Milliarcsecond Morphology of Scintillating & Non-scintillating Sources

Milliarcsecond Morphology of Scintillating & Non-scintillating Sources Roopesh Ojha, Alan Fey, David Jauncey, Jim Lovell, Ken Johnston Email: rojha@atnf.csiro.au Dwingeloo, April 7th, 2004

Outline Background Motivation-Objectives The three samples The observations Analysis Results Conclusions

The MASIV Survey 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. Dwingeloo, April 6th, 2004

Motivation 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

Milliarcsecond Scale Structure of Scintillating Sources Our VLBI observations address the following questions: • 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 ? Dwingeloo, April 7th, 2004

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 Dwingeloo, April 7th, 2004

The Three Samples Three 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 • Srms > ((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

HFDS HIGH 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 ?

Dwingeloo, April 7th, 2004

Analysis • Used three measures: • Core Fraction (core dominance) • Flux-weighted radial extent (core dominance) • Unweighted radial extent (overall source size) Dwingeloo, April 7th, 2004

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) Dwingeloo, April 7th, 2004

LFDS(HFDS) and HFDN 99% (95%) level LFDS-HFDS same pop 10% Dwingeloo, April 7th, 2004

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 Dwingeloo, April 7th, 2004

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. Dwingeloo, April 7th, 2004

Caveats • HFDS sample smallish (18) --- but large enough for K-S test to be valid • 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. Dwingeloo, April 7th, 2004

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 Dwingeloo, April 7th, 2004

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 Dwingeloo, April 7th, 2004

Conclusions Both 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 scintillate LFDN sample and prediction Dwingeloo, April 7th, 2004

Statistics of Source Angular Extent C R(mas) q0.95(mas) Dwingeloo, April 7th, 2004

K-S Statistics K-S probability Dwingeloo, April 7th, 2004

ABSTRACT 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. Dwingeloo, April 6th, 2004

Milliarcsecond Morphology of Scintillating & Non-scintillating Sources