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 \chi ^ { 2 } minima at orbital periods of 26-30 days for fixed mass functions of 5 , 10 , 15 , and 20 M _ { \odot } .
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 \pm 1 ) \times 10 ^ { 22 } cm ^ { -2 } , photon index = 1.01 \pm 0.08 , cutoff energy E _ { cut } = 11 \pm 1 keV , and folding energy E _ { fold } = 19 \pm 4 keV .
Estimated 2-10 keV peak fluxes , corrected for contributions from the galactic ridge and SGR 1900+14 , are 6 \times 10 ^ { -12 } ergs cm ^ { -2 } s ^ { -1 } and 1.1 \times 10 ^ { -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 .