XTE J1858+034

R. Remillard and A. Levine, Massachusetts Institute of Technology (MIT), report, for the RXTE ASM team at MIT and NASA Goddard Space Flight Center (GSFC): "The RXTE All Sky Monitor has detected a new, hard x-ray transient, which we designate XTE J1858+034. The source was detected in a correlation map constructed by superposing data collected during Feb. 6-12. The source is located at R.A. = 18h58m41s, Decl. = +3d24'.0 (equinox 2000.0), with an estimated uncertainty (90-percent confidence) of 6'. Reanalysis of recent ASM observations shows that the source flux (2-12 keV) was 15 mCrab on Jan. 30, 24 mCrab on Feb. 7, 15 mCrab on Feb. 16 and 10 mCrab on Feb. 26. The spectrum is very hard, resembling that of an x-ray pulsar." T. Takeshima and R. H. D. Corbet, GSFC and Universities Space Research Association; F. E. Marshall and J. H. Swank, GSFC; and D. Chakrabarty, MIT, report: "PCA/RXTE observations of XTE J1858+034 were performed on Feb. 20 (1 ks duration) and 24 (8 ks). The source flux (2-10 keV) was 13 and 12 mCrab on Feb. 20 and Feb. 24, respectively. The identification of this source as a pulsar is confirmed by the detection of pulsations in both observations, and a period of 221.0 +/- 0.5 s was determined from the Feb. 24 observations. The pulse profile is nearly sinusoidal with a pulsed fraction of about 25 percent (peak to peak). The pulsations and transient nature of XTE J1858+034 suggest that this is a Be/neutron star binary. Optical observations are strongly encouraged."