VLBI Observations of IDV Sources

Brian Moloney

Dr. Denise Gabuzda

University College Cork

Summary

What is IDV?

• Intra-Day Variability: Rapid variations in the I and/or P flux on a timescale of hours

How is it caused?

• Intrinsic

• Extrinsic

Intrinsic

• Within the source

• Implies compact size

– Unfeasibly high Brightness Temperature

• 1017 K which exceeds the Inverse Compton Limit of 1012 K.

Extrinsic

• Outside the source

• Similar to twinkling stars

Previous Results

• J1819+3845’s IDV behaviour has been proven to arise due to Extrinsic effects.

How?– Annual cycle– Time delay between 2 antennas

Dennett-Thorpe J., De Bruyn A.G. 2003, A&A, 404, 113

Dennett-Thorpe J., De Bruyn A.G. 2002, Nature 415, 57

My Experiment

Set of IDV sources in which the cause of variability is not yet certain.

Observed at 8 & 15 GHz in 1997

VLBA + VLA

My Sources

– 0917+624– 0954+658– 1150+812– 2007+777– 1642+690– 0804+499

Plan

• Use VLA data to determine behaviour

• Use all I data to make VLBA map, then create slices in time of P data

• PVLA - PVLBI should be constant

• Identify if variability can be attributed to a certain area of polarisation on VLBI scales

• Analyse properties of varying region

Some Vector Subtraction

PVLBA

VLBA PVLA

VLA PDIFF

DIFF

0917+62416 ->1/0 8 -88 13 77 7 -301/0 ->1/6 14 85 22 76 10 -27

1150+8128 ->13 11 -79 21 86 13 -1713 ->18 13 -81 21 86 11 -1818 ->22 13 -71 22 74 21 -34

1642+6908 ->12 31 -25 39 -31 11 4112 ->15 27 -24 40 -32 16 4415 ->18 33 -21 36 -31 12 2518 ->23 23 -31 32 -39 12 35

2007+7778 ->16 80 -67 92 -70 14 716 ->21 62 -65 78 -65 16 2321 ->1/7 80 -68 96 -70 18 10

0917+624 (VLA)

0917+624 (VLBI)

0917+624 Slices

1642+690 (VLA)

1642 +690 (VLBA)

Future Work

• More detailed analysis for 0917, 1642

• Analyse time slices for other sources

• Model-fitting P data for time slices to better identify varying component and estimate its properties

• Compare results with expected behaviour for scintillation

Acknowledgements

• UCC Radio Astronomy Lab

• The Audience