Milliarcsecond Morphology of Scintillating & Non-scintillating Sources

Milliarcsecond Morphology of Scintillating & Non-scintillating Sources

Email: [email protected]

Roopesh Ojha, Alan Fey, David Jauncey,

Jim Lovell, Ken Johnston

Dwingeloo, April 7th, 2004

Outline

Background

Motivation-Objectives

The three samples

The observations

Analysis

Results

Conclusions

The Micro Arcsecond Scintillation Induced Variability (MASIV) Survey, aims to construct a sample of 100 to 150 scintillating extragalactic source with which to examine both the microarcsecond structure and the parent populations of these sources, and to probe the turbulent ISM responsible for the scintillation.

VLA observations have been made, at 4.9 GHz, of 710 compact, flat-spectrum sources over the Northern sky. We have observed four epochs spread evenly over 13 months.

The first epoch revealed variability on timescales ranging from hours to days in 85 of the sources. The number of highly variable sources, those with rms flux densities above 4 %, increases with decreasing source flux density but rapid, large amplitude variables such as J1819+3845 are very rare.

The MASIV Survey


Increase in fraction of highly variable scintillators (those with rms flux density variations above 4%) with decreasing flux density

Increase in their fractional amplitude of variability with decreasing flux density

These results of the MASIV survey raised the possibility that the Low Flux Density Scintillating sources may form a distinct population with differing morphology

Motivation

• Are there any morphological differences, at mas scales, between high flux density, ~1 Jy, and low flux density, ~0.1 Jy, scintillating sources ?

• Are there any morphological differences, at mas scales, between Scintillating and Non-scintillating sources ?

Milliarcsecond Scale Structure of Scintillating Sources

Our VLBI observations address the following questions:


The Observations• Used the VLBA to make 8.4 GHz snapshot images of the 75 Low Flux Density Scintillating (LFDS) sources found with the MASIV survey.

• 4 scans of 7.5 minutes each, full polarization

•Usual AIPS, Difmap, Model fitting.

• Mean and median flux density of 120mJy and 110mJy

• Distribution of fitted core angular size (core assumed to be fitted component at origin of image) has mean (median) of 0.11 (0.10) mas

• 19 sources have completely unresolved cores

•Mean separation from the core 2.9 mas

Fig 1: Examples of LDSS sources


The Three SamplesThree uniformly selected samples in that:

•Point sources at all resolutions of the VLA

•Spectral index flatter than 0.5 (from 1.4 to 4.9 GHz)

•8.4 GHz with VLBA

•Two Scintillating samples were found to be scintillating with rms flux density variations during a 72 hour period of

S rms > ((0.0032 + (0.02*S)2))

LFDS

LOW FLUX DENSITY SCINTILLATING

Source flux density ranges from 50 to 370 mJy with mean of 120 mJy

75 sources

Our observations

HFDSHIGH FLUX DENSITY SCINTILLATING

Source flux density ~ 1 Jy

18 sources

Structural information from USNO’s, RRFID. Database of images of all radio reference frame sources at the same wavelengths as those used for precision astrometry. To monitor sources for variability or structural change.

HFDN

HIGH FLUX DENSITY NON-SCINTILLATING

No scintillating behaviour detected by MASIV

Source flux density ~ 1 Jy

144 sources

Structural information from USNO’s, RRFID.

SEA ?

Analysis

Used three measures:

• Core Fraction (core dominance)

• Flux-weighted radial extent (core dominance)

• Unweighted radial extent (overall source size)


Core Fraction

Ratio of core flux density to total flux density

C = Σi (Si)beam / Σi Si

Core flux density is defined as the sum of the CLEAN-ed flux density within one synthesized beam

Total flux density is defined as the total CLEAN-ed flux density (i.e. the sum of all CLEAN components)



LFDS(HFDS) and HFDN 99% (95%) level

LFDS-HFDS same pop 10%

Flux-weighted radial extent

Complementary measure of core dominance, which is defined as

R = Σi Siri / Σi Si

Where ri is the radius at which the ith CLEAN component has flux density Si

R has units of milliarcseconds


Unweighted radial extent

Straightforward measure of overall source size i.e. the maximum radial extent of the source. It is the angular radius that contains 95% of the total CLEAN-ed flux density.

We first calculate a quantity E, similar to C above

E = Σi (Si)θ / Σi Si

Where the numerator represents the flux density contained within an area of angular radius θ.


Caveats

1. HFDS sample smallish (18) --- but large enough for K-S test to be valid

2. Comparison of LFDS sources with HFDN sources: sensitivity limits could bias the results as weak extended structure in the LFDS sample might not be detected.


Results of Analysis

Both low and high flux density scintillator samples have significantly higher core-fraction and significantly smaller flux-weighted linear extent than the non-scintillating sample significantly more core dominant

Overall angular size of scintillating sources is significantly smaller than that of non-scintillators


Results of Analysis

There are no significant differences between the mas morphology of low and high flux density scintillators

Consistent with same parent core-fraction population


ConclusionsBoth low and high flux density scintillators have significantly different morphologies than non-scintillators:

have a higher proportion of their flux in a compact core

have a smaller overall angular size.

Low and high flux density scintillators do not have significantly different morphologies.

Cores scintillateLFDN sample and prediction


Statistics of Source Angular Extent

Sample Number mean median mean median mean median

LFDS 75 0.90 0.94 0.29 0.16 1.62 0.83

HFDS 18 0.86 0.91 0.29 0.22 1.59 1.04

HFDN 174 0.73 0.76 0.76 0.52 3.36 2.07

C R(mas) 0.95(mas)


K-S Statistics

C R 0.95

LFDA-HFDS 0.10 0.68 0.50

LFDS-HFDN 0.00 0.00 0.00

HFDS-HFDN 0.05 0.00 0.03

K-S probability


We present VLBA snap-shot images of low flux density scintillating sources and compare their morphologies with those of high flux density scintillating sources imaged by the USNO astrometry program.

It is now clear that interstellar scintillation (ISS) is the principal cause of the intra-day variability (IDV) seen in some compact, flat-spectrum radio sources at centimeter wavelengths. The MASIV (Micro-Arcsecond Scintillation-Induced Variability) survey has been looking for new scintillating radio sources. One result from this survey is that the fraction of scintillating sources increases strongly with decreasing flux density. Here we address the possibility that the above MASIV result arises because we are seeing different mas morphologies among the weaker sources.

ABSTRACT


Milliarcsecond Morphology of Scintillating & Non-scintillating Sources

Documents

Transcript of Milliarcsecond Morphology of Scintillating & Non-scintillating Sources