MCQC J162847-4152 - a Possible Brightest Microquasar found

ATel #80; G. S. Tsarevsky (ATNF, Sydney, & ASC, Moscow), N. S. Kardashev (ASC), R. A. Stathakis (AAO, Sydney), O. B. Slee (ATNF), R. Ojha (ATNF)
on 11 Dec 2001; 10:19 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Gregory S. Tsarevsky (Gregory.Tsarevsky@atnf.csiro.au)

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MCQC J162847-4152 is one of 40 objects found in the course of our systematic and complete survey for new microquasars in the whole Galactic plane (see project description in astro-ph/0110511). It is optically brightest and mostly definite microquasar candidate in the list, which we describe as follows: 1) It is a bright ROSAT source, 1RXS J162848.1-415241, in the Galactic plane (b=4.7deg), with a hard X-ray spectrum (HR1 = 1.00+/-0.04), which is characteristic of X-ray binaries (Motch et al., AAS 132, 341, 1998). 2) It was detected at 4.8 and 8.6 GHz by the Australia Telescope Compact Array (ATCA) in October 2000 as a ~7 mJy flat spectrum radio source with an unresolved core, but with no conclusive evidence for the presence of a jet on an arcsec scale. 3) Using precision radio coordinates, MCQC J1628-4152 was identified with a relatively bright star-like object as follows:

 
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 Source of data  RA J2000       DEC J2000    r-o   B    V    R
 
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  USNO (PMM)     16h28m47s.267 -41o52'38".63 0".5  15.5 13.4 11.4
 
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V-magnitude is an averaged visual estimation by B. Monard (VSNET). Apparently large color-indices measured in different epochs may suggest a possible strong source variability. 4) Low resolution spectroscopy with 4-m (AAO) and 2.3-m (MSSSO) telescopes, showed MCQC J162847-4152 as a K5 spectrum star with a strong and rapidly variable H-alpha emission, i.e. consistent with a low mass XRB (LMXB). 5) Our three observing sessions with the ATCA revealed the following variability in flux density (RMS ~0.1 mJy, 4-sigma limit of detection):

 
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 Epoch of ATCA  F_4.8  F_8.6  Spectral
 
 observations   [mJy]  [mJy]  Index
 
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  29.10.2000     7.2    6.6   -0.1
 
  05.09.2001     8.8   12.3   +0.5
 
  02.12.2001    <0.5   <0.4     -
 
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It is evident from the table that the source shows a violent transient- like radio activity. Significant variability on time scales of several hours is also present. The above X-ray, optical and radio characteristics are suggestive of microquasar behavior (which still is not well established as there are only a few microquasars known, cf Mirabel and Rodriguez, ARAA 37, 409, 1999). For all that, we cannot exclude the possibility that the object is an active K-star. So we call for observations of MCQC J162847-4152 in radio (flux monitoring and structure variability, preferably by VLBI), optical spectroscopy (to find evidence for the object's binarity), optical photometry (to monitor disc driven flare activity and evidence for possible eclipse), and in X-rays (high spatial, spectral and time-resolved observations by Chandra, XMM and RXTE may reveal the characteristic structure, spectral features and flux variations found in the few established microquasars). We have already sent out a call to VSNET for photometric observations of this object.