RXTE detects a coherent signal at ~ 401 Hz from IGR J17498-2921.

ATel #3556; A. Papitto (Univ. Cagliari), C. Ferrigno, E. Bozzo, L. Gibaud (ISDC, Switzerland), L. Burderi (Univ. Cagliari), T. Di Salvo (Univ. Palermo), A. Riggio (INAF OAC)
on 13 Aug 2011; 10:54 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

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A RXTE Target of Opportunity pointed observation of the newly discovered transient IGR J17498-2921 (ATsLs #3551, #3555) began at roughly 2011-09-13 02:19 UTC, for an exposure of 1.8 ks. The observed flux in the 2-30 keV energy band is 1.1(1)e-9 erg/s/cm². Such a value is probably an over-estimate of the source flux, because of the contribution by the emission of the Galactic ridge.

Power density spectra produced every 256s reveal the presence of a signal at a frequency between 400.95 and 401.0 Hz, making this source a candidate accreting millisecond pulsar. Taking into account the number of frequencies searched in each of the intervals (2**20), the signal is detected at a significance in excess of 4.7 sigma in all but one of the 256s interval considered. The frequency is observed to vary by ~0.05 Hz during the observation. Such a variation is possibly caused by the uncorrected orbital motion of the spacecraft and/or of the NS in the system. By considering only the intervals during which the signal is significantly detected, we set a lower limit of 0.027 on the RMS fractional amplitude of the signal (3 sigma confidence level).

Further observations at these and other wavelengths are strongly encouraged.

We thank the RXTE team for the prompt scheduling of this observation.