A large fraction of compact, extragalactic radio sources exhibit rapid variability at centimetre wavelengths as their radio emission is scattered by electron density fluctuations in the interstellar medium of the Galaxy. Next-generation wide-field radio telescopes will have to account for this in forming deep images of the radio sky. Interstellar scintillation offers a unique probe of very small-scale structure in both the ionized interstellar medium and the compact jets of the radio sources themselves. The effective resolution is two orders of magnitude higher than achievable with very long baseline interferometry. The recent Micro-Arcsecond Scintillation-Induced Variability Survey revealed a reduction in ISS at 4.9 GHz with increasing source redshift, implying either an increase in the apparent angular size of high-redshift radio cores beyond that expected due to a cosmological decrease in brightness, or a decrease in the microarcsecond-scale core dominance towards high redshift. The result could be due either to source-intrinsic evolution in the selected sample, or to scatter-broadening in the intergalactic medium

Bignall, Hayley

Dutka, M.

Jauncey, J.

Kedziora-Chudczer, L.

Lovell, J

Macquart, Jean-Pierre

Ojha, Roopesh

Pursimo, T.

Rickett, B.J.

Senkbeil, C.

Shabala, S.

English

espace@Curtin

Interstellar Scintillation, AGN Physics and the SKAH.E. Bignall1, J.E.J. Lovell2, B.J. Rickett3, J-P. Macquart1, D.L. Jauncey4, L. Kedziora-Chudczer5 , R. Ojha6, T.Pursimo7, C. Senkbeil2 , S. Shabala2 and M. Dutka61Curtin Institute of Radio Astronomy, Curtin University of Technology, Bentley WA, Australia.2School of Mathematics and Physics, University of Tasmania, Australia3Department of Electrical and Computer Engineering, University of California, San Diego, U.S.A.4CSIRO Australia Telescope National Facility, Epping NSW, Australia5School of Physics, University of Sydney, Australia6US Naval Observatory, Washington DC, U.S.A.7Nordic Optical Telescope, Santa Cruz de La Palma, SpainAbstractA large fraction of compact, extragalactic radio sources exhibit rapid variability at centimetre wavelengths as theirradio emission is scattered by electron density fluctuations in the interstellar medium of the Galaxy. Next-generationwide-field radio telescopes will have to account for this in forming deep images of the radio sky. Interstellar scin-tillation offers a unique probe of very small-scale structure in both the ionized interstellar medium and the compactjets of the radio sources themselves. The effective resolution is two orders of magnitude higher than achievable withvery long baseline interferometry. The recent Micro-Arcsecond Scintillation-Induced Variability Survey revealeda reduction in ISS at 4.9 GHz with increasing source redshift, implying either an increase in the apparent angularsize of high-redshift radio cores beyond that expected due to a cosmological decrease in brightness, or a decrease inthe microarcsecond-scale core dominance towards high redshift. The result could be due either to source-intrinsicevolution in the selected sample, or to scatter-broadening in the intergalactic medium.Keywords: interstellar medium; quasars; radio continuum; scattering; techniques: high angular resolutionIntroductionActive Galactic Nuclei (AGN) show variability across the whole electromagnetic spectrum. Observations of multi-wavelength variability help to constrain the emission mechanisms and physical properties of the region around thesupermassive black hole central engine. In addition to source-intrinsic variability, compact AGN (quasars and BL Lac-ertae objects) often show intensity variations at radio wavelengths due to interstellar scintillation (ISS). The theory ofinterstellar scattering has been reviewed by e.g. Rickett (1990) and Narayan (1992).For very compact AGN, the largest amplitude variations due to ISS are typically observed close to the transitionbetween weak and strong scattering, which occurs at observing frequencies of a few gigahertz (GHz) for most ofthe extragalactic sky. At frequencies below the transition, in the strong scattering regime, refractive scintillations arecommonly observed on timescales of typically weeks to months. Near the transition and in weak scattering, shortertimescale variability, so-called intra-day variability (IDV), may be observed. ISS is now recognised as the dominantcause of IDV at GHz frequencies, while intrinsic variations dominate on longer timescales and at higher frequencies.ISS probes source structure with effective resolution ∼ 10−50 microarcseconds (µas) at GHz frequencies, more thanan order of magnitude smaller than the resolution achievable with very long baseline interferometry (VLBI). ThusISS can provide unique information on the structure of the inner radio jet close to the AGN central engine. ISS alsoprobes small-scale structure in the local ionized interstellar medium (ISM).The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey aimed to provide a large, detailed statisticalstudy of the IDV AGN population drawn from a well-defined sample. The present paper highlights some results ofthe MASIV survey, most of which are described in detail in Lovell et al. (2003) and Lovell et al. (2008).Materials and MethodsThe MASIV Survey used the NRAO Very Large Array (VLA) observing at 4.9 GHz, which for many sources is closeto the transition frequency between weak and strong scattering where the largest amplitude ISS is observed. The firstfour epochs of observations were carried out in sessions of 72 or 96 hours at 4 month intervals between January 2002and January 2003. The VLA was split into 5 independent subarrays of typically 5 antennas each, observing subsets ofthe source sample. 710 compact sources were selected for the first epoch of MASIV Survey observations. The detailsof source selection are described in Lovell et al. (2003). In order to test the dependence of ISS on source flux densityS, “strong” (S8.6GHz > 0.6 Jy; 1 Jy= 10−26 W m−2 Hz−1) and “weak” (0.1 < S8.6GHz < 0.13 Jy) sub-samples wereselected. Additionally, a radio spectral index cutoff α > −0.3 (S ∝ να) was applied. Earlier surveys have shown thatsteep-spectrum radio AGN don’t exhibit short timescale radio variability (Heeschen 1984).For the MASIV Survey each source was observed for 1 minute every ∼ 2 hours while the elevation was above 15◦.After the first observing session the number of sources was reduced to a core sample of 578 sources, and the 5thsubarray used to better sample those sources which showed evidence of variability on timescales shorter than the 2hour sampling interval. The results presented here are from analysis of the data from the first four epochs and the firstfour VLA subarrays.Source flux densities were extracted in the visibility domain, rather than forming images for all scans. The fluxdensity scale is tied to the VLA primary calibrator 3C 286. Non-variable sources were used to correct gain andpointing errors for the nearby targets. The sources used as calibrators were then excluded from the analysis since theyare forced to have zero variation and would therefore bias the statistics if included. Sources that showed effects ofresolution or confusion have been excluded from the analysis. The data have a sufficiently high signal-to-noise ratiothat the source flux density as a function of time can then be readily measured from the (1-minute) scan-averagedtotal intensity amplitude averaged over all baselines. The uncertainty in flux density measurements is estimated usingσ2err,s,p = (s/S̄)2 + p2, where S̄ is mean flux density, s = 0.0013 Jy due to noise and confusion and p = 0.005 due topointing and gain errors. The presence of nearby confusing sources, although generally not a large effect for the VLAat 4.9 GHz, in practice varies from source to source and is often the dominant source of error in the light curves forweak sources. Sources were observed at approximately the same sidereal times on each day of an observing run, andthe presence of confusing sources is indicated by a daily repeating pattern in the data. Sources significantly affectedby confusion were excluded from the analysis, and we have used a conservative threshold for classifying sources asvariable. A total of 443 sources are included in the analysis presented here.Data analysisAs an initial step the modulation index µ (rms fractional variability) was calculated for each epoch separately, andsources whose modulation index exceeded 2σerr,s,p were identified as variable. The initial analysis used conservativeerror estimates s = 0.0015 Jy and p = 0.01. Additionally, all light curves were visually inspected and classifiedindependently by 2 members of the team, as many sources not satisfying the µ > 2σerr,s,p criterion were clearlyvariable on visual inspection. In particular, many sources showing slow variations did not make the initial cut, as µ isnot an ordered statistic and is not sensitive to low-level, slow monotonic changes in flux density. Light-curve inflectionpoints were counted as part of the visual inspection in order to estimate characteristic timescales of variability.In an independent analysis, the structure function (SF) was used to quantify the amplitude and timescale of variability.A simple model fit was used to estimate the amplitude of the SF at 2 days lag, D(2d), and the lag τ at which the SFreaches half its saturation value. The details of the fit are described in Lovell et al. (2008). In order to obtain reliableestimates of these quantities, the four epochs were combined to calculate the SF. In general time scales have largeuncertainties due to the limited sampling of the variability pattern, so sources were divided into just three timescalecategories: “fast”: 2h < τ < 0.5 days, “medium”: 0.5d < τ < 3 days, and “slow” (τ > 3 days). Most variablesources fall into the “slow” category. This implies that the amplitude and timescales of variability are commonlyunderestimated.Results and DiscussionBoth analysis methods found that approximately half of the MASIV sample showed rms variations between∼ 1−10%on timescales ranging between 2 hours and 3 days in one or more of the four epochs. At any given epoch, ∼ 30% ofthe sources were observed to vary.No new extreme, intra-hour variable (IHV) sources were discovered in the MASIV Survey. This was initially some-thing of a surprise to the team, as at the time the survey was initiated, two extreme IHV sources had been recentlyserendipitously discovered (Dennett-Thorpe and de Bruyn 2000; Bignall et al. 2003), both from relatively smallsource samples compared with the MASIV Survey sample, and from observations that were not intended to look forIDV. Another extreme IHV source, PKS 0405−385, had earlier been discovered in the Australia Telescope CompactArray IDV Survey which included 118 relatively bright compact sources (Kedziora-Chudczer et al. 1997; 2001). Itwas initially hoped that the MASIV survey would uncover several more such extreme IHVs, since these sources haveprovided rich datasets for detailed study of the ISS phenomenon and µas-scale source structure, due to the fact thattheir variability patterns can be well sampled with an interferometer in a relatively short observing session (typically∼ 12 hours). The IHV sources have been shown to lie behind discrete nearby regions of enhanced scattering (Dennett-Thorpe and de Bruyn 2000; Rickett et al 2002; Bignall et al. 2006), and the absence of IHV sources in the MASIVSurvey implies such nearby scattering “screens” cover only a tiny fraction (<< 1%) of the sky.In order to investigate the Galactic dependence of IDV, D(2d) was compared with the emission measure (columndensity of the square of the electron density) as estimated from observations of Hα emission. The intensity of Hαemission from the WHAM Northern sky survey was found on a 1 degree grid (Haffner et al. 2003) nearest to eachsource. The Hα intensity summed over all velocities is interpreted as proportional to the ISM emission measure onthat line of sight, assuming the temperature of the emitting gas does not vary by a large percentage (Haffner et al.2003). Figure 1 plots D(2d) against the WHAM Hα emission (in Rayleighs). There is a clear upward trend of D(2d)with emission measure, which establishes ISS as the dominant cause of the variability in the MASIV Survey. Thelower panel of Figure 1 shows that the fraction of slowly scintillating sources clearly increases with emission measureand vice-versa for the fast scintillators. This finding that the longer scintillation timescales occur when seen throughgreater column density of electrons is consistent with enhanced ISS from strongly ionized regions of the ISM, whichare typically found towards lower Galactic latitudes and at greater distances L.Figure 1: Upper panel: Mean value of D(2d) in the indicated bins of Hα emission. The vertical bars give the standarderror in the mean. Lower panel: Fraction of variable sources in each timescale class in each bin. Error bars assumebinomial distributions.The MASIV Survey found that weaker sources on average show larger fractional varaiations, and the fraction ofvariable sources increases towards lower flux densities. This is as expected for brightness-temperature limited sources(θsrc ∝ (S̄/Tb)0.5). At still lower flux densities, ISS may be even more dominant. A simple model of ISS fit to theMASIV observations (Lovell et al. 2008) implies maximum brightness temperatures in the range 1012 − 1014 K.This in turn requires substantial Doppler factors in the AGN jets, comparable with those estimated from VLBI andmillimetre-wavelength variability studies. Looking at the longer-term inter-epoch variability, which is likely to havea substantial contribution from source-intrinsic variability, no significant correlation with source flux density is seen.There seems to be a trend for sources detected at GeV energies with EGRET to scintillate more often than non-EGRET-detected sources. Taking only the 17 MASIV sources which are within the 95% confidence radius of anEGRET source position and firmly associated (Hartman et al. 1999), 11 sources showed IDV in 2 or more epochsof the MASIV Survey, a larger fraction than expected from sources with the same flux density distribution drawnrandomly from the MASIV sample. A simple contingency test shows this difference to be significant at the 95%confidence level, however the source numbers involved are small. The recently launched Fermi Large Area Telescopeis expected to detect many more of the MASIV sources and hence allow detailed comparison between ISS and gamma-ray emission.Figure 2 shows the mean value of D(2d) as a function of redshift for those sources which have measured redshifts,either in the literature or from observations with the Nordic Optical Telescope. This reveals a significant decrease inISS for sources above a redshift of z ≈ 2.5. The suppression of ISS of high redshift sources could be caused by eitheran increase in the apparent diameter of the source, or a decrease in the flux density of the compact fraction. Somedecrease in ISS with increasing source redshift is expected for sources limited to a constant rest-frame brightnesstemperature in the concordance cosmology, however our results indicate an excess suppression of ISS. It remains tobe determined whether the observed effect is due to source-intrinsic cosmological evolution and/or selection effects,or may be apparent angular broadening due to scattering in the ionized intergalactic medium. Scatter-broadeningwould be expected to have a strong frequency dependence. VLBI observations may indicate whether the effect islikely to be due to a fainter µas-scale core fraction if the high-redshift sources show significant milliarcsecond-scalestructure.Figure 2: Mean value of D(2d) in redshift bins for 271 sources with measured redshift.ConclusionsThe MASIV Survey showed that a large fraction of compact extragalactic radio sources show variations on timescalesof days due to interstellar scintillation at frequencies of a few GHz, implying angular sizes for the radio “cores”of ∼ 10 − 50µas. An excess suppression of ISS is seen for sources beyond redshift z ∼ 2. Towards lower fluxdensities, the fraction of scintillating sources increases. Next generation wide-field radio telescopes such as theSquare Kilometre Array will have to account for many variable sources in the field of view. Extracting measurementsof the variable sources will provide a rich database for large statistical ISS studies.AcknowledgementsThe National Radio Astronomy Observatory is a facility of the National Science Foundation operated under coopera-tive agreement by Associated Universities, Inc. This research has made use of the NASA/IPAC Extragalactic Database(NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract withthe National Aeronautics and Space Administration. We also made use of data from the Wisconsin H-Alpha Mapper,which is funded by the National Science Foundation.ReferencesBignall, H.E. et al. (2003). Rapid variability and annual cycles in the characteristic timescale of the scintillating source PKS 1257−326. TheAstrophysical Journal 585, 653–664.Bignall, H.E. et al. (2006). Rapid Interstellar Scintillation of PKS 1257-326: Two-Station Pattern Time Delays and Constraints on Scatteringand Microarcsecond Source Structure. The Astrophysical Journal 652, 1050–1058.Dennett-Thorpe, J. and de Bruyn, A.G. (2000). The Discovery of a Microarcsecond Quasar: J1819+3845. Astrophysical Journal Letters 529,L65–L68.Haffner, L.M. et al. (2003). The Wisconsin Hα Mapper Northern Sky Survey. The Astrophysical Journal Supplement Series 149, 405–422.Hartman, R.C. et al. (1999). The Third EGRET Catalog of High-Energy Gamma-Ray Sources. The Astrophysical Journal Supplement Series123, 79–202.Heeschen, D.S. (1984). Flickering of extragalactic radio sources. The Astronomical Journal 89, 1111–1123.Kedziora-Chudczer, L. et al. (1997). PKS 0405-385: The Smallest Radio Quasar? Astrophysical Journal Letters 490, L9.Kedziora-Chudczer, L. et al. (2001). The ATCA intraday variability survey of extragalactic radio sources. Monthly Notices of the RoyalAstronomical Society 325, 1411–1430.Lovell, J.E.J. et al. (2003). First Results from MASIV: The Micro-Arcsecond Scintillation-Induced Variability Survey. The AstronomicalJournal 126, 1699–1706.Lovell, J.E.J. et al. (2008, in press). The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey II: The First Four Epochs. Toappear in The Astrophysical Journal 689.Narayan, R. (1992). The physics of pulsar scintillation. Philosophical Transactions of the Royal Society of London A. 341, 151–165.Rickett, B.J. (1990). Radio propagation through the turbulent interstellar plasma. Annual Review of Astronomy and Astrophysics 28, 561–605.Rickett, B.J., Kedziora-Chudczer, L. and Jauncey, D.L. (2002). Interstellar Scintillation of the Polarized Flux Density in Quasar PKS 0405−385.The Astrophysical Journal 581, 103–126.

Interstellar scintillation, AGN physics and the SKA

https://espace.curtin.edu.au/bitstream/20.500.11937/29990/2/119198_10813_hayley-aip.pdf

Interstellar scintillation, AGN physics and the SKA

Abstract

Similar works

Full text

Available Versions

espace@Curtin