A Pointed RXTE Observation of XTE J1817-330

ATel #743; J. M. Miller (Michigan), J. Homan (MIT), D. Steeghs (CfA), M. A. P. Torres (CfA), R. Wijnands (Amsterdam)
on 21 Feb 2006; 17:26 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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We report on a pointed RXTE observation of XTE J1817-330, obtained on 2006-02-13 at 08:10:40 UT. After standard screening, the net good time for the PCA was 9.6 ksec, and the net good time for the HEXTE (cluster A) was 3.0 ksec. Joint fits were made to the PCU-2 and HEXTE-A spectra on the 2.8-25.0 keV and 20.0-200.0 keV range, respectively, with XSPEC.

For a variety of continuum spectral models, a low column density is implied. Error scans indicate that a column density in the 0.8-5.0 E+21 range (90% confidence limits), though it seems likely that the column density is as low as 2 E+21. This value corresponds well to the weighted average column density given by the N_H HEASARC tool, based on Dickey & Lockman 1990.

A phenomenological model consisting of disk blackbody and power-law continuum components and a Gaussian Fe K line component gives kT = 0.89(1) keV, Gamma = 2.20(3), and a line equivalent width of 240(40) eV (90% confidence errors). There is no evidence for a break or cut-off in the power-law. The line appears to be broadened, and the best fit Gaussian centroid energy lies between 5.5-6.0 keV. The source appears to be clearly detected out to 200 keV.

The above model gives an unabsorbed flux of 1.3 E-8 erg/cm^2/s in the 3-200 keV band, a flux of 4.6 E-8 erg/cm^2/s in the 0.5-200 keV band, and a flux of 4.4 E-8 erg/cm^2/s in the 0.5-10.0 keV band. The disk contributes 90% of the flux in the 0.5-10 keV band, and 77% of the flux in the 3-200 keV band.

These fit parameters and fluxes, and the 1/f nature of the power spectra we have observed, are consistent with the source being in the high/soft (= thermal-dominant) state on February 13. ATEL 738 indicates that the power-law softened after this RXTE observation.

Particularly given that the low column density implied in X-rays confirms optical inferences (see ATEL 740, ATEL 733), we strongly encourage joint optical and X-ray follow-up of this source.