SWIFT XRT Observations of 4U 1608-522 in Outburst
ATel #858; M. V. Greco (Univ. of Michigan), J. M. Miller (Univ. of Michigan), D. Steeghs (SAO)
on 17 Jul 2006; 17:26 UT
Credential Certification: Jon Miller (jonmm@umich.edu)
Subjects: X-ray, Binary, Neutron Star, Transient
Referred to by ATel #: 1113
We report on X-ray observations obtained with the SWIFT-XRT of the neutron star transient 4U1608-522. The source recently went into outburst (ATel #851). Our spectral fits were made using XSPEC v12 on archival Swift/XRT data (observation ID 00030791001). The observation was begun on July 11, 2006 at 21:08:54 UT. Its duration was 989.4s, after standard screening.
Our first fit was a simple power-law, and resulted in spectral index 1.7 (+/- 0.05) and an unabsorbed flux (0.5 - 10.0 keV) of 1.9 (+/- 0.1) *10^-9 erg cm^-2 s^-1. This flux, given a distance of 3.6 kpc (Wachter et al., 2002, ApJ), corresponds to a luminosity of 2.9 (+/- 0.2) *10^36 erg s^1. The flux is higher by a factor of ~2 than was measured over a similar bandpass (2 - 12 keV) by the RXTE All-Sky Monitor on July 6 (ATel #851). The reduced chi-squared statistic for this fit is 1.01, for 602 degrees of freedom; and thus this fit formally describes the data adequately.
However, the column density derived in this fit, 1.28 (+/- 0.06) *10^22 cm^-2, is lower than the weighted mean value predicted by the HEASARC tool, 1.92 *10^22 (Dickey & Lockman, 1990, ARAA). And though our derived value is within the range found in the literature (1-2 * 10^22 cm^-2) (Penninx et al., 1989, A&A; Yoshida et al., 1993, PASJ), we felt it worthwhile to investigate this second possibility.
A disk blackbody component is also needed to make the fit work with the column density frozen at the predicted value. The new spectral index was 1.96 (+/- 0.04), and the unabsorbed flux (0.5 - 10.0 keV) was 8.8 (+/- 0.4) *10^-9 erg cm^-2 s^-1. This spectral index is consistent with the index fit to Aql X-1, a similar source, in the hard state of its 2005 outburst; that index was 1.86 (+/- 0.05) (Rodriguez et al., 2006, A&A). The flux corresponds to a luminosity of 1.36 (+/- 0.07) *10^37 erg s^-1. The temperature at the inner disk radius is 0.13 (+/- 0.02) keV. This temperature is slightly lower than the first value fit to Aql X-1 in the hard state of its 1999 outburst, 0.19 (+0.08 / -0.03) keV (Maccarone & Coppi, 2003, A&A). The reduced chi-squared statistic for this fit is 0.98, for 601 degrees of freedom; thus this fit too formally describes the data adequately.
