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Register a new userAccretion-powered pulsations in an apparently quiescent neutron star binary
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Accreting millisecond X-ray pulsars are an important subset of low-mass X-ray binaries in which coherent X-ray pulsations can be observed during occasional, bright outbursts (X-ray luminosity $L_Xsim 10^{36}$ erg s$^{-1}$). These pulsations show that matter is being channeled onto the neutron stars magnetic poles. However, such sources spend most of their time in a low-luminosity, quiescent state ($L_Xlesssim 10^{34}$ erg s$^{-1}$), where the nature of the accretion flow onto the neutron star (if any) is not well understood. Here we report that the millisecond pulsar/low-mass X-ray binary transition object PSR J1023+0038 intermittently shows coherent X-ray pulsations at luminosities nearly 100 times fainter than observed in any other accreting millisecond X-ray pulsar. We conclude that in spite of its low luminosity PSR J1023+0038 experiences episodes of channeled accretion, a discovery that challenges existing models for accretion onto magnetized neutron stars.
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We describe observations of the seventh accretion-powered millisecond pulsar, HETE J1900.1-2455 made with the Rossi X-ray Timing Explorer during the year of activity that followed its discovery in 2005 June. We detected intermittent pulsations at a peak fractional amplitude of 3%, but only in the first two months of the outburst. On three occasions during this time we observed an abrupt increase in the pulse amplitude, approximately coincident with the time of a thermonuclear burst, followed by a steady decrease on a timescale of approx. 10 d. HETE J1900.1-2455 has shown the longest active period by far for any transient accretion-powered millisecond pulsar, comparable instead to the outburst cycles for other transient X-ray binaries. Since the last detection of pulsations, HETE J1900.1-2455 has been indistinguishable from a low-accretion rate, non-pulsing LMXB; we hypothesize that other, presently active LMXBs may have also been detectable initially as millisecond X-ray pulsars.
We discuss properties of the ultra-luminous $X$-ray source in the galaxy M82, NuSTAR J095551+6940.8, containing an accreting neutron star. The neutron star has surface magnetic field $ B_{NS} approx 1.4 times 10^{13 } , {rm G}$ and experiences accretion rate of $9 times 10^{-7} M_odot {rm , yr}^{-1} $. The magnetospheric radius, close to the corotation radius, is $sim 2 times 10^8$ cm. The accretion torque on the neutron star is reduce well below what is expected in a simple magnetospheric accretion due to effective penetration of the stellar magnetic field into the disk beyond the corotation radius. As a result, the radiative force of the surface emission does not lead to strong coronal wind, but pushes plasma along magnetic field lines towards the equatorial disk. The neutron star is nearly an orthogonal rotator, with the angle between the rotation axis and the magnetic moment $geq 80$ degrees. Accretion occurs through optically thick -- geometrically thin and flat accretion curtain, which cuts across the polar cap. High radiation pressure from the neutron star surface is nevertheless smaller than that the ram pressure of the accreting material flowing through the curtain, and thus fails to stop the accretion. At distances below few stellar radii the magnetic suppression of the scattering cross-section becomes important. The $X$-ray luminosity (pulsed and persistent components) comes both from the neutron star surface as a hard $X$-ray component and as a soft component from reprocessing by the accretion disk.
Ultraluminous X-ray sources (ULX) are off-nuclear point sources in nearby galaxies whose X-ray luminosity exceeds the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes. Their luminosity ranges from $10^{40}$ erg s$^{-1} < L_X$(0.5 - 10 keV) $<10^{40}$ erg s$^{-1}$. Since higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit theoretical models have focused on black hole rather than neutron star systems. The most challenging sources to explain are those at the luminous end ($L_X$ > $10^{40}$ erg s$^{-1}$), which require black hole masses MBH >50 solar masses and/or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries. Here we report broadband X-ray observations of the nuclear region of the galaxy M82, which contains two bright ULXs. The observations reveal pulsations of average period 1.37 s with a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to $L_X$(3 - 30 keV) = $4.9 times 10^{39}$ erg s$^{-1}$. The pulsating source is spatially coincident with a variable ULX which can reach $L_X$ (0.3 - 10 keV) = $1.8 times 10^{40}$ erg s$^{-1}$. This association implies a luminosity ~100 times the Eddington limit for a 1.4 solar mass object, or more than ten times brighter than any known accreting pulsar. This finding implies that neutron stars may not be rare in the ULX population, and it challenges physical models for the accretion of matter onto magnetized compact objects.
During the final stages of a compact object merger, if at least one of the binary components is a magnetized neutron star (NS), then its orbital motion substantially expands the NSs open magnetic flux -- and hence increases its wind luminosity -- relative to that of an isolated pulsar. As the binary orbit shrinks due to gravitational radiation, the power and speed of this binary-induced inspiral wind may (depending on pair loading) secularly increase, leading to self-interaction and internal shocks in the outflow beyond the binary orbit. The magnetized forward shock can generate coherent radio emission via the synchrotron maser process, resulting in an observable radio precursor to binary NS merger. We perform 1D relativistic hydrodynamical simulations of shock interaction in the accelerating binary NS wind, assuming that the inspiral wind efficiently converts its Poynting flux into bulk kinetic energy prior to the shock radius. This is combined with the shock maser spectrum from particle-in-cell simulations, to generate synthetic radio light curves. The precursor burst with a fluence of $sim1$ Jy$cdot$ms at $sim$GHz frequencies lasts $sim 1-500$ ms following the merger for a source at $sim3$ Gpc ($B_{rm d}/10^{12}$ G)$^{8/9}$, where $B_{rm d}$ is the dipole field strength of the more strongly-magnetized star. Given an outflow geometry concentrated along the binary equatorial, the signal may be preferentially observable for high-inclination systems, i.e. those least likely to produce a detectable gamma-ray burst.
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$_Xsimeq$3.9x10$^{33}$ erg/s) and a low-L$_X$ passive state (L$_Xsimeq$5.6x10$^{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.
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