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Broad relativistic iron line observed in SAX J1808.4-3658 by Suzaku

ATel #1844; E. Cackett (Michigan), D. Altamirano (Amsterdam), J. Miller, M. Reynolds, J. Blum (Michigan), M. Linares, A. Patruno, R. Wijnands (Amsterdam)
on 14 Nov 2008; 15:09 UT
Credential Certification: Edward Cackett (ecackett@umich.edu)

Subjects: X-ray, Binary, Neutron Star, Pulsar

Referred to by ATel #: 1846

The accreting millisecond X-ray pulsar SAX J1808.4-3658 recently exhibited another outburst in September and October 2008 (ATel #1728). During this outburst, on 2008 Oct 2, we obtained a ~40 ks observation with Suzaku. The XIS detectors were operated in 1/4 window mode, with 1 sec burst, leading to a good time exposure of 21 ks for the XIS 0, 1 and 3 detectors. No type-I X-ray bursts were detected in the lightcurves.

We fitted the spectra from all three XIS detectors jointly in XSPEC, finding that the continuum was described well by an absorbed disk-blackbody + blackbody + power-law model, with a column density of 0.5E21 +/- 0.3E21 cm^-2, kT(disk) = 0.50 +/- 0.01 keV, kT(bbody) = 1.05 +/- 0.04 keV, and power-law index = 1.85 +/- 0.09 (all uncertainties quoted here at the 90% confidence level). We find that a broad, relativistic Fe K emission line is present in the spectrum, with a clearly asymmetric profile, as has been seen recently in other neutron star low-mass X-ray binaries (e.g., Bhattacharyya & Strohmayer 2007, ApJ, 664, L103, Cackett et al., 2008, ApJ, 674, 415). This is the first time an asymmetric iron line has been observed in an accreting millisecond X-ray pulsar.

The iron line can be well fit by the diskline model (Fabian et al. 1989, MNRAS, 238, 729), with EW = 121 +/- 23 eV. From our initial analysis of the iron line, we find an inner accretion disk radius of 13 +/- 5 GM/c^2. During fitting, we only allowed the inclination to be within the range 36 - 67 degrees (Deloye et al., 2008, MNRAS, in press, arXiv:0810.0489), and found a best-fitting value of 50 +/- 6 degrees.

We thank the US and Japanese Suzaku teams for performing the observation of SAX J1808.4-3658.