[vsnet-alert 2246] XTE J1946+274 optical observations (Garcia)

[Taichi Kato <tkato>]

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From: Mike Garcia <garcia@head-cfa.harvard.edu>
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Date: Mon, 28 Sep 1998 04:26:50 -0400 (EDT)
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Dear Colleagues - We have just submitted the following for
inclusion in the next IAUC
-Mike Garcia
----------------------


Dear Dr. Green,

Please consider the following announcement for inclusion in the next
IAU Circular.  The publication charges will be covered by the High
Energy Astrophysics Division of CfA.

-Sincerely, Parviz Ghavamian
********************************************************



Parviz Ghavamian, Rice University, parviz@sparky.rice.edu
Michael Garcia, Harvard-Smithsonian Center
                for Astrophysics, garcia@cfa.harvard.edu


We report photometric and spectrocopic measurements of the source suggested by
G. L. Israel et al. (IAUC 7021) to be the optical counterpart of
the transient X-ray pulsar XTE J1946+274.  Spectroscopy was performed
with the FAST spectrograph on the 1.5 meter telescope of Fred Lawrence
Whipple Observatory, Mt. Hopkins, Arizona, U.S.A., using the position reported
by Israel et al. (R.A. 19h45m39s.4, Decl. +27o22'43" (2000), IAUC 7021),
on the night of Sept. 27.1 UT.  Integration time was 50 minutes, with
spectral resolution of 0.6 nm and spectral coverage 350-750 nm.  Two
point sources appear near the the coordinates of Israel et al. in the
Digital Sky Survey, both of which were covered by the slit (PA = 90o).
The ratio of the Hbeta / HeI 447.1 / Hgamma lines indicates spectral type of
B2 +/- 2 subclasses for the brighter of the pair, which is located at R.A.
19h45m39s.26, Decl. 27o22'45".3 (+/- 0.3").   Interstellar absorption lines
at 577.8, 589.0 (Na-D) and 628.4 nm are very prominent, with EW = 0.09, 0.2,
and 0.18 nm, respectively.  The fainter star lies approximately 7" to the
west of the brighter one.  The ratio of the Balmer series and MgB absorption
lines in the fainter star suggests a spectral type of F9 +/- 1.  The FAST
spectra of the B star fail to show the weak Halpha emission reported by
Israel et al.; instead Halpha absorption is seen with EW = 0.68 nm. Photometry
was performed on the B star using the CCD imager on the 1.2 meter Telescope of
Fred Lawrence Whipple Observatory on September 28.1 UT.  Using Guide Star
magnitudes for flux calibration, we find V = 14.4 +/- 0.3.  The lack of
emission lines and variability does not necessarily discount this star as
the optical counterpart of XTE J1946+274.  The strength of the NaD line
indicates that the brighter star is approximately 4 kpc distant, and implies
A(V)  ~ 8.  If this star is the optical counterpart of the X-ray pulsar,
the X-ray spectrum should show substantial low energy absorption
(N_H ~ 1.5e22 cm^-2).

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