The XMM-Newton/EPIC-pn Spectrum of the Millisecond X-ray Pulsar XTE J1751-305 in Outburst

ATel #90; Jon M. Miller (MIT), Eckhard Kendziorra (Tuebingen Univ.), Marcus Kirsch (SOC/ESA), Rudy Wijnands (MIT), Mariano Mendez (SRON), Michael Nowak (MIT), Bryan Gaensler (Harvard-Smithsonian CfA), Deepto Chakrabarty (MIT), and Walter H. G. Lewin (MIT)
on 26 Apr 2002; 19:33 UT
Credential Certification: Jon M. Miller (jmm@space.mit.edu)

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We have performed a preliminary analysis of the XMM-Newton/EPIC-pn spectrum of the accreting millisecond X-ray pulsar XTE J1751-305 (Markwardt and Swank, 2002, and in't Zand et al. 2002, IAUC 7867 ; Ehle et al. 2002, IAUC 7872 ; Markwardt and Swank, 2002, IAUC 7876 ). The observation started on 2002 April 07.54 and lasted for 34.2 ksec, and represents the first outburst spectrum from such a source at CCD resolution.

The 0.3-10.0 keV spectrum is well-described by black-body and power-law components, modified by photoelectric absorption in the ISM (with elemental abundances consistent with solar). We measure a 0.3-10.0 keV unabsorbed flux of 7.0 +/- 0.1 E-10 erg cm^-2 s^-1; 83% of the flux in this band is due to the hard power-law component. The equivalent neutral hydrogen column density is 0.99 +/- 0.01 E+22 atoms cm^-2. The black-body temperature is: kT = 1.06 +/- 0.01 keV, and the power-law index is: Gamma = 1.45 +/- 0.01. The normalization of the blackbody component gives an emitting radius: R = (D / 10 kpc) * 2.93 +/- 0.07 km. This radius may imply that the the emission region is a hot spot on the neutron star surface; however, the model is overly simple and we approach this interpretation with caution. (All errors are 90% confidence limits.)

We note broad deviations below 0.6 keV which appear as absorption. At present, it is unclear if this represents real, unresolved absorption or a defect in the instrumental response. The deviations are not removed by requiring enhanced elemental abundances in the photoelectric absorption model. Over the 0.3-10 keV band, we obtain a chi^2 fit statistic of 2771 for 1941 degrees of freedom; considering only the 0.6-10.0 keV band yields a chi^2 value of 2085 for 1883 degrees of freedom and component parameters are essentially unchanged.

A plot from our analysis may be obtained at: http://space.mit.edu/~jmm/xtej1751-305.ps