RXTE PCA Observations of XTE J1751-305

ATel #449; J. H. Swank (NASA/GSFC), C. B. Markwardt (U. Maryland & NASA/GSFC), E. A. Smith (L-3 Communications)
on 31 Mar 2005; 17:43 UT
Credential Certification: Jean Swank (swank@milkyway.gsfc.nasa.gov)

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The RXTE PCA observed the position of XTE J1751-305 on Mar 28.812 (UTC) for about two minutes, on Mar 29.229-29.273, and on Mar 30.478-30.517, as well as in a scan on Mar 30.379. The PCA flux is systematically uncertain due to the contributions from diffuse galactic emission and other nearby sources in the PCA field of view. The uncertainty is about 3 mCrab peak-to-peak, based on the regular PCA bulge scans of the region.

For the longer observation on Mar 30, the 2-10 keV total flux is 2.8 mCrab, consistent with the best background flux estimated in the bulge scan analysis. There is a strong Fe line in the spectrum, which is another indicator of contamination by diffuse galactic emission. The bulge scan an Mar 30 also shows a background-subtracted flux of < ~1 mCrab.

Assuming that the background is fixed by the Mar 30 observation, then XTE J1751-305 was detected on Mar 28 at a level of about 7.7 mCrab (2-10 keV). The background subtracted spectrum is consistent with a power law of with photon index ~2. The net X-ray fluxes are 18.4, 8.3, and 8.2 x 10E-11 erg/cm^2/s in the 2-10, 10-20 and 20-40 keV bands, respectively. Because the Mar 28 observation was done by realtime command, the PCA instrument was not in the proper mode to detect pulsations.

The fluxes on Mar 29 were 9.1, 3.8, and 3.9 x 10E-11 erg/cm^2/s, in the corresponding energy bands. No pulsations were detected. The flux was about at the level where the pulses became undetectable as the April 2002 outburst decayed.

Based on these results, the flux appears to have decreased to the point of non-detectability by the PCA, since the hard X-ray measurement on Mar 28.054-28.464 by Grebenev et al. (ATEL #446).