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IGR J17418-1212 - A Possible Microquasar with Multiple Identification

ATel #239; Gregory Tsarevsky (Australia Telescope National Facility, Sydney & Astro Space Center, Moscow)
on 19 Feb 2004; 04:52 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Gregory S. Tsarevsky (Gregory.Tsarevsky@atnf.csiro.au)

Subjects: Radio, Far-Infra-Red, Infra-Red, Optical, X-ray, Request for Observations, Binary, Black Hole, Neutron Star, Star, Transient

Referred to by ATel #: 264

Due to Bassani et al., ATEL #232, the source No. 5 = IGR J17418-1212 (b = 9.4 deg.) was first detected in the hard X-ray band (>20 keV) by the IBIS/ISGRI instrument of INTEGRAL, and is probably identified with 2E1739.1-1210.
I call attention that the INTEGRAL object coincides unambiguously with the bright ROSAT source 1RXS J174155.3-121157.
Its hardness ratios (HR1 = 1.00 +/- 0.03; and HR2 = 0.48 +/- 0.09) place it well within the characteristic narrow strip occupied by XRBs
in the 'two-colour' HR1 - HR2 diagram (Motch et al., A&AS, 132, 341, 1998). Such prominent behaviour, together with radio identification,
has been chosen as the main selection criteria to find new microquasars in the relatively wide, |b| < 20 deg., belt along the whole Galactic plane
(see Tsarevsky et al., ATEL #80, 2001, with references therein). So it actually is a microquasar candidate
MCQC J1741-12 -- one of sixty in our survey list.
Indeed, NVSS map showed a weak, S_1.4 = 3.5 mJy, unresolved radio source
NVSS J174155-121201 17h41m55.55s -12o12'01.3"
just 5.3 arcsec from the formal position of the ROSAT object, but with a total position error 6.6 arcsec,
which is too large for reliable optical identification in the crowded Galactic plane.
Nevertheless, the following tentative identifications have been found:

 
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Catalogue_Name RA_2000 DEC_2000 B R I J
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2MASS 17415525-1211566 17h41m55.25s -12o11'56.7" - - - 13.4
DENIS J174155.2-121156 17h41m55.26s -12o11'56.2" - - 15.0 13.4
USNO 0778-0495204 17h41m55.276s -12o11'56.9" 16.3 14.9 - -
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One can see all three issues well coinside each other, and the question is how they are related to other sources in the pack as above.
Note that the bright and badly studied source IRAS 17391-1210 is also a fairly likely identification and should be included into the pack.
The USNO object has a weaker, R = ~17, barely-resolved SE companion ~2 arcsec distant in PA = 135 d., see 3'x3' finder chart
(where the 10" radius circle is centered on the star-like USNO source). This satellite also could be suspected to be related to the matter.
So it is a case to encourage observers to obtain a sub-arcsec radio image of the NVSS source
in order to unambiguously identify it with the USNO-2MASS-DENIS-etc object as above.
Just a single moderate-resolution spectrogram can give a reliable classification of this relatively bright source (see magnitude set in the Table),
and, if found to be stellar, would confirm it as a likely microquasar-candidate for further spectroscopic and photometric investigation.