Optical Spectroscopy of XTE J1817-330 in Outburst
ATel #749; M. A.P. Torres, D. Steeghs, J. McClintock, M. Garcia (CfA), Alexis Brandeker, Duy Nguyen (Toronto), P. G.Jonker (SRON/CfA), J. M.Miller (Michigan)
on 25 Feb 2006; 22:50 UT
Credential Certification: Danny Steeghs (email@example.com)
Subjects: Optical, Binary, Black Hole, Transient
A single 30 min high-resolution spectrum of the optical counterpart to
the black hole candidate XTE J1817-330 (ATEL #714, #733) was acquired
with the MIKE echelle spectrograph on the Magellan-Clay telescope at
Las Campanas Observatory on Feb 20 2006 08:36 UT. In the blue (red),
MIKE covered the wavelength range 3530-5070 (4830-9420) Angstrom with
a dispersion varying between 0.03 and 0.05 (0.07 and 0.013) A/pix.
The only emission line detected in the spectrum is Halpha with
equivalent width (EW) of ~ 4 A and FWHM ~ 640 km/s. No other Balmer
lines (in emission/absorption) were detected, nor was there evidence
for HeII 4686 A or Bowen blend emission. The signal-to-noise ratio was
~40-45 at Hbeta, Hgamma and HeII.
We clearly detect the CaII H&K and the NaD interstellar lines in
absorption, but we do not find absorption line features that would be
expected from a local absorber. The EW of the NaD 5890,5996 A
components were measured to be 0.68 +/- 0.04 A and 0.48 +/- 0.04 A
respectively. The EW uncertainties were estimated by looking at the
scatter in the values when selecting different wavelength intervals to
reflect the local continuum level.
The ratio between the two Sodium lines is much less than the factor of
two expected at the lowest optical depths (Munari & Zwitter
1997,A&A,318,269). The total EW of the doublet corresponds to E(B-V) =
0.29 +/- 0.01 mag according to the empirical relationship of Barbon et
al. (1990,A&A,237,79). However, the calibration of Munari & Zwitter
(1997) would yield a much higher extinction of E(B-V) > 0.7 mag. For
comparison, the Hydrogen column as estimated from the Schlegel dust
maps gives E(B-V)=0.24.
We surveyed the MIKE spectrum to identify other possible
interstellar lines (IS) and diffuse interstellar bands (DIBs) that
could be used to estimate the reddening using
the list of Jenniskens & Desert (1994,A&AS,106,39). We were able
to identify the DIB at 5870 A and KI 7699 A with EWs of 0.18 +/-
0.04 A and 0.08 +/- 0.03 A, yielding E(B-V)= 0.44 +/- 0.11 and 0.3 +/-
0.2 respectively when using the calibrations from Herbig
(1993,ApJ,407,142) and Munari & Zwitter (1997).
The above estimates indicate that significant differences exist between the
various EW versus N_H calibrations. This is the limiting factor in
determining the N_H from the MIKE spectrum. We therefore constrain
the Hydrogen column to be in the range 1e21 < N_H < 3e21.
These estimates for N_H, together with the independent constraint
from the Chandra observations (ATEL #746), confirm that the column
towards XTE J1817-330 is modest.