The neutron star transient and millisecond pulsar in M28: from sub-luminous accretion to rotation-powered quiescence

Abstract

The X-ray transient IGR J18245–2452 in the globular cluster M28 contains the first neutron star (NS) seen to switch between rotation-powered and accretion-powered pulsations. We analyse its 2013 March–April 25 d long outburst as observed by Swift, which had a peak bolometric luminosity of ∼6 per cent of the Eddington limit (LEdd), and give detailed properties of the thermonuclear burst observed on 2013 April 7. We also present a detailed analysis of new and archival Chandra data, which we use to study quiescent emission from IGR J18245–2452 between 2002 and 2013. Together, these observations cover almost five orders of magnitude in X-ray luminosity (LX, 0.5–10 keV). The Swift spectrum softens during the outburst decay (photon index Γ from 1.3 above LX/LEdd = 10−2 to ∼2.5 at LX/LEdd = 10−4), similar to other NS and black hole transients. At even lower luminosities, LX/LEdd = [10−4–10−6], deep Chandra observations reveal hard (Γ = 1–1.5), purely non-thermal and highly variable X-ray emission in quiescence. We therefore find evidence for a spectral transition at LX/LEdd ∼ 10−4, where the X-ray spectral softening observed during the outburst decline turns into hardening as the source goes to quiescence. Furthermore, we find a striking variability pattern in the 2008 Chandra light curves: rapid switches between a high-luminosity ‘active’ state (LX ≃ 3.9 × 1033 erg s−1) and a low-luminosity ‘passive’ state (LX ≃ 5.6 × 1032 erg s−1), with no detectable spectral change. We put our results in the context of low-luminosity accretion flows around compact objects and X-ray emission from millisecond radio pulsars. Finally, we discuss possible origins for the observed mode switches in quiescence, and explore a scenario where they are caused by fast transitions between the magnetospheric accretion and pulsar wind shock emission regimes.