The MicroArcsecond ScintillationInduced Variability MASIV Survey

1
MASIV The Micro-Arcsecond Scintillation-Induced
Variability survey
Jim Lovell1, Dave Jauncey1, Hayley Bignall2,
Lucyna Kedziora-Chudczer3, J-P Macquart4,
Barney Rickett5, Tasso Tzioumis1 1 ATNF 2
JIVE 3 Sydney Uni 4 Kapteyn Institute,
Netherlands 5 Uni California San Diego
2
Aims of MASIV

• Obtain a statistically significant sample of
  Intra-Day Variables (IDVs), 100
• 2/3 extreme IDVs were discovered serendipitously
• How common are they?
• So far surveys have been small and limited to
  bright sources. What is the IDV population
  amongst weaker sources?
• What makes these objects different? Correlations
  with z, b, VLBI TB, ?, X-ray, ?-ray etc
• Structure of the ISM
• AGN structure at ?as resolution

3
VLA Observations

• Four 72 hr epochs Jan, May and Sep 2002 (96h),
  Jan 2003
• 5 sub-arrays of 5 or 6 antennas each.
• 710 compact, flat-spectrum sources at 5 GHz
• 60 sec on-source per scan
• 6 scans per source per day (10,000 scans per
  epoch)
• After first epoch, moved to core sample of 550
  sources with 4 subarrays and used subarray 5 for
  more intensive monitoring.

4
Current Status

• Observations complete
• First results published
• 85 of 710 variable
• TB lt 1012K
• Top 29 sources listed Lovell et al. 2003,
  AJ 126, 1699
• All 4 epochs now reduced
• Analysis underway
• Polarisation still to do

5
Variability Detection RMS/mean vs Mean

• Uncertainties made up of two components
• S Jy due to noise and confusion
• P due to pointing errors
• S 1.5 mJy, P 1
• First cut select sources with

6

• Extreme variables are rare
• More high modulation index sources in the weaker
  sample. Difference in mas-scale structure? See
  Roopeshs talk.

7
Variability Statistics

• After removing sources with structure 525 in
  sample common to all epochs.
• 146 (28) showed variations
• 74 varied in 1 epoch only
• 33 varied in 2 epochs
• 21 varied in 3 epochs
• 18 varied in all epochs
• Single epoch variables from Jan 2002 and Jan 2003
  may be stopping/starting so the 74 is an
  overestimate. In epochs 2 and 3, 25 of each were
  single-epoch variables

8
Variability Statistics cont

• Variability by epoch
• Jan 2002 63
• May 2002 71
• Sep 2002 81
• Jan 2003 60
• On any epoch, 11 to 15 variable
• Sep 2002 has most number of variables despite
  being in the nominal slowdown period for LSR
  screens.

9
Lifetime of Variables

• Jan 2002 and Jan 2003 useful as annual cycle
  effects removed. Gives a good idea of lifetime on
  1 year timescale.
• 92 variables in total
• Jan 2002 63 variables
• Jan 2003 60 variables
• 31 in common
• So (in January at least) expect 12 of sources to
  be variable but only half of them to be there a
  year later.

10
Variability Timescales

• Full range of timescales from hours to days but
  most are 0.5 d or longer
• Generally not enough scints to get a good
  measurement of Tchar
• Some with multiple timescales

11
A few lightcurves..
12
J18193845 J09495819
13
J08294018 B115629517 modulation
high RMS
14
Changes in timescale

• Difficult to determine with this dataset due to
  few scintles per epoch, but
• Look at 27 bright, large RMS variability sources
  in epoch 1 with 4-epoch coverage

15
JVAS J07204737
Screen moving with LSR
Tchar 2.5 d
Tchar 0.3 d
Tchar 0.4 d
16
JVAS J05021338
Change in structure? Screen not moving with LSR?
Tchar 1.2 d
Tchar 3.0 d
Tchar 1.6 d
17
J09160242
Tchar 2 d
Tchar 1.3 d
Tchar gt1.8 d
Not an annual cycle. Tchar increasing.
Tchar gt3 d
18
Timescale variations

• Of 27 bright, large RMS variability sources in
  epoch 1 with 4-epoch coverage
• 13 show evidence of annual cycles
• 4 tied to the LSR
• 9 non-LSR
• 8 show changes in Tchar not consistent with
  annual cycles
• Intrinsic and/or ISM changes?
• 4 varied only in epoch 1
• 2 unclassified

19
Sky distribution

• Isotropic or not?

20
Galactic distribution (nH)
21
Galactic Distribution
Latitude distribution
Longitude distribution
N
N
Fraction
Fraction
Longitude
Latitude
22
Galactic Distribution (nH)
23
Sky distribution

• Anisotropic
• No obvious correlations with HI, H-alpha etc.

24
Summary

• 28 of compact flat spectrum sources vary on at
  least one epoch
• Extreme variables are rare
• Timescales typically hours to days, not many
  short-period variables. Monitoring may require
  dedicated telescopes.
• Annual cycles seem common among persistent
  variables. Many non-LSR screens
• Large fraction of variables are one-off episodic.
• Non-isotropic sky distribution but no obvious
  correlation with HI, electron density dist etc.

25
The End
MASIV 2, May 2002
http//www.atnf.csiro.au/jlovell/masiv