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ATEL # 2197; C. B. Markwardt (U. Maryland & NASA/GSFC), D. Altamirano (UvA), J. H. Swank (NASA/GSFC), T. E. Strohmayer (NASA/GSFC), M. Linares (UvA), D. Pereira (WYLE & NASA/GSFC)
on 13 Sep 2009; 20:47 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Craig B. Markwardt (email@example.com)
Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Pulsars, Transients
Referred to by ATEL #: 2198, 2199
RXTE PCA Bulge scans detected activity of what appeared to be a known source. However, based on the results presented below, and the report of a new INTEGRAL source IGR J17511-305 (Baldovin et al. ATEL #2196), the activity detected by RXTE is due to a new 245 Hz X-ray pulsar.
The source was initially detected on 2009-09-11 at 17:50 UT as a rising flux attributed to two known sources, XTE J1751-305 and GRS 1747-312, the first of which is a known 435 Hz X-ray pulsar.
The position of XTE J1751-305 was observed by RXTE PCA on 2009-09-12 at 19:30 for 8.5 ksec. X-ray pulsations were detected at 245 Hz, which reveal that there is a new X-ray millisecond pulsar in the PCA field of view. Swift XRT also observed the position of XTE J1751-305 on 2009-09-12, at 18:40 UT and did not detect a source at the known Chandra position, or any other position in the 22 arcmin diameter field of view. A crude fit to the source position from the PCA bulge scan data agrees with the position of IGR J17511-305 (ATEL #2196) within ~10 arcmin. Given the coincidence of the newly reported source within the PCA field of view but not Swift XRT, we now attribute the pulsations to IGR J17511-305.
Doppler modulations are clearly visible in the barycentered data (barycentered with the IGR position). A preliminary orbital solution yields a barycentered pulse frequency of 244.8337(1) Hz, an orbital period of 207.4(8) minutes, and an a*sin(i) value of 274(1) lt-ms. (All uncertainties 1-sigma in the last digit.) The mass function is 0.00107(2) M_sun, giving a minimum companion mass of 0.13 solar masses. Unlike the nearby ultracompact system, XTE J1751-305, IGR J17511-305 is a more "typical" low mass X-ray binary.
The X-ray spectrum on 2009-09-12 is consistent with an absorbed power-law model with photon index 1.8, and a possible Fe line at 6.6 keV. However, given the potential for contamination from the galactic ridge, the line may be due to diffuse emission. Fluxes in the 2-10, 10-20, 20-40 keV bands are approximately 4, 2 and 2, in units of 1e-10 erg/cm^2/s. The flux history suggests a rising 2-10 keV flux, from 9 to 13 mCrab from 2009-09-11 to 2009-09-12.
While it may be surprising that two millisecond X-ray pulsars would be found so close together and not in a globular cluster, we find no reason to believe that XTE J1751-305 and IGR J17511-305 are not two distinct X-ray pulsars, as they have distinct spin and orbital periods. PCA Bulge scan intensities are based on fits to the X-ray scan light curve with templates of known sources. In crowded regions such as this one, it is possible for a new source to be confused with one or more other known sources, which explains the early misattribution.
Further observations are being planned by RXTE, and observations at other wavelengths are encouraged.
We appreciate rapid follow-up scheduling by both the RXTE and Swift teams.