Broad iron emission line and kilohertz quasi-periodic oscillations in the neutron star system 4U 1636–53

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% 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 (BH) 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.

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Identifiers

ORCID for Diego Altamirano: orcid.org/0000-0002-3422-0074

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