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 (dsteeghs@cfa.harvard.edu)

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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.