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Register a new userA state change in the low-mass X-ray binary XSS J12270-4859
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Authors
C.G. Bassa
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Millisecond radio pulsars acquire their rapid rotation rates through mass and angular momentum transfer in a low-mass X-ray binary system. Recent studies of PSR J1824-2452I and PSR J1023+0038 have observationally demonstrated this link, and they have also shown that such systems can repeatedly transition back-and-forth between the radio millisecond pulsar and low-mass X-ray binary states. This also suggests that a fraction of such systems are not newly born radio millisecond pulsars but are rather suspended in a back-and-forth state switching phase, perhaps for giga-years. XSS J12270-4859 has been previously suggested to be a low-mass X-ray binary, and until recently the only such system to be seen at MeV-GeV energies. We present radio, optical and X-ray observations that offer compelling evidence that XSS J12270-4859 is a low-mass X-ray binary which transitioned to a radio millisecond pulsar state between 2012 November 14 and 2012 December 21. Though radio pulsations remain to be detected, we use optical and X-ray photometry/spectroscopy to show that the system has undergone a sudden dimming and no longer shows evidence for an accretion disk. The optical observations constrain the orbital period to 6.913+-0.002 hr.
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XSS J12270-4859 is the only low mass X-ray binary (LMXB) with a proposed persistent gamma-ray counterpart in the Fermi-LAT domain, 2FGL 1227.7-4853. Here, we present the results of the analysis of recent INTEGRAL observations, aimed at assessing the long-term variability of the hard X-ray emission, and thus the stability of the accretion state. We confirm that the source behaves as a persistent hard X-ray emitter between 2003 and 2012. We propose that XSS J12270-4859 hosts a neutron star in a propeller state, a state we investigate in detail, developing a theoretical model to reproduce the associated X-ray and gamma-ray properties. This model can be understood as being of a more general nature, representing a viable alternative by which LMXBs can appear as gamma-ray sources. In particular, this may apply to the case of millisecond pulsars performing a transition from a state powered by the rotation of their magnetic field, to a state powered by matter in-fall, such as that recently observed from the transitional pulsar PSR J1023+0038. While the surface magnetic field of a typical NS in a LMXB is lower by more than four orders of magnitude than the much more intense fields of neutron stars accompanying high-mass binaries, the radius at which the matter in-flow is truncated in a NS-LMXB system is much lower. The magnetic field at the magnetospheric interface is then orders of magnitude larger at this interface, and as consequence, so is the power to accelerate electrons. We demonstrate that the cooling of the accelerated electron population takes place mainly through synchrotron interaction with the magnetic field permeating the interface, and through inverse Compton losses due to the interaction between the electrons and the synchrotron photons they emit. We found that self-synchrotron Compton processes can explain the high energy phenomenology of XSS J12270-4859.
We present the first detection of X-ray coherent pulsations from the transitional millisecond pulsar XSS J12270-4859, while it was in a sub-luminous accretion disk state characterized by a 0.5-10 keV luminosity of 5E33 erg/s (assuming a distance of 1.4 kpc). Pulsations were observed by XMM-Newton at an rms amplitude of (7.7 +/- 0.5)% with a second harmonic stronger than the the fundamental frequency, and were detected when the source is neither flaring nor dipping. The most likely interpretation of this detection is that matter from the accretion disk was channelled by the neutron star magnetosphere and accreted onto its polar caps. According to standard disk accretion theory, for pulsations to be observed the mass in-flow rate in the disk was likely larger than the amount of plasma actually reaching the neutron star surface; an outflow launched by the fast rotating magnetosphere then probably took place, in agreement with the observed broad-band spectral energy distribution. We also report about the non-detection of X-ray pulsations during a recent observation performed while the source behaved as a rotationally-powered radio pulsar.
We report on the first NuSTAR observation of the transitional millisecond pulsar binary XSS J12270-4859 during its current rotation-powered state, complemented with a 2.5yr-long radio monitoring at Parkes telescope and archival XMM-Newton and Swift X-ray and optical data. The radio pulsar is mainly detected at 1.4GHz displaying eclipses over about 40% of the 6.91h orbital cycle. We derive a new updated radio ephemeris to study the 3-79keV light curve that displays a significant orbital modulation with fractional amplitude of 28+/-3%, a structured maximum centred at the inferior conjunction of the pulsar and no cycle-to-cycle or low-high-flaring mode variabilities. The average X-ray spectrum, extending up to about 70keV without a spectral break, is well described by a simple power-law with photon index Gamma = 1.17+/-0.08 giving a 3-79keV luminosity of 7.6(-0.8;+3.8)x10**32 erg/s, for a distance of 1.37(-0.15;+0.69)kpc. Energy resolved orbital light curves reveal that the modulation is not energy dependent from 3keV to 25keV and is undetected with an upper limit of about 10% above 25keV. Comparison with previous X-ray XMM-Newton observations in common energy ranges confirms that the modulation amplitudes vary on timescales of a few months, indicative of a non-stationary contribution of the intrabinary shock formed by the colliding winds of the pulsar and the companion. A more detailed inspection of energy resolved modulations than previously reported gives hints of a mild softening at superior conjunction of the pulsar below 3keV, likely due to the contribution of the thermal emission from the neutron star. The intrabinary shock emission, if extending into the MeV range, would be energetically capable alone to irradiate the donor star.
XSS J12270-4859 (henceforth J12270) is the first low-mass X-ray binary to exhibit a transition, taking place at the end of 2012, from an X-ray active state to a radio pulsar state. The X-ray emission based on archival XMM-Newton observations is highly variable, showing rapid variations (10 s) from a high X-ray luminosity mode to a low mode and back. A flaring mode has also been observed. X-ray pulsations have been detected during the high mode only. In this work we present two possible interpretations for the rapid swings between the high and low modes. In the first scenario, this phenomenon can be explained by a rapid oscillation between a propeller state and a radio-ejection pulsar state, during which the pulsar wind prevents matter from falling onto the neutron star surface. In the second scenario, a radio pulsar is always active, the intra-binary shock is located just outside the light cylinder in the high mode, while it expands during the low mode. At variance with other transitional pulsars, J12270 shows two instances of the low mode: a low-soft and low-hard mode. Performing an X-ray spectral analysis, we show that the harder component, present in the low-hard spectra, is probably related to the tail of the flare emission. This supports the understanding that the flare mechanism is independent of the high-to-low mode transitions.
The X-ray source XSS J12270-4859 has been first suggested to be a magnetic cataclysmic variable of Intermediate Polar type on the basis of its optical spectrum and a possible 860 s X-ray periodicity. However further X-ray observations by the Suzaku and XMM-Newton satellites did not confirm this periodicity but show a very peculiar variability, including moderate repetitive flares and numerous absorption dips. These characteristics together with a suspected 4.3 h orbital period would suggest a possible link with the so- called dipping sources, a sub-class of Low-Mass X-ray Binaries (LMXB). Based on the released FERMI catalogues, the source was also found coincident with a very high energy (0.1-300 GeV) VHE source 2FGL J1227.7-4853. The good positional coincidence, together with the lack of any other bright X-ray sources in the field, makes this identification highly probable. However, none of the other standard LMXBs have been so far detected by FERMI. Most galactic VHE sources are associated with rotation-powered pulsars. We present here new results obtained from a 30 ksec high-time resolution XMM observations in January 2011 that confirm the flaring-dipping behaviour and provide upper limits on fast X-ray pulsations. We discuss the possible association of the source with either a microquasar or an accreting rotation powered pulsar.
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