsar processing (dynamic allocation of resources); allows pulse-phase dependant studies (binning) and “matched filtering” forrecovery ofoptimal S/N fromcomplexprofiles

8. Unique DiFX capabilities • Ease of adding new features has allowed low-overhead commensal functionality • One such feature produces ms time resolution spectrometer and spectral kurtosis data • The V-FASTR project has been approved to search for fast transient events during all DiFX correlations of VLBA data • Real-time pipeline captures, re-orders and flags data and searches for dispersed pulses

9. Unique DiFX capabilities raw filterbank data bandpass, tcal corrected data frequency time

10. Unique DiFX capabilities • V-FASTR has detected both normal and giant pulses from multiple (targeted) pulsars • Running near full-time now • Exploring an unknown area of parameter using a new technique at near-zero cost • Highly visible pathfinder for SKA transient searches • Also produces valuable RFI information for routine VLBA operations

11. Wide-field imaging • DiFX is the most capable VLBI correlator in the world for wide-field imaging, due to the attainable time and frequency resolution primary beam: 30’ primary beam: 30’ Time resolution:200 ms Freq. resolution:50 kHz 12hr VLBA dataset:240 GB Time resolution:2000 ms Freq. resolution:500 kHz 12hr VLBA dataset:2.4 GB phase centre phase centre Smearing-limitedfield of view2’ Smearing-limitedfield of view15” Calculations for 1.6 GHz, total smearing = 10%

12. Wide-field imaging • This ability has been widely used since the introduction of DiFX • However, full-beam VLBA imaging is still a logistical impracticality primary beam: 30’ Time resolution:20 ms Freq. resolution:4 kHz 12hr VLBA dataset:30,000 GB phase centre Smearing-limitedfield of view30’ Calculations for 1.6 GHz, total smearing = 10%

13. Wide-field imaging • Generally, however, the sky is almost entirely empty at VLBI resolution • Thus, usually do not want “full beam” imaging; rather, many targeted small “fields” • This can be achieved by uv shifting after correlation, but spectral/temporal resolution requirements are identical to imaging • DiFX has moved the uv shift inside the correlator, allowing “multi-field” correlation and avoiding the logistical problem

14. Multi-field imaging primary beam primary beam primary beam primary beam phase centre phase centre Smearing-limitedfield of view phase shift phase centre Smearing-limitedfield of view Smearing-limitedfield of view Correlateat highresolutionfor ~10ms Apply uv shift Averagein frequency Repeat for many phase centres THEN: Repeat for next ~10ms (average in time)

15. Multi-field imaging Satisfactory “finder” catalogs already exist for most applications of this technique primarybeam Image:Randomcutout, NRAO FIRSTsurvey VLBI fields still not to scale!

16. Multi-field imaging • Some computational overhead (factor of ~2.5) due to higher upfront spectral resolution, but additional fields are almost free (factor of <1.01) • Thus efficiency gain increases as number of targets per pointing increases • VLBA is unparalleled for multi-field VLBI applications due to homogeneous, relatively small dishes (large antennas or phased arrays reduce useful field of view)

17. Multi-field imaging • For mJy-sensitivity secondary calibrator searches (me, later) with ~20 targets/pointing, net factor of 7 increase • For sub-mJy sensitivity deep field AGN searches (e.g. Middelberg) with ~300 targets/pointing, net factor of ~100!

18. Multi-field imaging • Efficient VLBI surveys of mJy and sub-mJy objects are feasible for the first time • Middelberg et al. (2011) already published VLBA results on Chandra Deep Field South, more on the way covering variety of area and sensitivity ranges From Middelberg et al., 2011

19. Conclusions • In addition to facilitating the ongoing sensitivity upgrade, DiFX has opened a number of new areas of parameter space for the VLBA • Advanced pulsar processing • Commensal transient observations • Wide-field and multi-field observations • Of these, multi-field observations have the potential for opening up the most new applications - VLBI surveying is now practical