Discovery of kilohertz QPOs in XTE J1701-407

ATel #1635; T. E Strohmayer (NASA/GSFC); C. B. Markwardt (CRESST/U. Md./NASA/GSFC); J. H. Swank (NASA/GSFC)
on 29 Jul 2008; 17:06 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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We report the discovery with the RXTE PCA of kilohertz quasiperiodic oscillations (kHz QPOs) in the X-ray flux from the recently discovered neutron star transient XTE J1701-407 (Atel 1569, 1616, 1618, 1621,1630). Power spectral analysis of RXTE PCA Observations obtained on 2008-06-16 at 05:53 UTC, show a strong 30 Hz QPO as well as a high frequency kHz QPO centered at 1150 +- 7 Hz. These QPO frequencies are characteristic of the "atoll" class of accreting neutron stars. The 1150 Hz QPO has a coherence of about 30 (centroid / FWHM), and is very strong, with an average fractional r.m.s. amplitude of 25 +- 4 % (3-30 keV). The source flux during this pointing was approximately 15 mCrab. This makes XTE J1701-407 one of only two sources with such high amplitude kHz QPOs, the other being 1A 1246-588 (Jonker et al. MNRAS, 2007, 378, 1187).

In an observation on 2008-07-18 at 02:30 UTC, when the source was brighter (near 25 mCrab) we find evidence for a simultaneous pair of kHz QPO. The lower kHz feature is centered near 745 +- 9 Hz, with a coherence of about 40 and a fractional r.m.s. amplitude of approximately 9%. The upper QPO remains near 1150 Hz, but it is weaker than in the other observation. Using the upper frequency measurement from the 2008-06-16 observation and the lower frequency from the 2008-07-18 data we would infer a separation of 405 +- 11 Hz, which would suggest a frequency difference for the pair near the upper end of the distribution of observed frequency separations. We caution, however, that the simultaneous detection of kHz QPO pairs should be confirmed with additional data. The large r.m.s amplitude of the upper QPO and the apparently large frequency separation for the kHz QPO pair suggests that XTE J1701-407 has an interesting kHz QPO phenomenology. We urge further X-ray as well as multi-wavelength observations to further investigate this source.