[vsnet-alert 7352] V4641 Sgr: continued strong & variable radio emission

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

Date: Fri, 24 May 2002 01:00:16 -0600 (MDT)
From: Michael Rupen <mrupen@aoc.nrao.edu>
Subject: (correct) V4641 Sgr: continued strong & variable radio emission

Dear colleagues,
  I have reduced part of our 23may02 VLA run on V4641 Sgr, spanning 4.5
hours with a time resolution of 3.3s, time-sharing between 1.425, 4.86, 
8.46, 15, 22, and 43 GHz.  At 8.46 GHz the flux smoothly declined from
 ~170 to 110 mJy in the first 90 minutes, but rose equally smoothly from 
about 80 to about 140 mJy in the last 11 minutes.  There is no evidence 
for extremely rapid fluctuations, or for any dramatic extension to the 
source (0.72x0.46 asec beam, rms a few tenths of a mJy [varies with 
time]). A preliminary version of this light curve is available at

  http://vsnet.aoc.nrao.edu/~mrupen/XRT/V4641Sgr/v4641sgr.shtml 

  The only other frequency I've reduced so far is 43 GHz.  The flux
density here ranges from ~15 to ~130 mJy, with ~30% [smooth] variations on
timescales of a couple minutes.  Again there are no obvious extensions
(beam 0.14 x 0.09 arcsec), but I've not tried very hard to bring them up
yet.  The spectral index between 8.5 and 43 GHz can only be measured at a
couple times (due to that frequency time sharing I mentioned); on the 
first occasion it's around alpha~ -0.4 (S_nu going as nu^alpha), while on
the second it's basically flat (alpha~ 0).   This is consistent with 
multiple flares from a small source, quite likely a relativistic jet as we 
saw in Sep99.

  In sum, it's actively flaring on timescales of minutes to hours, though
I don't see any evidence (with admittedly poor SNR at the high 
frequencies!) for anything like the few-second oscillations reported in
the optical.  Even so, good temporal coverage at high time resolution is 
very helpful. 

  Note that this is all very different from either of the two previous 
radio flares.  In Sep99 we had a very rapid radio burst with evidence for 
extremely rapid (0.4 arcsec/day) expansion, followed by a couple weeks of
slow, smooth decay; in Jul00 we had an optically-thick and quite stable
radio source, until it rather suddenly disappeared.  This time around it's 
much less stable.

  Unfortunately it does not look like we'll be getting VLBA time for at
least a few days, as we too have been hit by the Chandra Sgr A* campaign.
Similarly there is not a great deal of VLA time available just now. 
The next definite observations will be on the 31st, from 0935 to 1135 UT.
Simultaneous optical/X-ray observations would be lovely :) 

  Good hunting --

                Michael Rupen, Amy Mioduszewsk, & Vivek Dhawan
                NRAO/Socorro

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