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Further radio observations of V4641 Sgr (= SAX J1819.3-2525)

ATel #175; M. P. Rupen, V. Dhawan, & A. J. Mioduszewski
on 9 Aug 2003; 06:23 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Michael P. Rupen (mrupen@nrao.edu)

Subjects: Radio, Request for Observations, Binary, Black Hole, Transient, Variables

We report further radio observations of the X-ray transient and likely black hole binary V4641 Sgr (ATEL #170, #171, #172, #173, #174). Observations with the Very Large Array (VLA) August 8 and 9, 2003 gave flux densities of 4.08+/-0.08 and 3.07+/-0.11 mJy, respectively, at 4.86 GHz; and 2.84+/-0.05 and 2.16+/-0.09 mJy at 8.46 GHz. The corresponding spectral index on both days is about -0.6, indicating optically-thin synchrotron emission.

This represents a very marked decline from the 545-665 mJy seen on August 6 (ATEL #172). Comparing the data from August 8 and 9 however suggests a much slower decay at both frequencies, which can be fit, for example, by an exponential with a time constant of 3.6 days. This could be a long-lived remnant as seen following the major flare in 1999 (cf. Hjellming et al. 2000, ApJ 544, 977-992), or the beginning of a series of rapid and chaotic events as in 2002 (e.g., IAUCs 7906 , 7908 , and 7928 ; ATEL #105).

Further VLA and VLBA observations are planned, and we urgently request complementary observations at other wavelengths. Simultaneous observations at radio, X-ray, and optical/infrared wavelengths are particularly valuable. To this end we have created a Web site summarizing the planned observations of which we are aware, particularly at X-ray and radio wavelengths:

http://www.aoc.nrao.edu/~mrupen/XRT/V4641Sgr/v4641sgr.shtml

Observations at other wavelengths during these intervals will be invaluable in deciphering the physical mechanism(s) behind the emission.

Planned Observations of V4641 Sgr