2002ApJ...565.1150W

In 1996 during Rossi X-Ray Timing Explorer (RXTE) observations of SGR 1900+14, the 89 s X-ray pulsar XTE J1906+09 was discovered. As a result of monitoring campaigns of SGR 1900+14, XTE J1906+09 was also monitored regularly in 1996 September, 1998 May-June, 1998 August-1999 July, and 2000 March-2001 January. A search for pulsations in these observations resulted in detections of only the two previously reported outbursts in 1996 September and 1998 August-September. Pulsed flux upper limits for the remaining observations indicate that XTE J1906+09 is a transient X-ray pulsar and therefore likely has a Be star companion. XTE J1906+09 was not detected with the RXTE all-sky monitor. Pulse-timing analysis of the second outburst revealed a sinusoidal signature in the pulse frequencies that is likely produced by periastron passage in an orbit. Fits to pulse phases using an orbital model and quadratic phase model have χ² minima at orbital periods of 26-30 days for fixed mass functions of 5, 10, 15, and 20M_☉. The pulse shape showed intensity- and energy-dependent variations. Pulse-phase spectroscopy was used to quantify the energy-dependent variations. The phase-averaged spectrum, using the pulse minimum spectrum as the background spectrum to eliminate effects from SGR 1900+14 and the Galactic ridge, was well fitted by an absorbed power law with a high-energy cutoff with column density N_H=(6±1)x10²² cm^–2, a photon index of 1.01±0.08, cutoff energy E_cut=11±1 keV, and e-folding energy E_fold=19±4 keV. Estimated 2-10 keV peak fluxes, corrected for contributions from the Galactic ridge and SGR 1900+14, are 6x10^–12 and 1.1x10^–10 ergs.cm^–2.s^–1 for the 1996 and 1998 outbursts, respectively. XTE J1906+09 may be a member of an unusual class of Be/X-ray binaries that do not lie on the general spin period versus orbital period correlation with the majority of Be/X-ray binaries.