Monitoring Supergiant Fast X-ray Transients with Swift. Rise to the outburst in IGR J16479-4514

摘要

IGR J16479-4514 is a Supergiant Fast X-ray Transient (SFXT), a new class ofHigh Mass X-ray Binaries, whose number is rapidly growing thanks to theobservations of the Galactic plane performed with the INTEGRAL satellite. IGRJ16479-4514 has been regularly monitored with Swift/XRT since November 2007, tostudy the quiescent emission, the outburst properties and their recurrence. Anew bright outburst, reaching fluxes above 10$^{-9}$ erg cm$^{-2}$ s$^{-1}$,was caught by the Swift/BAT. Swift immediately re-pointed at the target with the narrow-field instrumentsso that, for the first time, an outburst from a SFXT where a periodicity in theoutburst recurrence is unknown could be observed simultaneously in the 0.2--150keV energy band. The X-ray emission is highly variable and spans almost fourorders of magnitude in count rate during the Swift/XRT observations covering afew days before and after the bright peak. The X-ray spectrum in outburst ishard and highly absorbed. The power-law fit resulted in a photon index of0.98$\pm{0.07}$, and in an absorbing column density of $\sim5\times10^{22}$cm$^{-2}$. These observations demonstrate that in this source (similarly towhat was observed during the 2007 outburst from the periodic SFXT IGRJ11215-5952), the accretion phase lasts much longer than a few hours.