[vsnet-alert 5140] (fwd) Re: Renewed activity in V4641 Sgr (XTE J1819-254) ?

[Taichi Kato <tkato>]

(fwd) Re: Renewed activity in V4641 Sgr (XTE J1819-254)  ?

   Further follow-up information.  (plus XTE J1118+480)

Date: Fri, 21 Jul 2000 12:57:24 -0600
From: "Robert M. (Bob) Hjellming" <rhjellmi@cv3.cv.nrao.edu>
Subject: Re: Renewed activity in V4641 Sgr (XTE J1819-254)  ?

Lest someone be led astray by what I said was going on with V4641 Sgr, let me
describe the situation in the radio.

I sent an early report of a preliminary result of a detection of 1.6 mJy on 
one day, and
a detection of a slightly weaker source the next day.

The VLA has now observed V4641 Sgr on five days in succession, and
within the error bars, it is close to a constant source of about 1.2 mJy.
This is from daily observations at 4.9, 8.4, and 14. 9 GHz.  At 1.49 GHz there
are some very strong confusing sources nearby so I have given up trying
to get believable numbers for that frequency. There may be some slow
systematic variations in the last four days, but, I repeat, within the errors
there has not yet been a significant change in the radio source in the last 
five days.

Interestingly, I am monitoring XTE J1118+480 at the same time and it is 
noticeable that
most of the time its radio spectrum at high frequencies is very similar - a 
flat, slightly positive
spectral index - a state XTE J1118+480 has mostly been in for
months.  This is of course, also similar to the usual spectrum of Cyg X-1.

So, while V4641 Sgr has returned to producing a radio source, it is 
definitely not
a short time scale radio event.  This matches the lack of significant 
changes in
the optical and X-ray - now that the one 18 cts/sec data point on the RXTE 
Weather
Page has proven to be a single aberration.

Since the VLA is in its most compact 1 km configuration, there is a faint 
possibility
that we are seeing an source that was resolved out in the early 
observations, but
such a source almost never has a flat radio spectrum.

We have a puzzle, which we will continue to pursue.

Bob

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