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Date: Wed, 24 Sep 2003 00:01:05 -0700 (MST)
From: Brian Skiff <Brian.Skiff@lowell.edu>
Subject: [vsnet-chat 6895] Re: (fwd) On atel 178 - OR - xray pulsar orbits massive hidden supergiant!
I essentially recovered all of what John G outlined in his note.
I think, however, about all that he's shown is that the IRAS source is
the MSX source is the (southern) 2MASS/DENIS star. Everything is
characteristic of a strongly reddened red star, an M giant or supergiant.
Very large J-K colors are not unusual in this part of the sky because
of the strong reddening, which though only about one-tenth of the value
in the visible, is still a lot: 1/10th of 23 magnitudes is still ~3 mags
at K.
The thing that bothers me about the link between the red star and
the high-energy object is that the ATEL 178 notice indicates a 2' uncertainty
in the position. Even if you assume this is correct, there's still a lot
of real estate to consider given that this is only 9 deg from the
Galactic Center and dead-on the plane. What if the position uncertainty
is double the 2' value (which wouldn't surprise me at all)? There's
enough going here that's utterly obscured from us that existing catalogues
probably won't be of any use.
A note about 2MASS confusion: John notes there are two 2MASS sources
here (viz. at 17 25 22.75 -36 18 03.6, and another about 5" north),
both quite bright at K. A look at the K-band image shows that the southern
star is reliable, and that the northern one is a close companion that has
not been well resolved by the blob-finding routine, so the magnitudes
given are much brighter than the star actually is. A way to check for
this sort of thing in 2MASS is to look at the various flags, notably 'Qflg',
which gives a rough idea of the astrometric/photometric fit quality.
If 'Qflg' doesn't show 'AAA' (all three bands good), then it's reasonable
to worry about crowding or other problems. In this case, the northern
star is 'AAU', meaning the K band is lousy---when it actually should be
the best one, since that's where the data show it is brightest. So when
dealing with difficult cases, check those flags for clues as to possible
problems in the data.
A note in re IRAS versus MSX: note that the IRAS 12mu flux is the
only reliable detection of the four bands, and gives about 1.8 janskys.
MSX decected the source only in the three bands near 10 microns, which
like the IRAS 12-micron passband, covers the SiO emission feature in
evolved cool stars. The MSX A-band flux is 1.5 janskys, which is within
the uncertainty of the IRAS 12-micron flux---these are the same object,
and that object is almost certainly the red 2MASS/DENIS star.
\Brian
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