VLA Expansion Project Science Working Group:
The Transient Universe
(30 November 1998)

Summary by M. Rupen.

Chaired by Dale Frail
Also present:
  Tim Bastian, Alan Bridle, Chris Carilli, Vivek Dhawan, Chris Fassnacht,
  Miller Goss, John Hibbard, Bob Hjellming, Amy Mioduszewski, Frazer Owen,
  Rick Perley, Michael Rupen, Dick Sramek, one illegible (sorry).

Overall: desire for dynamic scheduling, and fast, real-time imaging
  Also A+, even when the rest of the array is not available: note that
    A+ with 8GHz/IF is roughly as sensitive as the ENTIRE current VLA

1- The Sun

2- Galactic Novae
  Follow-up: Bob Hjellming
  a. Imaging ***
    * Example: Nova Cyg 1992
      - radio shells mapped with MERLIN and VLA, 81-769 days after explosion
      - masses 3-5 times larger than thermoinuclear WD explosion should give
      - 30,000 to 40,000 K, but thermal (when tau<1, T~1e4 K)
        MERLIN showed north-south elongation, whereas VLA source is round;
        eventually HST kicked in, but by then it only saw 5-10% of the mass
      --> can get mass and clumpiness fairly directly from radio imaging
    * What the Expansion Project could do
      - SENSITIVITY
        (flux densities are intiailly a few mJy, and Snu goes as t^2 while
          the source is optically thick, so in a few cases can get 100s of
          mJy at peak.  43 GHz fluxes can reach the Jy level.)
      - RESOLUTION (A+) -- ***all the time***
      --> could image ALL novae in the entire Galaxy: perhaps 30 per year
        total, vs. current 6-10 found optically each year which would be
        suitable (and a few even fainter)
        (only 2 novae have been imaged so far!)
      --> early-time geometry progresses from outermost ejection shell
        inwards through the various layers, until the source is completely
        optically thin.  Optical folks *only* see this latter stage, so they
        miss all the interesting stuff.  Need A+ resolution...which will be
        there all the time!
    * vs. recurrent novae, which may have nonthermal emission -- those have
      not yet been imaged, because the VLA hasn't been in A config. at the
      right time (RS Oph was caught much later)
    * Might be able to see RRLs too; velocities are 100-3000km/s, so you
      need the correlator
    * "calibrate thermonuclear physics"
      - geometry
      - mass -- radio sees too much!
      - clumpiness
  
  b. X-ray binaries
    See also Galactic and stellar working groups
    * VLA-VLBA gap leads to missing lots of flux at the VLBA, and boring maps
      at the VLA
    * Expansion Project offers
      - SNAPSHOTS (quick response; good imaging; sensitivity)
      - A+ always present (good resolution)
      --> high-quality, high-resolution imaging
      --> much better light curves
      --> test ADAFs with "off" sources
    * Examples (pretty pics): 1655, 1748
      - 1655 VLA blobs, VLBA over-resolved
      - 1748 "laboratory for hot spot interactions and synchrotron jets"

3- Supernovae
  Follow-up: Dick Sramek
  a. Early detection --> figure out absorbing process (free-free vs. SSA;
    if you see high Tb, it's SSA)
  
  b. More SNe
    * Currently about 1/yr of >14th mag. (of which there are ~8/yr)
    * Could get beyond Virgo
    * Might detect dust ???
  
  c. Type Ia
    * So far undetected, though some predict they might be at a low level?
      - How far out could you see SN87A?
  
  d. Misc.
    * Ties in with X-ray/optical observations
  
  e. Expansion Project offers SENSITIVITY, esp. at high frequency

4- GRBs
  Follow-up: Dale Frail
  a. Physics of relativistic blast waves: radio contributions
    ISS+SSA --> size and expansion rate (beaming vs. spherical)
    reolve with VLBI for z<0.2
    blastwave parameters
    trace full evol'n, for > 1 year, covering all segments of the light-curve

  b. Nature of progenitors
    * NS-NS vs. hypernova
    * subarcsecond positions --> figure out where they live in host galaxies
      (based on optical follow-up)
    * Det. optically-obscured GRBs: dusty, or z>5

  c. Cosmological tools
    * high-z population: absorption, RM, etc. to z=6; maybe even lensing???
  
  d. New types of GRBs
    * SGR, SNe-GRB, classical GRB are all distinguishable at radio wavelengths

5- Surveys for variability
  Frazer: we have the data already
  * Looking for ESEs (one per 10^4 or so --> need 4 20cm beams observed for
    12hrs each to get one)
  * stars, lensing, low flux density AGNs, scintillation


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Last modified 08 December 1999

mrupen@nrao.edu