Fast Variability of Compact Radio Sources: Where do we stand ? T.P. Krichbaum MPIfR, Bonn

Involved Scientists: Bonn, Germany: A. Kraus, I. Agudo, E. Ros, A. Witzel, J.A. Zensus, (plus several PhD students) Urumqi, China: Jin Lin Han, Peter Müller, Xiao Hui Sun, Chen Wang, et al. Peking, China: S.J. Qian Heidelberg, Germany: S. Wagner et al. Perugia, Italy: L. Fuhrmann Hobart, Tasmania: G. Cimo Special Thanks !

early 60's: discovery of low frequency variability in compact radio sources, interpreted by interstellar scintillation (Shapirovskaya et al., Dent et al.) late 60's: too rapid cm-variability leads to prediction of bulk relativistic motion and subsequent discovery of apparent superluminal motion in radio jets with VLBI. Since then a huge variety of papers were published on both topics, source extrinsic and source intrinsic variability. Intraday Variability (IDV) was discovered with the 100m RT of the MPIfR in 1985, when a sample of flickering radio sources, previously observed with the Arecibo telescope, was observed in Effelsberg (Witzel et al. 1986, Heeschen et al. 1987) Brief History

Expected Size of a Radio Sources Brightness temperature T B ~ K (Energy equipartition, Inverse Compton Limit) Source Size This is much larger than the Fresnel scale at 100 pc, so naively one would not expect strong effects from interstellar scintillation at cm-wavelengths.

N2 N3 N4 N5 5 GHz: 2.7 Jy in 122 days  T B = 1.7 x K,  = GHz: 4.4 Jy in 214 days  T B = 2.0 x K,  = 6 90 GHz High brightness temperatures (T B  K) are not exceptional, but indicate bulk relativistic motion with Doppler–factors > 1.

2.8 cm 6 cm 11 cm Kraus et al., AA, 2003 A typical example of an IDV source (type II)

Sample averaged structure functions of Type I and Typ II IDV's Examples of Type I and Type II IDV: Type I Type II Heeschen et al. 1987, Quirrenbach et al Statistical Properties

The strength of IDV correlates with spectral index and source compactnes Quirrenbach et al. 1992, Cimo et al S ~ 

IDV in Quirrenbach et al Brightness temperatures derived from radio IDV: T b =10 15 K to K Qian et al. 1996, Wagner et al radio spectral index correlates with optical flux !

The kilo-parsec jet of the BL Lac S Slow Fast IDV in T B =3.6 x K (65 mJy in 1 day) T B =1.7 x K (5 mJy in 1hr)

Brightness Temperature Observe a change of  S in time  t. If variation is source intrinsic => size ~ c  t / (1+z) Brightness Temperature: T B var ≈ 5 x K  S Jy [ cm d Mpc ] 2 [  t day (1+z)] -2 Doppler Factor:  T B var ) 0.31 (1+z) Example: 0.1 Jy / day at 6 cm, for a z = 1 quasar T B var ≈ 4x10 16 K  ≈ Jy / hr at 6 cm, for a z = 1 quasar T B var ≈ 2x10 19 K  ≈ 360 But: VLBI observes only Lorentz – factors of  < 20 – 30. Big Question: How high can  be ? (formally:  = f  ≤ 2  max ).

Multi-frequency Studies and Spectral Properties

Power spectrum of electron density fluctuations: Scattering measure: Fresnel angle (strong, weak scattering) Characteristic variability time scale (RISS):Modulation Index (RISS): Remember for weak ISS: IDV occurs if source size < scattering size variability amplitude ~ 1/ (frequency) 2 variability time scale ~ screen distance / transverse velocity

Variability index versus frequency standard model: at low frequencies and for strong RISS: m ~  0.57 at high frequencies and for weak RISS: m ~  - 2

Modulation index versus time: - variable with time - strongly varies between sources - not simply related to strong or weak ISS

IDV of at 9mm wavelengths, 20% flux change in 7 hrs 100m RT EffelsbergKraus et al T B ~ 3.3 x K  ~ 32

m 6cm = 1.3 % m 2.8cm = 5.5% m 9mm = 11,2% m 3mm = 11,4% 9mm: 30 % increase in 6 days (T B  4.1 x K)

Example: Polarisation IDV in

Variations in polarisation are ~ 2 times faster than in total intensity Power density spectra

Polarization IDV and polarisation angle angle swings indicate polarized multi–component sub–structure : PA-swing without variations in I & P:

Uyaniker et al cm Effelsberg 100m RT

Polarisation and total Intensity are strongly correlated  I /  P / PP II corr. coeff cm This proportionality implies that RISS might be at work. At shorter wavelength, however, we see correlations and anti-correlations between I & P ! Qian et al P scaled by ~ 2.36

Residual polarisation at 20cm after subtraction of 3 component RISS model 3 peaks in P res of which two correlate with I and one anti- correlates.This cannot be explained by the same process ! P1P1 P2P2 P3P3 P1P1 P3P3 P2P2 Qian et al. 2002

3 polarized components (17 variable parameters) this fits the Q-U cross correlation (and therefore the polarisation angle) note: the closer the screen, the lower T B ! a better fit Rickett et al. 2002

29. Sep Oct Oct. 00 VLBA + Effelsberg + HALCA 3 epochs of 16hrs at 5 GHz, dual pol. nearly identical uv-coverage 0.25 mas resolution The observations VSOP observations of : S5 blazar, z > 0.3 intraday variable correlated variations in the X-ray, optical and radio bands

Space-VLBI of ° 41°53° 4 mas 29. Sep Oct Oct Bach et al., 2004 IPIP Oct. 4, 3-7 UTOct. 4, UT P=13.5 mJy P=9.5 mJy 2 mas

Jet Profiles Pronounced core polarization IDV of P and PA on timescales of > 0.5 day in VLBI core, but not in jet. Bach et al., 2004

Rapid IDV of Quasar J WSRT data from G de Bruyn & J Dennett-Thorpe Quasar J exhibits ISS with a pronounced annual cycle in its characteristic timescale ISM Raypath from QSO earth

Annual intensity variations due to orbital motion of Earth image:

Bignall et al anisotropic scintillation pattern: scale length ~ GHz axial ratio R ~5 time lag ATCA-VLA ~ s screen distance D = 30 pc scattering size 20  as

Characteristic scintillation timescale versus epoch Blue dotted line: No screen velocity (ecliptic latitude ~ 62 o ) Red line V_screen ~ 30 km/sec STILL POOR FIT Dennett-Thorpe & de Bruyn, 2003

….but excellent fit with anisotropic scintels (14:1) ~Nov 20 Dennett-Thorpe & de Bruyn, 2003

J : Diffractive 21cm detected ! Source size < 3-16  as Brightness temperature > K Doppler-factor  > 30 – 100 ! Dynamic Spectra 8 x 10 MHz Macquart & de Bruyn, 2004 & 2005 (submitted)

Published annual modulation due to orbital motion cannot be confirmed, since IDV stopped. Rickett et al

: no evidence yet for annual modulation of IDV due to orbital motion characteristic time vs. DOY variability index vs. DOY Fuhrmann, PhD 2004

characteristic time vs. DOY variability index vs. DOY : some evidence for annual modulation of IDV due to orbital motion Fuhrmann, PhD 2004

An Extreme Scattering Event (ESE) in Fiedler et al & 1994, Clegg et al Gaussian Plasma lens:

: An ESE with a time scale of only days ! size of the plasma lens: 0.04 AU electron density: n e ~ 100 Cimo et al. 2002, PASA

Evidence for ISS: time lags in rapid IDV sources annual modulation in at least some IDVs extreme scattering events (time scales: days – months) variability strength correlates with compactness, size angular broadening diffractive scattering (plus all from pulsars) So what do we know about the ISM ?

MASIV VLA Survey: Galactic distribution of IDV sources MASIV VLA survey: Lovell et al anisotropic distribution no obvious correlation with HI, H-alpha etc. grey scale: n(H)

No systematic dependence of variability strength from cosmological distance

Cimo et al no obvious dependence from galactic latitude

Sky-distribution of the original IDV sample of Heeschen et al.1987 and by Fuhrmann et al. 2004:

High Latitude Clouds as Origin of Scattering Material for IDVs? Fuhrmann et al. 2002

The local Vicinity of the Sun  large scale structures of compressed material produced by SN or star forming regions  turbulent interaction zones/walls Shells, Bubbles, Radio Loops: Local Vicinity of the Sun:  Local Bubble  surrounding “bubbles“  local “fluff“  Shells, Bubbles and Radio Loops  screen for fast scintillators ? Snowden et al. 1998

IDV Research with the Urumqi telescope – motivation: many aspects of the physical interpretation of IDV in CERS are still unclear (intrinsic vs. extrinsic, characterization of ISM) large radio-telescopes (like the 100 m RT) are overbooked and cannot observe long and regularly enough (typically < 1 week intervals every few months) repeated regular observations over the year are required to measure the variations of the variability properties (m, t var ) as function of DOY (i.e. annual modulation caused by orbital motion of the Earth) additional observing time at smaller telescopes is needed (but: telescope sensitivity must allow detection of small IDV amplitudes) feasibility study with the 25m Urumqi RT (December 2004, August 2005)

December 8 – 10, 2004 : cross-scans in (  ) with 2x4 subscans of 30 sec duration length 1.62 days, 17 cycles, each 2.2 hrs long 18 program sources + 3 primary calibrators partly successfull, main problem: tracking errors ~ 1/cos , calibrators shown an rms < 2 % IDV feasibility study with the 25m Urumqi RT August 15 – 18, 2004 : cross-scans in (Az, El) with 2x (4...8) subscans of 30 sec duration length 2.89 days, 28 cycles, each 2.5 hrs long 20 program sources + 4 primary calibrators successfull, IDV detected in several sources, calibrators shown an rms < 0.5 %

Basic Steps of Data Analysis remove baseline drift from each cross-scan (derive T sys (t) from baseline) apply Gauss-fits to individual subscans (parameters: amp, HPBW, offset) inspect amplitude, HPBW, position offset as function of AZ, ELV, time, temperature,... remove bad subscans (both automatic and manually), common problems: instable non-gaussian signal, jumps, interference, pointing error average all sub-scans in each slewing direction for each source (, ) apply pointing error correction to,, then average both directions determine gain-elevation dependence (amp vs. elv) and correct amp's for it apply Kelvin to Jansky conversion (primary calibration using 3C286, 3C295, NGC7027,...) determine mean flux and std. dev. for each source, identify secondary calibrators plot normalized flux of secondary calibrators vs. time, determine gain variation vs. time (evtl. apply moderate time-smoothing to combined calibrator curves) apply time correction to data (eventually 1st and 2nd order) and check/plot S(t) determine statistical properties ( t, , m, Y,  ) for each source plot S(t), derive auto-correlation function, structure function, etc.

Example of Cross-Scans ( , 0.7 Jy)

Example of Cross-Scans ( , 0.9 Jy)

Example of Scans with some Problems:

Example of Scans with Problems:

Measurement accuracy for Urumqi : suitable secondary calibrators in this exp. are: % % % % _______________ Mean: 0.37 ± 0.15 % ® m 0 = 0.52 % for comparison Effelsberg: m 0 = % Summary of Results

Confirmation of the annual modulation effect in

Summary General: 3 types of IDV sources known, classical Type I & II plus particular rapid r-IDVs (Type III) (i.e. J ) ISS responsible for IDV at least in r-IDVs, annual modulation in indicative of ISS also for Type II's radio-optical correlation, IDV at mm-, and frequency dependence of Y are inconsistent with simple ISS models space VLBI reveals that IDV component is located within the self-absorbed core, not in the jet observed polarisation IDV requires multiple polarized structural components with different sizes of tens of  as present ISS models have difficulties explaining polarisation IDV, particularly variations of P.A. (source intrinsic ?) diffractive scintillation in J yields rather high brightness temperature and Doppler-factor location of the screen(s) and type of ISM responsible for IDV still unknown (screen distance pc) IDV is a complex mixture of source intrinsic and extrinsic effects Particularly for Urumqi: IDV deteced in several sources, measurement accuracy ~ 0.5 % IDV detected in the new source , its IDV amplitude and timescale is likely to vary, is it an r-IDV ? slow-down of the variability in around day 220, confirms annual modulation (urgently need more data before day 300) the Urumqi 25m RT is well suited for IDV research, ready to begin with more systematic observations future improvements: driving program, baseline stability and calibration, pointing correction polarisation performance not yet checked, additional frequencies ? RM variability studies ?

Thank you for your attention !