National Aeronautics and Space Administration

XTE J1807-294

C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); and E. Smith and J. H. Swank, GSFC, report that RXTE PCA monitoring observations of the Galactic-center region have revealed a new millisecond pulsar, designated XTE J1807-294. PCA cross scans on Feb. 21.6 UT determined the position to be R.A. = 18h07m.0, Decl. = -29o24' (equinox 2000.0), with an uncertainty of approximately 1' due to systematic errors. Coherent pulsations were detected at a barycentered frequency of 190.625 Hz. While the pointing is interrupted by scans, there appears to be a significant sinusoidal modulation of the pulse frequency, at a period of 35 +/- 3 min, and with a half-amplitude of 2.2 +/- 0.6 mHz. This source is the fourth known accreting millisecond pulsar, and the orbital period, if confirmed, is the shortest yet of that group. The 2-10- keV x-ray fluxes (in mCrab) were: Feb. 16.7, 33; 19.8, 38; 21.6, 58; 22.6, 41; 25.4, 32 -- suggesting that the x-ray peak has passed. Further RXTE observations are planned. Observations at other wavelengths are encouraged.

X1901+031

XTE J0119-731

XTE J1720-318

C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); and J. H. Swank, GSFC, write: The best x-ray position of the transient XTE J1720-318, derived from RXTE PCA scans on Jan. 15.06 UT, is R.A. = 17h20m00s, Decl. = -31o44'.8 (equinox 2000.0). The estimated systematic uncertainty is 0'.5, consistent with the radio position (IAUC 8054). The x-ray flux declined 30 percent between Jan. 14 and 19, from a maximum of 1.0 x 10**-8 erg cm**-2 s**-1 (2-10 keV). The soft spectrum and low high-frequency variability are consistent with the high soft states of black-hole candidates."

XTE J1720-318

R. A. Remillard, A. M. Levine, and E. H. Morgan, Massachusetts Institute of Technology (MIT); and E. Smith and J. Swank, Goddard Space Flight Center (GSFC), for the RXTE All Sky Monitor Team at MIT and NASA's GSFC, report the discovery of a transient x-ray source at R.A. = 17h19m58s, Decl. = -31o46'.8 (equinox 2000.0; estimated 3' uncertainty at 90-percent confidence): "The source was first detected on 2003 Jan. 9, when the average flux (2-12 keV) was 130 +/- 20 mCrab. The intensity increased to 430 +/- 40 mCrab on Jan. 10, and to 400 +/- 20 mCrab on Jan. 13. A brief observation of the ASM position on Jan. 14 with the RXTE PCA confirms the presence of a bright transient. More extensive observations with the PCA and HEXTE, including raster scans to improve the source position, are planned starting Jan. 15. The error circle does not contain any noteworthy sources in the Simbad catalogs. The ASM hardness ratios indicate that the spectrum was moderately hard during Jan. 9-10 and relatively soft on Jan. 13. These results resemble the early spectral evolution of x-ray transients known to contain an accreting black hole. We encourage optical and radio observations of this new transient."