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