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[vsnet-alert 7680] IGR J17464-3213 = XTE J17464-3213: request for IR observations




We just submitted this as ATel 139.  This source may currently be 11-13th
mag. in the near IR, although the probable extinction rules out optical
observations; it may also be highly variable, though that's far from clear
at the moment.  Have a look if you can -- this is turning into a very
interesting source!

  Clear skies,

             Michael Rupen
             NRAO/Socorro
             mrupen@nrao.edu

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IGR J17464-3213 = XTE J17464-3213: Request for IR Observations

ATEL # 139: M.P. Rupen, A.J. Mioduszewski, and V. Dhawan (NRAO)
             on 4 Apr 2003; 21:08 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Michael P. Rupen (mrupen@nrao.edu) 

We report further radio observations of the X-ray transient and black hole 
candidate IGR J17464-3213 = XTE J17464-3213 (ATEL #132; ATEL #133; IAUC 8105)
from the Very Large Array (VLA). The source continues to brighten, and appears
now to be doubling in flux density every two days. Observations on 4 April 
2003 UT give flux densities of 20.0+/-0.2 mJy at 4.9 GHz, and 22.1+/-0.2 mJy 
at 8.5 GHz. The radio spectrum has remained remarkably constant at nu^+0.2, 
characteristic of optically-thick synchrotron emission. 

If this spectrum continues into the near infrared, as expected in some jet 
models (e.g. Markoff et al. 2003 A&A 397, 645), the K band magnitude would be 
between 9th and 11th. Even with 2-3 magnitudes of K band extinction (based on 
the X-ray absorption column density, ATEL #133), this should be easily 
detectable. Even upper limits would be very useful indeed, in constraining 
these models, and in showing whether there is a chance that some of the X-ray 
emission is synchrotron. 

Such associated infrared emission has already been observed in some sources 
(e.g., GRS 1915+105, Fender & Pooley 1998, MNRAS 300, 573; GX 339-4, Corbel &
Fender 2002, ApJL 573, L35), but never before in combination with such a
stable and slow flux increase. One expects rapid variations, simply because 
the size of the emitting region is so small, if the flat spectrum is due to 
synchrotron self-absorption. Simple calculations give sizes of order or less 
than a few tenths of a milliarcsecond; this corresponds to a few 10s of 
light-minutes at the distance of the Galactic center, so one might hope to 
see significant variations in some 10s of minutes in the near IR. 

We therefore urgently request infrared observations of this source, as soon as 
possible. The radio spectrum has held steady for about a week, and the flux 
density seems only to increase, but this is unlikely to continue for long. 

The best position so far (from the VLA, IAUC 8105) is 
       17 46 15.61 +/- 0.01s -32 13 59.9 +/- 1" (J2000)
(we have a heavily elongated NS beam). This will improve when we have a chance 
to look more carefully at the data. 

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