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Discovery of a short orbital period in the Supergiant Fast X-ray Transient IGR J16479-4514

We report here discovery of a 3.32 day orbital period in the Supergiant Fast X-ray Transient (SFXT) source IGR J16479-4514. Using the long term light curve of this source obtained with Swift-BAT in the energy range of 15-50 keV, we have clearly detected an orbital modulation including a full eclipse of duration ~0.6 day. In the hard X-ray band of the BAT instrument, the eclipse ingress and egress are rapid. We have also used the long term light curve obtained with the RXTE -ASM in the energy range of 1.5-12 keV. Taken independently, the detection of orbital modulation in the RXTE -ASM light curve is not significant. However, considering a clear detection of orbital modulation in the BAT light curve, we have used the ASM light curve for a more precise determination of the orbital period. IGR J16479-4514 has the shortest orbital period among the three SFXTs with measured/known orbital period. We discuss the implication of a short orbital period with the various mechanisms proposed to explain the transient nature of this class of sources.