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[vsolj-alert 417] GM Sgr radio update

GM Sgr radio update


From rhjellmi@zia.aoc.NRAO.EDU Fri Sep 24 06:46 JST 1999
Date: Thu, 23 Sep 1999 15:45:08 -0600
To: dasmith@space.mit.edu, michiel@astro.uva.nl, rpf@astro.uva.nl,
        rudy@space.mit.edu, gehrels@lheavx.gsfc.nasa.gov,
        giommi@napa.sdc.asi.it, jem@head-cfa.harvard.edu, lewin@space.mit.edu,
        rhjellmi@zia.aoc.NRAO.EDU, rr@space.mit.edu,
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        chaswell@star.cpes.susx.ac.uk, ehm@space.mit.edu,
        Michael.McCollough@msfc.nasa.gov, p.charles1@physics.oxford.ac.uk,
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        mnowak@rocinante.colorado.edu, wheel@alla.as.utexas.edu
From: "Robert M. (Bob) Hjellming" <rhjellmi@aoc.nrao.edu>
Subject: Update on radio information on GM Sgr
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   We probably will be sending out something more formal to IAUC or ...,
but this is
a quick (and cheap) update on information that might affect observing

   The last observations of GM Sgr with the VLA on Sept. 23.0 UT, together
the previous data from the VLA, ATCA, and GBI indicate that the radio
source has been undergoing an approximately power law decay since going
thin with a spectral index -0.8 and a power law index -2.1.  VLBA observations
were made about Sept. 18.0, 20.0, 22.0, and 23.0.  It will be  next week
before we have any idea about what we have from the VLBA because of the time
it takes for the tapes to get to the correlator and be correlated.

   I have made a web page with URL
that contains a preliminary image at 4.9 GHz from Sept. 16.02 UT data, and
a plot of the abovementioned radio data and approximate power law fits.
Work is continuing on the radio imaging at the other VLA epochs; however,
there are not enough hours in the day.

   The double radio source seen Sept. 17.93 UT was mentioned in a previous
E-mail.  The Sept. 23.0 images show that the only the strong southerly
is still present, ~0.3" south of the Sept. 16.02 location of the strongest
radio emission - and the USNO position for GM Sgr. Really good astrometry
is critical to deciding whether the true declination of the star is 
-25 24 25.6 as indicated by the Sept. 16.02 radio centroid and USNO
positions, or near -24 24 25.85.  It makes a difference between one
sided relativistic ejection, and two-sided ejection with one side possibly
decelerated while brightening - a la XTE J1748-288.  It also may
make a difference in whether the motion ends up superluminal or subluminal.

   In the web page I update what I know about the distance, which
indicates a distance between 0.4 and 1 kpc.  My own bet/guess is that
it will end up near 0.5 kpc because that makes the apparent relativisitic
motion closer to what is reasonable for a galactic object.

   The sky may have many objects with events like these that are missed
because of
their extremely short duration and unusual characteristics.  It is fascinating
to see the papers on CVs where GM Sgr appears in tables with the notation

Cheers, Bob

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