Spectroscopic Observations of MCQC J162847-4152
ATel #292; M. A. P. Torres (CfA), M. R. Garcia (CfA), D. Steeghs (CfA), J. E. McClintock (CfA), J. S. Bloom (CfA)
on 22 Jun 2004; 06:19 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)
Subjects: Binary, Transient
We obtained moderate/low resolution spectroscopy (2/5 AA FWHM) of the
microquasar candidate MCQC J162847-4152 (hereafter J1628) on 2003 May
5/6/13 (UT) and June 22/23/25 with the B&C spectrograph mounted on the
6.5-m Baade and Clay telescopes at LCO. In addition, high resolution
spectra (R~27000 and 36000 in the blue and red respectively) were
acquired with the MIKE echelle on the Baade telescope during 2003 May
12/13.
The optical spectrum of is dominated by broad absorption lines
with a FWHM of 57 +/- 3 km/s as measured from the MIKE
data. Inspection of the 5500-8420 AA sp. interval shows that the
sp. type/luminosity class of J1628 is in the range of K3 +/- 1
III-IV. RVs were measured by cross-correlating the observed spectra
with the spectrum of a K3III template. The RV variations, with a full
range of ~ 100 km/s, confirm the binary nature of J1628. We fit
sinusoids to the RVs using the photometric periods (P_ph=4.835 d or
~2xP_ph d; ATEL #234) as initial guesses for the orbital period. From
the best fit we find P=4.986 +/- 0.005 d; K_2 = 49 +/- 1 km/s and a
time of closest approach of the secondary to the observer of T_0 = HJD
2,452,767.895 +/- 0.035. The quoted uncertainties are 1-sigma and were
obtained after scaling the errors in the RVs in order to give
chi**2/dof=1. The value of P is discrepant with and 3% larger than
P_ph. We note the existence of significant deviations from the fit
(see phase-folded RV curve in URL link), and therefore caution that
our results are tentative. We find f(M)=0.061 +/- 0.004 Msun from the
above P and K_2. Under the assumption of synchronous rotation we set
a lower limit to the stellar radius of ~6 Rsun from the rotational
broadening and P. The MIKE spectra show the presence of the KI 7699
interstellar line blended with the broader KI photospheric line. We
measure an EW of about 0.08 AA for the interstellar feature after
fitting the blend with a double-Gaussian model. Using the calibration
of Munari & Zwitter (1997,AAp,318,269) we derive an interstellar
extinction to J1628 of E(B-V)~0.3.
The Halpha line is in emission, with FWHM~250 km/s and EW values
between 0.3-1.1 AA, except on May 6, when the EW increases to 4 AA
(see also ATEL #80). The Halpha profile obtained from the MIKE spectra
shows a double-peak shape that could be interpreted as emission
arising from an accretion disk. However, variable broad Halpha
emission with a self-reversal core is not unexpected in RS CVn
stars. The CaII H & K lines have emission cores with likely absorption
reversal at the top of the emission. We estimated a value of the
absolute visual magnitude of 1.6 for the secondary of J1628 by using
the Wilson-Bappu (WB) relation (eq. 6 in Montes et
al. 1994,AAp,285,609). This value is significantly above that for K3 V
stars (Mv=6.8) and close to the value of K3 giants (Mv=0.3). We
calculate a distance of 1.5 kpc to J1628 by using Mv=1.6, V=13.4 (ATEL
#80) and Av=0.93.
Assuming an absorbed Raymond-Smith model the ROSAT PSPC count rate
implies an unabsorbed flux of 2.8e-12 erg/cm**2/s (0.1-2.4 keV). A
distance of 1.5 kpc yields an intrinsic X-ray luminosity of ~8e32
erg/s, a value above the range of X-ray luminosities observed in RS
CVn stars (1e29-3e31 erg/s; Dempsey et al. 1993,APJSS,86,559). If
J1628 is a rapidly rotating RS CVn, it may deviate from the WB law due
to the influence of rotational broadening, explaining the discrepancy
in the X-ray luminosity. An X-ray luminosity of 3e31 erg/s implies
d~300 pc and Mv~5. Since the companion star is not detected in our
spectra, its visual luminosity should be ~100 times less than the
visible star, i.e. Mv>10 if we adopt Mv=5. On this basis, the unseen
companion is probably a M dwarf or a WD.
The above results suggest a RS CVn (not a microquasar) nature for
J1628. However, the paucity of RV measurements and the deviations from
the sinusoidal fit reported herein demonstrate the need for additional
data.
Phase-folded radial velocity curve of J1628
