Outburst Spectroscopy of Swift J1922.7-1716 and Optical Imaging in Quiescence
ATel #3742; J. P. Halpern (Columbia U.), Julie Skinner (Dartmouth College)
on 7 Nov 2011; 00:37 UT
Credential Certification: Jules Halpern (jules@astro.columbia.edu)
Subjects: Optical, X-ray, Binary, Transient
We obtained a spectrum of the optical counterpart of Swift J1922.7-1716
in outburst (Barthelmy et al., GCN #12522) using the Boller and Chivens CCD
spectrograph on the Hiltner 2.4m telescope of the MDM Observatory on 2011
November 4, 01:23 UT. A 30 minute exposure covering the wavelength range
3480-7130 Å at 10 Å resolution was obtained under partly cloudy conditions.
In agreement with the results of Wiersema et al. (ATel #3740), weak emission
lines of Hα, He II 4686, and the N III 4640 Bowen fluorescence blend
are seen at zero redshift, superposed on a blue continuum. These are
characteristic properties of low-mass X-ray binaries.
In a prior quiescent phase, imaging with RETROCAM on the 2.4m telescope
on 2009 May 29 UT for 5 minutes each in SDSS g and r filters did not
detect the optical counterpart of Swift J1922.7-1716 to a limit > 23.4 mag
in each band, which further supports the LMXB hypothesis and constrains
the type of the companion star. Adopting the distance upper limit of 5.4 kpc
from the X-ray burst analysis of Degenaar et al. (ATel #3741), the donor
star should be an M dwarf or a more compact, evolved star. The orbital
period should be < 5 hours.
