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 25d-long outburst as observed by Swift, which had a peak bolometric luminosity of ~6% of the Eddington limit (L${}_E$), 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 (L${}_X$, 0.5-10 keV). The Swift spectrum softens during the outburst decay (photon index $\Gamma$ from 1.3 above L${}_X$/L${}_E$=10${}^{-2}$ to ~2.5 at L${}_X$/L${}_E$=10${}^{-4}$), similar to other NS and black hole (BH) transients. At even lower luminosities, deep Chandra observations reveal hard ($\Gamma$=1-1.5), purely non-thermal and highly variable X-ray emission in quiescence. We therefore find evidence for a spectral transition at L${}_X$/L${}_E$~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-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 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.

Accepted for publication in MNRAS. 12 pages, 6 figures, 2 tables

References in corpus (10)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• A Radio Pulsar/X-ray Binary Link
• Models of crustal heating in accreting neutron stars
• Six new candidate ultracompact X-ray binaries
• The hard X-ray spectral evolution in X-ray binaries and its application to constrain the black hole mass of ultraluminous X-ray sources
• X-ray Variability and Evidence for Pulsations from the Unique Radio Pulsar/X-ray Binary Transition Object FIRST J102347.6+003841
• Faint X-ray Sources in the Globular Cluster Terzan 5
• SDSS J102347.6+003841: A Millisecond Radio Pulsar Binary That Had A Hot Disk During 2000-2001
• Studying Millisecond Pulsars in X-rays
• Swift observations of SAX J1808.4-3658: monitoring the return to quiescence

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