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Register a new userEvidence of Fast Magnetic Field Evolution in an Accreting Millisecond Pulsar
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Authors
A. Patruno
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The large majority of neutron stars (NSs) in low mass X-ray binaries (LMXBs) have never shown detectable pulsations despite several decades of intense monitoring. The reason for this remains an unsolved problem that hampers our ability to measure the spin frequency of most accreting NSs. The accreting millisecond X-ray pulsar (AMXP) HETE J1900.1--2455 is an intermittent pulsar that exhibited pulsations at about 377 Hz for the first 2 months and then turned in a non-pulsating source. Understanding why this happened might help to understand why most LMXBs do not pulsate. We present a 7 year long coherent timing analysis of data taken with the Rossi X-ray Timing Explorer. We discover new sporadic pulsations that are detected on a baseline of about 2.5 years. We find that the pulse phases anti-correlate with the X-ray flux as previously discovered in other AMXPs. We place stringent upper limits of 0.05% rms on the pulsed fraction when pulsations are not detected and identify an enigmatic pulse phase drift of ~180 degrees in coincidence with the first disappearance of pulsations. Thanks to the new pulsations we measure a long term spin frequency derivative whose strength decays exponentially with time. We interpret this phenomenon as evidence of magnetic field burial.
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We present results of targeted searches for signatures of non-radial oscillation modes (such as r- and g-modes) in neutron stars using {it RXTE} data from several accreting millisecond X-ray pulsars (AMXPs). We search for potentially coherent signals in the neutron star rest frame by first removing the phase delays associated with the stars binary motion and computing FFT power spectra of continuous light curves with up to $2^{30}$ time bins. We search a range of frequencies in which both r- and g-modes are theoretically expected to reside. Using data from the discovery outburst of the 435 Hz pulsar XTE J1751$-$305 we find a single candidate, coherent oscillation with a frequency of $0.5727597 times u_{spin} = 249.332609$ Hz, and a fractional Fourier amplitude of $7.46 times 10^{-4}$. We estimate the significance of this feature at the $1.6 times 10^{-3}$ level, slightly better than a $3sigma$ detection. We argue that possible mode identifications include rotationally-modified g-modes associated with either a helium-rich surface layer or a density discontinuity due to electron captures on hydrogen in the accreted ocean. Alternatively, the frequency could be identified with that of an inertial mode or an r-mode modified by the presence of a solid crust, however, the r-mode amplitude required to account for the observed modulation amplitude would induce a large spin-down rate inconsistent with the observed pulse timing measurements. For the AMXPs XTE J1814$-$338 and NGC 6440 X-2 we do not find any candidate oscillation signals, and we place upper limits on the fractional Fourier amplitude of any coherent oscillations in our frequency search range of $7.8times 10^{-4}$ and $5.6 times 10^{-3}$, respectively. We briefly discuss the prospects and sensitivity for similar searches with future, larger X-ray collecting area missions.
We report the detection of a possible gamma-ray counterpart of the accreting millisecond pulsar SAX J1808.4-3658. The analysis of ~6 years of data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (Fermi-LAT) within a region of 15deg radius around the position of the pulsar reveals a point gamma-ray source detected at a significance of ~6 sigma (Test Statistic TS = 32), with position compatible with that of SAX J1808.4-3658 within 95% Confidence Level. The energy flux in the energy range between 0.6 GeV and 10 GeV amounts to (2.1 +- 0.5) x 10-12 erg cm-2 s-1 and the spectrum is well-represented by a power-law function with photon index 2.1 +- 0.1. We searched for significant variation of the flux at the spin frequency of the pulsar and for orbital modulation, taking into account the trials due to the uncertainties in the position, the orbital motion of the pulsar and the intrinsic evolution of the pulsar spin. No significant deviation from a constant flux at any time scale was found, preventing a firm identification via time variability. Nonetheless, the association of the LAT source as the gamma-ray counterpart of SAX J1808.4-3658 would match the emission expected from the millisecond pulsar, if it switches on as a rotation-powered source during X-ray quiescence.
The accreting millisecond pulsars IGR J00291+5934 and SAX J1808.4-3658 are two compact binaries with very similar orbital parameters. The latter has been observed to evolve on a very short timescale of ~70 Myr which is more than an order of magnitude shorter than expected. There is an ongoing debate on the possibility that the pulsar spin-down power ablates the companion generating large amount of mass-loss in the system. It is interesting therefore to study whether IGR J00291+5934 does show a similar behaviour as its twin system SAX J1808.4-3658. In this work we present the first measurement of the orbital period derivative of IGR J00291+5934. By using XMM-Newton data recorded during the 2015 outburst and adding the previous results of the 2004 and 2008 outbursts, we are able to measure a 90% confidence level upper limit for the orbital period derivative of -5x10^-13<Pb_dot<6x10^-13. This implies that the binary is evolving on a timescale longer than ~0.5 Gyr, which is compatible with the expected timescale of mass transfer driven by angular momentum loss via gravitational radiation. We discuss the scenario in which the power loss from magnetic dipole radiation of the neutron star is hitting the companion star. If this model is applied to SAX J1808.4-3658 then the difference in orbital behavior can be ascribed to a different efficiency for the conversion of the spin-down power into energetic relativistic pulsar wind and X-ray/gamma-ray radiation for the two pulsars, with IGR J00291+5934 requiring an extraordinarily low efficiency of less than 5% to explain the observations. Alternatively, the donor in IGR J00291+5934 is weakly/not magnetized which would suppress the possibility of generating mass-quadrupole variations.
Millisecond spinning, low magnetic field neutron stars are believed to attain their fast rotation in a 0.1-1 Gyr-long phase during which they accrete matter endowed with angular momentum from a low-mass companion star. Despite extensive searches, coherent periodicities originating from accreting neutron star magnetospheres have been detected only at X-ray energies and in ~10% of the presently known systems. Here we report the detection of optical and ultraviolet coherent pulsations at the X-ray period of the transient low mass X-ray binary system SAX J1808.4-3658, during an accretion outburst that occurred in August 2019. At the time of the observations, the pulsar was surrounded by an accretion disc, displayed X-ray pulsations and its luminosity was consistent with magnetically funneled accretion onto the neutron star. Current accretion models fail to account for the luminosity of both optical and ultraviolet pulsations; these are instead more likely driven by synchro-curvature radiation in the pulsar magnetosphere or just outside of it. This interpretation would imply that particle acceleration can take place even when mass accretion is going on, and opens up new perspectives in the study of coherent optical/UV pulsations from fast spinning accreting neutron stars in low-mass X-ray binary systems.
The flow of a matter, accreting onto a magnetized neutron star, is accompanied by an electric current. The closing of the electric current occurs in the crust of a neutron stars in the polar region across the magnetic field. But the conductivity of the crust along the magnetic field greatly exceeds the conductivity across the field, so the current penetrates deep into the crust down up to the super conducting core. The magnetic field, generated by the accretion current, increases greatly with the depth of penetration due to the Hall conductivity of the crust is also much larger than the transverse conductivity. As a result, the current begins to flow mainly in the toroidal direction, creating a strong longitudinal magnetic field, far exceeding an initial dipole field. This field exists only in the narrow polar tube of $r$ width, narrowing with the depth, i.e. with increasing of the crust density $rho$, $rpropto rho^{-1/4}$. Accordingly, the magnetic field $B$ in the tube increases with the depth, $Bpropto rho^{1/2}$, and reaches the value of about $10^{17}$ Gauss in the core. It destroys super conducting vortices in the core of a star in the narrow region of the size of the order of ten centimeters. Because of generated density gradient of vortices they constantly flow into this dead zone and the number of vortices decreases, the magnetic field of a star decreases as well. The attenuation of the magnetic field is exponential, $B=B_0(1+t/tau)^{-1}$. The characteristic time of decreasing of the magnetic field $tau$ is equal to $tausimeq 10^3$ years. Thus, the magnetic field of accreted neutron stars decreases to values of $10^8 - 10^9$ Gauss during $10^7-10^6$ years.
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