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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
Credential Certification: Michael P. Rupen (mrupen@nrao.edu)

Subjects: Radio, Infra-Red, X-ray, Gamma Ray, Request for Observations, Binary, Black Hole, Transient

Referred to by ATel #: 141

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.