The X-ray transient IGR J18245-2452 in the globular cluster M28 contains thefirst neutron star (NS) seen to switch between rotation-powered andaccretion-powered pulsations. We analyse its 2013 March-April 25d-long outburstas observed by Swift, which had a peak bolometric luminosity of ~6% of theEddington limit (L$_{E}$), and give detailed properties of the thermonuclearburst observed on 2013 April 7. We also present a detailed analysis of new andarchival Chandra data, which we use to study quiescent emission from IGRJ18245-2452 between 2002 and 2013. Together, these observations cover almostfive orders of magnitude in X-ray luminosity (L$_X$, 0.5-10 keV). The Swiftspectrum softens during the outburst decay (photon index $\Gamma$ from 1.3above L$_X$/L$_{E}$=10$^{-2}$ to ~2.5 at L$_X$/L$_{E}$=10$^{-4}$), similar toother NS and black hole (BH) transients. At even lower luminosities, deepChandra observations reveal hard ($\Gamma$=1-1.5), purely non-thermal andhighly variable X-ray emission in quiescence. We therefore find evidence for aspectral transition at L$_X$/L$_{E}$~10$^{-4}$, where the X-ray spectralsoftening observed during the outburst decline turns into hardening as thesource goes to quiescence. Furthermore, we find a striking variability patternin the 2008 Chandra light curves: rapid switches between a high-L$_X$ "active"state (L$_X\simeq$3.9x10$^{33}$ erg/s) and a low-L$_X$ "passive" state(L$_X\simeq$5.6x10$^{32}$ erg/s), with no detectable spectral change. We putour results in the context of low luminosity accretion flows around compactobjects and X-ray emission from millisecond radio pulsars. Finally, we discusspossible origins for the observed mode switches in quiescence, and explore ascenario where they are caused by fast transitions between the magnetosphericaccretion and pulsar wind shock emission regimes.
展开▼
