Serendipitous Detections of XTE J1906+09 with theRossi X‐Ray Timing Explorer

In 1996 during Rossi X-ray Timing Explorer (RXTE) observations of SGR1900+14, the 89-s X-ray pulsar XTE J1906+09 was discovered. As a result ofmonitoring campaigns of SGR 1900+14, XTE J1906+09 was also monitored regularlyin 1996 September, 1998 May-June, 1998 August-1999 July, and 2000 March-2001January. A search for pulsations in these observations resulted in detectionsof only the two previously reported outbursts in 1996 September and 1998August- September. Pulsed flux upper limits for the remaining observationsindicate that XTE J1906+09 is a transient X-ray pulsar and therefore likely hasa Be star companion. XTE J1906+09 was not detected with the RXTE All SkyMonitor. Pulse timing analysis of the second outburst revealed a sinusoidalsignature in the pulse frequencies that is likely produced by periastronpassage in an orbit. Fits to pulse phases using an orbital model and quadraticphase model have chi^2 minima at orbital periods of 26-30 days for fixed massfunctions of 5, 10, 15, and 20 M_sun. The pulse shape showed intensity andenergy dependent variations. Pulse phase spectroscopy was used to quantify theenergy dependent variations. The phase averaged spectrum, using the pulseminimum spectrum as the background spectrum to eliminate effects from SGR1900+14 and the galactic ridge, was well fitted by an absorbed power law with ahigh energy cutoff. Estimated 2-10 keV peak fluxes, corrected for contributionsfrom the galactic ridge and SGR 1900+14, are 6E-12 ergs/cm^2/s and 1.1E-10ergs/cm^2/s for the 1996 and 1998 outbursts, respectively. XTE J1906+09 may bea member of an unusual class of Be/X-ray binaries that do not lie on thegeneral spin period versus orbital period correlation with the majority ofBe/X-ray binaries.Comment: 18 pages 12 figures, accepted for publication in Ap