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RXTE and Swift observations of XTE J1751-305 Show a Return to Quiescence

ATEL # 1051; C. B. Markwardt (CRESST/U. Maryland/NASA/GSFC), D. Pereira (RSIS), J. H. Swank (NASA/GSFC)
on 6 Apr 2007; 16:49 UT
Password Certification: Craig B. Markwardt (

Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients

The X-ray millisecond pulsar XTE J1751-305 was reported active on 04 April 2007 by RXTE (Markwardt & Swank, ATEL #1045) and INTEGRAL (Falanga et al., ATEL #1046).

Follow-up observations were obtained by RXTE and Swift, which show the source is already returning to quiescence.

The Swift observation began on 05 April 2007 16:25 UT and had a duration of about 2000 sec. The Swift XRT observation confirms the presence of XTE J1751-305, and no other bright source in the field of view. The 2-10 keV flux is comparable to 1 mCrab.

The RXTE observations occurred on April 5.16, 5.66 and 5.79. The 2-10 keV X-ray fluxes were 5 mCrab, 2 mCrab, and 2 mCrab respectively, with uncertainties of about 2 mCrab. The primary uncertainty is due to the contributions of diffuse galactic emission and nearby point sources. Given the flux of 10 mCrab on April 4.72 (ATEL #1045), the pulsar emission is already fading into quiescence.

No obvious pulsations were present in RXTE PCA observations. Based on the pointed observation on April 5.16, we can only set a pulsation upper limit of about 35% (fractional r.m.s.; 2 sigma confidence). Since the pulsation r.m.s. amplitude was typically 5% for this source, it is quite likely the pulsations were too faint to be detected.

Thus, the entire outburst appears to have lasted less than a 2-3 days. This behavior is similar to that reported in March 2005, when the source faded to non-detectability over the course of a few days (Grebenev et al. ATEL #446; Swank, Markwardt & Smith, ATEL #449).

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