Gravitational Microlensing

• O.J. Lodge - First reference to lensing in gravitational work ~ 1919.
• A. S. Eddington pointed out that multiple images may occur! ~ 1920
• Mandl wrote to Einstein on the possibility of gravitational lensing. Einstein felt it would not be possible to observe such an effect. ~ 1925
• Fritz Zwicky calculated deflection angles, image strengths, and came up with several reasons why astronomer should look for gravitational lenses. ~ 1937 1. Additional tests of General Relativity 2. Extension to fainter magnitudes due to magnification 3. Determining Nebular Masses
• Around 1963 - many researchers led by S. Refsdal, started looking at realistic scenarios for using lensing. This included the study of Quasi-stellar objects and distant galaxies. S. Refsdel - M.N.R.A.S. 128, 295, 1964 S. Refsdel - M.N.R.A.S. 134, 295, 1966 - possibility of determining distances and masses in lensing systems.
• Walsh, Carswell and Weymann, 1979, detected the first gravitational lens PKS 0957+56. (Nature, 279, 281, 1979.)

  Observers started observing these systems with optical telescopes.

• First International conference on Lensing in 1983.

RQ =Radio quiet BEL = Broad Emission Lines

RL = Radio Loud NEL = Narrow Emission Lines

PL = Power Law TH = Thermal

V = Variable OVV = Optically Violent Variables

Blazars = BL Lacs and OVV quasars

• Optically Violent variations in optical, radio, mm, x-ray, g-ray
• Highly Polarized continuum

Blazars are a subset of Quasi-Stellar Objects.

QSO's - Compact, star-like extragalactic objects with:

• Broad Highly redshifted spectral lines
• Emission over entire E-M spectrum
• Non-Thermal Continumm is Synchrotron Emission
• Varies Rapidly in Flux - small size if intrinsic
• Over 3,000 observed up to z=0.489?

QSS - Radio Quiet Quasars

• OVV Quasars - Very energetic quasars that exhibit rapid, high Amplitude outbursts and have variable optical polarization.

• BL Lac Objects - Similar properties to OVV's except no broad emission Lines, some other differences in Polzarization and gamma-ray emission.

• Blazars - OVV quasar + BL Lacs

  Blazar Models:

  • Supermassive Black hole in center
  • Accretion disk feeding hole
  • Relativistic Jet - Blazars have jets pointing at us?

  • Variability -
  • Processes in accretion disk
  • Processes in Jet (shocks)
  • extrinsic causes -microlensing!
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  Parameters of a gravitational Lens

  • x = impact parameter
  • Ds= Distance from Observer to source
  • Dds= Distance from deflector mass to source
  • Dd= Distance from Observer to lens

  • Einstein Angle = a = 4GM/c2 x = 2Rs/ x

  • Einstein Radius rE = [ (4Gm/c2) (DsDds)Dd-1]1/2

  Fitting Program written in IDL fits model light curves to data using an automated Levenberg-Marquardt least-squares minimization (ref: MinPack-1) (This version allows upper and lower bounding constraints to be placed on each parameter, or the parameter can be held fixed).

  Characteristics of a Lensing System

  • Distant source (Quasar or galaxy) and a nearby lens (intervening galaxy or objects in the galaxy?)
  • Multiple Images of Source Quasar or distant galaxy - 2 or more images, arcs, circles depending on orientation of lens, source, observer.
  • Microlensing - Image deflection is small and images fall on top of each other resulting in one magnified image. /UL>

    If the lens (or source) is in motion relative to the source (or lens) or observer moves, AND image resolution is below angular separation, we see amplitude or brightness changes, so called High Amplitude Events (HAE's).

    What to expect from a High Amplitude Lensing Event.

    • Symmetric Outburst
    • Frequency independence of outburst across spectrum
    • Duration of outburst is related to lens speed

    Requirements for a Blazar to be microlensed

    • Existence of intervening lensing mass and relative motion!

      The Case of AO 0235+164

      • Originally classified as a BL Lacertae Object (does have weak emission lines found later), z(emission) = 0.954.
      • Has multiple absorption line systems indicating intervening mass, zabs = 0.52 among others.
      • Shows intermittent dramatic High amplitude optical and radio outbursts.

      Lots of optical work done on this object:

      • Webb et al. 1988 - long-term optical monitoring LI>Webb and Smith 1989 - modeling of the 3.4 magnitude outburst which was observed in 1987.
      • Webb 1992, "The energetics and Morphologies of Blazar Outbursts"
      • Complex optical system - Jones et al. (1984)
      • Charles et al. (1994)
      • Stickel, Fried and Kuhr, A.Ap. 198, L13. showed the image of AO 0235+164 was actually convolved with a galaxy at z= 0.52, i.e. directly along line of sight to AO!

        Conclusions:i

        • Outbursts have complex morphologies, but not periodic and are undersampled, but could be consistent with superposition of several "symmetric pulses"
        • Radio emission also shows outbursts, with little delay in some cases (unlike most other Blazars).
        • Lensing material in intervening galaxy is present.

        AO 0235+164 as a Gravitational Lens system

        Ostriker and Vietri (1985) suggests that BL Lacs high brightness temperature might be due to microlensing.

        Paczynski (1986) and Kayser et al. (1987)Schneider and Weiss (1987) analyze scenarios for microlensing for blazars. Concludes that microlensing is a viable explanation for variability in at least some sources.

        Kayser (A.Ap. 206,L8, 1988) applied microlensing to AO 0235+164. They conclude that "… intense optical outbursts are unlikely to be high amplitude events due to microlensing…" Primary objection is timescales for radio and optical events, plus creation of superluminal blobs are inconsistent with microlensing.

        Stickel, Fried and Kuhr (A.Ap. 198, L13, 1988) conclude that the optical events are likely to be microlensing, the superluminal radio blobs are radio images of the central core, and not intrinsic expansion as previously thought!

        The 1997 Outburst of AO 0235+164: Evidence for a Microlensing Event. Webb, Howard, Benitez, Balonek, McGrath, Shrader, Robson and Jenkins. A. J. 120, 41, 2000.

        Monitoring data of AO 0235+164 throughout an energetic outburst in three frequency, optical, millimeter, and Ghz radio, plus an x-ray observation.

        A 5-magnitude outburst (~ factor of 100 in brightness) over 800 days (Earth ref. frame)

        Maximum Luminosity (assuming no beaming) of 9.8x1046 ergs/sec. (previous outbusrts exceeded 1049 ergs/sec.

        Outburst exhibited a double-peaked morphology, and durations, amplitudes and morphologies are similar in each frequency band.

        Outbursts are consistent with being simultaneous in all three frequency bands, optical, mm, and radio.

        • Most shock in jet models predict optical should peak first, then mm, then x-ray.
        • Microlensing predicts correlated, simultaneous outbursts in widely seperated frequency bands.
        • A look at the historical outbursts in light of grav. lensing: Not all optical outbursts are accompanied by radion outbursts with 0 time delay!
        • Are there both intrinsic and Lensing outbursts??

        Conclusions

        • AO 0235+164 is a great candidate for a microlensing with "line-of-sight" intervening galaxy.
        • 1997 outburst, by far the best observed, is consistent with microlensing.
        • Other previous outbursts may not be consistent with microlensing.
        • Awaiting actual model fits to AO 0235+164 data to determine if actual lensing properties, masses, velocities, are realistic.